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Entangled photons cross 144 km free-space link

04 Jun 2007

optics.org speaks to the team that has sent entangled photons over a record-breaking 144 km free-space link.

A team of researchers from across Europe has sent an entanglement-based quantum key over a 144 km free-space link. The distance is an order of magnitude longer than previous free-space results and is said to be a crucial step towards future satellite-based quantum communication. (Nature Physics advanced online publication)

"We used a source of entangled photons with the repetition rate necessary to perform optical downlinks from low-Earth satellites such as the International Space Station (ISS) to optical ground stations (OGSs) on Earth for the first time," Rupert Ursin from the University of Vienna told optics.org. "We also developed a closed-loop tracking system to guarantee a stable link for the entire measurement time and adapted an OGS to detect and analyze single photons."

Entangled photons shared between two parties (known as Alice and Bob) are used to establish a secure key. Any attempt by an eavesdropper to intercept and copy the key is obvious to the receiving party, who notices errors in the transmission.

The experiment was carried out in the Canary Islands. One photon is measured on the island of La Palma. The second is sent over the 144 km link to Tenerife where it is received by the adapted OGS, owned by the European Space Agency.

The team used spontaneous parametric down conversion to generate polarization-entangled photon pairs. "We used a picosecond 355 nm Nd:vanadate laser with a repetition rate of 249 MHz to produce two photons at 710 nm in a BBO crystal," explained Ursin.

As atmospheric turbulence was often a problem, developing a tracking system was essential. "The closed loop tracking system on La Palma consisted of an optical telescope capable of transmitting single photons and receiving a beacon laser simultaneously," explained Ursin.

The received beacon enters the tracking lens with a time-dependent angle of arrival, hitting a CCD camera at different positions. This position was read out by a computer and compared with a previous reference position. The calculated error signal was used to readjust the telescope with the single photon transmitter terminal pointing direction.

Finally, the OGS (Bob), a 1 m Richey-Chrétien/Coudé telescope with an effective focal length of 39 m (f/39), was used to collect the single photons with a field-of-view of 8 arcmin. The atmospheric turbulence caused significant beam wander in the focal plane of the telescope - up to 3 mm in the worst case according to Ursin.

"To prevent the beam from wandering off the detectors, we re-collimated with an additional 400 mm focal length lens to pass through the polarization analyzer and a 10 nm (FWHM) filter," said Ursin. "Finally, the single photons were focused onto silicon avalanche photodiodes with a quantum efficiency of about 40%. The resulting beam size and beam wander was smaller than the detector's active area of 500 microns in diameter. We had a loss of -30 dB."

Using the quantum entanglement between the photon pairs, the team says it obtained a secure key with a length of 178 bits in total.

"The distance in our experiment exceeds all previous free-space experiments by more than one order of magnitude and exploits the limit for ground-based free-space communication," concluded Ursin. "Significantly longer distances can only be reached using air- or space-based platforms. The range achieved here demonstrates the feasibility of quantum communication in space, involving satellites or the ISS."

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