Abstract
We investigate covert communication over general memoryless classical-quantum channels with fixed finite-size input alphabets. We show that the square root law (SRL) governs covert communication in this setting when product of n input states is used: L_ SRL√n+o(√n) covert bits (but no more) can be reliably transmitted in n uses of classical-quantum channel, where L_ SRL>0 is a channel-dependent constant that we call covert capacity. We also show that ensuring covertness requires J_ SRL√n+o(√n) bits secret shared by the communicating parties prior to transmission, where J_ SRL≥0 is a channel-dependent constant. We assume a quantum-powerful adversary that can perform an arbitrary joint (entangling) measurement on all n channel uses. We determine the single-letter expressions for L_ SRL and J_ SRL, and establish conditions when J_ SRL=0 (i.e., no pre-shared secret is needed). Finally, we evaluate the scenarios where covert communication is not governed by the SRL.