Experimental scheme and calculated fidelity for practical free-space quantum teleportation.

**(a)** Schematic drawing for practical free-space quantum teleportation. Charlie distributes an entangled pair of photons 2 and 3 to Alice and Bob, where Bob is at a distant location. Due to the finite size of the telescopes Bob and Charlie can use and the diffraction limit, Bob will receive the signal photon with very high loss. Alice then performs a joint Bell-state measurement (BSM) on the initial particle and one of the entangled photons from Charlie, projecting them onto an entangled state. After she has sent the result of her measurement as classical information to Bob, he can perform a unitary transformation (U) on his photon to obtain the initial state.

**(b)** Calculated fidelity for ideal optics and an entangled photon source with the generation probability used in Ref. [24] as a function of total channel loss and the dark count on Bob’s side. The red line shows the classical limit [25] of 2/3. The white line shows the lowest dark count rate currently achieved via free-space link [21].

**Juan Yin, He Lu, Ji-Gang Ren, Yuan Cao, Hai-Lin Yong, Yu-Ping Wu, Chang Liu, Sheng-Kai Liao, Yan Jiang, Xin-Dong Cai, Ping Xu, Ge-Sheng Pan, Jian-Yu Wang, Yu-Ao Chen, Cheng-Zhi Peng, Jian-Wei Pan**
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(9) % 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…

Read more: http://arxiv.org/pdf/1205.2024v1.pdf

Read also: **Chinese Physicists Smash Distance Record For Teleportation**

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