Transporting Cold Atoms towards a GaN-on-Sapphire Chip via an Optical Conveyor Belt

Lei Xu(徐磊)$^{1,2}$, Ling-Xiao Wang(王凌潇)$^{1,2}$, Guang-Jie Chen(陈广杰)$^{1,2}$, Liang Chen(陈梁)$^{1,2}$,\\ Yuan-Hao Yang(杨元昊)$^{1,2}$, Xin-Biao Xu(徐新标)$^{1,2}$, Aiping Liu(刘爱萍)$^{3}$, Chuan-Feng Li(李传锋)$^{1,2}$,\\ Guang-Can Guo(郭光灿)$^{1,2,4}$, Chang-Ling Zou(邹长铃)$^{1,2,4\hyperlink{s}{*}}$, and Guo-Yong Xiang(项国勇)$^{1,2,4\hyperlink{s}{*}}$

doi:10.1088/0256-307X/40/9/093701

⇦Chinese Physics Letters, 2023, Vol. 40, No. 9, Article code 093701⇨ Transporting Cold Atoms towards a GaN-on-Sapphire Chip via an Optical Conveyor Belt Lei Xu (徐磊)1,2, Ling-Xiao Wang (王凌潇)1,2, Guang-Jie Chen (陈广杰)1,2, Liang Chen (陈梁)1,2, Yuan-Hao Yang (杨元昊)1,2, Xin-Biao Xu (徐新标)1,2, Aiping Liu (刘爱萍)3, Chuan-Feng Li (李传锋)1,2, Guang-Can Guo (郭光灿)1,2,4, Chang-Ling Zou (邹长铃)1,2,4*, and Guo-Yong Xiang (项国勇)1,2,4* Affiliations 1CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei 230026, China 2CAS Center for Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China 3Institute of Quantum Information and Technology, Nanjing University of Posts and Telecommunications, Nanjing 210003, China 4Hefei National Laboratory, University of Science and Technology of China, Hefei 230088, China Received 26 May 2023; accepted manuscript online 27 July 2023; published online 28 August 2023 ^*Corresponding authors. Email: clzou321@ustc.edu.cn; gyxiang@ustc.edu.cn Citation Text: Xu L, Wang L X, Chen G J et al. 2023 Chin. Phys. Lett. 40 093701 Abstract Trapped atoms on photonic structures inspire many novel quantum devices for quantum information processing and quantum sensing. Here, we demonstrate a hybrid photonic-atom chip platform based on a GaN-on-sapphire chip and the transport of an ensemble of atoms from free space towards the chip with an optical conveyor belts. Due to our platform's complete optical accessibility and careful control of atomic motion near the chip with a conveyor belt, successful atomic transport towards the chip is made possible. The maximum transport efficiency of atoms is about $50\%$ with a transport distance of $500\,\mathrm{µ m}$. Our results open up a new route toward the efficient loading of cold atoms into the evanescent-field trap formed by the photonic integrated circuits, which promises strong and controllable interactions between single atoms and single photons.

