We study the nonequilibrium many-body dynamics of a cold gas of ground-state alkali-metal atoms weakly admixed by Rydberg states with laser light. On a time scale shorter than the lifetime of the dressed states, effective dipole-dipole or van der Waals interactions between atoms can lead to the formation of strongly correlated phases, such as atomic crystals. Using a semiclassical approach, we study the long-time dynamics where decoherence and dissipative processes due to spontaneous emission and blackbody radiation dominate, leading to heating and melting of atomic crystals as well as particle losses. These effects can be substantially mitigated by performing active laser cooling in the presence of atomic dressing.
We study the nonequilibrium many-body dynamics of a cold gas of ground-state alkali-metal atoms weakly admixed by Rydberg states with laser light. On a time scale shorter than the lifetime of the dressed states, effective dipole-dipole or van der Waals interactions between atoms can lead to the formation of strongly correlated phases, such as atomic crystals. Using a semiclassical approach, we study the long-time dynamics where decoherence and dissipative processes due to spontaneous emission and blackbody radiation dominate, leading to heating and melting of atomic crystals as well as particle losses. These effects can be substantially mitigated by performing active laser cooling in the presence of atomic dressing.
We discuss techniques to generate long-range interactions in a gas of ground state alkali atoms, by weakly admixing excited Rydberg states with laser light. This provides a tool to engineer strongly correlated phases with reduced decoherence from inelastic collisions and spontaneous emission. As an illustration, we discuss the quantum phases of dressed atoms with dipole-dipole interactions confined in a harmonic potential, as relevant to experiments. We show that residual spontaneous emission from the Rydberg state acts as a heating mechanism, leading to a quantum-classical crossover.
The interaction between atomic transition dipoles and photons leads to the formation of many-body states with collective dissipation and long-ranged interactions. Here, we put forward and explore a scenario in which a dense atomic gas---where the separation of the atoms is comparable to the transition wavelength---is weakly excited by an off-resonant laser field. We develop the theory for describing such a dressed many-body ensemble and show that collective excitations are responsible for the emergence of many-body interactions, i.e., effective potentials that cannot be represented as a sum of binary terms. We illustrate how collective effects may be probed experimentally through microwave spectroscopy. We analyze time-dependent line shifts, which are sensitive to the phase pattern of the dressing laser, and show that the strong interactions lead to a dramatic slowdown of the relaxation dynamics. Our study provides a so far little explored perspective on dense atomic ensembles interacting with light and promotes this platform as a setting for the exploration of dissipative nonequilibrium many-body physics.
This thesis explores the effects of decoherence and dephasing on single- and many- body dynamics of spin-systems. A particular realisation of the spin systems that the work focuses on are highly excited, Rydberg states of atoms. A software library ARC (Alkali Rydberg Calculator) for calculating properties of Rydberg states in alkali metals is presented, with particular attention to the multi- atom and multi-level effects that influence many-body dynamics in realistic systems, and properties related to terahertz imaging with alkali atom Rydberg states. Dressed-state electromagnetically induced transparency (EIT) is proposed as a way of preparing uniform-phase spin-waves in ladder excitation schemes, making the collective excitation storage insensitive to motional dephasing. Proof of concept dressed state EIT experiments are presented. Strong resonant dressing is also theoretically analysed as a way of preparing velocity superposition of spin-waves. The developed theoretical model is in a good agreement with existing experimental data on single-photon many-atom quantum beats in diamond excitation schemes. By modelling the strongly driven Rydberg ensembles, many-body dynamics of driven-dissipative spin systems is analysed. Working in the limit of strong dephasing, the effects of fluctuations, the shape of interaction potential, spatial correlations and motion on non-equilibrium phase diagrams and the occurrence of bistability are examined. An ensemble averaged mean field model is introduced as an exact solution for completely uncorrelated ensembles. It is shown that the van der Waals interaction does not allow the occurrence of bistability, for which a finite dipolar core is required. The short-range interaction potential shape is found to have a profound influence on non-equilibrium phase diagrams, controlling the size of fluctuations in the dynamics. For a frozen system, several methods for identifying and quantifying bistable phases are introduced, and phase diagrams are reconstructed. It is shown that the temperature of external degrees of freedom, i.e. spin motion, can drive a non-equilibrium transition into the bistable phase.
