# Flavio Del SantoUniversity of Geneva | UNIGE · Department of Applied Physics

Flavio Del Santo

Doctor of Philosophy

## About

65

Publications

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481

Citations

## Publications

Publications (65)

What is fundamentally quantum? We argue that most of the features, problems, and paradoxes -- such as the measurement problem, the Wigner's friend paradox and its proposed solutions, single particle nonlocality, and no-cloning -- allegedly attributed to quantum physics have a clear classical analogue if one is to interpret classical physics as fund...

Since the early days of humankind, people have been asking questions about Nature of two kinds: why did that happen? And how can that be used? In a broad sense, science was born that day. We show indeed that science has two complementary and interdependent souls that aim, respectively, to how to understand and how to control Nature. Through a broad...

Despite their importance in quantum theory, joint quantum measurements remain poorly understood. An intriguing conceptual and practical question is whether joint quantum measurements on separated systems can be performed without bringing them together. Remarkably, by using shared entanglement, this can be achieved perfectly when disregarding the po...

From a quantum information perspective, verifying quantum coherence in a quantum experiment typically requires adjusting measurement settings or changing inputs. A paradigmatic example is that of a double-slit experiment, where observing the interference pattern on the screen in a series of experimental settings where one, the other, and both slits...

Wigner's friend experiment and its modern extensions display the ambiguity of the quantum mechanical description regarding the assignment of quantum states. While the friend applies the state-update rule to the system upon observing an outcome of her measurement in a quantum system, Wigner describes the friend's measurement as a unitary evolution,...

While entanglement between distant parties has been extensively studied, entangled measurements have received relatively little attention despite their significance in understanding nonlocality and their central role in quantum computation and networks. We present a systematic study of entangled measurements, providing a complete classification of...

Naive attempts to put together relativity and quantum measurements lead to signaling between space-like separated regions. In QFT, these are known as impossible measurements . We show that the same problem arises in non-relativistic quantum physics, where joint nonlocal measurements (i.e., between systems kept spatially separated) in general lead t...

We propose an interpretation of physics named potentiality realism . This view, which can be applied to classical as well as to quantum physics, regards potentialities (i.e. intrinsic, objective propensities for individual events to obtain) as elements of reality, thereby complementing the actual properties taken by physical variables. This allows...

While entanglement between distant parties has been extensively studied, entangled measurements have received relatively little attention despite their significance in understanding non-locality and their central role in quantum computation and networks. We present a systematic study of entangled measurements, providing a complete classification of...

David Bohm has put forward the first deterministic interpretation of quantum physics, and for this he seems to be regarded as a champion of determinism by physicists (both his contemporaries and the supporters of his interpretation, the so-called "Bohmians") as well as by historians of physics. The standard narrative is that he underwent a "convers...

We propose an interpretation of physics named potentiality realism. This view, which can be applied to classical as well as to quantum physics, regards potentialities (i.e. intrinsic, objective propensities for individual events to obtain) as elements of reality, thereby complementing the actual values taken by physical variables. This allows one t...

A. Dragan and A. Ekert [New Journal of Physics, 22(3), p.033038.] have recently claimed that fundamental properties of quantum physics (e.g. fundamental indeterminism and the principle of superposition) can be derived solely from relativistic considerations, if one takes as physically meaningful superluminal reference frames. In this comment we sho...

Discussions on indeterminism in physics focus on the possibility of an open future, i.e. the possibility of having potential alternative future events, the realisation of one of which is not fully determined by the present state of affairs. Yet, can indeterminism affect also the past, making it open as well? We show that by upholding principles of...

Vienna today is one of the capitals of research on foundations of quantum physics. We reconstruct the development of modern physics in Vienna, with a focus on foundations of quantum mechanics (FQM), which is a sub-field of quantum mechanics. We show that the influence of Erwin Schrödinger and, in more recent years, the initiatives of Anton Zeilinge...

We show that the axiom of choice, a basic yet controversial postulate of set theory, leads to revise the standard understanding of one of the pillars of our best physical theories, namely the no-signaling principle. While it is well known that probabilistic no-signaling resources (such as quantum non-locality) are stronger than deterministic ones,...

Discussions on indeterminism in physics focus on the possibility of an open future, i.e. the possibility of having potential alternative future events, the realisation of one of which is not fully determined by the present state of affairs. Yet, can indeterminism affect also the past, making it open as well? We show that by upholding principles of...

