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Microcavity polaritons are bosonic light-matter particles that can emit coherent radiation without electronic population inversion via bosonic scattering. This phenomenon, known as polariton lasing, strongly depends on the polaritons' confinement. Shrinking the polaritons' mode volume increases the interactions mediated by their excitonic part, and thereby the density-dependent blueshift of the polariton to a higher energy is enhanced. Previously, polariton lasing has been demonstrated in micropillars with diameters larger than three microns, in grating based cavities, fiber cavities and photonic crystal cavities. Here we show polariton lasing in a micropillar with one micron diameter operating in a single transverse mode that can be optimally coupled to a singlemode fiber. We geometrically decouple the excitation with an angle from the collection. From the number of collected photons we calculate the number of polaritons and observe a blueshift large enough to qualify our device for novel schemes of quantum light generation such as the unconventional photon blockade. To that end, we also apply angled excitation to polaritonic molecules and show site-selective excitation and collection of modes with various symmetries., Comment: Submission to SciPost

Nonlinear frequency conversion is ubiquitous in laser engineering and quantum information technology. A long-standing goal in photonics is to integrate on-chip semiconductor laser sources with nonlinear optical components. Engineering waveguide lasers with spectra that phase-match to nonlinear processes on the same device is a formidable challenge. Here, we demonstrate difference-frequency generation in an AlGaAs Bragg reflection waveguide which incorporates the gain medium for the pump laser in its core. We include quantum dot layers in the AlGaAs waveguide that generate electrically driven laser light at ~790 nm, and engineer the structure to facilitate nonlinear processes at this wavelength. We perform difference-frequency generation between 1540 nm and 1630 nm using the on-chip laser, which is enabled by the broad modal phase-matching of the AlGaAs waveguide, and measure normalized conversion efficiencies up to $(0.64\pm0.21)$ %/W/cm$^2$. Our work demonstrates a pathway towards devices that utilize on-chip active elements and strong optical nonlinearities to enable highly integrated photonic systems-on-chip.

Due to their strict photon-number correlation, the twin beams produced in parametric down-conversion (PDC) work well for heralded state generation. Often, however, this state manipulation is distorted by the optical losses in the herald and by the higher photon-number contributions inevitable in the PDC process. In order to find feasible figures of merit for characterizing the heralded states, we investigate their normalized factorial moments of the photon number that can be accessed regardless of the optical losses in the detection. We then perform a measurement of the joint photon statistics of twin beams from a semiconductor Bragg-reflection waveguide with transition-edge sensors acting as photon-number-resolving detectors. We extract the photon-number parity of heralded single photons in a loss-tolerant fashion by utilizing the moment generating function. The photon-number parity is highly practicable in quantum state characterization, since it takes into account the complete photon-number content of the target state., Comment: 15 pages, 6 figures

Bragg-reflection waveguides (BRWs) fabricated from AlGaAs provide an interesting non-linear optical platform for photon-pair generation via parametric down-conversion (PDC). In contrast to many conventional PDC sources, BRWs are made of high refractive index materials and their characteristics are very sensitive to the underlying layer structure. First, we show that the design parameters like the phasematching wavelength and the group refractive indices of the interacting modes can be reliably controlled even in the presence of fabrication tolerances. We then investigate, how these characteristics can be taken advantage of when designing quantum photonic applications with BRWs. We especially concentrate on achieving a small differential group delay between the generated photons of a pair and then explore the performance of our design when realizing a Hong-Ou-Mandel interference experiment or generating spectrally multi-band polarization entangled states. Our results show that the versatility provided by engineering the dispersion in BRWs is important for employing them in different quantum optics tasks., Comment: 10 pages, 11 figures

Bragg-reflection waveguides emitting broadband parametric down-conversion (PDC) have been proven to be well suited for the on-chip generation of polarization entanglement in a straightforward fashion [R. T. Horn et al., Sci. Rep. 3, 2314 (2013)]. Here, we investigate how the properties of the created states can be modified by controlling the relative temporal delay between the pair of photons created via PDC. Our results offer an easily accessible approach for changing the coherence of the polarization entanglement, in other words, to tune the phase of the off-diagonal elements of the density matrix. Furthermore, we provide valuable insight in the engineering of these states directly at the source., Comment: 5 pages, 4 figures

