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Strongly Interacting Bose Gases

Periodic Reporting for period 1 - SIBG (Strongly Interacting Bose Gases)

Reporting period: 2015-07-20 to 2017-07-19

This project addresses questions of emergent phenomena: How do remarkable properties of matter emerge from complex correlations between the atomic constituents? Specifically, our research is focused on Strongly Interacting Bose Gases (SIBG). The Bose gas is of fundamental significance as a quintessential example of a quantum fluid -- that is, a fluid that occurs at extremely low temperature and whose properties can only be understood within the theory of quantum mechanics. SIBG occur when interactions play a dominant role in the gas when the scattering length or dipole length becomes large with respect to the interparticle spacing. Loss from three-body recombination and instability from attractive interactions present two major obstacles to the experimental creation of SIBG. Recombination processes involve a collision between three atoms that results in a deeply bound two-body dimer and a high-energy atom. Instability arises when attractive interactions overcome the kinetic pressure in the gas, leading to an implosion. This project focuses on improving our understanding of two promising pathways toward SIBG where loss and instability can be suppressed to a manageable level. The first scenario involves gases wherein one has a combination of dipole-dipole forces and s-wave collisions. The second involves temporal manipulation of the interactions in a periodic fashion via the dynamical utilization of a Fano-Feshbach resonance.

A complete theoretical understanding of SIBG presents an important fundamental advance in many-body physics. Strong correlations between the atoms make these systems remarkably unique and unusual in many respects, while also providing a formidable scientific challenge. The phase diagram, although not currently understood in its entirety, is likely to be very rich in exotic phases that involve Efimovian trimers, many-body droplets, roton modes, and supersolidity. The task (both theoretical and experimental) is made complicated by instability and loss, but navigating around these obstacles will provide unheralded examples of emergent quantum many-body physics that can play highly specialized roles in future quantum technologies.

The overarching objectives of this project are to improve our theoretical understanding of SIBG, with particular focus on scenarios that may feasibly come under experimental investigation in the near future. This includes developing a detailed theory of few-body physics in novel dynamical regimes using Floquet theory.
We have completed and published the two major separate work packages.

1. ``Anisotropic Expansion of a Thermal Dipolar Bose Gas'', Physical Review Letters 117, 155301 (2016); ``Anisotropic collisions of dipolar Bose-Einstein condensates in the universal regime'', New Journal of Physics 18, 113004 (2016); see also the accompanying “perspective article” by Igor Ferrier-Barbut, New J. Phys. 18, 111004 (2016) and our explanatory video at https://youtu.be/t3Oj_-WhbBo

In the first article we develop a complete quantitative theory of the free-expansion of a dipolar gas after it is released from a trap. The theory expresses the post-expansion aspect ratio (related to the difference of apparent temperatures along different directions, see Figure attached) in terms of temperature and microscopic collisional properties by incorporating Hartree-Fock mean-field interactions, hydrodynamic effects, and Bose-enhancement factors. These results extend the utility of expansion imaging by providing accurate thermometry for dipolar Bose gases. Furthermore, this provides a simple and efficient method to determine scattering lengths in dipolar Bose gases, including near a Fano-Feshbach resonance, through the observation of thermal gas expansion. In the second paper we demonstrated and further develop the simulation capabilities of the dipolar-direct-simulation-Monte-Carlo method. We have performed large-scale numerical simulations of a collision between two Bose-condensed clouds with strong dipolar interactions and compared the results to experimental data from the group of Benjamin Lev at Stanford University. We describe a novel regime of quantum scattering, relevant to dipolar interactions, in which a large number of angular momentum states become coupled during the collision. The comparison between theory and experiment was excellent and contributes strongly to the foundations of understanding in strongly dipolar gases.

2. ``Two- and three-body problem with Floquet-driven zero-range interactions'', Physical Review A 95, 062705 (2017).

Here we have studied few-body physics with temporally manipulated interactions, in particular, sinusoidally driven. A complete theory of this scenario is developed. The main results include the discovery of a family of curves in the drive-parameter space wherein the effective scattering amplitude is resonantly enhanced, thereby creating a strongly interacting system. The resonance width along these curves changes from narrow to wide, thereby providing a novel and new experimental tool for tuning resonance width in Bose gases. In addition, the inelastic scattering on these resonance curves vanishes, thereby eliminating heating. We have also considered the three-body problem of two heavy particles and one light particle. We have shown that the Floquet driving can be used to tune the three-body and inelasticity parameters.
This project has substantially advanced our understanding in several key areas within SIBG. By providing a quantitative theoretical basis for gas expansion dynamics of a gas with a combination of dipolar and s-wave interactions, the scattering length of dysprosium has been successfully measured, including in the vicinity of a Fano-Feshbach resonance. This is a crucial step required to understand the equation of state, in order to completely characterize the equilibrium properties of the gas. Furthermore, the direct-simulation-Monte-Carlo algorithm was improved and adjusted to simulate dipolar condensate collisions. In addition, the project has developed a new theory of few-body physics in the presence of periodically driven interactions. This progress has far reaching consequences in the fields of atomic/molecular physics, ultra cold quantum gases, and beyond. The knowledge gleaned from this research will enable future advances toward the experimental creation and further theoretical understanding of SIBG. Such advances will ultimately yield new and highly exotic examples of quantum matter with unique properties that will potentially play highly specialized roles in future quantum technologies.
Temperatures deduced from uncorrected (gray) and corrected (red) time-of-flight thermometry