Initialization by measurement of a superconducting quantum bit circuit.

We demonstrate initialization by joint measurement of two transmon qubits in 3D circuit quantum electrodynamics. Homodyne detection of cavity transmission is enhanced by Josephson parametric amplification to discriminate the two-qubit ground state from single-qubit excitations nondestructively and with 98.1% fidelity. Measurement and postselection of a steady-state mixture with 4.7% residual excitation per qubit achieve 98.8% fidelity to the ground state, thus outperforming passive initialization.

[1]  G. C. Hilton,et al.  Amplification and squeezing of quantum noise with a tunable Josephson metamaterial , 2008, 0806.0659.

[2]  Alexandre Blais,et al.  Tunable joint measurements in the dispersive regime of cavity QED , 2009, 0911.5322.

[3]  Todd A. Brun,et al.  Quantum Computing , 2011, Computer Science, The Hardware, Software and Heart of It.

[4]  Jens Koch,et al.  Coupling superconducting qubits via a cavity bus , 2007, Nature.

[5]  L. Tornberg,et al.  High-fidelity feedback-assisted parity measurement in circuit QED , 2010, 1003.5488.

[6]  G. Milburn,et al.  Quantum Measurement and Control , 2009 .

[7]  Alexandre Blais,et al.  Quantum trajectory approach to circuit QED: Quantum jumps and the Zeno effect , 2007, 0709.4264.

[8]  I. Chuang,et al.  Quantum Computation and Quantum Information: Introduction to the Tenth Anniversary Edition , 2010 .

[9]  Chad Rigetti,et al.  Josephson amplifier for qubit readout , 2011, 1103.1405.

[10]  Denis Vion,et al.  Single-shot qubit readout in circuit quantum electrodynamics , 2009, 1005.5615.

[11]  L. DiCarlo,et al.  Fast reset and suppressing spontaneous emission of a superconducting qubit , 2010, 1003.0142.

[12]  Antonio Corcoles,et al.  Protecting superconducting qubits from radiation , 2011 .

[13]  D. Wineland,et al.  High-fidelity adaptive qubit detection through repetitive quantum nondemolition measurements. , 2007, Physical review letters.

[14]  J. E. Mooij,et al.  Quantum non-demolition measurement of a superconducting two-level system , 2007 .

[15]  Jens Koch,et al.  Suppressing Charge Noise Decoherence in Superconducting Charge Qubits , 2007, 0712.3581.

[16]  Andrew G. Glen,et al.  APPL , 2001 .

[17]  B. Hensen,et al.  High-fidelity projective read-out of a solid-state spin quantum register , 2011, Nature.

[18]  S. Girvin,et al.  Strong coupling of a single photon to a superconducting qubit using circuit quantum electrodynamics , 2004, Nature.

[19]  J. Raimond,et al.  Seeing a single photon without destroying it , 1999, Nature.

[20]  S. Girvin,et al.  Quantum non-demolition detection of single microwave photons in a circuit , 2010, 1003.2734.

[21]  R. J. Schoelkopf,et al.  Resolving photon number states in a superconducting circuit , 2007, Nature.

[22]  Matthias Steiner,et al.  Single-Shot Readout of a Single Nuclear Spin , 2010, Science.

[23]  Denis Vion,et al.  Quantum nondemolition readout using a Josephson bifurcation amplifier , 2007 .

[24]  Austin G. Fowler,et al.  Quantum computing with nearest neighbor interactions and error rates over 1 , 2010, 1009.3686.

[25]  Erik Lucero,et al.  Implementing the Quantum von Neumann Architecture with Superconducting Circuits , 2011, Science.

[26]  R Patil Vijay,et al.  Observation of quantum jumps in a superconducting artificial atom. , 2010, Physical review letters.

[27]  F. Khalili,et al.  Quantum nondemolition measurements: the route from toys to tools , 1996 .

[28]  S. Deleglise,et al.  Quantum jumps of light recording the birth and death of a photon in a cavity , 2006, Nature.

[29]  K. Berggren,et al.  Microwave-Induced Cooling of a Superconducting Qubit , 2006, Science.

[30]  W. Marsden.  I and J , 2012 .

[31]  S. Girvin,et al.  Observation of high coherence in Josephson junction qubits measured in a three-dimensional circuit QED architecture. , 2011, Physical review letters.

[32]  Jens Koch,et al.  Controlling the spontaneous emission of a superconducting transmon qubit. , 2008, Physical review letters.

[33]  Shiro Saito,et al.  Quantum Zeno effect with a superconducting qubit , 2010, 1006.2133.

[34]  John Clarke,et al.  Heralded state preparation in a superconducting qubit. , 2012, Physical review letters.

[35]  S D Bartlett,et al.  Measuring a photonic qubit without destroying it. , 2004, Physical review letters.

[36]  Daniel Nigg,et al.  Experimental Repetitive Quantum Error Correction , 2011, Science.

[37]  S. Girvin,et al.  Cavity quantum electrodynamics for superconducting electrical circuits: An architecture for quantum computation , 2004, cond-mat/0402216.

[38]  M. Devoret,et al.  Invited review article: The Josephson bifurcation amplifier. , 2009, The Review of scientific instruments.

[39]  Spectral measurement of the thermal excitation of a superconducting qubit , 2009, 0912.1956.