title: Heisenberg-limited atom clocks based on entangled qubits.
abstract: We present a quantum-enhanced atomic clock protocol based on groups of sequentially larger Greenberger-Horne-Zeilinger (GHZ) states that achieves 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. The simultaneous interrogation of the laser phase with a cascade of GHZ states realizes an incoherent version of the phase estimation algorithm that enables Heisenberg-limited operation while extending the coherent interrogation time beyond the laser noise limit. We 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.
authors.affiliation: Physics Department, Harvard University, Cambridge, Massachusetts 02138, USA and ITAMP, Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA.
authors.forename: E M
authors.initials: EM
authors.lastname: Kessler
authors.affiliation: Physics Department, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: P
authors.initials: P
authors.lastname: Kómár
authors.affiliation: JILA, National Institute of Standards and Technology, Department of Physics, University of Colorado, Boulder, Colorado 80309-0440, USA.
authors.forename: M
authors.initials: M
authors.lastname: Bishof
authors.affiliation: Department of Applied Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: L
authors.initials: L
authors.lastname: Jiang
authors.affiliation: QUANTOP, Danish National Research Foundation Centre of Quantum Optics, Niels Bohr Institute, DK-2100 Copenhagen, Denmark.
authors.forename: A S
authors.initials: AS
authors.lastname: Sørensen
authors.affiliation: JILA, National Institute of Standards and Technology, Department of Physics, University of Colorado, Boulder, Colorado 80309-0440, USA.
authors.forename: J
authors.initials: J
authors.lastname: Ye
authors.affiliation: Physics Department, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: M D
authors.initials: MD
authors.lastname: Lukin