Minimum-error discrimination of quantum states: New bounds and   comparison
  The minimum-error probability of ambiguous discrimination for two quantum states is the well-known {\it Helstrom limit} presented in 1976. Since then, it has been thought of as an intractable problem to obtain the minimum-error probability for ambiguously discriminating arbitrary $m$ quantum states. In this paper, we obtain a new lower bound on the minimum-error probability for ambiguous discrimination and compare this bound with six other bounds in the literature. Moreover, we show that the bound between ambiguous and unambiguous discrimination does not extend to ensembles of more than two states. Specifically, the main technical contributions are described as follows: (1) We derive a new lower bound on the minimum-error probability for ambiguous discrimination among arbitrary $m$ mixed quantum states with given prior probabilities, and we present a necessary and sufficient condition to show that this lower bound is attainable. (2) We compare this new lower bound with six other bounds in the literature in detail, and, in some cases, this bound is optimal. (3) It is known that if $m=2$, the optimal inconclusive probability of unambiguous discrimination $Q_{U}$ and the minimum-error probability of ambiguous discrimination $Q_{E}$ between arbitrary given $m$ mixed quantum states have the relationship $Q_{U}\geq 2Q_{E}$. In this paper, we show that, however, if $m>2$, the relationship $Q_{U}\geq 2Q_{E}$ may not hold again in general, and there may be no supremum of $Q_{U}/Q_{E}$ for more than two states, which may also reflect an essential difference between discrimination for two-states and multi-states. (4) A number of examples are constructed. 
Daowen Qiu and Lvjun Li