Compression of photon wave packets in the soft-x-ray region

Using a tapered undulator and a double grating system we have compressed soft-x-ray photon wave packets into 37% of the original length. To confirm the shortening of the length of wave packets, we employed a special analog method to measure two-photon correlation as it is without elongating the wave-packet length, while the previous two-photon correlation measurements had adopted the method to largely elongate the wave-packet length up to several hundred fs using a monochromator. The present method gives a new prospect in that the compressed wave-packet length is extremely short and of the order of 1 fs, which enables various soft-x-ray spectroscopic experiments to detect in the time domain fast elementary excitations in a solid.