An experimental test of Bell's inequality allows ruling out any local-realistic description of nature by measuring correlations between distant systems. While such tests are conceptually simple, there are strict requirements concerning the detection efficiency of the involved measurements, as well as the enforcement of spacelike separation between the measurement events. Only very recently could both loopholes be closed simultaneously. Here we present a statistically significant, event-ready Bell test based on combining heralded entanglement of atoms separated by 398 m with fast and efficient measurements of the atomic spin states closing essential loopholes. We obtain a violation with S ¼ 2.221 AE 0.033 (compared to the maximal value of 2 achievable with models based on local hidden variables) which allows us to refute the hypothesis of local realism with a significance level P < 2.57 × 10

Quantum key distribution 1,2 (QKD) is the first commercial application in the new field of quantum information with first routine applications in governmental and financial sectors 3 and with successful demonstrations of trusted node networks 4,5. Today, the grand goal is efficient long range key distribution either via quantum repeaters 6 or via satellites 7-9 in order to enable global secure communication. On the way to QKD via satellites a free-space demonstration of secure key distribution was performed over 144 km between two ground stations 10. This scenario is comparable to links between satellites in low earth orbits (LEO) and ground stations with respect to both attenuation and fluctuations. However, we still miss key exchange with rapidly moving platforms. Here we prove for the first time the feasibility of BB84 quantum key distribution between an airplane and a ground station. Establishing a stable and low noise quantum communication channel with the plane moving with 290 km/h at a distance of 20 km, i.e., 4 mrad/s, our results are representative for typical communication links to satellites 11 or to high altitude platforms. Quantum key distribution provides a whole new level of information security. Any information gained by eavesdropping on the quantum channel can be quantified by the observed transmission noise, the quantum bit error ratio (QBER) 12. Security proofs, solely based on the laws of quantum mechanics, show how to determine the necessary amount of privacy amplification (i.e., key shrinkage according to the QBER) to eliminate the knowledge of a possible adversary 13. Starting with a first quantum channel of 30 cm length in 1989 14 quantum key distribution quickly was enabled on successively longer distances. Two main branches for communicating qubits encoded with quantum states of light were established: either via telecom fiber channels or via free-space transmission. In both cases increasing attenuation and noise limit the maximum distance for a successful key distribution to typically 150−200 km 10,15,16. So far, long range free-space quantum communication experiments used a direct line of sight either between two Canary Islands (144 km) or across a lake in China (95 km) to demonstrate free-space QKD, entanglement distribution 10,17-19 , or quantum teleportation 20,21. In spite of this remarkable progress, all quantum communication so far was performed with stationary systems only. Contrary, for classical optical free-space communication high bandwidth links to aircrafts and satellites have been shown to be feasible in recent years 11,22,23. Here we report on an experiment combining recent advances in classical and in quantum optical technologies to demonstrate the feasibility of quantum key distribution from an airplane to ground (fig. 1). Major challenges in this experiment are the higher pointing requirements compared to classical free-space communication, the development of a precise compensation technique to account for the relative rotations of airborne and ground station qubit en...

