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ATESIT Sintesi della relazione

Project ID: IST-2000-29681
Finanziato nell'ambito di: FP5-IST
Paese: Italy

Active teleportation

A qubit teleportation, experimentally demonstrated by Partner 1, in collaboration with Partner 4 with an unprecedented large 'fidelity' (F0.95) was one of the first ATESIT results. It is based on the concept of 'entanglement of one photon with the vacuum' by which each qubit can be physically implemented by a two dimensional subspace of Fock states of a mode of the electromagnetic field, specifically the space spanned by the QED 'vacuum' and the 1-photon state. The relevant conceptual novelty introduced here consists of the fact that the field's modes rather than the photons associated with them are taken as the information carriers, qubits. It is worth noting that, differently from the standard scheme of quantum bit teleportation, the present realization has the advantage of avoiding the difficult implementation of the final stage of QST: the unitary transformations U restoring the exact input qubit at Bob's site under Alice's control through the QST classical channel.

The active qubit teleportation process was realized in Rome by adopting this vacuum-1 photon configuration. In that case the main problem was to switch under single-photon excitation the high-voltage pulses driving the electro-optic Pockels-cells (EOP) which implemented the necessary U-unitaries at Bob's site. Of course, in order to preserve an appreciable QST fidelity it must be: T<<?, being ? the characteristic time of the decoherence process affecting the nonlocal channel. This scheme provided a discrete variable optical quantum teleportation as complete as possible using linear optics. A large value of teleportation 'fidelity' was attained: F=90+/-2%.

Furthermore the implementation of the unitary transformation driven by Alice’s detector represents the first experimental realization of single photon feedforward necessary for the all optical quantum computer proposed by Knill, Laflamme and Milburn (Nature (London) 409,46 (2001)).

As a final step, the availability of the linear momentum entanglement given by the high brilliance source of entangled photons, which will be described in the next paragraph, allows to realize the complete active teleportation protocol of a polarization single photon state. This experiment is going to be performed in the Laboratory of Rome.

In this experiment the two parties, Alice and Bob, receive a single photon of the momentum entangled pair, then the state to teleport is codified in the Alice’s photon polarization. At her site Alice performs a measurement in order to discriminate among the four momentum-polarization Bell state of a single particle, a task which can be accomplished by means of only linear optical devices, viz. a half-wave plate, a 50/50 beam splitter and two polarizing beam splitters (PBS).

On the Bob’s site, the momentum state is swapped into a polarization state by means of a PBS, then the photon is coupled to a single mode optical fiber. Two fast Pockels cells mounted at the output port of the fiber, properly triggered by the outcome of the Alice’s measurement, perform the Pauli transformation on Bob’s state, which concludes the protocol. The optical fiber delay experienced by the Bob’s photon must be long enough to get the proper activation of Pockels cells.


Francesco DE MARTINI, (head of unit)
Tel.: +39-064-9913518
Fax: +39-064-454778