Physical Resources, Entanglement, and the Power of Quantum Computation Carlton M. Caves, University of New Mexico

Requiring that a quantum computer not need an exponentially growing amount of any physical resource places stringent constraints on the systems that can act as quantum computers.  In particular, a quantum computer must be made up of subsystems, usually qubits, thus ruling out implementing a quantum computer in a single atom or by using the interference of classical waves.  Furthermore, if a quantum computer made up of subsystems performs some computation exponentially faster than any classical computer, there must be global entanglement among all the subsystems at some point during the computation.  Having thus led up to the conclusion that quantum entanglement is the essential ingredient for quantum computation, I will discuss why this conclusion isn't ironclad.