@article{13514,
  author = {Alec Cao and William Eckner and Theodor Yelin and Aaron Young and Sven Jandura and Lingfeng Yan and Kyungtae Kim and Guido Pupillo and Jun Ye and Nelson Oppong and Adam Kaufman},
  title = {Multi-qubit gates and Schrödinger cat states in an optical clock},
  abstract = {Many-particle entanglement is a key resource for achieving the fundamental precision limits of a quantum sensor1. Optical atomic clocks2, the current state of the art in frequency precision, are a rapidly emerging area of focus for entanglement-enhanced metrology3–6. Augmenting tweezer-based clocks featuring microscopic control and detection7–10 with the high-fidelity entangling gates developed for atom-array information processing11,12 offers a promising route towards making use of highly entangled quantum states for improved optical clocks. Here we develop and use a family of multi-qubit Rydberg gates to generate Schrödinger cat states of the Greenberger–Horne–Zeilinger (GHZ) type with up to nine optical clock qubits in a programmable atom array. In an atom-laser comparison at sufficiently short dark times, we demonstrate a fractional frequency instability below the standard quantum limit (SQL) using GHZ states of up to four qubits. However, because of their reduced dynamic range, GHZ states of a single size fail to improve the achievable clock precision at the optimal dark time compared with unentangled atoms13. Towards overcoming this hurdle, we simultaneously prepare a cascade of varying-size GHZ states to perform unambiguous phase estimation over an extended interval14–17. These results demonstrate key building blocks for approaching Heisenberg-limited scaling of optical atomic clock precision.},
  year = {2024},
  journal = {Nature},
  volume = {634},
  pages = {315-320},
  month = {2024-10},
  isbn = {1476-4687},
  url = {https://doi.org/10.1038/s41586-024-07913-z},
  doi = {10.1038/s41586-024-07913-z},
}