André Stefanov

Manipulation of multiphoton entanglement in waveguide quantum circuits

Precise control of single-photon states and multiphoton entanglement is demonstrated on-chip. Two- and four-photon entangled states have now been generated in a waveguide circuit and their interference tuned. These results open up adaptive and reconfigurable photonic quantum circuits not just for single photons, but for all quantum states of light.

Photon counting for quantum key distribution with peltier cooled InGaAs/InP APDs

Abstract The performance of three types of InGaAs/InP avalanche photodiodes is investigated for photon counting at 1550 nm in the temperature range of thermoelectric cooling. The best one yields a dark count probability of 2.8 × 10−5 per gate (2.4 ns) at a detection efficiency of 10% and a temperature of -60°C. The afterpulse probability and the timing jitter are also studied. The results obtained are compared with those of other papers and applied to the simulation of a quantum key distribution system. An error rate of 10% would be obtained after 54 km.

Experimental entanglement distillation and 'hidden' non-locality

Entangled states are central to quantum information processing, including quantum teleportation, efficient quantum computation and quantum cryptography. In general, these applications work best with pure, maximally entangled quantum states. However, owing to dissipation and decoherence, practically available states are likely to be non-maximally entangled, partially mixed (that is, not pure), or both. To counter this problem, various schemes of entanglement distillation, state purification and concentration have been proposed. Here we demonstrate experimentally the distillation of maximally entangled states from non-maximally entangled inputs. Using partial polarizers, we perform a filtering process to maximize the entanglement of pure polarization-entangled photon pairs generated by spontaneous parametric down-conversion. We have also applied our methods to initial states that are partially mixed. After filtering, the distilled states demonstrate certain non-local correlations, as evidenced by their violation of a form of Bells inequality. Because the initial states do not have this property, they can be said to possess ‘hidden’ non-locality.

Quantum correlations with spacelike separated beam splitters in motion: experimental test of multisimultaneity.

Multisimultaneity is a causal model of relativistic quantum physics which assigns a real time ordering to any set of events, much in the spirit of the pilot-wave picture. Contrary to standard quantum mechanics, it predicts a disappearance of the correlations in a Bell-type experiment when both analysers are in relative motion such that, each one in its own inertial reference frame, is first to select the output of the photons. We tested this prediction using acousto-optic modulators as moving beam-splitters and interferometers separated by 55 m. We didnt observe any disappearance of the correlations, thus refuting Multisimultaneity.

Faint laser quantum key distribution: Eavesdropping exploiting multiphoton pulses

Abstract The technological possibilities of a realistic eavesdropper are discussed. Two eavesdropping strategies taking advantage of multiphoton pulses in faint laser QKD are presented. We conclude that, as long as storage of qubits is technically impossible, faint laser QKD is not limited by this security issue, but mostly by the detector noise.

Quantum entanglement with acousto-optic modulators: Two-photon beats and Bell experiments with moving beam splitters

We present an experiment testing quantum correlations with frequency shifted photons. We test Bell inequality with two-photon interferometry where we replace the beam splitters with acousto-optic modulators, which are equivalent to moving beam splitters. We measure the two-photon beats induced by the frequency shifts, and we propose a cryptographic scheme in relation. Finally, setting the experiment in a relativistic configuration, we demonstrate that the quantum correlations are not only independent of the distance but also of the time ordering between the two single-photon measurements.

Quantum Information Science with Photons on a Chip

Quantum technologies based on photons will likely require integrated optics architectures for improved performance, miniaturization and scalability. We demonstrate high-fidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits.

Practical quantum key distribution

Quantum key distribution (QKD) is stepping out of the lab. We present a commercial fibre-optic QKD-prototype based on faint laser pulses and the results of field tests. Faint laser, single photon and entangled-photon based systems are compared with respect to possible bit rates, detector noise and security and their possible implementation in a commercial apparatus.