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Physical Review

Approximate Variational Principle in Quantum Statistics

William M. MacDonald, John M. Richardson

Published Oct 1, 1954• 5 citations
Publisher

Abstract

The density matrix of von Neumann can be used to formulate an exact variational principle for quantum statistics which embodies the principle of maximization of entropy. Using the formalism of second quantization, we can write this variational principle for fermions or bosons and can then derive from it an approximate variational procedure which yields the particle states of a system of interacting bosons or fermions as well as the distribution of particles in these states. These equations yield the generalization of the Hartree-Fock equations for nonzero temperature and the corresponding extension to bosons.

DOI
10.1103/physrev.96.18

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The density matrix of von Neumann can be used to formulate an exact variational principle for quantum statistics which embodies the principle of maximization of entropy. Using the formalism of second quantization, we can write this variational principle for fermions or bosons and can then derive from it an approximate variational procedure which yields the particle states of a system of interacting bosons or fermions as well as the distribution of particles in these states. These equations yield the generalization of the Hartree-Fock equations for nonzero temperature and the corresponding extension to bosons.
In solving the problem of finding a temperature distribution which, at zero temperature, corresponds to superfluidity, i.e., to nonzero energy, the author tried to quantize free energy. This was done on the basis of supersecondary quantization whose special case is the usual secondary quantization for bosons and with the help of which new representations of the Schrödinger equation were obtained. The supersecondary quantization allowed the author to construct a variational method whose zero approximation are the Hartree-Fock and Bogolyubov-BCSch variational principles. This method works especially well in the case of not a large number of particles. The new quantization and the variational method are of general character and can be used in the quantum field theory.
This chapter begins with the discussion of various meanings of the entropy: Boltzmann entropy, Shannon entropy, von Neumann entropy. It then presents the basic properties of the von Neumann entropy such as concavity, sub-additivity, strong sub-additivity, and continuity. It proves the existence of mean entropy for shift-invariant states on quantum spin chains, and then derives the expression for the mean entropy for quasi-free Fermions on a chain. The chapter defines the quantum relative entropy and presents its basic properties, including behaviour with respect to completely positive maps. The maximum entropy principle defines thermal equilibrium states (Gibbs states). This variational principle is illustrated by the Hartree–Fock approximation for a model of interacting Fermions. The entropy of an equilibrium state for a free quantum particle on a compact Riemannian manifold is also estimated. Finally, the notion of relative entropy is formulated in the algebraic setting using the relative modular operator.
The hierarchy equations for the quantum thermal density matrices of an assembly of interacting particles in an external potential are derived in a compact form. This approach can complement and finally replace the phenomenological treatment of the thermodynamic properties of quantum gasses, liquids, and solids. We also derive a temperature Hartree-Fock approximation for fermions based on the symmetry properties of the exact hierarchy and finally we discuss the present approximation in relation to that of Kaplan and Argyres [T. A. Kaplan and P. N. Argyres, Ann. Phys. (N.Y.) 92, 1 (1975)].
In its original formulation, Lieb's variational principle holds for fermion systems with purely repulsive pair interactions. As a generalization we prove for both fermion and boson systems with semi-bounded Hamiltonian that the infimum of the energy over quasifree states coincides with the infimum over pure quasifree states. In particular, the Hamiltonian is not assumed to preserve the number of particles. To shed light on the relation between our result and the usual formulation of Lieb's variational principle in terms of one-particle density matrices, we also include a characterization of pure quasifree states by means of their generalized one-particle density matrices.
The evolution of a quantum particle interacting with a classical system is described by a generalized variational principle. The dynamical variable is a quantum state vector which includes the classical action as a phase factor, and the common time is treated as a collective variable. Combined with the model of bilinear coupling, the variational principle is applied to the problem of a quantum system in a thermal environment. It is shown that the statistical ensemble of Brownian state vectors is described by the solution of a nonlinear quantum Fokker-Planck equation for the density matrix. Exact solutions of this equation are obtained for the case of a two-level system, considering both stationary and nonstationary initial states.