A. Dragan and A. Ekert [New Journal of Physics, 22(3), p.033038.] have recently claimed that fundamental properties of quantum physics (e.g. fundamental indeterminism and the principle of superposition) can be derived solely from relativistic considerations, if one takes as physically meaningful superluminal reference frames. In this comment we sho...

Quantum mechanics approaches its centenary with an impressive record. It became the backbone of most research in physics, led to applications such as the transistor and laser, and prompted an upheaval in the philosophy of science. Its scope and its precision have been constantly growing, and it is now promising more powerful computers and safer cry...

Karl Popper and Paul Feyerabend were among the most influential philosophers of science of the twentieth century. Extensive studies have been dedicated to the development of their controversial relationship, which saw Feyerabend turning from a student and supporter of Popper to one of his harshest critics. Yet, it is not as well known that the rift...

In a recent paper, S. Gao has claimed that, under the assumption that the initial state of the universe is a pure quantum state, only the many worlds interpretation can account for the observed arrow of time. We show that his argument is untenable and that if endorsed it potentially leads to undermine the search for a scientific explanation of cert...

In a recent paper, S. Gao has claimed that, under the assumption that the initial state of the universe is a pure quantum state, only the many worlds interpretation can account for the observed arrow of time. We show that his argument is untenable and that if endorsed it potentially leads to undermine the search for a scientific explanation of cert...

Vienna today is one of the capitals for the research on foundations of quantum physics. In this paper we reconstruct the main historical steps of the development of modern physics in Vienna, with an emphasis on quantum foundations. We show that the two main intuitive reasons, namely the influence of E. Schr\"odinger and the initiatives of A. Zeilin...

A long-standing tradition, largely present in both the physical and the philosophical literature, regards the advent of (special) relativity—with its block-universe picture—as the failure of any indeterministic program in physics. On the contrary, in this paper, we note that upholding reasonable principles of finiteness of information hints at a pi...

Karl Popper and Paul Feyerabend have been among the most influential philosophers of science of the twentieth century. Extensive studies have been dedicated to the development of their controversial relationship, which saw Feyerabend turning from a student and supporter of Popper to one of his harshest critics. Yet, it is not as well known that the...

A tradition handed down among physicists maintains that classical physics is a perfectly deterministic theory capable of predicting the future with absolute certainty, independently of any interpretations. It also tells that it was quantum mechanics that introduced fundamental indeterminacy into physics. We show that there exist alternative stories...

The quantum measurement problem can be regarded as the tension between the two alternative dynamics prescribed by quantum mechanics: the unitary evolution of the wave function and the state-update rule (or "collapse") at the instant a measurement takes place. The notorious Wigner's friend gedankenexperiment constitutes the paradoxical scenario in w...

It is generally known that the philosophy of science of Karl Popper, and in particular the concept of falsifiability as a criterion of demarcation, was heavily influenced by the revolutionary developments of physics at the beginning of the twentieth century. However, only in recent years some attention has been devoted to the many contributions tha...

The notorious Wigner’s friend thought experiment (and modifications thereof) has received renewed interest especially due to new arguments that force us to question some of the fundamental assumptions of quantum theory. In this paper, we formulate a no-go theorem for the persistent reality of Wigner’s friend’s perception, which allows us to conclud...

A long-standing tradition, largely present in both the physical and the philosophical literature, regards the advent of (special) relativity -- with its block-universe picture -- as the failure of any indeterministic program in physics. On the contrary, in this paper, we note that upholding reasonable principles of finiteness of information hints a...

A long-standing tradition, largely present in both the physical and the philosophical literature, regards the advent of (special) relativity –with its block-universe picture– as the failure of any indeterministic program in physics. On the contrary, in this paper, we note that upholding reasonable principles of finiteness of information hints at a...

In a standard interferometry experiment, one measures the phase difference between two paths by recombining the two wave packets on a beam splitter. However, it has been recognized that the phase can also be estimated via local measurements by using an ancillary particle in a known superposition state. In this work, we review and further explicate...

After World War II, a hyper-pragmatic paradigm was established in physics in most of the western countries, within which foundations of quantum mechanics were vastly dismissed as pointless speculations. In this paper, we show that in Italy, however, the interest toward quantum foundations was revived at the turn of the 1960s, mainly thanks to the i...

