Figure 1 : Adaptive quantum protocol.

From: Fundamental limits of repeaterless quantum communications

Figure 1

The first step is the preparation of the initial separable state of a and b by some adaptive LOCC Λ0. After the preparation of the local registers, there is the first transmission: Alice picks a system from her local register a1a, so that the register is updated as aaa1; system a1 is sent through the channel , with Bob getting the output b1; Bob includes the output in his local register, which is updated as b1bb; finally, Alice and Bob apply another adaptive LOCC Λ1 to their registers a,b. In the second transmission, Alice picks and sends another system a2a through channel with output b2 for Bob. The parties apply a further adaptive LOCC Λ2 to their registers and so on. This procedure is repeated n times, with output state for the Alice’s and Bob’s local registers.