Heisenberg-Limited Atom Clocks Based on Entangled Qubits

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E.  M. Kessler, P. Kómár, M. Bishof, L. Jiang, A.  S. Sørensen, J. Ye, M.  D. Lukin
Phys. Rev. Lett. 112, 190403 (2014)

The improvement of frequency standards using quantum resources, such as entanglement has been actively explored in recent years. The use of entangled resources, in principle, allows one to surpass the classical limit on precision. However, a characterization of the improvement obtainable by using entanglement requires a detailed investigation of the decoherence present in the system.

In their work, Kessler and co-workers present a quantum-enhanced atomic clock protocol based on sets of sequentially larger Greenberger-Horne-Zeilinger (GHZ) states that achieve the best clock stability allowed by quantum theory up to a logarithmic correction. Importantly, the protocol is designed to work under realistic conditions where the drift of the phase of the laser interrogating the atoms is the main source of decoherence. They compare and merge the new protocol with existing state of the art interrogation schemes, and identify the precise conditions under which entanglement provides an advantage for clock stabilization: it allows a significant gain in the stability for short averaging time.