Fiber-optic quantum interface with an array of more than 100 individually addressable atoms on an optical nanofiber

Abstract

Integrating the scalability of individually addressable arrays of optical-tweezer-trapped single atoms with the efficient light-matter interface provided by nanophotonic waveguides has been a long-standing challenge in quantum technologies based on atoms and photons. Here we realize a quantum interface between photons guided in an optical nanofiber with a diameter of 310 nm and an array of on average 155 individually addressable atoms. Using a spatial light modulator and an objective lens with NA = 0.45, single cesium atoms are trapped in a one-dimensional array of 200 optical tweezer spots with micrometer-scale trap sizes on the nanofiber. Individual atoms are addressed by spatially scanning an excitation laser beam, focused to a spot size comparable to that of the traps through the same objective lens, along the nanofiber. We confirm the single-atom nature of the individual trapping sites through photon-correlation measurements of the guided fluorescence, observing strong photon antibunching with $g^{(2)}(0) \approx 0.26$. We measure trap lifetimes of a few hundred milliseconds, with a maximum value of 460 ms, at an atom-surface separation of 670 nm without active cooling, representing an order-of-magnitude improvement over previous nanofiber traps. This platform opens a new regime for atom-photon interfaces, paving the way for scalable distributed quantum computing and quantum networks, as well as for the exploration of collective radiative effects in waveguide QED with individually addressable atoms.

Figures (7)

• Figure 1: Experimental setup for nanofiber-integrated optical tweezer array.a, Schematic of the optical arrangement. A spatial light modulator (SLM) generates the tweezer array, while a galvanometer scanner (GS) steers the excitation beam. Both are focused onto the optical nanofiber (ONF) via a high-NA objective lens (OL). The ONF is connected to a fiber detection setup with 99:1 fiber beam splitters (FBSs) and superconducting single-photon detectors (SSPDs). b, Magnified view of the trapping geometry. Fluorescence coupled into the guided mode is detected by the SSPDs. c, Reflection image of the optical tweezer array (shown in segments). The 200 sites exhibit a uniform $5$-$\mu\text{m}$ pitch and high intensity uniformity. d, Calculated distribution of the optical intensity, assuming a trapping laser ($\lambda = 935$ nm) focused to a beam waist of 1.0 $\mu$m. The trap laser forms a standing wave with its reflection from the ONF. e, Line profile of the trapping potential along the tweezer beam axis, given by the sum of the optical dipole potential and the attractive surface potential (see the Supplementary Note 1 for details).
• Figure 2: Site-resolved fluorescence traces from atoms trapped in 200 tweezer array.a, Photon detection traces recorded while linearly scanning the excitation laser along the optical tweezer array. Each row corresponds to a single scan, and 1000 repetitions are shown. Blue dots indicate photon detection events within 50 $\mu$s time bins. The data show that single atoms trapped at individual sites can be clearly detected within a single scan. b, Averaged photon counts per time bin obtained from the 1000 traces shown in a (blue solid line), together with control measurements taken in the absence of the MOT (black dotted line). The observation of 200 distinct peaks, corresponding to the sequential addressing of each tweezer site, is consistent with atom trapping across all 200 sites and demonstrates the capability for individual addressing over the entire array.
• Figure 3: Verification of single-atom trapping and estimation of filling factor.a, Second-order photon correlation function, $g^{(2)}(\tau)$, integrated over the array and 31000 scans. The blue line represents the experimental results, and the black line represents the theoretical calculation (see Supplementary Note 2). The observed antibunching $g^{(2)}(0) < 0.5$ confirms the single-atom nature of each trapped emitter. b-e, Histograms of photon counts recorded for individual trap sites. For b, the data aggregates the counts obtained from all 200 sites across 31000 measurement repetitions, while c-e are from 1000 repetitions. The panels correspond to the following experimental conditions: with optical tweezers and MOT (b); with optical tweezers but without MOT (c); without optical tweezers but with MOT (d); and without optical tweezers or MOT (e). The clear separation of the signal distribution in b from the control measurements (c-e) allows for the determination of the filling factor (loading efficiency), yielding a total estimated atom number of $N \approx 155$.
• Figure 4: Transmission measurements through the nanofiber.a, Optical depth spectrum measured via the nanofiber guided mode. The peak at the atomic resonance frequency in free space verifies the magic-wavelength operation of the 935-nm trapping light. b, Time evolution of the optical depth (OD) at the atomic resonance. The OD is directly proportional to the number of trapped atoms, $N$, serving as a probe for the trap lifetime. The data points represent the measured OD at the atomic resonance after a variable hold time in the optical tweezers with cooling beams extinguished. The solid line indicates a single-exponential fit to the data, yielding a $1/e$ lifetime of $\tau = 0.26~\text{s}$. This duration significantly exceeds the lifetimes typically achieved in conventional nanofiber-based trapping schemes, demonstrating the robust stability of the integrated platform. c, The observed results with the maximum value for the lifetime, $\tau = 0.46~\text{s}$, with 100 tweezer sites for another nanofiber with a diameter of 280 nm.
• Figure 5: Numerical analysis of the optical trapping potential and coupling efficiency. In all panels, the black filled areas (circles in a, b and shaded regions in c, d) indicate the optical nanofiber (diameter: 310 nm). a, Calculated distribution of the optical intensity, assuming a trapping laser ($\lambda = 935$ nm) focused to a beam waist of 1.0 $\mu$m. b, Line profiles of the optical potential (red solid line), vdW potential (black dotted line), and the total potential (blue dashed line). For the optical potential, we assume the trap beam power of $P = 1.5$ mW. For the vdW potential, we use the $C_3$ coefficient corresponding to a Cs metallic surfaceDerevianko1999, taking into account the adsorption of Cs atoms on the nanofiber surface. The first minimum of the optical potential at 190 nm from the surface is strongly affected by the vdW potential, resulting in the reduction of the effective trap depth to $V_1$, where as that for the second minimum at 671 nm from the surface is negligible. c, Calculated distribution of the coupling efficiency $\beta$ between the Cs atom and the nanofiber. d, Line profile of the coupling efficiency along the $z$-axis. The dashed lines indicate the position of the first and second trap sites.