Selective darkening of degenerate transitions demonstrated with two superconducting quantum bits

TL;DR

This paper presents a method to selectively suppress one transition in degenerate quantum states of coupled qubits, enabling frequency selectivity and control in quantum operations, demonstrated on superconducting flux qubits.

Contribution

Introducing a technique to suppress one transition in degenerate qubit pairs using simultaneous driving with specific amplitude and phase, creating artificial selection rules.

Findings

Successfully demonstrated on superconducting flux qubits.

Enables single-pulse controlled-NOT gate for transversely coupled qubits.

Applicable to various qubit types with individual driving capabilities.

Abstract

Controlled manipulation of quantum states is central to studying natural and artificial quantum systems. If a quantum system consists of interacting sub-units, the nature of the coupling may lead to quantum levels with degenerate energy differences. This degeneracy makes frequency-selective quantum operations impossible. For the prominent group of transversely coupled two-level systems, i.e. qubits, we introduce a method to selectively suppress one transition of a degenerate pair while coherently exciting the other, effectively creating artificial selection rules. It requires driving two qubits simultaneously with the same frequency and specified relative amplitude and phase. We demonstrate our method on a pair of superconducting flux qubits. It can directly be applied to the other superconducting qubits, and to any other qubit type that allows for individual driving. Our results provide a single-pulse controlled-NOT gate for the class of transversely coupled qubits.