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SCIENTIFIC GOALS

QUBO — SCIENTIFIC FRAMEWORK AND GOALS OF THE PROJECT

The mechanism at the basis of the transition from Quantum to Classical behavior is not embedded in the original QT and puzzles the scientific community since its formulation. In particular the superposition principle is a milestone of QT, accounting for a multitude of phenomena which can not be explained in Classical Mechanics. It is a consequence of the linearity of the Schrödinger equation, which has to break down at a certain scale, in order to avoid preposterous predictions concerning the macroscopic bodies dynamics.
For several decades, phenomenological dynamical models of wave function collapse have been developed [1,2,3,4,5,6,7], which explain the quantum-to-classical transition by a progressive reduction of the superposition, proportional to the increase of the mass of the many-body system under consideration.
Technological developments recently made it possible to bring such debate in the realm of experimental investigation. Several techniques are presently being employed, constraining the phenomenological parameters, necessarily introduced in the collapse dynamics (see e.g. [14] for a review on the argument).
Indirect tests of the collapse mechanism, which exploit the unavoidable random motion which is associated with it [15], can probe the effect of the collapse process on macroscopic objects, thus leading, thanks to the amplification mechanism, to extreme sensitivity bounds. This is the case of X and ɣ-rays measurements, which set the strongest constraints over broad ranges of the typical parameters space.
The most studies CMs are the gravity-related DP and the CSL:
Unification of QT and General Relativity is probably the main ambition of modern physics. The main pursued stream consists in attempts to quantize the gravitational field, which leads to several developments like string theory [16] or loop quantum gravity [17]. Motivated by the awareness that space-time fluctuations would induce decoherence in quantum systems [18,19] the opposite attitude, to “gravitize” QT, aroused growing interest in the last decades, especially for the privileged role which gravity may play to solve the measurement conundrum. Analogously to the collapse, the effect of the gravitational field is also universal and is magnified by the growing mass of the system; gravity may provide the non-linear coupling responsible for the breakdown of the quantum superposition. QUBO is concerned, in particular, on the visionary gravity related collapse model developed independently by Diosi and Penrose (DP) (see Refs. [1,2,8,9]). Diosi’s approach moves from the consideration that QT requires an absolute indeterminacy of the gravitational field, which leads to the conclusion that the local gravitational potential should be regarded as a stochastic variable, whose mean value coincides with the Newton potential in the non-relativistic limit. Penrose argues that the time translation operator is ill-defined for the superposition of two stationary states (two energy eigenstates with the same eigenvalue), in the presence of gravity. The superposition gives rise to the superposition of two, slightly different, space-time geometries. A slight error in the identification of the Schrödinger operators for the two space-times would correspond to a slight uncertainty in the energy of the superposition, which makes it unstable. The two approaches lead to the same prediction for the superposition collapse time, which is inversely proportional to the gravitational self-energy of the difference between two stationary mass distributions of the superposition. Considering that the gravitational self-interaction energy leads to divergences for point-like objects, a short-length cutoff is to be introduced, which represents a free parameter for the model.
CSL consists in a non-linear and stochastic modification of the Schröedinger equation; non-linearity is needed to suppress quantum superposition, stochasticity to avoid faster than light signaling and to recover the Born rule. The dynamics is characterized by the interaction with a continuous set of independent noises (one for each point of the space) having, under the simplest assumption, zero average and white correlation in time. The new terms require the introduction of two phenomenological quantities: a collapse strength, and a noise correlation length, which measures the spatial resolution of the collapse. Various theoretical considerations lead to different choices for the parameters, in particular Ghirardi, Rimini and Weber proposed the values 10-17 s-1 for the strength, and 10-7 m for the correlation length.
Besides collapsing the wave function in space, the interaction with the stochastic noise induces a diffusion in space, resulting in a Brownian-like motion. For a system of charged particles, this Brownian-like diffusion causes the particles to emit radiation, known as spontaneous radiation. The standard QT does not include such a phenomenon. The noise-induced radiation emission can then be used to test the CMs.
Within QUBO we are developing generalized, more realistic, dissipative and non-Markovian versions of the CMs. We are performing an intertwined theoretical and experimental investigation of the spontaneous radiation emission rate, which is predicted in the context of the generalized theories

 
 

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