arXiv subjects

Robin Kaiser

Publications and source records attributed to Robin Kaiser.

At least 37 records · Page 2Linked to original sources

arXiv2025

Stellar intensity interferometry in the photon-counting regime

Stellar intensity interferometry consists in measuring the correlation of the light intensity fluctuations at two telescopes observing the same star. The amplitude of the correlation is directly related to the luminosity distribution of the star, which would be unresolved by a single telescope. This technique is based on the well-known Hanbury Brown and Twiss effect. After its discovery in the 1950s, it was used in astronomy until the 1970s, and then replaced by direct (``amplitude'') interferometry, which is much more sensitive, but also much more demanding. However, in recent years, intensity interferometry has undergone a revival. In this article, we present a summary of the state-of-the-art, and we discuss in detail the signal-to-noise ratio of intensity interferometry in the framework of photon-counting detection.

astro-ph.IM↗

arXiv2025

Nonergodic extended phase for waves in three dimensions

Wave transport in complex media is determined by the nature of quasimodes at the microscopic level. In three dimensional disordered media, waves generally undergo a phase transition from diffusion to Anderson localization, characterized by exponentially localized modes. A remarkable exception are electromagnetic waves, whose vector-like nature prevents Anderson localization to occur. Here we demonstrate that both scalar and vector (electromagnetic) waves exhibit a non-ergodic extended phase characterized by fractal quasimodes, for a broad range of disorder strengths. While electromagnetic waves remain in the non-ergodic extended phase at high disorder strength, scalar waves eventually enter a localized regime. These results pave the way for the engineering of anomalous wave transport phenomena in disordered media without spatial correlations.

cond-mat.dis-nn↗

arXiv2025

Dimensional Control of the Coherence Time of Scattered Light in Cold Atom Clouds

Cold atomic clouds are promising platforms for generating correlated photons, but multiple scattering and associated Doppler broadening limit their temporal coherence. Here we demonstrate that cloud geometry provides a powerful means to extend the coherence time of scattered light. In the experiment, intensity-correlation measurements show that an elongated (quasi-1D) cloud exhibits systematically longer coherence times than a spherical (3D) cloud of the same on-axis optical thickness, as a direct consequence of the suppression of multiple scattering in the elongated geometry. Random-walk simulations reproduce this trend and further show that elongation drives the coherence time toward the single-scattering limit. The combined results establish cloud geometry as a robust control parameter for temporal coherence in cold-atom ensembles, with potential applications in quantum optics and communication.

physics.atom-ph↗

arXiv2025

Condensation of vanishing photon emission rates in random atomic clouds

In the collective photon emission from atomic clouds both the atomic transition frequency and the decay rate are modified compared to a single isolated atom, leading to the effects of superradiance and subradiance. In this article, we analyse the properties of the Euclidean random matrix associated to the radiative dynamics of a cold atomic cloud, previously investigated in the contexts of photon localization and Dicke super- and subradiance. We present evidence of a new type of phase transition, surprisingly controlled by the cooperativeness parameter, rather than the spatial density or the diagonal disorder. The numerical results corroborate the occurrence of such a phase transition at a critical value of the cooperativeness parameter, above which the lower edge of the spectrum vanishes exhibiting a macroscopic accumulation of eigenvalues. Independent evaluations based on the two phenomena provide the same value of the critical cooperativeness parameter.

quant-ph↗

arXiv2025

Probing subradiant dynamics in cold atomic ensembles via population and emitted light measurements

In this letter, we report on the time- and space-resolved measurement of subradiant excited state population in an ultra-cold atomic cloud of 174Yb atoms. We use a depletion imaging technique that exploits the V-type internal energy structure of alkaline-earth-like atoms to directly observe the time-resolved spatial distribution of excited state population. We characterize the decay dynamics of the subradiant modes using both the excited state population and scattered light intensity, finding good quantitative agreement with numerical predictions from simulations of two-level atomic ensembles.

physics.atom-ph↗

arXiv2025

Average height for Abelian sandpiles and the looping constant on Sierpinski graphs

