Storage tube

In mathematics, particularly in functional analysis, a projection-valued measure (PVM) is a function defined on certain subsets of a fixed set and whose values are self-adjoint projections on a Hilbert space. Projection-valued measures are used to express results in spectral theory, such as the spectral theorem for self-adjoint operators. In quantum mechanics, PVMs are the mathematical description of projective measurements. They are generalized by positive operator valued measures (POVMs) in the same sense that a mixed state or density matrix generalizes the notion of a pure state.

Formal definition

A projection-valued measure on a measurable space (X, M), where M is a σ-algebra of subsets of X, is a mapping π from M to the set of self-adjoint projections on a Hilbert space H such that

$\pi (X)={\mathrm{id}}_H$

and for every ξ, η ∈ H, the set-function

$A\mapsto ⟨\pi (A)\xi \mid \eta ⟩$

is a complex measure on M (that is, a complex-valued countably additive function). We denote this measure by ${\mathrm{S}}_{\pi }(\xi ,\eta )$.

If π is a projection-valued measure and

$A\cap B=\mathrm{\varnothing },$

then π(A), π(B) are orthogonal projections. From this follows that in general,

$\pi (A)\pi (B)=\pi (A\cap B).$

Example. Suppose (X, M, μ) is a measure space. Let π(A) be the operator of multiplication by the indicator function 1_A on L²(X). Then π is a projection-valued measure.

Extensions of projection-valued measures

If π is an additive projection-valued measure on (X, M), then the map

${\mathbf{1}}_A\mapsto \pi (A)$

extends to a linear map on the vector space of step functions on X. In fact, it is easy to check that this map is a ring homomorphism. In fact this map extends in a canonical way to all bounded complex-valued Borel functions on X.

Theorem. For any bounded M-measurable function f on X, there is a unique bounded linear operator T_π(f) such that

$⟨{\mathrm{T}}_{\pi }(f)\xi \mid \eta ⟩=\underset{X}{\int }f(x)d{\mathrm{S}}_{\pi }(\xi ,\eta )(x)$

for all ξ, η ∈ H. The map

$f\mapsto {\mathrm{T}}_{\pi }(f)$

is a homomorphism of rings.

Structure of projection-valued measures

First we provide a general example of projection-valued measure based on direct integrals. Suppose (X, M, μ) is a measure space and let {H_x}_{x ∈ X} be a μ-measurable family of separable Hilbert spaces. For every A ∈ M, let π(A) be the operator of multiplication by 1_A on the Hilbert space

$\underset{X}{\overset{\oplus }{\int }}{H}_{x}d\mu (x).$

Then π is a projection-valued measure on (X, M).

Suppose π, ρ are projection-valued measures on (X, M) with values in the projections of H, K. π, ρ are unitarily equivalent if and only if there is a unitary operator U:H → K such that

$\pi (A)=U^{*}\rho (A)U$

for every A ∈ M.

Theorem. If (X, M) is a standard Borel space, then for every projection-valued measure π on (X, M) taking values in the projections of a separable Hilbert space, there is a Borel measure μ and a μ-measurable family of Hilbert spaces {H_x}_{x ∈ X} , such that π is unitarily equivalent to multiplication by 1_A on the Hilbert space

$\underset{X}{\overset{\oplus }{\int }}{H}_{x}d\mu (x).$

The measure class of μ and the measure equivalence class of the multiplicity function x → dim H_x completely characterize the projection-valued measure up to unitary equivalence.

A projection-valued measure π is homogeneous of multiplicity n if and only if the multiplicity function has constant value n. Clearly,

Theorem. Any projection-valued measure π taking values in the projections of a separable Hilbert space is an orthogonal direct sum of homogeneous projection-valued measures:

$\pi =\underset{1\le n\le \omega }{⨁}(\pi |H_n)$

where

$H_n=\underset{X_n}{\overset{\oplus }{\int }}{H}_{x}d(\mu |X_n)(x)$

and

$X_n=\{x\in X:\dim H_x=n\}.$

Generalizations

The idea of a projection-valued measure is generalized by the positive operator-valued measure (POVM), where the need for the orthogonality implied by projection operators is replaced by the idea of a set of operators that are a non-orthogonal partition of unity. This generalization is motivated by applications to quantum information theory.

References

• G. W. Mackey, The Theory of Unitary Group Representations, The University of Chicago Press, 1976
• M. Reed and B. Simon, Methods of Mathematical Physics, vols I–IV, Academic Press 1972.
• G. Teschl, Mathematical Methods in Quantum Mechanics with Applications to Schrödinger Operators, http://www.mat.univie.ac.at/~gerald/ftp/book-schroe/, American Mathematical Society, 2009.
• V. S. Varadarajan, Geometry of Quantum Theory V2, Springer Verlag, 1970.