Julian Fischer

Free space quantum key distribution over 500 meters using electrically driven quantum dot single-photon sources?a proof of principle experiment

Highly efficient single-photon sources (SPS) can increase the secure key rate of quantum key distribution (QKD) systems compared to conventional attenuated laser systems. Here we report on a free space QKD test using an electrically driven quantum dot single-photon source (QD SPS) that does not require a separate laser setup for optical pumping and thus allows for a simple and compact SPS QKD system. We describe its implementation in our 500 m free space QKD system in downtown Munich. Emulating a BB84 protocol operating at a repetition rate of 125 MHz, we could achieve sifted key rates of 5–17 kHz with error ratios of 6–9% and g (0) (2) -values of 0.39–0.76.

A polariton condensate in a photonic crystal potential landscape

This work has been supported by the State of Bavaria and the Australian Research Council (ARC).

Spatial coherence properties of one dimensional exciton-polariton condensates

In this work, we combine a systematic experimental investigation of the power- and temperature-dependent evolution of the spatial coherence function, g^{(1)}(r), in a one dimensional exciton-polariton channel with a modern microscopic numerical theory based on a stochastic master equation approach. The spatial coherence function g^{(1)}(r) is extracted via high-precision Michelson interferometry, which allows us to demonstrate that in the regime of nonresonant excitation, the dependence g^{(1)}(r) reaches a saturation value with a plateau, which is determined by the intensity of the pump and effective temperature of the crystal lattice. The theory, which was extended to allow for treating incoherent excitation in a stochastic frame, matches the experimental data with good qualitative and quantitative agreement. This allows us to verify the prediction that the decay of the off-diagonal long-range order can be almost fully suppressed in one dimensional condensate systems.

Coherent polariton laser

S. K., Z. B., Z. W., and H. D. acknowledge support from the National Science Foundation (NSF) under Grant No. DMR 1150593 and the Air Force Office of Scientific Research under Grant No. FA9550-15-1-0240. C. S., S. B., M. K., and S. H. acknowledge support from the State of Bavaria, Germany.

Wiring up pre-characterized single-photon emitters by laser lithography

Future quantum optical chips will likely be hybrid in nature and include many single-photon emitters, waveguides, filters, as well as single-photon detectors. Here, we introduce a scalable optical localization-selection-lithography procedure for wiring up a large number of single-photon emitters via polymeric photonic wire bonds in three dimensions. First, we localize and characterize nitrogen vacancies in nanodiamonds inside a solid photoresist exhibiting low background fluorescence. Next, without intermediate steps and using the same optical instrument, we perform aligned three-dimensional laser lithography. As a proof of concept, we design, fabricate, and characterize three-dimensional functional waveguide elements on an optical chip. Each element consists of one single-photon emitter centered in a crossed-arc waveguide configuration, allowing for integrated optical excitation and efficient background suppression at the same time.

Electro-optical switching between polariton and cavity lasing in an InGaAs quantum well microcavity

We report on the condensation of microcavity exciton polaritons under optical excitation in a microcavity with four embedded InGaAs quantum wells. The polariton laser is characterized by a distinct non-linearity in the input-output-characteristics, which is accompanied by a drop of the emission linewidth indicating temporal coherence and a characteristic persisting emission blueshift with increased particle density. The temporal coherence of the device at threshold is underlined by a characteristic drop of the second order coherence function to a value close to 1. Furthermore an external electric field is used to switch between polariton regime, polariton condensate and photon lasing.

High-temperature thin-film calorimetry: a newly developed method applied to lithium ion battery materials

A thin-film calorimeter has been developed to investigate the thermodynamic properties of thin films including battery layer sequences. A new approach, i.e., the application of high-temperature stable piezoelectric resonators as highly sensitive planar temperature sensor, is chosen. Thin films with a thickness of several micrometers of the material of interest are deposited on the resonators. The production or consumption of latent heat by the active layer(s) results in temperature fluctuations with respect to surroundings, in our case the furnace in which the sensor is placed. The temperature fluctuations can be easily monitored in situ via changes of the resonance frequency of the resonator. This enables us to extract the temperature and time dependence of phase transformations as well as the associated enthalpies. To cover a temperature range from −20 to 1000xa0°C, high-temperature stable piezoelectric langasite (La3Ga5SiO14) resonators are applied. Initially, aluminum and tin layers are used to test the calorimeter. The temperature and enthalpy of the solid–liquid phase transformation agree well with the literature data. Further, the thermodynamic data of the battery materials to be used as cathode, solid electrolyte, and anode in lithium ion batteries are investigated by the newly developed method. The cathode materials Li(Ni0.8Co0.15Al0.05)O2-δ (NCA) and LiMn2O4-δ (LMO) are amorphous after deposition and crystallize during heating. NCA shows this transformation at 455xa0°C with an enthalpy of −4.8xa0J/g. LMO undergoes three phase transformations at 330, 410 and 600xa0°C. They require initially an activation which is followed by an exothermic enthalpy. The associated energies (activation; enthalpy) are (+67.2; −50.2) J/g, (+29.3; −29.3)xa0J/g, and (+20.4; −26.2)xa0J/g, respectively. The solid electrolyte Li3.4V0.6Si0.4O4-δ (LVSO) shows no phase transformation up to its decomposition at about 220xa0°C. The anode material molybdenum disulfide (MoS2) exhibits a phase transformation at 480xa0°C with an enthalpy of −183.2xa0J/g.

PMMA-micro goblet resonators for biosensing applications

We report on a new type of whispering gallery mode (WGM) resonator made of out of low-loss polymer poly (methyl methacrylate) (PMMA). These optical cavities are fabricated using standard semiconductor processing methods in combination with a specific thermal reflow process. During this subsequent thermal treatment, surface tension leads to the goblet like geometry of the resonator and to an ultra smooth surface. The Q-factor of these goblet resonators is above 2·106 in the 1310 nm wavelength range. In order to demonstrate the applicability of the goblet resonators for bio sensing, Bovine Serum Albumin was detected by monitoring the shift of resonator modes due to protein adsorption.

Electroluminescence from spatially confined exciton polaritons in a textured microcavity

We report on the formation of spatially confined exciton-polaritons under electrical injection in a textured microcavity. The trapping of polaritons in the diode sample is achieved through a locally elongated GaAs microcavity with a quality factor exceeding 6000. The polaritonic resonances of traps with diameters of 10u2009μm and 2u2009μm are studied by angular-resolved electroluminescence spectroscopy, revealing their hybrid light-matter nature.

Low dimensional GaAs/air vertical microcavity lasers

We report on the fabrication of gallium arsenide (GaAs)/air distributed Bragg reflector microresonators with indium gallium arsenide quantum wells. The structures are studied via momentum resolved photoluminescence spectroscopy which allows us to investigate a pronounced optical mode quantization of the photonic dispersion. We can extract a length parameter from these quantized states whose upper limit can be connected to the lateral physical extension of the microcavity via analytical calculations. Laser emission from our microcavity under optical pumping is observed in power dependent investigations.