DOI:10.1088/0256-307X/40/9/093701 © 2023 Chinese Physics Society Article Text By introducing neutral atoms to integrated photonic devices, hybrid photonic-atomic chips (PACs) have attracted extensive research in recent years.[1-5] Benefiting from the strongly enhanced light-matter interactions due to the tightly optical field confinement at the wavelength and even subwavelength scale, PACs hold great potential in many quantum-based applications, such as quantum memory,[6,7] novel quantum light sources,[8] chiral quantum optics devices,[9,10] nodes of quantum networks,[11-13] novel quantum optics phenomena with surface plasmons,[14,15] many-body physics,[16,17] and quantum sensing.[18,19] Early attempts to trap cold neutral atoms near surface microstructures were initially investigated above current carrying microstructures,[20,21] which can be tailored to create a variety of potential geometry and guiding schemes for cold atoms. Atoms are manipulated by magnetic field, and distances between atoms and the microstructure surface can be reduced to as close as $0.5\,\mathrm{µ m}$.[20] However, the magnetic field transport configuration limits optical access to the atoms in one or more directions. With the advancement of fabrication techniques in photonic structures, manipulation of single atoms at wavelength and even subwavelength scales is possible with the tightly confined optical field confinement near the photonic structures. Many ground-breaking experimental results in coupling atoms to photonic structures have been achieved in various nanophotonic platforms.[7,22-32] However, these studies move forward with some potential disadvantages. For instance, the platforms based on nanofibers[7,30-32] are suspended in vacuum, thus being potentially unstable and having poor thermal conductivity, which imposes limitations on the atom trap lifetime and atom coherence time.[33,34] In addition, vacuum feedthrough for the coupling of light in and out of the nanofiber brings complexity to fiber alignment and assembly. Other platforms based on solid-state microcavities, such as microtoroid or bottle microresonators,[9,28,29,35] unable to directly load laser cooled atoms into the evanescent-field trap,[36] face difficulties in deterministic loading and trapping of cold atoms. Although the reported single atom-photon interaction time has been improved from only a few microseconds[28] to $2\,\mathrm{ms}$,[29] the further extension of the system to more photonic structures and more atoms is very challenging. In order to achieve deterministic atom trapping on integrated photonic devices, important theoretical and experimental milestones have been reached with unsuspended waveguides and microring structures.[22-27,37,38] Atoms are loaded into evanescent field of photonic structures from free space with optical tweezers and optical conveyor belts. These methods exhibit a highly precise control of atomic motion near photonic structures, including photonic crystal waveguides[22-25] and microring resonators.[26,27,37] Additionally, these demonstrations are compatible with on-chip integrated devices for cooling, transport, and trapping of cold atoms.[39-41] In this Letter, we report on transporting free space cooled $^{87}$Rb atoms towards a GaN-on-sapphire chip[38] with an optical conveyor belt.[22,42-44] Successful atomic transport towards the chip is made possible by our platform's full optical accessibility and careful control of atomic motion with a conveyor belt. The conveyor belt consists of two focused beams, both of which pass through the sapphire substrate perpendicularly. After careful spatial calibration of the beams and phase stabilization, our conveyor belt directly transports $10^4$ trapped atoms with a temperature around $40\,\mathrm{µ K}$ towards the chip, without extra aberration of the focus beam from the sapphire substrate. The maximum transport efficiency of atoms is about $50\% $ with a transport distance of $500\,\mathrm{µ m}$. It paves the way for the further implementation of stable atom trapping on the GaN-on-sapphire chip, promotes the realization of deterministic loading of atoms into the evanescent-field trap, which is promising for realizing the on-chip single-photon-level optical nonlinearity.

Fig. 1. Schematic of our experimental system, with a GaN-on-sapphire chip ($5\,\mathrm{mm} \times 10\,\mathrm{mm}$) platform placed inside the vacuum cell. Two pairs of cooling laser beams with a crossing angle of 60$^\circ$ in the $x$–$y$ plane go parallel to the chip surface, while the third pair intersects the crossing of the beams in the $x$–$y$ plane and the chip surface at an angle of 60$^\circ$ to the chip surface. The $\mathrm{781\, nm}$ dipole laser beams are split by a polarizing beam splitter (PBS), pass through the acousto-optic modulator (AOM) and focus onto the chip by a pair of lens with a focus length $f = 100\,\mathrm{mm}$. (a) Photograph of our PAC platform in a vacuum cell. (b) Picture of the micro-fabricated microring resonator and the bus waveguides on top of the GaN-on-sapphire chip taken by a scanning electron microscope. (c) Photograph of our testing setup for the coupling to PAC. (d) Schematic of an optical conveyor belt for atom transport towards a GaN-on-sapphire chip. Trapped atoms are confined in the lattice antinodes and move with the temporally varying trap potential.

