Limits to the flux of a continuous atom laser

We present experimental results demonstrating that the flux of a continuous atom laser beam cannot be arbitrarily increased by increasing the strength of output coupling. We find that the continuous atom laser has large density fluctuations when the Rabi frequency becomes greater than the radial trapping frequency, due to the output coupling populating all accessible Zeeman states. In addition, we find that a state-changing output coupler for a continuous atom laser switches off as the output-coupling strength, parametrized in general by the Rabi frequency, is increased above a critical value. The numerical solution of a one-dimensional five-state Gross-Pitaevskii model for the atom laser is in good qualitative agreement with the experiment.

Figure 1

Optical depth plot (35 independent experiments) showing the spatial structure of a $3\text{−}\mathrm{ms}$ atom laser as a function of output-coupling strength. The system was left to evolve for a further $4\mathrm{ms}$ before the trap was switched off and $2\mathrm{ms}$ later the images were acquired. The field of view for each individual image is $0.7\times 0.3\mathrm{mm}$. The final image is taken with $\Omega =16\mathrm{kHz}$. Antitrapped Zeeman states are clearly visible in (a), (b), and (c), which show an extended view $(2.7\times 0.3\mathrm{mm})$ of the data in the main part figure (represented by the dashed box).

Figure 2

Outcoupled fraction in the Zeeman states of the $F=2$ single pulse $\left(50\mu \mathrm{s}\right)$ atom laser as a function of angular Rabi frequency. Solid lines: theoretical calculation with no free parameters from a 3D GP model of the five-state $F=2$ atom laser.

Figure 3

Atom number as a function of angular Rabi frequency for a $10\text{−}\mathrm{ms}$ atom laser. The upper plot shows the raw optical depth data averaged over six identical experiments. The field of view for each image is $1.5\mathrm{mm}$ by $0.5\mathrm{mm}$.

Figure 4

Trapped fraction as a function of angular Rabi frequency for a $100\text{−}\mathrm{ms}$ atom laser (open circles). Data for a $200\text{−}\mathrm{ms}$ output coupler are shown for comparison (black dots). The inset, on the same scale, shows the detail of low Rabi frequency behavior.

Figure 5

Theoretical results obtained by solution of the 1D-GP equation. The atomic population in the Zeeman states as a function of time for a $15\text{−}\mathrm{ms}$ atom laser for (a) $\Omega =800\mathrm{Hz}$ and (b) $\Omega =16\mathrm{kHz}$. In (c) we show the behavior of a pure two-state atom laser system for $\Omega =16\mathrm{kHz}$. (d) The total atomic population remaining in the magnetic trap after $100\mathrm{ms}$ of output coupling plotted as a function of angular Rabi frequency.