One for all and all for one (Image: MerryMoonMary/iStock)

A SEA of photons has been coaxed into acting as one for the first time. The feat, proposed by Albert Einstein and Satyendra Nath Bose in 1925, could help shrink the size of electronic devices.

Close to absolute zero, some atoms and molecules have been made to form a quantum material called a Bose-Einstein condensate (BEC). In this material, the particles are all in their lowest possible energy state and behave as a single entity.

However, despite Einstein and Bose’s prediction, cajoling photons to drop into their lowest energy state and form a BEC has proved difficult. This is because the unruly particles of light become absorbed by the surrounding material when cooled, rather than shedding their energy.


Now Martin Weitz of the University of Bonn in Germany and colleagues have managed to lower photons’ energies without losing them. “This is the main trick of the experiment,” Weitz says. And they did it at room temperature.

First, the team placed two concave mirrors 1 micrometre apart, and filled the lens-shaped cavity between them with a red liquid dye. They then fired a green laser at the cavity. The dye absorbed photons from the laser and re-emitted them at lower-energy yellow wavelengths, which the mirrors focused at the centre of the cavity. While some photons were indeed absorbed by the mirrors, the large number present in the laser more than made up for this.

When the low-energy photons at the centre of the cavity reached a density of about a trillion photons per cubic centimetre, they began to act as a single photon, shifting in appearance from a blurry glow to a bright point (Nature, DOI: 10.1038/nature09567). “All the photons marched in lockstep,” Weitz says.

The photons began to act as one, shifting in appearance from a blurry glow to a bright point

Zoran Hadzibabic of the University of Cambridge says the result completes the theoretical journey that Einstein and Bose started 85 years ago. “With this work, the circle is closed,” he says.

Weitz says the work could help further shrink electronic devices. Ultraviolet light has a short wavelength, making it an ideal tool to burn small patterns onto computer chips. But UV lasers are difficult to make. If UV photons can be cooled in the same way that the optical photons in this study were, a photon BEC could serve as a new high-energy UV photon source, Weitz says.