Strontium tweezer arrays for clocks and entanglement

Short answer: Strontium atoms in optical tweezers combine the long-lived optical clock transition of atomic clocks with the single-atom control of quantum computing platforms. A 2020 Science paper reported coherence times of 3.4 seconds in such arrays, and a 2022 Nature Physics paper reported Bell states with a fidelity of 92.8% after correcting for preparation and measurement errors, and a Bell-state coherence lifetime of 4.2 seconds. These are laboratory metrology experiments with small numbers of atoms.

2020: a clock built from tweezers

The Science abstract says leading optical atomic clocks interrogate high-quality-factor transitions in either single ions or ensembles of neutral atoms. The authors demonstrate a platform combining the strengths of both, based on arrays of individual strontium atoms in optical tweezers. They report coherence times of 3.4 seconds, single-ensemble duty cycles up to 96% through repeated interrogation, and a frequency stability of 4.7 times ten to the minus sixteen, scaling as the inverse square root of averaging time in seconds. They say the results establish tweezer arrays as a tool for coherent control of optical transitions for metrology and quantum information science.

2021: toward Rydberg entanglement

A 2021 conference abstract from the APS Division of Atomic, Molecular and Optical Physics meeting describes bosonic strontium in a hybrid apparatus that combines quantum gas microscope resolution with the programmable state preparation of tweezers. The authors added the ability to drive directly from the clock state to a Rydberg state at high Rabi frequency. They report generating entanglement on the optical clock transition with both detuned and adiabatically resonant Rydberg pulses, and discuss implications for a quantum speed-up in optical frequency metrology. They also connect the dynamics to a transverse field Ising model and to spin squeezed and GHZ states. This is a meeting abstract and not a peer-reviewed paper.

2022: Bell states on the clock transition

The Nature Physics abstract says generating long-lived entanglement in optical clocks is a main goal of quantum metrology. The authors use tweezers and a three-dimensional optical lattice to prepare an ensemble of strontium atom pairs in their motional ground state. They engineer global single-qubit gates on the clock transition and two-qubit entangling gates through adiabatic Rydberg dressing. They generate Bell states with a state-preparation-and-measurement-corrected fidelity of 92.8% (uncertainty 2.0), and 87.1% (uncertainty 1.6) without that correction.

They also find the Bell state coherence has a lifetime of 4.2 seconds (uncertainty 0.6), measured by parity correlations and comparisons between entangled and unentangled ensembles. They say such long-lived Bell states could enhance metrological stability and bandwidth. The abstract adds that in the future atomic rearrangement would enable many-qubit gates, cluster states and transverse field Ising model studies. Those are planned directions.

Limits

The goal is better clocks and sensors. None of the abstracts reports a quantum advantage in timekeeping, and the entanglement here involves pairs of atoms. The uncorrected 87.1% fidelity is the more conservative figure.

FAQ

Why use strontium?

Its optical clock transition has a long coherence time, 3.4 seconds in the 2020 experiment.

What is Rydberg dressing?

A technique used in the 2022 paper to create entangling gates between atoms.

How good were the Bell states?

92.8% after correction and 87.1% without, per the authors.

Is this a better clock?

Not yet. The abstracts discuss future benefits for metrology.

Is the 2021 item a paper?

No, it is a meeting abstract.