Open Problems in Quantum Mechanics

The project Open Problems in Quantum Mechanics (PAMQ) is dedicated to high sensitivity experimental tests of two pillars of Quantum Mechanics, those on which is based the stability of matter (from the subatomic to the macroscopic level) and our perception of physical reality. PAMQ is centered on two experiments exploring the Pauli Exclusion Principle and the collapse of the wave function. I am the National Responsible of these experiments.

According to the PEP a system can not hold two (or more) fermions with all quantum numbers identical. PEP stands as one of the fundamental and most solid cornerstones of modern physics, its validity explaining a plenty of phenomena ranging from the structure of atoms to the stability of neutron stars. PEP is a consequence of the spin statistics theorem which can be proven based on few assumptions (Lorentz/Poincaré and CPT symmetries, locality, unitarity and causality) which are deeply related to the nature of space and time. Extensions of the Quantum Theory of Fields (e.g. Quantum Gravity) admit, however, spin-statistics violations, hence experimental evidence of even a tiny violation of the PEP would be an undeniable indication of physics beyond the Standard Model.

The VIP-2 (VIolation of the Pauli Exclusion Principle) experiment looks for the signature of “impossible atoms” which would violate the Pauli Exclusion Principle (PEP) for the electrons. The experimental technique consists in circulating a DC current in a copper conductor and search for the X-rays signature of PEP-violating Kα transitions (2p → 1s in Cu when the 1s level is already occupied by two electrons). As a consequence of the shielding effect of the two electrons in the ground state, the Kα violating transition is shifted of about 300 eV with respect to the standard line. The signal is detectable by means of the Silicon Drift Detectors (SDDs) which are specifically developed and characterised for this experiment. The VIP-2 experiment sets the best limit on the PEP violation probability, challenging Quantum Gravity models with a sensitivity which is orders of magnitude higher than the energy scale which can be reached at the Large Hadron Collider.

In Quantum Mechanics the measurement process is not embedded in the theory, the collapse of the wave function is postulated: a quantum state consists of a superposition of all the allowed states, as long as it is observed and suddenly reduces to the state which is actually measured. The second experiment investigates the collapse models, evolutions of Quantum Mechanics which dynamically accounts for the collapse phenomenon. The collapse would produce a faint spontaneous radiation which we search by means of High Purity Germanium detectors and extreme radio-purity Roman lead targets, thus rejecting or setting the strongest bounds on these models.

The experiments are operated at the Gran Sasso Underground Laboratories. The six orders of magnitude cosmic rays suppression provides the cosmic silence indispensable to unveil the most shy mysteries of our quantum realm.

The implications of our studies extend to technology,  the SDD detectors can be used in medical and industrial  applications, moreover the collapse models tests have an impact on the emerging quantum technologies.