We study experimentally the dissipative dynamics of ultracold bosonic gases in a dynamic disorder potential with tunable correlation time. First, we measure the heating rate of thermal clouds exposed to the dynamic potential and present a model of the heating process, revealing the microscopic origin of dissipation from a thermal, trapped cloud of bosons. Second, for Bose-Einstein condensates, we measure the particle loss rate induced by the dynamic environment. Depending on the correlation time, the losses are either dominated by heating of residual thermal particles or the creation of excitations in the superfluid, a notion we substantiate with a rate model. Our results illuminate the interplay between superfluidity and time-dependent disorder and on more general grounds establish ultracold atoms as a platform for studying spatiotemporal noise and time-dependent disorder.We study experimentally the dissipative dynamics of ultracold bosonic gases in a dynamic disorder potential with tunable correlation time. First, we measure the heating rate of thermal clouds exposed to the dynamic potential and present a model of the heating process, revealing the microscopic origin of dissipation from a thermal, trapped cloud of bosons. Second, for Bose-Einstein condensates, we measure the particle loss rate induced by the dynamic environment. Depending on the correlation time, the losses are either dominated by heating of residual thermal particles or the creation of excitations in the superfluid, a notion we substantiate with a rate model. Our results illuminate the interplay between superfluidity and time-dependent disorder and on more general grounds establish ultracold atoms as a platform for studying spatiotemporal noise and time-dependent disorder.

arXiv:2007.11523Jul 22, 20205 citations

Dynamics of ultracold bosons in a disordered optical lattice

Carolyn Meldgin

"This thesis presents research on a variety of dynamical phenomena in strongly correlated systems. We use 87Rb atoms trapped in a disordered optical lattice to realize the 3D Bose Hubbard model, with independently, dynamically tunable lattice depth and disorder strength. The equilibrium behavior of the Bose Hubbard and disordered Bose Hubbard models are understood, but there are many outstanding questions about dynamics. One area of interest is in dynamical behavior near quantum phase boundaries, such as the generation of quench-induced excitations in the presence of disorder. Another area concerns heat flow, relaxation, and thermalization. The primary focus of this thesis is driving a gas across a disorder-induced quantum phase transition from the Bose glass to the superfluid phase via quenching the disorder. We measured excitations generated during the quench and related them to crossing the equilibrium phase transition determined by quantum Monte Carlo simulations of the trapped system. The behavior we observe is reminiscent of the quantum Kibble Zurek mechanism, where the relaxation timescale diverges and local fluctuations are ""frozen"" into the system near the phase boundary. Understanding how disorder impacts the Kibble Zurek paradigm about behavior near a phase boundary provides insight into the dynamics of strongly interacting, disordered systems. The three other works discussed in this thesis concern relaxation and energy transfer in clean optical lattices. Understanding these dynamical properties of lattice gases is key to developing tools such as cooling in a lattice. In the first result, a gas comprised of two spin states was loaded into a spin-dependent optical lattice, such that one spin state experienced the lattice, and the other did not. We found that when the lattice gas was heated, energy transfer to the non-lattice gas did not occur at higher lattice depths due to the mismatch in the dispersion relations. In the second result, lattice atoms were excited to a higher band and found to decay to the ground state via two mechanisms. One decay channel involved generating excitations in the non-lattice atoms, while the other channel involved collisions between lattice atoms, where one atom decayed to the ground state and the other was excited to a higher band. The final result measured relaxation timescales by removing atoms with low quasimomentum and measuring rethermalization. The observed timescales are faster than the tunneling time and interaction time of the lattice. Proof-of-principle cooling was also demonstrated by removing atoms with higher quasimomentum and allowing the remaining atoms rethermalize to a cooler temperature.""This thesis presents research on a variety of dynamical phenomena in strongly correlated systems. We use 87Rb atoms trapped in a disordered optical lattice to realize the 3D Bose Hubbard model, with independently, dynamically tunable lattice depth and disorder strength. The equilibrium behavior of the Bose Hubbard and disordered Bose Hubbard models are understood, but there are many outstanding questions about dynamics. One area of interest is in dynamical behavior near quantum phase boundaries, such as the generation of quench-induced excitations in the presence of disorder. Another area concerns heat flow, relaxation, and thermalization. The primary focus of this thesis is driving a gas across a disorder-induced quantum phase transition from the Bose glass to the superfluid phase via quenching the disorder. We measured excitations generated during the quench and related them to crossing the equilibrium phase transition determined by quantum Monte Carlo simulations of the trapped system. The behavior we observe is reminiscent of the quantum Kibble Zurek mechanism, where the relaxation timescale diverges and local fluctuations are ""frozen"" into the system near the phase boundary. Understanding how disorder impacts the Kibble Zurek paradigm about behavior near a phase boundary provides insight into the dynamics of strongly interacting, disordered systems. The three other works discussed in this thesis concern relaxation and energy transfer in clean optical lattices. Understanding these dynamical properties of lattice gases is key to developing tools such as cooling in a lattice. In the first result, a gas comprised of two spin states was loaded into a spin-dependent optical lattice, such that one spin state experienced the lattice, and the other did not. We found that when the lattice gas was heated, energy transfer to the non-lattice gas did not occur at higher lattice depths due to the mismatch in the dispersion relations. In the second result, lattice atoms were excited to a higher band and found to decay to the ground state via two mechanisms. One decay channel involved generating excitations in the non-lattice atoms, while the other channel involved collisions between lattice atoms, where one atom decayed to the ground state and the other was excited to a higher band. The final result measured relaxation timescales by removing atoms with low quasimomentum and measuring rethermalization. The observed timescales are faster than the tunneling time and interaction time of the lattice. Proof-of-principle cooling was also demonstrated by removing atoms with higher quasimomentum and allowing the remaining atoms rethermalize to a cooler temperature."

