Quantum data coding gets denser

US researchers have broken the record for the most amount of data sent by photons.

Using the "wiggling" and "twisting" of a pair of hyper-entangled photons, researchers at the University of Illinois have surpassed a fundamental limit on the channel capacity for dense coding with linear optics.

"Dense coding is arguably the protocol that launched the field of quantum communication," said Paul Kwiat, a John Bardeen professor of physics and electrical and computer engineering at Illinois.

"Today, however, more than a decade after its initial experimental realisation, channel capacity has remained fundamentally limited as conceived for photons using conventional linear elements."

The boffins explained that a single photon in classical coding will convey only one of two messages, or one bit of information. In dense coding, a single photon can convey one of four messages, or two bits of information.

"Dense coding is possible because the properties of photons can be linked to each other through a peculiar process called quantum entanglement," said Professor Kwiat.

"This bizarre coupling can link two photons even if they are located on opposite sides of the galaxy."

Using linear elements, however, the standard protocol is fundamentally limited to convey only one of three messages, or 1.58 bits.

The new experiment surpasses this threshold by employing pairs of photons entangled in more ways than one, i.e. hyper-entangled.

As a result, additional information can be sent and correctly decoded to achieve the full power of dense coding.

Professor Kwiat, graduate student Julio Barreiro and postdoctoral researcher Tzu-Chieh Wei describe the experiment in a paper published in Nature Physics.