Entanglement is one of the most puzzling phenomena in quantummechanics, and also one of the most difficult to illustrate, which could helpexplain why this image of entangled photons has proved so popular. Thephotograph was created by Paul Kwiat and Michael Reck at the Universityof Innsbruck in Austria in 1995.

To produce the entangled photons, the Innsbruck team shone anultraviolet laser beam at a crystal of beta barium borate. About one in ten billion of the photons were “down-converted” into two lower-energyphotons, which were emitted on opposite sides of the ultraviolet beamalong two cones. The photons on one cone were vertically polarized, whilethose on the other were horizontally polarized. Under certain conditions, thepolarizations of the photons were entangled – in other words thecorrelations between them were stronger than any correlations allowed byclassical physics.

The photograph is unusual in that it was obtained without a lens, with the down-converted light from the crystal falling straight onto thephotographic film. The image shown is actually a false-colour composite ofthree images – each requiring a 40 minute exposure – taken with differentfilters in front of the film: the blue rings correspond to light with awavelength of 681 nm, green is 702 nm and red is 725 nm. Entanglementwas observed for 702 nm photons travelling in directions that correspondto where the green circles overlap (P G Kwiat et al. 1995 Phys. Rev. Lett. 754337). About 1 in 500 of the down-converted photon pairs are entangled,which means that fewer than 1 in 1012 of the original ultraviolet photonsresult in entangled photons.

The long exposure time meant that Kwiat and Reck had to keep any straylight away from the film. The photographic company the researchers usedalso managed to ruin the first roll of film and Kwiat and Reck had to repeat

everything again. However, the firm is acknowledged in the original paperfor developing the second roll at night to ensure optimum conditions.

According to Kwiat, who is now at the University of Illinois at Urbana-Champaign, he and Reck did not pay particular attention to the aestheticappearance of the image, although they wanted the colours to be easilydistinguishable. They also deliberately chose red for the longest wavelengthphotons and blue for the shortest. Any other aesthetic considerations werepurely subconscious, says Kwiat: “We weren’t particularly thinking ‘Ha! Nowthis would look good on wallpaper’.”

Kwiat is interested in the interplay between physics and art, but headmits that the two communities often have different ideas about beauty:“There’s nothing like seeing a good sine wave on your oscilloscope at2.00 a.m. when you’ve been searching for it for months and months – it’sreally beautiful.” He also sees many similarities in the way that physicistsand artists work. “Artists often look at the same object from many differentperspectives – from different angles and using different media to representthem,” he says. “Physicists do the same, whether we’re solving somethingusing Maxwell’s equations or quantum mechanics or quantumelectrodynamics, working in different co-ordinate systems and so on.”

There are further similarities between artists and scientists, according to Kwiat: “The other thing that artists do – not just for physicists but foreveryone – is to have a real appreciation of beauty in the world and innature, and how we interact with that. I think that it’s important foreveryone to maintain this. It is even more important for scientists, so thatwe don’t become too abstracted away from the fact that what we really areis natural philosophers – we’re trying to understand nature, to unravel themysteries of the universe as it were.”Credit: M Reck and P G Kwiat

Entangled photons