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{{Short description|Tensor product space endowed with a special inner product}}In [[mathematics]], and in particular [[functional analysis]], the '''tensor product of [[Hilbert space]]s''' is a way to extend the [[tensor product]] construction so that the result of taking a tensor product of two Hilbert spaces is another Hilbert space. Roughly speaking, the tensor product is the [[metric space]] [[Complete metric space|completion]] of the ordinary tensor product. This is an example of a [[topological tensor product]]. The tensor product allows Hilbert spaces to be collected into a [[symmetric monoidal category]].<ref>[[Bob Coecke|B. Coecke]] and E. O. Paquette, Categories for the practising physicist, in: New Structures for Physics, B. Coecke (ed.), Springer Lecture Notes in Physics, 2009. [https://arxiv.org/abs/0905.3010 arXiv:0905.3010]</ref>

==Definition==

Since Hilbert spaces have [[Dot product|inner products]], one would like to introduce an inner product, and thereby a topology, on the tensor product that arises naturally from the inner products on the factors. Let <math>H_1</math> and <math>H_2</math> be two Hilbert spaces with inner products <math>\langle\cdot, \cdot\rangle_1</math> and <math>\langle\cdot, \cdot\rangle_2,</math> respectively. Construct the tensor product of <math>H_1</math> and <math>H_2</math> as vector spaces as explained in the article on [[tensor product]]s. We can turn this vector space tensor product into an [[inner product space]] by defining
<math display=block>\left\langle\phi_1 \otimes \phi_2, \psi_1 \otimes \psi_2\right\rangle = \left\langle\phi_1, \psi_1\right\rangle_1 \, \left\langle\phi_2, \psi_2\right\rangle_2 \quad \mbox{for all } \phi_1,\psi_1 \in H_1 \mbox{ and } \phi_2,\psi_2 \in H_2</math>
and extending by linearity. That this inner product is the natural one is justified by the identification of scalar-valued [[bilinear map]]s on <math>H_1 \times H_2</math> and [[linear functional]]s on their vector space tensor product. Finally, take the [[Complete metric space#Completion|completion]] under this inner product. The resulting Hilbert space is the tensor product of <math>H_1</math> and <math>H_2.</math>

===Explicit construction===

The tensor product can also be defined without appealing to the metric space completion. If <math>H_1</math> and <math>H_2</math> are two Hilbert spaces, one associates to every [[simple tensor]] product <math>x_1 \otimes x_2</math> the rank one operator from <math>H_1^*</math> to <math>H_2</math> that maps a given <math>x^*\in H^*_1</math> as
<math display=block>x^* \mapsto x^*(x_1) \, x_2.</math>

This extends to a linear identification between <math>H_1 \otimes H_2</math> and the space of finite rank operators from <math>H_1^*</math> to <math>H_2.</math> The finite rank operators are embedded in the Hilbert space <math>HS(H_1^*, H_2)</math> of [[Hilbert–Schmidt operator]]s from <math>H_1^*</math> to <math>H_2.</math> The scalar product in <math>HS(H_1^*, H_2)</math> is given by
<math display=block>\langle T_1, T_2 \rangle = \sum_n \left \langle T_1 e_n^*, T_2 e_n^* \right \rangle,</math>
where <math>\left(e_n^*\right)</math> is an arbitrary orthonormal basis of <math>H_1^*.</math>

Under the preceding identification, one can define the Hilbertian tensor product of <math>H_1</math> and <math>H_2,</math> that is isometrically and linearly isomorphic to <math>HS(H_1^*, H_2).</math>

===Universal property===

The Hilbert tensor product <math>H_1 \otimes H_2</math> is characterized by the following [[universal property]] {{harv|Kadison|Ringrose|1997|loc=Theorem 2.6.4}}:

{{math theorem | There is a weakly Hilbert–Schmidt mapping <math>p : H_1 \times H_2 \to H_1 \otimes H_2</math> such that, given any weakly Hilbert–Schmidt mapping <math>L : H_1 \times H_2 \to K</math> to a Hilbert space <math>K,</math> there is a unique bounded operator <math>T : H_1 \otimes H_2 \to K</math> such that <math>L = T p.</math>}}

