Coherent dynamics of a Josephson charge qubit

TL;DR

This paper demonstrates coherent control and measurement of a Josephson charge qubit, showing quantum oscillations, high charge state polarization, and a method to determine relaxation times.

Contribution

It introduces a Josephson charge qubit with RF-SET readout, achieving coherent manipulation and a new method to measure relaxation time T_1.

Findings

Quantum oscillations persist for ~10 ns.

Charge state polarization approaches 100%.

Charge quantization of 2e observed.

Abstract

We have fabricated a Josephson charge qubit by capacitively coupling a single-Cooper-pair box (SCB) to an electrometer based upon a single-electron transistor configured for radio-frequency readout (RF-SET). Charge quantization of 2e is observed and microwave spectroscopy is used to extract the Josephson and charging energies of the box. We perform coherent manipulation of the SCB by using very fast DC pulses and observe quantum oscillations in time of the charge that persist to ~=10ns. The observed contrast of the oscillations is high and agrees with that expected from the finite E_J/E_C ratio and finite rise-time of the DC pulses. In addition, we are able to demonstrate nearly 100% initial charge state polarization. We also present a method to determine the relaxation time T_1 when it is shorter than the measurement time T_{meas}.