  We demonstrate an optical quantum nondemolition (QND) interaction gate with a
bandwidth of about 100 MHz. Employing this gate, we are able to perform QND
measurements in real time on randomly fluctuating signals. Our QND gate relies
upon linear optics and offline-prepared squeezed states. In contrast to
previous demonstrations on narrow sideband modes, our gate is compatible with
non-Gaussian quantum states temporally localized in a wave-packet mode, and
thus opens the way for universal gate operations and realization of quantum
error correction.
