Significant news broke yesterday in the field of quantum computing.  The world record for the amount of data sent by a single photon was shattered using some intensely cerebral science. 

The new record for so called "dense coding" in linear optics, set by researchers at the University of Illinois, involves the use of hyperentanglement to achieve "superdense" coding levels.  Paul Kwiat, a John Bardeen Professor of Physics and Electrical and Computer Engineering, explains dense coding stating, "Dense coding is arguably the protocol that launched the field of quantum communication.  Today, however, more than a decade after its initial experimental realization, channel capacity has remained fundamentally limited as conceived for photons using conventional linear elements."

Classical photon coding works like silicon transistor coding in basic principle-- an element, in this case a photon, can convey one of two messages, or one bit of information.  Dense coding takes advantage of quantum mechanics to allow photons to encode up to four messages theoretically, or two bits of information.  Kwiat explains, "Dense coding is possible because the properties of photons can be linked to one another through a peculiar process called quantum entanglement.  This bizarre coupling can link two photons, even if they are located on opposite sides of the galaxy."

However, past efforts have fallen short of the theoretical 4 message limit, due to inherent 3 message limitation placed by the standard entanglement protocol using linear elements.  This means that in the past communication was limited to 1.58 bits/photon as opposed to the theoretical 2 bits/photon.

Kwiat's team took advantage of a more complicated form of entanglement known as hyperentanglement.  By hyperentangling photons, the team is able to cause the photons to become entangled in more ways than one, allowing for the full transmission of data and achievement of 2 bits sent per photon.

The achievement of superdense, 2 bit coding carried out by Kwiat, graduate student Julio Barreiro, and postdoctoral researcher Tzu-Chieh Wei (now at the University of Waterloo) described the results in a paper, which has been accepted for publication in the Nature Physics journal.

The new superdense coding relies on conveying both photon's "wiggle" polarization, and its "twisting", its orbital angular momentum.  In order to do this, spontaneous parametric down conversion is performed on a pair of nonlinear crystals, creating this phenomenon of dual-parameter hyperentanglement.  Data is then transferred from one photon to the other by changing the polarization by applying birefringent phase shifts utilizing liquid crystals. 

Barreiro warns that the method may impractical within the Earth's atmosphere.  Says Barreiro, "While hyper-entanglement in spin and orbital angular momentum enables the transmission of two bits with a single photon.  Atmospheric turbulence can cause some of the quantum states to easily decohere, thus limiting their likely communication application to satellite-to-satellite transmissions."