Ilja Gerhardt

@article{wang_optica_2022,
author = {Denis Uhland, Helena Dillmann, Yijun Wang, and Ilja Gerhardt},
journal = {New Journal of Physics},
keywords = {Atomic Filters, Faraday Filters, Alkali Atoms},
title = {How to build an optical filter with an atomic vapor cell},
year = {2023},
pages = {125001},
url = {https://iopscience.iop.org/article/10.1088/1367-2630/ad0fa8},
abstract = {The nature of atomic vapors, their natural alignment with interatomic transitions, and their ease of use make them highly suited for spectrally narrow-banded optical filters. Atomic filters come in two flavors: a filter based on the absorption of light by the Doppler broadened atomic vapor, i.e. a notch filter, and a bandpass filter based on the transmission of resonant light caused by the Faraday effect. The notch filter uses the absorption of resonant photons to filter out a small spectral band around the atomic transition. The off-resonant part of the spectrum is fully transmitted. Atomic vapors based on the Faraday effect allow for suppression of the detuned spectral fraction. Transmission of light originates from the magnetically induced rotation of linear polarized light close to an atomic resonance. This filter constellation allows selective acceptance of specific light frequencies. In this manuscript, we discuss these two types of filters and elucidate the specialties of atomic line filters. We also present a practical guide on building such filter setups from scratch and discuss an approach to achieve an almost perfect atomic spectrum backed by theoretical calculations.}
}

@article{wang_optica_2022,
author = {Yijun Wang, Vladislav Bushmakin, Guilherme Alexander Stein, Andreas W. Schell, and Ilja Gerhardt},
journal = {Optica},
keywords = {Single Molecules; Single Photons; Coherence; Ramsey Spectroscopy},
title = {Optical Ramsey spectroscopy on a single molecule},
year = {2022},
pages = {374-378},
url = {https://doi.org/10.1364/OPTICA.443727},
abstract = {Organic chemistry offers the potential to synthesize individual molecules for specific tasks in quantum information processing. One of their striking properties is the emission of single photons with nearly ideal coherence. Here, we implement Ramsey spectroscopy to measure the electronic state coherence of a single molecule. Conventionally, the emitter’s coherence is characterized by saturation or intensity auto-correlation measurements. However, both methods are under the steady influence of continuous interaction with the excitation laser. This influence can be eliminated by using a pump–probe sequence of two optical pulses to observe the decoherence. We have measured a near-transform-limited decoherence time of 17.6 ns, while the radiative lifetime is 10.1 ns. We also perform frequency-detuned excitation, gaining richer insights into the dephasing behavior of the molecule. The experiments exhibit that optical Ramsey spectroscopy is a promising tool for measuring the emitter’s coherence properties.}
}

@article{afach_np_2021,
author = {Samer Afach, Ben C. Buchler, Dmitry Budker, Conner Dailey, Andrei Derevianko, Vincent Dumont, Nataniel L. Figueroa, Ilja Gerhardt, Zoran D. Grujić, Hong Guo, Chuanpeng Hao, Paul S. Hamilton, Morgan Hedges, Derek F. Jackson Kimball, Dongok Kim, Sami Khamis, Thomas Kornack, Victor Lebedev, Zheng-Tian Lu, Hector Masia-Roig, Madeline Monroy, Mikhail Padniuk, Christopher A. Palm, Sun Yool Park, Karun V. Paul, Alexander Penaflor, Xiang Peng, Maxim Pospelov, Rayshaun Preston, Szymon Pustelny, Theo Scholtes, Perrin C. Segura, Yannis K. Semertzidis, Dong Sheng, Yun Chang Shin, Joseph A. Smiga, Jason E. Stalnaker, Ibrahim Sulai, Dhruv Tandon, Tao Wang, Antoine Weis, Arne Wickenbrock, Tatum Wilson, Teng Wu, David Wurm, Wei Xiao, Yucheng Yang, Dongrui Yu & Jianwei Zhang},
journal = {Nature Physics},
keywords = {dark matter detection, GNOME, magnetometer, atomic vapors},
title = {Search for topological defect dark matter with a global network of optical magnetometers},
year = {2021},
pages = {1--12},
url = {https://doi.org/10.1038/s41567-021-01393-y},
abstract = {Ultralight bosons such as axion-like particles are viable candidates for dark matter. They can form stable, macroscopic field configurations in the form of topological defects that could concentrate the dark matter density into many distinct, compact spatial regions that are small compared with the Galaxy but much larger than the Earth. Here we report the results of the search for transient signals from the domain walls of axion-like particles by using the global network of optical magnetometers for exotic (GNOME) physics searches. We search the data, consisting of correlated measurements from optical atomic magnetometers located in laboratories all over the world, for patterns of signals propagating through the network consistent with domain walls. The analysis of these data from a continuous month-long operation of GNOME finds no statistically significant signals, thus placing experimental constraints on such dark matter scenarios.}
}

@article{toninelli_nm_2021,
author = {C. Toninelli and I. Gerhardt and A. S. Clark and A. Reserbat-Plantey and S. G\"otzinger and Z. Ristanovic and M. Colautti and P. Lombardi and K. D. Major and I. Deperasinska and W. H. Pernice and F. H. L. Koppens and B. Kozankiewicz and A. Gourdon and V. Sandoghdar and M. Orrit },
journal = {Nature Materials},
keywords = {Single Molecules; Single photons; Quantum Technology},
title = {Single organic molecules for photonic quantum technologies},
year = {2021},
pages = {1--14},
url = {https://www.nature.com/articles/s41563-021-00987-4},
abstract = {Isolating single molecules in the solid state has allowed fundamental experiments in basic and applied sciences. When cooled down to liquid helium temperature, certain molecules show transition lines that are tens of megahertz wide, limited by only the excited-state lifetime. The extreme flexibility in the synthesis of organic materials provides, at low costs, a wide palette of emission wavelengths and supporting matrices for such single chromophores. In the past few decades, their controlled coupling to photonic structures has led to an optimized interaction efficiency with light. Molecules can hence be operated as single-photon sources and as nonlinear elements with competitive performance in terms of coherence, scalability and compatibility with diverse integrated platforms. Moreover, they can be used as transducers for the optical read-out of fields and material properties, with the promise of single-quanta resolution in the sensing of charges and motion. We show that quantum emitters based on single molecules hold promise to play a key role in the development of quantum science and technologies.}
}

@article{chen_pra_2021,
author = {Xing Chen, Kai Redeker, Robert Garthoff, Wenjamin Rosenfeld, J\"org Wrachtrup, and Ilja Gerhardt},
journal = {Phys. Rev. A},
keywords = {Randomness; Quantumness; Bell Test, QRNG},
title = {Certified randomness from a remote-state-preparation dimension witness},
year = {2021},
number = {103},
pages = {042211},
url = {https://doi.org/10.1103/PhysRevA.103.042211},
abstract = {Randomness in Bell test data can be device-independently certified by Bell's theorem without placing assumptions about the experimental devices. The device-independent randomness has very demanding requirements about the experimental devices and relatively lower output randomness. With the same Bell test data we can extract substantially more randomness without using Bell's theorem. To achieve this goal, we introduce a remote-state-preparation dimension witness and a semi-device-independent randomness certification model which is based on it. This is one important step towards practical use of the Bell test in randomness generation.}
}

@article{vural_prb_2020,
author = {H\"usseyin Vural and Julian Maisch and Ilja Gerhardt and Michael Jetter and Simone Luca Portalupi and Peter Michler},
journal = {Physical Review B},
keywords = {Single photons; Slow Light; Atomic Vapors; Quantum Dots},
pages = {161401},
title = {Characterization of spectral diffusion by slow-light photon-correlation spectroscopy},
volume = {101},
year = {2020},
url = {https://link.aps.org/doi/10.1103/PhysRevB.101.161401},
abstract = {Solid-state quantum emitters are playing a crucial role in the development of photonic quantum technologies. However, several decoherence mechanisms are known to impact the device performances and scalability. In various quantum emitter materials the fluctuation of the two-level system has been investigated by different methods to capture the evolution of coherence reduction. However, resolving the dynamics of spectral diffusion in the fundamental framework of on-demand single-photon generation remained unsolved. Hereby, the challenge is to observe with high time and energy resolution the impact of fluctuations directly in the properties of the emitted photons. In this Rapid Communication, we demonstrate the use of dispersion in a slow-light medium to map the photons frequency domain into time domain, observed as frequency-dependent time-of-flight. This allows for the measurement of the emission spectrum and the quantification of the spectral diffusion dynamics in one intensity correlation measurement. On exemplary semiconductor quantum dots, the impact of charge and spin noise on the spectral diffusion are revealed to follow an Ornstein-Uhlenbeck process. By a single measurement, broadening from the excitation repetition up to the stationary limit is resolved. This enables one to extract time-dependent two-photon interference visibilities for various timescales, which is a key performance measure for quantum emitters.}
}

@article{chen_srep_2019,
author = {Xing Chen and Johannes N. Greiner and J\"org Wrachtrup and Ilja Gerhardt},
journal = {Scientific Reports},
keywords = {Single photons; Non-classical light, Quantum randomness; Anti-bunching; Entropy},
pages = {18474},
title = {Single Photon Randomness based on a Defect Center in Diamond},
volume = {9},
year = {2019},
url = {https://doi.org/10.1038/s41598-019-54594-0},
abstract = {The prototype of a quantum random number generator is a single photon which impinges onto a beam splitter and is then detected by single photon detectors at one of the two output paths. Prior to detection, the photon is in a quantum mechanical superposition state of the two possible outcomes with -ideally- equal amplitudes until its position is determined by measurement. When the two output modes are observed by a single photon detector, the generated clicks can be interpreted as ones and zeros - and a raw random bit stream is obtained. Here we implement such a random bit generator based on single photons from a defect center in diamond. We investigate the single photon emission of the defect center by an anti-bunching measurement. This certifies the "quantumness" of the supplied photonic input state, while the random "decision" is still based on the vacuum fluctuations at the open port of the beam-splitter. Technical limitations, such as intensity fluctuations, mechanical drift, and bias are discussed. A number of ways to suppress such unwanted effects, and an a priori entropy estimation are presented. The single photon nature allows for a characterization of the non-classicality of the source, and allows to determine a background fraction. Due to the NV-center's superior stability and optical properties, we can operate the generator under ambient conditions around the clock. We present a true 24/7 operation of the implemented random bit generator.}
}

