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arXiv:2504.18648 · Physical review. D/Physical review. D.

Recoherence, adiabaticity, and Markovianity in Gaussian maps

Dimitrios Kranas, J. Grain, Vincent Vennin

Published Aug 27, 2025• 3 citations
arXivHTMLPDF

Abstract

Motivated by the recent discovery of situations where cosmological fluctuations recohere during inflation, we investigate the relationship between quantum recoherence (late-time purification after a transient phase of decoherence), adiabaticity, and Markovianity. To that end, we study a simple setup of two linearly coupled harmonic oscillators, and compute the purity of one oscillator when the interaction is switched off. We find that there exists a critical value for the coupling strength below which the purity oscillates and above which it decays exponentially. This decay cannot be captured by perturbation theory; hence, decoherence is always a nonperturbative phenomenon. When the interaction is turned off, the purity either freezes to its value prior to the turn-off, or it smoothly goes back to a value very close to one (recoherence). This depends on the rate at which the turn-off occurs. We thus develop a new adiabatic-expansion scheme and find complete recoherence at any finite order in the inverse turn-off time. Therefore, decoherence is always a nonadiabatic effect. The critical value of the turn-off time above which recoherence takes place is then expressed in terms of the other time scales of the problem. Finally, we show that the dynamics of the system is never Markovian, even when decoherence takes place. We introduce a new measure of Markovianity dubbed the Bures velocity and use it to optimize Markovian approximations.

