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Register a new userDesign and characterization of a quantum heat pump in a driven quantum gas
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We propose the implementation of a quantum heat pump with ultracold atoms. It is based on two periodically driven coherently coupled quantum dots using ultracold atoms. Each dot possesses two relevant quantum states and is coupled to a fermionic reservoir. The working principle is based on energy-selective driving-induced resonant tunneling processes, where a particle that tunnels from one dot to the other either absorbs or emits the energy quantum $hbaromega$ associated with the driving frequency, depending on its energy. We characterize the device using Floquet theory and compare simple analytical estimates to numerical simulations based on the Floquet-Born-Markov formalism. In particular, we show that driving-induced heating is directly linked to the micromotion of the Floquet states of the system.
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While ergodicity is a fundamental postulate of statistical mechanics and implies that driven interacting systems inevitably heat, ergodic dynamics can be disrupted by quantum interference. Despite a quarter-century of experimental studies, the effect of many-body interactions on the resulting dynamically localized state has remained unexplored. We report the experimental realization of a tunably-interacting kicked quantum rotor ensemble using a Bose-Einstein condensate in a pulsed optical lattice. We observe a prethermal localized plateau, which survives for hundreds of kicks, followed by interaction-induced anomalous diffusion. Echo-type time reversal experiments establish the role of interactions in destroying reversibility, and a mapping to kicked spin models illustrates connections to many-body dynamical localization in spin chains. These results demonstrate a dynamical transition to many-body quantum chaos, and illuminate and delimit possibilities for globally protecting quantum information in interacting driven quantum systems.
Scale-invariant fluxes are the defining property of turbulent cascades, but their direct measurement is a notorious problem. Here we perform such a measurement for a direct energy cascade in a turbulent quantum gas. Using a time-periodic force, we inject energy at a large lengthscale and generate a cascade in a uniformly-trapped Bose gas. The adjustable trap depth provides a high-momentum cutoff $k_{textrm{D}}$, which realises a synthetic dissipation scale. This gives us direct access to the particle flux across a momentum shell of radius $k_{textrm{D}}$, and the tunability of $k_{textrm{D}}$ allows for a clear demonstration of the zeroth law of turbulence: we observe that for fixed forcing the particle flux vanishes as $k_{textrm{D}}^{-2}$ in the dissipationless limit $k_{textrm{D}}rightarrow infty$, while the energy flux is independent of $k_{textrm{D}}$. Moreover, our time-resolved measurements give unique access to the pre-steady-state dynamics, when the cascade front propagates in momentum space.
By quenching the strength of interactions in a partially condensed Bose gas we create a super-saturated vapor which has more thermal atoms than it can contain in equilibrium. Subsequently, the number of condensed atoms ($N_0$) grows even though the temperature ($T$) rises and the total atom number decays. We show that the non-equilibrium evolution of the system is isoenergetic and for small initial $N_0$ observe a clear separation between $T$ and $N_0$ dynamics, thus explicitly demonstrating the theoretically expected two-step picture of condensate growth. For increasing initial $N_0$ values we observe a crossover to classical relaxation dynamics. The size of the observed quench-induced effects can be explained using a simple equation of state for an interacting harmonically-trapped atomic gas.
Three-body recombination in quantum gases is traditionally associated with heating, but it was recently found that it can also cool the gas. We show that in a partially condensed three-dimensional homogeneous Bose gas three-body loss could even purify the sample, that is, reduce the entropy per particle and increase the condensed fraction $eta$. We predict that the evolution of $eta$ under continuous three-body loss can, depending on small changes in the initial conditions, exhibit two qualitatively different behaviours - if it is initially above a certain critical value, $eta$ increases further, whereas clouds with lower initial $eta$ evolve towards a thermal gas. These dynamical effects should be observable under realistic experimental conditions.
We study the thermodynamics of Bose-Einstein condensation in a weakly interacting quasi-homogeneous atomic gas, prepared in an optical-box trap. We characterise the critical point for condensation and observe saturation of the thermal component in a partially condensed cloud, in agreement with Einsteins textbook picture of a purely statistical phase transition. Finally, we observe the quantum Joule-Thomson effect, namely isoenthalpic cooling of an (essentially) ideal gas. In our experiments this cooling occurs spontaneously, due to energy-independent collisions with the background gas in the vacuum chamber. We extract a Joule-Thomson coefficient $mu_{rm JT} > 10^9$ K/bar, about ten orders of magnitude larger than observed in classical gases.
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