Jean-Philippe Bourgoin

Free-space quantum key distribution to a moving receiver

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.

Attacks exploiting deviation of mean photon number in quantum key distribution and coin tossing

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-Ac´ 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 plug-and-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.

Security loophole in free-space quantum key distribution due to spatial-mode detector-efficiency mismatch

In free-space quantum key distribution (QKD), the sensitivity of the receivers detector channels may depend differently on the spatial mode of incoming photons. Consequently, an attacker can control the spatial mode to break security. We experimentally investigate a standard polarization QKD receiver and identify sources of efficiency mismatch in its optical scheme. We model a practical intercept-and-resend attack and show that it would break security in most situations. We show experimentally that adding an appropriately chosen spatial filter at the receivers entrance may be an effective countermeasure.

Creation of backdoors in quantum communications via laser damage

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 behavior on demand. 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.

Generating polarization-entangled photon pairs using cross-spliced birefringent fibers

We demonstrate a novel polarization-entangled photon-pair source based on standard birefringent polarization-maintaining optical fiber. The source consists of two stretches of fiber spliced together with perpendicular polarization axes, and has the potential to be fully fiber-based, with all bulk optics replaced with in-fiber equivalents. By modelling the temporal walk-off in the fibers, we implement compensation necessary for the photon creation processes in the two stretches of fiber to be indistinguishable. Our source subsequently produces a high quality entangled state having (92.2 ± 0.2) % fidelity with a maximally entangled Bell state.

Novel High-Speed Polarization Source for Decoy-State BB84 Quantum Key Distribution Over Free Space and Satellite Links

To implement the BB84 decoy-state quantum key distribution (QKD) protocol over a lossy ground-satellite quantum uplink requires a source that has high repetition rate of short laser pulses, long term stability, and no phase correlations between pulses. We present a new type of telecom optical polarization and amplitude modulator, based on a balanced Mach-Zehnder interferometer configuration, coupled to a polarization-preserving sum-frequency generation (SFG) optical setup, generating 532 nm photons with modulated polarization and amplitude states. The weak coherent pulses produced by SFG meet the challenging requirements for long range QKD, featuring a high clock rate of 76 MHz, pico-second pulse width, phase randomization, and 98% polarization visibility for all states. Successful QKD has been demonstrated using this apparatus with full system stability up to 160 minutes and channel losses as high 57 dB . We present the design and simulation of the hardware through the Mueller matrix and Stokes vector relations, together with an experimental implementation working in the telecom wavelength band. We show the utility of the complete system by performing high loss QKD simulations, and confirm that our modulator fulfills the expected performance.

Detailed performance analysis of the proposed QEYSSAT quantum receiver satellite

Transmission losses limit quantum key distribution (QKD) to distances of only a few hundred kilometres. We investigate performance aspects of the QEYSSAT proposal to demonstrate global QKD using a microsatellite as a trusted quantum receiver.

Airborne demonstration of a quantum key distribution receiver payload

Demonstrations of quantum key distribution (QKD) [1] with moving platforms are important to prove the viability of future satellite implementations. Thus far these demonstrations of QKD to aircraft have operated exclusively in the downlink configuration [2, 3], where the quantum source and transmitter are placed on the airborne platform. While this approach has the potential for higher key rates, it is more complex and is not as flexible as an uplink configuration, which places the quantum receiver on the airborne platform [4]. Here we present the first successful demonstration of QKD to a receiver on a moving aircraft.

Optimal pair-generation rate for entanglement-based quantum key distribution

In entanglement-based quantum key distribution (QKD), the generation and detection of multi-photon modes leads to a trade-off between entanglement visibility and two-fold coincidence events when maximizing the secure key rate (SKR). We produce a predictive model for the optimal two-fold coincidence probability per coincidence window given the channel efficiency and detector dark count rate of a given system. This model is experimentally validated and used in simulations for QKD with satellites as well as optical fibers.

A collinear nondegenerate source of entangled photon pairs in PM fiber

We demonstrate a source of entangled photon pairs based on standard polarization-maintaining fiber in a novel architecture. Sections of fibre are spliced together with perpendicular polarization axes, allowing single-path entanglement generation.