@article{stein_prappl_2020,
author = {Guilherme Stein and Vladislav Bushmakin and Yijun Wang and Andreas W. Schell and Ilja Gerhardt},
journal = {Phys. Rev. Applied},
keywords = {Single photons; Quantum information processing; Quantum optics; Fiber Optics;},
pages = {054042},
title = {Narrow-band fiber-coupled single photon source},
volume = {13},
year = {2020},
url = {https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.13.054042},
abstract = {A single-photon source is an essential tool for the emerging field of quantum technologies. Ideally, it should be spectrally compatible with other photonic devices while providing a high flux of narrow-band photons. The single organic dye molecule dibenzanthanthrene under cryogenic conditions possesses the given characteristics and therefore constitutes a prominent single-photon source. Nevertheless, the implementation of such a single-photon source requires a complex experimental setup involving a cryostat with a confocal microscope for the effective collection of the molecular emission. In the approach presented here we use a single emitter coupled directly to the end facet of an optical fiber. This has the potential to transfer a single-photon source based on a quantum emitter from a proof-of-principle type of setup to a scalable "plug-and-play" device. We present successful coupling of a single organic molecule to an optical fiber and record the excitation spectrum, measure the saturation curve, and analyze the contributions of Raman background fluorescence. The single-photon nature is proven by an antibunched autocorrelation recording, which reveals coherent Rabi oscillations.}
}

@article{rezai_qst_2019,
author = {Mohammad Rezai and Jan Sperling and Ilja Gerhardt},
journal = {Quantum Science and Technology},
keywords = {Single photons; Quantum information processing; Quantum optics;},
pages = {1--7},
title = {What can single photons do what lasers cannot do?},
volume = {},
year = {2019},
url = {https://dx.doi.org/10.1088/2058-9565/ab3d56},
doi = {},
abstract = {Coherence forms the foundation of quantum information processing and roots its bases in effects such as quantum interference and entanglement. In our classically perceived life, interference is a familiar notion, but what makes this phenomenon "quantum" is a challenging task to be quantitatively verified. In this contribution, we experimentally implement quantum interference and investigate the impact of different origins of an interference pattern by characterizing correlations obtained with distinct light sources, namely single photons and laser light. We present the correlation measurements on a general class of linear optical gates in a uniform format. Consequently, this modeling provides a precise characterization of different photonic sources. Specifically, we demonstrate how an interference pattern can be uniquely decomposed into a classical and quantum part. By extension, our approach renders it possible to perform a comprehensive analysis of the wave-particle duality in quantum-optical interference experiments.}
}

@article{maisch_aqt_2019,
author = {Julian Maisch and H\"useyin Vural and Michael Jetter and Peter Michler and Ilja Gerhardt and Simone Luca Portalupi},
journal = {Adv. Quantum Technol.},
keywords = {Quantum dots; Aomic vapor; Cesium; Single photons; Quantum information processing; Quantum optics; Slow light; Group velocity},
number = {3},
pages = {1900057},
title = {Controllable Delay and Polarization Routing of Single Photons},
volume = {},
year = {2020},
url = {https://doi.org/10.1002/qute.201900057},
doi = {},
abstract = {Full control of single photons is important in quantum information and quantum networking. In particular, controlling the photon–atom interaction can be an appealing means to realize more complex quantum experiments. As a matter of example, the storage of photons into atomic media represents one key approach to memory‐assisted quantum communication and computing. Here it is shown that the propagation of single photons from a semiconductor quantum dot can be deliberately controlled by an atomic vapor under the application of an external magnetic field. The present results enable the use of an atomic vapor as a precise and reliable wavelength selective delay and allows for routing the single photons according to their polarization and the external magnetic field. With an overall delay of 25 ns, it is possible to fine‐tune the arrival time of the photons by more than 600 ps which matches the scale of the quantum dot's lifetime. The experimental data are fully reproduced by a theoretical model.}
}

@article{rezai_procspie_2019,
author = {Mohammad Rezai and Guilherme Stein and Vladislav Bushmakin and J\"org Wrachtrup and Ilja Gerhardt},
title = {Polarization-entangled photon pairs from a single molecule},
journal = {Proc. SPIE}
volume = {10934},
pages = {1--5},
keywords = {Single Molecules; Single Photons; Entanglement; Hybrid Systems; Quantum Sensing; Microscopy);
abstract = {Single organic dye molecules have been studied for almost three decades. A majority of experiments are now conducted under ambient conditions and aim towards material science and micro-biology. The experiments under cryogenic conditions are often based on highly rigid polycyclic hydrocarbons, which have excellent fluorescing properties when they are operated at temperatures below 2 K. Their photonic advantages integrate their high flux, their narrow-band nature and their tunability over the entire visible spectrum. Therefore the use of organic molecules as efficient and narrow-band single photon sources facilitated a number of quantum optical advances in the past years. The results mostly cover quantum sensing, the formation of quantum hybrid systems and the optical combination of multiple photons in all-optical experiments. Here we present all three fields and outline our own experiments in the combination of single molecule studies, atomic spectroscopy and entanglement generation. The brightness and the narrow-band nature of the molecules are outlined and a delayed-choice quantum eraser is presented. The mode-mixing of two photons on a beam-splitter allows for the generation of a degenerate photon pair, which is resonant to the sodium D₂-line. Furthermore, its spectral width matches roughly to the natural linewidth of an isolated sodium atom, such that further experiments are feasible. The entanglement of the post-selected photon pair is testified by a violation of Bell's inequality. Even the raw collected clicks violate Bell's inequality by more than two standard deviations.},
year = {2019},
doi = {10.1117/12.2516608},
url = {https://doi.org/10.1117/12.2516608}
}

@article{rezai_njp_2019,
author = {Mohammad Rezai and J\"org Wrachtrup and Ilja Gerhardt},
journal = {New Journal of Physics},
keywords = {Single Molecules; Rabi Oscillations; Coherent Control; Single Photons; Quantum information processing; Quantum optics},
number = {},
pages = {},
title = {Detuning Dependent Rabi Oscillations of a Single Molecule},
volume = {},
year = {2019},
url = {https://iopscience.iop.org/article/10.1088/1367-2630/ab130e},
doi = {10.1088/1367-2630/ab130e},
abstract = {A single organic dye molecule at cryogenic conditions is resonantly excited in a confocal microscope. Under strong laser illumination it undergoes Rabi oscillations. Mathematically, this is well described and it has been experimentally implemented. These oscillations can be measured as side-bands on their resonance fluorescence, e.g. in the Mollow-Triplet. An alternative method is to research this effect by an analysis of the single molecule anti-bunched photon statistics. This has been performed in this work. Here we research on the detuning dependence of this signal -- it is experimentally demanding since the utilized laser might drift or single emitters are not necessarily spectral stable enough, such that the spectrum can be measured indefinitely. We therefore apply a measurement technique in which the photon correlation signal is acquired in detuning dependent steps. This is performed by continuous laser sweeps over the single molecule excitation spectrum. A single recording of the anti-bunched photons takes 20 to 50 ms. After approx. 1 hour of repetitive laser detunings a full anti-bunching curve is reconstructed for each spectral position. An alternative technique with 100 ns laser pulses allows to acquire a set of comparable data. Our study is derived from a single dibenzanthanthrene molecule. It emits under resonant excitation more than 380.000 photons per second. Under spectral detuning, Rabi-oscillations are observed up to \Omega_Rabi=2 Pi 160 MHz.}
}

@article{rezai_optica_2019,
author = {Mohammad Rezai and J\"org Wrachtrup and Ilja Gerhardt},
journal = {Optica},
keywords = {Magnetic fields; Parametric down conversion; Photon polarization; Photons; Quantum information processing; Quantum optics},
number = {1},
pages = {34--40},
publisher = {OSA},
title = {Polarization-entangled photon pairs from a single molecule},
volume = {6},
year = {2019},
url = {http://www.osapublishing.org/optica/abstract.cfm?URI=optica-6-1-34},
doi = {10.1364/OPTICA.6.000034},
abstract = {Photonic entanglement is one of the key resources in modern quantum optics. It opens the door to schemes such as quantum communication, quantum teleportation, and quantum-enhanced precision sensing. Sources based on parametric down-conversion or cascaded decays in atomic and atom-like emitters are limited because of their weak interaction with stationary qubits. This is due to their commonly broadband emission. Furthermore, these sources are commonly in the near-infrared such that quantum emitters in the blue spectral region, such as ions or many defect centers, cannot be addressed. Here, we present a sodium-resonant (589.0nm) and narrow-band (14MHz) degenerate entanglement source based on a single molecule. A beam-splitter renders two independently emitted photons into a polarization-entangled state. The quality of the entangled photon pairs is verified by the violation of Bell's inequality. We measure a Bell parameter of S=2.26+/-0.05. This attests that the detected photon pairs exceed the classical limit; it is reconfirmed by quantum-state tomography and an analysis of the raw detector counts, which result in a value of S=2.24+/-0.12. The tomography shows fidelity of 82% to a maximally entangled Bell state. This work opens the route to background-free solid-state entanglement sources which surpass the probabilistic nature of the commonly used sources and are free from unwanted multi-photon events. The source is ideal for combination with stationary qubits such as atoms, ions, quantum dots, or defect centers.}
}

@article{gerhardt_ol_2018,
author = {Ilja Gerhardt},
journal = {Opt. Lett.},
keywords = {Dispersion; Light beams; Magnetic fields; Optical activity; Polarized light; Quantum optics},
number = {21},
pages = {5295--5298},
publisher = {OSA},
title = {How anomalous is my Faraday filter?},
volume = {43},
year = {2018},
url = {http://ol.osa.org/abstract.cfm?URI=ol-43-21-5295},
doi = {10.1364/OL.43.005295},
abstract = {The Macaluso-Corbino effect describes the optical rotation of light in the spectral proximity to an atomic resonance. One use of this effect is narrowband optical filtering. So-called Faraday filters utilize the difference of the two components of the refractive indices, which are split by the Zeeman effect in a longitudinal magnetic field. This allows for a net rotation of a linearly polarized input beam within the medium. Placing it between crossed polarizers therefore only allows light near resonance to pass. Since any resonant spectrum implies anomalous dispersion on resonance, these filters are often characterized as being based on this anomalous dispersion. This Letter analyses to what extent the anomalous dispersion and the anomalous rotation are relevant for Faraday filters. Considering the sign of the anomalous rotation introduces a strict criterion if the filter is operated in the line center or in the spectral wing of an atomic resonance.}
}

@article{widmann_ptl_2018,
author = {M. Widmann and S. L. Portalupi and P. Michler and J. Wrachtrup and I. Gerhardt},
journal = {Photonic Technology Letters},
keywords = {Photonics;Cesium;Atom optics;Optical polarization;Temperature measurement;Magnetic flux;Amplitude modulation;Atomic filter;Faraday filter;Quantum dots;Quantum Hybrid Devices},
number = {},
pages = {1--1},
title = {Faraday Filtering on the Cs-D1-Line for Quantum Hybrid Systems},
year = {2018},
url = {https://dx.doi.org/10.1109/LPT.2018.2871770},
abstract = {Narrow-band filtering of light is widely used in optical spectroscopy. Atomic filters, which rely on the Faraday effect, allow for GHz-wide transmission spectra, which are intrinsically matched to an atomic transition. We present an experimental realization and a theoretical study of a Faraday filter based on cesium and its D1-line-transition (62S1/2 -> 62P1/2) around 894 nm. We also present the prospects and visions for combining this filter with the single photon emission of a single quantum dot (QD), which matches with the atomic transition. The option to lock a quantum dot is discussed at the end of the paper.}
}

