Quantum Mechanical Ensembles. Macroscopic Operators

Kunio Goto

Giuseppe Pileio

In this lecture, I lay down the basics for the study of the dynamics of a large collection of identical spin systems by introducing the concept of a density operator and an equation for its evolution. Ensembles of spin systems represent macroscopic samples; in quantum mechanics, these can only be described in statistical terms through a density operator. This latter can be expressed in terms of spin angular momentum operators. The density operator allows all measurable properties of an ensemble of spin systems to be determined.

Amit Vikram

We identify a general mechanism for the finite-time thermalization of macroscopic observables, such as coarse-grained charge densities, in terms of elementary forms of the quantum dynamics of initial states: (1) aperiodicity, which provides a computable measure of (2) a dynamical partially ergodic exploration of the Hilbert space. Specifically, this mechanism predicts the equilibration of all (concentrated) macroscopic observables, in almost all states in an initial ensemble and almost all times within finite and longer intervals, given only the observable-independent information that the return probability of the ensemble of initial states is small over a finite time range. As a special case, it also accesses standard results on equilibration over infinitely long times in terms of (stronger versions of) the effective dimension of initial state delocalization in the energy eigenbasis. Our results incorporate macroscopic thermalization into the domain of operational quantum statistical mechanics, recently developed to provide finitely computable criteria for microscopic thermalization. We discuss an overall characterization of this approach as establishing connections between (1) the decay of a (theoretically or experimentally) computable probe indicating memorylessness, (2) a fundamental invariant mechanism in terms of the alignment of observables or states in the Hilbert space, and (3) predicting different natural forms of (classical and) quantum thermalization, most of which rigorously recover conventional eigenstate-based descriptions of infinite-time thermalization as a special case but provide stronger accessible predictions over finite observation times in the thermodynamic limit.

John von Neumann

This chapter considers what would happen to a mixture with the statistical operator 𝗨 if a quantity 𝖱 with operator 𝖱 is measured in it. It then investigates the thermodynamics of quantum mechanical ensembles from two different points of view. First, the chapter assumes the validity of both of the fundamental laws of thermodynamics, and proceeds on this basis to calculation of the entropy of ensembles. In this effort the normal methods of phenomenological thermodynamics are applied, and quantum mechanics plays a role only insofar as these thermodynamic observations relate to objects whose behavior is regulated by the laws of quantum. Afterwards this chapter proves the validity of these fundamental laws in quantum mechanics. Next, the chapter introduces some reversibility and equilibrium problems. Finally, this chapter discusses the macroscopic measurement.

George H. Duffey