RQC Seminar

38th RQC Seminar

• Speaker

  Dr. Fabrizio Minganti　
  (Institute of Physics, and Center for Quantum Science and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland)

• Date

  16:00-17:00 (JST), April 10, 2023 (Monday)

• Venue

  Hybrid (Zoom / Wako 2F Large Conference Room, Admin. Headquarters Bldg. )

• Title

  Parametric processes, criticality, and quantum information: from cat codes to enhanced sensing

• Inquiries

  rqc_info[at]ml.riken.jp

Abstract
Parametric processes are a resource for quantum sensing and quantum information. Indeed, Schrödinger cat codes are a promising route to quantum error correction [1]. This encoding relies on the confinement of the system's dynamics onto the two-dimensional manifold spanned by Schrödinger cats of opposite parity, where two-photon drive and two-photon loss (dissipative cat qubits [2]) or two-photon drive and Kerr nonlinearity (Kerr cat qubits [3]) cooperate to confine the system to a two-fold degenerate manifold spanned by cat states of opposite parity. Dissipative, Hamiltonian, and hybrid confinement mechanisms have been investigated at resonance, i.e., for driving frequencies matching that of the cavity. In this talk, I will introduce the critical cat code [4], where both two-photon loss and Kerr nonlinearity are present, and the two-photon drive is allowed to be out of resonance. Using the Liouvillians spectral theory, we show that large detunings and small, but non-negligible, two-photon loss rates are fundamental to achieve optimal performance. We demonstrate that the competition between nonlinearity and detuning results in a first-order dissipative phase transition, leading to a squeezed vacuum steady state. We show that to achieve the maximal suppression of the logical bit-flip rate requires initializing the system in the metastable state emerging from the first-order transition, and we detail a protocol to do so. Efficiently operating over a broad range of detuning values, the critical cat code is also particularly resistant to random frequency shifts characterizing multiple-qubit operations, opening venues for the realization of reliable protocols for scalable and concatenated bosonic qubit architectures. The quantum critical properties of phase transitions of the parametrically driven Kerr resonator can be used as probes to estimate a physical parameter. I will also present a sensing protocol based on phase transitions of the Kerr resonator [5]. We show that, in spite of the critical slowing down, critical quantum optical probes can achieve quantum advantage in sensing applications exploiting the dissipative nature of the system. By going beyond the asymptotic regime of parameters, we show that the Heisenberg-limited precision can be achieved with current quantum technologies. Finally, we propose specific applications for quantum magnetometry and for superconducting-qubit readout.

[1] M. Mirrahimi et al, Dynamically protected cat-qubits: a new paradigm for universal quantum computation, New J. Phys. 16, 045014 (2014)
[2] Z. Leghtas et al., Confining the state of light to a quantum manifold by engineered two-photon loss, Science 347, 853 (2015)
[3] A. Grimm et al., Stabilization and operation of a Kerr-cat qubit, Nature 584, 205 (2020)
[4] L. Gravina, F. Minganti, and V. Savona, A critical Schrödinger cat qubit, arXiv:2208.04928 (2022)
[5] R. Di Candia, F. Minganti, K.V. Petrovnin, G.S. Paraoanu, and S. Felicetti, Critical parametric quantum sensing, npj Quantum Inf. 9, 23 (2023)