@article{rezai_prx_2018,
title = {Coherence Properties of Molecular Single Photons for Quantum Networks},
author = {Mohammad Rezai and J\"org Wrachtrup and Ilja Gerhardt},
journal = {Phys. Rev. X},
volume = {8},
issue = {3},
pages = {031026},
numpages = {9},
year = {2018},
publisher = {American Physical Society},
doi = {10.1103/PhysRevX.8.031026},
url = {https://link.aps.org/doi/10.1103/PhysRevX.8.031026},
keywords = {Quantum Hybrid Systems; Atomic Vapors; Sodium; Hong-Ou-Mandel; Quantum Interference; Single Molecules},
abstract = {Quantum mechanics implies that a single photon can be in the superposition of two distant spatial modes and enable nonlocal interferences. The most vivid example is the two-photon coalescence on a 50∶50 beam splitter, known as Hong-Ou-Mandel interference. In the past decade, this experiment has been used to characterize the suitability of different single-photon sources for linear optical quantum gates. This characterization alone cannot guarantee the suitability of the photons in a scalable quantum network. As for a deeper insight, we perform a number of nonclassical interference measurements of single photons emitted by a single organic molecule that are optimized by an atomic Faraday filter. Our measurements reveal near unity visibility of the quantum interference, and a one-port correlation measurement proves the ideal Fourier limited nature of our single-photon source. A delayed choice quantum eraser allows us to observe a constructive interference between the photons, and a Hong-Ou-Mandel peak is formed additionally to the commonly observed dip. These experiments comprehensively characterize the involved photons for their use in a future quantum Internet, and they attest to the fully efficient interaction of the molecular photons with a next subsequent quantum node. They can be adapted to other emitters and will allow us to gain insights to their applicability for quantum information processing. We introduce a quality number that describes the photon’s properties for their use in a quantum network; this states that effectively 97\% of the utilized molecular photons can be used in a scalable quantum optical system and interact with other quantum nodes. The experiments are based on a hybridization of solid state quantum optics, atomic systems, and all-optical quantum information processing.}
}

@ARTICLE{bodenstedt_a_2018,
author = {Sven Bodenstedt and Ingmar Jakobi and Julia Michl and Ilja Gerhardt and Philipp Neumann and J\"org Wrachtrup},
title = {Nanoscale spin manipulation with pulsed magnetic gradient fields from a hard disc drive writer},
journal = {Nano Letters},
year = {2018},
volume = {},
issue = {},
pages = {1--6},
doi = {},
url = {https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.8b01387},
keywords = {Hard Disk Drive Writer; Individual Spin Control; Nitrogen-Vacancy (NV) Center; Nonadiabatic Fast Passage; Pulsed Magnetic Fields; HDDs; ODMR; Superresolution; Magnetic Field Gradients},
abstract = {The individual and coherent control of solid-state based electron spins is important covering fields from quantum information processing and quantum metrology to material research and medical imaging. Especially for the control of individual spins in nanoscale networks, the generation of strong, fast, and localized magnetic fields is crucial. Highly engineered devices that demonstrate most of the desired features are found in nanometer size magnetic writers of hard disk drives (HDD). Currently, however, their nanoscale operation in particular comes at the cost of excessive magnetic noise. Here, we present HDD writers as a tool for the efficient manipulation of single as well as multiple spins. We show that their tunable gradients of up to 100 uT/nm can be used to spectrally address individual spins on the nanoscale. Their gigahertz bandwidth allows one to switch control fields within nanoseconds, faster than characteristic time scales such as Rabi and Larmor periods, spin–spin couplings, or optical transitions, thus extending the set of feasible spin manipulations. We used the fields to drive spin transitions through nonadiabatic fast passages or to enable the optical readout of spin states in strong misaligned fields. Building on these techniques, we further apply the large magnetic field gradients for microwave selective addressing of single spins and show its use for the nanoscale optical colocalization of two emitters.}
}

@ARTICLE{nagy_pra_2018,
title = {Quantum Properties of Dichroic Silicon Vacancies in Silicon Carbide},
author = {Nagy, Roland and Widmann, Matthias and Niethammer, Matthias and Dasari, Durga B. R. and Gerhardt, Ilja and Soykal, \"Oney O. and Radulaski, Marina and Ohshima, Takeshi and Vuckovic, Jelena and Son, Nguyen Tien and Ivanov, Ivan G. and Economou, Sophia E. and Bonato, Cristian and Lee, Sang-Yun and Wrachtrup, J\"org},
journal = {Phys. Rev. Applied},
volume = {9},
issue = {3},
pages = {034022},
numpages = {8},
year = {2018},
keywords = {Silicon carbide, Polarization, Silicon Vacancies},
publisher = {American Physical Society},
doi = {10.1103/PhysRevApplied.9.034022},
url = {https://link.aps.org/doi/10.1103/PhysRevApplied.9.034022}
}

@ARTICLE{vural_optica_2018,
author = {H\"useyin Vural and Simone L. Portalupi and Julian Maisch and Simon Kern and Jonas H. Weber and Michael Jetter and J\"org Wrachtrup and Robert L\"ow and Ilja Gerhardt and Peter Michler},
title = {Two-photon interference in an atom-quantum dot hybrid system},
journal = {Optica},
year = {2018},
volume = {5},
issue = {4},
pages = {367-373},
doi = {https://doi.org/10.1364/OPTICA.5.000367},
url = {https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-4-367},
keywords = {Quantum Hybrid Systems; Atomic Vapors; Cesium; Quantum Dots; Hong-Ou-Mandel},
abstract = {Future quantum networks will need flying qubits and stationary nodes. As for the generation of single photons which may act as flying qubits, resonantly-excited single semiconductor quantum dots are ideal in terms of their on-demand single-photon emission, their indistinguishability, and their brightness. Atomic systems can effectively act as mediators for photon-photon interactions, storage media, or building blocks for stationary qubits. Here we hybridize these two systems and investigate the non-classical interference of spectral Lorentzian-shaped photons, fine-tuned between the cesium (Cs)-D1 hyperfine resonances. The temporal delay in the hot dispersive atomic cesium vapor amounts up to 50 times the photons initial width and reveals beats on the single quanta. The photons' indistinguishability is preserved even after atomic-enabled delay. This proves that the interaction with the Cs-vapor conserves the photons' coherence. The role of spectral diffusion in the solid state emitter is studied via the strong dependence of the single and two-photon experiments on the frequency. Our results pave the way to efficient hybrid interfaces between quantum dots and hot atomic vapors as storage media in future quantum networks.}
}

@ARTICLE{steinle_prx_2017,
author = {Tobias Steinle and Johannes N. Greiner and J\"org Wrachtrup and Harald Giessen and Ilja Gerhardt},
title = {Unbiased All-Optical Random-Number Generator},
journal = {Phys. Rev. X},
year = {2017},
volume = {7},
issue = {4},
pages = {041050},
doi = {10.1103/PhysRevX.7.041050},
url = {https://doi.org/10.1103/PhysRevX.7.041050},
keywords = {Optical Bistability; Optical Parametric Oscillators; Quantum Randomness; Entropy; Information Security},
abstract = {The generation of random bits is of enormous importance in modern information science. Cryptographic security is based on random numbers which require a physical process for their generation. This is commonly performed by hardware random number generators. These exhibit often a number of problems, namely experimental bias, memory in the system, and other technical subtleties, which reduce the reliability in the entropy estimation. Further, the generated outcome has to be post-processed to "iron out" such spurious effects. Here, we present a purely optical randomness generator, based on the bi-stable output of an optical parametric oscillator. Detector noise plays no role and post-processing is reduced to a minimum. Upon entering the bi-stable regime, initially the resulting output phase depends on vacuum fluctuations. Later, the phase is rigidly locked and can be well determined versus a pulse train, which is derived from the pump laser. This delivers an ambiguity-free output, which is reliably detected and associated with a binary outcome. The resulting random bit stream resembles a perfect coin toss and passes all relevant randomness measures. The random nature of the generated binary outcome is furthermore confirmed by an analysis of resulting conditional entropies.}
}

@ARTICLE{docters_srep_2017,
author = {Bernd Docters and J\"org Wrachtrup and Ilja Gerhardt},
title = {Two Step Excitation in Hot Atomic Sodium Vapor},
journal = {Scientific Reports},
year = {2017},
pages = {11760},
doi = {10.1038/s41598-017-12089-w},
url = {https://www.nature.com/articles/s41598-017-12089-w},
keywords = {Sodium Vapor; Atomic Spectroscopy; Ladder Scheme;},
abstract = {A two step excitation scheme in hot atomic sodium vapor is experimentally investigated. The observed effects reflect a coupling between the 3S, 3P and the 3D states. We present the relative dependence on detuning of the two utilized lasers around lambda=589nm and 819nm. Unlike expected, we achieve a higher detuning dependence of the probe and the coupling laser by a factor of approximately three. The presented work aimed for a Rydberg excitation and quantum light storage. Such schemes are usually implemented with a red laser on the D-line transition and a coupling laser of shorter (typically blue) wavelength. Due to the fact that higher P-Rydberg states are approximately two times higher in energy than the 3D state, a two photon transition from the atomic excited 3P state to a Rydberg P state is feasible. This might circumvent laser frequency doubling whereby only two lasers might mediate a three photon process. The scheme of adding three k-vectors allows for electromagnetically induced transparency experiments in which the resulting k-vector can be effectively reduced to zero. By measurements utilizing electric fields and an analysis of the emission spectrum of the atomic vapor, we can exclude the excitation of the P-P two photon transition.}
}

@ARTICLE{gerhardt_jmp_2017,
author = {Ilja Gerhardt and Bernhard Grotz and Petr Siyushev and J\"org Wrachtrup},
title = {Coherent interaction of single molecules and plasmonic nanowires},
journal = {International Journal of Modern Physics B},
year = {2017},
pages = {1740004},
doi = {10.1142/S0217979217400045},
url = {http://www.worldscientific.com/doi/abs/10.1142/S0217979217400045},
keywords = {Plasmonics; Single Molecules; Silver Nanowires; Single Photons; Coherent Interaction}
abstract = {Quantum plasmonics opens the option to integrate complex quantum optical circuitry onto chip scale devices. In the past, often external light sources were used and nonclassical light was coupled in and out of plasmonic structures, such as hole arrays or waveguide structures. Another option to launch single plasmonic excitations is the coupling of single emitters in the direct proximity of, e.g., a silver or gold nanostructure. Here, we present our attempts to integrate the research of single emitters with wet-chemically grown silver nanowires. The emitters of choice are single organic dye molecules under cryogenic conditions, which are known to act as high-brightness and extremely narrow-band single photon sources. Another advantage is their high optical nonlinearity, such that they might mediate photon–photon interactions on the nanoscale. We report on the coupling of a single molecule fluorescence emission through the wire over the length of several wavelengths. The transmission of coherently emitted photons is proven by an extinction type experiment. As for influencing the spectral properties of a single emitter, we are able to show a remote change of the line-width of a single terrylene molecule, which is in close proximity to the nanowire.}
}

