Contributing to the pursuit of an experimental quantum computer, researchers at Università degli Studi di Roma "La Sapienza," Slovak Academy of Sciences in Bratislava and National University of Ireland in Maynooth have demonstrated an optical setup that approximates a quantum universal NOT logic gate. They reported their work in the Oct. 24 issue of Nature. In the experiment, an optical parametric amplifier generated two polarization-entangled photons by spontaneous emission in a 1.5-mm-thick BBO crystal. One photon was reinjected into the system on one of the two input modes, producing a photon in an orthogonal state on the other approximately 63 percent of the time. Because an exact realization of a NOT gate is impossible in quantum mechanics, an ideal quantum NOT gate would produce opposite polarization states two-thirds of the time.