Multi-qubit Gates and Metrology using GHZ states in an Optical Clock

In recent years, entanglement has become a key component in pushing the precision of sensing technology [1]. Using a neutral atom array of Strontium assembled with individually-controlled optical tweezers, we are able to realize a platform for studying quantum-enhanced metrology based upon Rydberg-mediated interactions. Through the creation of highly entangled multi-particle states, we push towards Heisenberg-limited scaling for quantum sensing precision [3].



Here, we report on a novel family of multiqubit gates to generate Greenberger-Horne-Zeilinger (GHZ) states of variable sizes, improvements over the standard quantum limit (SQL) in sensing using fixed-size GHZ states, and a protocol for using cascades of multiple sizes to increase the dynamic range of such measurements. We demonstrate the ability to prepare 2-qubit bell states with >98% raw fidelity, as well as GHZ states of up to 9 atoms. Using 4-qubit states, we demonstrate measurements with a fractional frequency instability of ~10^-14/sqrt(tau/s), representing an improvement of ~2dB over the SQL. Finally, using a newly developed set of multiqubit gates to simultaneously prepare GHZ states of multiple sizes, we demonstrate a proof-of-concept for increasing measurement range while maintaining the sensitivity of up-to-size-8 GHZ states.

References:

[1] L. Pezzè, A. Smerzi, M. K. Oberthaler, R. Schmied, and P. Treutlein, Quantum metrology with nonclassical states of atomic ensembles, Rev. Mod. Phys. 90, 035005 (2018).

[3] E. M. Kessler, P. Kómár, M. Bishof, L. Jiang, A. S. Sørensen, J. Ye, and M. D. Lukin, Heisenberg-Limited Atom Clocks Based on Entangled Qubits, Phys. Rev. Lett. 112, 190403 (2014).