@ARTICLE{portalupi_ncomm_2016,
author = {Simone Luca Portalupi and Matthias Widmann and Cornelius Nawrath and Michael Jetter and Peter Michler and J\"org Wrachtrup and Ilja Gerhardt},
title = {Simultaneous Faraday filtering of the Mollow triplet sidebands with the Cs-D1 clock transition},
journal = {Nature Communications},
year = {2016},
volume = {7},
pages = {13632},
doi = {10.1038/ncomms13632},
url = {http://www.nature.com/articles/ncomms13632},
keywords = {Faraday Filter; Atomic and molecular interactions with photons; Quantum optics; Single Photons and Quantum Effects;},
abstract = {Hybrid quantum systems integrating semiconductor quantum dots (QDs) and atomic vapours become important building blocks for scalable quantum networks due to the complementary strengths of individual parts. QDs provide on-demand single-photon emission with near-unity indistinguishability comprising unprecedented brightness - while atomic vapour systems provide ultra-precise frequency standards and promise long coherence times for the storage of qubits. Spectral filtering is one of the key components for the successful link between QD photons and atoms. Here we present a tailored Faraday anomalous dispersion optical filter based on the caesium-D1 transition for interfacing it with a resonantly pumped QD. The presented Faraday filter enables a narrow-bandwidth (D omega = 2 pi 1 GHz) simultaneous filtering of both Mollow triplet sidebands. This result opens the way to use QDs as sources of single as well as cascaded photons in photonic quantum networks aligned to the primary frequency standard of the caesium clock transition.}
}

@ARTICLE{arnold_rsi_2017,
author = {Daniel Arnold and Steven Siegel and Emily Grisanti and J\"org Wrachtrup and Ilja Gerhardt},
title = {A Rubidium Mx-magnetometer for Measurements on Solid State Spins},
journal = {Review of Scientific Instruments},
year = {2017},
volume = {88},
pages = {023103},
url = {http://aip.scitation.org/doi/abs/10.1063/1.4974845},
keywords = {Magnetic fields; Coils; Linewidths; Spin relaxation; Rubidium; Magnetometry;},
abstract = {The detection of environmental magnetic fields is well established by optically pumped atomic magnetometers. Another focus of magnetometry can be the research on magnetic or spin-active solid-state samples. Here we introduce a simple and compact design of a rubidium-based Mx-magnetometer, which allows for hosting solid-state samples. The optical, mechanical and electrical design is reported, as well as simple measurements which introduce the ground-state spin-relaxation time, the signal-to-noise ratio of a measurement, and subsequently the overall sensitivity of the magnetometer. The magnetometer is optimized for the most sensitive operation with respect to laser power and magnetic field excitation at the Larmor frequency.}
}

@ARTICLE{striebel_srep_2017,
author = {Moritz Striebel and J\"org Wrachtrup and Ilja Gerhardt},
title = {Absorption and Extinction Cross Sections and Photon Streamlines in the Optical Near-field},
journal = {Scientific Reports},
volume = {7},
number = {1},
pages = {15420},
year = {2017},
url = {https://doi.org/10.1038/s41598-017-15528-w},
keywords = {Light-matter interaction; Scattering; Absorption; Extinction},
abstract = {The optical interaction of light and matter is modeled as an oscillating dipole in a plane wave electromagnetic field. We analyze absorption, scattering and extinction for this system by the energy flow, visualized as streamlines of the Poynting vector. Depending on the dissipative damping of the oscillator, a part of the streamlines ends up in the dipole. Based on a graphical investigation of the streamlines, this represents the absorption cross section, and forms a far-field absorption aperture. In the near-field of the oscillator, a modification of the aperture is observed. As in the case for a linear dipole, we model the energy flow and derive the effective absorption apertures for an oscillator with a circular dipole characteristics - such as an atom in free space.}
}

@ARTICLE{tran_pra_2017,
author = {Thai Hien Tran and Petr Siyushev and J\"org Wrachtrup and Ilja Gerhardt},
title = {Extinction of Light and Coherent Scattering by a Single Nitrogen-Vacancy Center in Diamond},
journal = {Physical Review A},
year = {2017},
volume = {95},
pages = {053831},
url = {https://doi.org/10.1103/PhysRevA.95.053831},
keywords = {Single Emitters; NV-centers; Extinction; Spectroscopy; Gouy-Phase;},
abstract = {The efficient interaction of light and a single quantum system is required to implement a photon to spin interface. It is important to determine the amount of coherent and incoherent photons in such a scheme, since it is based on coherent scattering. In this paper an external laser field is efficiently coupled to a single nitrogen vacancy center in diamond. We detect the direct extinction signal and estimate the nitrogen vacancy's extinction cross section. The exact amount of coherent and incoherent photons is determined against the saturation parameter. This reveals the optimal point of interaction for further experiments. A theoretical model allows us to explain the deviation to an atom in free space. The introduced experimental techniques are used to determine the properties of the tight focusing in an interference experiment and allow for a direct determination of the Gouy phase in a strongly focused beam.}
}

@ARTICLE{chejanovsky_nl_2016,
author = {Nathan Chejanovsky and Mohammad Rezai and Federico Paolucci and Youngwook Kim and Torsten Rendler and Wafa Rouabeh and Felipe Favaro de Oliveira and Patrick Herlinger and Andrej Denisenko and Sen Yang and Ilja Gerhardt and Amit Finkler and Jurgen H. Smet and J\"org Wrachtrup}
title = {Structural Attributes and Photodynamics of Visible Spectrum Quantum Emitters in Hexagonal Boron Nitride},
journal = {Nano Letters},
volume = {16},
number = {11},
pages = {7037-7045},
year = {2016},
doi = {10.1021/acs.nanolett.6b03268},
url = {http://dx.doi.org/10.1021/acs.nanolett.6b03268},
keywords = {Hexagonal boron-nitride; Single quantum emitters; Van der Waals materials;},
abstract = {Newly discovered van der Waals materials like MoS2, WSe2, hexagonal boron nitride (h-BN), and recently C2N have sparked intensive research to unveil the quantum behavior associated with their 2D structure. Of great interest are 2D materials that host single quantum emitters. h-BN, with a band gap of 5.95 eV, has been shown to host single quantum emitters which are stable at room temperature in the UV and visible spectral range. In this paper we investigate correlations between h-BN structural features and emitter location from bulk down to the monolayer at room temperature. We demonstrate that chemical etching and ion irradiation can generate emitters in h-BN. We analyze the emitters' spectral features and show that they are dominated by the interaction of their electronic transition with a single Raman active mode of h-BN. Photodynamics analysis reveals diverse rates between the electronic states of the emitter. The emitters show excellent photo stability even under ambient conditions and in monolayers. Comparing the excitation polarization between different emitters unveils a connection between defect orientation and the h-BN hexagonal structure. The sharp spectral features, color diversity, room-temperature stability, long-lived metastable states, ease of fabrication, proximity of the emitters to the environment, outstanding chemical stability, and biocompatibility of h-BN provide a completely new class of systems that can be used for sensing and quantum photonics applications.}
}

@ARTICLE{oberreiter_lpor_2016,
author = {Lukas Oberreiter and Ilja Gerhardt},
title = {Light on a Beam Splitter: More Randomness with Single Photons},
journal = {Laser & Photonic Reviews},
volume = {10},
number = {1},
pages = {108--115},
year = {2016},
url = {http://dx.doi.org/10.1002/lpor.201500165},
keywords = {Quantum Randomness; Single Photon Detection; Single Photons; Avalanche Photodiodes; Waiting time distributions; Entropy},
abstract = {One of the most fundamental quantum random number generators is implemented with light impinging onto a beam splitter, and two single photon detectors at its output. Often, this generator is described as "a photon which takes one or the other path towards a detector". The input state of light in conjunction with the detector response is relevant for the amount, the pattern and the correlation of the generated clicks. Only a fraction of all generator outcomes, the min-entropy, can be used as a further resource for true randomness. This paper addresses the difference in the common description with incoming single photons and the often implemented scheme with a weak coherent light source, such as an attenuated laser. For this very fundamental and widely used configuration the amount of usable entropy is compared: If single photons from an anti-bunched light-source are supplied, the amount of entropy is higher than for the case of a supplied coherent state - although the latter can be arbitrarily bright unlike the single photon source. Furthermore, we can show that the use of a single photon emitter makes an injection of external information by an adversary into the device extremely challenging.}
}

@ARTICLE{kiefer_apb_2016,
author = {Wilhelm Kiefer and Mohammad Rezai and J\"org Wrachtrup and Ilja Gerhardt},
title = {An atomic spectrum recorded with a single molecule light source},
journal = {Applied Physics B: Lasers and Optics},
volume = {122},
number = {2},
pages = {1--12},
year = {2016},
url = {http://dx.doi.org/10.1007/s00340-015-6278-7},
keywords = {Single Molecules; Faraday Filter; Atomic Spectroscopy; Quantum Hybrid Systems; Sodium},
abstract = {A single molecule under cryogenic conditions allows one to realize an extremely bright and simultaneously narrow-band single photon source. We present a review on the different excitation schemes of a single molecule and present the corresponding single photon nature of the emitted light. Single molecule spectroscopy has been recently interlinked with atomic spectroscopy. This optical interconnect among the different quantum systems might be enhanced by a so-called Faraday anomalous dispersion optical filter -- an ideal tool for many experiments in quantum optics. We introduce our theoretical and experimental approach on these filters which are based on hot atomic sodium vapor. The electrical tunability together with the brightness of a single molecule allows us to record a full atomic spectrum of this filter with the single photons originating from a single molecular emitter.},
issn = {1432-0649}
}

@ARTICLE{widmann_arxiv_2015,
author = {Matthias Widmann and Simone Portalupi and Sang-Yun Lee and Peter Michler and J\"org Wrachtrup and Ilja Gerhardt},
title = {Faraday Filtering on the Cs-D1-Line for Quantum Hybrid Systems},
journal = {ArXiv},
volume = {},
pages = {1--4},
year = {2015},
url = {http://arxiv.org/abs/1505.01719},
keywords = {Faraday Filter; Atomic Spectroscopy; Quantum Dots; Quantum Hybrid Systems; Laser Locking; Cesium},
abstract = {Narrow-band filtering of light is widely used in optical spectroscopy. Atomic filters, which rely on the Faraday effect, allow for GHz-wide transmission spectra, which are intrinsically matched to an atomic transition. We present an experimental realization and a theoretical study of a Faraday anomalous dispersion optical filter (FADOF) based on cesium and its D1-line-transition (62S1/2 - 62P1/2) around 894nm. We also present the prospects and visions for combining this filter with the single photon emission of a single quantum dot (QD), which matches with the atomic transition. }
}

