Universal dynamics of a degenerate unitary Bose gas

Abstract

From neutron stars to high-temperature superconductors, strongly interacting many-body systems at or near quantum degeneracy are a rich source of intriguing phenomena. The microscopic structure of the first-discovered quantum fluid, superfluid liquid helium, is difficult to access due to limited experimental probes. While an ultracold atomic Bose gas with tunable interactions (characterized by its scattering length, a) had been proposed as an alternative strongly interacting Bose system [1][2][3][4][5][6][7][8] , experimental progress [9][10][11][12] has been limited by its short lifetime. Here we present time-resolved measurements of the momentum distribution of a Bose-condensed gas that is suddenly jumped to unitarity, i.e. to a = ∞. Contrary to expectation, we observe that the gas lives long enough to permit the momentum to evolve to a quasi-steady-state distribution, consistent with universality, while remaining degenerate. Investigations of the time evolution of this unitary Bose gas may lead to a deeper understanding of quantum many-body physics.A powerful feature of atom gas experiments that provides access to these new regimes is the ability to change the interaction strength using a magnetic-field Feshbach resonance [13]. In particular, at the resonance location, a is infinite. For atomic Fermi gases [14][15][16][17][18][19][20], accessing this regime by adiabatically changing a led to the achievement of superfluids of paired fermions and enabled investigation of the crossover from superfluidity of weakly bound pairs, analogous to the Bardeen-Cooper-Schrieffer (BCS) theory of superconductors, to Bose-Einstein condensation (BEC) of tightly bound molecules [16,17]. For bosonic atoms, however, this route to strong interactions is stymied by the fact that three-body inelastic collisions increase as a to the fourth power [21][22][23]. This circumstance has limited experimental investigation of Bose gases with increasing interaction strength to studying either non-quantum-degenerate gases [24,25] or BECs with modest interaction strengths (na 3 < 0.008, where n is the atom number density) [9][10][11][12].The problem is that the loss rate scales as n 2 a 4 while the equilibration rate scales as na 2 v, where v is the average velocity. Thus, it would seem that the losses will always dominate as a is increased to ∞. Even if we were to forsake thermal equilibrium and suddenly change a in order to project a weakly interacting BEC onto strong interactions [12,[26][27][28], one might expect that three-body losses would still dominate the ensuing dynamics for large a. In this work, however, we use this approach to take a BEC to the unitary gas regime, and we observe dynamics that in fact saturate on a timescale shorter than that set by three-body losses and that exhibit universal scaling with density.

Efimov-driven phase transitions of the unitary Bose gas

“…These questions are closely related to the very existence of the unitary Bose gas on time scales larger than its thermalization time. Very recent experimental works indicate that the ultra-cold unitary Bose gas can indeed be stabilized on appreciable time scales 11,14,15 .…”

“…However, the macroscopic many-body properties of the unitary Bose gas have remained unknown. The understanding of its thermodynamic behaviour is of great importance, especially as the experimental stability of the unitary Bose gas of cold atoms has been reported for appreciable time scales 11,14,15 .…”

Nonequilibrium states of a quenched Bose gas

“…At unitarity, on purely dimensional grounds, both the three-body loss and the equilibration rates will be of the same order of magnitude as the Fermi energy [38], ǫ F = (ω F = k 2 F /2m), where k F = (6π 2 n) 1/3 is the Fermi momentum. It was unclear which dominates until recently when a JILA experiment on 85 Rb by Makotyn et al [2] firmly established that the three-body loss rate is much slower than the equilibration rate. This pleasant surprise opens the door to the possibility of exploring the rich physics underlying strongly interacting Bose gases [29][30][31][32][33][34][35][36][37] and has motivated, together with experimental works such as [43], a flurry of theoretical studies concerning quenched nonequilibrium dynamics [1,[44][45][46][47][48][49].…”

Dynamics of correlations in a quasi-two-dimensional dipolar Bose gas following a quantum quench

“…For quenches to strong dipolar interactions, where τ roton > τ , the timescale for the fast modes to dephase relative to one another, the excited fraction also develops oscillations. These can be readily probed in time-of-flight [27]. The dynamics of the roton modes in this bosonic dipolar gas is analogous to that of Cooper pairs in an attractive Fermi gas, driven out of equilibrium following an interaction quench [55].…”

“…For example, the realization of quantum degenerate gases with large magnetic dipole moments [2][3][4][5][6], ongoing efforts to trap and cool polar molecules [7][8][9][10], experiments on Rydberg atoms [11], trapped ions [12,13], and atoms in high finesse optical cavities [14] have opened up the possibility of realizing ultra-cold atomic systems with long-range interactions, such as the anisotropic dipoledipole interaction [15][16][17][18]. Concurrently, better control over experimental parameters and high resolution imaging techniques have introduced new probes for exploring many-body physics, notable among which is the ability to study the dynamics of correlations following a nonadiabatic ramp (quench) of system parameters [19][20][21][22][23][24][25][26][27]. Remarkably, these quench experiments provide a wealth of information about the low energy properties of the underlying system such as the nature of the excitations, and the manner in which correlations develop in a system.…”