Error-corrected gates on an encoded qubit

To reach their full potential, quantum computers need to be resilient to noise and decoherence. In such a fault-tolerant quantum computer, errors must be corrected in real time to prevent them from propagating between components(1,2). This requirement is especially pertinent while applying quantum gates, where the interaction between components can cause errors to spread quickly throughout the system. However, the large overhead involved in most fault-tolerant architectures(2,3) makes implementing these systems a daunting task, motivating the search for hardware-efficient alternatives(4,5). Here, we present a gate enacted by an ancilla transmon on a cavity-encoded logical qubit that is fault-tolerant to ancilla decoherence and compatible with logical error correction. We maintain the purity of the encoded qubit by correcting ancilla-induced errors in real time, yielding a reduction of the logical gate error by a factor of two in the presence of naturally occurring decoherence. We also demonstrate a sixfold suppression of the gate error with increased ancilla relaxation errors and a fourfold suppression with increased ancilla dephasing errors. The results demonstrate that bosonic logical qubits can be controlled by error-prone ancilla qubits without inheriting the ancilla's inferior performance. As such, error-corrected ancilla-enabled gates are an important step towards fault-tolerant processing of bosonic qubits.

Error-corrected quantum gates that can tolerate dominant errors during the execution of quantum operations have been demonstrated. Substantial improvement of the gate fidelity sheds light on fault-tolerant universal quantum computation.