Stinespring factorization theorem

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In mathematics, Stinespring's dilation theorem, also called Stinespring's factorization theorem, named after W. Forrest Stinespring, is a result from operator theory that represents any completely positive map on a C*-algebra as a composition of two completely positive maps each of which has a special form:

  1. A *-representation of A on some auxiliary Hilbert space K followed by
  2. An operator map of the form TVTV*.

Formulation[edit]

In the case of a unital C*-algebra, the result is as follows:

Theorem. Let A be a unital C*-algebra, H be a Hilbert space, and B(H) be the bounded operators on H. For every completely positive
\Phi : A \to B(H),
there exists a Hilbert space K and a unital *-homomorphism
\pi : A \to B(K)
such that
\Phi(a) = V \pi (a) V^*,
where V: K \to H is a bounded operator. Furthermore, we have
\| \Phi(1) \| = \| V \|^2.

Informally, one can say that every completely positive map Φ can be "lifted" up to a map of the form V (\cdot) V^*.

The converse of the theorem is true trivially. So Stinespring's result classifies completely positive maps.

Sketch of proof[edit]

We now briefly sketch the proof. Let K = A \otimes H. For a \otimes h, b \otimes g \in K, define

 \langle a \otimes h, b \otimes g  \rangle _K := \langle \Phi(b^*a) h, g  \rangle _H = \langle h, \Phi(a^*b)g \rangle_H

and extend by (conjugate) linearity to all of K. We see that this is a Hermitian bilinear form by definition. By the completely positivity of Φ, it is also positive. The assumption that Φ preserves positivity means Φ commutes with the * operation in A, which can be used to show that \langle \cdot, \cdot \rangle _K is conjugate-symmetric. Therefore \langle \cdot, \cdot \rangle _K is a, possibly degenerate, Hermitian bilinear form. Since Hermitian bilinear forms satisfy the Cauchy Schwarz inequality, the subset

K' = \{x \in K | \langle x ,  x  \rangle _K = 0 \} \subset K

is a subspace. We can remove degeneracy by considering the quotient space K / K' . The completion of this quotient space is then a Hilbert space, also denoted by K. Next define \pi (a) (b \otimes g) = ab \otimes g and V^* h = 1 \otimes h, where 1 is the unit in A. One can check that π and V have the desired properties.

Notice that V^* is just the natural algebraic embedding of H into K. Direct calculation shows that, in the finite-dimensional case, V V^* can be identified with the algebraic identity map on H. The definitions of π(A) and K are also rather natural. Thus the key element of the proof is the introduction of \langle \cdot, \cdot \rangle _K. In particular, after the algebraic embedding, H is "re-normed" in the following sense: If h is identified with 1 ⊗ h, then

 \langle 1 \otimes h, 1 \otimes h \rangle _K = \langle V^* h, V^* h \rangle _K  = \langle V V^* h, h \rangle _K  = \langle \Phi (1) h, h \rangle _H.

This can be viewed as the restriction of \langle \cdot, \cdot \rangle _K to H.

When Φ is unital, i.e. Φ(1) = 1, we see that V^* is an isometry and H can be embedded, in the Hilbert space sense, into K. V, acting on K, becomes the projection onto H. Symbolically, we can write

\Phi (a) = P_H \; \pi(a) |_H.

In the language of dilation theory, this is to say that Φ(a) is a compression of π(a). It is therefore a corollary of Stinespring's theorem that every unital completely positive map is the compression of some *-homomorphism.

Minimality[edit]

The triple (π, V, K) is called a Stinespring representation of Φ. A natural question is now whether one can reduce a given Stinespring representation in some sense.

Let K1 be the closed linear span of π(A) V*H. By property of *-representations in general, K1 is an invariant subspace of π(a) for all a. Also, K1 contains V*H. Define

\pi _1 (a) = \pi (a) |_{K_1}.

We can compute directly

\pi_1 (a) \pi_1 (b) = \pi (a) |_{K_1} \pi (b) |_{K_1} = \pi (a) \pi (b) |_{K_1} = \pi (ab) |_{K_1} = \pi_1 (ab)

and if k and l lie in K1

\langle \pi_1 (a^*)k, l \rangle = \langle \pi (a^*)k, l \rangle = \langle \pi(a) ^* k, l \rangle = \langle k, \pi (a) l \rangle = \langle k, \pi_1 (a) l \rangle =\langle  \pi_1 (a) ^* k, l \rangle.

So (π1, V, K1) is also a Stinespring representation of Φ and has the additional property that K1 is the closed linear span of π(A) V*H. Such a representation is called a minimal Stinespring representation.

Uniqueness[edit]

Let (π1, V1, K1) and (π2, V2, K2) be two Stinespring representations of a given Φ. Define a partial isometry W : K1K2 by

\; W \pi_1 (a) V_1 h = \pi_2 (a) V_2 h.

On V1HK1, this gives the interwining relation

\; W \pi_1 = \pi_2 W.

In particular, when both Stinespring representations are minimal, W is unitary. Thus minimal Stinespring representations are unique up to a unitary transformation.

Some consequences[edit]

We mention a few of the results which can be viewed as consequences of Stinespring's theorem. Historically, some of the results below preceded Stinespring's theorem.

