# Hyper-entangled photons claim bit-encoding record

PORTLAND, Ore. -- A new world record has been claimed for encoding information onto a binary property of light, according to researchers at the University of Illinois. By "hyper-entangling" photons--that is, using quantum entanglement with multiple degrees of freedom--Professor Paul Kwiat, doctoral candidate Julio Barreiro, and postdoctoral researcher Tzu-Chieh Wei (now at the University of Waterloo) claim to have encoded 1.63 bits per photon. The previous world's records were 1.13 bits per photon without hyper-entanglement, out of a theoretical limit of 1.58 held by Professor Anton Zeilinger at the University of Vienna (and 1.18 with h hyper-entanglement out of a theoretical limit of 2 held by Professor Harald Weinfurter at the University of Munich, Germany).

By combining Hyper-entanglement and linear optics, Kwiat, Barreiro and Wei, claim that 2.81 bits per photon could be encoded to someday increase the channel capacity of satellite-to-satellite data transmissions by more than 3.5 times.

"The record amount of information encoded on the binary property of a photon used to be 1.18 bits," said Barreiro. "We encode only in polarization as the binary property, but have the orbital angular momentum as ancilla, to achieve 1.63 bits per photon."

Called "superdense" encodings, these techniques harness the quantum mechanical properties in pairs of entangled photons, and could theoretically be extended to more dimensions in future experiments to achieve an exponentially larger number of bits.

In classically encoded binary systems, a single photon can at most encode a single bit--either a one for horizontal polarization, or a zero for vertical polarization. With entanglement--the linking of the states of two photons--it is possible to encode more than a single bit per photon to achieve a theoretical high of 1.58 bits per photon. With hyper-entanglement--adding another degree of freedom--it is theoretically possible to encode two bits per photon; however, in real systems, unavoidable imperfections lead to the recording of only statistically valid encodings between 1 and 2 bits (here the researchers achieved 1.63 bits). By encoding also in the ancillary degree of freedom, a theoretical high of 2.81 bits per photon is possible--one of seven messages per photon—albeit, again, imperfections will limit real systems to somewhere between 2 and 2.81.

"With hyper-entanglement, you can get up to 2 bits per photon, but in our experiments we achieved 1.63 bits, limited by imperfections in our system," said Barreiro. "But if you use our two binary properties--polarization and orbital angular momentum--it would raise your theoretical limit to 2.81 bits per photon."

The researchers used a process called "spontaneous parametric down conversion" in a pair of nonlinear crystals to produce pairs of entangled photons, allowing bits to be encoded in the polarization of the photons by applying birefringent phase shifts.