Apr 1, 20161 citation

Out-of-equilibrium dynamics in a tuneable homogeneous Bose gas

Jake Glidden

This thesis describes studies of out-of-equilibrium dynamics in an interacting homogeneous Bose gas. We use the versatile platform of an ultracold gas of $^{39}$ K, prepared in an optical box trap. The experiment features both tuneable interaction strengths and a homogeneous atomic density. We examine first a weakly interacting near-equilibrium system, followed by far-from-equilibrium dynamics under moderate interactions, and finally combine strong interactions with highly non-equilibrium dynamics in studies of the unitary Bose gas. We start with a theoretical study of the effects of three-body recombination on the temperature of a weakly interacting, partially condensed gas. Contrary to the expectation of heating, we find that for appropriate values of the condensed fraction and gas parameter, the system temperature is expected to decrease. Not only do we predict lower temperatures, however, but also for the condensate fraction to grow. We then present experiments in which we engineer a far-from-equilibrium momentum distribution, and study the dynamics as the gas relaxes. As the distribution evolves, we observe bidirectional dynamic scaling, with a flow of particles to lower momenta while energy is transported to higher ones. These flows are characterised by spatiotemporal scaling exponents, which we find do not vary appreciably over a range of moderate interaction strengths. Finally, we look to the unitary Bose gas, quenching clouds into the strongly interacting regime. This regime offers insight into universal behaviour that depends only on the interparticle spacing, as well as being an avenue for the study of Efimov physics. However, few things come for free; strongly interacting Bose systems are plagued by high atom-loss and heating rates. We study atom-loss rates and the dynamics of correlations in our gases, isolate the coherent effects from the dissipative ones, and identify a quasi-equilibrium state that features a non-zero condensate fraction.This thesis describes studies of out-of-equilibrium dynamics in an interacting homogeneous Bose gas. We use the versatile platform of an ultracold gas of $^{39}$K, prepared in an optical box trap. The experiment features both tuneable interaction strengths and a homogeneous atomic density. We examine first a weakly interacting near-equilibrium system, followed by far-from-equilibrium dynamics under moderate interactions, and finally combine strong interactions with highly non-equilibrium dynamics in studies of the unitary Bose gas. We start with a theoretical study of the effects of three-body recombination on the temperature of a weakly interacting, partially condensed gas. Contrary to the expectation of heating, we find that for appropriate values of the condensed fraction and gas parameter, the system temperature is expected to decrease. Not only do we predict lower temperatures, however, but also for the condensate fraction to grow. We then present experiments in which we engineer a far-from-equilibrium momentum distribution, and study the dynamics as the gas relaxes. As the distribution evolves, we observe bidirectional dynamic scaling, with a flow of particles to lower momenta while energy is transported to higher ones. These flows are characterised by spatiotemporal scaling exponents, which we find do not vary appreciably over a range of moderate interaction strengths. Finally, we look to the unitary Bose gas, quenching clouds into the strongly interacting regime. This regime offers insight into universal behaviour that depends only on the interparticle spacing, as well as being an avenue for the study of Efimov physics. However, few things come for free; strongly interacting Bose systems are plagued by high atom-loss and heating rates. We study atom-loss rates and the dynamics of correlations in our gases, isolate the coherent effects from the dissipative ones, and identify a quasi-equilibrium state that features a non-zero condensate fraction.

