title: Tracking photon jumps with repeated quantum non-demolition parity measurements.
abstract: Quantum error correction is required for a practical quantum computer because of the fragile nature of quantum information. In quantum error correction, information is redundantly stored in a large quantum state space and one or more observables must be monitored to reveal the occurrence of an error, without disturbing the information encoded in an unknown quantum state. Such observables, typically multi-quantum-bit parities, must correspond to a special symmetry property inherent in the encoding scheme. Measurements of these observables, or error syndromes, must also be performed in a quantum non-demolition way (projecting without further perturbing the state) and more quickly than errors occur. Previously, quantum non-demolition measurements of quantum jumps between states of well-defined energy have been performed in systems such as trapped ions, electrons, cavity quantum electrodynamics, nitrogen-vacancy centres and superconducting quantum bits. So far, however, no fast and repeated monitoring of an error syndrome has been achieved. Here we track the quantum jumps of a possible error syndrome, namely the photon number parity of a microwave cavity, by mapping this property onto an ancilla quantum bit, whose only role is to facilitate quantum state manipulation and measurement. This quantity is just the error syndrome required in a recently proposed scheme for a hardware-efficient protected quantum memory using Schrödinger cat states (quantum superpositions of different coherent states of light) in a harmonic oscillator. We demonstrate the projective nature of this measurement onto a region of state space with well-defined parity by observing the collapse of a coherent state onto even or odd cat states. The measurement is fast compared with the cavity lifetime, has a high single-shot fidelity and has a 99.8 per cent probability per single measurement of leaving the parity unchanged. In combination with the deterministic encoding of quantum information in cat states realized earlier, the quantum non-demolition parity tracking that we demonstrate represents an important step towards implementing an active system that extends the lifetime of a quantum bit.
authors.affiliation: 1] Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA [2] Center for Quantum Information, Institute for Interdisciplinary Information Sciences, Tsinghua University, Beijing 100084, China (L.S.); Institut für Experimentalphysik, Universität Innsbruck, Technikerstraße 25, A-6020 Innsbruck, Austria and Institut für Quantenoptik und Quanteninformation, Österreichische Akademie der Wissenschaften, Otto-Hittmair-Platz 1, A-6020 Innsbruck, Austria (G.K.).
authors.forename: L
authors.initials: L
authors.lastname: Sun
authors.affiliation: Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: A
authors.initials: A
authors.lastname: Petrenko
authors.affiliation: Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: Z
authors.initials: Z
authors.lastname: Leghtas
authors.affiliation: Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: B
authors.initials: B
authors.lastname: Vlastakis
authors.affiliation: 1] Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA [2] Center for Quantum Information, Institute for Interdisciplinary Information Sciences, Tsinghua University, Beijing 100084, China (L.S.); Institut für Experimentalphysik, Universität Innsbruck, Technikerstraße 25, A-6020 Innsbruck, Austria and Institut für Quantenoptik und Quanteninformation, Österreichische Akademie der Wissenschaften, Otto-Hittmair-Platz 1, A-6020 Innsbruck, Austria (G.K.).
authors.forename: G
authors.initials: G
authors.lastname: Kirchmair
authors.affiliation: Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: K M
authors.initials: KM
authors.lastname: Sliwa
authors.affiliation: Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: A
authors.initials: A
authors.lastname: Narla
authors.affiliation: Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: M
authors.initials: M
authors.lastname: Hatridge
authors.affiliation: Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: S
authors.initials: S
authors.lastname: Shankar
authors.affiliation: Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: J
authors.initials: J
authors.lastname: Blumoff
authors.affiliation: Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: L
authors.initials: L
authors.lastname: Frunzio
authors.affiliation: 1] Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA [2] INRIA Paris-Rocquencourt, Domaine de Voluceau, BP 105, 78153 Le Chesnay Cedex, France.
authors.forename: M
authors.initials: M
authors.lastname: Mirrahimi
authors.affiliation: Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: M H
authors.initials: MH
authors.lastname: Devoret
authors.affiliation: Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: R J
authors.initials: RJ
authors.lastname: Schoelkopf