Molecular scale heat engines and scalable quantum computation
Leonard J. Schulman, Umesh V. Vazirani · 1999
We describe a quantum mechanical heat engine.Like its classical counterpart introduced by Carnot, this entine carries out a reversible process in which an input of energy to the system results in a separation of cold and hot regions.The method begins with a reinterpretation in thermodynamic terms of a simple step introduced by van Neumann to extract fair coin flips from sequences of biased coin flips.Some of the experimental set-ups proposed for implementation of quantum computers, begin with the quantum bits of the computer initially in a mixed state.Each qubit is L polarized -in the state IO) with probability 9, and in the state 11) with probability *, independently (or nearly so) of all other bits.The heat engine may be used to trans.form this initial collection of n qubits into a state in which a near-optimal m = n[ FIg(l +e) + %Ig(l -c) -o(l)] qubits are in the joint state IO"').These qubits can then be used as the register for a quantum computation.The heat engine is described at the level of an algorithm implementable in any quantum system capable of massive coherent states.A particular implementation is also described for a system of nuclear spins arranged in a chain.The temperature the cold qubits reach is inverse polynomial in n.