Quantum state diffusion is a framework within which measurement may be described as the continuous and gradual collapse of a quantum system to an eigenstate as a result of interaction with its environment. The irreversible nature of the quantum trajectories that arise may be characterized by the environmental stochastic entropy production associated with the measurement. We consider a system of two spin-1/2 particles undergoing either single-particle measurements or measurements of the total z -spin component S z . The mean asymptotic rates of environmental stochastic entropy production associated with collapse can depend on the eigenstate of S z selected and on the initial state of the system, offering an additional avenue for characterizing quantum measurement.
Quantum state diffusion is a framework within which measurement may be described as the continuous and gradual collapse of a quantum system to an eigenstate as a result of interaction with its environment. The irreversible nature of the quantum trajectories that arise may be characterized by the environmental stochastic entropy production associated with the measurement. We consider a system of two spin-1/2 particles undergoing either single-particle measurements or measurements of the total z -spin component S z . The mean asymptotic rates of environmental stochastic entropy production associated with collapse can depend on the eigenstate of S z selected and on the initial state of the system, offering an additional avenue for characterizing quantum measurement.
Abstract We investigate the total stochastic entropy production of a two-level bosonic open quantum system under protocols of time dependent coupling to a harmonic environment. These processes are intended to represent the measurement of a system observable, and consequent selection of an eigenstate, whilst the system is also subjected to thermalising environmental noise. The entropy production depends on the evolution of the system variables and their probability density function, and is expressed through system and environmental contributions. The continuous stochastic dynamics of the open system is based on the Markovian approximation to the exact, noise-averaged stochastic Liouville-von Neumann equation, unravelled through the addition of stochastic environmental disturbance mimicking a measuring device. Under the thermalising influence of time independent coupling to the environment, the mean rate of entropy production vanishes asymptotically, indicating equilibrium. In contrast, a positive mean production of entropy as the system responds to time dependent coupling characterises the irreversibility of quantum measurement, and a comparison of its production for two coupling protocols, representing connection to and disconnection from the external measuring device, satisfies a detailed fluctuation theorem.
The reduced density matrix that characterises the state of an open quantum system is a projection from the full density matrix of the quantum system and its environment, and there are many full density matrices consistent with a given reduced version. Without a specification of relevant details of the environment, the time evolution of a reduced density matrix is therefore typically unpredictable, even if the dynamics of the full density matrix are deterministic. With this in mind, we investigate a two-level open quantum system using a framework of quantum state diffusion. We consider the pseudorandom evolution of its reduced density matrix when subjected to an environment-driven process that performs a continuous quantum measurement of a system observable, invoking dynamics that asymptotically send the system to one of the relevant eigenstates. The unpredictability is characterised by a stochastic entropy production, the average of which corresponds to an increase in the subjective uncertainty of the quantum state adopted by the system and environment, given the underspecified dynamics. This differs from a change in von Neumann entropy, and can continue indefinitely as the system is guided towards an eigenstate. As one would expect, the simultaneous measurement of two non-commuting observables within the same framework does not send the system to an eigenstate. Instead, the probability density function describing the reduced density matrix of the system becomes stationary over a continuum of pure states, a situation characterised by zero further stochastic entropy production. Transitions between such stationary states, brought about by changes in the relative strengths of the two measurement processes, give rise to finite positive mean stochastic entropy production. The framework investigated can offer useful perspectives on both the dynamics and irreversible thermodynamics of measurement in quantum systems.
