Multi-qubit gates and Schr\"odinger cat states in an optical clock

TL;DR

This paper demonstrates the creation of multi-qubit Schr"odinger cat states in an optical clock using Rydberg gates, achieving sub-standard quantum limit frequency instability and exploring paths toward Heisenberg-limited clock precision.

Contribution

It introduces a family of multi-qubit Rydberg gates for generating GHZ states in optical clocks and shows their potential for enhanced precision.

Findings

Achieved GHZ states of up to 9 qubits in an optical clock.

Demonstrated fractional frequency instability below the standard quantum limit with 4-qubit GHZ states.

Prepared a cascade of GHZ states for extended phase estimation.

Abstract

Many-particle entanglement is a key resource for achieving the fundamental precision limits of a quantum sensor. Optical atomic clocks, the current state-of-the-art in frequency precision, are a rapidly emerging area of focus for entanglement-enhanced metrology. Augmenting tweezer-based clocks featuring microscopic control and detection with the high-fidelity entangling gates developed for atom-array information processing offers a promising route towards leveraging highly entangled quantum states for improved optical clocks. Here we develop and employ a family of multi-qubit Rydberg gates to generate Schr\"odinger cat states of the Greenberger-Horne-Zeilinger (GHZ) type with up to 9 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 using GHZ states of up to 4 qubits. However, due to their reduced dynamic range, GHZ states of a single size fail to improve the achievable clock precision at the optimal dark time compared to unentangled atoms. Towards overcoming this hurdle, we simultaneously prepare a cascade of varying-size GHZ states to perform unambiguous phase estimation over an extended interval. These results demonstrate key building blocks for approaching Heisenberg-limited scaling of optical atomic clock precision.