Optimal eavesdropping in quantum cryptography. II. A quantum circuit

It is shown that the optimum strategy of the eavesdropper, as described in the preceding paper, can be expressed in terms of a quantum circuit in a way which makes it obvious why certain parameters take on particular values, and why obtaining information in one basis gives rise to noise in the conjugate basis.
7 pages, 1 figure, Latex, the second part of quant-ph/9701039