We investigate the dispersion properties of ridge Bragg-reflection waveguides to deduce their phasematching characteristics. These are crucial for exploiting them as sources of parametric down-conversion (PDC). In order to estimate the phasematching bandwidth we first determine the group refractive indices of the interacting modes via Fabry-Perot experiments in two distant wavelength regions. Second, by measuring the spectra of the emitted PDC photons we gain access to their group index dispersion. Our results offer a simple approach for determining the PDC process parameters in the spectral domain and provide an important feedback for designing such sources, especially in the broadband case., Comment: 8 pages, 5 figures

Based on the interaction between different spatial modes, semiconductor Bragg-reflection waveguides provide a highly functional platform for non-linear optics. Therefore, the control and engineering of the properties of each spatial mode is essential. Despite the multimodeness of our waveguide, the well-established Fabry-Perot technique for recording fringes in the optical transmission spectrum can successfully be employed for a detailed linear optical characterization when combined with Fourier analysis. A prerequisite for the modal sensitivity is a finely resolved transmission spectrum that is recorded over a broad frequency band. Our results highlight how the features of different spatial modes, such as their loss characteristics and dispersion properties, can be separated from each other allowing their comparison. The mode-resolved measurements are important for optimizing the performance of such multimode waveguides by tailoring the properties of their spatial modes., Comment: 8 pages, 7 figures

Parametric downconversion (PDC) in semiconductor Bragg-reflection waveguides (BRW) is routinely exploited for photon-pair generation in the telecommunication range. Contrary to many conventional PDC sources, BRWs offer possibilities to create spectrally broadband but nevertheless indistinguishable photon pairs in orthogonal polarizations that simultaneously incorporate high frequency entanglement. We explore the characteristics of co-propagating twin beams created in a type-II ridge BRW. Our PDC source is bright and efficient, which serves as a benchmark of its performance and justifies its exploitation for further use in quantum photonics. We then examine the coalescence of the twin beams and investigate the effect of their inevitable multi-photon contributions on the observed photon bunching. Our results show that BRWs have a great potential for producing broadband indistinguishable photon pairs as well as multi-photon states., Comment: 10 pages, 5 figures

In this paper, we introduce two simple and inexpensive versions of the well-known Foucault method for measuring the speed of light. In a footprint of just 20 cm by 270 cm with readily available laboratory items and a webcam, we obtained $c=296720\pm3000$ km/s, and $c=302295\pm3000$ km/s, respectively, both within less than a per cent of the defined value. The experiment also prepares students to work with large amounts of data., Comment: 9 pages, 12 figures

The measurement of light characteristics at the single- and few photon level plays a key role in many quantum optics applications. Often photodetection is preceded with the transmission of quantum light over long distances in optical fibers with their low loss window near 1550nm. Nonetheless, the detection of the photonic states at telecommunication wavelengths via avalanche photodetectors has long been facing severe restrictions. Only recently, demonstrations of the first free-running detector techniques in the telecommunication band have lifted the demand of synchronizing the signal with the detector. Moreover, moderate cooling is required to gain single-photon sensitivity with these detectors. Here we implement a liquid-nitrogen cooled negative-feedback avalanche diode (NFAD) at telecommunication wavelengths and investigate the properties of this highly flexible, free-running single-photon sensitive detector. Our realization of cooling provides a large range of stable operating temperatures and has advantages over the relatively bulky commercial refrigerators that have been used before. We determine the region of NFAD working parameters most suitable for single-photon sensitive detection enabling a direct plug-in of our detector to a true photon counting task., Comment: 7 pages, 8 figures. The final publication is available at Springer via http://dx.doi.org/10.1007/s00340-015-6019-y

We experimentally analyze Rayleigh scattering in coupled planar microcavities. We show that the correlations of the disorder in the two cavities lead to inter-branch scattering of polaritons, that would otherwise be forbidden by symmetry. These longitudinal correlations can be inferred from the strength of the inter-branch scattering., Comment: 6 pages, 10 figures

