One of the greatest challenges in modern physics is to understand the behaviour of an ensemble of strongly interacting particles. A class of quantum many-body systems (such as neutron star matter and cold Fermi gases) share the same universal thermodynamic properties when interactions reach the maximum effective value allowed by quantum mechanics, the so-called unitary limit. This makes it possible in principle to simulate some astrophysical phenomena inside the highly controlled environment of an atomic physics laboratory. Previous work on the thermodynamics of a two-component Fermi gas led to thermodynamic quantities averaged over the trap, making comparisons with many-body theories developed for uniform gases difficult. Here we develop a general experimental method that yields the equation of state of a uniform gas, as well as enabling a detailed comparison with existing theories. The precision of our equation of state leads to new physical insights into the unitary gas. For the unpolarized gas, we show that the low-temperature thermodynamics of the strongly interacting normal phase is well described by Fermi liquid theory, and we localize the superfluid transition. For a spin-polarized system, our equation of state at zero temperature has a 2 per cent accuracy and extends work on the phase diagram to a new regime of precision. We show in particular that, despite strong interactions, the normal phase behaves as a mixture of two ideal gases: a Fermi gas of bare majority atoms and a non-interacting gas of dressed quasi-particles, the fermionic polarons.

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Exploring the Thermodynamics of a Universal Fermi Gas

Recent experiments with ultra-cold atoms have demonstrated the possibility of realizing fermionic superfluids with imbalanced spin populations experimentally. We discuss how these developments have shed new light on a half-century-old open problem in condensed matter physics, and raised new interrogations of their own.

https://iopscience.iop.org/article/10.1088/0034-4885/73/11/112401/pdf

Ultra-cold polarized Fermi gases

The behavior of two-dimensional (2D) atom localization is explored by monitoring the probe absorption in a microwave-driven four-level atomic medium under the action of two orthogonal standing-wave fields. Because of the position-dependent atom-field interaction, the information about the position of the atom can be obtained via the absorption measurement of the weak probe field. It is found that the localization behavior is significantly improved due to the joint quantum interference induced by the standing-wave and microwave-driven fields. Most importantly, the atom can be localized at a particular position and the maximal probability of finding the atom in one period of the standing-wave fields reaches unity by properly adjusting the system parameters. The proposed scheme may provide a promising way to achieve high-precision and high-resolution 2D atom localization.

Proposal for efficient two-dimensional atom localization using probe absorption in a microwave-driven four-level atomic system

We give in this lecture an introduction to the physics of two-dimensional (2d) Bose gases. We first discuss the properties of uniform, infinite 2d Bose fluids at non-zero temperature T. We explain why thermal fluctuations are strong enough to destroy the fully ordered state associated with Bose-Einstein condensation, but are not strong enough to suppress superfluidity in an interacting system at low T. We present the basics of the Berezinskii-Kosterlitz-Thouless theory, which provides the general framework for understanding 2d superfluidity. We then turn to experimentally relevant finite-size systems, in which the presence of residual “quasi–long-range” order at low temperatures leads to an interesting interplay between superfluidity and condensation. Finally we summarize the recent progress in theoretical understanding and experimental investigation of ultracold atomic gases confined to a quasi-2d geometry.

/pdf/two-dimensional-bose-fluids-an-atomic-physics-perspective-11b8mhdoxa.pdf

Two-dimensional Bose fluids: An atomic physics perspective

A scheme for two-dimensional (2D) subwavelength atom localization is proposed, in which the atom is in a four-level tripod configuration and driven by two orthogonal standing-wave lasers. Because of the spatial dependence of atom-field interaction, the spontaneously emitted photon carries information about the position of the atom in standing-wave fields. We exploit this fact to 2D atom localization conditioned on the measurement of spontaneously emitted photon at a particular frequency, and obtain a high precision and resolution in the position probability distribution. Moreover, an improvement by a factor of 2 in the detecting probability of an atom can be achieved by initially preparing the atom in the coherent population trapping state. Qualitatively, the high-precision, high-resolution atom localization can be attributed to the quantum interference effect between competitive multiple spontaneous decay channels.

Two-dimensional atom localization via controlled spontaneous emission from a driven tripod system

The effect of spontaneously generated coherence (SGC) on spontaneous emission and dynamical evolution of a microwave-driven four-level atomic system is studied. Interesting phenomena due to SGC, such as spectral-line narrowing, spectra-line enhancement, spectral-line suppression, and fluorescence quenching, are shown in the spontaneous emission spectra, which can be effectively modulated by amplitude and phase of the microwave field. Due to SGC, the total populations are partly, even completely, trapped in upper levels in steady state, the decay time of which is sensitive to the phase of the microwave field. In the dressed state picture, multiple SGC arise during the process of three close-lying states decaying to the same state. The corresponding experiment to observe the expected phenomena related to SGC can be more conveniently realized in atoms, since no rigorous conditions are required and the amplitude and phase of the microwave field can be conveniently controlled.

Effects of spontaneously generated coherence in a microwave-driven four-level atomic system

We experimentally and theoretically demonstrate that the atomic coherence can be completely transferred or arbitrarily contributed among the different levels in a four-level atomic (tripod) scheme by a group of coupled pulse sequences. This technique can be applied to the information conversion in slow-light storage, quantum logical gates, and so on, which is based on the atomic coherence effect.

Coherence transfer between atomic ground states by the technique of stimulated Raman adiabatic passage.

We experimentally and theoretically demonstrate that multioptical signals can be effectively stored and retrieved by fractional stimulated Raman adiabatic passage technique in a tripod-type four-level {sup 87}Rb atomic system. The optical pulses stored can be controllably released into two of the three different channels. The restored pulses have the same frequency, polarization, and propagation direction as the writing pulses. The experimental results fit very well with the numerical simulations.

Storage and switching of multiple optical signals among three channels

The authors report an experiment in which optical pulses are effectively stored via maximal atomic coherence in a three-level Λ-type Rb87 atomic system, and then selectively released at one of the two different wavelengths by turning on the retrieve control pulse at 794.9842 or 794.9698nm. The storage time up to 360ns was demonstrated.

Optical signal storage and switching between two wavelengths

We control the atomic coherence and the population transfer among Rb hyperfine atomic levels by the fractional stimulated Raman adiabatic passage (F-STIRAP) in a L-type configuration, and verify the theoretical predictions. Applying this technique, we are able to prepare the atoms with maximal coherence to enhance coherent anti-Stokes Raman scattering (CARS) signal. In our experiment by scanning the frequency of one laser from 794.9839 nm to 794.9844 nm with the 794.9698 nm laser frequency fixed to generate a maximum of the CARS signal we are able to obtain the energy level diagram of the sample.

Xiao-Li Song

Papers

Effects of spontaneously generated coherence in a microwave-driven four-level atomic system

Coherence transfer between atomic ground states by the technique of stimulated Raman adiabatic passage.

Storage and switching of multiple optical signals among three channels

Optical signal storage and switching between two wavelengths

Observation of CARS signal via maximal atomic coherence prepared by F-STIRAP in a three-level atomic system