Publications

Detailed polarization properties comparison for three completely different species of highly birefringent fibers

We present a detailed experimental investigation for four dgerent fiber samples: a standard PANDA pber, two index-guiding nonlinear photonic crystal fibers. and an air-core photonic bandgap $her. Although all fibers exhibit large birefringence, their polarization properties are found to be quite different. Most surprisingly, the principle states of polarization strongly vary with wavelength for the photonic bandgap pber, indicating the presence of polarization mode coupling.

Multimode and Long-Lived Quantum Correlations Between Photons and Spins in a Crystal

The realization of quantum networks and quantum repeaters remains an outstanding challenge in quantum communication. These rely on the entanglement of remote matter systems, which in turn requires the creation of quantum correlations between a single photon and a matter system. A practical way to establish such correlations is via spontaneous Raman scattering in atomic ensembles, known as the Duan-Lukin-Cirac-Zoller (DLCZ) scheme. However, time multiplexing is inherently difficult using this method, which leads to low communication rates even in theory. Moreover, it is desirable to find solid-state ensembles where such matter-photon correlations could be generated. Here we demonstrate quantum correlations between a single photon and a spin excitation in up to 12 temporal modes, in a ^{151}Eu^{3+}-doped Y_{2}SiO_{5} crystal, using a novel DLCZ approach that is inherently multimode. After a storage time of 1 ms, the spin excitation is converted into a second photon. The quantum correlation of the generated photon pair is verified by violating a Cauchy-Schwarz inequality. Our results show that solid-state rare-earth-ion-doped crystals could be used to generate remote multimode entanglement, an important resource for future quantum networks.

Experimental quantum communication

No abstract is provided for this article.

Aperitifs

No abstract is provided for this article.

Entangled states cannot be classically simulated in generalized Bell experiments with quantum inputs

Simulation tasks are insightful tools to compare information-theoretic resources. Considering a generalization of usual Bell scenarios where external quantum inputs are provided to the parties, we show that any entangled quantum state exhibits correlations that cannot be simulated using only shared randomness and classical communication, even when the amount and rounds of classical communication involved are unrestricted. We indeed construct explicit Bell-like inequalities that are necessarily satisfied by such classical resources but nevertheless violated by correlations obtainable from entangled quantum states, when measured a single copy at a time.

Device-independent demonstration of genuine multipartite entanglement

No abstract is provided for this article.

Un saut quantique en cryptographie

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.

Multiplexed on-demand storage of polarization qubits in a crystal

A long-lived and multimode quantum memory is a key component needed for the development of quantum communication. Here we present temporally multiplexed storage of five photonic polarization qubits encoded onto weak coherent states in a rare-earth-ion doped crystal. Using spin refocusing techniques we can preserve the qubits for more than half a millisecond. The temporal multiplexing allows us to increase the effective rate of the experiment by a factor of five, which emphasizes the importance of multimode storage for quantum communication. The fidelity upon retrieval is higher than the maximum classical fidelity achievable with qubits encoded onto single photons and we show that the memory fidelity is mainly limited by the memory signal-to-noise ratio. These results show the viability and versatility of long-lived, multimode quantum memories based on rare-earth-ion doped crystals.

Long-distance Bell-type tests using energy-time entangled photons

Long-distance Bell-type experiments are presented. The different experimental challenges and their solutions in order to maintain the strong quantum correlations between energy-time entangled photons over more than 10 km are reported and the results analyzed from the point of view of tests of fundamental physics as well as from the more applied side of quantum communication, especially quantum key distribution. Tests using more than one analyzer on each side are also presented.

Quantum Relays for Long Distance Quantum Cryptography

Quantum Cryptography is on the verge of commercial application. One of its greatest limitations is over long distance - secret key rates are low and the longest fibre over which any key has been exchanged is currently 100 km. We investigate the quantum relay, which can increase the maximum distance at which quantum cryptography is possible. The relay splits the channel into sections, and sends a different photon across each section, increasing the signal to noise ratio. The photons are linked as in teleportation, with entangled photon pairs and Bell measurements. We show that such a scheme could allow cryptography over hundreds of kilometers with today's detectors. It could not, however, improve the rate of key exchange over distances where the standard single section scheme already works. We also show that reverse key reconciliation, previously used in continuous variable quantum cryptography, gives a secure key over longer distances than forward key reconciliation.

Quantum key distribution over 67 km with a plug&play system

We present a fibre-optical quantum key distribution system. It works at 1550 nm and is based on the plug&play set-up. We tested the stability under field conditions using aerial and terrestrial cables and performed a key exchange over 67 km between Geneva and Lausanne.

Quantum Cryptography with Polarized Photons in Optical Fibres

An all-fibre quantum cryptography set-up based on the polarization of single photon pulses is described. The feasibility of establishing a key over more than 1 km has been experimentally demonstrated. The limits due to the topological effects, the birefringence, and the polarization mode dispersion of fibres are discussed. A set-up using only two non-orthogonal polarization states and polarizers instead of analysers is proposed.

Introduction to guided-wave quantum optics (GWQO): science, technology, and magic

In this paper we propose various applications of guided wave optics to quantum optics experiments. As quantum optics moves on to multiparticle and multiport configurations, guided wave optics, incorporating both integrated optics and fiber optics, could provide a realistic means of constructing experiments and devices.

Quantum cryptography

Entangled qutrits for quantum communication

We have experimentally realized a technique to generate, control and measure entangled qutrits, 3-dimensional quantum systems. This scheme uses spontaneous parametric down converted photons and unbalanced 3-arm fiber optic interferometers in a scheme analogous to the Franson interferometric arrangement for qubits. The results reveal a source capable of generating maximally entangled states with a net state fidelity, F = 0.985 $\pm$ 0.018. Further the control over the system reveals a high, net, 2-photon interference fringe visibility, V = 0.919 $\pm$ 0.026. This has all been done at telecom wavelengths thus facilitating the advancement towards long distance higher dimensional quantum communication.

Solutions of the dynamical equation for polarization dispersion

We model the local briefringence of a standard fiber by representing each component of the modal birefringence vector as colored noise (Ornstein-Uhlenbeck stochastic processes). For this statistical model we consider the dynamical equation for polarization dispersion and derive the distribution of the group delays between the principal states. We then compare statistical ensembles at a fixed wavelength ensembles with a fixed local birefringence. We show that under reasonable assumptions the average over both ensembles coincide. We conclude thus that the use of a large spectrum source, like a LED, permits measuring the rms deviation in a single run.

Non-realism: Deep Thought or a Soft Option?

No abstract is provided for this article.