@ARTICLE{uhland_arxiv_2015,
author = {Denis Uhland and Torsten Rendler and Matthias Widmann and Sang-Yun Lee and J\"org Wrachtrup and Ilja Gerhardt},
title = {Single Molecule DNA Detection with an Atomic Vapor Notch Filter},
journal = {EPJ Quantum Technology},
volume = {2},
pages = {20},
year = {2015},
url = {https://epjquantumtechnology.springeropen.com/articles/10.1140/epjqt/s40507-015-0033-1},
keywords = {Single Molecule Detection; Atomic Filters; Sodium spectroscopy; DNA detection},
abstract = {The detection of single molecules has facilitated many advances in life- and material-science. Commonly the fluorescence of dye molecules is detected, which are attached to a non-fluorescent structure under study. For fluorescence microscopy one desires to maximize the detection efficiency together with an efficient suppression of undesired laser leakage. Here we present the use of the narrow-band filtering properties of hot atomic sodium vapor to selectively filter the excitation light from the red-shifted fluorescence of dye labeled single-stranded DNA molecules. A statistical analysis proves an enhancement in detection efficiency of more than 15\% in a confocal and in a wide-field configuration.}
}

@ARTICLE{widmann_nmat_2014,
author = {Matthias Widmann and Sang-Yun Lee and Torsten Rendler and Nguyen Tien Son and Helmut Fedder and Seoyoung Paik and Li-Ping Yang and Nan Zhao and Sen Yang and Ian Booker and Andrej Denisenko and Mohammad Jamali and Seyed Ali Momenzadeh and Ilja Gerhardt and Takeshi Ohshima and Adam Gali and Erik Janzen and J\"org Wrachtrup},
title = {Coherent control of single spins in silicon carbide at room temperature},
journal = {Nature Materials},
volume = {},
pages = {1476-4660},
year = {2014},
url = {http://dx.doi.org/10.1038/nmat4145},
keywords = {Silicon Carbide; Single Quantum Emitters},
abstract = {Spins in solids are cornerstone elements of quantum spintronics. Leading contenders such as defects in diamond, or individual phosphorous dopants in silicon have shown spectacular progress but either miss established nanotechnology or an efficient spin-photon interface. Silicon carbide (SiC) combines the strength of both systems: It has a large bandgap with deep defects and benefits from mature fabrication techniques. Here we report the characterization of photoluminescence and optical spin polarization from single silicon vacancies in SiC, and demonstrate that single spins can be addressed at room temperature. We show coherent control of a single defect spin and find long spin coherence time under ambient conditions. Our study provides evidence that SiC is a promising system for atomic-scale spintronics and quantum technology.}
}

@ARTICLE{jamali_rsi_2014,
author = {Mohammad Jamali and Ilja Gerhardt and Mohammad Rezai and Karsten Frenner and Helmut Fedder and J\"org Wrachtrup},
title = {Microscopic diamond Solid-Immersion-Lenses fabricated around single defect ceneters by focussed ion beam milling},
journal = {Review of Scientific Instruments},
volume = {85},
pages = {123703},
year = {2014},
url = {http://dx.doi.org/10.1063/1.4902818},
keywords = {Solid Immersion Lenses; Single Photons; Focused Ion Beam Milling},
abstract = {Recent efforts to define microscopic solid-immersion-lenses (SIL) by focused ion beam milling into diamond substrates that are registered to a preselected single photon emitter are summarized. We show how we determine the position of a single emitter with at least 100 nm lateral and 500 nm axial accuracy, and how the milling procedure is optimized. The characteristics of a single emitter, a Nitrogen Vacancy (NV) center in diamond, are measured before and after producing the SIL and compared with each other. A count rate of 1.0 million counts per second is achieved with a [111] oriented NV center.}
}

@ARTICLE{kiefer_srep_2014,
author = {Wilhelm Kiefer and Robert L\"ow and J\"org Wrachtrup and Ilja Gerhardt},
title = {Na-Faraday rotation filtering: The optimal point},
journal = {Scientific Reports},
volume = {4},
pages = {6552},
year = {2014},
url = {http://dx.doi.org/10.1038/srep06552},
keywords = {Atmospheric optics; Atom optics; Atomic and molecular interactions with photons},
abstract = {Narrow-band optical filtering is required in many spectroscopy applications to suppress unwanted background light. One example is quantum communication where the fidelity is often limited by the performance of the optical filters. This limitation can be circumvented by utilizing the GHz-wide features of a Doppler broadened atomic gas. The anomalous dispersion of atomic vapours enables spectral filtering. These, so-called, Faraday anomalous dispersion optical filters (FADOFs) can be by far better than any commercial filter in terms of bandwidth, transition edge and peak transmission. We present a theoretical and experimental study on the transmission properties of a sodium vapour based FADOF with the aim to find the best combination of optical rotation and intrinsic loss. The relevant parameters, such as magnetic field, temperature, the related optical depth, and polarization state are discussed. The non-trivial interplay of these quantities defines the net performance of the filter. We determine analytically the optimal working conditions, such as transmission and the signal to background ratio and validate the results experimentally. We find a single global optimum for one specific optical path length of the filter. This can now be applied to spectroscopy, guide star applications, or sensing.}
}

@ARTICLE{siyushev_n_2014,
author = {Petr Siyushev and Guilherme Stein and J\"org Wrachtrup and Ilja Gerhardt},
title = {Molecular photons interfaced with alkali atoms},
journal = {Nature},
volume = {509},
pages = {66-70},
year = {2014},
issue = {7498},
url = {http://dx.doi.org/10.1038/nature13191},
keywords = {Nanophotonics and plasmonics; Atom optics; Quantum information},
abstract = {Future quantum communication will rely on the integration of single-photon sources, quantum memories and systems with strong single-photon nonlinearities. Two key parameters are crucial for the single-photon source: a high photon flux with a very small bandwidth, and a spectral match to other components of the system. Atoms or ions may act as single-photon sources—owing to their narrowband emission and their intrinsic spectral match to other atomic systems—and can serve as quantum nonlinear elements. Unfortunately, their emission rates are still limited, even for highly efficient cavity designs. Single solid-state emitters such as single organic dye molecules are significantly brighter and allow for narrowband photons; they have shown potential in a variety of quantum optical experiments but have yet to be interfaced with other components such as stationary memory qubits. Here we describe the optical interaction between Fourier-limited photons from a single organic molecule and atomic alkali vapours, which can constitute an efficient quantum memory. Single-photon emission rates reach up to several hundred thousand counts per second and show a high spectral brightness of 30,000 detectable photons per second per megahertz of bandwidth. The molecular emission is robust and we demonstrate perfect tuning to the spectral transitions of the sodium D line and efficient filtering, even for emitters at ambient conditions. In addition, we achieve storage of molecular photons originating from a single dibenzanthanthrene molecule in atomic sodium vapour. Given the large set of molecular emission lines matching to atomic transitions, our results enable the combination of almost ideal single-photon sources with various atomic vapours, such that experiments with giant single-photon nonlinearities, mediated, for example, by Rydberg atoms, become feasible.}
}

@ARTICLE{peoloso_jlt_2013,
author = {Matthew P. Peloso and Ilja Gerhardt},
title = {Statistical tests of randomness on quantum keys distributed through a free-space channel coupled to daylight noise},
journal = {Journal of Lightwave Technology},
pages = {3794-3805},
year = {2013},
issue = {23},
volume = {31},
url = {http://jlt.osa.org/abstract.cfm?URI=jlt-31-23-3794},
keywords = {Quantum Key Distribution; Quantum Cryptography; QKD; Randomness;}
abstract = {A suite of random number tests was used to evaluate the randomness of raw cryptographic keys generated using quantum key distribution. The cryptographic channel was established by coupling an entanglement source over a free-space link during both day and night for a period of 48~hours. External noise from sunlight both scattered in the atmosphere and directly reflected within the field-of-view of the receiving optic reduced the randomness of the raw cryptographic keys. Systematic influences on the supposed random key were observed due to both imbalances in the coupling of the source and detectors through the free-space quantum key distribution channel, as well as the introduction of noise counts into the raw key from which the final key was distilled. The introduction of noise in the key exchange may drastically reduce the rate of secure key bits extracted from the raw keys using privacy amplification. Synchronization limits for key generation using photon timing correlations were established by experimentally controlling the coupling ratio of unbiased noise to the coincidence counts. The influence of external optical noise on the quantum bit error ratio is discussed, whereby the suppression of external noise, a balance of optical hardware, and active coupling of the free-space link were identified for optimization of the extracted QKD key generation rates.}
}

@ARTICLE{liu_arxiv_2013,
author = {Qin Liu and Antia Lamas-Linares and Christian Kurtsiefer and Johannes Skaar and Vadim Makarov and Ilja Gerhardt},
title = {A universal setup for active control of a single-photon detector},
journal = {Rev. Sci. Instrum.},
pages = {013108},
volume = {85},
year = {2013},
url = {http://dx.doi.org/10.1063/1.4854615},
keywords = {Quantum Key Distribution; Quantum Cryptography; QKD; Quantum Hacking; Single Photon Detectors;}
abstract = {The influence of bright light on a single-photon detector has been described in a number of recent publications. The impact on quantum key distribution (QKD) is important, and several hacking experiments have been tailored to fully control single-photon detectors. Special attention has been given to avoid introducing further errors into a QKD system. We describe the design and technical details of an apparatus which allows to attack a quantum-cryptographic connection. This device is capable of controlling free-space and fiber-based systems and of minimizing unwanted clicks in the system. With different control diagrams, we are able to achieve a different level of control. The control was initially targeted to the systems using BB84 protocol, with polarization encoding and basis switching using beamsplitters, but could be extended to other types of systems. We further outline how to characterize the quality of active control of single-photon detectors.}
}

@ARTICLE{spieler_molphys_2013,
author = {S. Spieler and W. Zhong and P. Djuricanin and O. Nourbakhsh and I. Gerhardt and K. Enomoto and F. Stienkemeier and T. Momose},
title = {Microwave lens effect for the J=0 rotational state of CH3CN},
journal = {Molecular Physics, Volume 111, Issue 12-13, 2013},
pages = {1--16},
year = {2013},
keywords = {microwave, cold molecules, dipole force, counter-rotating nozzle, focusing},
abstract = {We demonstrate the manipulation of the translational motion of a polar molecule in its J=0 rotational ground state by the microwave (MW) dipole force combined with a counter-rotating nozzle. A cold molecular beam of CH3CN seeded in Kr with a longitudinal velocity of about 100m/s was created by a pulsed counter-rotating nozzle. The cold beam was then introduced into a cylindrical MW cavity, in which a standing wave, TM01p mode MW field, nearly resonant to the JK=10 00 rotational transition of CH3CN was created. By choosing an appropriate MW frequency, we successfully observed focusing and deflection of the cold beam of CH3CN due to the lens effect of the MW standing wave. The present result indicates that the combination of a counter-rotating nozzle and a MW cavity will be a versatile method for making cold and ultracold ensembles of various polar molecules in their rotational ground state.},
url = {http://dx.doi.org/10.1080/00268976.2013.798044}
}

