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arXiv:1904.00908 · Physical Review A

Pair formation in quenched unitary Bose gases

S. Musolino, V. E. Colussi, S. J. J. M. F. Kokkelmans

Published Jul 11, 2019• 13 citations
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Abstract

We study a degenerate Bose gas quenched to unitarity by solving a many-body model including three-body losses and correlations up to second order. As the gas evolves in this strongly interacting regime, the buildup of correlations leads to the formation of extended pairs bound purely by many-body effects, analogous to the phenomenon of Cooper pairing in the BCS regime of the Fermi gas. Through fast sweeps away from unitarity, we detail how the correlation growth and formation of bound pairs emerge in the fraction of unbound atoms remaining after the sweep, finding quantitative agreement with experiment. We comment on the possible role of higher-order effects in explaining the deviation of our theoretical results from experiment for slower sweeps and longer times spent in the unitary regime.

DOI
10.1103/physreva.100.013612

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•Jan 1, 2013•1 citation
We study a degenerate Bose gas quenched to unitarity by solving a many-body model including three-body losses and correlations up to second order. As the gas evolves in this strongly interacting regime, the buildup of correlations leads to the formation of extended pairs bound purely by many-body effects, analogous to the phenomenon of Cooper pairing in the BCS regime of the Fermi gas. Through fast sweeps away from unitarity, we detail how the correlation growth and formation of bound pairs emerge in the fraction of unbound atoms remaining after the sweep, finding quantitative agreement with experiment. We comment on the possible role of higher-order effects in explaining the deviation of our theoretical results from experiment for slower sweeps and longer times spent in the unitary regime.
As the quenched unitary Bose gas evolves, the buildup of correlations leads to the formation of extended pairs bound purely by many-body effects, analogous to the phenomenon of Cooper pairing in the BCS regime of the Fermi gas. We study how correlation growth, bound pairs, and three-body losses emerge in the fraction of unbound atoms remaining post sweep, finding quantitative agreement with experiment and speculate on discrepancies. We also highlight more recent efforts to study effects of higher-order correlations in the many-body dynamics, including the Efimov effect. NWO Grant 680-47-623.
We study the growth of two- and three-body correlations in an ultracold Bose gas quenched to unitarity. This is encoded in the dynamics of the two- and three-body contacts analyzed in this work. Via a set of relations connecting many-body correlation dynamics with few-body models, signatures of the Efimov effect are mapped out as a function of evolution time at unitarity over a range of atomic densities nnn. For the thermal resonantly interacting Bose gas, we find that atom bunching leads to an enhanced growth of few-body correlations. These atom-bunching effects also highlight the interplay between few-body correlations that occurs before genuine many-body effects enter on Fermi timescales.
We study the impact of three-body physics in quenched unitary Bose gases, focusing on the role of the Efimov effect. Using a local density model, we solve the three-body problem and determine three-body decay rates at unitarity, finding density-dependent, log-periodic Efimov oscillations, violating the expected continuous scale invariance in the system. We find that the breakdown of continuous scale invariance, due to Efimov physics, manifests also in the earliest stages of evolution after the interaction quench to unitarity, where we find the growth of a substantial population of Efimov states for densities in which the interparticle distance is comparable to the size of an Efimov state. This agrees with the early-time dynamical growth of three-body correlations at unitarity [Colussi et al., Phys. Rev. Lett. 120, 100401 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.100401]. By varying the sweep rate away from unitarity, we also find a departure from the usual Landau-Zener analysis for state transfer when the system is allowed to evolve at unitarity and develop correlations.
We study the structure of a Bose-condensed gas after quenching interactions to unitarity. Using the method of cumulants, we decompose the evolving gas in terms of clusters. Within the quantum depletion we observe the emergence of two-body clusters bound purely by many-body effects, scaling continuously with the atomic density. As the unitary Bose gas forms, three-body Efimov clusters are first localized and then sequentially absorbed into the embedded atom-molecule scattering continuum of the surrounding depletion. These results highlight the interplay of quantum depletion and evolving scaling laws in the formation of the unitary Bose gas.
The quenched unitary Bose gas is a paradigmatic example of a strongly interacting out-of-equilibrium quantum system, whose dynamics become difficult to describe theoretically due to the growth of non-Gaussian quantum correlations. We develop a conserving many-body theory capable of capturing these effects, allowing us to model the postquench dynamics in the previously inaccessible time regime where the gas departs from the universal prethermal stage. Our results show that this departure is driven by the growth of strong lossless three-body correlations, rather than atomic losses, thus framing the heating of the gas in this regime as a fully coherent phenomenon. We uncover the specific few-body scattering processes that affect this heating and show that the expected connection between the two-body and three-body contacts and the tail of the momentum distribution is obscured following the prethermal stage, explaining the absence of this connection in experiments. Our general framework, which reframes the dynamics of unitary quantum systems in terms of explicit connections to microscopic physics, can be broadly applied to any quantum system containing strong few-body correlations. Published by the American Physical Society 2024
Ultracold gases have become versatile systems to study quantum many-body effects. Their high degree of control and the tunability of the atomic interactions led to important advances in the understanding of strongly correlated matter. In the regime of strong interactions, unlike fermions, the study of bosons is hampered by three-particle recombination that leads to atom losses and keeps the system from reaching a real equilibrium state. In this thesis, we have performed the first quantitative comparison between theory and experiment on the three-particle loss coefficient L_3(a,T), for arbitrary scattering length a and temperature T. For unitary two-particle scattering (|a| → ∞), we show that the three-particle loss coefficient follows the law L_3(∞, T ) = λ_3/T^2, that we have tested using a trapped non-degenerate ^{7}Li gas maintained at constant temperature. The measured value of λ_3 = 2.5(3)_{stat}(6)_{cyst} × 10^{−20}(μK)^2cm^6s^{−1} is, within the error bars in good agreement with the theory prediction λ_3^{th} = 1.52 × 10^{−20}(μK)^2cm^6s^{−1}. We have extended our measurements to arbitrary values of the scattering length a. For a < 0, the theory predicts a smooth connection between the previously derived zero-temperature model for L_3(a, 0) and the unitarity limited loss coefficient L_3(∞, T ). We also show that a second Efimov resonance in 7Li should be observable near a = − 500 a0 for a temperature of 1 μK. Finally, we compare our theoretical prediction with measurements performed at Innsbruck with ^{133}Cs and at Cambridge with ^{39}K. The theory also shows remarkable quantitative agreement with these measurements.