Imaging and Localizing Individual Atoms Interfaced with a Nanophotonic Waveguide

Single particle-resolved fluorescence imaging is an enabling technology in cold-atom physics. However, so far, this technique has not been available for nanophotonic atom-light interfaces. Here, we image single atoms that are trapped and optically interfaced using an optical nanofiber. Near-resonant light is scattered off the atoms and imaged while counteracting heating mechanisms via degenerate Raman cooling. We detect trapped atoms within 150 ms and record image sequences of given atoms. Building on our technique, we perform two experiments which are conditioned on the number and position of the nanofiber-trapped atoms. We measure the transmission of nanofiber-guided resonant light and verify its exponential scaling in the few-atom limit, in accordance with Beer-Lambert’s law. Moreover, depending on the interatomic distance, we observe interference of the fields that two simultaneously trapped atoms emit into the nanofiber. The demonstrated technique enables postselection and possible feedback schemes and thereby opens the road toward a new generation of experiments in quantum nanophotonics.

Figure 1

Experimental scheme and atom imaging. (a) A few cesium atoms are trapped $\sim 270\mathrm{nm}$ away from the surface of an optical nanofiber. They are exposed to a near-resonant excitation laser field that freely propagates along the $+y$ direction. The atoms scatter this light and are imaged using a camera. A fraction of the scattered photons also couples to the nanofiber-guided mode. The atoms can also be excited or probed with light launched into the nanofiber. The light exiting the nanofiber can be detected with a single photon counting module (SPCM). (b) Typical raw images of two, one, and zero atoms for an exposure time of 150 ms. (c) Counts summed over regions of $3\times 3\mathrm{pixels}$, centered on the atoms shown in (b) as a function of time. A 45-ms waiting time between consecutive images is reserved. The count rates drop to the background level when the atoms are lost from the trap after 0.975 s (left atom) and 1.95 s (right atom). See Supplemental Material [31] for the full series of images underlying the plot.

Figure 2

Resonant transmission measurement with single-atom sensitivity. (a)–(d) show histograms of the number of detected photons transmitted through the nanofiber for zero to three nanofiber-coupled atoms. Each atom absorbs $\sim 4\%$ of the guided light and, thus, the peak in the histogram shifts to the left with every additional atom. The integration time for each histogram is 90 ms, see main text. The count distributions are approximately Gaussian (see dashed lines for a fit).

Figure 3

Excitation of the nanofiber-guided mode by scattering light on nanofiber-coupled atoms. The atoms are excited with a free-space laser beam, see Fig. 1. A fraction of the light scattered on the atoms enters the guided mode of the nanofiber. Histograms of the number of photons detected with the SPCM conditioned on having zero (a), one (b), and two (c) atoms detected in images taken with the camera. Panel (a) shows a peak of the SPCM count distribution for zero atoms, indicating the background level. In (b), a peak that is shifted to larger values by about 35 counts compared to (a) is visible. This indicates that we can detect the nanofiber-coupled emission of single atoms. When two atoms emit into the nanofiber mode [in (c)], the respective light fields interfere with each other. As confirmed by the histogram data, the counts vary between the background level for perfectly destructive interference (vertical dashed line on the left) to four times the single-atom signal for constructive interference (line on the right). The black, red, and cyan lines show model predictions, also accounting for experimental imperfections. (d) Power spectral density (PSD) of the Fourier spectrum of the interference pattern deduced from the counts detected with the SPCM as a function of atom-atom distance determined using camera images. A clear peak at a spatial frequency of $0.269(3)\mu {\mathrm{m}}^{-1}$ is observed, consistent with our experimental setting. Parameters: Imaging duration 150 ms; detuning of excitation light from the $D2$ cycling transition $-3\mathrm{\Gamma }$.