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Session P1b: Advances in Ground Atomic Clocks

Cold-Atom Two-Photon Clock of 88Sr on the 1S0-1D2 Transition with Magic Polarization
Kai Suekane, Hidetoshi Katori, The University of Tokyo
Location: Royal Ballroom AB
Date/Time: Tuesday, Jan. 27, 5:08 p.m.

We report the development of a cold-atom two-photon (TP) clock of 88Sr on the 1S0–1D2 transition. Optical lattice clocks have established record-breaking stability and accuracy, making them the benchmark for precision timekeeping. However, their reliance on long ultra-stable cavities imposes constraints on portability. To explore complementary architectures that are more compact and robust, we implement a TP clock using a narrow-linewidth diode laser combined with continuous atom interrogation, thereby eliminating dead time and enabling stability scaling as 1/tau at short averaging times.
Our approach is to develop a TP clock that does not rely on large ultrastable cavities. Instead, we assume a compact narrow-linewidth diode laser with an instability of approximately 10^-13 at 10 ms as the frequency reference [1]. To fully exploit such a laser, a fast servo attack time of about 10 ms is preferred, together with a continuous, dead-time-free interrogation scheme that allows the clock stability to improve proportionally to 1/tau.
The 1S0–1D2 TP transition has a natural linewidth of about 300 Hz, which is suitable for precise spectroscopy, and allows indirect detection through the strong 3P2–3D3 transition at 496 nm. Counterpropagating clock beams suppress the first-order Doppler shift, while choosing the m=0 - m=0 transition minimizes the first-order Zeeman sensitivity. A central challenge of TP clocks—the ac Stark shift induced by the clock laser—is mitigated by operating at a “magic polarization” angle [2], where the shift is canceled. This mechanism, previously predicted theoretically [3], is experimentally confirmed in our system.
In the experiment, TP spectroscopy is performed while a magneto-optical trap (MOT) on the 496 nm transition is continuously maintained. This configuration supplies a steady stream of cold 88Sr atoms in the ground state, allowing uninterrupted interrogation. Fluorescence from the MOT provides a sensitive readout channel, and wavelength-modulation spectroscopy generates an error signal used for frequency stabilization. The clock laser, pre-stabilized to an optical cavity, is further locked to the atomic transition through this error signal. The stability of the optical cavity is floored at 10^-16 and is much better than we expect for this clock, so we add a flicker noise to the laser that is stabilized to the cavity. This added noise worsens the stability of the pre-stabilized clock laser to 10^-13 level without feedback from atoms. Frequency comparison with a strontium optical lattice clock is performed via an optical frequency comb.
The TP clock demonstrates a servo response time of a few milliseconds, and its frequency stability improves proportionally to 1/tau at short averaging times, reaching the 10^-15 level within one second. This rapid approach to high stability is enabled by continuous interrogation without dead time. At longer timescales, the influence of the clock-laser-induced ac Stark shift is suppressed by operation at the magic polarization, ensuring robustness against intensity fluctuations. Cold atoms further suppress residual Doppler shifts caused by imperfect beam alignment, a limitation that often affects vapor-cell and beam-based TP clocks.
In summary, we demonstrate a cold-atom TP clock on the 88Sr 1S0–1D2 transition that offers several unique advantages. Continuous operation with cold atoms allows stability to improve as 1/tau, reaching the 10^-15 level at one second, while the experimentally verified magic polarization secures long-term stability against clock-laser-induced ac Stark shift. These results highlight that TP clocks can achieve both short- and long-term stability without reliance on long ultra-stable cavities, making them a promising compact and transportable platform for precision timekeeping in future scientific and technological applications.
[1] W. Liang, et al. Nat. Commun. 6, 7371 (2015).
[2] T. Ido, et al. PRL 91, 053001 (2003).
[3] S. Jackson, et al, PRA 99, 063422 (2019).

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