$ \newcommand{\ket}[1]{\left|#1\right\rangle} \newcommand{\bra}[1]{\left\langle#1\right|} % $Suppose that we have a density operator $\rho=\rho_1 \otimes\rho_2\otimes \rho_3$ where $\rho_1 ,\rho_3 $ are pure states and $\rho_2$ is a mixed state. If we perform a measurement on the whole system $\rho$ the state will remain mixed state. I am wondering how the probability outcomes of the state after the measurement can be computed.

For example suppose that the $\rho_1,\rho_3$ is a general state of a two qubit system where $$\rho=\ket{\psi}\bra{\psi}\,\,\text{with} \,\,\ket{\psi}=\cos(\theta/2)\ket{0}+e^{i\phi}\sin(\theta/2)\ket{1}$$ and the $\rho_2=(1-p)\ket{\psi}\bra{\psi}+p I$ where I is the identity operator. How can I compute the probabilities of the different outcomes of the density matrix due to measurement?

I am considering a projective measurement P where Pa =$\ket{+}\bra{+}\otimes I\otimes I$ or Pb= $I\otimes \ket{+}\bra{+}\otimes I$.

Is there a straightforward way to calculate the probability of the outcome in a statistical ensemble after the X basis measurement? Some posts claim that the answer uses the classical probability theory (Mixed state after measurement ).

  • $\begingroup$ There isn't enough information here (specifically, regarding what measurement you want to describe) for this question to be really answerable. $\endgroup$ Aug 30, 2021 at 10:23
  • $\begingroup$ Do you think it is still sloppy or undefined? If so I will erase it in order to write a specific example. $\endgroup$ Aug 30, 2021 at 10:34
  • $\begingroup$ Greetings and welcome! Note that the culture in our community is that it's normal and expected for a question to be edited and improved after it's posted. However it's generally frowned upon for a user to delete a question and soon afterwards ask a very similar question — we have problems with people using that technique to avoid our question-review system. In its current form (v5) I don't see any reason for you to hide this question, but it's also fine if you improve it and use the "undelete" button later on. $\endgroup$
    – rob
    Aug 30, 2021 at 12:38
  • $\begingroup$ Thanks a lot for the answer. I didn't know if the question is fair enought to be answered. That's why I deleted it. If anyone can help with any literature or example I would be glad. Also if anybody wants more clarification about the problem please comment. $\endgroup$ Aug 30, 2021 at 12:59


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