Observation of Phase Defects in Quasi-Two-Dimensional Bose-Einstein Condensates

We have observed phase defects in quasi-2D Bose-Einstein condensates close to the condensation temperature. Either a single or several equally spaced condensates are produced by selectively evaporating the sites of a 1D optical lattice. When several clouds are released from the lattice and allowed to overlap, dislocation lines in the interference patterns reveal nontrivial phase defects.

Figure 1

An array of individually addressable quasi-2D BECs. (a) A 1D optical lattice splits a cigar-shaped 3D condensate into 15–30 independent quasi-2D BECs. (b) A magnetic field gradient along the lattice axis allows us to selectively address the sites by an rf field. We evaporate the atoms from all the sites except those within a frequency gap $\Delta$. (c) Steps in the BEC atom number $N_{\mathrm{at}}$ as a function of $\Delta$, corresponding to 0, 1, and 2 sites spared from evaporation. Each data point represents a single measurement.

Figure 2

Phase defects in quasi-2D condensates. Interference of four (a)–(c) and seven (d) independent BECs is observed 12 ms after release from the $2.7\mu \mathrm{m}$ period lattice. Dislocation lines in the interference patterns (b)–(d) reveal the presence of phase defects in quasi-2D condensates.

Figure 3

Examples of numerical simulations of two (a) and four (b) interfering condensates. In both cases one randomly chosen BEC has a phase factor $e^{i\phi }$ corresponding to a centered vortex, and the others have randomly chosen uniform phases. For simplicity, we model the clouds as Gaussian wave packets and neglect interactions during the expansion. The images are convolved with a Gaussian of $4\mu \mathrm{m}$ rms width to simulate the finite imaging resolution.