The Fock space is an algebraic construction used in quantum mechanics to construct the quantum states space of a variable or unknown number of identical particles from a single particle Hilbert space H. It is named after V. A. Fock who first introduced it in his 1932 paper "Konfigurationsraum und zweite Quantelung" ("Configuration space and second quantization").[1][2]
Informally, a Fock space is the sum of a set of Hilbert spaces representing zero particle states, one particle states, two particle states, and so on. If the identical particles are bosons, the n-particle states are vectors in a symmetrized tensor product of n single-particle Hilbert spaces H. If the identical particles are fermions, the n-particle states are vectors in an antisymmetrized tensor product of n single-particle Hilbert spaces H (see symmetric algebra and exterior algebra respectively). A general state in Fock space is a linear combination of n-particle states, one for each n.
Technically, the Fock space is (the Hilbert space completion of) the direct sum of the symmetric or antisymmetric tensors in the tensor powers of a single-particle Hilbert space H,
![{\displaystyle F_{\nu }(H)={\overline {\bigoplus _{n=0}^{\infty }S_{\nu }H^{\otimes n}}}~.}](//wikimedia.org/api/rest_v1/media/math/render/svg/01dc19624d083c13e07aad8b1ec5add08942988b)
Here
is the operator which symmetrizes or antisymmetrizes a tensor, depending on whether the Hilbert space describes particles obeying bosonic
or fermionic
statistics, and the overline represents the completion of the space. The bosonic (resp. fermionic) Fock space can alternatively be constructed as (the Hilbert space completion of) the symmetric tensors
(resp. alternating tensors
). For every basis for H there is a natural basis of the Fock space, the Fock states.
The Fock space is the (Hilbert) direct sum of tensor products of copies of a single-particle Hilbert space ![{\displaystyle H}](//wikimedia.org/api/rest_v1/media/math/render/svg/75a9edddcca2f782014371f75dca39d7e13a9c1b)
![{\displaystyle F_{\nu }(H)=\bigoplus _{n=0}^{\infty }S_{\nu }H^{\otimes n}=\mathbb {C} \oplus H\oplus \left(S_{\nu }\left(H\otimes H\right)\right)\oplus \left(S_{\nu }\left(H\otimes H\otimes H\right)\right)\oplus \cdots }](//wikimedia.org/api/rest_v1/media/math/render/svg/4cb6ae3ba27d9cff8a391ca5de76de9b36a27549)
Here
, the complex scalars, consists of the states corresponding to no particles,
the states of one particle,
the states of two identical particles etc.
A general state in
is given by
where
is a vector of length 1 called the vacuum state and
is a complex coefficient,
is a state in the single particle Hilbert space and
is a complex coefficient,
, and
is a complex coefficient, etc.
The convergence of this infinite sum is important if
is to be a Hilbert space. Technically we require
to be the Hilbert space completion of the algebraic direct sum. It consists of all infinite tuples
such that the norm, defined by the inner product is finite
where the
particle norm is defined by
i.e., the restriction of the norm on the tensor product ![{\displaystyle H^{\otimes n}}](//wikimedia.org/api/rest_v1/media/math/render/svg/eb4edc8f2f9b1f139d89d626f3b5c06cad73053e)
For two general states
and
the inner product on
is then defined as
where we use the inner products on each of the
-particle Hilbert spaces. Note that, in particular the
particle subspaces are orthogonal for different
.
A product state of the Fock space is a state of the form
![{\displaystyle |\Psi \rangle _{\nu }=|\phi _{1},\phi _{2},\cdots ,\phi _{n}\rangle _{\nu }=|\phi _{1}\rangle \otimes |\phi _{2}\rangle \otimes \cdots \otimes |\phi _{n}\rangle }](//wikimedia.org/api/rest_v1/media/math/render/svg/c22fa364b63fd273f692c1a5fb095499e20bcfdd)
which describes a collection of
particles, one of which has quantum state
, another
and so on up to the
th particle, where each
is any state from the single particle Hilbert space
. Here juxtaposition (writing the single particle kets side by side, without the
) is symmetric (resp. antisymmetric) multiplication in the symmetric (antisymmetric) tensor algebra. The general state in a Fock space is a linear combination of product states. A state that cannot be written as a convex sum of product states is called an entangled state.
