Selective darkening of degenerate transitions for implementing quantum controlled-NOT gates

TL;DR

This paper analyzes a method for implementing quantum CNOT gates by selectively darkening degenerate transitions in transversely-coupled qubits, achieving high fidelity and scalability with microwave-only control.

Contribution

It provides a theoretical analysis and numerical validation of the selective darkening method for fast, high-fidelity CNOT gates applicable to fixed-frequency qubits.

Findings

Gate fidelities of 99% at 0.48J and 99.99% at 0.07J gate speeds

Method is scalable and compatible with fixed-frequency qubits

Effective for systems with weak anharmonicity and multiple qubits

Abstract

We present a theoretical analysis of the selective darkening method for implementing quantum controlled-NOT (CNOT) gates. This method, which we recently proposed and demonstrated, consists of driving two transversely-coupled quantum bits (qubits) with a driving field that is resonant with one of the two qubits. For specific relative amplitudes and phases of the driving field felt by the two qubits, one of the two transitions in the degenerate pair is darkened, or in other words, becomes forbidden by effective selection rules. At these driving conditions, the evolution of the two-qubit state realizes a CNOT gate. The gate speed is found to be limited only by the coupling energy J, which is the fundamental speed limit for any entangling gate. Numerical simulations show that at gate speeds corresponding to 0.48J and 0.07J, the gate fidelity is 99% and 99.99%, respectively, and increases further for lower gate speeds. In addition, the effect of higher-lying energy levels and weak anharmonicity is studied, as well as the scalability of the method to systems of multiple qubits. We conclude that in all these respects this method is competitive with existing schemes for creating entanglement, with the added advantages of being applicable for qubits operating at fixed frequencies (either by design or for exploitation of coherence sweet-spots) and having the simplicity of microwave-only operation.