Optical tachyons in parametric amplifiers: How fast can quantum information travel?

We show that optical tachyonic dispersion corresponding to superluminal (fa ster-than-light) group velocities characterizes parametrically amplifying m edia. The turn-on of parametric amplification in finite media, followed by illumination by spectrally narrow probe wavepackets, can give rise to trans ient tachyonic wavepackets. In the stable (sub-threshold) operating regime of an optical phase conjugator, it is possible to transmit probe pulses wit h a superluminally advanced peak, whereas conjugate reflection is always su bluminal. In the unstable (above-threshold) regime, superluminal response o ccurs both in reflection and in transmission, at times preceding the onset of exponential growth due to the instability. Remarkably, the quantum infor mation transmitted by probe or conjugate pulses, albeit causal, is confined to times corresponding to superluminal velocities. These phenomena are exp licitly analyzed for four-wave mixing, stimulated Raman scattering and para metric downconversion.