CE30 - Physique de la matière condensée et de la matière diluée

BOsonic QUAntum TRAnsport – BOQUATRA

Quantum transport in ultracold Bose gases

Study of the dynamics of a gas of ultra-cold bosons with one or two spin components in arbitrary-shaped light potentials.

Objectives

The stake of the project consists on the one hand, to study the quantum transport in a mesoscopic channel of atoms connected to two reservoirs and, on the other hand, to study the dynamics of a spin mixture in a two-dimensional cloud of atoms with controlled shape.<br /><br />The objectives associated with transport are the study of mass and heat transport in a multimode or single-mode channel with the possible prospect of observing the quantification of thermal conductance.<br /><br />The study of spin dynamics will allow us to characterize the demixion dynamics in a mixture of superfluids, to study the dynamics of interfaces between spin domains and to perform a quantum Brownian motion experiment in which individual impurities are made to diffuse in a superfluid bath.

We manipulate Bose gases in potentials of various and controllable geometries thanks to spatial light modulators. This allows us to trap one- or two-dimensional Bose gases with chosen shapes and thus to control the dynamics of these systems.

On the other hand, we can manipulate mixtures of different internal states of the same atom and thus access spin dynamics in these systems. In particular, we have developed tools that allow us to create clouds of spin impurities in a superfluid bath with a number of atoms, a spatial profile and an arbitrary global momentum.

Main results

- Dynamical symmetries and breathers in a 2D Bose gas.
Two-dimensional Bose gas exhibits a property of scale invariance. We have used this invariance to verify and observe many characteristics of the dynamics of this gas in harmonic traps. Surprisingly, we have observed breathers in this interacting N-body system. These are particular states that recur periodically at a multiple or submultiple period of the trap period. The origin of the breathers that we discovered and observed remains largely misunderstood at this date.

- Magnetic dipolar interaction in a planar Bose gas.
We have observed that a mixture of atoms in two non-magnetic internal states nevertheless exhibits dipole-dipole magnetic interaction effects. These effects have the consequence of modifying the interstate scattering length. This observation, specific to gases of reduced dimensionality, provides a completely new tool for controlling spin dynamics in superfluid mixtures.

- Contact measurement in a planar Bose gas around the superfluid transition
We have made the first experimental measurement of contact in a two-dimensional Bose gas. This quantity is related to the interaction energy of the system which varies by a factor of 2 when the regime changes from a (approximately) perfect gas of bosons in the high temperature regime to a degenerate gas at zero temperature. To date, no theoretical study has made it possible to completely describe all the regimes explored in our experiment, in particular around the critical point of the transition.

This project will make possible the exploration of the dynamics ofbosonic gases in new regimes.

Publications :

1. Phys. Rev. X 9 021035 (2020)
2. arXiv:2007.12385 (2020)
3. arXiv:2007.12389 (2020)

I will study out-of-equilibrium dynamics of two-dimensional Bose gases. I will create samples with spatial variations of density, temperature or internal state and investigate their relaxation towards an equilibrium or metastable state. The two main breakthroughs of this project will consist in the experimental realization of (i) quantum transport of bosons through a single-mode channel. (ii) diffusion of a few single-particle impurities in a bath of atoms in another internal state, realizing a quantum Brownian motion experiment.

This project is based on an experimental setup that I have recently developed in my team and which is fully operational today. This platform allows us to confine Bose gases in a light sheet restricting the motion of the atoms to a two-dimensional plane and to limit their in-plane motion to an arbitrary-shaped potential. Such a setup, allowing high resolution tailoring of optical potentials is rather original in our field and will offer us a large flexibility to investigate out-of-equilibrium dynamics.

A first line of research will be devoted to transport properties of an atomic channel. We will start our work two-dimensional channels of a few micron width, directly available in our setup and hosting many conducting modes. We will study particle and heat transport and characterize the influence of disorder in the channel, which is expected to modify much more strongly the behavior of the normal part with respect to the superfluid part. Then, we will decrease the thickness of the channel to enter the single mode regime and where dramatic effects, like the quantification of heat of conductance, are expected.

A second line of research will focus on spin dynamics. The rubidium atom that we use has several internal hyperfine states in the electronic ground state. These states can be easily coupled thanks to a microwave field or a two-photon Raman transition, the latter easily allowing spatial resolution. We will focus on binary mixtures. For instance, we will realize quenches by abruptly superimposing two immiscible internal states and monitor their relaxation, a situation for which we already obtained preliminary results. Then, we will move to the study of the dynamics of an impurity in a bath of atoms in another internal state with tunable interaction. We will develop new tools to measure the motion of a few individual particles with sub-micron resolution. By doing so, we will achieve an experimental configuration where we will observe the diffusive behavior of quantum Brownian particles. A superdiffusive behavior is expected in this regime, which goes beyond the memory-less Markov regime for diffusion.

The duration of the project will be of 48 months. A large part of the proposed objectives can be directly tackled in our team. In addition, we will develop new tools (optical aberration correction techniques to achieve single mode channels, spatially resolved Raman coupling with tunable momentum transfer, single-atom fluorescence imaging) to achieve original regimes that goes beyond the current state-of-the-art.

Project coordinator

Monsieur Jerome Beugnon (Laboratoire Kastler Brossel)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LKB Laboratoire Kastler Brossel

Help of the ANR 250,486 euros
Beginning and duration of the scientific project: December 2018 - 48 Months

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