Overview of the Photonic-Atom Chip. Figure 1 illustrates our experimental setup for studying the PAC, where a GaN-on-sapphire chip is placed inside the vacuum cell ($25\,\mathrm{mm}\times 25\,\mathrm{mm} \times 50\,\mathrm{mm}$). Figure 1(a) provides a photograph of our PAC platform in a vacuum cell. We use low vapor pressure epoxy (torr seal) to stick the chip onto a metal holder while maintaining a high vacuum and enduring the high temperature during the vacuum baking process. Half of the chip without fabrication is glued onto a 316-L stainless steel holder, and the remaining part of the chip is suspended in vacuum with waveguide and microring resonator structures fabricated on the surface. The metal holder is then connected to a CF35 vacuum cube, providing heat dissipation and stability for the chip. Here, we adopt the GaN-on-sapphire platform for the PAC following our previous theoretical proposal,[38] as the system is more stable without suspended photonic structures. In addition, both GaN and sapphire are wide-band-gap materials that are transparent to ultra-broadband wavelengths (260–1590 nm),[45,46] so our chip is compatible with lasers working in the visible and near-visible wavelengths for many alkali and alkaline-earth atoms, allowing full optical access for cold rubidium atom cooling, trapping, transport, and detection. Figure 1(b) shows the scanning electron microscope (SEM) image of a fabricated microring resonator and the bus waveguides on top of the GaN-on-sapphire chip. The size of the sapphire substrate is $5\,\mathrm{mm} \times 10\,\mathrm{mm}$. The microring resonator is vacuum-cladded for direct interaction between atoms and the evanescent field of the confined modes, with a major radius of $60\,\mathrm{µ m}$ and a cross-section of $700\,\mathrm{nm}\times420\,\mathrm{nm}$, and the optical modes of the microring are coupled to a bus waveguide through the evanescent field. Such microring resonators have been widely studied in photonics applications, due to their easy fabrication, high quality factor and small mode volume.[47] The realization of the on-chip single-photon-level optical nonlinearity highly depends on the cooperativity parameter $C= \frac{3\lambda^2}{4\pi^2}\frac{Q}{V_{\rm m}}$, where $\lambda = 780\,\mathrm{nm}$ is the wavelength of the D2 line of rubidium atoms. The cooperativity parameter is proportional to the ratio of the quality factor $Q$ and the mode volume $V_{\rm m}$ for the microring resonator. For our microring resonator parameter, the currently achieved quality factor $Q=3.75\times10^4$, mainly limited by the surface roughness. As shown in Fig. 1, our optical configurations of the experiments could be divided into three parts: (i) coupling to PAC, (ii) magneto-optical trap (MOT), and (iii) optical conveyor belt. First, at both ends the GaN-on-sapphire chip, light is coupled in and out of the photonic chip through a high numerical aperture (N.A.) objectives. Figure 1(c) shows the photograph of our testing setup for the coupling to the PAC. A coupling efficiency of about $20\%$ for the GaN waveguide in Fig. 1(b) for optical signals with $780\,\mathrm{nm}$ wavelength can be achieved with commercial N.A. = 0.4 objectives. Laser coupled into the waveguide can be utilized to provide an evanescent field trap or to couple input signal photons close to the transition frequency of the atoms to the transported atoms. The cold $^{87}$Rb atoms are then prepared through a standard six-beam magneto-optical trap.[48] The glass cell is connected to a mini cube and a $30\,\mathrm{L/s}$ ion pump, resulting in a pressure of $10^{-9}$ mbar measured by the ion-pump current. Three pairs of cooling laser beams are generated from a $780\,\mathrm{nm}$ laser, with the power of each beam being about $150\,\mathrm{µ W}$ and the beam waist being $1\,\mathrm{mm}$. The cooling laser beam is red detuned by $8\,\mathrm{MHz}$ from the ${{|F=2\rangle}\,\rightarrow{|F'=3\rangle}}$ D2 cycling transition. Additionally, $80\,\mathrm{µ W}$ of repump laser beams overlap with one of the cooling laser beams. The beams intersect at one point about $1\,\mathrm{mm}$ above the surface of the chip, with additional anti-Helmholtz coils aligned with the point providing a magnetic field gradient up to $10\,\mathrm{G/cm}$. To align with our PAC, two pairs of cooling laser beams with a crossing angle of 60$^\circ$ in the $x$–$y$ plane go parallel to the chip surface, while the third pair intersects the crossing of the beams in the $x$–$y$ plane and the chip surface at an angle of 60$^\circ$ to the chip surface. Although MOT beam pairs are not oriented orthogonally to each other, 3D atom confinement is still achieved as components of each beam are projected along the orthogonal axis.

Fig. 2. (a) Single-shot absorption images of MOT atoms $800\,\mathrm{µ m}$ away from the chip surface. The absorption imaging is performed with camera of $2048\times1080$ resolution and each pixel point on the camera corresponds to $15.6\,\mathrm{µ m}$ on the object plane. The red line denotes the interface between free space and photonic chip. (b) Single-shot absorption images of atoms transported toward the chip surface with the optical conveyor belt.

Following the 3D MOT procedure, the temperature of the atom ensemble around $40\,\mathrm{µ K}$ is finally achieved by a polarization-gradient cooling (PGC) process. With a duration of $2\,\mathrm{ms}$ for the PGC, the cooling laser beams detuning is ramped down to $-48\,\mathrm{MHz}$ from the cycling transition. Figure 2(a) shows a density contour plot of the atom ensemble, which is deduced from a single-shot free space absorption image.[49] The cold atoms are about $800\,\mathrm{µ m}$ away from the chip surface, and the atom number density is about $3\times 10^{10}\,\mathrm{cm^{-3}}$ with an atom cloud radius of about $190\,\mathrm{µ m}$. The distance between the atom cloud and the chip surface can be adjusted from $300\,\mathrm{µ m}$ to $1000\,\mathrm{µ m}$ by adjusting the offset coil and the alignment of the cooling beams. However, in close proximity to the chip, the density and shape of the atomic cloud are altered due to surface reflections, which is consistent with the previous observations.[50]

Fig. 3. (a) Atomic density $\rho$ on chip surface with different holding times $\tau$. We measure the trajectories of moving atoms in atom conveyor belt by taking a sequence of images with incremental holding times and record the atomic density on chip surface (averaged 10 times) with $\Delta\nu_{\max}=104\,\mathrm{kHz}$ and a maximum transport distance $d=500\,\mathrm{µ m}$. We observe a clear density peak at $\tau = 13\,\mathrm{ms}$. The error bar comes from three continuous measurement sequences. Solid curve is the fit result from our theoretical model of atomic accumulation dynamics on the chip surface (see details in the text). Inset is the laser detuning profile; the rising and descending times of the profile are set to 1 ms, and the holding time is $\tau$. (b) Atomic density distribution along the conveyor belt axis for different holding times.

