# Verify that a field operator creates a particle

In example 4.1 of Lancaster and Blundell's "Quantum field theory for the gifted amateur", we verify that a field operator creates a particle as follow:

Let $$|\Psi\rangle=\hat{\psi}^{\dagger}(\boldsymbol{x})|0\rangle$$ and check that it does indeed correspond to a single particle at a particular location. Note that $$|\Psi\rangle=\hat{\psi}^{\dagger}(\boldsymbol{x})|0\rangle=\frac{1}{\sqrt{\mathcal{V}}} \sum_{\boldsymbol{p}} \mathrm{e}^{-\mathrm{i} \boldsymbol{p} \cdot \boldsymbol{x}} \hat{a}_{\boldsymbol{p}}^{\dagger}|0\rangle .$$ To calculate the total number of particles in this state we can use the number operator $$\hat{n}_{p}=\hat{a}_{p}^{\dagger} \hat{a}_{p}$$ (which measures the number of particles in state $$\boldsymbol{p}$$ ) and then sum over all momentum states. Consider $$\sum_{\boldsymbol{q}} \hat{n}_{\boldsymbol{q}}|\Psi\rangle=\frac{1}{\sqrt{\mathcal{V}}} \sum_{\boldsymbol{q} \boldsymbol{p}} \hat{a}_{\boldsymbol{q}}^{\dagger} \hat{a}_{\boldsymbol{q}} \hat{a}_{\boldsymbol{p}}^{\dagger}|0\rangle \mathrm{e}^{-\mathrm{i} \boldsymbol{p} \cdot \boldsymbol{x}}$$ and again using $$\left\langle 0\left|\hat{a}_{\boldsymbol{q}} \hat{a}_{\boldsymbol{p}}^{\dagger}\right| 0\right\rangle=\delta_{\boldsymbol{p q}}$$ we deduce that $$\sum_{\boldsymbol{q}} \hat{n}_{\boldsymbol{q}}|\Psi\rangle=|\Psi\rangle$$ ...

I can't figure out why using the identity leads to the last equation.

• Could you elaborate what exactly you don't understand? Is it the last equation? Dec 30, 2021 at 21:22
• @Jakob Yes! I don't understand how it follows from the equation $\left\langle 0\left|\hat{a}_{\boldsymbol{q}} \hat{a}_{\boldsymbol{p}}^{\dagger}\right| 0\right\rangle=\delta_{\boldsymbol{p q}}$ Dec 30, 2021 at 21:23
• Ah okay. Btw are you dealing with fermions or bosons? Dec 30, 2021 at 21:23
• The book doesn't specify but I assume the default is boson? Dec 30, 2021 at 21:24

At this point: $$\sum_{{q}} \hat{n}_{{q}}|\Psi\rangle = \frac{1}{\sqrt{\mathcal{V}}} \sum_{{q} {p}} \hat{a}_{{q}}^{\dagger} \hat{a}_{{q}} \hat{a}_{{p}}^{\dagger} |0\rangle \mathrm{e}^{-\mathrm{i} {p} \cdot {x}}\,,$$ I would put the operators on the right-hand side in normal-order by switching the order of the last two operators, i.e., $$\hat{a}_{{q}}^{\dagger} \hat{a}_{{q}} \hat{a}_{{p}}^{\dagger} = \hat{a}_{{q}}^{\dagger} \left( [\hat{a}_{{q}}, \hat{a}_{{p}}^{\dagger}]_{\mp} \pm \hat{a}_{{p}}^{\dagger}\hat{a}_{{q}} \right)\,$$ where $$[\cdot,\cdot]_+$$ is the anti-commutator (for fermions), and $$[\cdot,\cdot]_-$$ is the commutator (for bosons). In both cases, the (anti-)commutator is $$\delta_{pq}$$, and the annihilation operator kills the vacuum state $$\lvert 0 \rangle$$, leaving $$\sum_{{q}} \hat{n}_{{q}}|\Psi\rangle = \frac{1}{\sqrt{\mathcal{V}}} \sum_{{q} {p}} \hat{a}_{{q}}^{\dagger}\delta_{pq} |0\rangle \mathrm{e}^{-\mathrm{i} {p} \cdot {x}} = \frac{1}{\sqrt{\mathcal{V}}} \sum_{{p}} \hat{a}_{{q}}^{\dagger} |0\rangle \mathrm{e}^{-\mathrm{i} {p} \cdot {x}} =\lvert\Psi\rangle\,.$$ This, this vector is a eigenvector of the number operator with eigenvalue 1.
We're not really using that the expectation value of that operator is $$\delta_{pq}$$; really, we're using the (anti-)commutation relations and the action of the annihilation operator on the vacuum.
• Although of course your answer is correct, I think one could also use the resolution of the identity of the Fock space between $a^\dagger_q$ and $a_q$ and then make use of the 'hint' given in the text book, no? However, this seems much more complicated after all... I just wonder why they'd give this hint instead of referring to the (anti)-commutation relations?! Dec 30, 2021 at 21:43
• @Jakob Seems like a lot more work: you'd have to act with the operators on all of the number states, and you'll get more than just the expectation value in the vacuum state. It's straight-forward to show that all those other matrix elements ($\langle n | \hat{a}_q\hat{a}_p^{\dagger} | 0 \rangle$), so I suppose that way is fine. Seems like the (anti-)commutator way is more idiomatic of QFT, though. Dec 30, 2021 at 21:48