Tanja Behrle (ETH Zurich)

Title: Repeated multi-qubit parity measurement, feedback and stabilization using a mixed-species ion crystal

Abstract: Quantum error correction involves repeated rounds of syndrome extraction and recovery, involving multi-qubit non-demolition measurements along with conditional feedback. This requires the use of systems in which, measurement and decision times are short compared to relevant decoherence timescales, and in which the act of measurement does not destroy subspace coherence or disrupt future operations. Using a mixed-species ion chain, we demonstrate repeated parity measurement on two beryllium ion “clock” qubits by coupling these to a co-trapped calcium ion. Fluorescence readout of the calcium ancilla has no direct effect on the internal states of beryllium ions but heats up the ions’ motion, from which we re-cover by sympathetically cooling the ion chain using calcium. Using the ability to rapidly make sequence branches in our classical computer control, we perform feedback on the beryllium qubits conditioned on the ancilla readout, which we use to prepare and stabilize entangled states and parity subspaces. Our work takes place in a multi-zone segmented trap setup in which we have demonstrated full quantum control of both species and multi-well ion shuttling. The methods demonstrated here could be applied to quantum error correcting codes as well as quantum metrology and are key ingredients for realizing a hybrid universal quantum computer based on trapped ions.