  We introduce the study of quantum protocols that probabilistically simulate
quantum channels from a sender in the future to a receiver in the past.
  The maximum probability of simulation is determined by causality and depends
on the amount and type (classical or quantum) of information that the channel
can transmit. We illustrate this dependence in several examples, including
ideal classical and quantum channels, measure-and-prepare channels, partial
trace channels, and universal cloning channels. For the simulation of partial
trace channels, we consider generalized teleportation protocols that take N
input copies of a pure state in the future and produce M < N output copies of
the same state in the past. In this case, we show that the maximum probability
of successful teleportation increases with the number of input copies, a
feature that was impossible in classical physics. In the limit of
asymptotically large N, the probability converges to the probability of
simulation for an ideal classical channel.
  Similar results are found for universal cloning channels from N copies to M >
N approximate copies, exploiting a time-reversal duality between universal
cloning and partial trace.
