Publications

On the inequivalence of the CH and CHSH inequalities according to representation theory

Interference of Spontaneous Emission of Light from Two Atomic Ensembles

We report an interference experiment of spontaneous emission of light from two distant solid-state ensembles of atoms that are coherently excited by a short laser pulse. The ensembles are Erbium ions doped into two LiNbO3 crystals with channel waveguides, which are placed in the two arms of a Mach-Zehnder interferometer. The light that is spontaneously emitted after the excitation pulse shows first-order interference. By a strong collective enhancement of the emission, the atoms behave as ideal two-level quantum systems and no which-path information is left in the atomic ensembles after emission of a photon. This results in a high fringe visibility of 95%, which implies that the observed spontaneous emission is highly coherent.

Temporal Multimode Storage of Entangled Photon Pairs

Multiplexed quantum memories capable of storing and processing entangled photons are essential for the development of quantum networks. In this context, we demonstrate the simultaneous storage and retrieval of two entangled photons inside a solid-state quantum memory and measure a temporal multimode capacity of ten modes. This is achieved by producing two polarization entangled pairs from parametric down conversion and mapping one photon of each pair onto a rare-earth-ion doped (REID) crystal using the atomic frequency comb (AFC) protocol. We develop a concept of indirect entanglement witnesses, which can be used as Schmidt number witness, and we use it to experimentally certify the presence of more than one entangled pair retrieved from the quantum memory. Our work puts forward REID-AFC as a platform compatible with temporal multiplexing of several entangled photon pairs along with a new entanglement certification method useful for the characterisation of multiplexed quantum memories.

Alternative configuration for refracted near-field measurements of refractive index on glass-integrated-optics waveguides

We present experimental investigations of two configurations for refractive-index profiling and for geometry measurements of glass-integrated waveguides with the refracted near-field technique. An original configuration is proposed that simplifies the measurement cell and the handling.

Les promesses de l'information quantique

No abstract is provided for this article.

Error filtration and entanglement purification for quantum communication (17 pages)

Certification of non-classicality in all links of a photonic star network without assuming quantum mechanics

Networks composed of independent sources of entangled particles that connect distant users are a rapidly developing quantum technology and an increasingly promising test-bed for fundamental physics. Here we address the certification of their post-classical properties through demonstrations of full network nonlocality. Full network nonlocality goes beyond standard nonlocality in networks by falsifying any model in which at least one source is classical, even if all the other sources are limited only by the no-signaling principle. We report on the observation of full network nonlocality in a star-shaped network featuring three independent sources of photonic qubits and joint three-qubit entanglement-swapping measurements. Our results demonstrate that experimental observation of full network nonlocality beyond the bilocal scenario is possible with current technology.

Telecommunication-Wavelength Solid-State Memory at the Single Photon Level

We demonstrate experimentally the storage and retrieval of weak coherent light fields at telecommunication wavelengths in a solid. Light pulses at the single photon level are stored for a time up to 600 ns in an erbium-doped Y2SiO5 crystal at 2.6 K and retrieved on demand. The memory is based on photon echoes with controlled reversible inhomogeneous broadening, which is realized here for the first time at the single photon level. This is implemented with an external field gradient using the linear Stark effect. This experiment demonstrates the feasibility of a solid-state quantum memory for single photons at telecommunication wavelengths, which would represent an important resource in quantum information science.

The Quantum State Diffusion Model of Open Systems

No abstract is provided for this article.

Decay time measurement of fluorescent fibers with photon counting

In order to characterize fluorescent light, we present a decay time measurement method based on the use of a commercial photon counting module. This module, adapted for all fiber measurements, is well adapted for the developing field of optodes for chemical and biochemical sensing. Moreover, it improves the detection sensitivity to the femtowatt and picowatt range. It is thus possible to excite the fluorescence with very low power pump pulses, down to a few nanowatts, and to measure the temporal distribution of the fluorescent signal. As sources we can use LEDs, or white lamps plus a monochromator, avoiding unwanted phenomena such as saturation, amplified spontaneous emission (ASE), and excited state absorption (ESA). We obtain a measurement accuracy of 0.1% in a couple of minutes.

Entanglement, uncertainty and dispersion: a simple experimental demonstration of non-classical correlations

We perform an experiment to verify the presence of entanglement in photon pairs. By sending down-converted light through dispersive media with opposite group velocity dispersion coefficients, we overcome the minimum broadening of temporal correlation for classical light. Entanglement is proved via an inequality derived directly from Heisenberg's uncertainty principle. We present a criterion to select a suitable down-conversion source. Moreover, we introduce a new method to analyze the temporal correlations between the entangled photons.

General properties of nonsignaling theories

This article identifies a series of properties common to all theories that do not allow for superluminal signaling and predict the violation of Bell inequalities. Intrinsic randomness, uncertainty due to the incompatibility of two observables, monogamy of correlations, impossibility of perfect cloning, privacy of correlations, bounds in the shareability of some states; all these phenomena are solely a consequence of the no-signaling principle and nonlocality. In particular, it is shown that for any distribution, the properties of (i) nonlocal, (ii) no arbitrarily shareable and (iii) positive secrecy content are equivalent.

Implementation of continuous variable quantum cryptography in optical fibres using a go-&-return configuration

We demonstrate an implementation of quantum key distribution with continuous variables based on a go-&-return configuration over distances up to 14km. This configuration leads to self-compensation of polarisation and phase fluctuations. We observe a high degree of stability of our set-up over many hours.

Second order polarization mode dispersion

Unravellings of the master equation and the emergence of a classical world

The relevance for the classical limit of four different stochastic unravellings of the master equations is illustrated on a classically chaotic kicked anharmonic oscillator.

Purification of single-photon entanglement with linear optics

We show that single-photon entangled states of the form $\ensuremath{\mid}0⟩\ensuremath{\mid}1⟩+\ensuremath{\mid}1⟩\ensuremath{\mid}0⟩$ can be purified with a simple linear-optics-based protocol, which is eminently feasible with current technology. Besides its conceptual interest, this result is relevant for attractive quantum repeater protocols.

Distributed Temperature Sensor Interrogator Based on Polarization-Sensitive Reflectometry

In this paper, a polarization-sensitive reflectometry technique has been investigated in order to serve as an interrogating device in distributed sensing applications. As the sensing fiber has a beat length of about a few centimeters, a high-resolution reflectometer was required. A coherent optical frequency-domain reflectometer (OFDR) was used from this perspective. The principle is based on the linear dependence of fiber beat length on temperature. The reported experimental results detail the calibration method and discuss the features of the interrogating device in terms of temperature threshold and spatial resolution. The proposed system, exploiting the Rayleigh backscatter signal, offers an interesting alternative to existing systems, mainly based on nonlinear effects.

Creating high dimensional entanglement using mode-locked lasers

We present a new scheme to generate high dimensional entanglement between two photonic systems. The idea is based on parametric down conversion with a sequence of pump pulses generated by a mode-locked laser. We prove experimentally the feasibility of this scheme by performing a Franson-type Bell test using a 2-way interferometer with path-length difference equal to the distance between 2 pump pulses. With this experiment, we can demonstrate entanglement for a two-photon state of at least dimension d=11. Finally, we propose a feasible experiment to show a Fabry-Perot like effect for a high dimensional two-photon state.