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arXiv:2406.07902

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

Samuel P. Carman, Jan Rudolph, Benjamin E. Garber, Michael J. Van de Graaff, Hunter Swan, Yijun Jiang, Megan K. T. Nantel, Mahiro Abe, Rachel L. Barcklay, Jason M. Hogan

Published Jun 12, 2024• 2 citations
arXivHTMLPDF

Abstract

The ${{^1\mathrm{S}_0}\!-\!{^3\mathrm{P}_0}}$ 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 $^{87}$Sr but strongly forbidden in bosonic isotopes. Here, we demonstrate coherent excitation of the clock transition in bosonic ${}^{88}$Sr using a novel collinear three-photon process in a weak magnetic field. We observe Rabi oscillations with frequencies of up to $50~\text{kHz}$ using $\text{W}/\text{cm}^{2}$ 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.

DOI
10.48550/arxiv.2406.07902

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The 1S0 ⁣− ⁣3P0{{^1\mathrm{S}_0}\!-\!{^3\mathrm{P}_0}}1S0​−3P0​ 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 87^{87}87Sr but strongly forbidden in bosonic isotopes. Here, we demonstrate coherent excitation of the clock transition in bosonic 88{}^{88}88Sr using a novel collinear three-photon process in a weak magnetic field. We observe Rabi oscillations with frequencies of up to 50 kHz50~\text{kHz}50 kHz using W/cm2\text{W}/\text{cm}^{2}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.
The S 1 0 − P 3 0 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 Sr 87 but strongly forbidden in bosonic isotopes. Here, we demonstrate coherent excitation of the clock transition in bosonic Sr 88 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 / cm 2 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.
We report direct excitation of the strongly forbidden ${5s}^{2}{}^{1}{S}_{0}\ensuremath{-}5s5p{}^{3}{P}_{0}$ transition in 87Sr.{}^{87}\mathrm{Sr}.87Sr. Its frequency is 429 228 004 235(20) kHz. A resonant laser creates a small leak in a magneto-optical trap (MOT): atoms build up to the metastable 3P0{}^{3}{P}_{0}3P0​ state and escape the trapping process, leading to a detectable decrease in the MOT fluorescence. This line has a natural width of ${10}^{\ensuremath{-}3}\mathrm{Hz}$ and can be used for a new generation of optical frequency standards using atoms trapped in a light-shift-free dipole trap.
We report an experimental observation of the weak 'SO-~PO transition in 171,173Ybl and new approaches for qubits and optical clocks with this single-photon resonance and a three-photon resonance in an even isotope, respectively. Introduction - Optical clocks show great promise as the next generation of precision frequency standards due to the high frequency and correspondingly high line-Q of weakly allowed atomic transitions. Thanks to femtosecond comb technology (l), clocks based on optical transitions in both single trapped ions and neutral atom clouds are poised to exceed the already stringent bounds on precision set by microwave frequency standards. A recent proposal combining the Lamb-Dicke confinement of single ion frequency standards with the high signal-tenoise of neutral atom cLouds, and exploiting a highly forbidden transition insensitive to external magnetic fields and trap light polarization has the potential to do even better (2). The scheme makes use of the 'SO-~P~ electric dipole transition in alkaline-earth atoms, weakly allowed in odd isotopes by the hyperfine interaction of the nuclear spin and completely forbidden by a single-photon process in even isotopes. Trapping the atoms in a Stark-free optical httice will allow a large ensemble of atoms to exhibit a spectrum free of Doppler and recoil shifts, leading to an estimated accuracy of a few parts in lo1* (3, 41. As an additional application, the long atomic coherence times in optical lattices also suggest using the 'SO-~PO pairs of states as an array of qubits for quantum information (5, 61. Yb and Sr are both strong candidates for an optical lattice clock as well as for creating useful qubits. Both atoms have experimentally accessible energy level transitions, allowing the atoms to be readily laser-cooled and trapped, and in the case of Yb, Eose-Einstein condensed. While the Sr