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arXiv:1008.4428 · Physical Review A

Localized collapse and revival of coherence in an ultracold Bose gas

J. M. McGuirk, Lydia Neary‐Zajiczek

Published Jan 28, 2011• 4 citations
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Abstract

We study the collapse and revival of coherence induced by dipolar spin waves in a trapped gas of $^{87}\mathrm{Rb}$ atoms. In particular, we observe spatially localized collapse and revival of Ramsey fringe contrast and show how the pattern of coherence depends on the strength of the spin-wave excitation. We find that the spatial character of the coherence dynamics is incompatible with a simple model based only on position-space overlap of wave functions. We show that this phenomenon requires a full phase-space description of the atomic spin using a quantum Boltzmann transport equation, which highlights spin-wave-induced coherent spin currents and the ensuing dynamics they drive.

DOI
10.1103/physreva.83.013625

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We study the collapse and revival of coherence induced by dipolar spin waves in a trapped gas of 87Rb^{87}\mathrm{Rb}87Rb atoms. In particular, we observe spatially localized collapse and revival of Ramsey fringe contrast and show how the pattern of coherence depends on the strength of the spin-wave excitation. We find that the spatial character of the coherence dynamics is incompatible with a simple model based only on position-space overlap of wave functions. We show that this phenomenon requires a full phase-space description of the atomic spin using a quantum Boltzmann transport equation, which highlights spin-wave-induced coherent spin currents and the ensuing dynamics they drive.
Atomic four-wave mixing (FWM) is an important nonlinear process in ultracold physics. Despite numerous works on atomic FWM, the phase coherence effect has not been extensively studied. Here, by preparing multiple spin-momentum modes with ultracold 87Rb^{87}\mathrm{Rb}87Rb atoms in a crossed dipole trap and applying an external magnetic field to regulate the atomic motions and interactions, we extensively investigate how the coherence fluctuation affects the atomic FWM process. We observe that the atomic motion and spin-exchange collision involve a periodic collapse and revival of the yield fraction of the atomic FWM. The oscillation period is mainly determined by the spin-exchange rate and thus the atomic scattering cross section. Our work demonstrates that atomic FWM can be used to probe the coherence and the atomic interactions, which should benefit various areas related to multicomponent atomic interactions and spin dynamics in the future.
We generate spin currents in an 87^{87}87Rb spin-2 Bose-Einstein condensate by application of a magnetic field gradient. The spin current destroys the spin polarization, leading to a sudden onset of two-body collisions. In addition, the spin coherence, as measured by the fringe contrast using Ramsey interferometry, is reduced drastically but experiences a weak revival due to in-trap oscillations. The spin current can be controlled using periodic πππ pulses (bang-bang control), producing longer spin coherence times. Our results show that spin coherence can be maintained even in the presence of spin currents, with applications to quantum sensing in noisy environments.
In this review we discuss the dynamics of the phase of trapped Bose–Einstein condensates. In particular we consider the phenomena of phase decoherence (termed also as phase collapse, or diffusion), and phase revival in systems of interacting atoms. We analyse the dependence of the collapse and revival times on the trap potential, dimensionality of the gas, atom number fluctuations, and on the coherent dynamics of the condensate. We show that in a class of experimentally relevant systems, the collapse time is relatively short, and in some cases vanishes in the limit of a large number of atoms, implying that the trapped Bose gas cannot sustain a well-defined quantum phase, and that the phase memory is lost on a relatively short time scale. Furthermore, we calculate the relative atom number fluctuations or a model of two interacting condensates, and show that the fluctuations are generically sub-Poissonian.
We demonstrate a technique for exciting spin waves in an ultracold gas of 87 Rb atoms on the basis of tunable ac Stark potentials. This technique allows us to excite normal modes of spin waves with arbitrary amplitudes in the trapped gas, including dipole, quadrupole, octupole and hexadecapole modes. These modes exhibit strong nonlinearities, which manifest as amplitude dependence of the excitation frequencies and departure from sinusoidal behavior. Our results are in good agreement with a full treatment of a quantum Boltzmann transport equation.
We generate spin currents in an 87Rb^{87}\mathrm{Rb}87Rb spin-2 Bose-Einstein condensate by application of a magnetic field gradient. The spin current destroys the spin polarization, leading to a sudden onset of two-body collisions. In addition, the spin coherence, as measured by the fringe contrast using Ramsey interferometry, is reduced drastically but experiences a weak revival due to in-trap oscillations. The spin current can be controlled using periodic $\ensuremath{\pi}$ pulses (bang-bang control), producing longer spin-coherence times. Our results show that spin coherence can be maintained even in the presence of spin currents, with applications to quantum sensing in noisy environments.
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.