Abstract
A cat state is a superposition of two coherent states with amplitudes α0 and −α0. Recent experiments create cat states in a microwave cavity field using superconducting circuits. As with degenerate parametric oscillation (DPO) in an adiabatic and highly nonlinear limit, the states are formed in a signal cavity mode via a two-photon dissipative process induced by the down conversion of a pump field to generate pairs of signal photons. The damping of the signal and the presence of thermal fluctuations rapidly decoheres the state, and the effect on the dynamics is to either destroy the possibility of a cat state or else to sharply reduce the lifetime and size of the cat states that can be formed. In this paper, we study the effect on both the DPO and microwave systems of a squeezed reservoir coupled to the cavity. While the threshold nonlinearity is not altered, we show that the use of squeezed states significantly lengthens the lifetime of the cat states. This improves the feasibility of generating cat states of large amplitudes and with a greater degree of quantum macroscopic coherence, which is necessary for many quantum technology applications. Using current experimental parameters for the microwave setup, which requires a modified Hamiltonian, we further demonstrate how squeezed states enhance the quality of the cat states that could be formed in this regime. Squeezing also combats the significant decoherence due to thermal noise, which is relevant for microwave fields at finite temperature. By modeling a thermal squeezed reservoir, we show that the thermal decoherence of the dynamical cat states can be inhibited by a careful control of the squeezing of the reservoir. To signify the quality of the cat state, we consider different signatures including fringes and negativity, and the Cl1 measure of quantum coherence.14 MoreReceived 31 May 2020Accepted 20 October 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.043387Published 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 SocietyPhysics Subject Headings (PhySH)Research AreasNonlinear opticsOpen quantum systems & decoherenceQuantum channelsQuantum coherence & coherence measuresQuantum foundationsQuantum information processing with continuous variablesQuantum-to-classical transitionQuantum InformationGeneral PhysicsAtomic, Molecular & Optical
Highlights
Schrödinger raised the question of how to interpret a macroscopic quantum superposition state in his essay of 1935 [1]
The coupling between the system and its environment tends to destroy the quantum coherence of the superposition state, as the two states involved in the superposition become macroscopically distinct [2,9,17]
We demonstrate how squeezed states may enhance the formation of cat states by modifying the decoherence
Summary
Schrödinger raised the question of how to interpret a macroscopic quantum superposition state in his essay of 1935 [1]. Meccozi and Tombesi [47,48] and Kennedy and Walls [26] suggested using squeezed states to engineer the environment coupled to a macroscopic superposition state, and showed that squeezing in an optimally selected quadrature can suppress the decoherence that otherwise destroys the cat state This was further explored by Munro and Reid [49], who studied the effect of a squeezed reservoir on the dynamical creation of cat states generated in a highly nonlinear degenerate parametric oscillator (DPO). With squeezed input states into the cavity, we explain that the threshold nonlinearity is not reduced, decoherence can be compensated for to allow cat states of longer lifetimes and with larger amplitudes α0 to be formed, which is relevant to the resource theory of non-Gaussianity [60]. We certify cat states using three different criteria: We use interference fringes [4,9], the negativity of the Wigner function [66], and the Cl1 measure of quantum coherence [67]
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