At the end of a computation we need to measure the output in order to read
out the result of the computation. This leads to obtaining classical information
(in bits) out of the quantum system. One sets up an experiment that measures
an appropriate physical quantity to give one of its eigenvalues as the result
(recall Section 3.3 and the nature of measurements in quantum mechanics). We
denote this process generically by a measurement gate . The double
line for the output state is to emphasize that it is a classical state. By default
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the measurement is assumed to be in the computational basis. The state just
prior to measurement encodes the probabilities of its collapsing to |0i or |1i
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