arXiv subjects

Julien Chabé

Publications and source records attributed to Julien Chabé.

17 recordsLinked to original sources

arXiv2026

GUEST: Gravitational Universe Exploration with Satellite Tracking. A passive satellite laser-ranging mission for the dark gravitational Universe

GUEST is a space mission concept whose central objective is the detection of gravitational waves (GWs) in the microhertz band -- a physics-rich frequency window that no other present or planned detector can reach at a significant level. The concept is simple: two dense, passive spheres, covered with cube-corner retroreflectors, deployed in {highly eccentric} Earth orbits ($e \gtrsim 0.7$, period $P \gtrsim 33$ h), tracked continuously by the global network of satellite laser-ranging stations over a minimum observation time of 10 years, with an expected total duration of 30 years. The orbits themselves act as resonant detectors of the oscillating gravitational perturbations, with the microhertz sensitivity emerging from the selected orbital parameters. From the same data stream, GUEST delivers a programme of fundamental and applied science that cuts across particle physics, gravitational-wave astronomy, cosmology, astrophysics, and geodesy: the first coherent search for GWs from supermassive black-hole binaries in the $μ$Hz band, the exploration of primordial GW backgrounds in the unexplored energy-scale gap between pulsar-timing arrays and LISA, a dedicated probe of ultra-light dark matter in a parameter region untouched by any other experiment, a new way to search for ultra-light bosons, order-of-magnitude-improved tests of new gravitational interactions at astronomical ranges, and a step change in the absolute determination of $GM_\oplus$ that underpins the Global Geodetic Observing System and future navigation and Earth-observation missions. This white paper presents the motivation, scientific reach, and mission concept of GUEST.

astro-ph.CO↗

arXiv2025

Deployed quantum key distribution network: further, longer and more users

Entanglement-based quantum links are the backbone of future quantum internet networks, enabling secure communication between distant cities. Realizing such networks requires addressing multiple practical challenges in long-distance quantum key distribution : time synchronisation, interferometer stabilisation and automation. Here, we report several advances. First, we maintained an operational QKD link continuously for 325hours over 50km between two remotes locations, demonstrating the feasibility of long-duration key generation. We further extended secure key distribution up to a 100km operational link connecting the University of Nice to a ground-based optical station, a setup compatible with future quantum satellite connections. Finally, by employing wavelength demultiplexing to separate photons of entangled pairs, we performed QKD across multiple ITU channels, achieving secure key exchange via the BBM92 protocol and time-energy observables.

quant-ph↗

arXiv2024

Searching for large dark matter clumps using the Galileo Satnav clock variations

This study presents bounds on transient variations of fundamental constants, with typical timescales ranging from minutes to months, using clocks in space. The underlying phenomenology describing such transient variations relies on models for Dark Matter (DM) which suggest possible encounters of macroscopic compact objects with the Earth, due to the motion of the solar system in the galactic halo. If such compact objects possess an effective feeble interaction with the ordinary matter beyond the gravitational one, it may result in effective transient variations of fundamental constants. Such variations leave signatures on clocks onboard GNSS satellites. In this paper, we introduce a phenomenological study dedicated to the search for such DM transient objects using the network of passive hydrogen masers (H-Masers) onboard Galileo satellites. We first model the signature of transient variations of fundamental constants as a frequency modulation in the difference between two satellite clocks, considering the satellite trajectories relative to the transient event. Then, we present first results based on a fast analysis method, the maximum reach analysis. The main result is a significant extension of the discovery range for DM transients, with a sensitivity never achieved before. We investigate indeed the range of transient sizes from $10^5$ to $10^9$ kilometres, which, apart from indirect and model-dependent non-transient effects, has never been explored previously.

astro-ph.CO↗

arXiv2023

Intensity Interferometry observations of the H$α$ envelope of $γ$ Cas with MéO and a portable telescope

We report on observations of the extended environment of the bright Be star $γ$-Cas performed using intensity interferometry measurements within its H$α$ emission line. These observations were performed using a modified version of the I2C intensity interferometry instrument installed onto the 1.54 meter MéO optical metrology telescope and a portable 1-meter telescope (T1M). In order to better constrain the extent of the H$α$ envelope, observations were performed for two different positions of the T1M telescope, corresponding to an intermediate and long baselines in which the extended region was partially and fully resolved. We find that the observed data are consistent with past interferometric observations of $γ$-Cas. These observations demonstrate the capability to equip optical telescopes of different optical designs with intensity interferometry capabilities and illustrate the potential to scale a similar system onto many additional telescopes.