Transporting Atoms towards the Chip. The optical conveyor belt is realized by an optical dipole trap, which consists of two linearly polarized counter-propagating Gaussian beams with beam waist $w_0 = 20\,\mathrm{µ m}$, and the waist is located in the middle of the MOT atom cloud and the chip surface. The intensities of both beams are equal, and their optical frequencies are different by detuning $\Delta\nu$. For the realization of the moving standing wave, the phase fluctuations between the two beams must be minimized. Fluctuations of the relative phase with an rms value of roughly $2\pi/1000$ between the two beams are directly translated into position fluctuations of the dipole trap potential, which limit the lifetime to 3 s, as reported in Ref. [51]. Thus in our experiment, the two counterpropagating dipole laser beams come from a single laser, with $\lambda = 781\,\mathrm{nm}$ corresponding to a frequency $2.3\,\mathrm{THz}$ red detuned to the $^{87}$Rb atom D2 transition. The laser is then split into two paths and passed through a double-pass 80 MHz acousto-optic modulator (AOM), with $\Delta\nu$ controllable via the phase-synchronized RF signals applied to the AOMs. By moving atoms back and forth within the Rayleigh length in free space, the phase stability is empirically determined with a retention probability of approximately 90% for a single cycle of movement. Therefore, the two beams generate a spatially and temporally varying trap potential $U(z,t) = U_0\cos^2(\frac{2\pi\Delta\nu}{2}t-\frac{2\pi}{\lambda}z)$, where $U_0$ is the local trap depth and $z$ is the position of atoms along the beam axis. Trapped atoms are confined in the lattice antinodes and moved with the temporally varying trap potential without significant phase noise heating [see Fig. 1(d)]. In order to transport atoms towards the surface of the PAC, we first load approximately $10^4$ atoms into a standing-wave dipole trap with $\Delta\nu=0$ by overlapping both dipole beams with the 3D MOT for $150\,\mathrm{ms}$. Here, each beam has a power of $9\,\mathrm{mW}$, which corresponds to a trap depth of about $1.3\,\mathrm{mK}$. After the loading process, atoms are trapped in a series of lattice antinodes along the beam axis, with an axial distribution of about $150\,\mathrm{µ m}$ limited by the size of the atom cloud. Then, a frequency chirping sequence of $\Delta\nu$ [see the inset of Fig. 3(a)], which is achieved by the sweep mode of the signal generator, is sent to one of the AOMs to create a moving optical conveyor belt, and the antinodes move at a velocity[42] \begin{align} v=\frac{1}{2}\lambda\Delta\nu. \tag {1} \end{align} To verify the transport of atoms towards the chip surface, we take an absorption image of cold atoms in the optical conveyor belt after the transport process. Figure 2(b) shows the results indicating the ensemble of atoms near the chip surface (the dashed red line). Compared with the image of the atom cloud prepared by MOT, our conveyor belt has successfully delivered atoms towards the chip. Then, the transport of atoms in the optical conveyor belt is systematically investigated. Through a sequence of $\Delta\nu$, as illustrated by the inset of Fig. 3(a), we could transport the atoms over a certain distance by ramping up $\Delta \nu$ in $\tau_{\mathrm{ramp}}=1\,\mathrm{ms}$ to $\Delta\nu_{\max}$, holding the detuning for a duration of $\tau$, and then ramping down $\Delta \nu$ in $1\,\mathrm{ms}$. The distance $\Delta z =\frac{1}{2}\lambda\Delta\nu_{\max}(\tau_{\mathrm{ramp}}+\tau)$. Figure 3(a) summarizes the measured atomic density on the chip surface for different hold times $\tau$, with $\Delta\nu_{\max}=104\,\mathrm{kHz}$ and a maximum transport distance $d=500\,\mathrm{µ m}$ limited by the block of the chip. The atomic density of the single pixel value above the red line in Fig. 2(b) is used to extract the measured atomic density for each of the various hold times. We observed a clear atomic density peak of $2.5\times10^{10}\,\mathrm{cm^{-3}}$ when $\tau =13\,\mathrm{ms}$, which almost