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Laser & Photonics Review

Continuous Strong Coupling of Cold Atoms to an Optical Microcavity and Heralded Single‐Photon Emission

Yi‐Jia Liu, Jian Wang, Yu‐Shu Chen, Xiaolong Zhou, Dong‐Yu Huang, Ze‐Min Shen, Si‐Jian He, Qi‐Yang Huang, Quan Jiang, Chuan‐Feng Li, Guang-Can Guo

Published Oct 28, 2026• 0 citations
Publisher

Abstract

ABSTRACT Neutral atoms coupled to optical microcavities provide an efficient atom‐photon interface for quantum computing, quantum networking and quantum metrology. However, the finite trapping lifetime of atoms generally requires cyclic atom transfer, which interrupts the atom‐cavity interaction and limits the applications of cavity QED experiments. Here, we realize continuous coupling of rubidium atoms to an optical microcavity. Continuous transfer of atoms from a magneto‐optical trap into the optical microcavity is achieved using a near‐resonant optical dipole trap, enabling strong coupling between an atomic ensemble and the microcavity with a vacuum Rabi splitting of 395(2) MHz. To demonstrate a potential application, we further realize continuous heralded single‐photon emission and measure a minimum value of the heralded second‐order correlation function, . These results pave the way for the continuous transfer of atoms into optical microcavities and broaden the application prospects of laser‐cooled atoms coupled to optical microcavities.

