Quantum Subdivision Capacities and Continuous-Time Quantum Coding

Quantum memories can be regarded as quantum channels that transmit information through time without moving it through space. Aiming at a reliable storage of information, we may thus not only encode at the beginning and decode at the end, but also intervene during the transmission-a possibility not captured by the ordinary capacities in quantum Shannon theory. In this paper, we introduce capacities that take this possibility into account and study them, in particular, for the transmission of quantum information via dynamical semigroups of Lindblad form. When the evolution is subdivided and supplemented by additional continuous semigroups acting on arbitrary block sizes, we show that the capacity of the ideal channel can be obtained in all cases. If the supplementary evolution is reversible, however, this is no longer the case. Upper and lower bounds for this scenario are proven. Finally, we provide a continuous coding scheme and simple examples showing that adding a purely dissipative term to a Liouvillian can sometimes increase the quantum capacity.