agrees with the calculated time (dashed vertical line) for transporting atoms from the center of the MOT to the chip surface. The accumulation of atomic density $\rho$ on the chip surface can be described by a simple rate equation \begin{align} \frac{d\rho}{dt} =J(t)-\varGamma\rho, \tag {2} \end{align} where $J(t)$ represents the atomic flux to the chip surface by the optical conveyor belt and $\varGamma$ is the linear atomic loss coefficient. Here, $\varGamma$ is mainly attributed to the atom collision and absorption loss on the chip surface, heating by the optical dipole trap, and the vacuum gas collisions. Since the atomic density is relatively low, the nonlinear atomic loss due to atomic collisions is neglected. According to the atomic cloud shape, we made the assumption that atomic flux is a Gaussian function \begin{align} J(t)=J_0 \exp\Big[-\Big(\frac{t-\tau_0}{\sigma_0}\Big)^2\Big], \tag {3} \end{align} where $J_0$ is the maximum atomic flux density, $\tau_0$ is the time of atomic peak flux arriving at the chip surface without the block of the chip, and $\sigma_0$ describes the width of the Gaussian function in the time domain. Using the above rate equation, we fitted the atomic density (solid line) as shown in Fig. 3(a), which agrees excellently with the experimental results. Further investigations of the influence of the chip surface on the transport of atoms are shown in Fig. 3(b), where the atomic density distribution along the atom conveyor belt axis for different holding times $\tau$ is plotted. A string of pixel values along the conveyor belt axis above the red line in Fig. 2(b) is used to extract the measured atomic density distribution. The origin of the $z$-axis is set to the chip surface. The parameters of the conveyor belt are the same as in Fig. 3(a). We find that when atoms are close to the chip surface within $100\,\mathrm{µ m}$, the peak height of atomic density distribution along the atom conveyor belt axis decreases, which indicates an increasing atom loss rate. The unevenness of the flat surface, which needs to be super-ground to obtain a super-flat mirror surface with a roughness of a few angstroms, is due to the enhanced atomic loss within $100\,\mathrm{µ m}$, which has been studied by other groups. For future PAC experiments, a high density of atoms on the surface of the GaN-on-sapphire chip is desired. Therefore, we experimentally varied the location of the MOT center, and optimized the $\Delta\nu_{\max}$ for atom transportation. The results for maximum transport distance $d = 1000\,\mathrm{µ m}$, $750\,\mathrm{µ m}$, and $500\,\mathrm{µ m}$ are summarized in Fig. 4, with the atomic density on the chip surface calculated from the fitting results, as shown by the solid line in Fig. 3(a). The transport efficiency $\eta$ is defined as the ratio of the maximum atomic density on the chip surface to the initial atomic density in the standing wave dipole trap. In comparison of the different $\Delta \nu_{\max}$ in Fig. 4(a), the transport efficiency reaches the optimum when $\Delta \nu_{\max}$ is 100–160 kHz, and the transport efficiency drops when $\Delta \nu_{\max}$ is further increased. In particular, the efficiency dramatically decreases when $\Delta \nu_{\max}$ is less than $80\,\mathrm{kHz}$. The dependence of $\eta$ on $\Delta \nu_{\max}$ may be attributed to two different reasons. If $\Delta \nu_{\max}$ is too large, the acceleration and deceleration process of the conveyor belt may induce a significant loss of atoms. The trap potential gives transported atoms an acceleration boost. Depending on the phase of the atom's oscillation, abrupt changes in the potential either enhance or decrease the atom's kinetic energy. Such phenomena have been studied in many other experimental works in detail.[51,52] In contrast, if $\Delta \nu_{\max}$ is too small, the required $\tau$ is too large, and the atomic density is limited by the intrinsic atomic loss in the dipole trap. In practical cases, we aim to obtain higher transport efficiency along with less transport time.

