Publications

Quantum State Diffusion: from Foundations to Applications

Deeper insight leads to better practice. We show how the study of the foundations of quantum mechanics has led to new pictures of open systems and to a method of computation which is practical and can be used where others cannot. We illustrate the power of the new method by a series of pictures that show the emergence of classical features in a quantum world. We compare the development of quantum mechanics and of the theory of (biological) evolution.

Long distance quantum teleportation in quantum relay configuration

Experimental realization of a quantum relay is reported, where the source creating the qubit to be teleported and the EPR source are both connected to the BSM (Bell state measurement) by 2 km of optical fibers.

Automated 'plug & play' quantum key distribution

An improved plug & play interferometric system for quantum key distribution is presented. Self-alignment and compensation of birefringence remain, while limitations due to reflections are overcome. Original electronics implementing the BB84 protocol makes adjustment simple. Key creation with 0.1 photon per pulse at a rate of 325 Hz with a 2.9 percent QBER - corresponding to a net rate of 210Hz - over a 23 Km installed cable was performed.

Advisory Board

No abstract is provided for this article.

Classical Simulation of Entanglement Swapping with Bounded Communication

Entanglement appears under two different forms in quantum theory, namely, as a property of states of joint systems and as a property of measurement eigenstates in joint measurements. By combining these two aspects of entanglement, it is possible to generate nonlocality between particles that never interacted, using the protocol of entanglement swapping. We show that even in the more constraining bilocal scenario where distant sources of particles are assumed to be independent, i.e., to share no prior randomness, entanglement swapping can be simulated classically with bounded communication, using only 9 bits in total. Our result thus provides an upper bound on the nonlocality of the entanglement swapping process.

Tunable upconversion photon detector

We introduce a simple approach for a tunable upconversion detector. This scheme is relevant for both single photon detection or anywhere where low light levels at telecom wavelengths need to be detected with a high degree of temporal resolution or where high count rates are desired. A system combining a periodically poled lithium niobate waveguide for the nonlinear wavelength conversion and a low jitter silicon avalanche photodiode is used in conjunction with a tunable pump source. We report more than a tenfold increase in the detectable bandwidth using this tuning scheme.

Determination of the phase and group birefringence of single-mode optical fibers based on the twist

A novel method for the measurement of the phase and group birefingence is described. The method is based on the evolution of the differential group delay as firnetion of the twist applied on the fiber. It gives the birefringence values for zero twist for fibers within the beatlength range of 5mm-lm. The technique is very simple and found advantageous for the study of special fibers like PCF.

Mathematical languages shape our understanding of time in physics

No abstract is provided for this article.

On the simulation of two-qubit states with non-local resources

Device-independent quantum random number generator with a photon pair source

What is the most efficient way to generate random numbers deviceindependently using a photon pair source based on spontaneous parametric down conversion (SPDC)? We consider this question by comparing two implementations of a detection-loophole-free Bell test. In particular, we study in detail a scenario where a heralded single photon source (HSPS) is used to herald path-entangled states, i.e. entanglement between two spatial modes sharing a single photon and where nonlocality is revealed using photon counting preceded by small displacement operations. ar X iv :1 40 9. 80 51 v1 [ qu an tph ] 2 9 Se p 20 14 Device-independent quantum random number generator with a photon pair source 2 We start by giving a theoretical description of such a measurement. We then show how to optimize the Bell-CHSH violation through a non-perturbative calculation, taking the main experimental imperfections into account. We finally bound the amount of randomness that can be extracted and compare it to the one obtained with the conventional scenario using photon pairs entangled e.g. in polarization and analyzed through photon counting. While the former requires higher overall detection efficiencies, it is far more efficient in terms of both the entropy per experimental run and the rate of random bit generation. PACS numbers: 03.65.Ud Device-independent quantum random number generator with a photon pair source 3

Using quantum key distribution for cryptographic purposes: a survey

The appealing feature of quantum key distribution (QKD), from a cryptographic viewpoint, is the ability to prove the information-theoretic security (ITS) of the established keys. As a key establishment primitive, QKD however does not provide a standalone security service in its own: the secret keys established by QKD are in general then used by a subsequent cryptographic applications for which the requirements, the context of use and the security properties can vary. It is therefore important, in the perspective of integrating QKD in security infrastructures, to analyze how QKD can be combined with other cryptographic primitives. The purpose of this survey article, which is mostly centered on European research results, is to contribute to such an analysis. We first review and compare the properties of the existing key establishment techniques, QKD being one of them. We then study more specifically two generic scenarios related to the practical use of QKD in cryptographic infrastructures: 1) using QKD as a key renewal technique for a symmetric cipher over a point-to-point link; 2) using QKD in a network containing many users with the objective of offering any-to-any key establishment service. We discuss the constraints as well as the potential interest of using QKD in these contexts. We finally give an overview of challenges relative to the development of QKD technology that also constitute potential avenues for cryptographic research.

Implementation of Device-Independent Quantum Key Distribution based on a Heralded Qubit Amplification

In device-independent quantum key distribution (DIQKD), the violation of a Bell inequality is exploited to establish a shared key that is secure independently of the internal workings of the QKD devices. An experimental implementation of DIQKD, however, is still awaited, since hitherto all optical Bell tests are subject to the detection loophole, making the protocol unsecured. In particular, photon losses in the quantum channel represent a fundamental limitation for DIQKD. Here, we introduce a heralded qubit amplifier based on single-photon sources and linear optics that provides a realistic solution to overcome the problem of channel losses in Bell tests.

Long-distance teleportation of qubits at telecommunication wavelengths

No abstract is provided for this article.

Experimental Bit Commitment Based on Quantum Communication and Special Relativity

Bit commitment is a fundamental cryptographic primitive in which Bob wishes to commit a secret bit to Alice. Perfectly secure bit commitment between two mistrustful parties is impossible through asynchronous exchange of quantum information. Perfect security is however possible when Alice and Bob split into several agents exchanging classical and quantum information at times and locations suitably chosen to satisfy specific relativistic constraints. Here we report on an implementation of a bit commitment protocol using quantum communication and special relativity. Our protocol is based on [A. Kent, Phys. Rev. Lett. 109, 130501 (2012)] and has the advantage that it is practically feasible with arbitrary large separations between the agents in order to maximize the commitment time. By positioning agents in Geneva and Singapore, we obtain a commitment time of 15 ms. A security analysis considering experimental imperfections and finite statistics is presented.

Anwendungen

No abstract is provided for this article.

Optical storage on the timescale of a second in a solid-state atomic frequency comb memory using dynamical decoupling

Demonstration of genuine multipartite entanglement with device-independent witnesses

No abstract is provided for this article.

Determination of nonlinear coefficient n2/Aeff using self-aligned interferometer and Faraday mirror

A method for measuring the nonlinear coefficient n2/Aeff in telecom fibres at 1550 nm is presented. The method is based on determining the Kerr phase shift detected by a self-aligned interferometer incorporating a Faraday mirror. This makes the setup very robust, and different test fibres can be measured without the need for any further readjustments.