Phase-coherent manipulation of cold biexcitonic waves

By using femtosecond optical pulses we create and manipulate a long-lived highly coherent biexcitonic wave in CuCl. Phase space compression by a factor 102 in the two-photon absorption process yields a quasiparticle ensemble with a density of up to 10(13) cm(-3) and more than 10(4) biexcitons per mode, which shows harmonic scalar wave behavior for 70 ps. The coherent biexciton ensemble acts as a phase-dependent two-photon amplifier for optical pulses. Phase sensitivity of the light pulse transmission allows us to phase lock the two successive pulses and demonstrates the coherent control of biexciton waves. By measuring the visibility of the biexcitonic interference pattern we confirm a high degree of coherence of the stored excitation.