The security of quantum key distribution (QKD) can easily be obscured if the eavesdropper can utilize technical imperfections of the actual implementation. Here we describe and experimentally demonstrate a very simple but highly effective attack which even does not need to intercept the quantum channel at all. Only by exploiting the dead time effect of single photon detectors the eavesdropper is able to gain (asymptotically) full information about the generated keys without being detected by state-of-the-art QKD protocols. In our experiment, the eavesdropper inferred up to 98.8% of the key correctly, without increasing the bit error rate between Alice and Bob significantly. Yet, we find an evenly simple and effective countermeasure to inhibit this and similar attacks.The communication of sensitive data has become part of our everyday life resulting in a growing need for mechanisms which ensure secure transmissions of these data. The secrecy of the information transfer can be guaranteed using a classical cryptographic method called one-time-pad. This method enables unconditionally secure communicationprovided that the exchange of the cryptographic key has been perfectly secure. In 1984 Ch. Bennett and G. Brassard showed that this indeed can be achieved using quantum cryptography, or more precisely quantum key distribution (QKD) [1,2], an approach which employs non-orthogonal quantum states for encoding information. Over the past years there have been remarkable QKD experiments pushing both the limits in distance and/or key rate [3][4][5][6] as well as the level of applicability achieving network functionality [7,8] with first systems for quantum secured communication being commercially available.Yet, what does "secure" mean? Today there exist security proofs [9,10] showing that the ideal protocol is secure in the sense that any knowledge of an eavesdropper about the key can be quantified and consequently made negligibly small. However, these proofs rarely specify requirements for QKD hardware and, if they do, real implementations will usually not fully comply with these specifications. This can lead to new types of attacks which are not covered by the proofs and hence won't be revealed by standard security tests. Recently, considerable effort has been made to reveal those potential threats [11][12][13][14][15] and to find countermeasures against them [16][17][18][19][20][21][22]. Many attacks are designed only for very specific systems and/or require sophisticated technology which is not yet (public) state-of-the-art.In this paper we introduce a novel type of attack which even does not need to intercept the qubits sent over the quantum channel. We show how to utilize an imperfection, which almost all QKD-systems display, namely the fact that common single photon detectors are rendered inactive for a period of time (called dead time) after a detection event. This enables the eavesdropper to unveal the full key without significantly changing the quantum bit error ratio using very simple equipment. On the one hand we...

Abstract. We report on in-lab free space quantum key distribution (QKD) experiments over 40 cm distance using highly efficient electrically driven quantum dot single-photon sources emitting in the red as well as near-infrared spectral range. In the case of infrared emitting devices, we achieve sifted key rates of 27.2 kbit s −1 (35.4 kbit s −1 ) at a quantum bit error rate (QBER) of 3.9% (3.8%) and a g (2) (0) value of 0.35 (0.49) at moderate (high) excitation. (2) (0) value of 0.49. This first successful proof of principle QKD experiment based on electrically operated semiconductor single-photon sources can be considered as a major step toward practical and efficient quantum cryptography scenarios. Contents

Currently most quantum key distribution (QKD) experiments are focusing on efficient long-distance implementations. Yet the recent development of miniaturized photonic modules and integrated quantum optics circuits could open new perspectives toward secure short-distance communication for daily-life applications. Here, we present the design of a new integrated optics architecture with an effective size of 25 mm\, \times \,2 mm\, \times \, 1\,mm. Our objective is to obtain an ultraflat microoptics QKD add-on suitable for integration into handheld platforms such as smartphones. In this context, we evaluated the suitability of various optical subsystems. We tested an array of four vertical cavity surface emitting lasers (VCSEL) with highly similar emission properties capable of producing subnanosecond near-infrared pulses at 100-MHz repetition rate. As short pulses exhibit a low polarization degree, their polarization can be externally controlled by a micropolarizer array. The fabrication of such elements is quite straightforward using standard lithographic techniques and extinction ratios up to 29 \,dB have been measured. To guarantee spatial indistinguishability of the qubits, we investigate the option of using low-birefringence, single-mode waveguide array manufactured via femtosecond laser micromachining

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.

Most polarization-based BB84 quantum key distribution (QKD) systems utilize multiple lasers to generate one of four polarization quantum states randomly. However, random bit generation with multiple lasers can potentially open critical side channels, which significantly endangers the security of QKD systems. In this paper, we show unnoticed side channels of temporal disparity and intensity fluctuation, which possibly exist in the operation of multiple semiconductor laser diodes. Experimental results show that the side channels can enormously degrade security performance of QKD systems. An important system issue for the improvement of quantum bit error rate (QBER) related with laser driving condition is furtherly addressed with experimental results. "Informatic analysis for hidden pulse attack exploiting spectral characteristics of optics in plug-and-play quantum key distribution system", Quant. Inf. Proc. 15, 4265-4282 (2016). 3. Nauerth, S., Furst, M., Schmitt-Manderbach, T., Weier, H., and Weinfurter, H., "Information leakage via side channels in freespace BB84 quantum cryptography", New J. full-scale experimental verifications towards ground-satellite quantum key distribution", Nat. Photon. 7, 387-393 (2013). 9

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