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research article

Critical Schrodinger Cat Qubit

Gravina, Luca  
•
Minganti, Fabrizio  
•
Savona, Vincenzo  
June 7, 2023
Prx Quantum

Encoding quantum information onto bosonic systems is a promising route to quantum error correc-tion. In a cat code, this encoding relies on the confinement of the dynamics of the system onto the two-dimensional manifold spanned by Schrodinger cats of opposite parity. In dissipative cat qubits, an engineered dissipation scheme combining two-photon drive and two-photon loss has been used to autonomously stabilize this manifold, ensuring passive protection against, e.g., bit-flip errors regardless of their origin. Similarly, in Kerr-cat qubits, where highly performing gates can be engineered, two-photon drive and Kerr nonlinearity cooperate to confine the system to a twofold-degenerate ground-state 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. Here, we pro-pose a critical cat code, where both two-photon loss and Kerr nonlinearity are present and the two-photon drive is allowed to be out of resonance. The performance of this code is assessed via the spectral theory of Liouvillians in all configurations ranging from the purely dissipative to the Kerr limit. We show that large detunings and small, but non-negligible, two-photon loss rates are fundamental to achieve optimal performance. We further 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 particularly resistant to random frequency shifts charac-terizing multiple-qubit operations, opening avenues for the realization of reliable protocols for scalable and concatenated bosonic qubit architectures.

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Type
research article
DOI
10.1103/PRXQuantum.4.020337
Web of Science ID

WOS:001012081100001

Author(s)
Gravina, Luca  
•
Minganti, Fabrizio  
•
Savona, Vincenzo  
Date Issued

2023-06-07

Publisher

AMER PHYSICAL SOC

Published in
Prx Quantum
Volume

4

Issue

2

Article Number

020337

Subjects

Quantum Science & Technology

•

Physics, Applied

•

Physics, Multidisciplinary

•

Physics

•

quantum error-correction

•

generation

•

states

•

light

Editorial or Peer reviewed

REVIEWED

Written at

EPFL

EPFL units
LTPN  
Available on Infoscience
July 17, 2023
Use this identifier to reference this record
https://infoscience.epfl.ch/handle/20.500.14299/199107
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