Can one be thought to be a more powerful resource than the other? It seems to me like you could use QSDC to achieve the same thing as QKD (sharing of a secret key) without the use of a classical channel. What exactly is the trade-off between the two?
Finally, are there established bounds on the capacities of various quantum channels for QSDC, and if so, how do they relate (do they differ?) from that of the secret key capacity? It seems to me in many cases there could be a reduction between the two, but I haven’t seen any literature on the capacities of channels for QSDC.