# Research lines

We work in quantum mechanics and its foundations: the measurement problem, the meaning of the wave function, entaglement and nonlocality, the quantum-to-classical transition, the relation between quantum theory and relativity/gravity. The major research lines are:

## Foundations of Quantum Mechanics and Collapse Models

Quantum mechanics is undoubtedly a successful theory. Nevertheless, it is still puzzling the scientific community with its unsolved problems. Where does the border between the quantum (microscopic) and the classical (macroscopic) world lay? How can one reconcile quantum linearity with the lack of macroscopic superpositions? What is the role of the wave function? How does it collapse? These are only few of the open problems on the foundations of quantum mechanics.

The group is engaged in developing and testing models of spontaneous wave function collapse, which aim at giving a coherent answer to the questions above. After the seminal model by Ghirardi, Rimini, and Weber [1], several models describing the wave function collapse were developed in the following years [2]. The group is focused on two research directions: testing current collapse models, working in close contact with experimental physicists, and developing their extensions to dissipative and non-Markovian dynamics, and to the relativistic framework [3].

The experimental testing of collapse models in particular is giving important results, greatly reducing the allowed parameter space, as shown in Figure 1.

Figure1- Parameter space for the Continuous Spontaneous Localization (CSL) model [2]. is the collapse rate and is the correlation distance of the noise providing the collapse. All the shaded areas correspond to experimentally and theoretically excluded values of the parameters. Here, all the upper bounds but that denoted by the red line were inferred from our group, mostly in collaboration with other experimental and theoretical researchers. The orange area is inferred from a cold atom experiment [4], the light blue area from x-ray emission from a Germanium sample [5], the two purple areas from millikelvin nanomechanical cantilever experiments [6], green and blue areas respectively from LISA Pathfinder and LIGO noise analysis [7], the grey area from theoretical considerations [8], the blue and red lines (and corresponding upper bounds) from the spectrum excitations of fermion and phonons respectively [9].

## Decoherence and Open Quantum Systems

Even in the most sophisticated experimental laboratories, quantum systems are unavoidably affected by the surrounding environment. Such an action can overtone the effects one desire to observe. Beside phenomena like dissipation and approach to thermal equilibrium, which are also present in classical systems, in the context of open quantum systems environmental decoherence plays the most significant role. This is indeed responsible for the loss of the quantum coherence and thus of the quantum traits of the system dynamics. To reduce the environmental action on the system, it is essential an accurate derivation and characterization of effective equations of motion embedding such effects.

The group works on modelling and quantifying these phenomena and, in liaison with experimental collaborators, aims at testing them. A recently developed model concerns gravitational decoherence [10], where gravity plays the role of the environment causing the loss of quantum coherence.

## Interplay between Quantum Theory and Gravity

The conciliation of relativity and quantum theories has always been problematic. The reasons are mainly two. On the one side, quantum nonlocality (exemplified by the violation of Bell inequalities) creates a direct conflict with special relativistic requirements. On the other side, the unification of quantum and gravitational phenomena has not yet reached the desired goal. On top of this, one should not forget that existing relativistic quantum field theories are plagued by infinities. Crucial questions are still open: how can our world be nonlocal but at the same time be relativistic? Does gravity really need to be quantized? How is the gravitational field generated by a quantum superposition shaped? In recent years, several scientists have been proposing ideas which differ from the dominant view. The group is engaged in understanding the source of friction between quantum theory and relativity/gravity [11].

### References:

[1] G.C. Ghirardi, A. Rimini, and T. Weber, Phys. Rev. D 34, 470 (1986).

[2] A. Bassi and G.C. Ghirardi, Phys. Rep. 379, 257 (2003); A. Bassi *et al.*, Rev. Mod. Phys. 85, 471 (2013).

[3] J. Nobakht *et al.*, Phys. Rev. A 98, 042109 (2018); M. Carlesso, L. Ferialdi, and A. Bassi, Eur. Phys. J. D, 72, 159 (2018).

[4] M. Bilardello, *et al.*, Physica A 462, 764 (2016).

[5] K. Piscicchia *et al.,*Entropy 19(7), 319 (2017).

[6]A. Vinante *et al.,*Phys. Rev. Lett. 116, 090402 (2016); A. Vinante *et al., ibid*. 119, 110401 (2017).

[7] M. Carlesso *et al.*, Phys. Rev. D 94, 124036 (2016).

[8] M. Toros, G. Gasbarri, and A. Bassi, Phys. Lett. A 381, 3921 (2017).

[9] S.L. Adler *et al.*,Phys. Rev. D 99, 103001 (2019); S. L. Adler and A. Vinante, Phys. Rev. A 97, 052119 (2018). M. Bahrami, *ibid*. 97, 052118 (2018).

[10] A. Bassi *et al*., Class. Quantum Grav. 34, 193002 (2017); M. Carlesso and A. Bassi, Phys. Lett. A, 380, 31-32 (2016).

[11] A. Bassi,Nat. Phys. 11, 626 (2015); G. Gasbarri *et al.*,Phys. Rev. D 96, 104013 (2017).

## Collaborations

Active collaborations include:

S. L. Adler, Institute for Advanced Study, Princeton (USA)

C. Curceanu, LNF-INFN, Frascati (Italy)

D. Dürr, Ludwig-Maximilan University, Munich (Germany)

D.A. Deckert, U.C. Davies (USA)

M. Paternostro, School of Mathematics and Physics, Queen's University Belfast (UK)

T. P. Singh, Tata Institute for Fundamental Research, Mumbai (India)

H. Ulbricht, Department of Physics and Astronomy, University of Southampton (UK)