0

Let $\psi$ and $\phi$ be two uniformly random pure state $\psi, \phi \sim\mathbb{C}^d$. The the following equality holds \begin{align} \mathbb{E}_{\psi, \phi \sim \mathbb{C}^d} {\rm Tr}[\vert \phi \rangle \phi \vert \psi \rangle \langle \psi \vert] = \mathbb{E}_{\psi, \phi \sim \mathbb{C}^d} \vert \langle \phi \vert\psi \rangle\vert^2 = \frac{1}{d} \tag{1}. \end{align} How to prove Eq. (1)?

glS
  • 24,708
  • 5
  • 34
  • 108
Michael.Andy
  • 559
  • 2
  • 7
  • 3
    possible duplicate of one of https://quantumcomputing.stackexchange.com/q/28615/55, https://quantumcomputing.stackexchange.com/q/25883/55, or https://quantumcomputing.stackexchange.com/q/28707/55 (observing that by symmetry the question is equivalent to fixing one of the states and only averaging wrt the other one) – glS Oct 09 '23 at 12:43
  • Thank you gIS. The Eq. (1) can be obtained using the properties of Haar measure from the above link. – Michael.Andy Oct 10 '23 at 07:22

0 Answers0