In this paper we theoretically study how structural disorder in coupled semiconductor heterostructures influences single-particle scattering events that would otherwise be forbidden by symmetry. We extend the model of V. Savona to describe Rayleigh scattering in coupled planar microcavity structures, and answer the question, whether effective filter theories can be ruled out. They can., Comment: 10 pages, 7 figures

Quantum correlations are critical to our understanding of nature, with far-reaching technological and fundamental impact. These often manifest as violations of Bell's inequalities, bounds derived from the assumptions of locality and realism, concepts integral to classical physics. Many tests of Bell's inequalities have studied pairs of correlated particles; however, the immense interest in multi-particle quantum correlations is driving the experimental frontier to test systems beyond just pairs. All experimental violations of Bell's inequalities to date require supplementary assumptions, opening the results to one or more loopholes, the closing of which is one of the most important challenges in quantum science. Individual loopholes have been closed in experiments with pairs of particles and a very recent result closed the detection loophole in a six ion experiment. No experiment thus far has closed the locality loopholes with three or more particles. Here, we distribute three-photon Greenberger-Horne-Zeilinger entangled states using optical fibre and free-space links to independent measurement stations. The measured correlations constitute a test of Mermin's inequality while closing both the locality and related freedom-of-choice loopholes due to our experimental configuration and timing. We measured a Mermin parameter of 2.77 +/- 0.08, violating the inequality bound of 2 by over 9 standard deviations, with minimum tolerances for the locality and freedom-of-choice loopholes of 264 +/- 28 ns and 304 +/- 25 ns, respectively. These results represent a significant advance towards definitive tests of the foundations of quantum mechanics and practical multi-party quantum communications protocols., Comment: 13 pages, 8 figures

Cryptography's importance in our everyday lives continues to grow in our increasingly digital world. Oblivious transfer (OT) has long been a fundamental and important cryptographic primitive since it is known that general two-party cryptographic tasks can be built from this basic building block. Here we show the experimental implementation of a 1-2 random oblivious transfer (ROT) protocol by performing measurements on polarization-entangled photon pairs in a modified entangled quantum key distribution system, followed by all of the necessary classical post-processing including one-way error correction. We successfully exchange a 1,366 bits ROT string in ~3 min and include a full security analysis under the noisy storage model, accounting for all experimental error rates and finite size effects. This demonstrates the feasibility of using today's quantum technologies to implement secure two-party protocols., Comment: 14 pages, 3 figures, 3 tables

We propose and theoretically analyze a new scheme for generating hyper-entangled photon pairs in a system of polaritons in coupled planar microcavities. Starting from a microscopic model, we evaluate the relevant parametric scattering processes and numerically simulate the phonon-induced noise background under continuous-wave excitation. Our results show that, compared to other polariton entanglement proposals, our scheme enables the generation of photon pairs that are entangled in both path and polarization degrees of freedom, and simultaneously leads to a strong reduction of the photoluminesence noise background. This can significantly improve the fidelity of the entangled photon pairs under realistic experimental conditions., Comment: Main paper + Supplementary Material

Entanglement generation in microcavity exciton-polaritons is an interesting application of the peculiar properties of these half-light/half-matter quasiparticles. In this paper we theoretically investigate their luminescence dynamics and entanglement formation in single, double, and triple cavities. We derive general expressions and selection rules for polariton-polariton scattering. We evaluate a number of possible parametric scattering schemes in terms of entanglement, and identify the ones that are experimentally most promising., Comment: 25 pages, 8 figures

The statistical fluctuations in free-space links in the turbulent atmosphere are important for the distribution of quantum signals. To that end, we first study statistics generated by the turbulent atmosphere in an entanglement based free-space quantum key distribution (QKD) system. Using the insights gained from this analysis, we study the effect of link fluctuations on the security and key generation rate of decoy state QKD concluding that it has minimal effect in the typical operating regimes. We then investigate the novel idea of using these turbulent fluctuations to our advantage in QKD experiments. We implement a signal-to-noise ratio filter (SNRF) in our QKD system which rejects measurements during periods of low transmission efficiency, where the measured quantum bit error rate (QBER) is temporarily elevated. Using this, we increase the total secret key generated by the system from 78,009 bits to 97,678 bits, representing an increase of 25.2% in the final secure key rate, generated from the same raw signals. Lastly, we present simulations of a QKD exchange with an orbiting LEO satellite and show that an SNRF will be extremely useful in such a situation, allowing many more passes to extract a secret key than would otherwise be possible., Comment: 9 pages, 6 figures

