Photons made to change colour and shape.
FORGET the X-Men – photons are the true superheroes. Not only do they travel at the universe’s fastest possible speed, now they have been made to both change colour and shape-shift. The feat brings the dream of ultrafast quantum computers a step closer.
Photons are waves of electromagnetic energy that come in different wavelengths, or colours. The wave patterns also vary in shape, depending in part on how they came into being. The wavelength of a photon produced by a laser resembles a bell curve, for example, while a photon emitted spontaneously by an atom when an electron loses energy has a peak that rises quickly and tails off slowly. The shape can affect how a photon interacts in collisions.
Photons normally maintain their size and shape until they are absorbed by matter. Now Matthew Rakher at the National Institute of Standards and Technology in Gaithersburg, Maryland, has made photons behave like shape-shifting
chameleons. They piped infrared photons with a wavelength of 1300 nanometres into a crystal, into which they also pumped photons from a 1550-nm-wavelength laser. Each had different shapes.
The crystal acted as a waveguide, reflecting and channelling the photons to hit each other at a specific angle and place, making them blend together to form photons with a wavelength of 710 nm with the same shape as the laser photons (Physical Review Letters, DOI: 10.1103/physrevlett.107.083602).
Such transformations will be crucial for developing networked quantum computers. These replace binary bits with quantum bits, or qubits, which can exist in a multitude of quantum states at once, allowing multiple simultaneous calculations.
Quantum computers could send and store data using photons’ quantum properties, such as polarisation, which is a measure of a photon’s angular momentum. The problem is that the fibre-optic cables that would transmit the photons between computers operate most efficiently at infrared wavelengths, while quantum-memory devices – made of atoms that would absorb the photons – work best with visible photons of a given shape.
The new work would minimise the data lost in translation, says Rakher. “Our research provides a method to take telecommunications-band single photons and change their wavelength and shape so they can be stored in visible-wavelength quantum memories,” he says.
Hayden McGuinness at the University of Oregon in Eugene calls the work “a clever method for tackling two difficult problems”.