Deterministic coupling of ultracold atomic lattice to a suspended photonic waveguide

TL;DR

This work tackles the challenge of interfacing neutral-atom quantum systems with integrated nanophotonic circuitry by deterministically coupling a two-dimensional ultracold Cs lattice to light guided in suspended photonic crystal waveguides. The authors transport atoms from programmable optical tweezer lattices into the evanescent field of a 1-D APCW, achieving high delivery fidelity and enabling direct, in-situ probing of the evanescent field and waveguide geometry. Key results include deterministic atom delivery with survival $ ext{> }90 ext{%}$, spatially resolved mapping of the evanescent field with ~tens of nanometer precision, and a quantitative model based on the Lamb-Dicke parameter $\\eta \\approx 0.26$ and a decay length $L \\approx 743 \\mathrm{nm}$ that agrees with measurements. The findings establish a scalable quantum interface between ultracold atomic arrays and on-chip nanophotonics, with implications for fast readout, subwavelength interaction zones, and quantum sensing; future work aims to enhance coupling strength, extend to more complex photonic geometries, and explore multi-atom Green’s-function measurements and quantum networks.

Abstract

The deterministic control of light-matter interactions at the level of single particles and on subwavelength scales is central to quantum optics and hybrid integrated quantum technologies. However, combining cold atom research with nanophotonic devices in a fully controllable platform remains a major experimental challenge. Here, we demonstrate the deterministic coupling of an ultracold atomic lattice to light propagating in suspended on-chip photonic circuits. These capabilities open avenues to address scalability challenges in neutral-atom quantum computers and simulators, enabling fast optical readout, efficient and subwavelength non-diffracting interaction zones, and genuine compatibility with integrated solid-state photon sources, detectors, and stop-band modulators. Beyond controllable quantum matter, the platform also enables in-situ imaging of evanescent fields of light and nanoscale structures, including prospects for three-dimensional scanning microscopy with non-invasive single-atom probes for quantum sensing applications.

Figures (4)

• Figure 1: Details of the experimental setup.a Overview of the experimental apparatus. A 2D array of atoms is trapped in optical tweezers at 933nm produced by a pair of crossed AODs. The tweezers are focused using a microscope objective mounted on a motorized stage allowing the atomic lattice to be brought from the loading region (3.6mm from the chip) to the nanophotonic device. The atoms are imaged with a CMOS camera. The inset shows an example image of the 8x8 atomic array with a histogram of photon statistics. b Scanning electron microscope (SEM) image of a SiN device similar to the one used for this experiment. Our waveguides are suspended over V-grooves etched into the silicon substrate. c Scattered light from impurities in the waveguide used in this work. Due to the TE bandgap of the APCW we can see a dependence of the waveguide transmission on the input polarization. Top: TM input polarization is transmitted through APCW. Bottom: TE input polarization is reflected by APCW. d Schematic of the experimental sequence. 1. The atoms are initially imaged in the loading region and are subsequently moved to the device. 2. A pulse of light on the D2 line is sent through the waveguide to interact with the atoms. 3. Finally, the atoms are imaged again after retracting the objective to evaluate the interaction.
• Figure 2: Scanning atom microscope. a Plot of the survival probability ($P_s$) of a line of eight atoms as a function of the displacement ($\delta y$) with no light sent through the waveguide. Vertical dashed lines indicate the optical tweezer waist ($1/\mathrm{e}^2$ intensity radius of 1.2µ m). b Color map showing the survival probability of each atom in the array as a function of $\delta y$. Note the slight tilt of the waveguide with respect to the atomic array. c Schematic of the atomic array motion relative to the waveguide. The dashed line represents the trajectory the atoms follow when moved to the waveguide and back. d Images of the atomic array scanned across the y-axis after being brought in the proximity of the waveguide. Localized losses due to the presence of the waveguide are clearly observed. Furthermore, we can see a tilt similar to that shown in panel b.
• Figure 3: Atomic survival probability as a function of light pulse duration.a. Image of the atomic array with a white-light image of the device superimposed. The white arrow indicates the direction of propagation of light through the waveguide. b. Survival probability for the inner (orange) and outer (blue) rows in the array as a function of the duration of the pulse of D2 light sent through the waveguide, for a constant light power (see main text). Data series are normalized to the zero pulse duration survival probability. Symbols differentiate the individual rows, see panel c. Dashed (dotted) lines serve as a visual aid. c. Averaged images of the atomic array for a 0ms pulse duration (top) and a 40ms pulse duration (bottom). The images illustrate the spatial dependence of the losses.
• Figure 4: Measurement of the evanescent field with atomic arrays.a. Atomic survival probability as a function of the displacement (AOD frequency) across the waveguide with (orange) and without (blue) light. Within the range of distances shaded in red we observe atomic loss solely due to interaction with the evanescent field, while no atomic loss is detectable at those distance without light in the waveguide. We are able to distinguish differences in the evanescent field with spatial resolution in the order of 30nm. The solid line shows a model prediction for the atomic survival probability using an asymptotic decay length of 743nm, inital temperature of $T=40µ k$ and optical power $P=400pW$. Error-bars are given by the standard error on the mean. b. Intensity distribution of the fundamental TE mode in a $180nm\times200nm$ waveguide, obtained from MPB simulations. Gray areas indicate the waveguide. Left panel: The intensity distribution of the evanescent field in the $yz$ plane of the device. Right panel: Single atom OD along the direction of polarization of the evanescent field (indicated by the dashed line in the left panel) as a function of the y-position (blue) overlapped with a fit to equation \ref{['eq:Evanescent Decay']} (orange). c. Single-atom survival probability for the atomic array when light is sent through the waveguide.