VIOLATION OF BELL INEQUALITIES BY PHOTONS MORE THAN 10 KM APART

A Franson-type test of Bell inequalities by photons 10.9 km apart is p resented. Energy-time entangled photon pairs are measured using two-ch annel analyzers, leading to a violation of the inequalities by 16 stan dard deviations without subtracting accidental coincidences. Subtracti ng them, a two-photon interference visibility of 95.5% is observed, de monstrating that distances up to 10 km have no significant effect on e ntanglement. This sets quantum cryptography with photon pairs as a pra ctical competitor to the schemes based on weak pulses. [S0031-9007(98) 07478-X].