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May 11, 2012

Teleporting independent qubits 97 kilometers opens the way to satellite based quantum communication

Technology Review - The ability to teleport photons through 100 kilometres of free space opens the way for satellite-based quantum communications, say researchers.

Juan Yin at the University of Science and Technology of China in Shanghai, and a bunch of mates say they have teleported entangled photons over a distance of 97 kilometres across a lake in China.

That's an impressive feat for several reasons. The trick these guys have perfected is to find a way to use a 1.3 Watt laser and some fancy optics to beam the light and receive it.

Inevitably photons get lost and entanglement is destroyed in such a process. Imperfections in the optics and air turbulence account for some of these losses but the biggest problem is beam widening (they did the experiment at an altitude of about 4000 metres). Since the beam spreads out as it travels, many of the photons simply miss the target altogether.

So the most important advance these guys have made is to develop a steering mechanism using a guide laser that keeps the beam precisely on target. As a result, they were able to teleport more than 1100 photons in 4 hours over a distance of 97 kilometres.

That's interesting because it's the same channel attenuation that you'd have to cope with when beaming photons to a satellite with, say, 20 centimetre optics orbiting at about 500 kilometres. "The successful quantum teleportation over such channel losses in combination with our high-frequency and high-accuracy [aiming] technique show the feasibility of satellite-based ultra-long-distance quantum teleportation," say Juan and co.


Bird's-eye view and schematic diagram for free-space quantum teleportation. a, Entanglement generation and distribution on Charlie's side. A near infrared pulse (788 nm) is focused on an LBO crystal to create an ultraviolet laser pulse, which is then focused with two cylindrical lenses (CL) and passed through a 2 mm nonlinear BBO crystal. By an SPDC process, an entangled photon pair is created. An interferometric Bell-state synthesizer is utilized to disentangle the temporal from the polarization information. While photon 2 is then directly sent to Alice for BSM, photon 3 is guided to a refractor telescope through a fiber and sent to Bob. A HWP sandwiched between two QWPs constitute the fi ber polarization compensation. Coaxial with the telescope, there is a green laser (532 nm) for system tracking and a red laser (1064 nm) for synchronization. The green arrows indicate the ne tracking system which consists of a four-quadrant detector and a fast steering mirror driven by piezo ceramics (PI). The blue arrows indicate the coarse tracking system which consists of a wide-angle camera and a two-dimensional rotatable platform. b, Initial state preparation and BSM on Alice's side.

Arxiv - Teleporting independent qubits through a 97 km free-space channel




With the help of quantum entanglement, quantum communication can be achieved between arbitrarily distant places without passing through intermediate locations by quantum teleportation. In the laboratory, quantum teleportation has been demonstrated over short distance by photonic and atomic qubits. Using fiber links, quantum teleportation has been achieved over kilometer distances. Long distance quantum teleportation is of particular interest and has been one of the holy grails of practical quantum communication. Most recently, quantum teleportation over 16 km free-space link was demonstrated. However, a major restriction in this experiment is that the unknown quantum state cannot directly come from outside. Here, based on an ultra-bright multi-photon entanglement source, we demonstrate quantum teleportation, closely following the original scheme, for any unknown state created outside, between two optical free-space links separated by 97 km. Over a 35-53 dB high-loss quantum channel, an average fidelity of 80.4 % is achieved for six distinct initial states. Besides being of fundamental interest, our result represents an important step towards a global quantum network. Moreover, the high-frequency and high-accuracy acquiring, pointing and tracking (APT) technique developed in our experiment can be directly utilized for future satellite-based quantum communication.


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