Expected Value Partial Trance Density Matrix

[VVS]

Hey I am currently studying Quantum Mechanics and I have difficulty grasping a concept.

I don't understand the following step in the derivation:

$\langle X_{A} \rangle=tr\left[\left(X_{A}\otimes I_{B}\right)\rho_{AB}\right]$
$=tr_{A}\left[X_{A} tr_{B}\left[\rho_{AB}\right]\right]$

Thanks

 

[SteamKing]

I believe 'tr' stands for 'trace', not 'trance'.

 

[VVS]

yes of course

 

[WWGD]

I believe 'tr' stands for 'trace', not 'trance'.

Note if you're computing it while listening to techno music :).