Abstract
Classical identification via a noisy channel is a communication task in which the receiver is not required to reconstruct the full transmitted message, but only to decide whether it coincides with a message of interest. This relaxation allows the number of identifiable messages to grow doubly exponentially with the blocklength. For quantum channels, the resulting (doubly exponential) identification capacity C_ID can strictly exceed the ordinary (exponential) transmission capacity C. In this paper, we prove that the entanglement-assisted transmission capacity C_E is a strong converse bound for this task: C_ID≤ C_E. For sufficiently low-noise channels, this bound can also be achieved via the Hayden-Winter (quantum) identification + fingerprinting codes. This yields an exact characterization C_ID=C_E of identification capacity for such channels. However, for general channels, we prove that this upper bound can be strict. We exhibit an explicit family of transpose-depolarizing channels for which C_ID<C_E. As a consequence, we also obtain the first example of strict superadditivity of the identification capacity C_ID.