Lobe Dynamics, Phase-Space Transport, and Non-Adiabatic Leakage Thresholds in the Nonautonomous Kerr-Cat Qubit

TL;DR

This paper investigates the nonautonomous dynamics of Kerr-cat qubits, revealing how time-dependent pulses induce non-adiabatic leakage through phase-space lobe dynamics, and introduces analytical tools for understanding state formation and gate transport.

Contribution

It develops a combined stability analysis and Melnikov's method approach to characterize nonautonomous state preparation and gate-induced transport in Kerr-cat qubits, extending beyond static equilibrium models.

Findings

Identifies symmetric post-threshold branches organizing state formation dynamics.

Derives a transport criterion using Melnikov's method for gate-induced phase-space leakage.

Provides a geometric threshold curve for non-adiabatic leakage onset.

Abstract

The Kerr-nonlinear parametric oscillator (KPO) provides a foundational semiclassical model for cat-state quantum hardware. Standard analyses of the KPO typically rely on autonomous, frozen-time approximations to describe the stabilization of macroscopic coherent states. However, state preparation and gate manipulation are driven by explicitly time-dependent microwave pulses, so the operational dynamics are inherently nonautonomous. In this paper, we show that static algebraic equilibrium pictures are incomplete for describing both state formation and gate-induced transport in the Kerr-cat qubit. For nonautonomous state preparation, we analyze the ramped resonant model by combining a linear nonautonomous stability analysis with a local invariant-graph reduction near the vacuum trajectory. This yields a quintic reduced normal form in the critical direction and identifies two symmetric post-threshold moving branches that organize the local state-formation dynamics. The associated diagnostics separate the reduced branch dynamics from the full two-dimensional phase-twist relaxation observed in the hardware coordinates. For gate execution, we model a fast pulse as a weak aperiodic perturbation of the conservative resonant figure-eight separatrix and apply Melnikov's method to derive a leading-order transport criterion. In this framework, transient lobe dynamics emerge as a semiclassical mechanism for non-adiabatic leakage, and the resulting amplitude-width threshold curve provides a leading-order geometric indicator for the onset of gate-pulse-induced transport.