Fault-tolerant measurement of a quantum error syndrome

Quantum error correction allows quantum computers to operate despite the presence of noise and imperfections. A critical component of any error correcting scheme is the mapping of a quantum error syndrome onto an ancilla qubit. However, errors occurring in the ancilla can propagate through the mapping operation onto the logical qubit, and irreversibly corrupt the encoded information. A fault-tolerant measurement protocol, which prevents the occurrence of such uncorrectable errors, is therefore a prerequisite for scaling up quantum error correction. 

I will present our recent demonstration of the fault-tolerant measurement of an error syndrome on a logical qubit encoded in a superconducting cavity. We achieve fault tolerance hardware-efficiently by coupling the logical qubit to a single multilevel ancilla transmon. The cavity-ancilla interaction is modified in-situ using off-resonant sideband drives to make the logical qubit transparent to all first-order ancilla errors. We achieve a sevenfold increase in the average number of syndrome measurements performed without destroying the logical qubit. These results demonstrate that hardware-efficient approaches which exploit system-specific error models can yield important advances towards fault-tolerant quantum computation.