Abstract
Hosting nonclassical states of light in three-dimensional microwave cavities has emerged as a promising paradigm for continuous-variable quantum information processing. Here we experimentally demonstrate high-fidelity generation of a range of Wigner-negative states useful for quantum computation, such as Schrödinger-cat states, binomial states, Gottesman-Kitaev-Preskill states, as well as cubic phase states. The latter states have been long sought after in quantum optics and have never been achieved experimentally before. We use a sequence of interleaved selective number-dependent arbitrary phase (SNAP) gates and displacements. We optimize the state preparation in two steps. First we use a gradient-descent algorithm to optimize the parameters of the SNAP and displacement gates. Then we optimize the envelope of the pulses implementing the SNAP gates. Our results show that this way of creating highly nonclassical states in a harmonic oscillator is robust to fluctuations of the system parameters such as the qubit frequency and the dispersive shift.
1 More- Received 10 December 2021
- Revised 19 April 2022
- Accepted 8 June 2022
DOI:https://doi.org/10.1103/PRXQuantum.3.030301
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 ability to store and manipulate quantum states is one of the milestones on the road to building a quantum computer. However, the "quantumness" of quantum states, which is measured by a property called coherence, can be easily destroyed by interactions with the environment. A solution to this problem is to encode the quantum states redundantly and perform error correction. Generally, quantum error correction requires some hardware overhead, for example, many physical quantum bits are used to encode a logical bit of information. An alternative, promising way to implement error correction is to encode the quantum information in a superposition of the many states that a single resonator inherently offers. In this way we get a compact logical qubit that has only one dominant source of decoherence to correct for, namely, photon loss in the resonator.
In our work, we show a technique to create any non-trivial quantum state of microwave photons in a 3D-cavity by coupling it to a superconducting qubit. The states are created by applying a sequence of a few displacement pulses, sent to the cavity, and qubit pulses. The latter exploit the dispersive qubit-cavity interaction to realize arbitrary phase rotations of the cavity field, conditioned on the photon number in the cavity, which are known as "SNAP" gates. We optimize the parameters of the SNAP and displacement pulses, in a two-step optimization sequence. We leverage this approach to demonstrate fast and robust generation of GKP, cat, and binomial states that were previously used for quantum error correction. We also generate the cubic phase state, which has not previously been generated and is a valuable resource in continuous variable quantum computing.