Experimental Quantum Teleportation of Qubits and Entanglement Swapping

Abstract

We implemented the quantum teleportation scheme, proposed by Bennett et al. [1], using single photons and entangled photons as produced by parametric down-conversion. The teleportation scheme is illustrated in the figure below. The idea is that Alice has particle 1 in a certain quantum state, in our experiment a single photon with a certain polarization, and she wishes to transfer this quantum state to Bob. but she cannot deliver the particle directly to him. Any quantum measurement will destroy the quantum state at hand without revealing the necessary information. So how can she provided Bob with the information of the quantum state? The way out is to use an ancillary pair of entangled particles 2 and 3 (EPR pair), where particle 2 is given to Alice and particle 3 is given to Bob. If Alice now performs a Bell state measurement (BSM) on particles 1 and 2. that is. she projects the two particles onto an entangled state, then Bob s particle will simultaneously be projected onto a state which is directly related to the initial state. All that Alice has to tell to Bob is in which entangled state her two particles have been projected by the Bell state measurement. This is enough information for Bob to transform the state of his particle by a unitary transformation (U) into the initial state. In our experiments we created the EPR pair of photons by pulsed tvpe-II parametric down-conversion. Creating the initial photon by the same pulse and using additional optical elements enabled us to overlap the initial photon with one of the photons from the EPR pair onto a beamsplitter. Detecting one photon in each output port of the beamsplitter yields a projection onto the Ψ− state, which is one of the four Bell-states. The corresponding unitary transformation that Bob has to perform in order to obtain a replica of the initial photon is simply the unitary transformation. Analysing the polarization properties of the photons received by Bob under the condition that Alice measures a projection onto the Ψ− provides the experimental proof of quantum teleportation [2].