@ARTICLE{gerhardt_arxiv_2012,
author = {I. Gerhardt and T. Hanke},
title = {Homomorphic Payment Addresses and the Pay-to-Contract Protocol},
journal = {ArXiv},
volume = {1212.3257},
issue = {},
pages = {1--11},
url = {http://arxiv.org/abs/1212.3257},
year = {2012},
keywords = {financial cryptography, online electronic payment protocol, payment scheme, homomorphic keypairs, ElGamal, bitcoin, deterministic wallet, labeled wallet, offline anonymous merchant},
abstract = {We propose an electronic payment protocol for typical customer-merchant relations which does not require a trusted (signed) payment descriptor to be sent from the merchant to the customer. Instead, the destination "account" number for the payment is solely created on the customer side. This eliminates the need for any encrypted or authenticated communication in the protocol and is secure even if the merchant's online infrastructure is compromised. Moreover, the payment transaction itself serves as a timestamped receipt for the customer. It proves what has been paid for and who received the funds, again without relying on any merchant signatures. In particular, funds and receipt are exchanged in a single atomic action. The asymmetric nature of the customer-merchant relation is crucial. The protocol is specifically designed with bitcoin in mind as the underlying payment system. Thereby, it has the useful benefit that all transactions are public. However, the only essential requirement on the payment system is that "accounts" are arbitrary user-created keypairs of a cryptosystem whose keypairs enjoy a homomorphic property. All ElGamal-type cryptosystems have this feature. For use with bitcoin we propose the design of a deterministic bitcoin wallet whose addresses can be indexed by clear text strings.},
}

@ARTICLE{enomoto_apb_2012,
author = {K. Enomoto and P. Djuricanin and I. Gerhardt and O. Nourbakhsh and Y. Moriwaki and W. Hardy and T. Momose},
title = {Superconducting microwave cavity towards controlling the motion of polar molecules},
journal = {Applied Physics B},
volume = {109},
issue = {1},
pages = {149--157},
url = {http://dx.doi.org/10.1007/s00340-012-5192-5},
year = {2012},
keywords = {Molecular Beams; Cooling; Stopping; Focusing; Cavities; Superconducting; Electroplating},
abstract = {We propose the use of superconducting microwave cavities for the focusing and deceleration of cold polar molecular beams. A superconducting cavity with a high quality factor produces a large ac Stark shift in polar molecules, which allows us to efficiently control molecular motion. Our discussion is based on the experimental characterization of a prototype cavity: a lead-tin coated cylindrical copper cavity, which has a quality factor of 10^6 and tolerates several watts of input power. Such a microwave device provides a powerful way to control molecules not only in low-field-seeking states, but also in high-field-seeking states such as the ground rotational state.}
}

@ARTICLE{gerhardt_jcp_2012,
author = {I. Gerhardt and Kyungseob (Brian) Sin and Takamasa Momose},
title = {Excitation and emission spectra of rubidium in rare-gas thin-films},
journal = {The Journal of Chemical Physics},
volume = {137},
issue = {1},
pages = {014507},
url = {http://dx.doi.org/10.1063/1.4730032},
year = {2012},
keywords = {Matrix isolation; Excitation spectra; Emission spectra; Rubidium; Rare-Gases; Atomic Spectroscopy},
abstract = {To understand the optical properties of atoms in solid state matrices, the absorption, excitation and emission spectra of rubidium doped thin-films of argon, krypton and xenon were investigated in detail. A two-dimensional spectral analysis extends earlier reports on the excitation and emission properties of rubidium in rare-gas hosts. We found that the doped crystals of krypton and xenon exhibit a simple absorption-emission relation, whereas rubidium in argon showed more complicated spectral structures. Our sample preparation employed in the present work yielded different results for the Ar crystal, but our peak positions were consistent with the prediction based on the linear extrapolation of Xe and Kr data. We also observed a bleaching behavior in rubidium excitation spectra, which suggests a population transfer from one to another spectral feature due to hole-burning. The observed optical response implies that rubidium in rare-gas thin-films is detectable with extremely high sensitivity, possibly down to a single atom level, in low concentration samples.}
}

@ARTICLE{gerhardt_prl_2011,
author = {I. Gerhardt and Q. Liu and A. Lamas-Linares and V. Scarani and J. Skaar and V. Makarov and C. Kurtsiefer},
title = {Experimentally faking the violation of Bells inequalities},
journal = {Physical Review Letters},
volume = {107},
issue = {17},
pages = {170404},
url = {http://link.aps.org/doi/10.1103/PhysRevLett.107.170404},
year = {2011},
keywords = {Quantum Key Distribution; Quantum Cryptography; QKD; Quantum Hacking; Single Photon Detectors; Bell's Inequality},
abstract = {Entanglement witnesses such as Bell inequalities are frequently used to prove the non-classicality of a light source and its suitability for further tasks. By demonstrating Bell inequality violations using classical light in common experimental arrangements, we highlight why strict locality and efficiency conditions are not optional, particularly in security-related scenarios.}
}

@ARTICLE{gerhardt_ncomms_2011,
author = {I. Gerhardt and Q. Liu and A. Lamas-Linares and J. Skaar and C. Kurtsiefer and V. Makarov},
title = {Full field implementation of a perfect eavesdropper on a quantum cryptography system},
journal = {Nature Comm.},
url = {http://www.nature.com/ncomms/journal/v2/n6/full/ncomms1348.html},
year = {2011},
volume = {2},
pages = {349},
keywords = {Quantum Key Distribution; Quantum Cryptogrpahy; QKD; Quantum Hacking; Single Photon Detectors},
abstract = {Quantum key distribution (QKD) allows two remote parties to grow a shared secret key. Its security is founded on the principles of quantum mechanics, but in reality it significantly relies on the physical implementation. Many technological imperfections in QKD systems were explored, whereas no attack on an established QKD connection has been realized so far. Here we show the first full field implementation of a complete attack on a running QKD connection. An installed eavesdropper obtains the entire `secret' key, while none of the parameters monitored by the legitimate parties indicate a security breach. This confirms that non-idealities in physical implementations of QKD can be fully practically exploitable, and must be given increased scrutiny if quantum cryptography is to become highly secure.}
}

@ARTICLE{gerhardt_sensors_2011,
author = {I. Gerhardt and L. Mai and A. Lamas-Linares and C. Kurtsiefer},
title = {Detection of Single Molecules illuminated by a Light-Emitting Diode},
journal = {Sensors},
number = {11},
volume = {1},
pages = {905--916},
url = {http://dx.doi.org/10.3390/s110100905},
year = {2011},
keywords = {Single Molecules; Fluorescence Microscopy; Light-Emitting Diode; LED; Signal to Noise Ratio; Single Photon Detection},
abstract = {Optical detection and spectroscopy of single molecules has become an indispensable tool in biological imaging and sensing. The success of this technique is based on intense fluorescence of organic dye molecule under carefully engineered laser illumination. In this paper we demonstrate the optical detection of single molecules on a wide-field microscope with an illumination based on a commercially available green light-emitting diode. The results are compared to laser illumination in the same experimental configuration. The setup and the limiting factors, such as light transfer to the sample, spectral filtering and the resulting signal-to-noise ratio are discussed. A theoretical and an experimental approach to estimate these parameters are presented. The results can be adapted to other single emitter and illumination schemes.}
}

@ARTICLE{siyushev_sil_2010,
author = {P. Siyushev and F. Kaiser and V. Jacques and I. Gerhardt and S. Bischof and H. Fedder and J. Dodson and M. Markhamand D. Twitchen and F. Jelezko and J. Wrachtrup},
title = {Monolithic diamond optics for single photon detection},
journal = {Applied Physics Letters},
year = {2010},
volume = {97},
pages = {241902},
keywords = {SIL; NV-centers},
url = {http://link.aip.org/link/APPLAB/v97/i24/p241902/s1},
abstract = {In this work, we experimentally demonstrate a novel and simple approach that uses off-the-shelf optical elements to enhance the collection efficiency from a single emitter. The key component is a solid immersion lens made of diamond, the host material for single color centers. We improve the excitation and detection of single emitters by one order of magnitude, as predicted by theory.}
}

@ARTICLE{gerhardt_pra_2010,
author = {I. Gerhardt and G. Wrigge and J. Hwang and G. Zumofen and V. Sandoghdar},
title = {Coherent Nonlinear Single Molecule Microscopy},
journal = {Physical Review A},
volume = {82},
pages = {063823},
number = {6},
year = {2010},
keywords = {Coherent state preparation; Rabi imaging; Quantum Optics; Gradient Imaging},
url = {http://link.aps.org/doi/10.1103/PhysRevA.82.063823},
abstract = {We investigate a nonlinear localization microscopy method based on Rabi oscillations of single emitters. We demonstrate the fundamental working principle of this new technique using a cryogenic far-field experiment in which subwavelength features smaller than $\lambda$/10 are obtained. Using Monte Carlo simulations, we show the superior localization accuracy of this method under realistic conditions and a potential for higher acquisition speed or a lower number of required photons as compared to conventional linear schemes. The method can be adapted to other emitters than molecules and allows for the localization of several emitters at different distances to a single measurement pixel.}
}

@ARTICLE{gerhardt_mp_2009,
author = {I. Gerhardt and G. Wrigge and V. Sandoghdar},
title = {Control and imaging of single-molecule spectral dynamics using a nano-electrode},
journal = {Molecular Physics},
year = {2009},
volume = {107},
pages = {1975-1979},
url = {http://arxiv.org/abs/0906.1624},
keywords = {Physics - Chemical Physics; Condensed Matter - Mesoscale and Nanoscale Physics; Physics - Optics },
abstract = {We study the influence of a scanning nano-electrode on fluorescence excitation spectra of single terrylene molecules embedded in thin p-terphenyl films at cryogenic temperatures. We show that applied voltages less than 10 V can result in reversible Stark shifts of up to 100 times and linewidth increase greater than 10 times the natural linewidth. We discuss the potential of our experimental scheme for direct imaging of individual two-level systems (TLS) in the nanometer vicinity of single molecules. }
}

@ARTICLE{peloso_njp_2009,
author = {M.P. Peloso and I. Gerhardt and C. Ho and A. Lamas-Linares and C. Kurtsiefer},
title = {Daylight operation of a free space, entanglement-based quantum key distribution system},
journal = {New Journal of Physics},
year = {2009},
volume = {11},
pages = {045007 (13pp)},
number = {4},
keywords = {Quantum cryptography; Quantum key distribution},
abstract = {Many quantum key distribution (QKD) implementations using a free space transmission path are restricted to operation at night time in order to distinguish the signal photons used for a secure key establishment from the background light. Here, we present a lean entanglement-based QKD system overcoming that limitation. By implementing spectral, spatial and temporal filtering techniques, we establish a secure key continuously over several days under varying light and weather conditions.},
url = {http://stacks.iop.org/1367-2630/11/045007}
}

