Enhancing the Lifetime of Quantum Information with Cat States in Superconducting Cavities

Abstract

The field of quantum computation faces a central challenge that has thus far impeded the fullscale realization of quantum computing machines: decoherence. Decoherence is a general process by which quantum bits, or qubits, interact in unknown ways with their environment and thereby corrupt the information encoded within them. Remarkably, however, protocols for Quantum Error Correction (QEC) exist, and their discovery was a critical advance in the pursuit to build practical quantum computers. To implement QEC, one redundantly encodes a qubit in a higher dimensional space using quantum states with carefully tailored symmetry properties. Projective measurements of these parity-type observables provide error syndrome information with which errors can be corrected via simple operations. Reaching the “break-even” point, at which a logical qubit’s lifetime exceeds the lifetime of the system’s qubit constituents, has thus far remained an outstanding goal. In this work, we implement QEC within a superconducting cavity Quantum Electrodynamics (cQED) architecture that exploits the advantages of encoding quantum information in superpositions of coherent states, or cat states, in highly coherent superconducting cavities. This hardware-ecient approach, termed the cat code, simplifies the encoding scheme and requires the extraction of just one error syndrome via single-shot photon number parity measurements. By implementing the cat code within a full QEC system, we demonstrate for the first time quantum computing that reaches the break-even point. Beyond applications to error correction, logical qubit encodings based on the cat code paradigm can be used to probe more fundamental questions of quantum entanglement between physical qubits and coherent states. Specifically, we demonstrate the violation of a Bell inequality in such a setup, which underscores our ability to eciently extract information from continuous variables encodings. These results highlight the power of novel, hardware-ecient qubit encodings over traditional QEC schemes. Furthermore, they advance the field of experimental error correction from confirming the basic concepts to exploring the metrics that drive system performance and the challenges in implementing a fault-tolerant system.