A symmetry-protected pseudo-Hermitian phase of quantum memory-kernel generators

TL;DR

This paper analytically demonstrates that a non-Hermitian memory-kernel generator in the Jaynes-Cummings model has a strictly real spectrum, revealing a symmetry-protected pseudo-Hermitian phase with implications for quantum memory stability.

Contribution

It introduces a hidden pseudo-Hermiticity in the memory-kernel generator and maps the phase boundary analytically, extending results to a broad class of U(1)-conserving models.

Findings

Spectrum of the generator is strictly real at all couplings and truncations.

Characteristic polynomial factorizes, reproducing JC dressed states for n >= 2.

Identifies a symmetry-protected phase with real spectrum bubbles and universal level spacings.

Abstract

The Jaynes-Cummings (JC) model, introduced in 1963 and central to cavity quantum electrodynamics, describes a two-level system coupled to a single bosonic mode under the rotating-wave approximation. When the mode is projected out via the Nakajima-Zwanzig (NZ) formalism, the memory-kernel generator QLQ is manifestly non-Hermitian -- yet we prove analytically that its spectrum is strictly real at every coupling and every finite truncation, for both vacuum and thermal baths. For the vacuum bath the characteristic polynomial factorises completely; the nonzero eigenvalues reproduce the JC dressed-state ladder for n >= 2, while the lowest mode is suppressed by exactly sqrt(2) relative to the bare-Hamiltonian prediction. For any thermal Gibbs state, the squared generator reduces to an asymmetric rank-one perturbation symmetrised by a closed-form diagonal metric, with eigenvalues guaranteed real by Cauchy interlacing. We construct a positive-definite metric eta_osc intertwining QLQ with its adjoint on the nonzero spectral subspace, proving hidden pseudo-Hermiticity. A classification theorem extends these results to the full U(1)-conserving single-photon-exchange class with arbitrary complex couplings. The phase boundary under counter-rotating deformation is mapped analytically at resonance and numerically across the coupling-truncation plane, revealing a weak-coupling protected wedge, re-entrant real-spectrum bubbles, and N-universal plateaus organised by a three-family band catalog with closed-form level spacings. The full phase structure is proved well-defined in the N -> infinity thermodynamic limit. The sqrt(2) suppression provides a platform-independent experimental falsification target spanning seven orders of magnitude in coupling strength.