We describe an experimental system that allows one to easily access the dispersion curve of exciton-polaritons in a microcavity. Our approach is based on two spatial light modulators (SLM), one for changing the excitation angles (momenta), and the other for tuning the excitation wavelength. We show that with this setup, an arbitrary number of states can be excited accurately and that re-configuration of the excitation scheme can be done at high speed., Comment: 4 pages, 5 figures

We investigate a quantum key distribution (QKD) scheme which utilizes a biased basis choice in order to increase the efficiency of the scheme. The optimal bias between the two measurement bases, a more refined error analysis, and finite key size effects are all studied in order to assure the security of the final key generated with the system. We then implement the scheme in a local entangled QKD system that uses polarization entangled photon pairs to securely distribute the key. A 50/50 non-polarizing beamsplitter with different optical attenuators is used to simulate a variable beamsplitter in order to allow us to study the operation of the system for different biases. Over 6 hours of continuous operation with a total bias of 0.9837/0.0163 (Z/X), we were able to generate 0.4567 secure key bits per raw key bit as compared to 0.2550 secure key bits per raw key bit for the unbiased case. This represents an increase in the efficiency of the key generation rate by 79%., Comment: v2: Revised paper based on referee reports, Theory section was revised (primarily regarding finite key effects), Results section almost completely rewritten with more experimental data. 16 pages, 5 figures. v1: 14 pages, 6 figures, higher resolution figures will be available in the published article

Poly(ethylene imine) functionalized carbon nanotube thin films, prepared using the vacuum filtration method, were decorated with Au nanoparticles by in situ reduction of HAuCl4 under mild conditions. These Au nanoparticles were subsequently employed for the growth of GaAs nanowires (NWs) by the vapor-liquid-solid process in a gas source molecular beam epitaxy system. The process resulted in the dense growth of GaAs NWs across the entire surface of the single-walled nanotube (SWNT) films. The NWs, which were orientated in a variety of angles with respect to the SWNT films, ranged in diameter between 20 to 200 nm, with heights up to 2.5 um. Transmission electron microscopy analysis of the NW-SWNT interface indicated that NW growth was initiated upon the surface of the nanotube composite films. Photoluminescence characterization of a single NW specimen showed high optical quality. Rectifying asymmetric current-voltage behavior was observed from contacted NW ensembles and attributed to the core-shell pn-junction within the NWs. Potential applications of such novel hybrid architectures include flexible solar cells, displays, and sensors.

Any characterization of a single-photon source is not complete without specifying its second-order degree of coherence, i.e., its $g^{(2)}$ function. An accurate measurement of such coherence functions commonly requires high-precision single-photon detectors, in whose absence, only time-averaged measurements are possible. It is not clear, however, how the resulting time-averaged quantities can be used to properly characterize the source. In this paper, we investigate this issue for a heralded source of single photons that relies on continuous-wave parametric down-conversion. By accounting for major shortcomings of the source and the detectors--i.e., the multiple-photon emissions of the source, the time resolution of photodetectors, and our chosen width of coincidence window--our theory enables us to infer the true source properties from imperfect measurements. Our theoretical results are corroborated by an experimental demonstration using a PPKTP crystal pumped by a blue laser, that results in a single-photon generation rate about 1.2 millions per second per milliwatt of pump power. This work takes an important step toward the standardization of such heralded single-photon sources., Comment: 18 pages, 9 figures; corrected Eq. (11) and the description follows Eq. (22)

We report on the first real-time implementation of a quantum key distribution (QKD) system using entangled photon pairs that are sent over two free-space optical telescope links. The entangled photon pairs are produced with a type-II spontaneous parametric down-conversion source placed in a central, potentially untrusted, location. The two free-space links cover a distance of 435 m and 1,325 m respectively, producing a total separation of 1,575 m. The system relies on passive polarization analysis units, GPS timing receivers for synchronization, and custom written software to perform the complete QKD protocol including error correction and privacy amplification. Over 6.5 hours during the night, we observed an average raw key generation rate of 565 bits/s, an average quantum bit error rate (QBER) of 4.92%, and an average secure key generation rate of 85 bits/s., Comment: v2: Revised paper based on referee report, primarily in the Security assumption section and towards the end of the Experimental implementation section. 14 pages, 5 figures, higher resolution images will be available in the published article. v1: 13 pages, 5 figures, higher resolution images will be available in the published article

