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Relaxation Effects in Cooper Pair Box Qubits

April 25, 2011 - 12:30pm
Ben Palmer
Laboratory for Physical Sciences

 I will describe and contrast two different sets of measurements we have made on the lifetime of the excited state of a charge qubit. In our case, the charge qubit is a superconducting Al/AlOx/Al single-Cooper-pair box that is cooled to 20 mK. Using a dissipative measurement and decoupling our qubit from charge noise, we were able to obtain lifetimes of a few microseconds. This measurement technique allowed us to use the Cooper-pair box as a “quantum spectrum analyzer” and showed that anomalous charge fluctuators in the Josephson tunnel junction can cause the lifetime of the qubit to decrease to a few hundred nanoseconds.† More recently, we have used a circuit QED scheme to dispersively measure the lifetime of a charge qubit. Using this scheme we have found a strong correlation between the lifetime of the qubit and the inverse of the coupling to the dissipative environment. At the smallest coupling, we have measured a maximum lifetime of 200 microseconds which represents an order of magnitude improvement from previous results.‡ This long lifetime places a bound in the dielectric loss of the Josephson junction barrier less than tan < 10-7.

  1. † Z. Kim et al., “Anomalous avoided level crossings in a Cooper-pair box spectrum,” Physical Review B 78, 144506 (2008).
  2. ‡ Z. Kim et al., “Decoupling a Cooper-pair box to enhance the lifetime to 0.2 ms,” arXiv:1101.4692v1 (2011).

* In collaboration with Zaeill Kim,1,2,4 Baladitya Suri,1,2 Vitaley Zaretskey,1,2 Sergey Novikov,1,2 Ari Mizel,1 Kevin Osborn,1 Matt Shaw,3 Pierre Echternach,3 and Fred Wellstood.2,4

  1. [1] Laboratory for Physical Sciences, College Park, MD 20740
  2. [2] Department of Physics, University of Maryland, College Park, MD 20742
  3. [3] Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109
  4. [4] Joint Quantum Institute and Center for Nanophysics and Advanced Materials, College Park, MD 20742
1201 Physics Building
College Park, MD 20742