Decoherence-Driven Cooling of a Degenerate Spinor Bose Gas

We investigate the relationship between the coherence of a partially Bose-condensed spinor gas and its temperature. We observe cooling of the normal component driven by decoherence as well as the effect of temperature on decoherence rates.

Figure 1

(a) Temperature of the normal component, which is initially in a superposition of states, as a function of time. The temperature initially decreases as the condensate contributes atoms with nearly zero energy to the normal component. At later times, the temperature increases due to heating from inelastic collisions. The solid line is a model as explained in the text. The single error bar is representative of the statistical error on all points. (b) The apparent phase-space density (PSD) is shown as a function of time. The open circles are the PSD of the normal component in a single state, whereas the solid circles represent the PSD of the normal component in an initially fully coherent superposition of states. The superposition-state PSD approaches $2\times 2.61$ in a time scale of 21(3) ms. Each point is a weighted average of six independent measurements. The standard deviations of the six measurements are equal to or smaller than the point size. The dashed lines are at PSDs of 2.61 and 5.22. (c) Magnitude of the spin vector as a function of time for the normal component initially in a fully coherent superposition of states. The entire spin vector is reconstructed from measurements of the longitudinal and transverse spin components similar to those described in Ref. 13. The coherence time of the normal component is 3.9(4) ms. The offset in the value of the spin vector from zero at long times is due to imaging and shot-to-shot number noise.

Figure 2

Coherence times of the condensate region (○) and the normal component (●) as a function of $T/T_C$. The condensate and normal component regions are defined using the Thomas-Fermi radius, $R_{\mathrm{T}\mathrm{F}}$. The rectangular regions in the sketch of the expanded density profile (not to scale) indicate the regions over which the local coherence of the normal and of the condensate components are probed. The axial (horizontal) extent of the rectangles is $4\mu \mathrm{m}$.