Solid State and Optics Seminar

Tuesday, December 20, 2005

11:00 a.m.

Becton 4th Floor Alcove

"Quantum Capacitance for Quantum Computation"

Dr. Christopher M. Wilson,
Chalmers University of Technology

Abstract

We present measurements of superconducting quantum bit (qubit) circuits utilizing the quantum capacitance (QC) of a single cooper-pair box (SCB). The QC is essentially the curvature of the SCB energy bands near the charge degeneracy point, and has recently been measured by our group and others. The QC can be much larger than the geometric capacitance, and changes sign between the ground and excited states. We present ground state measurements of one and two qubit circuits with an integrated readout that embeds the SCBs in a resonant circuit and detects changes in the QC as changes in the phase of a reflected microwave signal. We also analyze the readout theoretically using a formalism complementary to that developed at Yale. Our approach is applicable for large detuning between the qubit and oscillator, and valid for arbitrary values of the quality factor (Q) of the the resonator. We have calculated that this readout method is strictly quantum-limited independent of Q. Calculations also show that the method should be able to achieve single-shot discrimination of the qubit state under realistic experimental conditions.

Host: Dan Prober