- #1
jfy4
- 649
- 3
Homework Statement
Let [itex]U[/itex] and [itex]V[/itex] be the complementary unitary operators for a system of [itex]N[/itex] eingenstates as discussed in lecture. Recall that they both have eigenvalues [itex]x_n=e^{2\pi in/N}[/itex] where [itex]n[/itex] is an integer satisfying [itex]0\leq n\leq N[/itex]. The operators have forms
[tex]
U=\sum_{n}|n_u\rangle\langle n_u |e^{2\pi in/N}\quad\quad V=\sum_{m}|m_v\rangle\langle m_v |e^{2\pi i m/N}
[/tex]
These operators can be expressed as exponentials of complementary self-adjoint operators [itex]A[/itex] and [itex]B[/itex]:
[tex]
U=e^{i2\pi A}\quad\quad V=e^{i2\pi B}
[/tex]
where the operators [itex]A[/itex] and [itex]B[/itex] are
[tex]
A=\frac{n}{N}\sum_{n}|n_u\rangle\langle n_u| \quad\quad B=\frac{m}{N}\sum_{m}|m_v\rangle\langle m_v |
[/tex]
Calculate the commutator [itex][A,B][/itex].
Homework Equations
[tex]
\mathbb{I}=\sum |n\rangle\langle n|
[/tex]
for complementary observables
[tex]
\frac{1}{\sqrt{N}}e^{i2\pi mn/N}=\sum_{n}\sum_{m}\langle n_u |m_v\rangle
[/tex]
The Attempt at a Solution
First I have tried to work out AB and BA separately then combine them. Here is AB
[tex]
\begin{align}
AB &= \sum_{n}|n_u\rangle\langle n_u |\frac{n}{N}\sum_{m}|m_v\rangle\langle m_v |\frac{m}{N} \\
&= \sum_{n}\sum_{m}|n_u\rangle\langle n_u|m_v\rangle\langle m_v| \frac{nm}{N^2} \\
&= \sum_{n}\sum_{m}|n_u\rangle \frac{1}{\sqrt{N}}e^{i 2\pi nm/N}\langle m_v |\frac{nm}{N^2}
\end{align}
[/tex]
for BA:
[tex]
\begin{align}
BA &=\sum_{m}\sum_{n}|m_v\rangle\langle m_v | n_u\rangle\langle n_u |\frac{nm}{N^2} \\
&= \sum_{m}\sum_{n}|m_v\rangle \frac{1}{\sqrt{N}}e^{-i2\pi nm/N}\langle n_u |\frac{nm}{N^2}
\end{align}
[/tex]
Then
[tex]
AB-BA=\sum_{m}\sum_{n}\frac{nm}{N^2}\frac{1}{\sqrt{N}}\left( e^{i2\pi nm/N}|n_u\rangle\langle m_v |-|m_v\rangle\langle n_u |e^{-i2\pi nm/N}\right)
[/tex]
I'm stuck here more or less. I can put either the u basis vectors into the v basis or visa versa, but I don't know if that is right. Where should I go from here?
Thanks,