'Superdense' coding gets denser

Mar 24, 2008

The record for the most amount of information sent by a single photon has been broken by researchers at the University of Illinois. Using the direction of “wiggling” and “twisting” of a pair of hyper-entangled photons, they have beaten 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. “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.”

In classical coding, a single photon 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 one another through a peculiar process called quantum entanglement,” Kwiat said. “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 that threshold by employing pairs of photons entangled in more ways than one (hyper-entangled). As a result, additional information can be sent and correctly decoded to achieve the full power of dense coding.

Kwiat, graduate student Julio Barreiro and postdoctoral researcher Tzu-Chieh Wei (now at the University of Waterloo) describe their recent experiment in a paper accepted for publication in the journal Nature Physics, and posted on its Web site.

Through the process of spontaneous parametric down conversion in a pair of nonlinear crystals, the researchers first produce pairs of photons simultaneously entangled in polarization, or “wiggling” direction, and in orbital angular momentum, or “twisting” direction. They then encode a message in the polarization state by applying birefringent phase shifts with a pair of liquid crystals.

“While hyper-entanglement in spin and orbital angular momentum enables the transmission of two bits with a single photon,” Barreiro said, “atmospheric turbulence can cause some of the quantum states to easily decohere, thus limiting their likely communication application to satellite-to-satellite transmissions.”

ource: University of Illinois at Urbana-Champaign

Explore further: Thinner capsules yield faster implosions

Related Stories

QR codes engineered into cybersecurity protection

Feb 27, 2015

QR, or Quick Response, codes – those commonly black and white boxes that people scan with a smartphone to learn more about something – have been used to convey information about everything from cereals ...

Rapid data transfer thanks to quantum physics

Feb 25, 2015

RUB engineers have developed a new concept for accelerating data transfer in server farms. To this end, the team at the Chair of Photonics and Terahertz Technology applies a quantum-mechanical variable, i.e. ...

Recommended for you

Thinner capsules yield faster implosions

3 hours ago

In National Ignition Facility (NIF) inertial confinement fusion (ICF) experiments, the fusion fuel implodes at a high speed in reaction to the rapid ablation, or blow-off, of the outer layers of the target ...

Direct visualization of magnetoelectric domains

6 hours ago

A novel microscopy technique called magnetoelectric force microscopy (MeFM) was developed to detect the local cross-coupling between magnetic and electric dipoles. Combined experimental observation and theoretical ...

Upside down and inside out

7 hours ago

Researchers have captured the first 3D video of a living algal embryo turning itself inside out, from a sphere to a mushroom shape and back again. The results could help unravel the mechanical processes at ...

Heat makes electrons spin in magnetic superconductors

Apr 24, 2015

Physicists have shown how heat can be exploited for controlling magnetic properties of matter. The finding helps in the development of more efficient mass memories. The result was published yesterday in Physical Review Le ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

fleem
5 / 5 (1) Mar 25, 2008
Although this article has lots of interesting information, the implication that Shannon's limit has been beat or that a SINGLE photon can contain more information, is simply false. The quickest way to explain this is with a classical analogy of entangled photon communication: Entangled photons go two ways--which is, classically, a full duplex link. One can send one-way data twice as fast over a full duplex link because the back-channel can be used to send reverse error correction codes and thus no forward error correction need be coded into the forward channel.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.