For the Abelian sandpile model on Sierpinski graphs, we investigate several statistics such as average height, height probabilities and looping constant. In particular, we calculate the expected average height of a recurrent sandpile on the finite iterations of the Sierpinski gasket and we also give an algorithmic approach for calculating the height probabilities of recurrent sandpiles under stationarity by using the connection between recurrent configurations of the Abelian sandpile Markov chain and uniform spanning trees. We also calculate the expected fraction of vertices of height $i$ for $i\in\{0,1,2,3\}$ of sandpiles under stationarity and relate the bulk average height to the looping constant on the Sierpinski gasket.

math.PR↗

arXiv2024

Bridging Rayleigh-Jeans and Bose-Einstein condensation of a guided fluid of light with positive and negative temperatures

We consider the free propagation geometry of a light beam (or fluid of light) in a multimode waveguide. As a result of the effective photon-photon interactions, the photon fluid thermalizes to an equilibrium state during its conservative propagation. In this configuration, Rayleigh-Jeans (RJ) thermalization and condensation of classical light waves have been recently observed experimentally in graded index multimode optical fibers characterized by a 2D parabolic trapping potential. As well-known, the properties of RJ condensation differ substantially from those of Bose-Einstein (BE) condensation: The condensate fraction decreases quadratically with the temperature for BE condensation, while it decreases linearly for RJ condensation. Furthermore, for quantum particles the heat capacity tends to zero at small temperatures, and it takes a constant value in the classical particle limit at high temperatures. This is in contrast with classical RJ waves, where the specific heat takes a constant value at small temperatures, and tends to vanish above the condensation transition in the normal (uncondensed) state. Here, we reconcile the thermodynamic properties of BE and RJ condensation: By introducing a frequency cut-off inherent to light propagation in a waveguide, we derive generalized expressions of the thermodynamic properties that include the RJ and BE limits as particular cases. We extend the approach to encompass negative temperatures. In contrast to positive temperatures, the specific heat does not display a singular behavior at negative temperatures, reflecting the non-critical nature of the transition to a macroscopic population of the highest energy level. Our work contributes to understanding the quantum-to-classical crossover in the equilibrium properties of light, within a versatile experimental platform based on nonlinear optical propagation in multimode waveguides.

physics.optics↗

arXiv2024

In-situ measurements of light diffusion in an optically dense atomic ensemble

This study introduces a novel method to investigate in-situ light transport within optically thick ensembles of cold atoms, exploiting the internal structure of alkaline-earth metals. A method for creating an optical excitation at the center of a large atomic cloud is demonstrated, and we observe its propagation through multiple scattering events. In conditions where the cloud size is significantly larger than the transport mean free path, a diffusive regime is identified. We measure key parameters including the diffusion coefficient, transport velocity, and transport time, finding a good agreement with diffusion models. We also demonstrate that the frequency of the photons launched inside the system can be controlled. This approach enables direct time- and space-resolved observation of light diffusion in atomic ensembles, offering a promising avenue for exploring new diffusion regimes.

physics.atom-ph↗

arXiv2024

An optically accelerated extreme learning machine using hot atomic vapors

Machine learning is becoming a widely used technique with a impressive growth due to the diversity of problem of societal interest where it can offer practical solutions. This increase of applications and required resources start to become limited by present day hardware technologies. Indeed, novel machine learning subjects such as large language models or high resolution image recognition raise the question of large computing time and energy cost of the required computation. In this context, optical platforms have been designed for several years with the goal of developing more efficient hardware for machine learning. Among different explored platforms, optical free-space propagation offers various advantages: parallelism, low energy cost and computational speed. Here, we present a new design combining the strong and tunable nonlinear properties of a light beam propagating through a hot atomic vapor with an Extreme Learning Machine model. We numerically and experimentally demonstrate the enhancement of the training using such free-space nonlinear propagation on a MNIST image classification task. We point out different experimental hyperparameters that can be further optimized to improve the accuracy of the platform.

