2)

2) OUR EXPERIMENT

We report shortly the main features of the teleportation experiment performed in our lab, quite different indeed from the BBCJPW original idea. The interested reader can find details at...

GENERAL DESCRIPTION

This is our apparatus:

It consists of an Ou\&Mandel's interferometer (Alice's box) and of a Shih\&Alley's one (Verifier's box). Bob's box is composed by one wave retarder + one wave rotator, and can thus perform any unitary transformation on the polarization state of a photon. The EPR source is a type II-BBO crystal pumped by an argon laser at , producing SPDConverted photons at the wavelenght of .

The Preparer acts on the two s to create the state to be teleported and give it to Alice. She performs a Bell-type measurement upon this state using the BS and the four detectors and sends the result to Bob, who is waiting for it at about 4 mt of distance. He then makes one of unitary operations (because 2 is the dimension of the Hilbert space of the polarizations of a photon) upon his photon and gives it to the Verifier, whose task is to control that teleported state is the same that entered Alice's box.

2 PHOTONS INSTEAD OF 3

Alice's photon of the EPR pair has a double rule: it is both half of the quantum channel and the quantum state itself at the same time. How is it possible? There's no tricks at all, just space products. The spaces of 2 independent particles factorize just as the spaces of 2 different degrees of freedom of the same particle. In our case the calcite crystals in the figure make the photons entangled in their k-vector degree of freedom, while the Preparer rotate the polarization of one of them, leaving thus uneffected the entanglement. So 2 photons in all are involved instead of the usual 3.

PROS\&CONS.

The main advantage of our scheme is a 100% successful teleportation in the ideal case. The main problem with all other schemes is to distinguish which of the 4 Bell states comes out from Alice box. In our case the 4 orthonormal states directly analogous to the Bell ones are:

where and are respectively the paths and the polarizations of the entangled photons. It is easy to see how a click registered at detectors corresponds to the states , while a click at corresponds to the s. Once overcome this difficult step the whole process' efficiency is limited only by the quantum efficiency of the single detectors. The main \emph{handicap }is that the distinction among Bell states ``...is obtained at the expense of having to encode the state we wish to teleport onto a local member of the momentum-entangled two-photons state; thus this method, linear in nature, cannot be applied for the fundamental (and more general) case of teleporting the unknown state of an external particle, although it is conceptually apt to realize total teleportation of a known (prepared beforehand) state...''(E.DelRe).

A PASSIVE APPROACH

As in all the teleporting-photon experiments achieved until now , even in ours the classical channel is substitued by a ``passive'' approach to the original protocol; the easiest way to explain it is the following: Let's think the ''Preparer-box'' and the ''Verifier-box'' are managed by the same person: it's an easy task then for this person to verify that the teleported state is the same state he himself had prepared at the beginning of the process.

FUTURE IMPROVEMENTS

Next goal to achieve is active teleportation: the 4 detectors at Alice side are in fast electronic communication with Bob box, represented by 2 Pockel's cells in series. After detection an electronic pulse leaves the detector involved and reach the Pockel's cells, that will rotate polarization according to the provenience of the signal. Classical channel will be then represented by a normal electric cable(wire).