Cavity QED detection of interfering matter waves

We observe the build-up of a matter wave interference pattern from single atom detection events in a double-slit experiment. The interference arises from two overlapping atom laser beams extracted from a rubidium Bose-Einstein condensate. Our detector is a high-finesse optical cavity which realizes a quantum measurement of the presence of an atom and thereby projects delocalized atoms into a state with zero or one atom in the resonator. The experiment reveals simultaneously the granular and the wave nature of matter. We present a setup which is suited for applications in atom interferometry and cavity QED.

Figure 1

(Color online) Schematic of the experimental setup. From two well defined regions in a Bose-Einstein condensate (BEC), we couple atoms to an untrapped state. The real parts of the resulting atom laser wave functions are sketched on the right-hand side. The absorption image shows an interference pattern corresponding to $\Delta f=1\mathrm{kHz}$ and an atom flux $\sim {10}^6$ times larger than in the actual single atom interference experiment. Monitoring the transmission of a probe laser through a high-finesse optical cavity with a photon counter, single atom transits are detected.

Figure 2

(Color online) (a) Cavity single atom detection principle. An atom detunes the high-finesse cavity from resonance and the cavity transmission consequently drops. (b) Photon flux through the high-finesse optical cavity when an atom is detected. The photon count rate is averaged over $20\mu \mathrm{s}$. The detection threshold is set to be four times the standard deviation of the photon shot noise (dashed line).

Figure 3

(Color online) (a) Normalized transmission as a function of coupling strength. The solid line corresponds to our probe strength of five photons in the cavity in the absence of an atom. The dashed line is the weak probe limit. The dotted line corresponds to ten photons in the cavity. (b) Coherence between the states with one and no atom as a function of time. The initial coherence is normalized to 1. Solid line: $g=2\pi \times 10\mathrm{MHz}$. Dashed line: $g=2\pi \times 6.5\mathrm{MHz}$. Dotted line: $g=2\pi \times 3\mathrm{MHz}$.

Figure 4

(Color online) Histograms of the atoms detected in $5\mathrm{ms}$ time intervals. (a) Single experimental run. (b) Sum of four runs. (c) Sum of 16 runs. (d) Sum of 191 runs, the line is a sinusoidal fit. Please note the different scales.