Realization of threequbit quantum error correction with superconducting circuits
Abstract
Quantum computers could be used to solve certain problems exponentially faster than classical computers, but are challenging to build because of their increased susceptibility to errors. However, it is possible to detect and correct errors without destroying coherence, by using quantum error correcting codes. The simplest of these are threequantumbit (threequbit) codes, which map a onequbit state to an entangled threequbit state; they can correct any single phaseflip or bitflip error on one of the three qubits, depending on the code used. Here we demonstrate such phase and bitflip error correcting codes in a superconducting circuit. We encode a quantum state, induce errors on the qubits and decode the error syndromea quantum state indicating which error has occurredby reversing the encoding process. This syndrome is then used as the input to a threequbit gate that corrects the primary qubit if it was flipped. As the code can recover from a single error on any qubit, the fidelity of this process should decrease only quadratically with error probability. We implement the correcting threequbit gate (known as a conditionalconditional NOT, or Toffoli, gate) in 63 nanoseconds, using an interaction with the third excited state of a single qubit. We find 85 +/ 1 per cent fidelity to the expected classical action of this gate, and 78 +/ 1 per cent fidelity to the ideal quantum process matrix. Using this gate, we perform a single pass of both quantum bit and phaseflip error correction and demonstrate the predicted firstorder insensitivity to errors. Concatenation of these two codes in a ninequbit device would correct arbitrary singlequbit errors. In combination with recent advances in superconducting qubit coherence times, this could lead to scalable quantum technology.
 Publication:

Nature
 Pub Date:
 February 2012
 DOI:
 10.1038/nature10786
 arXiv:
 arXiv:1109.4948
 Bibcode:
 2012Natur.482..382R
 Keywords:

 Quantum Physics;
 Condensed Matter  Mesoscale and Nanoscale Physics;
 Condensed Matter  Superconductivity
 EPrint:
 10 pages, 7 figures