New System Could Send Entangled Photons into Space

By Michael Lucibella

A research team from Italy is developing a system that will let physicists beam quantum information to and from space. In a talk at the March Meeting, the team described a way to transmit an entangled photon, using a particular kind of polarized light, to an imperfectly aligned receiver akin to the orientation of a passing satellite.

Scientists have been investigating the “spooky” quantum properties of photons in hopes of setting up a long distance communication system. Among other advantages, it would be impossible for a nefarious third party to intercept and decrypt the message without alerting the sender and receiver.

In order for such a system to work, a link first needs to be set up between the two correspondents. One correspondent entangles two photons, and sends one still in its entangled state over a distance to the second correspondent. When the quantum state of one photon is measured, the wave function of the distant photon also collapses instantaneously. With some manipulation, scientists hope to be able to encode information into this collapse, possibly by entangling a third photon.

“We encode the quantum information in some degree of freedom of the photons, and send the photon from one partner to the other,” said Fabio Sciarrino from Sapienza–Università di Roma. “The most common approach exploits the polarization of light.”

Scientists have not quite gotten to the point where useful messages can be sent through such a system. However they have been making significant strides transmitting entangled photons over great distances. An experiment conducted in the Canary Islands set a new distance record in 2012, transmitting an entangled photon to another island 144 kilometers away. However, at some point the curvature of Earth will block the transmission’s line of sight, thus requiring a satellite to relay the signal.

Orbiting spacecraft would run into a problem when trying to receive signals from the ground. The satellite’s constantly changing position and orientation makes it nearly impossible for it to accurately receive traditional beams of polarized light. Most of the time the satellite’s receiver would be out of alignment with the transmitter on the ground, distorting the transmission.

“If you have two satellites which are moving, one with respect to the other, it is non-trivial to align the horizontal axis of one satellite with the horizontal axis of the other satellite,” Sciarrino said. “Our approach is to combine together two different degrees of freedom of light.”

Sciarrino’s solution was to use circularly polarized photons. “The phase of the beam is not a plane-wave. Instead it is a helix, rotating either clockwise, or counterclockwise,” Sciarrino said.

To generate the circularly polarized photon, Sciarrino shined the light through a liquid crystal display, dubbed a “q-plate.” He directed the beam at moving receivers, essentially mini-telescopes, to gauge how faithful the transmission was of the entangled photons.

Initial experiments carried out in his lab were encouraging. To follow up, Sciarrino partnered with a team from the Università degli Studi di Padova known for transmitting entangled photons over long distances. Tests so far at 100 meters have likewise yielded positive results, and Sciarrino said he hopes to push transmission distances up to a kilometer soon. The lowest satellites orbit at about 160 kilometers above Earth’s surface.

At those distances other factors could potentially interfere with the transmission. The effects of atmospheric disturbances in particular are what Sciarrino and his team will soon be investigating. In addition, in the future the team will have to look at the relativistic effects of orbiting satellites, but Sciarrino said he didn’t think that would be a difficult complication to surmount.