Light is intrinsically very difficult to store in a small space. The ability to trap photons for a long time (photon lifetime, ph) and to slow the propagation of light plays a significant role in quantum information and optical processing. Photonic-crystal cavities with an ultrahigh quality factor (Q) are attracting attention because of their extremely small volume; however, high-Q demonstrations have been accomplished only with spectral measurements. Here we describe time-domain measurements on photonic-crystal cavities with the highest Q among wavelength-scale cavities, and show directly that photons are trapped for one nanosecond. These techniques constitute clear and accurate ways of investigating ultrasmall and long ph systems. We also show that optical pulses are delayed for 1.45ns, corresponding to light propagation at 2×105c the speed of light in a vacuum, which is the slowest for any dielectric slow-light medium. Furthermore, we succeeded in dynamically changing the Q within the ph, which is key to realizing the dynamic control of light and photon-trapping memory.
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