Quantum cryptography protocols using entangled qubit pairs can increase the rate and security of information exchange through a quantum channel. Usually those protocols are non-deterministic, i.e. a classical bit is encoded in a quantum state, but the actual bit value cannot be determined. Therefore they request the transmission of additional information through a public channel. By contrast, the ping-pong coding scheme [1] based on entangled photons allows for deterministic and direct key distribution. No qubit has to be discarded and information can be decoded without the need for a subsequent transmission of additional information. Communication through a public channel nevertheless is necessary to ensure asymptotical security against arbitrary eavesdropping attacks. We realized the ping-pong protocol experimentally using polarisation-entangled photon pairs generated by parametric down conversion. A beta-Barium Borate crystal is pumped at 355 nm with laser pulses of 8 ps duration and 3.5 nJ pulse energy. With this source around 30,000 |ψ{sup}+>-states per second were detected. The polarization entanglement of the photons was characterized yielding a Bell parameter S=2.66 ± 0.0025, thus violating the CHSH inequality by 240 standard deviations. One photon of each pair is stored in a delay line, the other one is used to encode one bit of information by choosing whether or not to transform the |ψ{sup}+>-state into a |ψ{sup}->-state by the unitary transformation via a Pockels cell.
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