Ask here about the “Trace It and Forget It” Codebook topic from the “Noisy Quantum Theory” module.

Hey @CatalinaAlbornoz,

The coding part of this is not that difficult, but the theory just went over my head!!!

Are there any other sources that could help me understand it??

Thank you again for creating this amazing platform

Also @CatalinaAlbornoz I’m confused with N.3,3 where it says to create a state and measure the expected values of the parity operators. I don’t know what state to create and how it can change the expected val of parity operators.

Please help!!

Hey @CatalinaAlbornoz,

I understood the coding problem. I just had to create state using the “QubitDensityMatrix” and then just simply return the expval of the parity_operator.

But the theory part is still a bit confusing.

Any help would be appreciated!!

Hi @Krishna_Bhatia ,

I’ll forward the question to a colleague who can follow up with some links or something that can help you with the theory. We’ll get back to you on this next week!

The lesson that this exercise wants to teach is the following. You can have the whole two-qubit system in two different **two-qubit** states, say \rho_1 and \rho_2. It could happen that the reduced states on either individual (one-qubit system) are the same. That is

\text{Tr}_{A}(\rho_1) = \text{Tr}_{A}(\rho_2)

and

\text{Tr}_{B}(\rho_1) = \text{Tr}_{B}(\rho_2).

This exercise gives an example where this happens. Now the question is, is there a way to distinguish \rho_1 from \rho_2? Not if you only do measurements on one qubit!

But if you perform measurements on the two-qubit system, then you can distinguish the two states. The problem just tells you to implement some measurement to verify that the measurement outcomes would indeed be different.

Hope this helps!

Alvaro