Long storage time of collective coherence in an optically trapped Bose-Einstein condensate

Our recent work has demonstrated the superradiant writing and reading of collective coherence in a Bose-Einstein condensate [Phys. Rev. Lett. 99, 220407 (2007)]. This time we report the drastic improvement of the storage time realized by loading the atoms into an optical dipole trap, wherein an unfavorable spin-dependent phase shift and spatial diffusion of atoms can be suppressed. The measured storage time was $0.57\left(2\right)\mathrm{ms}$, which is limited by the temporal variation of atomic momentum due to harmonic oscillations in the trap.

Figure 1

(Color online) (a) Geometry and (b) energy-level diagram of the experiment. $L1$, $L2$, achromatic lens; DM, dichroic mirror; IF, interference filter; $\lambda ∕4$, quarter-wave plate; PBS, polarization beam splitter; and PMT, photomultiplier tube.

Figure 2

(Color online) (a) Example of an experimental timing and observed wave forms of the superradiant Raman pulses. (b) Conversion efficiency versus interval time between the write and read beams. Open and solid circles represent the results taken in free space and in the optical trap, respectively. The solid lines are Gaussian fit to the data.

Figure 3

(Color online) (a) Trajectory of the grating vector $\mathit{q}\left(t\right)$ in an anisotropic harmonic potential. The initial grating vector $\mathit{q}\left(0\right)$ is assumed to be $(k_{\rho },k_z)=(k_0,-k_0)$ with $k_0=2\pi ∕\lambda$. The shaded region represents the tolerance of the phase-matching condition. (b) Dependence of the storage time on the radial trap frequency ${\omega }_{\rho }∕\left(2\pi \right)$. The solid line is the phase-matching time given by Eq. (3).