Observation of Free-Space Single-Atom Matter Wave Interference

We observe matter wave interference of a single cesium atom in free fall. The interferometer is an absolute sensor of acceleration and we show that this technique is sensitive to forces at the level of $3.2\times {10}^{-27}\mathrm{N}$ with a spatial resolution at the micron scale. We observe the build up of the interference pattern one atom at a time in a free-space interferometer where the mean path separation extends far beyond the coherence length of the atom. Using the coherence length of the atom wave packet as a metric, we directly probe the velocity distribution and measure the temperature of a single atom in free fall.

Figure 1

Diagram depicting the apparatus for observing a single-atom interferometer. A single atom is trapped in an optical tweezer. The florescence from the atom is coupled to an avalanche photodiode (APD) for detection, showing (bottom left) the two discrete levels of photon counts that are characteristic of collisionally blockaded loading of single atoms into an optical tweezer. A wave packet trajectory is shown for an atom in free fall under the influence of gravity and a light pulse atom interferometer sequence.

Figure 2

Emergence of the interference fringe for $T=74.5\mu \mathrm{s}$. The plots show the cumulative number of Cs atoms (per phase) detected in $|F=3⟩$ after $N$ independent single atom experiments for (a) $N=1$, (b) $N=2$, (c) $N=17$, and (d) $N=813$. Note individual scaling of vertical axes. To generate the fringe, the phase of the atom is mapped out by scanning the phase of the Raman coupling field after the first $\pi /2$ pulse.

Figure 3

Atom interferometer phase evolution as a function of interrogation time for two cases: $k_{\mathrm{eff}}$ pointed along (upper curve) and opposite (lower curve) the direction of gravity. The measured phase shift (blue squares) is determined at each interrogation time by measuring the interferometric fringe, as in Fig. 2. The points are fit (blue line) to determine the acceleration of the atom due to gravity. The inset shows a two-sample Allan deviation of the force resolution as a function of the number of measurements for $T=363.9\mu \mathrm{s}$.

Figure 4

Measuring atom temperature by probing coherence length. The inset shows a $\pi /2–\pi –\pi /2$ sequence whose temporal symmetry is broken by the addition of a delay time $\delta T$ before the final pulse. The contrast of the interference fringe (squares) is measured for each $\delta T$ and fit (solid line) to the convolution of two Gaussians to give the wave packet coherence length.