astro-ph.IM↗

arXiv2022

Intensity Interferometry at Calern and beyond: progress report

We present the current status of the I2C stellar intensity interferometer used towards high angular resolution observations of stars in visible wavelengths. In these proceedings, we present recent technical improvements to the instrument, and share results from ongoing campaigns using arrays of small diameter optical telescopes. A tip-tilt adaptive optics unit was integrated into the optical system to stabilize light injection into an optical fiber. The setup was successfully tested with several facilities on the Calern Plateau site of the Observatoire de la Côte d'Azur. These include one of the 1 m diameter telescopes of the C2PU observatory, a portable 1 m diameter telescope, and also the 1.5 m MéO telescope. To better constrain on-sky measurements, the spectral transmission of instrument was characterized in the laboratory using a high resolution spectrograph. The system was also tested with two of the auxiliary telescopes of the VLTI resulting in successful temporal and spatial correlation measurements of three stars.

astro-ph.IM↗

arXiv2020

Turbulence monitoring at Calern observatory with the Generalised Differential Image Motion Monitor

The Generalised Differential Image Motion Monitor (GDIMM) was proposed a few years ago as a new generation instrument for turbulence monitoring. It measures integrated parameters of the optical turbulence, i.e the seeing, isoplanatic angle, scintillation index, coherence time and wavefront coherence outer scale. GDIMM is based on a fully automatic small telescope (28cm diameter), equipped with a 3-holes mask at its entrance pupil. The instrument is installed at the Calern observatory (France) and performs continuous night-time monitoring of turbulence parameters. In this communication we present long-term and seasonnal statistics obtained at Calern, and combine GDIMM data to provide quantities such as the equivalent turbulence altitude and the effective wind speed.

astro-ph.IM↗

arXiv2020

Extending Science from Lunar Laser Ranging

The Lunar Laser Ranging (LLR) experiment has accumulated 50 years of range data of improving accuracy from ground stations to the laser retroreflector arrays (LRAs) on the lunar surface. The upcoming decade offers several opportunities to break new ground in data precision through the deployment of the next generation of single corner-cube lunar retroreflectors and active laser transponders. This is likely to expand the LLR station network. Lunar dynamical models and analysis tools have the potential to improve and fully exploit the long temporal baseline and precision allowed by millimetric LLR data. Some of the model limitations are outlined for future efforts. Differential observation techniques will help mitigate some of the primary limiting factors and reach unprecedented accuracy. Such observations and techniques may enable the detection of several subtle signatures required to understand the dynamics of the Earth-Moon system and the deep lunar interior. LLR model improvements would impact multi-disciplinary fields that include lunar and planetary science, Earth science, fundamental physics, celestial mechanics and ephemerides.

astro-ph.IM↗

arXiv2020

Monitoring daytime and nighttime optical turbulence profiles with the PML instrument

The Profiler of Moon Limb is a recent instrument dedicated to the monitoring of optical turbulence profile of the atmosphere. Fluctuations of the Moon or the Sun limb allow to evaluate the index refraction structure constant C_n^2(h) and the wavefront coherence outer scale L_0(h) as a function of the altitude $h$. The atmosphere is split into 33 layers with an altitude resolution varying from 100m (at the ground) to 2km (in the upper atmosphere). Profiles are obtained every 3mn during daytime and nighttime. We report last advances on the instrument and present some results obtained at the Plateau de Calern (France).

astro-ph.IM↗

arXiv2018

A new generalized differential image motion monitor

We present first results of a new instrument, the Generalized Differential Image Motion Monitor (GDIMM), aiming at monitoring parameters of the optical turbulence (seeing, isoplanatic angle, coherence time and outer scale). GDIMM is based on a small telescope equipped with a 3-holes mask at its entrance pupil. The seeing is measured by the classical DIMM technique using two sub-pupils of the mask (6 cm diameter separated by a distance of 20 cm), the isoplanatic angle is estimated from scintillation through the third sub-pupil (its diameter is 10 cm, with a central obstruction of 4 cm). The coherence time is deduced from the temporal structure function of the angle of arrival (AA) fluctuations, thanks to the high-speed sampling rate of the camera. And the difference of the motion variances from sub-apertures of different diameters makes it possible to estimate the outer scale. GDIMM is a compact and portable instrument, and can be remotely controlled by an operator. We show in this paper the first results of test campaigns obtained in 2013 and 2014 at Nice observatory and the Plateau de Calern (France). Comparison with simultaneous data obtained with the Generalized Seeing Monitor (GSM) are also presented.

astro-ph.IM↗

arXiv2018

Turbulence monitoring at the Plateau de Calern with the GDIMM instrument

We present some statistics of turbulence monitoring at the Plateau de Calern (France), with the Generalised Differential Image Motion Monitor (GDIMM). This instrument allows to measure integrated parameters of the atmospheric turbulence, i.e. seeing, isoplanatic angle, coherence time and outer scale, with 2 minutes time resolution. It is running routinely since November 2015 and is now fully automatic. A large dataset has been collected, leading to the first statistics of turbulence above the Plateau de Calern.

