The protonic diffusion in ice was measured for the first time using a spectroscopic technique newly developed. The mutual diffusion process of proton (H⁺) and deuteron (D⁺) in an H₂O/D₂O ice bilayer, which was pressurized to several tens of GPa (1 GPa~10⁴ bar) and heated to 100-200 ^oC in a high-pressure optical cell, was monitored by time-dependent measurement of infrared reflection spectra. Applying high pressure produced a dense structure not allowing molecular diffusion, which is the dominant diffusion process in ice at atmospheric pressure. In "hot ice" realized at temperatures above 100 ^oC, the protonic diffusion was thermally activated and became the dominant diffusion process instead of the molecular one.

The diffusion coefficient at 127 ^oC and 10 GPa was determined to be 10^-15m²/s larger by a factor of 105 than the value estimated for ambient pressure ice. Protons can move around in ice at a rate of 30 nm (1 nm=10^-9 m) per second on average. In other words, the proton jumps successively into the neighboring water molecules every 10 ms (1 ms=0.001 s). The results were reported in Science (Vol. 295, No.5558, 15 February 2002).

Ice is a key material of protonics and can compare with silicon, which was a key material in electronics. Further study of the protonic diffusion in ice at extended high temperatures and pressures would construct the basis for protonics.