Abstract The quantum measurement problem arises from the coexistence of unitary Schr¨odinger evolution with the apparent nonunitary collapse of quantum states during observation. Despite extensive theoretical development, no consensus has been reached on a microscopic physical mechanism for state reduction within standard quantum theory. In this work, we present a complete dynamical theory of quantum measurement in which wavefunction collapse emerges from irreversible information transfer and entropy production in realistic system–detector–environment interactions. Starting from microscopic Hamiltonian models, we derive a stochastic nonlinear evolution equation for conditioned quantum states without introducing additional axioms or phenomenological parameters. We demonstrate that the effective collapse rate is uniquely determined by environmental entropy production and remains well-defined in thermal, non-Markovian, chaotic, and zero-temperature regimes. Using stochastic calculus and martingale theory, we establish the dynamical emergence of the Born probability rule and prove almost-sure convergence of measurement trajectories to definite outcomes. The theory exhibits robust many-body amplification, universal behavior in nonlinear detectors, and mathematical well-posedness in both finite- and infinite-dimensional settings. Compatibility with algebraic quantum field theory, renormalization theory, and stochastic semiclassical gravity is established, ensuring consistency with relativistic and high-energy physics. Extensive numerical simulations confirm analytical predictions, and a comprehensive experimental program is proposed, together with rigorous validation and replication protocols. Logical analysis demonstrates compatibility with Bell nonlocality, resolution of Wigner’s friend and Frauchiger–Renner paradoxes, and compliance with information-theoretic constraints. These results provide a unified physical explanation of quantum measurement as an emergent nonequilibrium process governed by universal thermodynamic and informational principles, integrating state reduction into the standard dynamical framework of physics.
We investigate stochastic entropy production in a two-level quantum system that performs Rabi oscillations while undergoing quantum measurement brought about by continuous random disturbance by an external measuring device or environment. The dynamics produce quantum Zeno and anti-Zeno effects for certain measurement regimes, and the stochastic entropy production is a measure of the irreversibility of the behaviour. When the strength of the measurement disturbance is time-dependent, the stochastic entropy production separates into three components. Two represent relaxational behaviour, one being specific to systems represented by coordinates that are odd under time reversal symmetry, and a third characterises the nonequilibrium stationary state arising from breakage of detailed balance in the dynamics. The study illustrates how the ideas of stochastic thermodynamics may be applied in similar ways to both quantum and classical systems.
Irreversible behavior in open stochastic dynamical systems is quantified by stochastic entropy production, a property that measures the difference in likelihoods of forward and subsequent backward system evolution. But for a closed system, governed by deterministic dynamics, such an approach is not appropriate. Instead, we can consider the difference in likelihoods of forward and "obverse" behavior: the latter being a backward trajectory initiated at the same time as the forward trajectory. Such a comparison allows us to define "dissipation production," an analog of stochastic entropy production. It quantifies the breakage of a property of the evolution termed "obversibility" just as stochastic entropy production quantifies a breakage of reversibility. Both are manifestations of irreversibility. In this study we discuss dissipation production in a quantum system. We consider a simple, deterministic, two-level quantum system characterized by a statistical ensemble of state vectors, and we provide numerical results to illustrate the ideas. We consider cases that both do and do not satisfy an Evans-Searles Fluctuation Theorem for the dissipation production, and hence identify conditions under which the system displays time-asymmetric average behavior: an arrow of time.
We investigate the entropic consequences of the relaxation of an open two-level quantum system towards a thermalised statistical state, using a framework of quantum state diffusion with evolution described by a minimal set of raising and lowering Lindblad operators. We demonstrate that thermalisation is typically accompanied by a persistent non-zero mean rate of change of the environmental component of stochastic entropy production. This thermodynamic signature can be associated with the purification of the reduced density matrix ρ of the randomly evolving state under these dynamics, which may be contrasted with the impurity of the more frequently considered ensemble average of ρ. The system adopts stationary statistics, with zero stochastic entropy production, after a further stage of relaxation once purity has been achieved. We show that apparent pathological mathematical difficulties in the computation of stochastic entropy production emerge in this evolution towards stationarity if ρ is represented using a certain set of coordinates, though these problems can be removed by choosing a different set. We conclude that frameworks for modelling open systems must be carefully selected to provide satisfactory thermodynamic as well as dynamic behaviour.