Strontium optical lattice clocks have the potential to simultaneously interrogate millions of atoms with a spectroscopic quality factor Q ≈ 4 × 10^17. Previously, atomic interactions have forced a compromise between clock stability, which benefits from a large atom number, and accuracy, which suffers from density-dependent frequency shifts. Here, researchers demonstrate a scalable solution which takes advantage of the high, correlated density of a degenerate Fermi gas in a three-dimensional optical lattice to guard against on-site interaction shifts. Using a state-of-the-art ultra-stable laser, they achieve an unprecedented level of atom-light coherence, reaching Q = 5.2 × 10^15 with 1 × 10^4 atoms. They investigate clock systematics unique to this design; in particular, they show that contact interactions are resolved so that their contribution to clock shifts is orders of magnitude lower than in previous experiments, and they measure the combined scalar and tensor magic wavelengths for state-independent trapping along all three lattice axes.
The work here takes an initial but important step towards reaching the ultimate limit of performance for strontium optical lattice clocks, where millions of atoms could be interrogated for coherence times greater than 100 s.
A Fermi-degenerate 3D optical lattice clock. (A) Momentum distribution data of a two-spin Fermi gas after being
released from the crossed optical dipole trap. (B) Schematic showing propagation direction (large arrows) and polarization (double arrows) of the 3D lattice and clock laser beams. (C)Motional sideband spectroscopy using the oblique clock laser shows no observable red sidebands, illustrating that atoms are predominantly in the ground band of the lattice.
Optical lattice clocks have now entered the quantum degenerate regime. With atoms that are frozen into a 3D cubic lattice, we have advanced the state-of-the art in coherent atom-light interrogation times. Further improvements will be enabled by the next generations of ultra-stable optical reference cavities based on crystalline materials. The latest advances in the frequency references and local oscillators that together constitute atomic clocks will lead to a new era for clock performance, resulting in new measurement capabilities.
Quantum degenerate clocks also provide a promising platform for studying many-body physics. Future studies of dipolar interactions will not only be necessary for clock accuracy, but will also provide insight into long-range quantum spin systems in a regime distinct from those explored by polar molecules, Rydberg gases and highly magnetic atoms. When clocks ultimately confront the natural linewidth of the atomic frequency reference, degenerate Fermi gases may be useful for engineering longer coherence times through Pauli blocking of spontaneous emission or collective radiative effects. Ultracold quantum gases provide new capabilities for precision metrology.
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