DOI
10.1103/r8pq-kms4

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Motivated by the recent discovery of situations where cosmological fluctuations recohere during inflation, we investigate the relationship between quantum recoherence (late-time purification after a transient phase of decoherence), adiabaticity, and Markovianity. To that end, we study a simple setup of two linearly coupled harmonic oscillators, and compute the purity of one oscillator when the interaction is switched off. We find that there exists a critical value for the coupling strength below which the purity oscillates and above which it decays exponentially. This decay cannot be captured by perturbation theory; hence, decoherence is always a nonperturbative phenomenon. When the interaction is turned off, the purity either freezes to its value prior to the turn-off, or it smoothly goes back to a value very close to one (recoherence). This depends on the rate at which the turn-off occurs. We thus develop a new adiabatic-expansion scheme and find complete recoherence at any finite order in the inverse turn-off time. Therefore, decoherence is always a nonadiabatic effect. The critical value of the turn-off time above which recoherence takes place is then expressed in terms of the other time scales of the problem. Finally, we show that the dynamics of the system is never Markovian, even when decoherence takes place. We introduce a new measure of Markovianity dubbed the Bures velocity and use it to optimize Markovian approximations.
Abstract Despite being created through a fundamentally quantum-mechanical process, cosmological structures have not yet revealed any sign of genuine quantum correlations. Among the obstructions to the direct detection of quantum signatures in cosmology, environmental-induced decoherence is arguably one of the most inevitable. Yet, we discover a mechanism of quantum recoherence for the adiabatic perturbations when they couple to an entropic sector. After a transient phase of decoherence, a turning point is reached, recoherence proceeds and adiabatic perturbations exhibit a large amount of self-coherence at late-time. This result is also understood by means of a non-Markovian master equation, which reduces to Wilsonian effective-field theory in the unitary limit. This allows us to critically assess the validity of open-quantum-system methods in cosmology and to highlight that re(de)coherence from linear interactions has no flat-space analogue.
To study the phenomenon of decoherence the effective Lagrangian of a test particle interacting with an ideal gas is derived in the leading order of the perturbation expansion and in order $\ord{\partial^2_t}$. Two concepts of decoherence are distinguished and investigated in details: The general dynamical decoherence and the stationary decoherence, an approximation of the dynamical phenomenon corresponding to the assumption of a static test particle trajectory. Comparison of the stationary decoherence described by the present effective theory with the traditional description of decoherence based on collision theory reveals a qualitative similarity within their domain of applicability. However, the effective theory predicts comparable time scales for decoherence and dissipation in contrast to the deviation of this scales by many orders of magnitudes, estimated by referring to the collision approach. In particular the extremely fast decoherence predicted by the collisional approach can only be reproduced by the effective theory for parameter values which violate the applicability of both approaches. On the other hand, in harmonic systems, the true dynamical decoherence is indeed an extremely fast process involving a double exponential time dependence. It is argued that, in realistic systems, the dissipative effective dynamics is non-perturbative in nature and its asymptotic, relaxed state incorporates strong quantum fluctuations.
Current observations constrain primordial perturbations to be adiabatic, approximately Gaussian, and nearly-scale invariant. However, a generic mixed state satisfying these constraints has additional unconstrained degrees of freedom, which can be parameterized by the purity of the state and its momentum variance. This allowable parameter space reveals a unified geometric landscape of mixed states, allowing us to map and relate distinct models of decoherence and their respective pointer bases. Within this landscape we show that decoherence models that "classicalize" -- in the sense of admitting a regular, positive-definite Glauber-Sudarshan PPP-function -- must actively inject momentum into the system, exceeding that of the vacuum. The enhanced momentum sources the decaying mode of the curvature perturbation, backreacting on the Newtonian potential and radiation-era geometry. While this mode decays away fast enough to preserve the temporal coherence of the CMB acoustic peaks, requiring the potential to remain within linear perturbation theory places a model-independent bound on the momentum variance generated by any model of decoherence. This bound is definitively violated by decohered thermal states with more than 141414 e-folds of inflation, while a strong dependence on the number of e-folds restricts amplitude-basis decoherence to fewer than roughly 646464 e-folds of inflation in order to stay in the linear regime. Altogether, we present a unifying framework for evaluating the quantum-to-classical transition of the early universe.
The adiabatic theorem and shortcuts to adiabaticity for time-dependent open quantum systems are explored in this paper. Starting from the definition of dynamical stable decoherence-free subspace, we show that, under a compact adiabatic condition, the quantum state remains in the time-dependent decoherence-free subspace with an extremely high purity, even though the dynamics of the open quantum system may not be adiabatic. The adiabatic condition mentioned here in the adiabatic theorem for open systems is very similar to that for closed quantum systems, except that the operators required to change slowly are the Lindblad operators. We also show that the adiabatic evolution of decoherence-free subspaces depends on the existence of instantaneous decoherence-free subspaces, which requires that the Hamiltonian of open quantum systems be engineered according to the incoherent control protocol. In addition, shortcuts to adiabaticity for adiabatic decoherence-free subspaces are also presented based on the transitionless quantum driving method. Finally, we provide an example that consists of a two-level system coupled to a broadband squeezed vacuum field to show our theory. Our approach employs Markovian master equations and the theory can apply to finite-dimensional quantum open systems.
Description: This theoretical study revisits the interpretation of cosmological end scenarios—such as heat death and asymptotic de Sitter expansion—through the framework of quantum decoherence and energy conservation in expanding spacetimes. Rather than treating late-time cosmology as an absolute termination of physical reality, the paper distinguishes between dynamical cessation and ontological nonexistence. Using explicit calculations of decoherence rates in FLRW backgrounds, we demonstrate that when environmental particle densities dilute as (a(t)^{-3}), the integrated decoherence over infinite future time can remain finite. Under such conditions, quantum superpositions formed beyond a critical cosmological time may never fully decohere. This does not imply recoherence, cyclic cosmology, or violation of thermodynamic irreversibility. Instead, it reflects the asymptotic disappearance of environmental interaction channels that dynamically sustain classical pointer states. The work therefore proposes that late-time cosmic evolution may be more coherently interpreted as a transition from environmentally enforced classicality to a regime of dominant quantum description. The analysis integrates principles from quantum field theory in curved spacetime, decoherence theory, and cosmological expansion dynamics, offering a conceptual clarification of what is meant by a “cosmic end.” 🔹 Keywords: Quantum cosmology; Quantum superposition; Decoherence; Early universe; Late universe; Environmental noise; Quantum–classical transition; Interaction density; Foundations of physics; Conceptual physics
Directed Recoherence in Structured Non-Markovian Environments presents a rigorously defined operating regime of open quantum system dynamics in which environmental memory leads not to oscillatory revivals, but to directed, monotonic recoherence toward a long-lived metastable coherence plateau. The work introduces a minimal and physically standard two-bath pure-dephasing model composed of a broadband Markovian noise bath and a narrowband non-Markovian reference bath. Within this framework, a dimensionless recoherence parameter R = λ_R² / [ γ_N Γ (1 + (δ/Γ)² ) ] is shown to govern a sharp transition between ordinary decoherence and a directed recoherence regime. When R exceeds a threshold of order unity, the system exhibits a single coherence minimum followed by monotonic recoherence and a metastable phase-locked plateau over experimentally relevant timescales. Exact closed-form solutions for the coherence dynamics are derived, alongside falsifiable predictions including bounded phase deviation relative to the reference bath. Numerical simulations confirm the existence of a critical threshold and distinguish this regime from known non-Markovian revivals, decoherence-free subspaces, driven steady states, and many-body prethermalization. The manuscript reframes Self-Initiated Phase Re-Coherence (SIPR) not as a new physical principle, but as a previously under-characterized regime within standard quantum open-system theory. Potential experimental realizations are discussed for superconducting qubits with Purcell-filtered environments and trapped ions with engineered sidebands. A companion conceptual document explores the epistemic and interpretational implications of measurement, observation, and environmental structure without modifying the physical results presented here.