Relaxation Effects in Cooper Pair Box Qubits
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.
- † Z. Kim et al., “Anomalous avoided level crossings in a Cooper-pair box spectrum,” Physical Review B 78, 144506 (2008).
- ‡ 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
-  Laboratory for Physical Sciences, College Park, MD 20740
-  Department of Physics, University of Maryland, College Park, MD 20742
-  Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109
-  Joint Quantum Institute and Center for Nanophysics and Advanced Materials, College Park, MD 20742
Subscribe to A Quantum Bit
Quantum physics began with revolutionary discoveries in the early twentieth century and continues to be central in today’s physics research. Learn about quantum physics, bit by bit. From definitions to the latest research, this is your portal. Subscribe to receive regular emails from the quantum world. Previous Issues...
Sign Up Now
Sign up to receive A Quantum Bit in your email!