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arXiv:2610.05983 · Physical Review Applied

Sub-Doppler cooling in a compact conical hollow mirror integrated with anti-Helmholtz coils

Shintaro Nagase, Naoya Ozawa, Teruhito Nakashita, Masaki Nakazawa, Kota Abe, Yu Nezu, Ryosuke Tsutsui, Hideki Ueno, Hiromitsu Haba, Y. Sakemi

Published Sep 30, 2026• 0 citations
arXivHTMLPDF

Abstract

We demonstrate sub-Doppler cooling of rubidium ($^{87}$Rb) atoms using a compact, single-beam configuration based on a conical hollow mirror integrated with anti-Helmholtz coils. The conical geometry converts a single incident beam into an effective six-beam magneto-optical trap (MOT) configuration while focusing the light towards the cone axis, thereby reducing the required laser power. The number of atoms in the conical MOT is maximized at an incident intensity of 6.5 $\mathrm{mW/cm^2}$ in our conical MOT, several times lower than in conventional six-beam MOTs. Under optimal conditions, we trap $\sim 1 \times 10^7$ $^{87}$Rb atoms. We obtain sub-Doppler temperature of $T_r=13.4\pm 1.2\ \mathrm{μK}$ and $T_z=106\pm 11\ \mathrm{μK}$ by polarization gradient cooling (PGC), revealing a strong anisotropy that we attribute to the polarization pattern created by the conical mirror. In addition, a modified release-and-recapture measurement in the MOT yields a radial temperature $T_r = 1.20\pm 0.02$ mK, approximately five times higher than that expected for comparable light-shift parameters in a six-beam MOT, in agreement with previous simulations. Our results establish conical hollow mirrors as a practical and power-efficient platform for compact cold-atom sources and integrated quantum sensors.

