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arXiv:1401.6648 · New Journal of Physics

Collapse and revivals for systems of short-range phase coherence

Khan W. Mahmud, Lei Jiang, P R Johnson, Eite Tiesinga

Published Oct 8, 2014• 8 citations
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Abstract

We predict the existence of novel collapse and revival oscillations that are a distinctive signature of the short-range off-diagonal coherence. Starting with an atomic Mott state in a one-dimensional optical lattice, suddenly raising the lattice depth freezes particle–hole pairs in place and induces phase oscillations. The peak of the quasi-momentum distribution, revealed through time-of-flight interference, oscillates between a maximum occupation at zero quasi-momentum (the Γ point) and the edge of the Brillouin zone. We show that the population enhancements at the edge of the Brillouin zone are due to short-range coherence due to particle–hole pairs, and we find similar effects for fermions and Bose–Fermi mixtures in a lattice. Our results open a new dynamic probe for strongly correlated many-body states with short-range phase coherence that is distinct from the matter–wave collapse and revivals previously observed in the long-range coherent superfluid regime.

DOI
10.1088/1367-2630/16/10/103009

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We predict the existence of novel collapse and revival oscillations that are a distinctive signature of the short-range off-diagonal coherence. Starting with an atomic Mott state in a one-dimensional optical lattice, suddenly raising the lattice depth freezes particle–hole pairs in place and induces phase oscillations. The peak of the quasi-momentum distribution, revealed through time-of-flight interference, oscillates between a maximum occupation at zero quasi-momentum (the Γ point) and the edge of the Brillouin zone. We show that the population enhancements at the edge of the Brillouin zone are due to short-range coherence due to particle–hole pairs, and we find similar effects for fermions and Bose–Fermi mixtures in a lattice. Our results open a new dynamic probe for strongly correlated many-body states with short-range phase coherence that is distinct from the matter–wave collapse and revivals previously observed in the long-range coherent superfluid regime.
Quantum-phase collapse and revival are studied in a binary mixture of bosonic atomic species loaded in a one-dimensional optical lattice. Oscillations in the quasimomentum distribution are usually observed after a sudden quench of the lattice parameters from the superfluid regime to the Mott-insulator regime. The influence of one species dynamics on the collapse and revival oscillations in the quasimomentum distribution of the second species is examined. The nontrivial dependencies on the interspecies and intraspecies interactions are illustrated distinguishing between the zero-tunneling and nonzero-tunneling cases.
We study the quantum dynamics of cold Bose atoms in a double well. It is shown that self-trapping as well as population oscillations are common phenomena associated with nonlinear interactions. For larger U/tU/tU/t, multiparticle tunneling is damped and the quantum dynamics is dominated by single-particle tunneling. The many-body system can be effectively described in a truncated Fock space. It exhibits coherence-decoherence oscillations in the temporal evolution. We predict an interesting phenomenon of dynamical revival and collapse of matter wave fields in optical lattices in regimes near the superfluid--Mott-insulator phase boundary.
Quantum tunneling is one of the most fundamental processes in nature. Single particle hopping of ultracold atoms in optical lattices changes its character dramatically when the ensemble is prepared in strongly correlated quantum phases due to atom-atom interactions. Correlated hopping in a Mott-insulating chain of bosons that is tilted to the Mott gap has recently been employed to study long-range order in the 1D transvers Ising model [1,2]. We study correlated tunneling dynamics for an ensemble of tilted 1D Mott chains after a sudden quench to the vicinity of the Ising paramagnetic to antiferromagnetic phase transition point [3]. The quench results in coherent oscillations for the orientation of effective Ising spins, detected via oscillations in the number of doubly occupied lattice sites. We characterize the quench by varying the system parameters. We report significant modification of the tunneling rate induced by interactions and show clear evidence for collective effects in the oscillatory response. We observe higher-order many-body tunneling processes over up to five lattice sites when the tilt per site is tuned to integer fractions of the Mott gap. Secondand third-order tunneling shows up in the transient response after the quench, from which we extract the characteristic scaling in accordance with perturbation theory and numerical simulations. In a second set of experiments we study the response of an ensemble of 1D superfluids in the Bose-Hubbard regime when subject to a tilt [4]. For large values of the tilt, we observe interaction-induced coherent decay and matter-wave quantum phase revivals of the Bloch oscillating ensemble. We analyze the revival period dependence on interactions by means of a Feshbach resonance. When reducing the value of the tilt, we observe the disappearance of the quasi-periodic phase revival signature towards an irreversible decay of Bloch oscillations, indicating the transition from regular to quantum chaotic dynamics.
Motivated by the experimental observation of collapses and revivals of Bose matter wave field, we investigate correlation functions of cold bosons in an optical lattice. Within a simple model we examine two kinds of states: one that employs the commonly used notion of coherent states, and one that obeys the total number of atoms conservation. We identify rare situations at which these states behave differently. Typically, however, their predictions coincide and so: As a function of ``interaction time'' the interference pattern in the density undergoes collapse and revival. Exactly at revival times the system mimics the ideal gas case, in which all correlation functions factorize, while in the collapsed phase of the evolution the system effectively behaves as if initially there was no long-range coherence. Even in the latter case though, an interference pattern should be seen in a single experiment. We stress the role of column averaging, which in fact corresponds to an averaged observation of an ensemble of two-dimensional realizations. We also note that, contrary to the common belief, an interference pattern should also be seen in a single observation of a Mott state.
We study the dynamics of spin-1 atoms in a periodic optical-lattice potential and an external magnetic field in a quantum quench scenario where we start from a superfluid ground state in a shallow lattice potential and suddenly raise the lattice depth. The time evolution of the nonequilibrium state shows collective collapse-and-revival oscillations of matter-wave coherence as well as oscillations in the spin populations. We show that the complex pattern of these two types of oscillations reveals details about the superfluid and magnetic properties of the initial many-body ground state. Furthermore, we show that the strengths of the spin-dependent and spin-independent atom-atom interactions can be deduced from the observations. The Hamiltonian that describes the physics of the final deep lattice not only contains two-body interactions but also effective multibody interactions, which arise due to virtual excitations to higher bands. We derive these effective spin-dependent three-body interaction parameters for spin-1 atoms and describe how spin mixing is affected. Spinor atoms are unique in the sense that multibody interactions are directly evident in the in situ number densities in addition to the momentum distributions. We treat both antiferromagnetic (e.g., 23{}^{23}23Na) and ferromagnetic (e.g., 87{}^{87}87Rb and 41{}^{41}41K) condensates.
Recent experiments have shown a remarkable number of collapse and revival oscillations of the matter-wave coherence of ultracold atoms in optical lattices [Will et al., Nature (London) 465, 197 (2010)]. Using a mean-field approximation to the Bose-Hubbard model, we show that the visibility of collapse and revival interference patterns reveals number squeezing of the initial superfluid state. To describe the dynamics, we use an effective Hamiltonian that incorporates the intrinsic two-body and induced three-body interactions, and we analyze in detail the resulting complex pattern of collapse and revival frequencies generated by virtual transitions to higher bands, as a function of lattice parameters and mean-atom number. Our work shows that a combined analysis of both the multiband, nonstationary dynamics in the final deep lattice and the number squeezing of the initial superfluid state explains important characteristics of optical lattice collapse and revival physics. Finally, by treating the two- and three-body interaction strengths and the coefficients describing the initial superposition of number states as free parameters in a fit to the experimental data, it should be possible to go beyond some of the limitations of our model and obtain insight into the breakdown of the mean-field theory for the initial state or the role of nonperturbative effects in the final-state dynamics.