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We introduce the study of quantum protocols that probabilistically simulatequantum channels from a sender in the future to a receiver in the past.  The maximum probability of simulation is determined by causality and dependson the amount and type (classical or quantum) of information that the channelcan transmit. We illustrate this dependence in several examples, includingideal classical and quantum channels, measure-and-prepare channels, partialtrace channels, and universal cloning channels. For the simulation of partialtrace channels, we consider generalized teleportation protocols that take Ninput copies of a pure state in the future and produce M < N output copies ofthe same state in the past. In this case, we show that the maximum probabilityof successful teleportation increases with the number of input copies, afeature that was impossible in classical physics. In the limit ofasymptotically large N, the probability converges to the probability ofsimulation 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 universalcloning and partial trace.

Optimal Probabilistic Simulation of Quantum Channels from the Future to the Past