Entanglement cuts through the noise

Quantum entanglement, by itself, cannot be used to communicate, but it can enhance the capacity, accuracy, or security of communication. In one example, researchers led by William Matthews (University of Waterloo, Canada) and Andreas Winter (University of Bristol, UK) showed last year that when two communicating parties—Alice and Bob—share pairs of entangled qubits, they can improve their chances of successfully transmitting a message over a complex noisy classical communication channel. Now, Robert Prevedel, Kevin Resch , Matthews, and other Waterloo colleagues have developed a similar protocol for a simpler channel, and they’ve implemented that protocol experimentally. The researchers used an electronic circuit to create a noisy classical channel that takes a pair of bits from Alice and tells Bob either the first bit, the second bit, or the parity of their sum—each with equal probability—and which of the three he received. Without the benefit of shared entanglement, Alice’s best strategy for transmitting a single bit to Bob succeeds 5/6, or 0.833, of the time. But if Alice and Bob each possess one of a pair of entangled qubits—polarization-entangled photons, in the experiment—then they can make better use of the channel and achieve a theoretical success rate of (2 + 2^-1/2)/3, or 0.902. The experimental success rate was 0.891 ± 0.002, much higher than the classical limit and not much lower than the entanglement-enhanced maximum. (R. Prevedel et al., Phys. Rev. Lett., in press.)—Johanna Miller