Quantum control and noise protection of a Floquet $0-\pi$ qubit

TL;DR

This paper introduces the Kapitzonium, a Floquet superconducting qubit inspired by the Kapitza pendulum, which offers exponential protection against decoherence and includes methods for stabilization and measurement.

Contribution

It proposes a new Floquet qubit design, the Kapitzonium, with built-in noise protection, passive cooling, and measurement techniques, advancing the development of protected quantum systems.

Findings

Exponential protection against bit and phase flips.

Passive cooling stabilizes the qubit subspace.

Feasible implementation with current superconducting technology.

Abstract

Time-periodic systems allow engineering new effective Hamiltonians from limited physical interactions. For example, the inverted position of the Kapitza pendulum emerges as a stable equilibrium with rapid drive of its pivot point. In this work, we propose the $\textit{Kapitzonium}$: a Floquet qubit that is the superconducting circuit analog of a mechanical Kapitza pendulum. Under periodic driving, the emerging qubit states are exponentially protected against bit and phase flips caused by dissipation, which is the primary source of decoherence of current qubits. However, we find that dissipation causes leakage out of the Floquet qubit subspace. We engineer a passive cooling scheme to stabilize the qubit subspace, which is crucial for high fidelity quantum control under dissipation. Furthermore, we introduce a hardware-efficient fluorescence-based method for qubit measurement and discuss the experimental implementation of the Floquet qubit. The proposed Kapitzonium is one of the simplest Floquet qubits that can be realized with current technology -- and it already has many intriguing features and capabilities. Our work provides the first steps to develop more complex Floquet quantum systems from the ground up to realize large-scale protected engineered dynamics.