Partial trace (${\mathrm{Tr}}_B$)

The concept of the partial trace arises in the context of quantum mechanics and quantum information theory, particularly when dealing with composite quantum systems. It is a mathematical operation used to obtain the reduced density matrix of a subsystem from the density matrix of a larger composite system. Let's break it down further: ### Quantum States and Density Matrices In quantum mechanics, a system can be described by a density matrix, which encodes the statistical state of the system.