When we speak of one particle in state
, we must bear in mind that in quantum mechanics identical particles are indistinguishable. In the same Fock space, all particles are identical. (To describe many species of particles, we take the tensor product of as many different Fock spaces as there are species of particles under consideration). It is one of the most powerful features of this formalism that states are implicitly properly symmetrized. For instance, if the above state
is fermionic, it will be 0 if two (or more) of the
are equal because the antisymmetric (exterior) product
. This is a mathematical formulation of the Pauli exclusion principle that no two (or more) fermions can be in the same quantum state. In fact, whenever the terms in a formal product are linearly dependent; the product will be zero for antisymmetric tensors. Also, the product of orthonormal states is properly orthonormal by construction (although possibly 0 in the Fermi case when two states are equal).
A useful and convenient basis for a Fock space is the occupancy number basis. Given a basis
of
, we can denote the state with
particles in state
,
particles in state
, ...,
particles in state
, and no particles in the remaining states, by defining
![{\displaystyle |n_{0},n_{1},\ldots ,n_{k}\rangle _{\nu }=|\psi _{0}\rangle ^{n_{0}}|\psi _{1}\rangle ^{n_{1}}\cdots |\psi _{k}\rangle ^{n_{k}},}](//wikimedia.org/api/rest_v1/media/math/render/svg/1850c6c09c276095a9692cd944c253357f549d36)
where each
takes the value 0 or 1 for fermionic particles and 0, 1, 2, ... for bosonic particles. Note that trailing zeroes may be dropped without changing the state. Such a state is called a Fock state. When the
are understood as the steady states of a free field, the Fock states describe an assembly of non-interacting particles in definite numbers. The most general Fock state is a linear superposition of pure states.
Two operators of great importance are the creation and annihilation operators, which upon acting on a Fock state add or respectively remove a particle in the ascribed quantum state. They are denoted
for creation and
for annihilation respectively. To create ("add") a particle, the quantum state
is symmetric or exterior- multiplied with
; and respectively to annihilate ("remove") a particle, an (even or odd) interior product is taken with
, which is the adjoint of
. It is often convenient to work with states of the basis of
so that these operators remove and add exactly one particle in the given basis state. These operators also serve as generators for more general operators acting on the Fock space, for instance the number operator giving the number of particles in a specific state
is
.
Often the one particle space
is given as
, the space of square-integrable functions on a space
with measure
(strictly speaking, the equivalence classes of square integrable functions where functions are equivalent if they differ on a set of measure zero). The typical example is the free particle with
the space of square integrable functions on three-dimensional space. The Fock spaces then have a natural interpretation as symmetric or anti-symmetric square integrable functions as follows.
Let
and
,
,
, etc.
Consider the space of tuples of points which is the disjoint union
![{\displaystyle X^{*}=X^{0}\bigsqcup X^{1}\bigsqcup X^{2}\bigsqcup X^{3}\bigsqcup \cdots .}](//wikimedia.org/api/rest_v1/media/math/render/svg/d8862c76b522b653c12c3220380cd022db5d0d06)
It has a natural measure
such that
and the restriction of
to
is
.
The even Fock space
can then be identified with the space of symmetric functions in
whereas the odd Fock space
can be identified with the space of anti-symmetric functions. The identification follows directly from the isometric mapping
.
Given wave functions
, the Slater determinant
is an antisymmetric function on
. It can thus be naturally interpreted as an element of the
-particle sector of the odd Fock space. The normalization is chosen such that
if the functions
are orthonormal. There is a similar "Slater permanent" with the determinant replaced with the permanent which gives elements of
-sector of the even Fock space.
Define the Segal–Bargmann space
[3] of complex holomorphic functions square-integrable with respect to a Gaussian measure:
where
Then defining a space
as the nested union of the spaces
over the integers
, Segal[4] and Bargmann showed[5][6] that
is isomorphic to a bosonic Fock space. The monomial
corresponds to the Fock state
![{\displaystyle |n_{0},n_{1},\ldots ,n_{k}\rangle _{\nu }=|\psi _{0}\rangle ^{n_{0}}|\psi _{1}\rangle ^{n_{1}}\cdots |\psi _{k}\rangle ^{n_{k}}.}](//wikimedia.org/api/rest_v1/media/math/render/svg/4efbcca79caa3e42296d1ae72b73c369cfb69fe3)
M.C. Reed, B. Simon, "Methods of Modern Mathematical Physics, Volume II", Academic Press 1975. Page 328. Segal, I. E. (1963). "Mathematical problems of relativistic physics". Proceedings of the Summer Seminar, Boulder, Colorado, 1960, Vol. II. Chap. VI.