arXiv subjects

Romain Veyron

Publications and source records attributed to Romain Veyron.

9 recordsLinked to original sources

arXiv2026

A single atom emitting resonance fluorescence into a coherent beam, and its use for non-destructive atom thermometry

Using a far-off-resonance optical dipole trap, we place a single neutral $^{87}$Rb atom in a weak, atom-resonant coherent beam, while also strongly illuminating it from an orthogonal direction to produce resonance fluorescence. The atom-modified coherent beam is then collected and its photon statistics analyzed. We observe first-order interference that can increase or decrease the beam flux, depending on the relative phase of the coherent beam and resonance fluorescence. This confirms predictions of Goncalves et al. [Phys. Rev. A 104, 013724]. The interference visibility is also shown to be a sensitive, time-resolved, non-destructive thermometer: by fitting the resulting photon count distributions, we infer the center-of-mass localization of the atom within the trap. With $1200$ atoms and integration time of $80\mathrm{ms}$ per atom, we demonstrate temperature uncertainties of $4 %$ for $\sim 30 \mu\mathrm{K}$ temperatures at $\sim 200\mu\mathrm{s}$ time resolution.

physics.atom-ph↗

arXiv2026

Distinguishing synthetic unravelings on quantum computers

Distinct monitoring or intervention schemes can produce different conditioned stochastic quantum trajectories while sharing the same unconditional (ensemble-averaged) dynamics. This is the essence of unravelings of a given Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) master equation: any trajectory-ensemble average of a function that is linear in the conditional state is completely determined by the unconditional density matrix, whereas applying a nonlinear function before averaging can yield unraveling-dependent results beyond the average evolution. A paradigmatic example is resonance fluorescence, where direct photodetection (jump/Poisson) and homodyne or heterodyne detection (diffusive/Wiener) define inequivalent unravelings of the same GKSL dynamics. In earlier work, we showed that nonlinear trajectory averages can distinguish such unravelings, but observing the effect in that optical setting requires demanding experimental precision. Here we translate the same idea to a digital setting by introducing synthetic unravelings implemented as quantum circuits acting on one and two qubits. We design two unravelings - a projective measurement unraveling and a random-unitary "kick" unraveling - that share the same ensemble-averaged evolution while yielding different nonlinear conditional-state statistics. We implement the protocols on superconducting-qubit hardware provided by IBM Quantum to access trajectory-level information. We show that the variance across trajectories and the ensemble-averaged von Neumann entropy distinguish the unravelings in both theory and experiment, while the unconditional state and the ensemble-averaged expectation values that are linear in the state remain identical. Our results provide an accessible demonstration that quantum trajectories encode information about measurement backaction beyond what is fixed by the unconditional dynamics.

quant-ph↗

arXiv2025

Detection of photon-level signals embedded in sunlight with an atomic photodetector

The detection of few-photon signals in a broadband background is an extreme challenge for photon counting, requiring filtering that accepts a narrow range of optical frequencies while strongly rejecting all others. Recent work [Zarraoa et. al, Phys. Rev. Res. 6, 033338 (2024)] demonstrated that trapped single atoms can act as low dark-count narrow-band photodetectors. Here we show that this ``quantum jump photodetector'' (QJPD) approach can also detect photon-level signals embedded in strong sunlight. Using a single rubidium atom as a QJPD, we count arrivals of individual narrow-band laser photons embedded in sunlight powers of order $10^{10}$ photons/s. We derive a rate-equation model for the atom's internal-state dynamics in sunlight, and find quantitative agreement with experiment. Using this model, we calculate the channel capacity over a noisy communication channel when sending weak coherent states and detecting them in the presence of sunlight, achieving a representative rate of 0.5 bits per symbol when sending 150 probe photons per 10 ms time-bin, embedded in 1 nW of sunlight (of order $10^{10}$ photons/s in the visible and near-infrared bands). The demonstration may benefit background-limited applications such as daytime light detection and ranging (LIDAR), remote magnetometry, and free-space classical and quantum optical communications.

quant-ph↗

arXiv2025

All-optical bubble trap for ultracold atoms in microgravity

In this paper, we present an all-optical method to produce shell-shaped traps for ultracold atoms in microgravity. Our scheme exploits optical double dressing of the ground state to create a short range strongly repulsive central potential barrier. Combined with a long range attractive central potential, this barrier forms the shell trap. We demonstrate that a pure spherical bubble, reaching the quasi 2D regime for standard atom numbers, could be formed from two crossed beams with a parabolic profile. An analytical study shows that the relevant characteristics of the trap depend on the ratio of the ground and excited state polarisabilities and the lifetime of the excited state. As a benchmark, we provide quantitative analysis of a realistic configuration for rubidium ensembles, leading to a 250 Hz transverse confinement for a 35 $\mu$m radius bubble and a trap residual scattering rate of less than 10 s$^{-1}$.

