Optical tweezers can already arrange thousands of atoms into arbitrary patterns, and cavities enhance light-matter coupling to the single-photon level. Related work in this area explores quantum squeezing of molecules, on-chip entanglement, and ion cooling without cryogenics, all centered on precise control of light and atoms. This paper combines two mature technologies: tweezer arrays and a high-cooperativity ring cavity. Previously, collective light scattering was studied in free space or in standing waves of two-mirror cavities, where boundary conditions were set by the mirrors. Here, the traveling waves of the ring cavity make interference depend only on the relative positions of the atoms.
Cesium atoms were trapped in optical tweezers at 937 nm and aligned along the axis of the ring cavity. A transverse probe beam was tuned near the cesium D2 line (852 nm), and scattered light was detected in the two counter-propagating cavity modes, clockwise and counterclockwise. The interatomic spacing was varied in steps smaller than a nanometer using an acousto-optic deflector. To suppress thermal jitter, the team applied cavity cooling: by tuning the probe detuning to the red sideband of the atomic motion, each scattered photon carried away one phonon. Cooling was performed sequentially on the radial and axial degrees of freedom, alternating the two illumination directions.
As the atom number increased from two to four, the constructive interference peak narrowed as expected for Bragg scattering, and the scattering rate grew faster than linearly. For two atoms, the oscillation period in the clockwise mode was 783.1(4) nm, and in the counterclockwise mode 935.0(6) nm; the ratio of these periods gave the probe incidence angle to within hundredths of a degree. Radial motion was cooled to a mean phonon number of 0.17(4), corresponding to a temperature of 2.1(3) µK and an 85% probability of being in the motional ground state. Axial motion reached 3.4(2.4) phonons. In the chiral regime, a single atom prepared in the state of maximum spin projection scattered light in one direction 23 times more strongly than in the other.
This work stands apart: the nearest neighbors focus on quantum squeezing of molecules, on-chip entanglement, and ion cooling, but no one has combined tweezer arrays with a ring cavity to control the direction of scattering. The key difference is the use of traveling waves instead of standing waves, which removes the dependence on mirror positions and makes interference purely geometric. This is the first realization of cavity cooling of single atoms in tweezers to the ground state in the resolved sideband regime.
The platform naturally extends to studies of non-Hermitian and nonreciprocal physics, where the directionality of scattering plays a key role. Adding fast state readout and atom counting, already demonstrated previously, will allow assembling molecules from atoms and monitoring their collisions in real time. Cavity cooling does not depend on the internal structure of the particles, so it can be applied to ions, molecules, and levitated nanoparticles.
The results directly impact the fidelity of quantum gates in Rydberg arrays, where residual temperature limits fidelity. The sub-nanometer distance calibration method will be useful in quantum metrology.
The immediate next step is to cool the axial motion to the level of the radial motion and check how this improves the contrast of the Bragg peaks. Then, collective oscillation modes in arrays with identical trap frequencies should be studied.
For quantum optics, the open question remains how to scale directional scattering to tens and hundreds of atoms without losing coherence. Also unresolved is the problem of suppressing heating from probe beams during long experiments.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study combines optical tweezer arrays with a ring resonator: cesium atoms are aligned along the resonator axis, a side probe beam near the D2 line scatters into two counterpropagating modes, and the interatomic spacing is controlled with subnanometer precision. Adding atoms from two to four narrows the interference peak, demonstrating Bragg scattering, while cavity cooling brings the radial motion down to 0.17 phonons. Chiral scattering by a single atom is also shown, with a 23-fold asymmetry.
Where its strength is
Using the traveling waves of a ring resonator removes the dependence of interference on mirrors, making it purely geometric, which opens the way to programmable control of scattering. Achieving ground-state cooling in the resolved sideband regime for single atoms in tweezers is a rare result, laying the groundwork for quantum optics on this platform.
Where it could go next
Worth trying
Why this may matter
If the platform can be scaled to tens of atoms and quantum correlations controlled, it could provide a programmable source of nonclassical light for quantum networks and computing, as well as a length standard based on geometric interference.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.56, against a usual 0.64. It looks like few have drilled nearby.
The precision of the interatomic distance in this experiment is such that a shift of about ten femtometers, the diameter of an atomic nucleus, can be detected. The cavity cools atoms to a temperature one hundred million times lower than room temperature.
The idea of controlling light scattering with an array of atoms echoes the 'optical lattices' in Greg Egan's novel 'Permutation City', where ordered structures of matter serve as computational elements.