A weakly Hilbert-Schmidt mapping <math>L : H_1 \times H_2 \to K</math> is defined as a bilinear map for which a real number <math>d</math> exists, such that 
<math display=block>\sum_{i,j=1}^\infty \bigl| \left\langle L(e_i, f_j), u  \right \rangle\bigr|^2 \leq d^2\,\|u\|^2</math> 
for all <math>u \in K</math> and one (hence all) orthonormal bases <math>e_1, e_2, \ldots</math> of <math>H_1</math> and <math>f_1, f_2, \ldots</math> of <math>H_2.</math>

As with any universal property, this characterizes the tensor product ''H'' uniquely, up to isomorphism. The same universal property, with obvious modifications, also applies for the tensor product of any finite number of Hilbert spaces. It is essentially the same universal property shared by all definitions of tensor products, irrespective of the spaces being tensored: this implies that any space with a tensor product is a [[symmetric monoidal category]], and Hilbert spaces are a particular example thereof.

===Infinite tensor products===
Two different definitions have historically been proposed for the tensor product of an arbitrary-sized collection <math display=inline>\{H_n\}_{n\in N}</math> of Hilbert spaces.  [[John von Neumann|Von Neumann]]'s traditional definition simply takes the "obvious" tensor product: to compute <math display=inline>\bigotimes_n{H_n}</math>, first collect all simple tensors of the form <math display=inline>\bigotimes_{n\in N}{e_n}</math> such that <math display=inline>\prod_{n\in N}{\|e_n\|}<\infty</math>.  The latter describes a pre-inner product through the [[polarization identity]], so take the closed span of such simple tensors modulo that inner product's isotropy subspaces.  This definition is almost never separable, in part because, in [[quantum field theory|physical applications]], "most" of the space describes impossible states.  Modern authors typically use instead a definition due to Guichardet: to compute <math display=inline>\bigotimes_n{H_n}</math>, first select a unit vector <math display=inline>v_n\in H_n</math> in each Hilbert space, and then collect all simple tensors of the form <math display=inline>\bigotimes_{n\in N}{e_n}</math>, in which only finitely-many <math display=inline>e_n</math> are not <math display=inline>v_n</math>.  Then take the <math>L^2</math> completion of these simple tensors.<ref><!--[[WP:SELFPUB]] src, but author is a recognized expert in the field-->[[Nik Weaver]] (8&nbsp;March&nbsp;2020).  [https://mathoverflow.net/revisions/354457/4 Answer] to [https://mathoverflow.net/posts/354418/revisions Result of continuum tensor product of Hilbert spaces].  ''[[MathOverflow]]''.  [[StackExchange]].</ref><ref name="BR">Bratteli, O. and Robinson, D: ''Operator Algebras and Quantum Statistical Mechanics v.1, 2nd ed.'', page 144. Springer-Verlag, 2002.</ref>

===Operator algebras===

Let <math>\mathfrak{A}_i</math> be the [[von Neumann algebra]] of bounded operators on <math>H_i</math> for <math>i=1,2.</math> Then the von Neumann tensor product of the von Neumann algebras is the strong completion of the set of all finite linear combinations of simple tensor products <math>A_1\otimes A_2</math> where <math>A_i \in \mathfrak{A}_i</math> for <math>i = 1, 2.</math> This is exactly equal to the von Neumann algebra of bounded operators of <math>H_1\otimes H_2.</math> Unlike for Hilbert spaces, one may take infinite tensor products of von Neumann algebras, and for that matter [[C*-algebra]]s of operators, without defining reference states.<ref name="BR"/> This is one advantage of the "algebraic" method in quantum statistical mechanics.

==Properties==

If <math>H_1</math> and <math>H_2</math> have [[Orthonormal basis|orthonormal bases]] <math>\left\{\phi_k\right\}</math> and <math>\left\{\psi_l\right\},</math> respectively, then <math>\left\{\phi_k \otimes \psi_l\right\}</math> is an orthonormal basis for <math>H_1\otimes H_2.</math> In particular, the Hilbert dimension of the tensor product is the product (as [[cardinal number]]s) of the Hilbert dimensions.

==Examples and applications==

The following examples show how tensor products arise naturally.