Mar 1, 20211 citation

Dynamical behavior of ultracold atomic gases

P. Pedri

The experimental achievement of Bose-Einstein condensation (BEC) in 1995 has aroused a large interest among the community of the physics of ultracold atomic gases.Recently new topics have attracted the attention of this very active field of research , in particular we could mention low dimensional systems, atoms in lattices, and classical and Fermi gases.As for the case of BEC, the study of the dynamical aspects of these systems provides a very valuable information.In particular time-of-flight experiments and measurements of collective excitations have been successfully realized providing a clear signature of the BEC and the regime of interaction.Following this line, in this thesis, we want to study some dynamical aspects of these new systems nowadays avaliable due to the remarkable development of the experimental techniques.In the first chapter we present the state of the art of the field, providing the historical path and the motivation for the work developed in the next chapters.In the second chapter, we discuss some general concepts in order to introduce the formalism and notation employed throughout the thesis, as well as those topics analyzed in more detail in the further chapters.In chapter three, we explain some general techniques which are useful to deal with one dimensional systems, and analyze in detail the expansion of a one dimensional Bose gas in a guide, which differs significantly from the expansion of a BEC.In particular we are able to predict the range of parameters for which the self-similarity of the expansion is violated.Chapter four is devoted to the physics of atoms in a periodic potential.We start, as in the previous chapter, introducing some general concepts and techniques employed further.First we discuss the physics of the superfluid case in a trap, showing that under certain conditions a three dimensional gas exhibits one dimensional properties.viii In particular, static properties and the lowest collective excitations differ significantly from those of a BEC.We extend the analysis to the insulating phase showing that correlation properties depend on the on-site one-dimensional regime.These correlations can be measured in current time of flight experiments.In the fifth chapter we analyze the dynamics of classical and Fermi gases.We show in detail how expansion and collective excitations depend on the interaction regime and the trap anisotropy.In particular, the expansion can be used to distinguish between a normal Fermi gas and a superfluid one.We summarize in the last chapter the new results presented in this thesis.The experimental achievement of Bose-Einstein condensation (BEC) in 1995 has aroused a large interest among the community of the physics of ultracold atomic gases.Recently new topics have attracted the attention of this very active field of research , in particular we could mention low dimensional systems, atoms in lattices, and classical and Fermi gases.As for the case of BEC, the study of the dynamical aspects of these systems provides a very valuable information.In particular time-of-flight experiments and measurements of collective excitations have been successfully realized providing a clear signature of the BEC and the regime of interaction.Following this line, in this thesis, we want to study some dynamical aspects of these new systems nowadays avaliable due to the remarkable development of the experimental techniques.In the first chapter we present the state of the art of the field, providing the historical path and the motivation for the work developed in the next chapters.In the second chapter, we discuss some general concepts in order to introduce the formalism and notation employed throughout the thesis, as well as those topics analyzed in more detail in the further chapters.In chapter three, we explain some general techniques which are useful to deal with one dimensional systems, and analyze in detail the expansion of a one dimensional Bose gas in a guide, which differs significantly from the expansion of a BEC.In particular we are able to predict the range of parameters for which the self-similarity of the expansion is violated.Chapter four is devoted to the physics of atoms in a periodic potential.We start, as in the previous chapter, introducing some general concepts and techniques employed further.First we discuss the physics of the superfluid case in a trap, showing that under certain conditions a three dimensional gas exhibits one dimensional properties.viii In particular, static properties and the lowest collective excitations differ significantly from those of a BEC.We extend the analysis to the insulating phase showing that correlation properties depend on the on-site one-dimensional regime.These correlations can be measured in current time of flight experiments.In the fifth chapter we analyze the dynamics of classical and Fermi gases.We show in detail how expansion and collective excitations depend on the interaction regime and the trap anisotropy.In particular, the expansion can be used to distinguish between a normal Fermi gas and a superfluid one.We summarize in the last chapter the new results presented in this thesis.

