 
Summary: Molecular Scale Heat Engines and Scalable Quantum Computation
Leonard J. Schulman \Lambda Umesh V. Vazirani y
Abstract
We describe a quantum mechanical heat engine. Like its
classical counterpart introduced by Carnot, this engine car
ries 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 thermody
namic terms of a simple step introduced by von Neumann
to extract fair coin flips from sequences of biased coin flips.
Some of the experimental setups proposed for imple
mentation of quantum computers, begin with the quantum
bits of the computer initially in a mixed state. Each qubit
is ffl polarized  in the state j0i with probability 1+ffl
2 , and in
the state j1i with probability 1\Gammaffl
2 , 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 nearoptimal m = n[ 1+ffl
