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Nanosecond Ramsey spectroscopy for Ca beam optical clocks

Jingming Chen, Tianyu Liu, Jie Miao, Zheyi Ge, Haosen Shang, Yabei Su, Xiaobo Xue, Duo Pan, Jingbiao Chen

Published Nov 11, 2025• 0 citations
Publisher

Abstract

Atomic beam optical clocks offer continuous operation, simplified structure, and excellent short- to mid-term stability, making them promising candidates for next-generation optical flywheel clocks. However, the atomic utilization efficiency in current thermal beam optical clocks remains below 1%, limiting their further performance enhancement. Therefore, maximizing the utilization of atoms within the atomic beam has become a critical problem. In this work, we propose an innovative nanosecond Ramsey spectroscopy for Ca beam optical clocks, which enables the full utilization of atoms with all transverse velocities. The spectral amplitude is enhanced 2300 times compared to conventional Ca beam optical clocks, and the frequency stability can reach the quantum projection noise limit of $$1.2\times 1{0}^{-17}/\sqrt{\tau }$$ in theory, comparable to the best cold-atom optical clocks. This approach paves the way for high-precision, transportable flywheel optical clocks.

DOI
10.1038/s41534-025-01114-w

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Atomic beam optical clocks offer continuous operation, simplified structure, and excellent short- to mid-term stability, making them promising candidates for next-generation optical flywheel clocks. However, the atomic utilization efficiency in current thermal beam optical clocks remains below 1%, limiting their further performance enhancement. Therefore, maximizing the utilization of atoms within the atomic beam has become a critical problem. In this work, we propose an innovative nanosecond Ramsey spectroscopy for Ca beam optical clocks, which enables the full utilization of atoms with all transverse velocities. The spectral amplitude is enhanced 2300 times compared to conventional Ca beam optical clocks, and the frequency stability can reach the quantum projection noise limit of 1.2×10−17/τ1.2\times 1{0}^{-17}/\sqrt{\tau }1.2×10−17/τ​ in theory, comparable to the best cold-atom optical clocks. This approach paves the way for high-precision, transportable flywheel optical clocks.
We demonstrate an optical clock based on Ramsey-Bordé interferometry in a laser-cooled ^{40}Ca beam. The mean velocity is reduced by an order of magnitude relative to a thermal beam and the transverse temperature approaches the Doppler limit, enabling the measurement of sub-kHz linewidth fringes in a compact interferometer. Using tailored phase and intensity modulation of the spectroscopy laser to add uniform frequency sidebands, we interrogate atoms throughout the transverse velocity distribution, increasing the Ramsey-Bordé fringe amplitude by a factor of 14 and improving the Allan deviation to 3.4×10^{-15} at one second averaging time.
A thermal atomic ensemble-based laser source with superior frequency stability is proposed that relies on the accumulated contributions from an abundance of nonzero-transverse-velocity atomic ensembles. Compared with the traditional case in which only atoms with near-zero transverse velocities are utilized, the amplitude of the optical Ramsey fringes for a thermal calcium beam can be dramatically enhanced by three orders of magnitude or more, thus, the signal-to-noise ratio can be improved 33-fold. Based on the recent results of atomic interferometry-based laser stabilization, a quantum projection noise-limited frequency instability less than 2E-17/tau^0.5 is feasible. Such an ultrastable laser has promising applications in diverse areas, including metrology and astronomy.
We demonstrate how to realize an optical clock with neutral atoms that is competitive to the currently best single ion optical clocks in accuracy and superior in stability. Using ultracold atoms in a Ca optical frequency standard, we show how to reduce the relative uncertainty to below 10(-15). We observed atom interferences for stabilization of the laser to the clock transition with a visibility of 0.36, which is 70% of the ultimate limit achievable with atoms at rest. A novel scheme was applied to detect these atom interferences with the prospect to reach the quantum projection noise limit at an exceptional low instability of 4 x 10(-17) in 1 s.
Herein, we report the scheme of an optically pumped atomic clock based on a cold cesium atomic beam source. We propose the laser system and physical mechanism of this atomic clock, wherein the atomic beam travels in an upper parabolic trajectory, thereby eliminating the light shift effect. In the experiments, when the length of the free evolution region was 167 mm, the line width of the Ramsey fringe was 37 Hz. When the expected signal-to-noise ratio of the Ramsey fringe that can be achieved is 36,000, the expected short-term frequency stability is about 3.6 × 10–14/√τ, which is significantly higher than that of a conventional optically pumped cesium clock of similar volume.
We propose an atomic fountain clock with a truncated cold atomic beam to achieve both a low collisional frequency shift and high frequency stability. In this clock, the launching velocity of a cold atomic beam can be swept to reduce the atomic density in the interrogation region for the Ramsey resonance and to increase the atomic density in the detection region. Before the top of the beam arrives at the interrogation region, the cold atomic beam is truncated by turning off the cooling laser beams to remove the unnecessary light shift. The atomic density in the interrogation region is theoretically evaluated to be 0.04 times that in an ordinary atomic fountain with optical molasses for the same number of detected atoms. The frequency stability limit due to quantum projection noise is calculated to reach $6.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ in 1 s from the number of detected atoms while the fractional collisional shift is estimated to be $~\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$.
Abstract We propose a longitudinal Ramsey spectroscopy of atoms in a moving optical lattice. Spatially separated magnetic fields, which open the forbidden 1 S 0 − 3 P 0 transition by mixing with the dipole-allowed states, create Ramsey pulses for a probe laser introduced along the optical lattice. Excitation of atoms in the Lamb-Dicke regime by a single probe laser avoids residual Doppler effects as well as mechanical instabilities that deteriorate Ramsey spectroscopy on an atomic beam. Continuous measurement and control of the probe laser improves the Allan deviation as τ −1 for an averaging time τ until the quantum projection noise limit is reached. For 88 Sr atoms with an atomic flux of 10 4 s −1 , we speculate a clock instability of 5 × 10 −16 ( τ s −1 ) −1 down to its floor at around 10 −18 .