Quantum information processing with circuit quantum electrodynamics

TL;DR

This paper analyzes single and two-qubit gates in circuit QED systems, emphasizing operations at the charge-degeneracy point to enhance coherence and proposing various gate schemes for scalable quantum computing.

Contribution

It introduces methods for implementing high-fidelity quantum gates in circuit QED at the charge-degeneracy point without circuit modification.

Findings

Multiple two-qubit gate schemes are feasible at the charge-degeneracy point.

Gates based on virtual photons, real resonator excitation, and geometric phase are discussed.

Operating at the charge-degeneracy point improves qubit coherence times.

Abstract

We theoretically study single and two-qubit dynamics in the circuit QED architecture. We focus on the current experimental design [Wallraff et al., Nature 431, 162 (2004); Schuster et al., Nature 445, 515 (2007)] in which superconducting charge qubits are capacitively coupled to a single high-Q superconducting coplanar resonator. In this system, logical gates are realized by driving the resonator with microwave fields. Advantages of this architecture are that it allows for multi-qubit gates between non-nearest qubits and for the realization of gates in parallel, opening the possibility of fault-tolerant quantum computation with superconduting circuits. In this paper, we focus on one and two-qubit gates that do not require moving away from the charge-degeneracy `sweet spot'. This is advantageous as it helps to increase the qubit dephasing time and does not require modification of the original circuit QED. However these gates can, in some cases, be slower than those that do not use this constraint. Five types of two-qubit gates are discussed, these include gates based on virtual photons, real excitation of the resonator and a gate based on the geometric phase. We also point out the importance of selection rules when working at the charge degeneracy point.