The security of quantum communication using a weak coherent source requires an accurate knowledge of the source's mean photon number. Finite calibration precision or an active manipulation by an attacker may cause the actual emitted photon number to deviate from the known value. We model effects of this deviation on the security of three quantum communication protocols: the Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol without decoy states, Scarani-Acín-Ribordy-Gisin 2004 (SARG04) QKD protocol, and a coin-tossing protocol. For QKD, we model both a strong attack using technology possible in principle, and a realistic attack bounded by today's technology. To maintain the mean photon number in two-way systems, such as plugand-play and relativistic quantum cryptography schemes, bright pulse energy incoming from the communication channel must be monitored. Implementation of a monitoring detector has largely been ignored so far, except for ID Quantique's commercial QKD system Clavis2. We scrutinize this implementation for security problems, and show that designing a hack-proof pulse-energy-measuring detector is far from trivial. Indeed the first implementation has three serious flaws confirmed experimentally, each of which may be exploited in a cleverly constructed Trojan-horse attack. We discuss requirements for a loophole-free implementation of the monitoring detector.

Decoy-state quantum key distribution (QKD) is a standard technique in current quantum cryptographic implementations. Unfortunately, existing experiments have two important drawbacks: the state preparation is assumed to be perfect without errors and the employed security proofs do not fully consider the finite-key effects for general attacks. These two drawbacks mean that existing experiments are not guaranteed to be secure in practice. Here, we perform an experiment that for the first time shows secure QKD with imperfect state preparations over long distances and achieves rigorous finite-key security bounds for decoy-state QKD against coherent attacks in the universally composable framework. We quantify the source flaws experimentally and demonstrate a QKD implementation that is tolerant to channel loss despite the source flaws. Our implementation considers more real-world problems than most previous experiments and our theory can be applied to general QKD systems. These features constitute a step towards secure QKD with imperfect devices.Comment: 12 pages, 4 figures, updated experiment and theor

Practical quantum communication (QC) protocols are assumed to be secure provided implemented devices are properly characterized and all known side channels are closed. We show that this is not always true. We demonstrate a laser-damage attack capable of modifying device behaviour ondemand. We test it on two practical QC systems for key distribution and coin-tossing, and show that newly created deviations lead to side channels. This reveals that laser damage is a potential security risk to existing QC systems, and necessitates their testing to guarantee security.Cryptography, an art of secure communication, has traditionally relied on either algorithmic or computational complexity [1]. Even the most state-of-the-art classical cryptographic schemes do not have a strict mathematical proof to ascertain their security. With the advance of quantum computing, it may be a matter of time before the security of the most widely used public-key cryptography protocols is broken [2]. Quantum communication (QC) protocols, on the other hand, have theoretical proofs of being unconditionally secure [3][4][5][6][7][8][9]. In theory, their security is based on the assumption of modeled behaviour of implemented equipment. In practice, the actual behaviour often deviates from the modeled one, leading to a compromise of security as has been seen so far in case of quantum key distribution (QKD) [10][11][12][13][14][15][16]. However, it is widely assumed that as long as these deviations are properly characterized and security proofs are updated accordingly [5,17], implementations are unconditionally secure. In this work we show that satisfying this during the initial installation only is not enough to guarantee security. Even if a system is perfectly characterized and deviations are included into the security proofs, an adversary can still create a new deviation ondemand and make the system insecure.Before going into details on how the adversary may do it, let's consider a few examples of deviations and their consequences. For example, a calibrated optical attenuator is required to set a precise value of the outgoing mean photon number µ in the implementations of ordinary QKD [18, 19] [9] protocols. An unexpected increase of this * makarov@vad1.com optical component's attenuation may cause a denial-ofservice. However, a reduction in attenuation will increase µ, leading to a compromise of security via attacks that rely on measurement of multi-photon pulses [25,26]. E.g., in QKD and secret-sharing this will allow eavesdropping of the key, and in bit commitment cheating the committed bit value. Some implementations use a detector for time synchronization [8,9,18, 19,[21][22][23][24]. Desensitizing it may result in the denial-of-service. However, several implementations require a calibrated monitoring detector for security purposes [8,9,18, 19,21,23,24]. A reduction in its sensitivity may lead to security vulnerabilities such as a Trojan-horse attack that reads the state preparation [27]. This leaks the key in QKD, increases the cheating p...

Technological realities limit terrestrial quantum key distribution (QKD) to single-link distances of a few hundred kilometers. One promising avenue for global-scale quantum communication networks is to use low-Earth-orbit satellites. Here we report the first demonstration of QKD from a stationary transmitter to a receiver platform traveling at an angular speed equivalent to a 600 km altitude satellite, located on a moving truck. We overcome the challenges of actively correcting beam pointing, photon polarization and time-of-flight. Our system generates an asymptotic secure key at 40 bits/s. 33. J.-P. Bourgoin, "Experimental and theoretical demonstration of the feasibility of global quantum cryptography using satellites," Ph.D. thesis, University of Waterloo (2014).

Satellite-based quantum terminals are a feasible way to extend the reach of quantum communication protocols such as quantum key distribution (QKD) to the global scale. To that end, prior demonstrations have shown QKD transmissions from airborne platforms to receivers on ground, but none have shown QKD transmissions from ground to a moving aircraft, the latter scenario having simplicity and flexibility advantages for a hypothetical satellite. Here, we demonstrate QKD from a ground transmitter to a receiver prototype mounted on an airplane in flight. We have specifically designed our receiver prototype to consist of many components that are compatible with the environment and resource constraints of a satellite. Coupled with our relocatable ground station system, optical links with distances of 3-10 km were maintained and quantum signals transmitted while traversing angular rates similar to those observed of low-Earth-orbit satellites. For some passes of the aircraft over the ground station, links were established within 10 s of position data transmission, and with link times of a few minutes and received quantum bit error rates typically ≈3%-5% , we generated secure keys up to 868 kb in length. By successfully generating secure keys over several different pass configurations, we demonstrate the viability of technology that constitutes a quantum receiver satellite payload and provide a blueprint for future satellite missions to build upon.