clock transition has been studied extensively, both in and out of an optical lattice, the same transition in Yb has thus far remained unexplored experimentally. Here we report the direct excitation of the 578 nm 'SO-~P~ transition in odd isotope 171,173Yb. (We understand that a group at NIST has also observed this transition (7)). We also discuss the possible application of this type of transitions in making qubits for quantum computation and a clock scheme based on three-photon transition in an even isotope (8, 91. - The level structure of Yb and the experimental apparatus are diagrammed in Fig. 1. The experiment is performed on a sample of cold atoms collected in a magneto- optical trap (MOT). To excite the clock transition, we shut off the MOT beams and irradiate the cloud of atoms with a 578 nm probe. JVe t,hen turn the MOT beams back on, continuously monitoring 399 nm fluorescence from the atom cloud. As the probe laser frequency is scanned over the dock transition, there is an observable decrease in MOT fluorescence as atoms are excited into the metastable 3Po state and escape the trap. By chopping trap and probe beams out of phase, we avoid light shifts of the clock transition due to near-resonant 399 nm light while maintaining a steady-state MOT population. The MOT fluorescence reduction of 171Yb is shown in Fig. 2 (a). Since iodine lines have been well measured in Observation of the 1So-3P, Transition in 1713173Yb
We demonstrate two Doppler-free (DF) excitation schemes for coherent manipulation of the 1 ⁣S0−3 ⁣P0{}^1\!S_0 - {}^3\!P_01S0​−3P0​ clock transition in 88^{88}88Sr that achieve microsecond-scale excitation times. The first approach uses three-photon excitation with distinct phase-coherent spectral components to couple the ground and clock states while canceling the first-order Doppler shift. The second approach is a sequential protocol that combines a single-photon excitation with a two-photon Raman excitation, reducing coherent clock transition manipulation times to below a microsecond. With both methods, we perform high-contrast Ramsey spectroscopy on thermal ensembles of 3×1063 \times 10^{6}3×106 atoms in free space. We observe three orders of magnitude suppression of Doppler dephasing compared to single-photon excitation, relaxing the need for tight confinement or ultra-low temperatures. These broadly applicable techniques enable fast, coherent manipulation of narrow-line transitions, with implications for optical atomic clocks, matter-wave interferometers, quantum-enhanced metrology, and quantum information processing.
We observe strong collective coupling between an optical cavity and the forbidden spin singlet to triplet optical transition 1S0^{1}S_{0}1S0​ to 3P1^{3}P_{1}3P1​ in an ensemble of 88Sr^{88}\text{Sr}88Sr. Despite the transition being 1000 times weaker than a typical dipole transition, we observe a well-resolved vacuum Rabi splitting. We use the observed vacuum Rabi splitting to make nondestructive measurements of atomic population with the equivalent of projection-noise limited sensitivity between subsequent measurements and with minimal heating [<0.01<0.01<0.01 (photon recoils)/atom]. This technique may be used to enhance the performance of optical lattice clocks by generating entangled states and reducing dead time.
We present detailed analyses of our recent experiment on the three-dimensional (3D) optical lattice clock with bosonic 88Sr^{88}\mathrm{Sr}88Sr atoms in which the collisional frequency shift was suppressed by applying a single-occupancy lattice. Frequency shifts in magnetically induced spectroscopy on the 1S0−−3P0{}^{1}{S}_{0}--{}^{3}{P}_{0}1S0​−−3P0​ clock transition ($\ensuremath{\lambda}=698$ nm) of 88Sr^{88}\mathrm{Sr}88Sr were experimentally investigated by referencing a one-dimensional (1D) lattice clock based on spin-polarized 87Sr^{87}\mathrm{Sr}87Sr atoms. We discuss that the clock stability is limited by the current laser stability as well as the experimental sequence of the clock operation, which may be improved to ${\ensuremath{\sigma}}_{y}(\ensuremath{\tau})=2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}/\sqrt{\ensuremath{\tau}}$ by optimizing the cycle time of the clock operation.