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...
We present a fully integrated, ready-for-use quantum random number generator (QRNG) whose stochastic model is based on the randomness of detecting single photons in attenuated light. We show that often annoying deadtime effects associated with photomultiplier tubes (PMT) can be utilized to avoid postprocessing for bias or correlations. The random numbers directly delivered to a PC, generated at a rate of up to 50 Mbit/s, clearly pass all tests relevant for (physical) random number generators.
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