In some standardized tests, the graders add in a penalty factor for wrong answers to discourage students from randomly guessing. It turns out that there's a lesson here for advanced communications as well. In some experimental communications schemes where information is carried by light pulses containing one or even less than one photon on average, it's better not to guess if you're not sure.

These systems, which could be used in applications as varied as the internal messaging in a quantum computer or ultra-secure long distance communications, would encode information as one of four different phase states of the photons—a property they have when considered as waveforms. Cool, yes, but there's a certain amount of overlap in the phases that could lead to ambiguous answers. In those cases you want your photon detector to be a smart student and not guess, because in secure communications, wrong is worse than "I don't know."

In new work reported in Nature Communications* researchers from the National Institute of Standards and Technology (NIST) and the Joint Quantum Institute (JQI) have built a single-photon detector that does just that, making highly accurate measurements of incoming photons while knowing when not to give a conclusive answer. Their system achieves error rates as much as nine times lower than more conventional measurement systems.

Their work is lucidly explained in "Quantum Information in Low Light" at the JQI Web site: http://jqi.umd.edu/news/quantum-information-low-light.

*F.E. Becerra, J. Fan, A.L. Migdall. "Implementation of generalized quantum measurements for unambiguous discrimination of multiple non-orthogonal coherent states," Nature Communications, 4, 2028 (2013).