Demonstrating Highly Symmetric Single-Mode, Single-Photon Heralding Efficiency in Spontaneous Parametric Downconversion
A JQI/PFC experiment establishes a new record for symmetric single-mode, single-photon, heralding efficiency for a pair of entangled photons produced during parametric downconversion. About 84% of the time they observe photon A in one detector they also observe photon B just where it should be in a second detector, and vice versa.
The aim of the JQI experiment is not itself to test the Bell criteria for entanglement (as it turns out the polarizations of photons A and B are known beforehand) but rather to optimize the process of heralding---the ability to say that if A is here then B is there. Some theorists hold that the heralding efficiency must be at least 82% if entanglement loopholes are to be closed. The JQI result has exceeded this criterion.
The JQI detection scheme will be useful for a number of reasons: it should help experiments to tighten remaining loopholes over the fundamental sway of quantum reality; with the use of a shutter, it should help to establish a reliable source of single photons; and it might be an ingredient in the creation of a source of random numbers.