Abstract
Qubit readout is an indispensable element of any quantum information processor. In this work, we experimentally demonstrate a nonperturbative cross-Kerr couplingbetween a transmon and polariton mode which enables an improved quantum nondemolition (QND) readout for superconducting qubits. The new mechanism uses the same experimental techniques as the standard QND qubit readout in the dispersive approximation, but due to its nonperturbative nature, it maximizes the speed, the single-shot fidelity, and the QND properties of the readout. In addition, it minimizes the effect of unwanted decay channels such as the Purcell effect. We observe a single-shot readout fidelity of 97.4% for short 50-ns pulses and we quantify a QND-ness of 99% for long measurement pulses with repeated single-shot readouts.
2 More- Received 3 May 2019
- Accepted 24 December 2019
DOI:https://doi.org/10.1103/PhysRevX.10.011045
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The measurement of a quantum bit (qubit) must compromise between two apparently contradictory facts: We want a reliable measurement, and thus the qubit should be strongly coupled to its detector, but the fragile quantum information must be protected from sources of noise, including the detector itself. Here, we propose and experimentally demonstrate a new readout scheme for superconducting qubits that preserves the underlying quantum states while enabling fast measurement times.
The standard readout scheme relies on a transverse coupling between a qubit and a readout cavity, however, this interaction does not preserve the probabilities of the qubit’s ground and excited states. When the readout cavity is highly off-resonant from the qubit, it approximately preserves these probabilities, but readout speed and fidelity are sacrificed. Our readout scheme, based on a new realization of a native energy-energy interaction, or “cross-Kerr coupling,” allows us to maximize all of these characteristics. In a first experimental demonstration of our technique, we measure 99% preservation of probabilities, a readout fidelity of 97.4%, and a short measurement time of 50 ns.
The refinement of our new readout scheme may allow measurements with a high signal-to-noise ratio even without amplification of the signals. This may reduce the overhead and complexity of superconducting quantum chips and thus facilitate the scaling up of these quantum technologies to large sizes, as well as the implementation of quantum error correction and fault-tolerant quantum computation in the future.