In this thesis, we develop a probabilistic description of the time-symmetric formulation of quantum mechanics.We derive a generalized form of the probability distribution of a quantum process which is determined by performing pre-and post-selections on a system.It gives the well-known value for an observable associated to a quantum process, called weak value, as an expectation value.We show that the generalized probability satisfies the law of total probability and admits the transitive form by means of conditional probabilities.This motivates us to step forward to a classical ensemble picture in quantum mechanics.In addition, we present the description of quantum measurement in terms of the generalized probability.It is suggested that the difference between the projective measurement and the weak measurement corresponds to the choice of selections of the forward and backward propagating processes. Consistency of the Probability in the Forward and Backward Processes 6 Conclusion and Discussion Bibliography
In this thesis, we develop a probabilistic description of the time-symmetric formulation of quantum mechanics.We derive a generalized form of the probability distribution of a quantum process which is determined by performing pre-and post-selections on a system.It gives the well-known value for an observable associated to a quantum process, called weak value, as an expectation value.We show that the generalized probability satisfies the law of total probability and admits the transitive form by means of conditional probabilities.This motivates us to step forward to a classical ensemble picture in quantum mechanics.In addition, we present the description of quantum measurement in terms of the generalized probability.It is suggested that the difference between the projective measurement and the weak measurement corresponds to the choice of selections of the forward and backward propagating processes. Consistency of the Probability in the Forward and Backward Processes 6 Conclusion and Discussion Bibliography
We explore further the suggestion to describe a pre- and postselected system by a two-state, which is determined by two conditions. Starting with a formal definition of a two-state Hilbert space and basic operations, we systematically recast the basics of quantum mechanics---dynamics, observables, and measurement theory---in terms of two-states as the elementary quantities. We find a simple and suggestive formulation that "unifies" two complementary observables: probabilistic observables and nonprobabilistic "weak" observables. Probabilities are relevant for measurements in the "strong-coupling regime." They are given by the absolute square of a two-amplitude (a projection of a two-state). Nonprobabilistic observables are observed in sufficiently weak measurements and are given by linear combinations of the two-amplitude. As a subclass they include the "weak values" of Hermitian operators. We show that in the intermediate regime, one may observe a mixing of probabilities and weak values. A consequence of the suggested formalism and measurement theory is that the problems of nonlocality and Lorentz noncovariance, of the usual prescription with a "reduction," may be eliminated. We exemplify this point for the Einstein-Podolsky-Rosen experiment and for a system under successive observations.
We investigate the sequential composition of weak values in the framework of time-symmetric quantum mechanics. Specifically, we consider a forward'' weak measurement from a preselected state ∣ψ⟩ to a post-selected state ∣ϕ⟩, followed by a reverse'' weak measurement. We show that the product of these two weak values corresponds to the normalized expectation value of a strong, state-conditioned observable B=APψA, where Pψ=∣ψ⟩⟨ψ∣ is the projector onto the preselected state. Analyzing the structure of B, we demonstrate how it encodes interference information, particularly when ∣ψ⟩ is a superposition rather than an eigenstate of A. This formulation extends naturally to mixed states by replacing Pψ with a generic density matrix ρ, linking the construction to the formalism of generalized quantum measurements. We illustrate practical applications in quantum information, including state-specific error witnessing in quantum computing, and show how the phase of a weak value can be inferred via strong measurements in the pure-state case.
The two-state vector formalism provides a complete description of pre- and post-selected quantum systems and has uncovered a host of new quantum phenomena which were previously hidden. The most important feature is that any weak coupling to a pre- and post-selected system is effectively a coupling to a "weak value" which is given by a simple expression depending on the two-state vector. In particular, weak values, are the outcomes of so called "weak measurements" which have recently become a very powerful tool for ultra-sensitive measurements. Using weak values, I will show that the classical limit of quantum mechanics is a far more complicated issue; it is in fact dramatically more involved and it requires a complete revision of all our intuitions. The revised intuitions can then serve as a guide to finding novel quantum effects.
This study proposes a time-symmetric framework for quantum measurement that restores microscopic reversibility at the level of the dynamical description while remaining compatible with causality and thermodynamic consistency. Instead of invoking a stochastic wavefunction collapse, the measurement process is modeled as a bidirectional informational update between a forward-evolving state and a backward-propagating effect, governed by a completely positive generator and its adjoint. Within this operator-based formalism, pre- and post-selected statistics are treated on an equal footing, yielding a unified description of both. The proposed scheme rigorously preserves complete positivity, normalization, and the no-signalling principle, and it is shown to satisfy Spohn's inequality for the associated quantum Markov semigroup, thereby ensuring non-negative entropy production within this setting. The framework admits a direct experimental interpretation across a range of scenarios, including weak measurements, EPR-Bell tests, homodyne detection, and photon counting. Furthermore, in the classical limit, the bidirectional update is demonstrated to reduce to the well-established Kalman filter and Rauch-Tung-Striebel (RTS) smoother used in classical estimation theory. These results support the view that the apparent temporal asymmetry of quantum measurement arises not from fundamental dynamical irreversibility, but from informational conditioning, specifically, the one-sided way in which measurement outcomes are incorporated into our description. In this sense, the arrow of time in measurement theory may be understood as an arrow of information.
One of the remarkable notions in the recent development of quantum physics is the weak value related to weak measurements. We emulate it as a two-time conditional expectation in a classical stochastic model. We use the well known symmetrized form of the master equation, which is formally equivalent to the wave equation in quantum mechanics apart from the fact that wave functions are always real. The origin of the unusual behaviors of the weak value such as the negative probability and the abnormal enhancement of some expectations becomes clearer in the present case, where the two-time conditional probability has no ambiguity of imaginary/complex values.
One of the remarkable notions in the recent development of quantum physics is the weak value related to weak measurements.We emulate it as a two-time conditional expectation in a classical stochastic model.We use the well known symmetrized form of the master equation, which is formally equivalent to the wave equation in quantum mechanics apart from the fact that wave functions are always real.The origin of the unusual behaviors of the weak value such as the negative probability and the abnormal enhancement of some expectations becomes clearer in the present case, where the two-time conditional probability has no ambiguity of imaginary/complex values.