We present cross-correlation measurement of ultrafast switching in a symmetric Mach–Zehnder all-optical switch. A full width at half-maximum of 2 ps has been achieved in the cross-correlation trace, corresponding to the estimated switching time of 1.5 ps. Both the switch-on and switch-off time of the measured traces are limited only by the control pulse duration of 1.1 ps. These results indicate that the effects of group velocity dispersion and intraband carrier relaxation are negligible within the present time scale.