quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...

, and especially quantum information processing, the entropy exchange of a

quantum operation In quantum mechanics, a quantum operation (also known as quantum dynamical map or quantum process) is a mathematical formalism used to describe a broad class of transformations that a quantum mechanical system can undergo. This was first discusse ...

\phi \,

acting on the density matrix

\rho_Q \,

of a system

Q \,

is defined as :

= S(\rho_')

where

S(\rho_') \,

is the von Neumann entropy of the system

Q \,

and a fictitious purifying auxiliary system

R \,

after they are operated on by

\phi \,

. Here, :

\rho_ = , QR\rangle\langle QR,  \quad,

\mathrm_R

rho_ Rho (uppercase Ρ, lowercase ρ or ; el, ρο or el, ρω, label=none) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician letter res . Its uppercase form uses the sam ...

= \rho_Q \quad, and :

\rho_' = \phi

\quad, where in the above equation

\phi

acts on

Q

leaving

R

unchanged.

References

* Quantum information science {{Quantum-stub