CNR-INO researchers, in collaboration with colleagues from Korea and UK, extended a fundamental principle in statistical mechanics, called microscopic reversibility, to the quantum world and experimentally verified it.

The second law of thermodynamics states that the entropy of an isolated system tends to increase. But at the microscopic level, transient reductions in entropy can occur with a tiny probability. These fluctuations are described by the classical microscopic reversibility principle.

Now we have demonstrated a quantum version of this principle. Since the uncertainty principle prevents the position and momentum of quantum systems from being measured precisely at the same time and the phase-space trajectories of such systems are affected by quantum coherence, we derived a modified relation between the probabilities for a phase-space trajectory and its time-reversed version.

We experimentally demonstrated quantum microscopic reversibility using a simple optical setup in which coherent laser light was mixed with thermal light states. We found that the derived probability relation accurately predicted our experimental results and that quantum effects were important only when the equivalent temperature of the thermal bath was low. By raising this temperature, we observed a transition from quantum to classical microscopic reversibility.

These results have just appeared in PRL and were selected as an Editor’s Suggestion and for a Synopsis in Physics.