© (2006) APS

Entanglement is perhaps one of the most intriguing aspects of quantum mechanics. The long-distance connection between two particles calls in to question our localized interpretation of the world around us. But it is much more than just a philosophical novelty; entanglement has many technological applications: it is key to quantum teleportation and seems set to become important in quantum computation and quantum cryptography. Now, after work by T. B. Pittman and J. D. Franson at John Hopkins University, entanglement is about to get even weirder with the concept of entangled pairs of ‘missing’ photons.

The idea of missing particles will be familiar to anyone with a knowledge of semiconductor theory: here, an absence of an electron in a ‘sea’ of electrons, referred to as a hole, is invested with particle properties such as an effective mass. In the optical case, imagine two laser beams of different wavelengths intersecting in a perfect two-photon absorbing material. One photon will be removed from each beam at the same time and hence the missing photons, or photon holes, will be entangled.

Is a real experiment that achieves the same thing as this idealized example possible? Pittman and Franson have achieved it using the interference of weak laser pulses and pairs of photons at a 50:50 beamsplitter1. By monitoring the constant intensity at both output ports with single-photon detectors, the absence of simultaneous detections is evidence of correlated photon holes. Further work is required to prove that the photon holes are also entangled, but the current work shows that it could be possible.