How often do quantum systems violate the second law of thermodynamics?

The likelihood of seeing quantum systems violating the second law of thermodynamics has been calculated by UCL scientists. In two papers, published in this week's issue of  Physical Review X  and funded by the Engineering and Physical Sciences Research Council, the team determined a more precise version of a basic law of physics - which says that disorder tends to increase with time unless acted on by an outside force - and applied it to the smallest quantum systems. 'The vast majority of the time, the second law of thermodynamics is obeyed. It says that a cup of hot coffee in a cold room will cool down rather than heat up, and a collection of coins all initially heads up will likely produce a mixture of heads and tails when given a shake. In fact, it is thanks to the second law of thermodynamics that we instantly recognise when we are watching a movie backwards,' explained PhD student Alvaro M. Alhambra (UCL Physics & Astronomy). The team say that situations which break the second law of thermodynamics are not ruled out in principle, but are rare. 'We wanted to find out by how much disorder increases, and if disorder sometimes decrease with some probability.