Physics – Quantum Physics
Scientific paper
2001-11-15
Journal of Mathematical Physics, Vol 43(9), September 2002, pp. 4376 - 4381
Physics
Quantum Physics
LATEX, 8 pages
Scientific paper
To implement any quantum operation (a.k.a. ``superoperator'' or ``CP map'') on a d-dimensional quantum system, it is enough to apply a suitable overall unitary transformation to the system and a d^2-dimensional environment which is initialized in a fixed pure state. It has been suggested that a d-dimensional environment might be enough if we could initialize the environment in a mixed state of our choosing. In this note we show with elementary means that certain explicit quantum operations cannot be realized in this way. Our counterexamples map some pure states to pure states, giving strong and easily manageable conditions on the overall unitary transformation. Everything works in the more general setting of quantum operations from d-dimensional to d'-dimensional spaces, so we place our counterexamples within this more general framework.
Rieffel Eleanor
Zalka Christof
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