DOI
10.1002/lpor.72141

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•Dec 5, 2022•0 citations
ABSTRACT Neutral atoms coupled to optical microcavities provide an efficient atom‐photon interface for quantum computing, quantum networking and quantum metrology. However, the finite trapping lifetime of atoms generally requires cyclic atom transfer, which interrupts the atom‐cavity interaction and limits the applications of cavity QED experiments. Here, we realize continuous coupling of rubidium atoms to an optical microcavity. Continuous transfer of atoms from a magneto‐optical trap into the optical microcavity is achieved using a near‐resonant optical dipole trap, enabling strong coupling between an atomic ensemble and the microcavity with a vacuum Rabi splitting of 395(2) MHz. To demonstrate a potential application, we further realize continuous heralded single‐photon emission and measure a minimum value of the heralded second‐order correlation function, . These results pave the way for the continuous transfer of atoms into optical microcavities and broaden the application prospects of laser‐cooled atoms coupled to optical microcavities.
The cavity quantum electrodynamics (QED) system is a promising platform for quantum optics and quantum information experiments.Its core is the strong coupling between atoms and optical cavity, which causes difficulty in the overlap between the atoms and the antinode of optical cavity mode.Here, we use a programmable movable optical dipole trap to load a cold atomic ensemble into an optical fiber microcavity and realize the strong coupling between the atoms and the optical cavity in which the coupling strength can be improved by polarization gradient cooling and adiabatic loading.By the measurement of vacuum Rabi splitting, the coupling strength can be as high as g N = 2π × 400 MHz, which means the effective atom number is N eff = 16 and the collective cooperativity is C N = 1466.These results show that this experimental system can be used for cold atomic ensemble and cold molecule based cavity QED research.
Continuously operating atom-light interfaces represent a key prerequisite for steady-state quantum sensors and efficient quantum processors. Here, we demonstrate continuous accumulation of sub-Doppler-cooled atoms in a shallow intracavity dipole trap, realizing this regime. The key ingredient is a light-shift manipulation that creates spatially varying cooling parameters, enabling efficient capture and accumulation of atoms within a cavity mode. Demonstrated with rubidium atoms, a continuous flux from a source cell is funneled through the magneto-optical trap into the cavity mode, where the atoms are cooled and maintained below 10 μK10~μ\text{K}10 μK in steady state without time-sequenced operation. We characterize the resulting continuously maintained ensemble of millions of atoms and its collective coupling to the cavity field, establishing a route toward continuously operated cavity-QED systems and long-duration atomic and hybrid quantum sensors.
Cavity quantum electrodynamic system with strongly coupled single atoms provides a good platform for studying quantum information processing, quantum simulation, quantum network, and distributed quantum computing. Cooling and trapping single atoms is a crucial technique in the quantum technology. At present, in a high-finesse cavity with finite space, cooling and trapping single atoms is a big challenge, even though it is a mature technique for free space. Great efforts have been made to cool and trap single atoms inside a cavity, and for a trapped atom its lifetime has reached as long as tens of seconds. Developing a more flexible method of cooling and trapping single atoms in a cavity is still essential for a strongly coupled cavity quantum electrodynamic system. In this work, we demonstrate experimentally that a single cesium atom in a cavity can be trapped by utilizing a single optical tweezer settled in cavity mode, and its lifetime is (2.60 ± 0.18) s. The experiment is carried out in a Fabry-Perot cavity, which is assembled by two concave mirrors each with a curvature radius of 100 mm, and cavity length of 335 μm. The concave surfaces are highly reflective, and the cavity has a finesse of 6.1 × 104. The 1080 nm optical tweezer with a waist of 2 μm is formed by an achromatic lens group with a numerical aperture of 0.4. At first, the precooled atomic assemble released from the magneto-optical trap (MOT) is transferred into cavity mode by an optical dipole trap with a waist of 36 μm. Then, one of the successfully transferred atoms is captured by the optical tweezer with the aid of cavity cooling mechanism. A blue detuned cavity locking laser is used as a standing-wave optical trap along the cavity axis. The signal of successfully trapped one atom is obtained by recording transmission of the cavity that will decrease owing to the strong coupling induced vacuum Rabi splitting. Finally, we demonstrate the precise manipulation of the atom-cavity coupling strength, which is achieved by scanning the position of the trapped atom step by step by using a high-precision translation stage. The system realized in this work can be used to study the dynamics of single atom-photon interactions with adjustable coupling strength. In addition, the mechanism adopted in this work is compatible with constructing tweezer arrays inside cavity mode, and thus possesses more flexibility and great potentials in cavity-based quantum entanglement and quantum simulation.
This paper presents a setup capable of producing a high-flux continuous beam of cold rubidium atoms for cavity quantum electrodynamics experiments in the region of strong coupling. A 2D + magneto-optical trap (MOT), loaded with rubidium getters in a dry-film-coated vapor cell, fed a secondary moving-molasses MOT (MM-MOT) at a rate greater than 2 × 10 10 atoms/s. The MMMOT provided a continuous beam with a tunable velocity. This beam was then directed through the waist of a cavity with a length of 280 µm, resulting in a vacuum Rabi splitting of more than ±10 MHz. The presence of a sufficient number of atoms in the cavity mode also enabled splitting in the polarization perpendicular to the input. The cavity was in the strong coupling region, with an atom-photon dipole coupling coefficient g of 7 MHz, a cavity mode decay rate κ of 3 MHz, and a spontaneous emission decay rate γ of 6 MHz.
We present trapping and cooling of single cesium atoms inside a microcavity by means of an intracavity far-off-resonance trap (FORT). By the 'magic' wavelength FORT, we achieve state-insensitive single-atom trapping and cooling in a microcavity. The cavity transmission of the probe beam strongly coupled to single atoms enables us to continuously observe the intracavity atom trapping. The average atomic localization time inside the bright FORT is about 7 ms by introducing cavity cooling with appropriate detuning. This experiment presents great potential in coherent state manipulation for strongly coupled atom–photon systems in the context of cavity quantum electrodynamics.
Generating and manipulating multiparticle quantum entangled states is an exciting challenge of modern physics. Along this line, we have built a platform where cold rubidium atoms are strongly coupled to a fiber-based microcavity, under a quantum gas microscope. The atoms are trapped in a lattice at 1560nm, which allows strong and homogeneous coupling to the commensurate probe lattice at 780nm. First, we observe a coherent interaction between the cavity and the atoms, in the strong coupling regime, in spite of a large frequency inhomogeneity induced by the trap. It is the first report of such cavity protection effect with less than 200 emitters. We measure the growing coupling of the dark states to the cavity as the disorder takes over the collective coupling. In the protected regime we engineer frequency-modulated polaritons through a modulation of the atomic frequency. Then, we setup optical tweezers in the microcavity to obtain and manipulate several single atoms. The cavity enables fast and non demolition detection of a single atom in a tweezer. We measure the vacuum Rabi splitting of this atom, demonstrating its strong coupling with an effective cooperativity of 30. Finally, we obtain single atoms in a chain of 9 tweezers. This work opens to way to manipulating many individual atoms strongly coupled to the cavity, with applications in multiparameter quantum metrology, using spatially distributed and locally measurable entanglement as a resource, and quantum simulation of a chain of addressable spins with cavity-mediated infinite range interactions.