# How to prove $\hat{p}|x\rangle=i\hbar\frac{\partial}{\partial x}|x\rangle$,using $[\hat{x},\hat{p}]=i\hbar$? [duplicate]

How to prove $$\hat{p}|x\rangle=i\hbar\frac{\partial}{\partial x}|x\rangle,$$ using $$[\hat{x},\hat{p}]=i\hbar~?$$ The question seems to be uncomplete because for any $f(x)$ $$[\hat{x},\hat{p}+f(x)]=i\hbar.$$ But that's my homework. Can someone add some appropriate assumptions and prove the question?

Here is a seemingly right answer:

$$\hat{Q} = e^{-i \frac{u}{\hbar} \hat{p}},[\hat{x},\hat{Q}]=u \hat{Q}$$ $$\hat{x}\hat{Q}|x\rangle = (x+u)\hat{Q}|x\rangle$$ $$\hat{Q}|x\rangle = |x+u\rangle$$ $$\hat{T} = e^{i \frac{u}{\hbar} \hat{x}},[\hat{p},\hat{T}]=u \hat{T}$$ $$\hat{p}\hat{T}|p\rangle = (p+u)\hat{T}|p\rangle$$ $$\hat{T}|p\rangle = |p+u\rangle$$

Therefore

$$\langle x | p \rangle = e^{i\frac{px}{\hbar}} \langle 0 |_x |0 \rangle_p$$

$$\delta(p-p') = \langle p | p' \rangle = 2\pi \hbar |\langle 0 |_x |0 \rangle_p|^2 \delta(p-p')$$ $$\langle 0 |_x |0 \rangle_p = \sqrt{\frac{1}{2 \pi \hbar}}$$ $$\hat{p}|x\rangle = \int \hat{p} |p\rangle\langle p |x\rangle= \int \hat{p} |p\rangle e^{-i\frac{px}{\hbar}}\sqrt{\frac{1}{2 \pi \hbar}} = i\hbar\frac{\partial}{\partial x}|x\rangle$$ Which assumption is made here?

## marked as duplicate by BMS, John Rennie, Qmechanic♦Oct 14 '14 at 9:05

• Related: physics.stackexchange.com/q/41880/2451 , physics.stackexchange.com/q/45248/2451 and links therein. – Qmechanic Oct 14 '14 at 6:46
• Comment to the question (v3): OP writes: $\hat{Q}|x\rangle = |x+u\rangle$. This is not necessarily true: There could be a phase factor in that equation. – Qmechanic Oct 16 '14 at 22:50
• I personally prefer the fourier transform method as it is used all the time in QFT if you progress on with your studies although your notation is a little dodgy in your integral – Alexander McFarlane May 12 '16 at 22:18