Wiesner's quantum money scheme is based on the principle that coins
encoded with quantum states cannot be cloned by a counterfeiter, but
can be verified easily by a central bank. The scheme has a number of
variants that describe how the bank verifies the coins. We show how it
is possible to use weak measurements and exploit a version the bank's
verification procedure to learn the quantum state encoded in the coin
without the bank ever finding out. The attack works for both Wiesner's
original scheme and for generalizations of the scheme and remains
efficient as long as the encoded states have a product structure. To
counter the attack, the bank would need to replace the quantum state
each time the coins are sent for verification.
Based on joint work with Daniel Nagaj, Or Sattath and Dominique Unruh http://arxiv.org/abs/1404.1507 .