GNS construction[edit]

Let H in Stinespring's theorem be 1-dimensional, i.e. the complex numbers. So Φ now is a positive linear functional on A. If we assume Φ is a state, that is, Φ has norm 1, then the isometry V^* : H \to K is determined by

V^* 1 = \xi

for some \xi \in K of unit norm. So

\Phi(a) = V \pi (a) V^* = \langle V \pi (a) V^* 1, 1 \rangle _H = \langle \pi (a) V^* 1, V^* 1 \rangle _K = \langle \pi (a) \xi, \xi \rangle _K

and we have recovered the GNS representation of states. This is one way to see that completely positive maps, rather than merely positive ones, are the true generalizations of positive functionals.

A linear positive functional on a C*-algebra is absolutely continuous with respect to another such (called reference) functional if it is zero on any positive element on which the reference positive functional is zero. This leads to a noncommutative generalization of Radon-Nikodym theorem. The usual density operator of states on the matrix algebras with respect to the standard trace is nothing but the Radon-Nikodym derivative when the reference functional is chosen to be trace. Belavkin introduced the notion of complete absolute continuity of one completely positive map with respect to another (reference) map and proved an operator variant of the noncommutative Radon-Nikodym theorem for completely positive maps. A particular case of this theorem corresponding to a tracial completely positive reference map on the matrix algebras leads to the Choi operator as a Radon-Nikodym derivative of a CP map with respect to the standard trace (see Choi's Theorem).

Choi's theorem[edit]

It was shown by Choi that if \Phi: B(G) \to B(H) is completely positive, where G and H are finite-dimensional Hilbert spaces of dimensions n and m respectively, then Φ takes the form:

\Phi (a) =  \sum _{i = 1} ^{nm} V_i a V_i ^*.

Choi proved this using linear algebra techniques, but his result can also be viewed as a special case of Stinespring's theorem: Let (π, V, K) be a minimal Stinespring representation of Φ. By minimality, K has dimension less than that of C^{n \times n} \otimes C^m. So without loss of generality, K can be identified with

K = \oplus _{i = 1} ^{nm} C_i ^n.

Each C_i^n is a copy of the n-dimensional Hilbert space. From \pi (a) (b \otimes g) = ab \otimes g, we see that the above identification of K can be arranged so \; P_i \pi(a) P_i = a, where Pi is the projection from K to C_i ^n. Let V_i ^* = P_i V^*. We have

\Phi (a) = \sum_{i = 1}^{nm} (V P_i) (P_i \pi(a) P_i) (P_i V^*) = \sum _{i = 1} ^{nm} V_i a V_i ^*

and Choi's result is proved.

Choi's result is a particular case of noncommutative Radon-Nikodym theorem for completely positive (CP) maps corresponding to a tracial completely positive reference map on the matrix algebras. In strong operator form this general theorem was proven by Belavkin in 1985 who showed the existence of the positive density operator representing a CP map which is completely absolutely continuous with respect to a reference CP map. The uniqueness of this density operator in the reference Steinspring representation simply follows from the minimality of this representation. Thus, Choi's operator is the Radon-Nikodym derivative of a finite-dimensional CP map with respect to the standard trace.

Notice that, in proving Choi's theorem, as well as Belavkin's theorem from Stinespring's formulation, the argument does not give the Kraus operators Vi explicitly, unless one makes the various identification of spaces explicit. On the other hand, Choi's original proof involves direct calculation of those operators.

Naimark's dilation theorem[edit]

Naimark's theorem says that every B(H)-valued, weakly countably-additive measure on some compact Hausdorff space X can be "lifted" so that the measure becomes a spectral measure. It can be proved by combining the fact that C(X) is a commutative C*-algebra and Stinespring's theorem.

Sz.-Nagy's dilation theorem[edit]

This result states that every contraction on a Hilbert space has a unitary dilation with the minimality property.

Application[edit]

In quantum information theory, quantum channels, or quantum operations, are defined to be completely positive maps between C*-algebras. Being a classification for all such maps, Stinespring's theorem is important in that context. For example, the uniqueness part of the theorem has been used to classify certain classes of quantum channels.

For the comparison of different channels and computation of their mutual fidelities and information another representation of the channels by their "Radon-Nikodym" derivatives introduced by Belavkin is useful. In the finite-dimensional case, Choi's theorem as the tracial variant of the Belavkin's Radon-Nikodym theorem for completely positive maps is also relevant. The operators \{ V_i \} from the expression

\Phi (a) = \sum_{i = 1}^{nm} V_i a V_i ^*.

are called the Kraus operators of Φ. The expression

\sum_{i = 1}^{nm} V_i ( \cdot ) V_i ^*

is sometimes called the operator sum representation of Φ.

References[edit]

  • M. Choi, Completely Positive Linear Maps on Complex matrices, Linear Algebra and Its Applications, 285–290, 1975
  • V. P. Belavkin, P. Staszewski, Radon-Nikodym Theorem for Completely Positive Maps, Reports on Mathematical Physics, v.24, No 1, 49–55, 1986.
  • V. Paulsen, Completely Bounded Maps and Operator Algebras, Cambridge University Press, 2003.
  • W. F. Stinespring, Positive Functions on C*-algebras, Proceedings of the American Mathematical Society, 211–216, 1955