# Constructor-Based Irreversibility: reconciling irreversibility with quantum mechanics

A team of **INRiM** researchers, in collaboration with researchers from **Oxford University**, shows in the study “** Emergence of Constructor-Based Irreversibility in Quantum Systems: Theory and Experiment**”, published in

*, how the compatibility between irreversibility and quantum mechanics time-reversal symmetric laws can emerge within the constructor theory framework.*

**Physical Review Letters**

The question about irreversibility has been always tackled with different methods, from statistical mechanics methods to information-theoretic descriptions of logically irreversible tasks, as well as classical and quantum thermodynamics second laws. Anyway, a tension always arises between the modelization of irreversible phenomena and the time-reversible quantum laws at the microscopic scale.

In this work, **INRiM and Oxford University researchers formulate irreversibility as the fact that a transformation T can be realized arbitrarily well by a cyclic machine, but the same does not hold for its inverse T~**. A typical example of this irreversibility definition is given by Joule’s experiment: a volume of water can be mechanically-only heated, but not cooled down. The concept of a cyclic machine performing a transformation was generalized by von Neumann to a *constructor*, i.e., a system able to perform a certain task on another system while remaining capable of repeating the procedure. Hence, a transformation is possible only if there exists a constructor able to realize it. In this study, this irreversibility, generalizing the one of *Joule’s experiment*, is called *constructor-based irreversibility*.

To show the compatibility between this newly-defined irreversibility and the time-reversal-symmetric laws of quantum mechanics, the researchers study a qubit-based toy model centered on the quantum homogenizer, i.e., a machine composed of a set of N qubits each identically prepared in a specific state. An experimental demonstration of this model at work has been realized in INRiM labs, by exploiting single-photon qubits.

To learn more about the experiment, read the **full article.**