Experiment demonstrates the viability of 'quantum dense coding'

Mar 24, 2008 10:15 GMT  ·  By

You can't transfer much information with a photon, we've learned that from our experience with today's existing optic communication devices. If a single photon of light is manipulated in a classical way, then photons are not too different from electrons in the matter of information transfer, except maybe for the fact that photons of light travel slightly faster than electrons. Basically, a photon can carry only one bit of information, whether an '1' or a '0'.

Fortunately, the quantum world is not dominated by classical laws but by quantum mechanics laws, meaning that information transfer efficiency can be further improved with the help of quantum mechanics. For example, researchers say that they have been able recently to perform an experiment in which a photon carried 1.63 bits of information. Theoretically, the sky is the limit.

Just by altering the wavelength, a photon could easily carry several bits of information at a time. The problem is that there is no practical way to do that with a single photon. Alternatively, researchers argue that single photon communication could achieve greater efficiency with the help of one of light's most basic properties, namely light polarization. Light polarization is defined by the oscillation of the electrical field as light travels through space. Thus, an electromagnetic wave having the electrical field oscillating on an up and down direction would be vertically polarized, while side to side movement translates into horizontal polarization.

Reading the light polarization of a single photon of light is achievable with today's technology, thus just by exploiting this property of light one could easily encode an extra bit of information. In the scientific community, this encoding process is known as 'quantum dense coding'.

Light entanglement

However, there is a problem. For example, let's consider two emitters/receivers communicating with each other through photon pairs. The first emits a photon pair in an entangled state and the second receives it and sends it back. While the photon pair carries four possible states, the first receiver will only be able to distinguish three of those meaning that, out of 2 bits of information, only 1.585 bits are received.

By giving the photon orbital angular momentum, University of Illinois researcher Julio Barreiro successfully boosted the amount of retrieved information to 1.63 bits. The equivalent to the quantum term of orbital angular momentum in trivial talk would be that, as the photon travels through space, it follows a corkscrew trajectory.

In fact, the orbital angular momentum does not carry any information itself, but it helps the receiver to make a better distinction between the four possible states of the photon pair. "In principle, we can now encode two bits per photon," said Barreiro.

Improved efficiency

The main reason for the current inefficiency of optic communications is the imperfect construction of the optical components used in the build of the devices. Polarizing beam splitters, for example, are far from perfect, this is why most of the time they require compensating schemes to make them work well.

In an experiment during this month, a team of researchers from Austria and Italy made the first successful attempt to communicate information with single photons between a satellite and a ground facility. However, Barreiro believes that the technique would be more appropriate for satellite-to-satellite communication. The ability to encode information through orbital angular momentum would provide in the future with even more encoding capabilities to transfer more than two bits of information.