Hybrid Quantum Teleportation
The laws of quantum mechanics enable optical communications with the ultimate capacity and quantum computers to solve certain problems with unprecedented speed. A key ingredient in such quantum information processing is quantum teleportation: the act of transferring quantum information from a sender to a spatially distant receiver by utilizing shared entanglement and classical communications. Especially, optical quantum teleportation is essential for various quantum communication protocols. Quantum logic gates based on optical quantum teleportation are one of the building blocks of optical quantum computers.
After its original proposal in 1993 , a research group in Austria succeeded in teleporting photonic quantum bits (qubits) in 1997 . However, this scheme involved several features which hindered its applications to optical quantum information processing. 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. Another is that this scheme required post-selection of 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 related experiments followed, most of which withhold the same disadvantages.
In 2013, 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 our experiment in overcoming previous limitations lies in a hybrid technique of photonic qubits and continuous-variable (CV) quantum teleportation [5,6,7]. The strength of CV teleportation, first demonstrated in 1998 , is its deterministic nature due to the on-demand availability of entangled waves and the complete joint measurement of two waves. It has long been used to teleport the amplitude and phase signals of optical waves, rather than photonic qubits. 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 CV teleporter, which works only on narrow frequency sidebands. We overcame this incompatibility by developing an innovative technology: a broadband CV teleporter  and a narrowband qubit compatible with that teleporter . Furthermore, we discovered that qubit information can be faithfully transferred with the help of gain adjustment mechanism in CV teleportation .
This hybrid technique enabled the realization of completely deterministic quantum teleportation of photonic qubits without post-selection. The transfer accuracy (fidelity) ranged from 79 to 82 percent for four different qubits, all of which exceed the classical limit of teleportation. This is a decisive breakthrough in the field of optical teleportation 16 years after the first experimental realizations. We hope our work stimulates the further development of hybrid quantum information processing to overcome the current limitations in both the qubit and CV regimes.
Fig. 1: Concept of our hybrid technique for quantum teleportation. Single-photon-based qubits are combined with CV quantum teleportation to transfer optical waves.
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