We demonstrate collectively enhanced vacuum-induced quantum beat dynamics from a three-level V-type atomic system. Exciting a dilute atomic gas of magneto-optically trapped ^{85}Rb atoms with a weak drive resonant on one of the transitions, we observe the forward-scattered field after a sudden shut-off of the laser. The subsequent radiative dynamics, measured for various optical depths of the atomic cloud, exhibits superradiant decay rates, as well as collectively enhanced quantum beats. Our work is also the first experimental illustration of quantum beats arising from atoms initially prepared in a single excited level as a result of the vacuum-induced coupling between excited levels.
We demonstrate collectively enhanced vacuum-induced quantum beat dynamics from a three-level V-type atomic system. Exciting a dilute atomic gas of magneto-optically trapped ^{85}Rb atoms with a weak drive resonant on one of the transitions, we observe the forward-scattered field after a sudden shut-off of the laser. The subsequent radiative dynamics, measured for various optical depths of the atomic cloud, exhibits superradiant decay rates, as well as collectively enhanced quantum beats. Our work is also the first experimental illustration of quantum beats arising from atoms initially prepared in a single excited level as a result of the vacuum-induced coupling between excited levels.
We analyze the dynamics of quantum beats in a system of two V-type three-level atoms coupled to a waveguide. We show that quantum beats can be collectively enhanced or suppressed, akin to Dicke super- and subradiance, depending on the interatomic separation and the initial correlations between the atoms. In particular, the interference properties of the collective beats are determined by the distance between the atoms modulo the beat wavelength. We study the collective atomic and field dynamics, illustrating a crossover from a Markovian to a non-Markovian regime as the atomic separation becomes sufficiently large to bring memory effects of the electromagnetic environment into consideration. In such a non-Markovian regime, collective quantum beats can be enhanced beyond the Markovian limit as a result of retardation effects. Our results demonstrate the rich interplay between multilevel and multiatom quantum interference effects arising in a system of distant quantum emitters.
We experimentally investigate the collective radiative decay of a fully inverted ensemble of two-level atoms for a chiral, i.e., propagation direction-dependent light-matter coupling. Despite a fundamentally different interaction Hamiltonian which has a reduced symmetry compared to the standard Dicke case of superradiance, we do observe a superradiant burst of light. The burst occurs above a threshold number of atoms, and its peak power scales faster with the number of atoms than in the case of free-space Dicke superradiance. We measure the first-order coherence of the burst and experimentally distinguish two regimes, one dominated by the coherence induced during the excitation process and the other governed by vacuum fluctuations. Our results shed light on the collective radiative dynamics of cascaded quantum many-body systems, i.e., systems in which each quantum emitter is only driven by light radiated by emitters that are upstream in the cascade. Our findings may turn out useful for generating multiphoton Fock states as a resource for quantum technologies. Published by the American Physical Society 2024
The collective scattering of light by a large number of coupled scatterers yields a rich many-body physics, yet treating such a problem in the quantum regime is a challenge.In this thesis, we consider a large atomic cloud in free space and driven by a monochromatic light, where the vacuum modes induce long-range dipole-dipole interactions.In order to study clouds of hundreds of particles, higher order correlation terms are neglected, keeping only quantum correlations between pair of atoms: this allows to reduce the number of degrees of freedom from 2 2N in the full quantum model to N 2 .Most of the works in the literature on dipole-dipole interactions have been performed in the linear optics limit, and our techniques allow to compare classical to beyond-semi-classical results.In particular, superradiance and subradiance have been reported in the decay dynamics of the cloud.Differently, by considering a system in the ground state and switching on the pump, we show that superradiance is also present in the Rabi oscillations at a rate obtained from using a single-dipole model for the radiated intensity.A frequency shift in the Rabi oscillations is also reported, which can be interpreted, in a linear dispersion theory, as a signature of a collective multimode vacuum Rabi splitting.While the classical dipoles model captures correctly the dynamics in the low-intensity regime, it fails for higher saturation, where semi-classical methods can be applied successfully.In particular, we observe a quantum subradiant decay in the intensity of the radiated field in the saturated regime.Furthermore, considering the decay dynamics starting from an initially strongly driven cloud, we observed that the states with n < N/2 excited atoms decays much slower than the ones with n > N/2: in other words, the upper part of the Dicke ladder is characterized by a superradiant emission, and the lower part by a subradiant one.Finally, investigating the fluorescence spectrum, we obtained quantum cooperative effects that modify the steady-state spectrum: additional sidebands at twice the Rabi frequency for the system driven at resonance and, out of resonance, an asymmetry in the peaks at the generalized Rabi frequency, and for all detection angles.We also present preliminary results of a sensor model that can capture the time evolution of the spectrum in the decay dynamics, a situation that the quantum regression theorem fails to describe.Several of these results were discussed in parallel with experimental data obtained by an experimental group of collaborators, and others aim to guide future experiments.
We have studied the decay of Rydberg excitations in a cold Rb gas. A 10 ns, pulsed-dye-amplified diode laser excites Rb atoms at $70\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in a magneto-optical trap to ns or np Rydberg states with principal quantum numbers $26\ensuremath{\le}n\ensuremath{\le}40$. Time-delayed state-selective field ionization is used to directly monitor the population in the initial and neighboring Rydberg levels. The measured time dependence of the Rydberg population is well described by numerical simulations which consider only spontaneous emission and population transfer by blackbody radiation. No evidence for collective decay is found at atom densities up to $3\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. This result is in contrast to a previous study [Wang et al., Phys. Rev. A 75, 033802 (2007)], in which superradiant decay was theoretically predicted and experimentally inferred for atom density and laser focal volume conditions very similar to those considered here. Suppression of collective emission is likely due to variations in transition energies within the atom sample, dominated by inhomogeneities in dipole-dipole exchange interactions for initial s states, or by a combination of dipole-dipole and electric field inhomogeneities for the initial p states.
We discuss various aspects of the incoherent spontaneous emission in atomic few-level systems arising from the coupling of the atom to the surrounding vacuum. First, we consider systems where the decoherence due to spontaneous emission acts as a limiting factor. Here, we combine collective effects in larger samples of atoms with control mechanisms known from single-atom schemes, or modify the system dynamics by externally inducing multiphoton quantum interference effects. In the second part, we discuss ground-state laser cooling of trapped atoms and ions. Here, the momentum transfer in the spontaneous emission events is required to cool the particles, but needs to be controlled in order to achieve a low cooling limit. In our scheme, we make use of double electromagnetically induced transparency in order to design the absorption spectrum of the trapped particle. In the final part, we show that the incoherent part of the resonance fluorescence spectrum of a two-level system may serve as an interesting candidate for high-precision spectroscopy. For this, we discuss relativistic and radiative corrections to the resonance fluorescence spectra of laser-driven few-level systems.
Collective interaction of light with an atomic gas can give rise to superradiant instabilities. We experimentally study the sudden buildup of a reverse light field in a laser-driven high-finesse ring cavity filled with ultracold thermal or Bose-Einstein condensed atoms. While superradiant Rayleigh scattering from atomic clouds is normally observed only at very low temperatures (i.e., well below 1 microK), the presence of the ring cavity enhances cooperativity and allows for superradiance with thermal clouds as hot as several 10 microK. A characterization of the superradiance at various temperatures and cooperativity parameters allows us to link it to the collective atomic recoil laser.