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arXiv:gr-qc/9505011

REALITY AND GEOMETRY OF STATES AND OBSERVABLES IN QUANTUM THEORY

J. Anandan

Published May 10, 1995• 2 citations
arXivHTMLPDF

Abstract

The determination of the quantum state of a single system by protective observation is used to justify operationally a formulation of quantum theory on the quantum state space (projective Hilbert space) $\cal P$. Protective observation is extended to a more general quantum theory in which the Schrodinger evolution is generalized so that it preserves the symplectic structure but not necessarily the metric in $\cal P$. The relevance of this more general evolution to the apparant collapse of the state vector during the usual measurement, and its possible connection to gravity is suggested. Some criticisms of protective observation are answered. A comparison is made between the determination of quantum states using the geometry of $\cal P$ by protective measurements, via a reconstruction theorem, and the determination of space-time points by means of the space-time geometry, via Einstein's hole argument. It is argued that a protective measurement may not determine a time average.

DOI
10.48550/arxiv.gr-qc/9505011

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The determination of the quantum state of a single system by protective observation is used to justify operationally a formulation of quantum theory on the quantum state space (projective Hilbert space) P\cal PP. Protective observation is extended to a more general quantum theory in which the Schrodinger evolution is generalized so that it preserves the symplectic structure but not necessarily the metric in P\cal PP. The relevance of this more general evolution to the apparant collapse of the state vector during the usual measurement, and its possible connection to gravity is suggested. Some criticisms of protective observation are answered. A comparison is made between the determination of quantum states using the geometry of P\cal PP by protective measurements, via a reconstruction theorem, and the determination of space-time points by means of the space-time geometry, via Einstein's hole argument. It is argued that a protective measurement may not determine a time average.
This paper proposes a novel approach to quantum measurement theory based on the framework of information geometry. We argue that quantum measurement, traditionally viewed through the lens of projection postulate and wave function collapse, can be elegantly described as an optimization problem within the information geometry of the Hilbert space. Specifically, we posit that optimal quantum measurements are those which minimize the information-theoretic distance between the measured state and the manifold representing the possible measurement outcomes. This approach provides a geometric interpretation of measurement, allowing us to design and analyze measurement strategies with a deeper understanding of their underlying geometry. We explore key concepts such as the Fisher information metric, the Riemannian structure of the Hilbert space, and the role of the Born rule as a consequence of minimizing the information-theoretic distance. The core claim is that this geometric perspective offers a more profound and potentially more effective way to understand and implement quantum measurement. Furthermore, we outline a methodology for constructing measurement operators based on the curvature of the information geometry, suggesting avenues for designing novel quantum measurement protocols. ---
We consider the space of probabilities {P(x)}, where the x are coordinates of a configuration space. Under the action of the translation group, P(x) →P(x + θ), there is a natural metric over the parameters θ given by the Fisher-Rao metric. This metric induces a metric over the space of probabilities. Our next step is to set the probabilities in motion. To do this, we introduce a canonically conjugate field S and a symplectic structure; this gives us Hamiltonian equations of motion. We show that it is possible to extend the metric structure to the full space of the (P,S), and this leads in a natural way to introducing a Kähler structure; i.e., a geometry that includes compatible symplectic, metric and complex structures. The simplest geometry that describes these spaces of evolving probabilities has remarkable properties: the natural, canonical variables are precisely the wave functions of quantum mechanics; the Hamiltonian for the quantum free particle can be derived from a representation of the Galilean group using purely geometrical arguments; and it is straightforward to associate with this geometry a Hilbert space which turns out to be the Hilbert space of quantum mechanics. We are led in this way to a reconstruction of quantum theory based solely on the geometry of probabilities in motion.
“Protective measurement” refers to two related schemes for finding the expectation value of an observable without disturbing the state of a quantum system, given a single copy of the system that is subject to a “protecting” operation. There have been several claims that these schemes support interpreting the quantum state as an objective property of a single quantum system. Here we provide three counter-arguments, each of which we present in two versions tailored to the two different schemes. Our first argument shows that the same resources used in protective measurement can be used to reconstruct the quantum state in a different way via process tomography. Our second argument is based on exact analyses of special cases of protective measurement, and our final argument is to construct explicit “휓-epistemic” toy models for protective measurement, which strongly suggest that protective measurement does not imply the reality of the quantum state. The common theme of the three arguments is that almost all of the information comes from the “protection” operation rather than the quantum state of the system, and hence the schemes have no implications for the reality of the quantum state.
The state space of a quantum mechanical system is a complex projective space, the space of rays in the associated Hilbert space. The state space comes equipped with a natural Riemannian metric (the Fubini-Study metric) and a compatible symplectic structure. The operations of ordinary quantum mechanics can thus be reinterpreted in the language of differential geometry. It is interesting in this spirit to scrutinize the probabilistic assumptions that are brought in at various stages in the analysis of quantum dynamics, particularly in connection with state vector reduction. A promising approach to understanding reduction, studied recently by a number of authors, involves the use of nonlinear stochastic dynamics to modify the ordinary linear Schrodinger evolution. Here we use stochastic differential geometry to give a systematic geometric formulation for such stochastic models of state vector collapse. In this picture, the conventional Schrodinger evolution, which corresponds to the unitary flow associated with a Killing vector of the Fubini-Study metric, is replaced by a more general stochastic flow on the state manifold. In the simplest example of such a flow, the volatility term in the stochastic differential equation for the state trajectory is proportional to the gradient of the expectation of the Hamiltonian. The conservation of energy is represented by the requirement that the actual process followed by the expectation of the Hamiltonian, as the state evolves, should be a martingale. This requirement implies the existence of a nonlinear term in the drift vector of the state process, which is always oriented opposite to the direction of increasing energy uncertainty. As a consequence, the state vector necessarily collapses to an energy eigenstate, and a martingale argument can be used to show that the probability of collapse to a given eigenstate, from any particular initial state, is, in fact, given by precisely the usual quantum mechanical probability.
Protective measurement, in the language of standard quantum mechanics, is a method to measure the expectation value of an observable on a single quantum system (Aharonov and Vaidman, 1993; Aharonov, Anandan and Vaidman, 1993). For a conventional impulsive measurement, the state of the measured system is strongly entangled with the state of the measuring device during the measurement, and the measurement result is one of the eigenvalues of the measured observable. By contrast, during a protective measurement, the measured state is protected by an appropriate procedure so that it neither changes nor becomes entangled with the state of the measuring device appreciably. In this way, such protective measurements can measure the expectation values of observables on a single quantum system, and in particular, the wave function of the system can also be measured as expectation values of certain observables. It is expected that protective measurements can be performed in the near future with the rapid development of weak measurement technologies (e.g. Kocsis et al., 2011; Lundeen et al., 2011). In this chapter, we will give a clear introduction to protective measurement in quantum mechanics. Standard quantum mechanics and impulsive measurement The standard formulation of quantum mechanics, which was first developed by Dirac (1930) and von Neumann (1955), is based on the following basic principles. 1 Physical states The state of a physical system is represented by a normalized wave function or unit vector | ψ ( t )〉 in a Hilbert space. The Hilbert space is complete in the sense that every possible physical state can be represented by a state vector in the space. 2 Physical properties Every measurable property or observable of a physical system is represented by a Hermitian operator on the Hilbert space associated with the system. A physical system has a determinate value for an observable if and only if it is in an eigenstate of the observable (this is often called the eigenvalue–eigenstate link).