ИСТИНА

It is widely known that one cannot discriminate between the set of non-orthogonal quantum states, this fact is essential for quantum cryptography. However, unambiguous state discrimination (USD) is possible, which whether perfectly discriminates between the states, or yields inconclusive result. This operation can be used for eavesdropping strategy in quantum cryptography which is called USD attack. This attack becomes possible at some critical loss level in the channel between the legitimate users. In this work, we consider a generalization for unambiguous state discrimination, which makes the states more distinguishable in case of success, but success probability can be higher than the one for USD. With this transformation, we design an attack strategy for quantum cryptography based on symmetric coherent states.