Jan 1, 20051 citation

Two-dimensional Bose gases in systems with pointlike disorder

Garrick, David

The effect of disorder on many-body physics is important for our understanding of real-world systems. In particular, the behaviour of phase transitions between superfluid, normal and insulating phases is an area with many unanswered questions and experimental evidence remains scarce in certain important cases. In this thesis, we focus on systems of two-dimensional (2D) Bose gases subjected to a pointlike disorder potential. We trap atoms in radiofrequency dressed potentials and apply the technique of multiple-radiofrequency dressing to create a double-well potential into which we load the ultracold atoms. Matter-wave interferometry allows us to directly probe the local relative phase and its fluctuations between the clouds in the two potential wells. A pointlike disordered optical potential is produced by a high-intensity laser beam which is shaped by a programmable mirror array and passes through clouds in both of the wells. We adiabatically apply disordered potentials of increasing strength to observe their effect on the superfluids in equilibrium in a double-well. We observe that above a critical disorder strength the superfluid undergoes a transition to the normal phase at a lower temperature than that of the disorder-free case, and we characterise this disorder-induced transition. We also study the flow of a 2D superfluid cloud around both a single defect and a random potential and measure the dissipation induced by the obstacles. The absence of dissipation is observed below a critical velocity in the case of a single defect but we find no such dissipationless regime for the disordered potential. We describe the development of a software simulation package for modelling cold-atom systems and the implementation of particle collisions, magnetic trapping and evaporative cooling. Finally, we report on the initial work towards construction of a new cold-atom apparatus, capable of faster sequence times and greater atom numbers, to continue our investigation of many-body physics.The effect of disorder on many-body physics is important for our understanding of real-world systems. In particular, the behaviour of phase transitions between superfluid, normal and insulating phases is an area with many unanswered questions and experimental evidence remains scarce in certain important cases. In this thesis, we focus on systems of two-dimensional (2D) Bose gases subjected to a pointlike disorder potential. We trap atoms in radiofrequency dressed potentials and apply the technique of multiple-radiofrequency dressing to create a double-well potential into which we load the ultracold atoms. Matter-wave interferometry allows us to directly probe the local relative phase and its fluctuations between the clouds in the two potential wells. A pointlike disordered optical potential is produced by a high-intensity laser beam which is shaped by a programmable mirror array and passes through clouds in both of the wells. We adiabatically apply disordered potentials of increasing strength to observe their effect on the superfluids in equilibrium in a double-well. We observe that above a critical disorder strength the superfluid undergoes a transition to the normal phase at a lower temperature than that of the disorder-free case, and we characterise this disorder-induced transition. We also study the flow of a 2D superfluid cloud around both a single defect and a random potential and measure the dissipation induced by the obstacles. The absence of dissipation is observed below a critical velocity in the case of a single defect but we find no such dissipationless regime for the disordered potential. We describe the development of a software simulation package for modelling cold-atom systems and the implementation of particle collisions, magnetic trapping and evaporative cooling. Finally, we report on the initial work towards construction of a new cold-atom apparatus, capable of faster sequence times and greater atom numbers, to continue our investigation of many-body physics.