Fig. 4. (a) Atom transport efficiency $\eta$ with maximum frequency difference $\mathrm{\Delta \nu_{\max}}$ for different transport distance $d$. The transport efficiency is defined as the ratio of the maximum atomic density on the chip surface to the initial atomic density in the standing dipole trap. The transport efficiency reaches the optimum when $\Delta \nu_{\max}$ is 100–160 $\mathrm{kHz}$. (b) Atom transport efficiency $\eta$ with transport time $t$ for different transport distance $d$. Higher transport efficiency and shorter transport distance are achieved with shorter transport distance.

In Fig. 4(b), we present the transport efficiency with corresponding transport time for different transport parameters. Shortening the transport distance can improve both the transport efficiency and transport time, and a maximum transport efficiency close to $50\,\%$ is achieved for a maximum transport distance of $d = 500\,\mathrm{µ m}$. We fit our results with an empirical equation in the form \begin{align} \eta = e^{-at}(bt+c) \tag {4} \end{align} to describe the relation between transport efficiency and the corresponding transport time. The exponential decay accounts for the intrinsic loss irrelevant to the transport velocity, while the linear term accounts for the improvement of transport efficiency with slow transport velocity. It is anticipated that the transport efficiency can be improved by reducing the transport distance. However, the disturbing of the MOT when it is in the proximity of the chip surface prevent us from achieving a shorter distance. We also notice that the maximum transport efficiency is currently limited to about $50\%$. While nearly lossless atom transport was achieved for a transport distance of a few millimeters in free space,[51] the reflection of the dipole trap beams on the chip surface may destroy the moving lattice antinodes in our cases. On the one hand, the interference of the reflected light with the conveyor belt modulates the depth of the dipole trap and reduces the effective trap depth. On the other hand, the vibrational noise of the chip is transmitted to the atoms with the change in the phase of the reflected light. These issues may be mitigated by choosing an appropriate polarization of the dipole trap beams with the incident angle satisfying the Brewster angle, since the reflection can be greatly suppressed. Finally, the intrinsic heating of atoms in the conveyor belt is tested in a static standing wave dipole trap ($\Delta\nu_{\max}=0$). As shown in Fig. 5, a heating rate of $12.4\,\mathrm{mK/s}$ is extracted from the measurement of the trap lifetime for different trap depths, which explains the severe atomic loss when $\Delta\nu_{\max} < 80\,\mathrm{kHz}$.

Fig. 5. Atom lifetime in the standing wave dipole trap for different trap depths. The solid line is a linear fit of our measured atom lifetime for different trap depth, the fitted total heating rate from the measured data is $12.4\,\mathrm{mK/s}$.

In summary, we have demonstrated a hybrid photonic-atomic chip platform and successfully transported an ensemble of cold atoms from free space towards the chip with an optical conveyor belt. Our PAC platform is made of transparent materials, allowing full optical access for free space laser cooling, atom transport, and efficient coupling to on-chip photonic waveguides. The maximum transport efficiency of atoms is about $50\%$ with a transport distance of $500\,\mathrm{µ m}$. In the future, by combining on-chip MOT laser cooling, optical transport and evanescent-field trapping of cold atoms, a more compact PAC platform is attainable through the implementation of advanced photonic structure fabrication and design strategies. Our PAC platform holds great potential for research of atom-photon interactions and realization of single-photon-level optical nonlinearity, which could find applications in quantum information science and quantum sensing. Acknowledgments. This work was supported by the National Key R&D Program (Grant No. 2021YFF0603701), the National Natural Science Foundation of China (Grant Nos. U21A20433, U21A6006, 92265210, 12104441, 12134014, 61905234, and 11974335), and the USTC Research Funds of the Double First-Class Initiative (Grant No. YD2030002007). CLZ was also supported by the Fundamental Research Funds for the Central Universities, and USTC Research Funds of the Double First-Class Initiative. This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication. References Colloquium : Quantum matter built from nanoscopic lattices of atoms and photonsThe Integration of Photonic Crystal Waveguides with Atom Arrays in Optical TweezersAdvanced apparatus for the integration of nanophotonics and cold atomsOn‐Chip Light–Atom Interactions: Physics and ApplicationsSystematic design of a robust half-W1 photonic crystal waveguide for interfacing slow light and trapped cold atomsOptical quantum memoryDemonstration of a Memory for Tightly Guided Light in an Optical NanofiberUniversal photonic quantum computation via time-delayed feedbackQuantum optical circulator controlled by a single chirally coupled atomChiral quantum opticsThe quantum internetNanophotonic quantum phase switch with a single atomOvercoming erasure errors with multilevel systemsCooperative coupling of ultracold atoms and surface plasmonsPlasmonically tailored micropotentials for ultracold atomsQuantum many-body models with cold atoms coupled to photonic crystalsSubwavelength vacuum lattices and atom–atom interactions in two-dimensional photonic crystalsNanoscale atomic suspended waveguides for improved vapour coherence times and optical frequency referencingChip-Scale Integration of Nanophotonic-Atomic Magnetic SensorsImpact of the Casimir-Polder Potential and Johnson Noise on Bose-Einstein Condensate Stability Near SurfacesSurface effects in magnetic microtrapsClocked atom delivery to a photonic crystal waveguideCoupling a Single Trapped Atom to a Nanoscale Optical CavityStrong Coupling of Two Individually Controlled Atoms via a Nanophotonic CavityEntanglement transport and a nanophotonic interface for atoms in optical tweezersTrapping single atoms on a nanophotonic circuit with configurable tweezer latticesCoupling Single Atoms to a Nanophotonic Whispering-Gallery-Mode Resonator via Optical GuidingObservation of strong coupling between one atom and a monolithic microresonatorCoupling a Single Trapped Atom to a Whispering-Gallery-Mode MicroresonatorOptical Interface Created by Laser-Cooled Atoms Trapped in the Evanescent Field Surrounding an Optical NanofiberDemonstration of a State-Insensitive, Compensated Nanofiber TrapWaveguide-coupled single collective excitation of atomic arraysCoherence Properties of Nanofiber-Trapped Cesium AtomsHeating in Nanophotonic Traps for Cold AtomsAll-optical routing of single photons by a one-atom switch controlled by a single photonSubstrate-based atom waveguide using guided two-color evanescent light fieldsMicroring resonators on a suspended membrane circuit for atom–light interactionsProposal for low-power atom trapping on a GaN-on-sapphire chipMultigrating design for integrated single-atom trapping, manipulation, and readoutA nanowaveguide platform for collective atom-light interactionPlanar-Integrated Magneto-Optical TrapDeterministic Delivery of a Single AtomSubmicron Positioning of Single Atoms in a MicrocavityA technique for individual atom delivery into a crossed vortex bottle beam trap using a dynamic 1D optical latticeOptical properties of wurtzite structure GaN on sapphire around fundamental absorption edge (0.78–4.77 eV) by spectroscopic ellipsometry and the optical transmission methodAbsorption Coefficient and Refractive Index of GaN, AlN and AlGaN AlloysEmerging material platforms for integrated microcavity photonicsGraduate Texts in Contemporary PhysicsMaking, probing and understanding Bose-Einstein condensatesExperimental investigation of transparent silicon carbide for atom chipsAn optical conveyor belt for single neutral atomsSpeed, retention loss, and motional heating of atoms in an optical conveyor belt

