# 'Twisting' light packs more information into one photon

By Stephen Battersby How much can you say with a single photon? Classically, the limit has been one bit of information (a choice of 0 or 1), but by adding a quantum twist, physicists have now managed to make a photon carry 1.63 bits. The team says the technique might help maximise the efficiency of satellite communications. In theory, the information limit is much greater. If the sender could tune the wavelength of a photon and then the receiver could measure that wavelength with great accuracy, the information content could easily be several bits. But current technology cannot do that with a single photon. Instead, single-photon communication uses another property of light, its polarisation. Every photon is polarised in one of two states, with its electrical field moving either up and down, or side to side. The polarisation can be read out using simple optical devices. Classically, that’s enough to encode one bit of information. With a process called “quantum dense coding”, however, the information content can be increased. The idea is that the eventual receiver, Alice, prepares a pair of photons in a quantum-mechanically entangled state. She then sends one to Bob. The information is finally transmitted when Bob sends his photon back to Alice. She can measure the state of the pair of photons, which has four possibilities. That’s two bits of information. Another practical problem emerges, however. Devices employing ordinary optical components cannot distinguish all four entangled states of the photon pair. At best, Alice can only distinguish three states this way. That’s equivalent to 1.585 bits. Julio Barreiro of the University of Illinois at Urbana-Champaign and his colleagues have now managed to boost that number a little by twisting the light. In quantum terms, it involves giving the photon what is called orbital angular momentum, which makes it travel on a corkscrew path. The researchers don’t actually encode any information in the photon twist, but they can use the twist at the receiving end to tease apart the four ordinary states of quantum dense coding. “In principle, we can now encode two bits per photon,” says Barreiro. So far they are only managing to get 1.63 bits, however. That’s already a record for any binary property of a particle, but falls short of the ideal number because the available optical components are not perfect. “The polarising beam-splitters are not very good,” Barreiro told New Scientist. They are working on a scheme to compensate for that, and squeeze in still more information. The technique could help maximise the amount of information carried per unit of energy in satellite communications, says Barreiro. Earlier this month, a team from Austria and Italy made the first single-photon communication from ground to satellite; although Barreiro believes that the twisted-light scheme is more resilient for satellite-to-satellite transmissions. Future schemes should be able to do better still. If the twist is actually used to encode information, that would enable even more than two bits to be sent with each photon. Quantum World – Learn more in our comprehensive special report. Journal reference: Nature Physics (DOI: