Each Photon Sends 1.63 Bits

The record for the most amount of information sent by a single photon has been broken in new research that reports an average transmission of 1.63 bits of information per photon, allowing "superdense" coding for quantum communication to become even denser.

Researchers at the University of Illinois at Urbana-Champaign (UIUC) used pairs of photons entangled in more than one way -- so-called "hyper-entangled" photons -- taking advantage of the direction of "wiggling" and "twisting" of the pair to beat a fundamental limit on the channel capacity for dense coding using only 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 hyper-entangled photons. 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 in Canada) describe their recent experiment in a paper accepted for publication in the journal Nature Physics.

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.

The UIUC researchers found that, on average, they achieved a channel-capacity threshold of 1.63 bits of information per photon. Imperfections in the alignment, input states and components keep the channel capacity from achieving the 2-bit maximum, the researchers said.

Barreiro said that while the transmission of two bits with a single photon is possible, its application could likely be limited to transmissions between satellites because turbulence in Earth's atmosphere could cause some of the quantum states to break apart.

For more information, visit: www.uiuc.edu