There is a worldwide research effort to demonstrate the power of quantum computers by using them to solve carefully chosen mathematical problems. The problem should be chosen such that it is easy to solve for a quantum computer, but difficult enough for an ordinary (classical) computer that the quantum computer is finished long before the classical computer is. In this way, the power of a quantum computer would be demonstrated.

One such problem in which the quantum computer has an advantage is boson sampling. Boson sampling is a problem concerning the interference of individual particles of light (photons). NQIT researchers from Oxford University have shown that there is a lower limit on the quality of the photons used in this experiment. If the photons are not of high enough quality, the computational advantage that the quantum machine has disappears. 

This result has implications for research efforts into optical quantum machines. While before, it seemed possible to compensate imperfections by using more photons, this result shows that this is impossible: quantity is no substitute for quality. This means that scientists who are building such a machine will have to use photon sources which deliver fewer photons of high quality, rather than many photons, as was the trend in recent years.

The result has appeared in PRL this week.