Publications

2025

1. ArXiv

   

   Parametric pair production of collective excitations in a Bose-Einstein condensate

   Victor Gondret, Rui Dias, Clothilde Lamirault, and 7 more authors

   ArXiv 2508.01654, Jul 2025

   Abs HTML

   By exciting the transverse breathing mode of an elongated Bose-Einstein condensate, we parametrically produce longitudinal collective excitations in a pairwise manner. This process also referred to as Faraday wave generation, can be seen as an analog to cosmological particle production. Building upon single particle detection, we investigate the early time dynamics of the exponential growth and compare our observation with a Bogoliubov description. The growth rate we observe experimentally is in very good agreement with theoretical predictions, demonstrating the validity of the Bogoliubov description and thereby confirming the smallness of quasiparticle interactions in such an elongated gas. We also discuss the presence of oscillations in the atom number, which are due to pair correlations and to the rate at which interactions are switched off.

2. ArXiv

   Observation of entanglement in a cold atom analog of cosmological preheating

   Victor Gondret, Clothilde Lamirault, Rui Dias, and 8 more authors

   arXiv 2506.22024, Jun 2025

   Abs HTML PDF

   We observe entanglement between collective excitations of a Bose-Einstein condensate in a configuration analogous to particle production during the preheating phase of the early universe. In our setup, the oscillation of the inflaton field is mimicked by the transverse breathing mode of a cigar-shaped condensate, which parametrically excites longitudinal quasiparticles with opposite momenta. After a short modulation period, we observe entanglement of these pairs which demonstrates that vacuum fluctuations seeded the parametric growth, confirming the quantum origin of the excitations. As the system continues to evolve, we observe a decrease in correlations and a disappearance of non-classical features, pointing towards future experimental probes of the less understood interaction-dominated regime.

3. Phys. Rev. Lett.

   

   Quantifying two-mode entanglement of bosonic Gaussian states from their full counting statistics

   Victor Gondret, Clothilde Lamirault, Rui Dias, and 4 more authors

   Accepted in Phys. Rev. Lett., Mar 2025

   Abs DOI HTML PDF

   We study the entanglement properties of two-mode bosonic Gaussian states based on their multi-mode counting statistics. We exploit the idea that measuring high-order correlations of particle numbers can reveal entanglement without making any assumptions about the coherence of the fields. We show that the two- and four-body number correlations are sufficient to fully characterize the entanglement of two-mode bosonic Gaussian states for which each mode exhibits a thermal distribution. In addition, we derive an entanglement witness based on two-body correlations alone. Our findings are of great importance because it becomes possible to reveal entanglement in a series of recent experiments.

4. Phys. Rev. A

   

   Coherent coupling of momentum states: Selectivity and phase control

   Charlie Leprince, Victor Gondret, Clothilde Lamirault, and 4 more authors

   Phys. Rev. A, Jun 2025

   Abs DOI PDF

   We demonstrate the effect of pulse shaping in momentum-selective atomic Bragg diffraction. We compare temporal square pulses, which produce sidelobes in momentum space, with other shapes that can produce more nearly square momentum distributions. We produce pulses that simultaneously address two sets of velocity classes and demonstrate that we can control the differential phase imprinted on them in a way that is insensitive to laser phase fluctuations. Our work marks a significant step forward in testing Bell inequalities using massive particles entangled in momentum.

5. PhD thesis

   On the entanglement of quasi-particles in a Bose-Einstein Condensate

   Victor Gondret

   Université Paris-Saclay, Jan 2025

   Thesis work conducted under the supervision of Denis Boiron and Christoph I. Westbrook

   Abs HTML PDF

   This thesis focuses on the non-separability of pairs of quasi-particles excited by parametric resonance. The experimental setup used here allows the production of a Bose-Einstein condensate of metastable helium. The use of an ultra-cold atomic gas makes it possible to reach sufficiently low temperatures to observe intrinsically quantum phenomena: the non-separability of the state. In this work, we use the condensate as a coherent reservoir to populate two momentum modes. The advantage of metastable helium is its high internal energy, which allows the electronic detection of single particles. We therefore measure the position and the time of impact of the particles after a time of flight of 308 ms, which allows us to reconstruct the in-trap momentum distribution. In the first theoretical contribution of this work, we demonstrate that measuring the two- and four-body correlation functions not only attests to, but also quantifies the non-separability of a Gaussian state. We also derive a new entanglement witness using only the two-body correlation function. In the experimental part, we improve the machine used to produce our ultra-cold gas and enhance its stability. We implement original techniques to deflect part of the atoms and avoid the saturation of our detector. These improvements allow us to observe the non-separability of the state.

2024

1. Phys. Rev. A

   Sub-shot-noise interferometry with two-mode quantum states

   Quentin Marolleau, Charlie Leprince, Victor Gondret, and 2 more authors

   Phys. Rev. A, Feb 2024

   Abs DOI PDF

   We study the feasibility of sub-shot-noise interferometry with imperfect detectors, starting from twin Fock states and two-mode squeezed vacuum states. We derive analytical expressions for the corresponding phase uncertainty. We find that one can achieve phase shift measurements below the standard quantum limit, as long as the losses are smaller than a given threshold, and that the measured phase is close enough to an optimal value. We provide our analytical formulas in a python package, accessible online.

2019

1. Scientific Reports

   Relevant heating of the quiet solar corona by Alfvén waves: a result of adiabaticity breakdown

   D. F. Escande, V. Gondret, and F. Sattin

   Scientific Reports, Oct 2019

   Abs DOI HTML PDF

   Ion heating by Alfvén waves has been considered for long as the mechanism explaining why the solar corona has a temperature several orders of magnitude higher than the photosphere. Unfortunately, as the measured wave frequencies are much smaller than the ion cyclotron frequency, particles were expected to behave adiabatically, impeding a direct wave-particle energy transfer to take place, except through decorrelating stochastic mechanisms related to broadband wave spectra. This paper proposes a new paradigm for this mechanism by showing it is actually much simpler, more general, and very efficient. Indeed, for measured wave amplitudes in the quiet corona, ion orbits are shown to cross quasi-periodically one or several slowly pulsating separatrices in phase space. Now, a separatrix is an orbit with an infinite period, thus much longer than the pulsation one. Therefore, each separatrix crossing cancels adiabatic invariance, and yields a very strong energy transfer from the wave, and thus particle heating. This occurs whatever be the wave spectrum, even a monochromatic one. The proposed mechanism is so efficient that it might lead to a self-organized picture of coronal heating: all Alfvén waves exceeding a threshold are immediately quenched and transfer their energy to the ions.