In this short note we reply to a Comment by Callegaro et al. [Callegaro, Pennecchi, and Bich, Phys. Rev. A 102, 036201 (2020)] that points out some weakness of the model of indeterministic physics that we proposed in our work [Del Santo and Gisin, Phys. Rev. A 100, 062107 (2019)], based on what we named “finite information quantities” (FIQs). While...

In this short note we reply to a comment by Callegaro et al. [1] (arXiv:2009.11709) that points out some weakness of the model of indeterministic physics that we proposed in Ref. [2] (Physical Review A, 100(6), p.062107), based on what we named "finite information quantities" (FIQs). While we acknowledge the merit of their criticism, we maintain th...

The notorious Wigner's friend thought experiment (and modifications thereof) has in recent years received renewed interest especially due to new arguments that force us to question some of the fundamental assumptions of quantum theory. In this paper, we formulate a no-go theorem for the persistent reality of Wigner's friend's perception, which allo...

In this paper, we reject commonly accepted views on fundamentality in science, either based on bottom-up construction or top-down reduction to isolate the alleged fundamental entities. We do not introduce any new scientific methodology, but rather describe the current scientific methodology and show how it entails an inherent search for foundations...

In classical theory, the trajectory of a particle is entirely predetermined by the complete set of initial conditions via dynamical laws. Based on this, we formulate a no-go theorem for the dynamics of classical particles, i.e., a Bell inequality for trajectories, and discuss its possible violation in a quantum scenario. A trajectory, however, is n...

In this Letter, we introduce a “coherence equality” that is satisfied by any classical communication—i.e., conveyed by a localized carrier traveling along well defined directions. In contrast, this equality is violated when the carrier is prepared in coherent quantum superposition of communication directions. This is phrased in terms of the success...

In this work, we further analyse these protocols for different types of particles (bosons or fermions, charged or uncharged), with a particular emphasis on the subtleties that arise when the phase is due to the coupling to an abelian gauge field. In that case, we show that the measurable quantities are spacetime loop integrals of the 4-vector poten...

A tradition handed down among physicists maintains that classical physics is a perfectly deterministic theory capable of predicting the future with absolute certainty, independently of any interpretations. It also tells that it was quantum mechanics that introduced fundamental indeterminacy into physics. We show that there exist alternative stories...

Karl Popper published, in 1968, a paper that allegedly found a flaw in a very influential article of Birkhoff and von Neumann, which pioneered the field of “quantum logic”. Nevertheless, nobody rebutted Popper's criticism in print for several years. This has been called in the historiographical literature an “unsolved historical issue”. Although Po...

In classical theory, the trajectory of a particle is entirely predetermined by the complete set of initial conditions via dynamical laws. Based on this, we formulate a no-go theorem for the dynamics of classical particles, i.e. {\it a Bell's inequality for trajectories}, and discuss its possible violation in a quantum scenario. A trajectory, howeve...

Classical physics is generally regarded as deterministic, as opposed to quantum mechanics that is considered the first theory to have introduced genuine indeterminism into physics. We challenge this view by arguing that the alleged determinism of classical physics relies on the tacit, metaphysical assumption that there exists an actual value of eve...

The quantum measurement problem can be regarded as the tension between the two alternative dynamics prescribed by quantum mechanics: the unitary evolution of the wave function and the state-update rule (or "collapse") at the instant a measurement takes place. The notorious Wigner's friend gedankenexperiment constitutes the paradoxical scenario in w...

Karl Popper published, in 1968, a paper that allegedly found a flaw in a very influential article of Birkhoff and von Neumann, which pioneered the field of "quantum logic". Nevertheless, nobody rebutted Popper's criticism in print for several years. This has been called in the historiographical literature an "unsolved historical issue". Although Po...

In this paper, we introduce a "Bell's equality" that is satisfied by any classical communication -- i.e., conveyed by a localized carrier traveling along well defined directions. In contrast, this equality is violated when the carrier is prepared in a quantum superposition of communication directions. This is phrased in terms of the success probabi...

Superposition of two or more states is one of the most fundamental concepts of quantum mechanics and provides a basis for several advantages offered by quantum information processing. This work reports the experimental demonstration of two‐way communication between two distant parties that can exchange only a single particle once, an impossible tas...