Jan 1, 20220 citations

Dissipation in a finite-temperature atomic Josephson junction

Klejdja Xhani, Nick P. Proukakis

We numerically demonstrate and characterize the emergence of distinct dynamical regimes of a finite-temperature bosonic superfluid in an elongated Josephson junction generated by a thin Gaussian barrier over the entire temperature range where a well-formed condensate can be clearly identified. Although the dissipation arising from the coupling of the superfluid to the dynamical thermal cloud increases with increasing temperature as expected, the importance of this mechanism is found to depend on two physical parameters associated (i) with the initial chemical potential difference, compared to some characteristic value, and (ii) the ratio of the thermal energy to the barrier amplitude. The former determines whether the superfluid Josephson dynamics are dominated by gradually damped plasmalike oscillations (for relatively small initial population imbalances), or whether dissipation at early times is instead dominated by vortex- and sound-induced dissipation (for larger initial imbalances). The latter defines the effect of the thermal cloud on the condensate dynamics, with a reversal of roles, i.e., the condensate being driven by the oscillating thermal cloud, being observed when the thermal particles acquire enough energy to overcome the barrier. Our findings are within current experimental reach in ultracold superfluid junctions.We numerically demonstrate and characterize the emergence of distinct dynamical regimes of a finite-temperature bosonic superfluid in an elongated Josephson junction generated by a thin Gaussian barrier over the entire temperature range where a well-formed condensate can be clearly identified. Although the dissipation arising from the coupling of the superfluid to the dynamical thermal cloud increases with increasing temperature as expected, the importance of this mechanism is found to depend on two physical parameters associated (i) with the initial chemical potential difference, compared to some characteristic value, and (ii) the ratio of the thermal energy to the barrier amplitude. The former determines whether the superfluid Josephson dynamics are dominated by gradually damped plasmalike oscillations (for relatively small initial population imbalances), or whether dissipation at early times is instead dominated by vortex- and sound-induced dissipation (for larger initial imbalances). The latter defines the effect of the thermal cloud on the condensate dynamics, with a reversal of roles, i.e., the condensate being driven by the oscillating thermal cloud, being observed when the thermal particles acquire enough energy to overcome the barrier. Our findings are within current experimental reach in ultracold superfluid junctions.

arXiv:2110.00877Physical Review ResearchSep 13, 20228 citations

This thesis describes studies of out-of-equilibrium dynamics in an interacting homogeneous Bose gas. We use the versatile platform of an ultracold gas of

^{39}

K, prepared in an optical box trap. The experiment features both tuneable interaction strengths and a homogeneous atomic density. We examine first a weakly interacting near-equilibrium system, followed by far-from-equilibrium dynamics under moderate interactions, and finally combine strong interactions with highly non-equilibrium dynamics in studies of the unitary Bose gas. We start with a theoretical study of the effects of three-body recombination on the temperature of a weakly interacting, partially condensed gas. Contrary to the expectation of heating, we find that for appropriate values of the condensed fraction and gas parameter, the system temperature is expected to decrease. Not only do we predict lower temperatures, however, but also for the condensate fraction to grow. We then present experiments in which we engineer a far-from-equilibrium momentum distribution, and study the dynamics as the gas relaxes. As the distribution evolves, we observe bidirectional dynamic scaling, with a flow of particles to lower momenta while energy is transported to higher ones. These flows are characterised by spatiotemporal scaling exponents, which we find do not vary appreciably over a range of moderate interaction strengths. Finally, we look to the unitary Bose gas, quenching clouds into the strongly interacting regime. This regime offers insight into universal behaviour that depends only on the interparticle spacing, as well as being an avenue for the study of Efimov physics. However, few things come for free; strongly interacting Bose systems are plagued by high atom-loss and heating rates. We study atom-loss rates and the dynamics of correlations in our gases, isolate the coherent effects from the dissipative ones, and identify a quasi-equilibrium state that features a non-zero condensate fraction.