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arXiv:2407.03139

Is the Born rule a result of measurement noise?

Torres, Frank

Published Jul 3, 2024• 0 citations
arXivHTMLPDF

Abstract

The Born rule asserts the probability distribution of eigenstates observed in unbiased quantum measurements, but the reason it holds remains elusive. This manuscript discusses how the Born rule might be explained by Schrodinger equation dynamics, if a measurement comprises a system responding to random fluctuations until it is within an arbitrarily small tolerance of a measurement eigenstate. We describe the random walk dynamics that produce this behavior in terms of a class of time-dependent, stochastic unitary matrices U(t). We also discuss the class of stochastic potential energies in the Schrodinger equation that is equivalent to this class of unitary matrices. This analysis raises some questions worth considering, including how to determine if any measurements actually follow the predicted random walk mechanism and whether a reliable measurement apparatus could be designed that deviates from Born rule probabilities. Interestingly, if any measurements do follow this random walk mechanism, then exposing a quantum system to stochastic 'noise' is an intrinsic part of such a measurement, not merely an unwanted side effect. This characteristic would have implications for reducing noise in quantum sensing and quantum computing. This is a draft of a work in progress. Questions and suggestions are welcome.

DOI
10.48550/arxiv.2407.03139

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The Born rule asserts the probability distribution of eigenstates observed in unbiased quantum measurements, but the reason it holds remains elusive. This manuscript discusses how the Born rule might be explained by Schrodinger equation dynamics, if a measurement comprises a system responding to random fluctuations until it is within an arbitrarily small tolerance of a measurement eigenstate. We describe the random walk dynamics that produce this behavior in terms of a class of time-dependent, stochastic unitary matrices U(t). We also discuss the class of stochastic potential energies in the Schrodinger equation that is equivalent to this class of unitary matrices. This analysis raises some questions worth considering, including how to determine if any measurements actually follow the predicted random walk mechanism and whether a reliable measurement apparatus could be designed that deviates from Born rule probabilities. Interestingly, if any measurements do follow this random walk mechanism, then exposing a quantum system to stochastic 'noise' is an intrinsic part of such a measurement, not merely an unwanted side effect. This characteristic would have implications for reducing noise in quantum sensing and quantum computing. This is a draft of a work in progress. Questions and suggestions are welcome.
Abstract It was recently demonstrated that the Born rule for transition probabilities can be modeled using the linear Schrödinger equation with a Hamiltonian represented by a random matrix from the Gaussian unitary ensemble. The matrices representing the Hamiltonian at different time points throughout the observation period are assumed to be independent. Instead of suppressing superpositions, such Schrödinger evolution makes the state perform an isotropic random walk on the projective space of states. The relative frequency of reaching different eigenstates of an arbitrary observable in the random walk is shown to satisfy the Born rule. Here, we apply this methodology to investigate the behavior of a particle in the context of the double-slit experiment with measurement, offering new insights into its potential underlying mechanisms.
The goal of this paper is to apply the collection of mathematical tools known as the method of arbitrary functions to analyze how the Born rule probabilities arise from quantum dynamics. We argue that in a toy model of quantum measurement the Born rule probabilities can be derived from the unitary Schrödinger dynamics when certain dynamical parameters are treated as random variables with initial probability distributions. Specifically, we study the perturbed double well model, in which the perturbation is treated as a random variable, and we show that for arbitrary initial distributions within a certain class, the dynamics yield the Born rule probabilities in the joint limits given by long times and small values of Planck’s constant (the classical limit). Our results establish the Born rule as a type of universal limiting behavior that is independent of the precise initial dynamical parameters. We suggest that this provides a proof of concept for an explanatory schema for the Born rule, relying only on Schrödinger dynamics, which could generate a full explanation when supplemented with further interpretive details.