DOI
10.1103/t6hg-bmk9

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We demonstrate sub-Doppler cooling of rubidium (87^{87}87Rb) atoms using a compact, single-beam configuration based on a conical hollow mirror integrated with anti-Helmholtz coils. The conical geometry converts a single incident beam into an effective six-beam magneto-optical trap (MOT) configuration while focusing the light towards the cone axis, thereby reducing the required laser power. The number of atoms in the conical MOT is maximized at an incident intensity of 6.5 mW/cm2\mathrm{mW/cm^2}mW/cm2 in our conical MOT, several times lower than in conventional six-beam MOTs. Under optimal conditions, we trap ∼1×107\sim 1 \times 10^7∼1×107 87^{87}87Rb atoms. We obtain sub-Doppler temperature of Tr=13.4±1.2 μKT_r=13.4\pm 1.2\ \mathrm{μK}Tr​=13.4±1.2 μK and Tz=106±11 μKT_z=106\pm 11\ \mathrm{μK}Tz​=106±11 μK by polarization gradient cooling (PGC), revealing a strong anisotropy that we attribute to the polarization pattern created by the conical mirror. In addition, a modified release-and-recapture measurement in the MOT yields a radial temperature Tr=1.20±0.02T_r = 1.20\pm 0.02Tr​=1.20±0.02 mK, approximately five times higher than that expected for comparable light-shift parameters in a six-beam MOT, in agreement with previous simulations. Our results establish conical hollow mirrors as a practical and power-efficient platform for compact cold-atom sources and integrated quantum sensors.
We present a novel and simple vapour-cell magneto-optical atom trap in a pyramidal and a conical hollow mirror cavity. A single laser beam having modulation sidebands at microwaves is used for cooling, trapping and repumping of rubidium atoms. When the laser is circularly polarized and sent into the hollow region, three pairs of counterpropagating beams are automatically produced therein, having the same polarization configuration as in the conventional six beam magneto-optical trap. The precooled atom sources thus produced may be used to obtain much colder and denser atoms for study of their quantum statistical properties.
We present a novel and simple magneto-optical trap in pyramidal and in conical hollow mirrors, using a single beam. A diode laser having modulation sidebands at microwaves is used for cooling, trapping, and repumping of rubidium atoms in a vapor cell. When the laser is circularly polarized and sent into the hollow region, three pairs of counterpropagating beams are automatically produced therein that have the same polarization configuration as that of a conventional six-beam magneto-optical trap. The fluorescence by the trapped atoms and its mirror image are observed simultaneously. This system may be useful for atom-manipulation applications such as gravitational atom traps and atom waveguides.
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Abstract In order to realize a convenient and efficient laser cooling of a single atom, we propose a simple and promising scheme to cool a single neutral atom in a blue-detuned localized hollow-beam trap by intensity-gradient induced Sisyphus cooling, and study the dynamic process of the intensity-gradient cooling of a single 87 Rb atom in the localized hollow-beam trap by using Monte-Carlo simulations. Our study shows that a single 87 Rb atom with a temperature of 120 μ K from a magneto-optical trap (MOT) can be directly cooled to a final temperature of 4.64 μ K in our proposed scheme. We also investigate the dependences of the cooling results on the laser detuning δ of the localized hollow-beam, the power RP 0 of the re-pumping laser beam, the sizes of both the localized hollow-beam and the re-pumping beam, and find that there is a pair of optimal cooling parameters ( δ and RP 0 ) for an expected lowest temperature, and the cooling results strongly depend on the size of the re-pumping beam, but weakly depend on the size of the localized hollow-beam. Finally, we further study the cooling potential of our localized hollow-beam trap for the initial temperature of a single atom, and find that a single 87 Rb atom with an initial temperature of higher than 1 mK can also be cooled directly to about 6.6 μ K.
ENGLISH ABSTRACT: A Magneto Optical Trap (MOT) is a configuration formed by three orthogonal pairs of counterpropagating circularly polarized laser beams and a magnetic field gradient. A MOT is used to cool, capture and trap large numbers of atoms in vacuum at very low temperature in K range. In this thesis the development of an experimental setup for realising a MOT of 87Rb atoms is presented. The atomic structure of Rb and the theoretical background of laser cooling and magneto optical trapping was reviewed. The influence of rubidium background pressure in the vacuum system, the laser beam size and the power and frequency on the number of the trapped atoms were studied in literature. The trapping and repumping lasers were characterised experimentally. Six circularly polarised trapping beams with equal power were formed and properly aligned to intersect at the center of the trapping cell. Two optical setups were designed and exploited to investigate and optimise the trapping beam polarisation. The repumping laser beam was successfully aligned and colinearly combined into all the trapping beams. Three different experimental setups for saturated absorption spectroscopy were developed. Saturated absorption spectra showing the hyperfine structure of both 85Rb and 87Rb isotopes were measured and are discussed. Using two saturated absorption spectroscopy setups, the frequencies of both lasers were successfully locked to the trapping and repumping transitions of 87Rb respectively. A rectangular trapping cell was designed and attached to the vacuum system. A pressure of about 10­¯7 mbar was achieved. The magnetic field coils were characterised and affixed on both sides of the cell in an anti-Helmholtz configuration. Setups for imaging and quantification of the 87Rb atoms in the MOT were designed. Finally, the procedures for demonstrating a MOT are presented. In conclusion, the current status of the project is reported, with recommendations for the future work.
The magneto-optical trap (MOT) is an essential tool for collecting and preparing cold atoms with a wide range of applications. We demonstrate a planar-integrated MOT by combining an optical grating chip with a magnetic coil chip. The flat grating chip simplifies the conventional six-beam configuration down to a single laser beam, and the flat coil chip replaces the conventional anti-Helmholtz coils of cylindrical geometry. Up to 106{10}^{6}106 cold 87Rb{}^{87}\mathrm{Rb}87Rb atoms are trapped by the planar-integrated MOT, with the atom cloud being approximately 6mm6\phantom{\rule{0.2em}{0ex}}\mathrm{mm}6mm above the chip surface. This configuration effectively reduces the volume, weight, and complexity of the MOT, bringing benefits to applications including gravimeter, clock, and quantum memory devices.