Qubit-photon interactions in a cavity: Measurement-induced dephasing and number splitting

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Jay Gambetta,¹ Alexandre Blais,^1,2 D. I. Schuster,¹ A. Wallraff,^1,3 L. Frunzio,¹ J. Majer,¹ M. H. Devoret,¹ S. M. Girvin,¹ and R. J. Schoelkopf¹
¹Department of Applied Physics and Department of Physics, Yale University, New Haven, Connecticut 06520, USA
²Département de Physique et Regroupement Québécois sur les Matériaux de Pointe, Université de Sherbrooke, Sherbrooke, Québec, Canada, J1K 2R1
³Department of Physics, ETH Zurich, CH-8093 Zürich, Switzerland

Received 13 February 2006; published 17 October 2006

Wetheoretically study measurement-induced dephasing of a superconducting qubit in thecircuit QED architecture and compare the results to those obtainedexperimentally by Schuster et al. [Phys. Rev. Lett. 94, 123602 (2005)].Strong coupling of the qubit to the resonator leads toa significant ac Stark shift of the qubit transition frequency.As a result, quantum fluctuations in the photon number populatingthe resonator cause dephasing of the qubit. We find goodagreement between the predicted line shape of the qubit spectrumand the experimental results. Furthermore, in the strongly dispersive limit,where the Stark shift per photon is large compared tothe cavity decay rate and the qubit linewidth, we predictthat the qubit spectrum will be split into multiple peaks,with each peak corresponding to a different number of photonsin the cavity.

URL:	http://link.aps.org/doi/10.1103/PhysRevA.74.042318
DOI:	10.1103/PhysRevA.74.042318
PACS:	03.67.Lx; 73.23.Hk; 74.50.+r; 32.80.-t 03.67.Lx Quantum computation 73.23.Hk Coulomb blockade; single-electron tunneling 74.50.+r Tunneling phenomena; point contacts, weak links, Josephson effects in superconductors 32.80.-t Photon interactions with atoms YEAR: 2006
KEYWORDS:	quantum computing, quantum optics, Stark effect, fluctuations, superconducting devices

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