An international group of physicists managed to reverse the course of time for a pair of interconnected particles. Researchers have shown that for quantum interconnected qubits (quantum bits) the second law of thermodynamics is spontaneously violated, according to which in isolated systems all processes go only in the direction of increasing entropy. This is reported in a preprint published in the arXiv.org repository.
According to the second law of thermodynamics, time goes only in one direction, at which disorder (entropy) increases in macroscopic systems. For example, heat is transferred from heated to cold bodies, but never transferred from cold bodies to heated ones. Unidirectionality is explained in terms of statistics, since there are much more states of disorder in which bodies can be than ordered ones. The reversal of the arrow of time, that is, the transition from disorder to order, is thus much less likely.
However, in quantum systems, such a transition is considered feasible. It is shown that for a system consisting of two interconnected (correlated) qubits, which are particles with half-integer spin, the arrow of time is capable of reversing. Scientists using nuclear magnetic resonance, in which the nuclei of atoms absorb electromagnetic energy, "heated" both qubits to different temperatures, changing the energy of their spins. After that, physicists experimentally tracked changes in their temperature and, thus, determined the direction of the heat flow.
The nuclei of carbon-13 and hydrogen in a chloroform solution were taken as qubits. The solution was placed inside a superconducting magnet, which generated a static electromagnetic field directed in the longitudinal direction. The interconnected particle system was manipulated using transverse radio frequency fields. Scientists have tracked the process of energy transfer between nuclei on a scale of several milliseconds, which is much less than the time it takes for the correlation to break down.
The researchers found that in the condition when the particles are not interconnected, the arrow of time has the usual direction. A cold qubit heated up, and a hot one cooled. In the case when the qubits were correlated, that is, quantum entangled, the heat spontaneously flowed in the opposite direction. According to scientists, this phenomenon should also occur in systems consisting of a larger number of interconnected particles.