Measuring the Decoherence of a Quantronium Qubit with the Cavity Bifurcation Amplifier

TL;DR

This paper introduces a dispersive readout method for superconducting qubits using the Cavity Bifurcation Amplifier, demonstrating its effectiveness in measuring qubit decoherence and its potential for scalable quantum computing.

Contribution

The development and application of the Cavity Bifurcation Amplifier for qubit readout, showing its advantages and coherence limitations due to 1/f charge noise.

Findings

CBA exhibits two metastable states dependent on qubit state

Coherence limited by 1/f gate charge noise at the sweet spot

Architecture supports scalable, multiplexed quantum readouts

Abstract

Dispersive readouts for superconducting qubits have the advantage of speed and minimal invasiveness. We have developed such an amplifier, the Cavity Bifurcation Amplifier (CBA) [10], and applied it to the readout of the quantronium qubit [2]. It consists of a Josephson junction embedded in a microwave on-chip resonator. In contrast with the Josephson bifurcation amplifier [17], which has an on-chip capacitor shunting a junction, the resonator is based on a simple coplanar waveguide imposing a pre-determined frequency and whose other RF characteristics like the quality factor are easily controlled and optimized. Under proper microwave irradiation conditions, the CBA has two metastable states. Which state is adopted by the CBA depends on the state of a quantronium qubit coupled to the CBA's junction. Due to the MHz repetition rate and large signal to noise ratio we can show directly that the coherence is limited by 1/f gate charge noise when biased at the sweet spot - a point insensitive to first order gate charge fluctuations. This architecture lends itself to scalable quantum computing using a multi-resonator chip with multiplexed readouts.