[1]	Chang D E, Douglas J S, González-Tudela A, Hung C L, and Kimble H J 2018 Rev. Mod. Phys. 90 031002
[2]	Luan X S, Béguin J B, Burgers A P, Qin Z, Yu S P, and Kimble H J 2020 Adv. Quantum Technol. 3 2000008
[3]	Béguin J B, Burgers A P, Luan X, Qin Z, Yu S P, and Kimble H J 2020 Optica 7 1
[4]	Wang W Y, Xu Y T, and Chai Z 2022 Adv. Photon. Res. 3 2200153
[5]	Bouscal A, Kemiche M, Mahapatra S, Fayard N, Berroir J, Ray T, Greffet J J, Raineri F, Levenson A, Bencheikh K, Sauvan C, Urvoy A, and Laurat J 2023 arXiv:2301.04675 [quant-ph]
[6]	Lvovsky A I, Sanders B C, and Tittel W 2009 Nat. Photon. 3 706
[7]	Gouraud B, Maxein D, Nicolas A, Morin O, and Laurat J 2015 Phys. Rev. Lett. 114 180503
[8]	Pichler H, Choi S, Zoller P, and Lukin M D 2017 Proc. Natl. Acad. Sci. USA 114 11362
[9]	Scheucher M, Hilico A, Will E, Volz J, and Rauschenbeutel A 2016 Science 354 1577
[10]	Lodahl P, Mahmoodian S, Stobbe S, Rauschenbeutel A, Schneeweiss P, Volz J, Pichler H, and Zoller P 2017 Nature 541 473
[11]	Kimble H J 2008 Nature 453 1023
[12]	Tiecke T, Thompson J D, de Leon N P, Liu L, Vuletić V, and Lukin M D 2014 Nature 508 241
[13]	Muralidharan S, Zou C L, Li L, Wen J, and Jiang L 2017 New J. Phys. 19 013026
[14]	Stehle C, Zimmermann C, and Slama S 2014 Nat. Phys. 10 937
[15]	Stehle C, Bender H, Zimmermann C, Kern D, Fleischer M, and Slama S 2011 Nat. Photon. 5 494
[16]	Douglas J S, Habibian H, Hung C L, Gorshkov A V, Kimble H J, and Chang D E 2015 Nat. Photon. 9 326
[17]	González-Tudela A, Hung C L, Chang D E, Cirac J I, and Kimble H J 2015 Nat. Photon. 9 320
[18]	Zektzer R, Mazurski N, Barash Y, and Levy U 2021 Nat. Photon. 15 772
[19]	Sebbag Y, Naiman A, Talker E, Barash Y, and Levy U 2021 ACS Photon. 8 142
[20]	Lin Y J, Teper I, Chin C, and Vuletić V 2004 Phys. Rev. Lett. 92 050404
[21]	Fortágh J, Ott H, Kraft S, Günther A, and Zimmermann C 2002 Phys. Rev. A 66 041604
[22]	Burgers A P, Peng L S, Muniz J A, McClung A C, Martin M J, and Kimble H J 2019 Proc. Natl. Acad. Sci. USA 116 456
[23]	Thompson J D, Tiecke T, de Leon N P, Feist J, Akimov A, Gullans M, Zibrov A S, Vuletić V, and Lukin M D 2013 Science 340 1202
[24]	Samutpraphoot P, Đorđević T, Ocola P L, Bernien H, Senko C, Vuletić V, and Lukin M D 2020 Phys. Rev. Lett. 124 063602
[25]	Đorđević T, Samutpraphoot P, Ocola P L, Bernien H, Grinkemeyer B, Dimitrova I, Vuletić V, and Lukin M D 2021 Science 373 1511
[26]	Kim M E, Chang T H, Fields B M, Chen C A, and Hung C L 2019 Nat. Commun. 10 1647
[27]	Zhou X C, Tamura H, Chang T H, and Hung C L 2023 Phys. Rev. Lett. 130 103601