physics.atom-ph↗

arXiv2024

Sandpiles on the Vicsek fractal explode with probability 1/4

Vicsek fractal graphs are an important class of infinite graphs with self similar properties, polynomial growth and treelike features, on which several dynamical processes such as random walks or Abelian sandpiles can be rigorously analyzed and one can obtain explicit closed form expressions. While such processes on Vicsek fractals and on Euclidean lattices $\mathbb{Z}^2$ share some properties for instance in the recurrence behaviour, many quantities related to sandpiles on Euclidean lattices are still poorly understood. The current work focuses on the stabilization and explosion of Abelian sandpiles on Vicsek fractal graphs, and we prove that a sandpile sampled from the infinite volume limit plus one additional particle stabilizes with probability 3/4, that is, it does not stabilize almost surely and it explodes with the complementary probability 1/4. We prove the main result by using two different approaches: one of probabilistic nature and one of algebraic flavor. The first approach is based on investigating the particles sent to the boundary of finite volumes and showing that their number stays above four with positive probability. In the second approach we relate the question of stabilization and explosion of sandpiles in infinite volume to the order of elements of the sandpile group on finite approximations of the infinite Vicsek graph. The method applies to more general state spaces and by employing it we also find all invariant factors of the sandpile groups on the finite approximations of the infinite Vicsek fractal.

math.PR↗

arXiv2024

Detecting entanglement from macroscopic measurements of the electric field and its fluctuations

To address the outstanding task of detecting entanglement in large quantum systems, entanglement witnesses have emerged, addressing the separable nature of a state. Yet optimizing witnesses, or accessing them experimentally, often remains a challenge. We here introduce a family of entanglement witnesses for open quantum systems, based on the electric field -- its quadratures and the total fluorescence. More general than spin-squeezing inequalities, it can detect new classes of entangled states, as changing the direction for far-field observation opens up a continuous family of witnesses, without the need for a state tomography. Their efficiency is demonstrated by detecting, from almost any direction, the entanglement of collective single-photon states, such as long-lived states generated by cooperative spontaneous emission. Able to detect entanglement in large quantum systems, these electric-field-based witnesses can be used on any set of emitters described by the Pauli group, such as atomic systems (cold atoms and trapped ions), giant atoms, color centers, and superconducting qubits.

quant-ph↗

arXiv2024

Temporal coherences of atomic chaotic light sources: the Siegert relation and beyond

Light is characterized by its electric field, yet quantum optics has revealed the importance of monitoring photon-photon correlations at all orders. We here present a comparative study of two experimental setups, composed of cold and warm Rubidium atoms, respectively, which allow us to probe and compare photon correlations up to the fourth order. The former operates in the quantum regime where spontaneous emission dominates, whereas the latter exhibits a temperature-limited coherence time. While both setups present almost-chaotic light statistics, we discuss how the access to different orders of photon correlations allows one to better characterize the mechanisms responsible for deviations from those statistics.

physics.atom-ph↗

arXiv2024

Random rotor walks and i.i.d. sandpiles on Sierpinski graphs

We prove that, on the infinite Sierpinski gasket graph SG, rotor walk with random initial configuration of rotors is recurrent. We also give a necessary condition for an i.i.d. sandpile to stabilize. In particular, we prove that an i.i.d. sandpile with expected number of chips per site greater or equal to three does not stabilize almost surely. Furthermore, the proof also applies to divisible sandpiles and shows that divisible sandpile at critical density one does not stabilize almost surely on SG.

math.PR↗

arXiv2024

Topological constraints on the dynamics of vortex formation in a two-dimensional quantum fluid

We present experimental and theoretical results on formation of quantum vortices in a laser beam propagating in a nonlinear medium. Topological constrains richer than the mere conservation of vorticity impose an elaborate dynamical behavior to the formation and annihilation of vortex-antivortex pairs. We identify two such mechanisms, both described by the same fold-Hopf bifurcation. One of them is particularly efficient although it is not observed in the context of liquid helium films or stationary systems because it relies on the compressible nature of the fluid of light we consider and on the non-stationarity of its flow.

