  We present truly ultimate limits on covert quantum communication by
exploiting quantum-mechanical properties of the Minkowski vacuum in the quantum
field theory framework. Our main results are the following: We show how two
parties equipped with Unruh-DeWitt detectors can covertly communicate at large
distances without the need of hiding in a thermal background or relying on
various technological tricks. We reinstate the information-theoretic security
standards for reliability of asymptotic quantum communication and show that the
rate of covert communication is strictly positive. Therefore, contrary to the
previous conclusions, covert and reliable quantum communication is possible.