@ARTICLE{gerhardt_phiuz_2009,
author = {I. Gerhardt and G. Wrigge},
title = {Im Schatten eines Molek\"uls. Extinktionsmessung an einzelnen Quantensystemen},
journal = {Physik in unserer Zeit},
year = {2009},
volume = {40},
pages = {182--188},
number = {4},
abstract = {Das Ph\"anomen ist allt\"aglich: Licht durchl\"auft ein optisches Medium und wird dabei abgeschw\"acht. Der Effekt heisst Extinktion. Zu dieser "Ausl\"oschung" tragen zwei Mechanismen bei: Streuung und Absorption. Dabei ist besonders die Frage interessant, welchen "Schatten" einzelne Quantensysteme wie Atome oder Molek\"ule werfen. Inzwischen gelingt es, experimentell in diesen Gr\"ossenbereich vorzudringen. Die Streuung am System ist dabei entscheidend: Die destruktive Interferenz zwischen einfallendem und gestreutem Licht kann das einfallende Licht in Ausbreitungsrichtung sogar vollst\"andig ausl\"oschen. Ob auch im Experiment ein einzelnes Atom zu dieser vollst\"andigen Ausl\"oschung f\"ahig ist, geh\"ort zu den noch offenen Fragen. Das Forschungsgebiet hat zudem f\"ur die Quanteninformationsverarbeitung Bedeutung. Diese ben\"otigt eine m\"oglichst effiziente Kopplung eines Lichtfeldes an ein einzelnes Quantensystem.},
issn = {1521-3943},
keywords = {Absorption; Dipol; Einzelmolek\"ulspektroskopie; Extinktion; Farbstoffmolek\"ul; {Licht-Materie-Wechselwirkung;} Quanteninformationsverarbeitung; Schattenwurf; Streuung; {Zwei-Niveau-System}},
url = {http://dx.doi.org/10.1002/piuz.200801203}
}

@ARTICLE{gerhardt_pra_2009,
author = {I. Gerhardt and G. Wrigge and G. Zumofen and J. Hwang and A. Renn and V. Sandoghdar},
title = {Coherent state preparation and observation of Rabi oscillations in a single molecule},
journal = {Physical Review A},
year = {2009},
volume = {79},
pages = {011402},
number = {1},
eid = {011402},
doi = {10.1103/PhysRevA.79.011402},
keywords = {Dyes; Excited States; Fluorescence; High-speed Optical Techniques; Molecule-photon Collisions; Photoexcitation; Spectral Line Intensity; Spectral Line Shift},
numpages = {4},
publisher = {APS},
url = {http://link.aps.org/abstract/PRA/v79/e011402},
abstract = {We report on the excitation of single molecules via narrow zero-phonon transitions using short laser pulses. By monitoring the Stokes-shifted fluorescence, we studied the excited state population as a function of the delay time, laser intensity, and frequency detuning. A π-pulse excitation was demonstrated with merely 500 photons, and 5 Rabi cycles were achieved at higher excitation powers. Our findings are in good agreement with theoretical calculations and provide a first step toward coherent manipulation of the electronic states of single molecules with few photons.}
}

@ARTICLE{wrigge_oe_2008,
author = {G. Wrigge and J. Hwang and I. Gerhardt and G. Zumofen and V. Sandoghdar},
title = {Exploring the limits of single emitter detection in fluorescence and extinction},
journal = {Optics Express},
year = {2008},
volume = {16},
pages = {17358--17365},
number = {22},
keywords = {Coherent optical effects; Absorption; Spectroscopy, molecular; Quantum information and processing},
publisher = {OSA},
url = {http://www.opticsexpress.org/abstract.cfm?URI=oe-16-22-17358},
abstract = {We present an experimental comparison and a theoretical analysis of the signal-to-noise ratios in fluorescence and extinction spectroscopy of a single emitter. We show that because of its homodyne nature the extinction measurements can be advantageous if the emitter is weakly excited. Furthermore, we discuss the potential of this method for the detection and spectroscopy of weakly emitting systems such as rare earth ions. }
}

@ARTICLE{wrigge_np_2008,
author = {G. Wrigge and I. Gerhardt and J. Hwang and G. Zumofen and V. Sandoghdar },
title = {Efficient coupling of photons to a single molecule and the observation of its resonance fluorescence},
journal = {Nature Physics},
year = {2008},
volume = {4},
number = {},
pages = {60-66},
keywords = {Single Molecules; Quantum Optics; Mollow Triplet; Resonance Fluorescence},
url = {http://www.nature.com/nphys/journal/v4/n1/full/nphys812.html},
abstract = {Single dye molecules at cryogenic temperatures exhibit many spectroscopic phenomena known from the study of free atoms and are thus promising candidates for experiments in fundamental quantum optics. However, the existing techniques for their detection have either sacrificed information on the coherence of the excited state or have been inefficient. Here, we show that these problems can be addressed by focusing the excitation light near to the extinction cross-section of a molecule. Our detection scheme enables us to explore resonance fluorescence over nine orders of magnitude of excitation intensity and to separate its coherent and incoherent parts. In the strong excitation regime, we demonstrate the first direct observation of the Mollow fluorescence triplet from a single solid-state emitter. Under weak excitation, we report the detection of a single molecule with an incident power as faint as 600aW, paving the way for studying nonlinear effects with only a few photons. }
}

@ARTICLE{gerhardt_ol_2007,
author = {I. Gerhardt and G. Wrigge and M. Agio and P. Bushev and G. Zumofen and V. Sandoghdar},
title = {Scanning near-field optical coherent spectroscopy of single molecules at 1.4K},
journal = {Optics Letters},
year = {2007},
volume = {32},
pages = {1420--1422},
number = {11},
keywords = {Scanning microscopy; Coherent optical effects; Extinction; Spectroscopy, condensed matter},
publisher = {OSA},
url = {http://ol.osa.org/abstract.cfm?URI=ol-32-11-1420},
abstract = {We present scanning near-field extinction spectra of single molecules embedded in a solid matrix. By varying the tip-molecule separation, we modify the line shape of the spectra, demonstrating the coherent nature of the interaction between the incident laser light and the excited state of the molecule. We compare the measured data with the outcome of numerical calculations and find a very good agreement.}
}

@ARTICLE{gerhardt_prl_2007,
author = {I. Gerhardt and G. Wrigge and P. Bushev and G. Zumofen and M. Agio and R. Pfab and V. Sandoghdar},
title = {Strong Extinction of a Laser Beam by a Single Molecule},
journal = {Physical Review Letters},
year = {2007},
volume = {98},
pages = {033601},
number = {3},
eid = {033601},
doi = {10.1103/PhysRevLett.98.033601},
numpages = {4},
publisher = {APS},
url = {http://link.aps.org/abstract/PRL/v98/e033601},
keywords = {Single Molecules; Extinction Spectroscopy; Absorption; Condensed Matter; Homodyning},
abstract = {We present an experiment where a single molecule strongly affects the amplitude and phase of a laser field emerging from a subwavelength aperture. We achieve a visibility of −6\% in direct and +10\% in cross-polarized detection schemes. Our analysis shows that a close to full extinction should be possible using near-field excitation.}
}

@ARTICLE{ilja_phd_2006,
author = {Ilja Gerhardt},
title = {Scattering and absorption of light by a single molecule under a subwavelength aperture},
journal = {Dissertation},
year = {2006},
volume = {},
pages = {1-100},
abstract = {In this dissertation an experiment is introduced, in which a single dye molecule is excited in the optical near field of a subwavelength aperture under cryogenic conditions. The response of the molecule is observed as a fingerprint on the excitation beam and by its red-shifted fluorescence. The experiment reveals an efficient coupling between the excitation light and the molecule. Single molecule spectroscopy was developed at the end of the 1980s. The first experiments were performed in an absorptive configuration. The detection of molecules was achieved by focusing a laser beam down to a few micrometers and cooling down the sample to liquid helium temperatures. In order to record the weak fingerprint of the molecule on the laser beam, a double lock-in detection was used. Today single molecule spectroscopy is usually performed as fluorescence excitation spectroscopy. The molecules are detected by their red-shifted fluorescence and the excitation light is blocked by an optical long-pass filter. This allows a high signal-to-noise ratio in the detection and spectroscopy of single molecules, although at the cost of sacrificing the information on the narrow-band emission of the zero-phonon line. In many applications it is desirable to detect a quantum emitter directly in transmission. Textbook formulae suggest that in order to observe the effect of a single molecule on a laser beam, one has to focus it tightly on a region comparable to the absorption cross-section. It turns out, that in practice, this is more subtle. In this thesis the main issues are discussed and experimental results in the method of targeting the absorption cross-section of a single molecule are shown. In our configuration we use a pulled and metal coated glass fiber with a subwavelength aperture at its end to excite single DBATT (dibenzanthanthrene) molecules in the near field. By cooling the sample to below 2K, we achieve narrow zero-phonon transitions and therefore large absorption cross-sections. Our experiments show the first extinction measurements of light in the near field of a single molecule with no further noise suppressing elements. The results show an effect that is more than 3 orders of magnitude larger than in other absorption type experiments on a single molecule, demonstrating the efficient coupling of light with a single emitter in the near field. This technique might also be applied to quantum dots or systems with very small Stokes shifts.},
keywords = {Single Molecule Spectroscopy; Near-Field Optics; Applied Optics; Light Scattering; Optics; Low-temperature Physics; Production Of Low Temperatures; Thermophysics},
url = {http://e-collection.ethbib.ethz.ch/view/eth:29104},
}

@ARTICLE{sandoghdar_chimia_2006,
author = {V. Sandoghdar and E. Klotzsch and V. Jacobsen and A. Renn and U. Hakanson and M. Agio and I. Gerhardt and J. Seelig and and G. Wrigge},
title = {Optical Detection of Very Small Nonfluorescent Nanoparticles},
journal = {Chimia},
year = {2006},
volume = {60},
pages = {761-764},
abstract = {We discuss an interferometric method for the optical detection of very small nonfluorescent nanoparticles. In particular, we show that single gold nanoparticles with a diameter as small as 5 nm can be detected. We discuss the potential of such tiny particles as optical labels for biological studies. Furthermore, we show that our interferometric method can be also used for the detection and tracking of unlabelled biological nano-entities such as viruses or microtubuli.},
file = {sandoghdar_chimia_2006.pdf:pdf/sandoghdar_chimia_2006.pdf:PDF},
keywords = {Interferometric Detection; Nano-particles; Gold Nano Particles; Plasmonics; Tracking},
url = {http://dx.doi.org/10.2533/chimia.2006.761}
}

@ARTICLE{pfab_cpl_2004,
author = {R.J. Pfab and J. Zimmermann and C. Hettich and I. Gerhardt and A. Renn and V. Sandoghdar},
title = {Aligned terrylene molecules in a spin-coated ultrathin crystalline film of \emph{p}-terphenyl},
journal = {Chemical Physics Letters},
year = {2004},
volume = {387},
pages = {490-495},
keywords = {Terrylene, Single Molecules, Thin Films, Crystalline Films},
url = {http://dx.doi.org/10.1016/j.cplett.2004.02.040}
}