physics.optics↗

arXiv2023

Localization of light in three dimensions: a mobility edge in the imaginary axis in non-Hermitian Hamiltonians

Searching for Anderson localization of light in three dimensions has challenged experimental and theoretical research for the last decades. Here the problem is analyzed through large scale numerical simulations, using a radiative Hamiltonian i.e. a non-Hermitian long-range hopping Hamiltonian, well suited to model light-matter interaction in cold atomic clouds. Light interaction in atomic clouds is considered in presence of positional and diagonal disorder. Due to the interplay of disorder and cooperative effects (sub- and super-radiance) a novel type of localization transition is shown to emerge, differing in several aspects from standard localization transitions which occur along the real energy axis. The localization transition discussed here is characterized by a mobility edge along the imaginary energy axis of the eigenvalues which is mostly independent from the real energy value of the eigenmodes. Differently from usual mobility edges it separates extended states from hybrid localized states and it manifest itself in the large moments of the participation ratio of the eigenstates. Our prediction of a mobility edge in the imaginary axis, i.e. depending on the eigenmode lifetime, paves the way to achieve control both in the time and space domain of open quantum systems.

cond-mat.mes-hall↗

arXiv2023

Cooperative photon emission rates in random atomic clouds

We investigate the properties of the cooperative decay modes of a cold atomic cloud, characterized by a Gaussian distribution in three dimensions, initially excited by a laser in the linear regime. We study the properties of the decay rate matrix $S$, whose dimension coincides with the number of atoms in the cloud, in order to get a deeper insight into properties of cooperative photon emission. Since the atomic positions are random, $S$ is a Euclidean random matrix whose entries are function of the atom distances. We show that, in the limit of a large number of atoms in the cloud, the eigenvalue distribution of $S$ depends on a single parameter $b_0$, called the cooperativeness parameter, which can be viewed as a quantifier of the number of atoms that are coherently involved in an emission process. For very small values of $b_0$, we find that the limit eigenvalue density is approximately triangular. We also study the nearest-neighbour spacing distribution and the eigenvector statistics, finding that, although the decay rate matrices are Euclidean, the bulk of their spectra mostly behaves according to the expectations of classical random matrix theory. In particular, in the bulk there is level repulsion and the eigenvectors are delocalized, therefore exhibiting the universal behaviour of chaotic quantum systems.

quant-ph↗

arXiv2023

Laser frequency stabilization by modulation transfer spectroscopy and balanced detection of molecular iodine for laser cooling of ${}^{174}$Yb

We report laser frequency stabilization by the combination of modulation transfer spectroscopy and balanced detection of a relatively weak hyperfine transition of the R(158)25-0 line of molecular iodine (${}^{127}$I$_{2}$), which is used as a new frequency reference for laser trapping and cooling of ${}^{174}$Yb on the $^{1}S_{0} - {}^{3}P_{1}$ transition. The atomic cloud is characterized by time-of-flight measurements, and an on-resonance optical depth of up to 47 is obtained. We show laser noise reduction and characterize the short-term laser frequency instability by the Allan deviation of the laser fractional frequency. The minimum measured value is 3.9 x $10^{-13}$ at 0.17 s of averaging time.

physics.atom-ph↗

arXiv2023

Nonlinear effects in Anderson localization of light by two-level atoms

While Anderson is a single-particle wave effect, guaranteeing a single excitation in the system can be challenging. We here tackle this limitation in the context of light localization in three dimensions in disordered cold atom clouds, in presence of several photons. We show that the presence of these multiple excitations does not affect substantially the abnormal intensity fluctuations which characterize the Anderson localization transition, provided that the radiated light is frequency filtered. Due to their narrow linewidth, long-lived modes, and particularly the localized ones, are strongly saturated even for a weak resonant pump, leading to a large increase of the inelastic scattering and to reduced fluctuations in the total radiation. Yet the atomic coherences and the resulting elastic scattering remain a proper witness of the Anderson localization transition. Hence, frequency filtering allows one to investigate the single-excitation sector, dismissing the many-body effects showing up in the fluorescence spectrum.

physics.atom-ph↗