How much information can be transmitted over a noisy channel? Noise plagues nearly every channel, and thus error-correction must be used to maximize the transmission rate; by adding more and more channels the error can be driven to zero. However, some channels are so noisy that, like a blocked drainpipe, nothing useful can get through. Extending the analogy, it would seem crazy to think that combining two such blocked pipes would let the water flow, yet in the case of quantum mechanics, theorists have shown that the phenomenon of entanglement can clear out the information stoppage.
Writing in Physical Review Letters, Jianxin Chen, at the University of Guelph and the University of Waterloo, Canada, and colleagues show theoretically that pairs of “noise-blocked” quantum channels can, in combination, transmit classical information with zero error. This is a form of “superadditivity,” an effect in which the upper limit of information capacity of two combined channels can exceed the summed capacities of the individual channels. The authors show that not only can noise be entirely defeated for transmitting classical information over quantum channels, but that the result also holds for quantum information, which is more fragile and beset by decoherence. – David Voss