Hybrid Quantum Teleportation
Authors: Shuntaro Takeda, Maria Fuwa, and Akira Furusawa
Affiliation: Department of Applied Physics, School of Engineering, University of Tokyo, Japan
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The principles of quantum mechanics allow us to realize ultra-high-capacity optical communication and ultra-high-speed quantum computation beyond the limits of current technology. One of the most fundamental steps towards this goal is to transfer quantum bits (qubits) carried by photons through “quantum teleportation” . Quantum teleportation is the act of transferring qubits from a sender to a spatially distant receiver by utilizing shared entanglement and classical communications.
After its original proposal in 1993 , a research group in Austria succeeded in its realization in 1997 . However, this scheme involved several deficiencies. One is its low transfer efficiency, estimated to be far below 1%. This is due to the probabilistic nature of entanglement generation and the joint measurement of two photons. This scheme also required post-selection of the successful events by measuring the output qubits after teleportation . The transferred qubits are destroyed in this process, and thus cannot be used for further information processing. Various other related experiments have been reported thus far, but most withhold the same disadvantages. This problem has been a major limitation in the development of optical quantum information processing.
In our recent publication , we demonstrated “deterministic” quantum teleportation of photonic qubits for the first time. That is, photonic qubits are always teleported in each attempt, in contrary to the former probabilistic scheme. In addition, it does not require post-selection of the successful events. The success of the experiment lies in a hybrid technique of photonic qubits and continuous-variable quantum teleportation [5,6,7]; this required the combination of two conceptually different and previously incompatible approaches.
Figure 1: Concept of our hybrid technique for quantum teleportation. Single-photon-based qubits are combined with continuous-variable quantum teleportation to transfer optical waves.
Continuous-variable quantum teleportation, first demonstrated in 1998 , has long been used to teleport the amplitude and phase signals of optical waves, rather than photonic qubits. The advantage of continuous-variable teleportation is its deterministic success due to the on-demand availability of entangled waves and the complete joint measurement of two waves. However, its application to photonic qubits had long been hindered by experimental incompatibilities: typical pulsed-laser-based qubits have a broad frequency bandwidth that is incompatible with the original continuous-wave-based continuous-variable teleporter, which works only on narrow frequency sidebands. We overcame this incompatibility by developing an innovative technology: a broadband continuous-variable teleporter  and a narrowband qubit compatible with that teleporter .
Figure 2: Configuration of the teleportation experiment. Laser sources and non-linear optical processes supply the qubit and the required entanglement. More than 500 mirrors and beam splitters constitute the teleportation circuit.
This hybrid technique enabled the realization of completely deterministic and unconditional quantum teleportation of photonic qubits. The transfer accuracy (fidelity) ranged from 79 to 82 percent in four different qubits. Another strength of our hybrid scheme lies in the fact that the qubits were teleported much more efficiently than the previous scheme, even with low degrees of entanglement. This is a decisive breakthrough in the field of optical teleportation 16 years after the first experimental realizations.
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