A Quantum Capacitance Qubit

We study a new type of superconducting quantum bit (qubit) based on the quantum capacitance (QC) of a cooper-pair box (CPB). Recent developments have shown the importance of manipulating qubits at optimal working points where they are insensitive to environmental noise. The next step is to develop readout schemes that also function at these optimal points. Our approach embeds a CPB in a resonant circuit and detects changes in the QC as changes in the phase of a reflected microwave signal. The QC is simply the curvature of the energy bands near the charge degeneracy point. The curvature arises from the avoided level crossing induced by the Josephson coupling of the CPB. The QC can be much larger than the geometric capacitance, and changes sign between the ground and excited states. Calculations show that the method should be able to achieve single-shot discrimination of the qubit state. We will also present preliminary measurements. Our approach is a lumped-circuit analogy of the recent circuit cQED experiment done at Yale, except our qubit and cavity our not resonant.