Abstract Spontaneous collapse models use non-linear stochastic modifications of the Schrödinger equation to suppress superpositions of eigenstates of the measured observable and drive the state to an eigenstate. It was recently demonstrated that the Born rule for transition probabilities can be modeled using the linear Schrödinger equation with a Hamiltonian represented by a random matrix from the Gaussian unitary ensemble. The matrices representing the Hamiltonian at different time points throughout the observation period are assumed to be independent. Instead of suppressing superpositions, such Schrödinger evolution makes the state perform an isotropic random walk on the projective space of states. The relative frequency of reaching different eigenstates of an arbitrary observable in the random walk is shown to satisfy the Born rule. Here, we apply this methodology to investigate the behavior of a particle in the context of the double-slit experiment with measurement. Our analysis shows that, in this basic case, the evolution of the particle’s state can be effectively captured through a random walk on a two-dimensional submanifold of the state space. This random walk reproduces the Born rule for the probability of finding the particle near the slits, conditioned on its arrival at one of them. To ensure that this condition is satisfied, we introduce a drift term representing a change in the variance of the position observable for the state. It is argued that the drift term accounts for the energy transfer and trapping incurred during the particle’s interaction with the detector. A drift-free model, based on equivalence classes of states indistinguishable by the detector, is also considered. The resulting random walk, with or without drift, serves as a suitable model for describing the transition from the initial state to an eigenstate of the measured observable in the experiment, offering new insights into its potential underlying mechanisms.
The goal of this paper is to apply the collection of mathematical tools known as the "method of arbitrary functions" to analyze how probability arises from quantum dynamics. We argue that in a toy model of quantum measurement the Born rule probabilities can be derived from the unitary Schrödinger dynamics when certain dynamical parameters are treated as themselves random variables with initial probability distributions. Specifically, we study the perturbed double well model, in which the perturbation is treated as a random variable, and we show that for arbitrary initial distributions within a certain class, the dynamics yields the Born rule probabilities in the joint limits given by long times and small values of Planck's constant (the classical limit). Our results establish the Born rule as a type of universal limiting behavior that is independent of the precise initial dynamical parameters.
Projective measurement is used as a fundamental axiom in quantum mechanics, even though it is discontinuous and cannot predict which measured operator eigenstate will be observed in which experimental run. The probabilistic Born rule gives it an ensemble interpretation, predicting proportions of various outcomes over many experimental runs. Understanding gradual weak measurements requires replacing this scenario with a dynamical evolution equation for the collapse of the quantum state in individual experimental runs. We revisit the quantum trajectory framework that models quantum measurement as a continuous nonlinear stochastic process. We describe the ensemble of quantum trajectories as noise fluctuations on top of geodesics that attract the quantum state towards the measured operator eigenstates. In this effective theory framework for the ensemble of quantum trajectories, the measurement interaction can be specific to each system-apparatus pair, a context necessary for understanding weak measurements. Also in this framework, the constraint to reproduce projective measurement as per the Born rule in the appropriate limit requires that the magnitudes of the noise and the attraction are precisely related, in a manner reminiscent of the fluctuation-dissipation relation. This relation implies that both the noise and the attraction have a common origin in the underlying measurement interaction between the system and the apparatus. We analyze the quantum trajectory ensemble for the scenarios of quantum diffusion and binary quantum jump, and show that the ensemble distribution is completely determined in terms of a single evolution parameter. This trajectory ensemble distribution can be tested in weak measurement experiments. We also comment on how the required noise may arise in the measuring apparatus.
It is shown that Schrodinger's equation and Born's rule are sufficient to ensure that the states of macroscopic collective coordinate subsystems are microscopically localized in phase space and that the localized state follows the classical trajectory with random quantum noise that is indistinguishable from the pseudo-random noise of classical Brownian motion. This happens because in realistic systems the localization rate determined by the coupling to the environment is greater than the Lyapunov exponent that governs chaotic spreading in phase space. For realistic systems, the trajectories of the collective coordinate subsystem are at the same time an "unravelling" and a set of "consistent/decoherent histories". Different subsystems have their own stochastic dynamics that generally knit together to form a global dynamics, although in certain contrived thought experiments, most notably Wigner's friend, in the contrary, there is observer complementarity.