Aharon Brodutch, Macquarie University
Abstract
Entanglement is a paradigmatic example of quantum correlations, a
presumed reason for the superior performance of quantum computation
and an obvious divider of states and processes into classical and
quantum. In the last decade all these notions were challenged.
Entanglement does not capture the totality of non-classical behavior.
Quantum discord (in its different versions) is a more general measure
of quantum correlations. It can be related to the advantage in some
tasks like the extraction of work from a Szilrad heat engine using
Maxwell's demons with various resources. The discord turns out to be
the difference between the work extracted from a given bipartite
system using a a global and a local strategy. Different strategies
relate to different definitions of discord, but all definitions agree
on zero, so "classical" systems are universal to all heat engines.
One way of identifying a task as quantum or classical is by examining
the quantum resources required to implement it. This can be done by
examining the entanglement required for a LOCC implementation of the
task. Creation (or non-creation) of entanglement as a result of its
implementation is not enough to identify these resources. An example
is a bi-local implementation of an entangling quantum gate (C-NOT) by
LOCC with unentagled input and output states. This lack of
entanglement does not guarantee the LOCC implementability. A method to
determine if entanglement resources are required is to track the
change in quantum discord during the process.
A. Brodutch and K. Modi, arXiv:1108.3649
A. Brodutch and D. R. Terno, Phys. Rev. A 83, 010301 (2011); arXiv:1009.2571
A. Brodutch and D. R. Terno, Phys. Rev. A 81, 062103 (2010); arXiv:1002.4913