physics.atom-ph↗

arXiv2024

Quantum jump photodetector for narrowband photon counting with a single atom

Using a single neutral 87Rb atom held in an optical trap, and "quantum jump" detection of single-photon-initiated state changes, we demonstrate a single-photon quantum jump photodetector (QJPD) with intrinsically narrow bandwidth and strong rejection of out-of-band photons, of interest for detecting weak optical signals in the presence of a strong broadband background. By analyzing fluorescence photon count distributions for the bright and dark states with and without excitation, we measure quantum efficiency of 2.9(2)$\times 10^{-3}$, a record for single-pass quantum jump production, and signal-photon-unprovoked "dark jump" rate - analogous to the dark count rate of other detectors - 3(10)$\times 10^{-3}$ jumps per second during passive accumulation plus 4.0(4)$\times 10^{-3}$ jumps per readout, orders of magnitude below those of traditional single-photon detectors. Available methods can substantially improve QJPD quantum efficiency, dark jump rate, bandwidth and tunability.

quant-ph↗

arXiv2024

Telling different unravelings apart via nonlinear quantum-trajectory averages

The Gorini-Kossakowski-Sudarshan-Lindblad master equation (ME) governs the density matrix of open quantum systems (OQSs). When an OQS is subjected to weak continuous measurement, its state evolves as a stochastic quantum trajectory, whose statistical average solves the ME. The ensemble of such trajectories is termed an unraveling of the ME. We propose a method to operationally distinguish unravelings produced by the same ME in different measurement scenarios, using nonlinear averages of observables over trajectories. We apply the method to the paradigmatic quantum nonlinear system of resonance fluorescence in a two-level atom. We compare the Poisson-type unraveling, induced by direct detection of photons scattered from the two-level emitter, and the Wiener-type unraveling, induced by phase-sensitive detection of the emitted field. We show that a quantum-trajectory-averaged variance is able to distinguish these measurement scenarios. We evaluate the performance of the method, which can be readily extended to more complex OQSs, under a range of realistic experimental conditions.

quant-ph↗

arXiv2023

In situ subwavelength microscopy of ultracold atoms using dressed excited states

In this work, we implement a new method for imaging ultracold atoms with subwavelength resolution capabilities and determine its regime of validity. It uses the laser driven interaction between excited states to engineer hyperfine ground state population transfer in a three-level system on scales much smaller than the optical resolution. Subwavelength imaging of a quantum gas is atypical in the sense that the measurement itself perturbs the dynamics of the system. To avoid induced dynamics affecting the measurement, one usually measures "rapidly" the wavefunction in a so-called strong imaging regime. We experimentally illustrate this regime using a thermal gas ensemble, and demonstrate subwavelength resolution in quantitative agreement with a fully analytical model. Additionally, we show that, counter-intuitively, the opposite weak imaging regime can also be exploited to reach subwavelength resolution. As a proof of concept, we demonstrate that this regime is a robust solution to select and spatially resolve a 30 nm wide wavefunction, which was created and singled out from a tightly confined 1D optical lattice. Using a general dissipation-included formalism, we derive validity criteria for both regimes. The formalism is applicable to other subwavelength methods.

quant-ph↗

arXiv2021

Quantitative absorption imaging of optically dense effective two-level systems

Absorption imaging is a commonly adopted method to acquire, with high temporal resolution, spatial information on a partially transparent object. It relies on the interference between a probe beam and the coherent response of the object. In the low saturation regime, it is well described by a Beer Lambert attenuation. In this paper we theoretically derive the absorption of a $σ$ polarized laser probe by an ensemble of two-level systems in any saturation regime. We experimentally demonstrate that the absorption cross section in dense $^{87}$Rb cold atom ensembles is reduced, with respect to the single particle response, by a factor proportional to the optical density b of the medium. To explain this reduction, we developed a model that incorporates, in the single particle response, the incoherent electromagnetic background emitted by the surrounding ensemble. We show that it qualitatively reproduces the experimental results. Our calibration factor that has a universal dependence on optical density $b$ for $σ$ polarized light : $α$ = 1.17(9) + 0.255(2)b allows to obtain quantitative and absolute, in situ, images of dense quantum systems.

quant-ph↗

arXiv2021

Effective two-level approximation of a multi-level system driven by coherent and incoherent fields

The numerical simulation of multiple scattering in dense ensembles is the mostly adopted solution to predict their complex optical response. While the scalar and vectorial light mediated interactions are accurately taken into account, the computational complexity still limits current simulations to the low saturation regime and ignores the internal structure of atoms. Here, we propose to go beyond these restrictions, at constant computational cost, by describing a multi-level system (MLS) by an effective two-level system (TLS) that best reproduces the coherent and total scattering properties in any saturation regime. The correspondence of our model is evaluated for different experimentally realistic conditions such as the modification of the driving field polarization, the presence of stray magnetic fields or an incoherent resonant electromagnetic field background. The trust interval of the model is quantified for the D2-line of 87Rb atoms but it could be generalized to any closed transition of a multi-level quantum system.

physics.atom-ph↗