  Fluctuations of local fields cause decoherence of quantum objects. It is
generally believed that at high temperatures, thermal noises are much stronger
than quantum fluctuations unless the thermal effects are suppressed by certain
techniques such as spin echo. Here we report the discovery of strong
quantum-fluctuation effects of nuclear spin baths on free-induction decay of
single electron spins in solids at room temperature. We find that the
competition between the quantum and thermal fluctuations is controllable by an
external magnetic field. These findings are based on Ramsey interference
measurement of single nitrogen-vacancy center spins in diamond and numerical
simulation of the decoherence, which are in excellent agreement.