[28]	Aoki T, Dayan B, Wilcut E, Bowen W P, Parkins A S, Kippenberg T, Vahala K, and Kimble H 2006 Nature 443 671
[29]	Will E, Masters L, Rauschenbeutel A, Scheucher M, and Volz J 2021 Phys. Rev. Lett. 126 233602
[30]	Vetsch E, Reitz D, Sagué G, Schmidt R, Dawkins S, and Rauschenbeutel A 2010 Phys. Rev. Lett. 104 203603
[31]	Goban A, Choi K, Alton D, Ding D, Lacroûte C, Pototschnig M, Thiele T, Stern N, and Kimble H 2012 Phys. Rev. Lett. 109 033603
[32]	Corzo N V, Raskop J, Chandra A, Sheremet A S, Gouraud B, and Laurat J 2019 Nature 566 359
[33]	Reitz D, Sayrin C, Mitsch R, Schneeweiss P, and Rauschenbeutel A 2013 Phys. Rev. Lett. 110 243603
[34]	Hümmer D, Schneeweiss P, Rauschenbeutel A, and Romero-Isart O 2019 Phys. Rev. X 9 041034
[35]	Shomroni I, Rosenblum S, Lovsky Y, Bechler O, Guendelman G, and Dayan B 2014 Science 345 903
[36]	Barnett A H, Smith S P, Olshanii M, Johnson K S, Adams A W, and Prentiss M 2000 Phys. Rev. A 61 023608
[37]	Chang T H, Fields B M, Kim M E, and Hung C L 2019 Optica 6 1203
[38]	Liu A P, Xu L, Xu X B, Chen G J, Zhang P F, Xiang G Y, Guo G C, Wang Q, and Zou C L 2022 Phys. Rev. A 106 033104
[39]	Liu A P, Liu J W, Peng W, Xu X B, Chen G J, Ren X, Wang Q, and Zou C L 2022 Phys. Rev. A 105 053520
[40]	Meng Y, Lee J, Dagenais M, and Rolston S 2015 Appl. Phys. Lett. 107 091110
[41]	Chen L, Huang C J, Xu X B, Zhang Y C, Ma D Q, Lu Z T, Wang Z B, Chen G J, Zhang J Z, Tang H X, Dong C H, Liu W, Xiang G Y, Guo G C, and Zou C L 2022 Phys. Rev. Appl. 17 034031
[42]	Kuhr S, Alt W, Schrader D, Muller M, Gomer V, and Meschede D 2001 Science 293 278
[43]	Nußmann S, Hijlkema M, Weber B, Rohde F, Rempe G, and Kuhn A 2005 Phys. Rev. Lett. 95 173602
[44]	Dinardo B A and Anderson D Z 2016 Rev. Sci. Instrum. 87 123108
[45]	Yu G, Wang G, Ishikawa H, Umeno M, Soga T, Egawa T, Watanabe J, and Jimbo T 1997 Appl. Phys. Lett. 70 3209
[46]	Muth J, Brown J D, Johnson M, Yu Z, Kolbas R, Cook J, and Schetzina J 1999 MRS Int. J. Nitride Semicond. Res. 4 502
[47]	Liu J, Bo F, Chang L, Dong C H, Ou X, Regan B, Shen X, Song Q, Yao B, Zhang W, Zou C L, and Xiao Y F 2022 Sci. Chin. Phys. Mech. & Astron. 65 104201
[48]	Metcalf H J and van der Straten P 1999 Laser Cooling and Trapping (New York: Springer)
[49]	Ketterle W, Durfee D S, and Stamper-Kurn D 1999 arXiv:cond-mat/9904034
[50]	Huet L, Ammar M, Morvan E, Sarazin N, Pocholle J P, Reichel J, Guerlin C, and Schwartz S 2012 Appl. Phys. Lett. 100 121114
[51]	Schrader D, Kuhr S, Alt W, Muller M, Gomer V, and Meschede D 2001 Appl. Phys. B 73 819
[52]	Hickman G T and Saffman M 2020 Phys. Rev. A 101 063411