Two-way communication is possible with a single quantum particle

P. M. Intallura

Yakir Aharonov, Lev Vaidman A

The possibility to communicate between spatially separated regions, without even a single photon passing between the two parties, is an amazing quantum phenomenon. The possibility of transmitting one value of a bit in such a way, the interaction-free measurement, has been known for quarter of a century. The protocols of full communication, including transmitting unknown quantum states, were proposed only a few years ago, but it was shown that in all these protocols the particle was leaving a weak trace in the transmission channel, the trace being larger than the trace left by a single particle passing through the channel. This made the claim of counterfactuality of these protocols at best controversial. However, a simple modification of these recent protocols eliminates the trace in the transmission channel, making all these protocols counterfactual.

Mark Timothy Sheldrick

It is possible to use two particles (A and B) with entangled properties to transmit information at faster than light speeds. This can be done, not by trying to modulate the results of how particle A is measured, but by modulating whether particle A is measured or not measured. The effect of this modulating method is to place particle B in either a single value (but arbitrary) state or leave it in a superposition of states. It is then possible for the reciever of particle B to distinguish between these two states by the use of an appropriately designed interferometer. Such a device can be designed to produce an interference pattern only when particle B is in a superposition of values and a straight simple image when particle B has a single (arbitrary) defined value. Under the Copenhagen interpretation and existing experimental results, this method will produce a way of signalling fater than light. It will require the use of multiple entangled pairs of particles to effectively transmit a single bit of information. Under the Multi-Worlds Interpretation, it will not be so easy to communicate faster than light by this mechanism, as detector B will be detecting photon B as a superposition of values in both cases.

Anton Zeilinger

Harald Weinfurter

Flavio Del Santo, Borivoje Dakić

In this Letter we show that communication when restricted to a single information carrier (i.e., single particle) and finite speed of propagation is fundamentally limited for classical systems. On the other hand, quantum systems can surpass this limitation. We show that communication bounded to the exchange of a single quantum particle (in superposition of different spatial locations) can result in "two-way signaling," which is impossible in classical physics. We quantify the discrepancy between classical and quantum scenarios by the probability of winning a game played by distant players. We generalize our result to an arbitrary number of parties and we show that the probability of success is asymptotically decreasing to zero as the number of parties grows, for all classical strategies. In contrast, quantum strategy allows players to win the game with certainty.