It is well known that photons can be pass through such gratings by converting into ‘surface plasmons’ – surface excitations that involve billions of electrons. Now Woerdman and colleagues believe that their experiment has revealed the quantum nature of surface plasmons – which are macroscopic objects – for the first time.

Entanglement is a feature of quantum mechanics that allows particles to share a much closer relationship than classical physics allows. A measurement on one particle in an entangled system reveals the properties of the other part, even if they are widely separated. A pair of entangled photons can be produced by ‘down-converting’ an ultraviolet photon into two infrared photons in a crystal with nonlinear optical properties. If the polarization of the first photon is horizontal, the polarization of the second photon will be vertical, and vice versa.

Woerdman and colleagues wanted to see if entangled photons with a wavelength of 813 nm could survive transmission through a gold film containing an array of holes that measured just 200 nm across. When photons hit such a subwavelength grating, they are converted into surface plasmons that can tunnel through the film and be re-emitted on the other side as photons.

When the researchers compared the counts registered by two single-photon detectors on the far side of the grating, they found that most of the photons were still entangled – despite their temporary conversion into excitations containing around 1010 electrons.

According to Woerdman and colleagues, the demonstration of quantum properties on a macroscopic scale is very significant. “In general, it is very difficult to maintain entanglement in a system that contains a great number of particles,” team member Erwin Altewischer told PhysicsWeb.

The researchers cannot yet fully explain their observations, but they hope that by combining the fields of quantum information and nanostructured metal optics, they will prompt further studies of entanglement in condensed matter systems.