Sebastien Sauge

Controlling an actively-quenched single photon detector with bright light

We control using bright light an actively-quenched avalanche single-photon detector. Actively-quenched detectors are commonly used for quantum key distribution (QKD) in the visible and near-infrared range. This study shows that these detectors are controllable by the same attack used to hack passively-quenched and gated detectors. This demonstrates the generality of our attack and its possible applicability to eavsdropping the full secret key of all QKD systems using avalanche photodiodes (APDs). Moreover, the commercial detector model we tested (PerkinElmer SPCM-AQR) exhibits two new blinding mechanisms in addition to the previously observed thermal blinding of the APD, namely: malfunctioning of the bias voltage control circuit, and overload of the DC/DC converter biasing the APD. These two new technical loopholes found just in one detector model suggest that this problem must be solved in general, by incorporating generally imperfect detectors into the security proof for QKD.

Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks

A brighter, more narrowband and portable source using two 50 mm long orthogonally oriented, quasi-phase-matched crystals is built. The new source generates polarization-entangled photon pairs at 809 nm and 1555 nm at a maximum rate of 1.2 x 106s-1THz-1mW-1 after coupling to single-mode fiber. For transmission experiments, the quantum and synchronization channels are multiplexed in a WDM environment, and with only 0.8 nm spacing between them, a detected rate of 1.1 x 103 s-1 photon pairs is achieved with a raw visibility of 85 % in all measurement bases after transmission over 27 km of single mode fiber. With the current isolation of about 100 dB achieved between channels, the quantum channel could even be multiplexed with a classical data channel.

Ultra-low noise single-photon detector based on Si avalanche photodiode

We report operation and characterization of a lab-assembled single-photon detector based on commercial silicon avalanche photodiodes (PerkinElmer C30902SH, C30921SH). Dark count rate as low as 5 Hz was achieved by cooling the photodiodes down to -80 °C. While afterpulsing increased as the photodiode temperature was decreased, total afterpulse probability did not become significant due to detectors relatively long deadtime in a passively-quenched scheme. We measured photon detection efficiency >50% at 806 nm.

Single crystal source of polarization entangled photons at non-degenerate wavelengths

We demonstrate a bright, narrowband, compact single-crystal source of polarization entangled photon pairs at non-degenerate wavelength. This work is instrumental for quantum key distribution and entanglement transfer from photonic to atomic qubits.

Quantum hacking: how Eve can exploit component imperfections to control yet another of Bob's single-photon qubit detectors

Security and integrity of the network and the traffic it carries is a key requirement in modern communication systems. Over the past twenty years, quantum key distribution (QKD) has gained world-wide attention. This is because security of its implementations is based on the impossibility, in principle, to reliably copy an a-priori unknown quantum state (no-cloning theorem). However, security also relies on the assumption that electrooptical devices which are part of quantum cryptosystems do not deviate from model assumptions made to establish security proofs. This second range of security threats, which target component imperfections, has already been successfully exploited by one of the authors to take control of commonly used single photon detectors, namely InGaAs-based modules at telecom wavelengths [1] and Silicon-based passively-quenched modules in the visible - near infrared range [2].

A single-crystal source of path/polarization entanglement at non-degenerate wavelengths

We demonstrate a bright, narrowband, compact, quasi-phase-matched single-crystal source generating path-polarization-entangled photon pairs at 810 nm and 1550 nm at a maximum rate of 3 × 106 s−1 THz−1 mW−1 after coupling to single-mode fiber, and with two-photon interference visibility above 90%. While the source can already be used to implement quantum communication protocols such as quantum key distribution, this work is also instrumental for narrowband applications such as entanglement transfer from photonic to atomic qubits, or entanglement of photons from independent sources.

Robust decoy-state quantum key distribution with heralded single photon source

We have experimentally demonstrated a decoy-state quantum key distribution scheme (QKD) with a heralded single-photon source based on parametric down-conversion. What we used is a one-way BB84 protocol with a four-state and one-detector phase-coding scheme.

Quantum Communication in Optical Networks: an Overview and Selected Recent Results

We discuss recent work in quantum communication, and in some details present a bright, narrowband, portable, quasi-phase-matched two-crystal source generating polarization-entangled photon pairs at 809 nm and 1555 nm. We also show how the single-photon quantum channel at 1555 nm and a classical synchronization signal gating the single photon detector at the receiving side can be multiplexed in the same optical fiber of length 27 km by means of wavelength division multiplexers (WDM) having 100 GHz (0.8 nm) spacing between channels. This illustrates how single-photon quantum communication applications is compatible with current high-speed optical networks.

Single-photon correlations for secure communication

We present two types of photon sources designed for secure quantum communication, e.g. for quantum cryptography. Both types are based on the creation of photon pairs by spontaneous parametric downconversion in nonlinear crystals. The first is a heralded single photon source and the second is a source of polarization-entangled photon pairs. For the heralded single photon source the detection of one of the photons of a downconversion pair is used as a trigger to announce the presence of the other: the single photon. The source is characterized by a highly sub-Poisson photon number statistics making it very suitable for use in quantum cryptography protocols using single photonic qubits to create correlated information between a sender and a receiver. The entanglement source instead uses the inherent non-classical correlations between entangled qubits. We also present a hybrid-encoding where the sender uses polarization to encode information while the receiver uses time-bins. Both sources create photons with highly non-degenerate wavelengths of 810 nm and 1550 nm, taking advantage of the efficient detectors at near-infrared and the low transmission loss of optical fibers at telecommunication wavelengths.