# Scalar product between Fock states

Suppose to have a chain (of size $L$) with bosons, and $\hat{a}_i^\dagger$,$\hat{a}_i$ are the associated creation and annihilation operators at site $i$. A Fock state can be written as: $$| n_1 \dots n_L \rangle = \prod_{i} \frac{1}{\sqrt{n_i!}} \left( \hat{a}_i^\dagger \right)^{n_i} |\rangle$$ where $|\rangle$ is the empty state. Now we define a new set of bosons: \begin{align} \hat{b}_{k} &= \frac{1}{\sqrt{L}} \sum_{j} e^{-ikj} \hat{a}_j & \hat{b}_{k}^\dagger &= \frac{1}{\sqrt{L}} \sum_{j} e^{ikj} \hat{a}_j^\dagger \end{align} where $k$ are such that some boundary condition is fulfilled. Now a Fock state can be written as: $$| \dots \tilde{n}_k \dots \rangle = \prod_{k} \frac{1}{\sqrt{\tilde{n}_k!}} \left( \hat{b}_k^\dagger \right)^{\tilde{n}_k} |\rangle$$ The question is: is there a simple formula to express or compute the scalar product $\langle \dots \tilde{n}_k \dots | n_1 \dots n_L \rangle$?

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Do you mean the scalar product $<original | new>$ or $<new | new>$? Introduction of some different notation for the new bosons would help, eg. $|n_1' \ldots n_k' \ldots >$. –  au700 Nov 30 '12 at 10:30
You are right, I edited the question and changed the notation. Anyway, the question is about Fock states in different bases otherwise the answer would be trivial. And of course the two states have the same number of bosons: $\sum_k \tilde{n}_k = \sum_j n_j$. –  Hari Nov 30 '12 at 13:32
You can easily make $< new | original > = \sum c_{\ldots} < original | original >$ by substituting the definition of new creation / anihilation operators in $< new |$. –  au700 Nov 30 '12 at 13:50
If I use your recipe I get: \begin{align} \langle \dots \tilde{n}_k\dots |n_1 \dots n_L \rangle & = \langle | \prod_k \left( \hat{b}_k \right)^{\tilde{n}_k } |n_1 \dots n_L \rangle \\ & = \langle | \prod_k \left( \frac{1}{\sqrt{L}} \sum_j e^{-ikj} \hat{a}_j \right)^{\tilde{n}_k } |n_1 \dots n_L \rangle \end{align} even if now it is quite straightforward which are the terms that survive (the ones that annihilate $|n_1 \dots n_L \rangle$ ) I'm not able to find a "clean" and useful equation ... :( –  Hari Nov 30 '12 at 14:15

Define the operators $\hat a(f)=\sum f_j\hat a_j$ and $|f_1,...,f_n\rangle:=\hat a(f_1)...\hat a(f_n)|vac\rangle$. Then $\langle g_1,...,g_m|f_1,...,f_n\rangle$ vanishes for $m\ne n$ and is a sum of the products $\langle g_1|f_{j_1}\rangle...\langle g_n|f_{j_n}\rangle$ for all possible permutations $(j_1,...,j_n)$ of $1,...,n$. Note that the $\langle g|f\rangle$ are easy to compute.
I realise the question has already been answered, but just a suggestion to the topic starter: finite sums of the form $(\sum_i x_i)^N$ are expanded using objects called multinomial coefficients. They should make your life a little easier.