May 2018

Abstracts of the QSIT Lunch Seminar, Thursday, May 3, 2018

Fast and Unconditional All-Microwave Reset of a Superconducting Qubit

Paul Magnard – Quantum Device Lab (Wallraff group), ETH Zurich

Resetting qubits with high fidelity and on a short time-scale is of primary importance for quantum computation. As superconducting quantum processors tackle the hard task of scaling up, it is essential that reset protocols put as little constraints on the processor architecture and parameters while remaining fast and efficient. Using an all-microwave, dissipative Lambda scheme, we demonstrate the reset of a transmon-qubit coupled to a large bandwidth resonator, with a protocol that combines these properties. We experimentally explore the reset parameter space and reveal its fundamental features with a comprehensive non-Hermitian analysis. Additionally, we show that the presented reset protocol reaches or exceeds all existing ones in speed, efficiency and practicality, making it an ideal candidate for an easy implementation in scaled-up, fault-tolerant architectures.

Repeated multi-qubit readout and feedback with a mixed-species trapped-ion register

Matteo Marinelli – Trapped Ion Quantum Information (Home group), ETH Zurich

Quantum error correction involves repeated rounds of error detection 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 up to 50 sequential measurements of correlation between two beryllium ion qubits coupled to a co-trapped calcium ion. State readout of the calcium ancilla has no direct effect on the internal states of beryllium.Using the ability to rapidly make in-sequence processing in our classical computer control, we perform feedback on the beryllium qubits conditioned on the ancilla readout, which we use to prepare and stabilize Bell states and parity subspaces. 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.