**Quantum statistical mechanics** is the study of statistical ensembles of

quantum mechanical systemsQuantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

. A statistical ensemble is described by a

density operatorIn quantum mechanics, a density matrix is a self-adjoint positive-semidefinite matrix of trace one, that describes the statistical state of a quantum system...

*S*, which is a non-negative, self-adjoint, trace-class operator of trace 1 on the

Hilbert spaceThe mathematical concept of a Hilbert space, named after David Hilbert, generalizes the notion of Euclidean space. It extends the methods of vector algebra and calculus from the two-dimensional Euclidean plane and three-dimensional space to spaces with any finite or infinite number of dimensions...

*H* describing the quantum system. This can be shown under various

mathematical formalisms for quantum mechanicsThe mathematical formulations of quantum mechanics are those mathematical formalisms that permit a rigorous description of quantum mechanics. Such are distinguished from mathematical formalisms for theories developed prior to the early 1900s by the use of abstract mathematical structures, such as...

. One such formalism is provided by

quantum logicIn quantum mechanics, quantum logic is a set of rules for reasoning about propositions which takes the principles of quantum theory into account...

.

## Expectation

From classical probability theory, we know that the

expectationIn probability theory, the expected value of a random variable is the weighted average of all possible values that this random variable can take on...

of a

random variableIn probability and statistics, a random variable or stochastic variable is, roughly speaking, a variable whose value results from a measurement on some type of random process. Formally, it is a function from a probability space, typically to the real numbers, which is measurable functionmeasurable...

*X* is completely determined by its

distributionIn probability theory, a probability mass, probability density, or probability distribution is a function that describes the probability of a random variable taking certain values....

D

_{X} by

assuming, of course, that the random variable is integrable or that the random variable is non-negative. Similarly, let

*A* be an

observableIn physics, particularly in quantum physics, a system observable is a property of the system state that can be determined by some sequence of physical operations. For example, these operations might involve submitting the system to various electromagnetic fields and eventually reading a value off...

of a quantum mechanical system.

*A* is given by a densely defined self-adjoint operator on

*H*. The spectral measure of

*A* defined by

uniquely determines

*A* and conversely, is uniquely determined by

*A*. E

_{A} is a boolean homomorphism from the Borel subsets of

**R** into the lattice

*Q* of self-adjoint projections of

*H*. In analogy with probability theory, given a state

*S*, we introduce the

*distribution* of

*A* under

*S* which is the probability measure defined on the Borel subsets of

**R** by

Similarly, the expected value of

*A* is defined in terms of the probability distribution D

_{A} by

Note that this expectation is relative to the mixed state

*S* which is used in the definition of D

_{A}.

**Remark**. For technical reasons, one needs to consider separately the positive and negative parts of

*A* defined by the

Borel functional calculusIn functional analysis, a branch of mathematics, the Borel functional calculus is a functional calculus , which has particularly broad scope. Thus for instance if T is an operator, applying the squaring function s → s2 to T yields the operator T2...

for unbounded operators.

One can easily show:

Note that if

*S* is a pure state corresponding to the vector ψ,

## Von Neumann entropy

Of particular significance for describing randomness of a state is the von Neumann entropy of

*S* *formally* defined by

.

Actually, the operator

*S* log

_{2} *S* is not necessarily trace-class. However, if

*S* is a non-negative self-adjoint operator not of trace class we define Tr(

*S*) = +∞. Also note that any density operator

*S* can be diagonalized, that it can be represented in some orthonormal basis by a (possibly infinite) matrix of the form

and we define

The convention is that

, since an event with probability zero should not contribute to the entropy. This value is an extended real number (that is in [0, ∞]) and this is clearly a unitary invariant of

*S*.

**Remark**. It is indeed possible that H(

*S*) = +∞ for some density operator

*S*. In fact

*T* be the diagonal matrix

*T* is non-negative trace class and one can show

*T* log

_{2} *T* is not trace-class.

**Theorem**. Entropy is a unitary invariant.

In analogy with classical entropy (notice the similarity in the definitions), H(

*S*) measures the amount of randomness in the state

*S*. The more dispersed the eigenvalues are, the larger the system entropy. For a system in which the space

*H* is finite-dimensional, entropy is maximized for the states

*S* which in diagonal form have the representation

For such an

*S*, H(

*S*) = log

_{2} *n*. The state

*S* is called the maximally mixed state.

Recall that a pure state is one of the form

for ψ a vector of norm 1.

**Theorem**. H(

*S*) = 0 if and only if

*S* is a pure state.

For

*S* is a pure state if and only if its diagonal form has exactly one non-zero entry which is a 1.

Entropy can be used as a measure of

quantum entanglementQuantum entanglement occurs when electrons, molecules even as large as "buckyballs", photons, etc., interact physically and then become separated; the type of interaction is such that each resulting member of a pair is properly described by the same quantum mechanical description , which is...

.

## Gibbs canonical ensemble

Consider an ensemble of systems described by a Hamiltonian

*H* with average energy

*E*. If

*H* has pure-point spectrum and the eigenvalues

of

*H* go to + ∞ sufficiently fast, e

^{-r H} will be a non-negative trace-class operator for every positive

*r*.

The

*Gibbs canonical ensemble* is described by the state

Where β is such that the ensemble average of energy satisfies

and

is the quantum mechanical version of the canonical partition function. The probability that a system chosen at random from the ensemble will be in a state corresponding to energy eigenvalue

is

Under certain conditions, the Gibbs canonical ensemble maximizes the von Neumann entropy of the state subject to the energy conservation requirement.

## See also

- Density matrix
In quantum mechanics, a density matrix is a self-adjoint positive-semidefinite matrix of trace one, that describes the statistical state of a quantum system...

- Gibbs measure
In mathematics, the Gibbs measure, named after Josiah Willard Gibbs, is a probability measure frequently seen in many problems of probability theory and statistical mechanics. It is the measure associated with the Boltzmann distribution, and generalizes the notion of the canonical ensemble...

- Partition function (mathematics)
The partition function or configuration integral, as used in probability theory, information science and dynamical systems, is an abstraction of the definition of a partition function in statistical mechanics. It is a special case of a normalizing constant in probability theory, for the Boltzmann...

- Weyl quantization
In mathematics and physics, in the area of quantum mechanics, Weyl quantization is a method for systematically associating a "quantum mechanical" Hermitian operator with a "classical" kernel function in phase space invertibly...