Jean-Marc Merolla

Phase-modulation transmission system for quantum cryptography.

We describe a new method for quantum key distribution that utilizes phase modulation of sidebands of modulation by use of integrated electro-optic modulators at the transmitting and receiving modules. The system is shown to produce constructive or destructive interference with unity visibility, which should allow quantum cryptography to be carried out with high flexibility by use of conventional devices.

Demonstration of nonlocal dispersion cancelled two-photon Bessel interference in frequency domain

Entangled photons have appeared to be a promising way both for fundamental tests of physical principles and for quantum information applications such as Quantum Key Distribution. In particular, using entangled photons could potentially allow the realization of key distribution protocols over distances greater than a few hundreds of kilometers and security certification without a priori trust in the devices employed. Recently we reported experiments in which entanglement is manipulated in the frequency domain using electro-optic phase modulators driven by radio-frequency signals. Such method allows observation of an accurate and stable high-visibility (better than 99%) two-photon Bessel interference patterns. However in long distance fibre implementation this scheme remains sensitive to chromatic dispersion effect. We report a simple non local dispersion cancellation method built entirely from off-the-shelf components. The figure 1 a) depicts a setup allowing a realization of a non local dispersion cancellation in frequency domain. Pairs of photon are created by parametric down conversion in a Periodically Poled Lithium Niobate (PPLN) waveguide symmetrically around the 1550 nm wavelength (pulsation ω0). A 3dB coupler separates photon pairs. One of the photons is launched in a fibre spool. The second is launched in equivalent fibre spool following by a commercial programmable optical filter based on solid-state Liquid Crystal on Silicon switching element (Waveshaper 4000S). This equipment allows introducing a negative dispersion necessary to cancel the positive dispersion effect experienced by the photons propagating in the fiber spools. Both photons are then modulated by independent electro-optic phase modulators (EOPM) at 25 GHz. The relative phases between the 25 GHz signals are independently controlled by two phase shifters PS1 and PS2. After modulation, the photons are sent through narrowband tunable filters (F1,2) centred on angular frequency ω0. The photons are detected by two Avalanche Photodiodes (D1, 2). The phase modulators perform Bessel-type interference between photons separately detected on D1 and D2 (see [3] for details). The figure 1 b) and 1c) shows respectively the theoretical evolution of the interference patterns versus the distance when the phase shift is varying between 0 and 2π without (b) and with (c) negative dispersion (the normalized amplitude of modulation is 2.74). The introduction of negative dispersion allows cancelling the dispersion effect on the observed interference patterns.

Electro-optic nonlinear oscillator for ultra-fast secure chaos communication

Chaos-based secure communication is intended to operate as an additional encryption mean at the physical layer, with the unique capability for operating at several 10s of Gb/s. Different chaos generators have been recently explored for optical telecommunications, among which three setups were particularly interesting in terms of encryption speed capability: the all-optical external cavity semiconductor laser, the direct optoelectronic feedback semiconductor laser, and the nonlinear electrooptic feedback. The latter solution was mainly investigated by our group during the last 5 years. We report its numerous advantages in terms of chaos complexity (attractor dimension as high as several hundreds have been obtained), architecture reliability (different chaos generator have been performed experimentally), encryption bandwidth capability (more than 30GHz chaotic spectrum has been observed), stability, and decoding quality. The potential for high speed operation was recently demonstrated with an electrooptic chaos generator at 3Gb/s, with BER as low as 10-9}.

Recent advances in quantum cryptography

We report a particular implementation of a quantum cryptographic device operating at 1540 nm wavelength and involving interference between phase-modulated sidebands produced by a pair of phase modulators in the transmitting and receiving modules. The principle of operation is described in terms of both classical and quantum optics. The method has been demonstrated experimentally using a strong- attenuated semiconductor laser diode. Single photon interference has been obtained with a fringe visibility greater than 90%, including that the system can be used for quantum key distribution.

Classical and quantum coherence modulation

Coherence modulation of light consists in coding a signal as an optical delay greater than the coherence length of light. It makes use of interferometers powered by sources which exhibit a short coherence length. Such sources can be operated either as classical sources or quantum sources. We describe some applications dealing with optical communications and quantum cryptography.