G. B. Xavier

Full polarization control for fiber optical quantum communication systems using polarization encoding

A real-time polarization control system employing two non-orthogonal reference signals multiplexed in either time or wavelength with the data signal is presented. It is shown, theoretically and experimentally, that complete control of multiple polarization states can be attained employing polarization controllers in closed-loop configuration. Experimental results on the wavelength multiplexing setup show that negligible added penalties, corresponding to an average added optical Quantum Bit Error Rate of 0.044%, can be achieved with response times smaller than 10 ms, without significant introduction of noise counts in the quantum channel.

Experimental polarization encoded quantum key distribution over optical fibres with real-time continuous birefringence compensation

In this paper we demonstrate an active polarization drift compensation scheme for optical fibres employed in a quantum key distribution experiment with polarization encoded qubits. The quantum signals are wavelength multiplexed in one fibre along with two classical optical side channels that provide the control information for the polarization compensation scheme. This set-up allows us to continuously track any polarization change without the need to interrupt the key exchange. The results obtained show that fast polarization rotations of the order of 40??rad?s?1 are effectively compensated for. We demonstrate that our set-up allows continuous quantum key distribution even in a fibre stressed by random polarization fluctuations. Our results pave the way for Bell-state measurements using only linear optics with parties separated by long-distance optical fibres.

Stable single-photon interference in a 1 km fiber-optic Mach-Zehnder interferometer with continuous phase adjustment.

We experimentally demonstrate stable and user-adjustable single-photon interference in a 1 km long fiber-optic Mach-Zehnder interferometer, using an active phase control system with the feedback provided by a classical laser. We are able to continuously tune the single-photon phase difference between the interferometer arms using a phase modulator, which is synchronized with the gate window of the single-photon detectors. The phase control system employs a piezoelectric fiber stretcher to stabilize the phase drift in the interferometer. A single-photon net visibility of 0.97 is obtained, yielding future possibilities for experimental realizations of quantum repeaters in optical fibers and violation of Bells inequalities using genuine energy-time entanglement.

Scattering Effects on QKD Employing Simultaneous Classical and Quantum Channels in Telecom Optical Fibers in the C‐band

Almost all Quantum Key Distribution experiments in optical fibers performed so far have used the so called “dark fibers,” which are exclusively used as the quantum channel. As quantum communications make its way towards commercial applications, there is a clear advantage in using fibers with simultaneous classical signals, via wavelength multiplexing, for that purpose. We show what is the noise impact of transmitting classical optical signals alongside a quantum channel for a fixed fiber length.

A heralded single-photon source for quantum communications compatible with long-distance optical fibers

This paper reports the implementation of a heralded single-photon source for quantum communications. Pump photons are parametric down-converted into pairs of photons at 810 and 1550 nm inside a non-linear crystal. The former acts as a heralding signal to the telecom wavelength photon. The temporal correlation between the photons is measured, as well as the compromise between source visibility and maximum achievable transmission distance.

Two-Way Quantum Communication in a Single Optical Fiber with Active Polarization Compensation

We experimentally demonstrate a two-way stable transmission of polarization encoded qubits over 23 km of spooled dispersion-shifted fiber with active polarization control in both directions, while simultaneously exchanging classical data. Two classical reference channels (one containing a telecom 10 Gb/s data stream), wavelength-multiplexed with the quantum signal, are used as feedback. The feasibility of quantum communication is demonstrated in the two opposite directions over 6 hours of continuous operation, as well as a classical error rate better than 1.0 x 10− 9.

Polarization coded quantum key distribution through telecom single mode optical fibers: Problems and solutions

In this paper we briefly comment some technological limitations of the implementation of quantum cryptography employing polarization coding. We perform an experiment to demonstrate a compensation scheme for polarization coded QKD. We then apply a procedure to characterize an APD working as a photon counter operating in the single-photon regime. We finally perform a polarization coding photon counting experiment.