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Collinear Three-Photon Excitation of a Strongly Forbidden Optical Clock Transition

Samuel P. Carman1,*, Jan Rudolph1,2,*, Benjamin E. Garber1,*, Michael J. Van de Graaff1, Hunter Swan1, Yijun Jiang (姜一君)3, Megan Nantel3, Mahiro Abe1, Rachel L. Barcklay4 et al.

Jason M. Hogan1,†

  • *These authors contributed equally to this work.
  • †Contact author: hogan@stanford.edu
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Phys. Rev. X 15, 031051 – Published 22 August, 2025

DOI: https://doi.org/10.1103/qk3v-46y8

Abstract

The S10−P30 clock transition in strontium serves as the foundation for the world’s best atomic clocks and for gravitational wave detector concepts in clock atom interferometry. This transition is weakly allowed in the fermionic isotope Sr87 but strongly forbidden in bosonic isotopes. Here, we demonstrate coherent excitation of the clock transition in bosonic Sr88 using a novel collinear three-photon process in a weak magnetic field. We observe Rabi oscillations with frequencies of up to 50 kHz using W/cm2 laser intensities and Gauss-level magnetic field amplitudes. The absence of nuclear spin in bosonic isotopes offers decreased sensitivity to magnetic fields and optical lattice light shifts, enabling atomic clocks with reduced systematic errors. The collinear propagation of the laser fields permits the interrogation of spatially separated atomic ensembles with common laser pulses, a key requirement for dark matter searches and gravitational wave detection with next-generation quantum sensors.

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Popular Summary

Many cutting-edge instruments in timekeeping and quantum sensing depend on narrow optical transitions, which determine how precisely we can measure time or small changes in the environment. Today’s most precise atomic clocks rely on ultranarrow transitions—“forbidden” transitions—that naturally occur only in fermions. However, bosonic atoms, which are often more abundant and easier to handle, do not naturally support these transitions. In this work, we demonstrate a new three-photon excitation method that works in any isotope and enables strong coupling to long-lived clock states without being limited by the usual lifetime constraints.

Our approach builds on a concept proposed nearly two decades ago but never fully realized. The key challenge lies in ensuring that all three laser beams used to drive the transition can be aligned in the same direction, which is crucial for long-baseline sensors like atomic interferometers. Angular momentum selection rules typically prohibit this setup, but we overcome that limitation by applying a small magnetic field, allowing collinear propagation. Using this strategy, we successfully implement a bosonic clock atom interferometer driven by a single trichromatic laser pulse.

This new technique enables the use of bosonic atoms in quantum sensors that require long coherence times and precise control, such as gravitational wave detectors and dark matter experiments. It also speeds up transition rates for a given laser intensity, making sensors more efficient and sensitive. Our results pave the way for broader applications of bosonic atoms in quantum technologies, including more scalable and robust tools in quantum sensing and quantum information.

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Phys. Rev. X 15, 031051– Published 22 August, 2025
  • Received 27 June 2024
  • Accepted 2 June 2025
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