Single-photon sources (SPSs) are mainly characterized by the minimum value of their second-order coherence function, viz. their $g^{(2)}$ function. A precise measurement of $g^{(2)}$ may, however, require high time-resolution devices, in whose absence, only time-averaged measurements are accessible. These time-averaged measures, standing alone, do not carry sufficient information for proper characterization of SPSs. Here, we develop a theory, corroborated by an experiment, that allows us to scrutinize the coherence properties of heralded SPSs that rely on continuous-wave parametric down-conversion. Our proposed measures and analysis enable proper standardization of such SPSs., Comment: 4 pages, 4 figures, corrected Eq. (10)

This letter reports on the growth, structure and luminescent properties of individual multiple quantum well (MQW) AlGaAs nanowires (NWs). The composition modulations (MQWs) are obtained by alternating the elemental flux of Al and Ga during the molecular beam epitaxy growth of the AlGaAs wire on GaAs (111)B substrates. Transmission electron microscopy and energy dispersive X-ray spectroscopy performed on individual NWs are consistent with a configuration composed of conical segments stacked along the NW axis. Micro-photoluminescence measurements and confocal microscopy showed enhanced light emission from the MQW NWs as compared to non-segmented NWs due to carrier confinement and sidewall passivation.

Al(0.37)Ga(0.63)As nanowires (NWs) were grown in a molecular beam epitaxy system on GaAs(111)B substrates. Micro-photoluminescence measurements and energy dispersive X-ray spectroscopy indicated a core-shell structure and Al composition gradient along the NW axis, producing a potential minimum for carrier confinement. The core-shell structure formed during the growth as a consequence of the different Al and Ga adatom diffusion lengths., Comment: 20 pages, 7 figures

This is a collection of statements gathered on the occasion of the Quantum Physics of Nature meeting in Vienna., Comment: 3 pages, Quantum Physics of Nature (QUPON) Conference, Vienna, Austria, May 22nd-26th, 2005; v4: more contributions

Hess and Philipp have recently claimed that proofs of Bell's theorem have overlooked the possibility of time dependence in local hidden variables, hence the theorem has not been proven true. Moreover they present what is claimed to be a local realistic model of the EPR correlations. If this is true then Bell's theorem is not just unproven, but false. We refute both claims. Firstly we explain why time is not an issue in Bell's theorem, and secondly show that their hidden variables model violates Einstein separability. Hess and Philipp have overlooked the freedom of the experimenter to choose settings of a measurement apparatus at will: any setting could be in force during the same time period., Comment: 5 pages, submitted to P.N.A.S; some overlap with our earlier quant-ph/0204169

A recent Letter by Hess and Philipp claims that Bell's theorem neglects the possibility of time-like dependence in local hidden variables, hence is not conclusive. Moreover the authors claim that they have constructed, in an earlier paper, a local realistic model of the EPR correlations. However, they themselves have neglected the experimenter's freedom to choose settings, while on the other hand, Bell's theorem can be formulated to cope with time-like dependence. This in itself proves that their toy model cannot satisfy local realism, but we also indicate where their proof of its local realistic nature fails., Comment: Latex needs epl.cls

The phenomenon of quantum interrogation allows one to optically detect the presence of an absorbing object, without the measuring light interacting with it. In an application of the quantum Zeno effect, the object inhibits the otherwise coherent evolution of the light, such that the probability that an interrogating photon is absorbed can in principle be arbitrarily small. We have implemented this technique, demonstrating efficiencies exceeding the 50% theoretical-maximum of the original ``interaction-free'' measurement proposal. We have also predicted and experimentally verified a previously unsuspected dependence on loss; efficiencies of up to 73% were observed and the feasibility of efficiencies up to 85% was demonstrated., Comment: 4 pages, 3 postscript figures. To appear in Phys. Rev. Lett; submitted June 11, 1999