@ARTICLE{hettich_aip_2003,
author = {C. Hettich and Th. Kalkbrenner and I. Gerhardt and S. K\"uhn and V. Sandoghdar},
editor = {Paul M. Koenraad and Martijn Kemerink},
collaboration = {},
title = {Single Molecules, Single Nanoparticles and Their Optical Interaction},
publisher = {AIP},
year = {2003},
journal = {SCANNING TUNNELING MICROSCOPY/SPECTROSCOPY AND RELATED TECHNIQUES: 12th International Conference STM'03},
volume = {696},
number = {1},
pages = {127-135},
location = {Eindhoven (NETHERLANDS)},
keywords = {Stark effect; near-field scanning optical microscopy; dyes; molecules; metals; nanostructured materials},
url = {http://link.aip.org/link/?APC/696/127/1},
doi = {10.1063/1.1639686},
abstract = {We have applied a new method to localize single dye molecules with nanometer resolution in three dimensions by means of the Stark effect. With this method we were able to resolve two molecules which were so close to each other that they underwent a strong dipole-dipole interaction. A quantitative spectral study of this system allowed us to determine the coupling parameters. In the second part of this paper we discuss our research on metal nanoparticles. We have developed a method to mount single nanoparticles at the very end of a fibre tip. We have used such probes as well defined scattering centers for apertureless scanning near-field optical microscopy. In this article we also discuss our efforts to examine the interaction of a single metal nanoparticle and a single molecule in a controlled manner.}
}

@ARTICLE{hettich_science_2002,
author = {C. Hettich and C. Schmitt and J. Zitzmann and S. K\"uhn and I. Gerhardt and V. Sandoghdar},
title = {Nanometer resolution and coherent optical dipole coupling of two individual molecules},
journal = {Science},
year = {2002},
volume = {298},
pages = {385},
abstract = {By performing cryogenic laser spectroscopy under a scanning probe electrode that induces a local electric field, we have resolved two individual fluorescent molecules separated by twelve nanometers in an organic crystal. The two molecules undergo a strong coherent dipole-dipole coupling that gives rise to entangled sub- and superradiant states. Under intense laser illumination, both molecules are excited via a two-photon transition, and the fluorescence from this doubly excited system displays photon bunching. Our experimental scheme can be used to both optically resolve molecules at the nanometer scale and manipulate the degree of entanglement among them.},
baseurl = {http://intra.ethz.ch},
file = {hettich01.pdf:pdf/hettich01.pdf:PDF},
keywords = {Dipole-Dipole Coupling; Stark Microscopy; Gradient Microscopy; Single Molecule Studies; Quantum Optics},
shortdesc = {No shortdesc available},
url = {http://dx.doi.org/10.1126/science.1075606}
}

@ARTICLE{george_sensa_2000,
author = {M. George and W. J. Parak and I. Gerhardt and W. Moritz and F. Kaesen and H. Geiger and I. Eisele and H. E. Gaub},
title = {Investigation of the spatial resolution of the light-addressable potentiometric sensor},
journal = {Sensors and Actuators A: Physical},
year = {2000},
volume = {86},
pages = {187 - 196},
number = {3},
abstract = {The spatial resolution of the light-addressable potentiometric sensor (LAPS) is investigated both theoretically and experimentally. For a theoretical analysis, the diffusion equation for minority charge carriers in the semiconductor was solved. The results suggest that by thinning the semiconductor wafer, the spatial resolution of the LAPS is no longer limited by the bulk minority charge carrier diffusion length. Spatial resolution in the micrometer range should thus be possible. For an experimental analysis, the effective diffusion length of light-generated charge carriers parallel to the sensor surface was measured. The results show that by increasing the doping density and by thinning the semiconductor substrate, spatial resolution of about 15 [mu]m is obtained.},
doi = {DOI: 10.1016/S0924-4247(00)00455-6},
issn = {0924-4247},
keywords = {Light-addressable potentiometric sensor},
url = {http://www.sciencedirect.com/science/article/B6THG-41H9KP6-7/2/3598b3ce451e623b333ccfbc1aa2b757}
}

@ARTICLE{moritz_fresenius_2000,
author = {W. Moritz and I. Gerhardt and D. Roden and M. Xu and S. Krause},
title = {Photocurrent measurements for laterally resolved interface characterization},
journal = {Fresenius J. Anal. Chem.},
year = {2000},
volume = {367},
pages = {329--333},
abstract = {A miniaturized optical set-up based on a CD-ROM player optic was developed for LAPS (light ad-dressable potentiometric sensors). A focus of 2.6~m$\mu$m was achieved using this easy to handle device. The lateral res-olution of LAPS measurements can be improved by using GaAs as the semiconductor material instead of Si. The diffusion length of the minority charge carriers was deter-mined to be smaller than 3.1$\mu$m. A new method called SPIM (scanning photo-induced impedance microscopy) is described. Using this technique, the impedance of thin films can be measured with lateral resolution.},
baseurl = {http://intra.ethz.ch},
file = {moritz01.pdf:pdf/moritz01.pdf:PDF},
shortdesc = {No shortdesc available},
keywords = {Potentiometric Sensing, Interface Potentials, Optical Resolution in Semiconductors},
url = {http://dx.doi.org/10.1007/s002160000409}
}

@Article{gerhardt_master_1999,
author = {Ilja Gerhardt},
title = {Untersuchung an D\"unnschichtsystemen auf Basis des Fotoeffekts im Halbleiter},
journal = {Diplomarbeit},
volume = {1},
number = {},
pages = {1--97},
abstract = {Bestrahlt man einen Halbleiter mit Licht, werden in diesem Elektronen Loch Paare erzeugt. An einer Raumladungszone, wie sie z.B. durch einen Dotierungswechsel oder ein angelegtes elektrisches Feld erzeugt werden kann, k\"onnen sich die einzelnen Ladungstr\"ager trennen. Dieser Effekt wird z.B. in einer Solarzelle genutzt um Strom zu erzeugen. Nutzt man eine sogenannte MIS Struktur, welche aus einer Schichtenfolge Metall Isolator Halbleiter (engl. Semiconductor) besteht, kann die Ladungstrennung durch eine angelegte Vorspannung erfolgen. Legt man an einen dotierte Struktur ein starkes negatives Feld an, sammeln sich positive Ladungstr\"ager an der Halbleiter Isolator Grenzschicht. Bei Bestrahlung mit Licht werden Elektronen Loch Paare erzeugt, die sich in dieser Raumladungszone verteilen. Jedoch k\"onnen die getrennten Ladungstr\"ager nicht ungest\"ort abflie\ssen. Dies wird durch den in der Struktur befindlichen Isolator verhindert. Aus diesem Grund l\"adt sich der von Metall Isolator Halbleiter gebildete Kondensator auf. Ist dieser aufgeladen, k\"onnen keine weiteren Ladungstr\"ager mehr flie\ssen. Lediglich die Rekombination der Ladungstr\"ager an der Halbleiter Isolator Grenzschicht oder im Bulk bietet einen Gegenspieler zu diesem Vorgang. Bei wiederholtem Ein und Ausschalten des Lichts wird somit der Kondensator wechselweise be und entladen; das l\"asst sich an einem Wechselstromfluss detektieren. Beleuchtet man von einem Halbleiterst\"uck nur einen kleinen Teil, wird nur dort ein Fotostrom erzeugt. Dieser beinhaltet Informationen \"uber den beleuchteten Bereich der Struktur. Dieses Prinzip wird im LAPS dem Licht Adressierbaren Potenziometrischen Sensor genutzt. Dabei wird eine chemisch sensitive Schicht auf den Isolator aufgebracht. Ver\"andert man die chemischen Eigenschaften einer Elektrolytl\"osung \"uber dieser Schicht, kann hieraus eine Potenzialverschiebung resultieren. Diese Potenzialverschiebung addiert (bzw. subtrahiert) sich zur angelegten Spannung. Ver\"andert sich \"ortlich auf dem Sensor das Potenzial der chemisch sensitiven Schicht, wird dies in dem entstehenden Fotostrom widergespiegelt. Misst man diese Potenzial\"anderungen ortsaufgel\"ost, indem man nur einen Teil der Struktur mit Licht bestrahlt, erh\"alt man ein Bild der Oberfl\"ache. Mit diesem Aufbau lassen sich ortsaufgel\"oste Bilder z.B. von Zellkulturen oder deren Metaboliten erhalten. D\"unne Filme von nichtmetallischen Festk\"orpern haben h\"aufig kapazitive Eigenschaften. Diese Eigenschaften kann man mit Ersatzschaltbildern beschreiben. Leitet der Film den elektrischen Strom auch im Gleichstromfall, ver\"andert sich das Ersatzschaltbild der Struktur. Der Widerstand der Struktur l\"asst sich nicht mehr durch einen einfachen Kondensator beschreiben. Dieser komplexe Wechselstromwiderstand wird als Impedanz bezeichnet. In der Chemie wird die Untersuchung von Impedanzen zur Bestimmung von Grenzfl\"achenparametern genutzt. Bei der Impedanzspektroskopie lassen sich elektrische Ersatzschaltbilder von verschiedenen Grenzfl\"achen bestimmen. Mit diesen Schaltbildern k\"onnen Teile dieser Grenzfl\"achen beschrieben werden. Die dieser Arbeit zu Grunde liegende Idee ist, die Technik des LAPS mit der Untersuchungsm\"oglichkeit der Impedanzspektroskopie zu kombinieren. Beleuchtet man nur einen Teil der Halbleiterstruktur, Bisweilen dient auch der Isolator selbst als chemisch sensitive Schicht gibt der entstehende Fotostrom nicht nur Ausk\"unfte \"uber die jeweiligen chemischen Parameter der \"au\sseren Grenzfl\"ache, auch die Impedanz der Grenzschicht l\"asst sich bestimmen. F\"ur dieses Verfahren wird von W. MORITZ und S. KRAUSE die Bezeichnung SPIM f\"ur Scanning Photoinduced Impedance Microscopy vorgeschlagen. Ziel der Arbeit war der Aufbau eines kompletten Versuchsaufbaus, der lateral aufgel\"oste Messungen an MIS Halbleiterstrukturen erm\"oglicht. Ein wesentlicher Aspekt war die Miniaturisierung des optischen Aufbaus. Als L\"osungsansatz wurde hier die Verwendung einer CD Spieler Optik verfolgt. Dar\"uber hinaus sollten die Hauptparameter, mit denen Impedanzmessungen an diesen MIS Strukturen durchgef\"uhrt werden, mittels halbleitertheoretischen \"Uberlegungen optimiert werden. Im Fokus standen hierbei die H\"ohe des Fotostromsignals und eine ideale laterale Aufl\"osung. Die hieraus gewonnenen Erkenntnisse sollten, soweit zug\"anglich, praktisch \"uberpr\"uft werden. Zu diesem Zweck wurden Messungen an Silizium mit verschiedenen Dotierkonzentrationen durchgef\"uhrt. Auch das Halbleitermaterial selbst wurde variiert. S\"amtliche Proben wurden auf Leckstrom, die HF CV Kurve und Fotostrom untersucht.},
keywords = {LAPS; SPIM; Halbleiter; Chemische Sensoren},
shortdesc = {No shortdesc available},
url = {http://gerhardt.ch/pdf/gerhardt01.pdf},
baseurl = {http://intra.ethz.ch},
pdf = {/pdf/gerhardt01.pdf},
year = {1999},
}