Extending the lifetime of a quantum bit with error correction in superconducting circuits
Abstract
Quantum error correction (QEC) can overcome the errors experienced by qubits and is therefore an essential component of a future quantum computer. To implement QEC, a qubit is redundantly encoded in a higherdimensional space using quantum states with carefully tailored symmetry properties. Projective measurements of these paritytype observables provide error syndrome information, with which errors can be corrected via simple operations. The ‘breakeven’ point of QEC—at which the lifetime of a qubit exceeds the lifetime of the constituents of the system—has so far remained out of reach. Although previous works have demonstrated elements of QEC, they primarily illustrate the signatures or scaling properties of QEC codes rather than test the capacity of the system to preserve a qubit over time. Here we demonstrate a QEC system that reaches the breakeven point by suppressing the natural errors due to energy loss for a qubit logically encoded in superpositions of Schrödingercat states of a superconducting resonator. We implement a full QEC protocol by using realtime feedback to encode, monitor naturally occurring errors, decode and correct. As measured by full process tomography, without any postselection, the corrected qubit lifetime is 320 microseconds, which is longer than the lifetime of any of the parts of the system: 20 times longer than the lifetime of the transmon, about 2.2 times longer than the lifetime of an uncorrected logical encoding and about 1.1 longer than the lifetime of the best physical qubit (the 0>_{f} and 1>_{f} Fock states of the resonator). Our results illustrate the benefit of using hardwareefficient qubit encodings rather than traditional QEC schemes. Furthermore, they advance the field of experimental error correction from confirming basic concepts to exploring the metrics that drive system performance and the challenges in realizing a faulttolerant system.
 Publication:

Nature
 Pub Date:
 August 2016
 DOI:
 10.1038/nature18949
 Bibcode:
 2016Natur.536..441O