astro-ph.IM↗

arXiv2018

Intensity Interferometry revival on the Côte d'Azur

Recent advances in photonics have revived the interest in intensity interferometry for astronomical applications. The success of amplitude interferometry in the early 1970s, which is now mature and producing spectacular astrophysical results (e.g. GRAVITY, MATISSE, CHARA, etc.), coupled with the limited sensitivity of intensity interferometry stalled any progress on this technique for the past 50 years. However, the precise control of the optical path difference in amplitude interferometry is constraining for very long baselines and at shorter wavelengths. Polarization measurements are also challenging in amplitude interferometry due to instrumental effects. The fortuitous presence of strong groups in astronomical interferometry and quantum optics at Universite Cote d'Azur led to the development of a prototype experiment at Calern Observatory, allowing the measure of the temporal correlation g(2)(τ, r=0) in 2016 and of the spatial correlation g^(2)(r) in 2017 with a gain in sensitivity (normalized in observing time and collecting area) of a factor ~100 compared to Hanbury Brown and Twiss's original Narrabri Interferometer. We present possible ways to further develop this technique and point to possible implementations on existing facilities, such as CTA, the VLTI ATs or the summit of Maunakea, which offer a unique scientific niche.

astro-ph.IM↗

arXiv2014

Phase space tomography of cold-atoms dynamics in a weakly corrugated potential

We demonstrate tomographic reconstruction of the phase space distribution of atoms oscillating in a harmonic trap with weak potential corrugation caused by nanoscale imperfections in an atom chip. We find that deformations in these distributions are highly sensitive to anharmonic components of the potential. They are explained in terms of angular velocity dispersion of isoenergetic phase space trajectories. We show that the method is applicable for probing classical and quantum dynamics of cold atoms, and we note its importance for future technological applications.

quant-ph↗

arXiv2009

Observation of the Anderson Metal-Insulator Transition with Atomic Matter Waves: Theory and Experiment

Using a cold atomic gas exposed to laser pulses -- a realization of the chaotic quasiperiodic kicked rotor with three incommensurate frequencies -- we study experimentally and theoretically the Anderson metal-insulator transition in three dimensions. Sensitive measurements of the atomic wavefunction and the use of finite-size scaling techniques make it possible to unambiguously demonstrate the existence of a quantum phase transition and to measure its critical exponents. By taking proper account of systematic corrections to one-parameter scaling, we show the universality of the critical exponent $ν=1.59\pm0.01,$ which is found to be equal to the one previously computed for the Anderson model.

quant-ph↗

arXiv2008

Experimental observation of the Anderson transition with atomic matter waves

We realize experimentally an atom-optics quantum chaotic system, the quasiperiodic kicked rotor, which is equivalent to a 3D disordered system, that allow us to demonstrate the Anderson metal-insulator transition. Sensitive measurements of the atomic wavefunction dynamics and the use of finite-size scaling techniques make it possible to extract both the critical parameters and the critical exponent of the transition, which is in good agreement with the value obtained in numerical simulations of the 3D Anderson model.

quant-ph↗

arXiv2007

Improving Raman velocimetry of laser-cooled cesium atoms by spin-polarization

We study the peformances of Raman velocimetry applied to laser-cooled, spin-polarized, cesium atoms. Atoms are optically pumped into the F=4, m=0 ground-state Zeeman sublevel, which is insensitive to magnetic perturbations. High resolution Raman stimulated spectroscopy is shown to produce Fourier-limited lines, allowing, in realistic experimental conditions, atomic velocity selection to one-fiftieth of a recoil velocity.

quant-ph↗

arXiv2006

Quantum scaling laws in the onset of dynamical delocalization

We study the destruction of dynamical localization, experimentally observed in an atomic realization of the kicked rotor, by a deterministic Hamiltonian perturbation, with a temporal periodicity incommensurate with the principal driving. We show that the destruction is gradual, with well defined scaling laws for the various classical and quantum parameters, in sharp contrast with predictions based on the analogy with Anderson localization.

quant-ph↗

arXiv2005

Reversible Destruction of Dynamical Localization

Dynamical localization is a localization phenomenon taking place, for example, in the quantum periodically-driven kicked rotor. It is due to subtle quantum destructive interferences and is thus of intrinsic quantum origin. It has been shown that deviation from strict periodicity in the driving rapidly destroys dynamical localization. We report experimental results showing that this destruction is partially reversible when the deterministic perturbation that destroyed it is slowly reversed. We also provide an explanation for the partial character of the reversibility.

physics.atom-ph↗