  Quantum effects in condensed matter normally only occur at low temperatures.
Here we show a large quantum effect in high-pressure liquid hydrogen at
thousands of Kelvins. We show that the metallization transition in hydrogen is
subject to a very large isotope effect, occurring hundreds of degrees lower
than the equivalent transition in deuterium. We examined this using path
integral molecular dynamics simulations which identify a liquid-liquid
transition involving atomization, metallization, and changes in viscosity,
specific heat and compressibility. The difference between H$_2$ and D$_2$ is a
quantum mechanical effect which can be associated with the larger zero-point
energy in H$_2$ weakening the covalent bond. Our results mean that experimental
results on deuterium must be corrected before they are relevant to
understanding hydrogen at planetary conditions.
