Session B2: Focus Session: Quench Dynamics and Defect Formation in Ultracold Atoms

Dynamics of Symmetry Breaking Phase Transitions

In the course of a non-equilibrium continuous phase transition dynamics ceases to be adiabatic in the vicinity of the critical point as a result of the critical slowing down. Consequently, the choice of the broken symmetry has to be made locally - sonic horizon set by the speed of the relevant sound. The resulting disparate local choices of broken symmetry lead to excitations and often result in the topological defects. The Kibble-Zurek mechanism (KZM) was developed to capture the essence of the associated non-equilibrium dynamics and to estimate the density of defects as a function of the quench rate through the transition. I will review and analyze the KZM focusing in particular on BEC's. \\[4pt] del Campo, Adolfo; Zurek, Wojciech H. INTERNATIONAL JOURNAL OF MODERN PHYSICS A Volume: 29 Issue: 8 Article Number: 1430018 Published: MAR 30 2014   

Formation of topological defects in a two-dimensional Bose gas through quench cooling

We have shown experimentally that quench cooling of a Bose gas in a quasi two-dimensional geometry leads to the formation of topological defects. We trapped atoms in an original geometry with a strong confinement in the vertical direction and a flat-bottom in-plane potential. The in-plane confinement is realized by an optical dipole potential created by imaging a dark pattern onto the atomic plane whose shape can be easily changed. In a uniform square potential we demonstrated the creation of quantized vortices and for atoms confined in a ring we observed the creation of quantized currents along the ring. For both cases we investigated the statistics of this stochastic creation of defects as a function of the quench cooling time. We found results compatible with the prediction of Kibble-Zurek mechanism for Bose-Einstein condensation. In this specific geometry we also demonstrated that the transition crossed during the quench cooling is described by a transverse condensation mechanism.   

Bound-state effects in diabatically quenched BECs

We investigate the dynamics of a uniform Bose-Einstein condensate following a sudden quench of the scattering length. Our focus is the time evolution of short-range correlations via the dynamical contact. We compute the dynamics using a combination of two- and many-body models, and we propose a connection between them that unifies their short-time, short-range predictions. Our two-body models are exactly solvable and, when properly calibrated, lead to analytic formulas for the contact dynamics. We observe high-contrast oscillations of the contact, and their time average is typically much larger than the Bogoliubov prediction. The condensate fraction shows similar oscillations, whose amplitude we are able to estimate. Such pronounced effects originate from the Feshbach-molecular bound state, often ignored in BEC quench calculations.   

Critical dynamics of spontaneous symmetry breaking in a homogeneous Bose gas

Kibble-Zurek theory models the dynamics of spontaneous symmetry breaking, which plays an important role in a wide variety of physical contexts, ranging from cosmology to superconductors. We explored these dynamics in a homogeneous system by thermally quenching an atomic gas with short-range interactions through the Bose-Einstein phase transition. Using homodyne matter-wave interferometry to measure first-order correlation functions, we verified the central quantitative prediction of the Kibble-Zurek theory, namely the homogeneous-system power-law scaling of the coherence length with the quench rate. Moreover, we directly confirmed its underlying hypothesis, the freezing of the correlation length near the transition. Our measurements agree with a beyond-mean-field theory and support the expectation that the dynamical critical exponent for this universality class is z=3/2.   

Quantum Quench Dynamics of a Fermi Impurity

We investigate the dynamics of a $^{40}$K impurity in a $^{6}$Li Fermi sea after a quench into the strongly interacting regime. Using atom interferometry, we observe the formation dynamics of both the attractive and the repulsive polarons. For resonant $^{40}$K-$^{6}$Li interactions, we observe quantum beats due to a simultaneous excitation of the upper and lower branches of the interacting many-body system.   

Prethermalized states of quenched spinor condensates

Due to the interplay between spin and charge degrees of freedom, spinor Bose condensates exhibit a rich tapestry of magnetically ordered phases and topological defects. The non-equilibrium properties of these fluids have been the topic of recent interest. We have previously shown that quenched spinor condensates exhibit robust prethermalized states characterized by asymptotic correlations that differ from thermodynamic predictions [1]. These non-equilibrium states arise due to the disparate energy scales between the phonon and magnon excitations. The identification of a microscopic origin of prethermalization makes this system a promising platform for studies of prethermalization and possible universal scaling relations that characterize these nonequilibrium many-body states. We elaborate on our studies of prethermalized spinor condensates and the prospects of observing a dynamical Kosterlitz-Thouless transition in this system.\\[4pt] [1] R. Barnett, A. Polkovnikov, and M. Vengalattore, Phys. Rev. A 84, 023606 (2011)   

Interaction Quench Induced Multimode Dynamics of Finite Atomic Ensembles

The correlated non-equilibrium dynamics of few-boson systems in one-dimensional finite lattices is investigated. Starting from weak interactions we perform a sudden interaction quench and employ the numerically exact Multi-Layer Multi-Configuration time-dependent Hartree method for bosons to obtain the resulting quantum dynamics. Focusing on the low-lying modes of the finite lattice we observe the emergence of density-wave tunneling, breathing and cradle-like processes. In particular, the tunneling induced by the quench leads to a ``global'' density-wave oscillation. The resulting breathing and cradle modes are inherent to the local intrawell dynamics and connected to excited-band states [1]. Moreover, the interaction quenches couple the density-wave and the cradle modes allowing for resonance phenomena. These are associated with an avoided-crossing in the respective frequency spectrum and lead to a beating dynamics for the cradle. \\[4pt] [1] S. I. Mistakidis, L. Cao, and P. Schmelcher, J. Phys. B: At. Mol. Opt. Phys. 47 225303 (2014).