The work presented in this dissertation concerns dynamics of Rydberg atom lattices in the presence of noise and dissipation. Rydberg atoms possess a number of exaggerated properties, such as a strong van der Waals interaction. The interplay of that interaction, coherent driving and decoherence leads to intriguing non-equilibrium phenomena. Here, we study the non-equilibrium physics of driven atom lattices in the presence of decoherence caused by either laser phase noise or strong decay. In the first case, we compare between global and local noise and explore their effect on the number of excitations and the full counting statistics. We find that both types of noise give rise to a characteristic distribution of the Rydberg excitation number. The main method employed is the Langevin equation but for the sake of efficiency in certain regimes, we use a Markovian master equation and Monte Carlo rate equations, respectively. In the second case, we consider dissipative systems with more general power-law interactions. We determine the phase diagram in the steady state and analyse its generation dynamics using Monte Carlo rate equations. In contrast to nearest-neighbour models, there is no transition to long-range-ordered phases for realistic interactions and resonant driving. Yet, for finite laser detunings, we show that Rydberg atom lattices can undergo a dissipative phase transition to a long-range-ordered antiferromagnetic phase. We identify the advantages of Monte Carlo rate equations over mean field predictions. Having studied the dynamics of Rydberg atom lattices, we study an application of the strong interactions in such systems for quantum information processing. We investigate the coherent exchange of a single photon between a superconducting microwave cavity and a lattice of strongly interacting Rydberg atoms in the presence of local electric field fluctuations plaguing the cavity surface. We show that despite the increased sensitivity of Rydberg states to electric fields, as compared to ground state atoms, the Rydberg dipole-dipole interaction can be used to protect the system against the dephasing induced by the local noise. Using 1/f and laser phase noise models, we show that compared to the case with non-interacting atoms, our system exhibits longer coherence lifetimes and larger retrieval efficiency of the photon after storing into the atoms.
This thesis describes studies on a Rydberg-dressed ultracold atom system as a versatile platform for non-equilibrium physics research, atom-light interactions analysis and atomic sensing applications. This work includes a description of the experimental system, an ultracold potassium atom setup where we can prepare a gas of atoms in an optical dipole trap and precisely control their ground state properties. To excite the atoms to strongly-interacting Rydberg states, we set up and characterize a high-power and widely tunable excitation laser system, for driving both single and two-photon Rydberg excitations. Together this establishes a unique system for studying the effects of driving, long-range interactions and dissipation (due to the relatively short lifetime of excited states) on many-body dynamics. Using this setup we study the phase structure of a driven-dissipative system out of equilibrium , where exploiting the atom loss rate as an observable,we discover different power-laws depending on the system parameters (i.e driving,dissipation and interaction). We study further in detail a regime which shows loss rates much larger than those for single particles, and discover that due to the Rydberg population decay the system evolves to a self-organized critical state.Our key observations can be well described by effectively classical models for the many-body dynamics, which we understand as a consequence of rapid dephasing of atomic coherences. To quantify the coherence of the atom-light interactions, we realize a Rydberg dressed interferometer. This technique combines the precision of atomic clock transitions with the exaggerated properties of Rydberg atoms such as their long-range interactions and extreme response to electric fields. Using the interferometer, we are able to characterize the Rydberg-dressed ensemble, including the effects of population decay and dephasing both of which affect the coherence time. This enables us to identify power fluctuations in the excitation laser as the dominant effects limiting the coherence in the system, which will be used in the experiment for future improvements of the coherence time. As an additional application, we demonstrate that the Rydberg dressed interferometer can be used to precisely measure static electric fields down to 17 mV/cm which is comparable to state-of-the-art electrometers. These results together highlight the versatility of the Rydberg platform and pave the way towards a better understanding of long-range interacting systems out-of-equilibrium. This work paves the way to new studies of non-equilibrium phenomena and applications of many-body quantum systems which make use of both quantum coherent and dissipative interactions.
We study many-body correlations in the stationary state of a continuously driven, strongly interacting dissipative system. Specifically, we examine resonant optical excitations of Rydberg states of atoms interacting via long-range dipole-dipole and van der Waals potentials employing numerical and analytical techniques. Collection of atoms within a blockade distance form a ``superatom'' that can accommodate at most one Rydberg excitation. The superatom excitation probability saturates to 21 for coherently driven atoms but is significantly higher in the presence of dephasing, approaching unity as the number of atoms increases. Using the exact numerical solution of the density-matrix equations for a small system, we demonstrate that strong dephasing of the optically driven dipoles renders the many-body problem amenable to semiclassical Monte Carlo simulations. We employ the Monte Carlo algorithm for a large number of atoms and find that in the steady state of a uniformly driven, extended one-dimensional system, the saturation of superatoms leads to quasicrystallization of Rydberg excitations whose correlations exhibit damped spatial oscillations. We show that the behavior of the system under the van der Waals interaction potential can be approximated by a rate-equations model based on a ``hard-rod'' interatomic potential, and by solving it we obtain the period and correlation length for the density wave of Rydberg excitations.