Given two [[measure space]]s <math>X</math> and <math>Y</math>, with measures <math>\mu</math> and <math>\nu</math> respectively, one may look at <math>L^2(X \times Y),</math> the space of functions on <math>X \times Y</math> that are square integrable with respect to the product measure <math>\mu\times\nu.</math> If <math>f</math> is a square integrable function on <math>X,</math> and <math>g</math> is a square integrable function on <math>Y,</math> then we can define a function <math>h</math> on <math>X\times Y</math> by <math>h(x, y) = f(x) g(y).</math> The definition of the product measure ensures that all functions of this form are square integrable, so this defines a [[bilinear map]]ping <math>L^2(X) \times L^2(Y) \to L^2(X\times Y).</math> [[Linear combination]]s of functions of the form <math>f(x) g(y)</math> are also in <math>L^2(X \times Y).</math> It turns out that the set of linear combinations is in fact dense in <math>L^2(X \times Y),</math> if <math>L^2(X)</math> and <math>L^2(Y)</math> are separable.<ref>{{cite book|at=p.&nbsp;100, ex.&nbsp;3.|title=Elements of the theory of functions and functional analysis|volume=2: Measure, the Lebesgue integral, and Hilbert space|first1=A.&nbsp;N.|last1=Kolmogorov|author-link1=Andrei Kolmogorov|first2=S.&nbsp;V.|last2=Fomin|author-link2=Sergei Fomin|orig-date=1960|translator-first1=Hyman|translator-last1=Kamel|translator-first2=Horace|translator-last2=Komm|publisher=[[Graylock Press|Graylock]]|location=[[Albany, NY]]|year=1961|lccn=57-4134}}</ref> This shows that <math>L^2(X) \otimes L^2(Y)</math> is [[isomorphic]] to <math>L^2(X \times Y),</math> and it also explains why we need to take the completion in the construction of the Hilbert space tensor product.

Similarly, we can show that <math>L^2(X;H)</math>, denoting the space of square integrable functions <math>X \to H,</math> is isomorphic to <math>L^2(X) \otimes H</math> if this space is separable. The isomorphism maps <math>f(x) \otimes \phi \in L^2(X)\otimes H</math> to <math>f(x) \phi \in L^2(X;H)</math> We can combine this with the previous example and conclude that <math>L^2(X) \otimes L^2(Y)</math> and <math>L^2(X \times Y)</math> are both isomorphic to <math>L^2\left(X; L^2(Y)\right).</math>

Tensor products of Hilbert spaces arise often in [[quantum mechanics]]. If some particle is described by the Hilbert space <math>H_1,</math> and another particle is described by <math>H_2,</math> then the system consisting of both particles is described by the tensor product of <math>H_1</math> and <math>H_2.</math> For example, the state space of a [[quantum harmonic oscillator]] is <math>L^2(\R),</math> so the state space of two oscillators is <math>L^2(\R) \otimes L^2(\R),</math> which is isomorphic to <math>L^2\left(\R^2\right).</math> Therefore, the two-particle system is described by wave functions of the form <math>\psi\left(x_1, x_2\right).</math> A more intricate example is provided by the [[Fock space]]s, which describe a variable number of particles.

==References==

{{reflist}}

==Bibliography==

* {{Cite book | last1=Kadison | first1=Richard V. |author1link = Richard Kadison| last2=Ringrose | first2=John R. |author2link = John Robert Ringrose| title=Fundamentals of the theory of operator algebras. Vol. I | publisher=[[American Mathematical Society]] | location=Providence, R.I. | series=[[Graduate Studies in Mathematics]] | isbn=978-0-8218-0819-1 | mr= 1468229 | year=1997 | volume=15}}.
* {{Cite book | last1=Weidmann | first1=Joachim | title=Linear operators in Hilbert spaces | publisher=[[Springer-Verlag]] | location=Berlin, New York | series=[[Graduate Texts in Mathematics]] | isbn=978-0-387-90427-6 |mr=566954 | year=1980 | volume=68}}.

{{Hilbert space}}
{{Topological tensor products and nuclear spaces}}
{{Functional analysis}}

[[Category:Functional analysis]]
[[Category:Hilbert spaces]]
[[Category:Linear algebra]]
[[Category:Operator theory]]
[[Category:Topological tensor products]]