Coupling Single Atoms to a Nanophotonic Whispering-Gallery-Mode Resonator via Optical Guiding

Xinchao Zhou, Hikaru Tamura, Tzu-Han Chang, and Chen-Lung Hung

Phys. Rev. Lett. 130, 103601 – Published 7 March 2023

Abstract

We demonstrate an efficient optical guiding technique for coupling cold atoms in the near field of a planar nanophotonic circuit, and realize large atom-photon coupling to a whispering-gallery mode in a microring resonator with a single-atom cooperativity $C ≳ 8$ . The guiding potential is created by diffracted light on a nanophotonic waveguide that smoothly connects to a dipole trap in the far field for atom guiding with subwavelength precision. We observe atom-induced transparency for light coupled to a microring, characterize the atom-photon coupling rate, extract guided atom flux, and demonstrate on-chip photon routing by single atoms. Our demonstration promises new applications with cold atoms on a nanophotonic circuit for chiral quantum optics and quantum technologies.

Received 2 November 2021
Revised 8 November 2022
Accepted 6 February 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.103601

Physics Subject Headings (PhySH)

Atoms, ions, & molecules in cavities Cavity quantum electrodynamics Hybrid quantum systems Nanophotonics

Atoms Optical microcavities

Atom & ion trapping & guiding Whispering gallery mode resonators

Atomic, Molecular & Optical

Authors & Affiliations

Xinchao Zhou¹, Hikaru Tamura ¹, Tzu-Han Chang ¹, and Chen-Lung Hung ^1,2,*

¹Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, USA
²Purdue Quantum Science and Engineering Institute, Purdue University, West Lafayette, Indiana 47907, USA

^*clhung@purdue.edu

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Issue

Vol. 130, Iss. 10 — 10 March 2023

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Images

Figure 1
Optical funnel on a nanophotonic microring circuit. (a) Schematic of the setup. An optical funnel is formed by a red-detuned, bottom-illuminating beam. WGMs in the microring are excited by a probe field copropagating with a blue-detuned beam. The latter forms a repulsive potential barrier to plug the optical funnel. Transmitted light is directed to a single photon counting module (SPCM) after wavelength filtering. (b) Cross section of the funnel potential $U (x, z)$ (left) and a zoom in view near the ${Si}_{3} N_{4}$ waveguide (right). Inset shows the potential without a barrier (unplugged funnel). Gravity is along the $- z$ direction. (c) Scanning electron micrograph of a microring (left), and an optical micrograph (right, same field of view) showing fluorescence from guided atoms (bounded by dashed box). Other bright spots are unfiltered scattered light from the waveguide. (d) Potential line cuts $U_{tot} (0, z)$ with (solid curve) and without (dashed curve) the repulsive barrier. $U_{tot} = U + U_{cp}$ includes the atom-surface Casimir-Polder potential $U_{cp}$ [44].
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Figure 2
Atom guiding in the optical funnel. Resonant transmission $T$ versus guiding time $Δ t$ in a plugged (circles) and unplugged (triangles) funnel, respectively. The experimental sequence is illustrated in the inset.
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Figure 3
Atom-WGM photon interaction. (a) Sample trajectories (solid curves) in the near field. False color map shows unpolarized atom-photon coupling strength $\bar{g}$ . (b) $\bar{g}$ versus time for sample trajectories; the blue dash-dotted curve is the mean. Black dotted curves in (a) and (b) show a typical case without the repulsive barrier. The time origin is aligned with the time to have the largest coupling strength for each trajectory. (c) Measured transmission $T$ versus laser detuning $Δ ν$ for unpolarized atoms with (blue circles) and without (gray triangles) the repulsive barrier, and $T_{0}$ for bare resonator without atoms. Solid blue (gray) curve is a single parameter fit using Eq. (1) and input from trajectory calculations as in (a) and (b) with (without) the repulsive barrier. Shaded band shows 95% pointwise confidence level. (d) Measured and fitted transmission $T$ versus laser detuning $Δ ν$ for polarized atoms in a plugged funnel. Insets in (c) and (d) illustrate the levels involved in the $F = 4 \leftrightarrow F^{'} = 5$ transition.
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Figure 4
Correlation measurement for the resonant transmission. (a) Cross-correlation $ξ_{12} (τ)$ of two detector counts in the Hanbury Brown–Twiss setup. Gray triangles (blue circles) show the data obtained with guided atoms in the unplugged (plugged) optical funnel, while black squares show the background $ξ_{12} \approx 1$ obtained without guided atoms. Black and blue lines are bidirectional exponential fits to the data. The inset provides an enlarged view in the range of $| τ | \leq 100 ns$ . During the detection of atom transits, normalized intensity correlation functions $g^{(2)} (τ)$ show clear antibunching for (b) unplugged and (c) plugged optical funnels, respectively. Solid lines are theoretical fits [44], giving effectively time-averaged single-atom cooperativities $C^{+} \approx 8$ and 3 for atom transits in (b) and (c), respectively. Black (blue) bars in (d) show the measured $g^{(2)} (0)$ as a function of guiding time $Δ t$ in the unplugged (plugged) optical funnel.
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