Classical physics is generally regarded as deterministic, as opposed to quantum mechanics that is considered the first theory to have introduced genuine indeterminism into physics. We challenge this view by arguing that the alleged determinism of classical physics relies on the tacit, metaphysical assumption that there exists an actual value of eve...

In this comment we critically review an argument against the existence of objective physical outcomes, recently proposed by Healey [1]. We show that his gedankenexperiment, based on a combination of “Wigner’s friend” scenarios and Bell’s inequalities, suffers from the main criticism, that the computed correlation functions entering the Bell’s inequ...

It is not generally well known that the philosopher Karl Popper has been one of the foremost critics of the orthodox interpretation of quantum physics for about six decades. This paper reconstructs in detail most of Popper's activities on foundations of quantum mechanics (FQM) in the period of 1950s and 1960s, when his involvement in the community...

In this comment we critically review an argument against the existence of objective physical outcomes, recently proposed by R. Healey [Foundations of Physics, 48(11), 1568-1589]. We show that his gedankenexperiment, based on a combination of "Wigner's friend" scenarios and Bell's inequalities, suffers from the main criticism, that the computed corr...

It is not very well known that the philosopher Karl Popper has been one of the foremost critics of the orthodox interpretation of quantum physics for about six decades. This paper reconstructs in detail most of Popper's activities on foundations of quantum mechanics (FQM) in the period of 1950s and 1960s, when his involvement in the community of qu...

In this paper I show that, while Einstein and Bohm both pursued a deterministic description of quantum mechanics, their philosophical concern was in fact primarily realism and not determinism. Their alleged firm adherence to determinism is based on a long-lasting misleading narrative and it should therefore be greatly scaled down.

In this paper I show that, while Einstein and Bohm both pursued a deterministic description of quantum mechanics, their philosophical concern was in fact primarily realism and not determinism. Their alleged firm adherence to determinism is based on a long-lasting misleading narrative and it should therefore be greatly scaled down.

The philosopher Karl R. Popper has been, since as early as 1934, one of the foremost critics of the Copenhagen interpretation of quantum me-chanics, accusing it of subjectivism. In 1967, Popper published the paper “Quantum Mechanics without the Observer”, which had a certain resonance among the physicists and gathered appreciations from many illust...

Franco Selleri (1936-2013) has been the initiator and foremost proposer of a revival, in the 1970s, of the research on foundations of quantum mechanics in Italy. He criticised the dogmatism of standard quantum physics and supported the idea of hidden variables to re-establish realism in physics. Moved by similar realistic motivations, the preeminen...

Commonly accepted views on foundations of science, either based on bottom-up construction or top-down reduction of fundamental entities are here rejected. We show how the current scientific methodology entails a certain kind of research for foundations of science, which are here regarded as insurmountable limitations. At the same time, this methodo...

Commonly accepted views on foundations of science, either based on bottom-up construction or top-down reduction of fundamental entities are here rejected. We show how the current scientific methodology entails a certain kind of research for foundations of science, which are here regarded as insurmountable limitations. At the same time, this methodo...

We experimentally demonstrate that quantum superposition permits two-way communication between two distant parties that can exchange only one photon and show that the transmitted bits and the direction of communication remain private.

La contestazione studentesca del 1968 e le successive lotte operaie e sociali in Italia ebbero un forte riflesso tra gli scienziati e i tecnici. La giovane generazione dei fisici, oltre a partecipare in parte a quelle lotte, manifestò negli anni Settanta una crescente insoddisfazione verso i contenuti e l'organizzazione della ricerca scientifica, e...

In this letter we show that communication when restricted to a single information carrier (i.e. single particle) and finite speed of propagation is fundamentally limited for classical systems. On the other hand, quantum systems can surpass this limitation. We show that communication bounded to the exchange of a single quantum particle (in superposi...

I present the reconstruction of the involvement of Karl Popper in the community of physicists concerned with foundations of quantum mechanics, in the 1980s. At that time Popper gave active contribution to the research in physics, of which the most significant is a new version of the EPR thought experiment, alleged to test different interpretations...

We present a reconstruction of the studies on the Foundations of Quantum Mechanics (FQM) carried out in Italy at the turn of the 1960s. Actually, they preceded the revival of the interest of the American physicists towards the FQM around mid-1970s, recently reconstructed by David Kaiser in a book of 2011. An element common to both cases is the role...