# Workshop Quantum Foundations. The physics of "what happens" and the measurement problem

Europe/Rome
Aula Seminari (Laboratori Nazionali di Frascati)

### Aula Seminari

#### Laboratori Nazionali di Frascati

Description

This workshop is organized in the framework of the Foundational Questions Institute, FQXI, project: “Events” as we see them: experimental test of the collapse models as a solution of the measurement problem, and is sponsored by INFN.
The aim of the workshop is to discuss the possible limits of validity of quantum mechanics, the collapse models and, more generally,  theories which go beyond the standard quantum mechanics, as well as experiments aiming to test them. Also, in this context, the role that gravity may play will be discussed.
From the theoretical point of view, since the Einstein-Bohr debate, quantum mechanics never stopped raising questions about its meaning. In particular, the transition from the microscopic world, where systems are observed in a superposition of different quantum states, to the macroscopic world, where systems have well defined positions (the so-called “measurement problem”), never stopped to puzzle the scientific community. For this reason, scientists are pushed to look for theories beyond the standard quantum formulation.
From the experimental point of view, quantum mechanics is the best verified available theory. It is therefore a very compelling challenge to look for possible small violations predicted by alternative quantum theories. The aim is either to put stronger observational bounds on the new theories, i.e. on model's parameters, or, much more exciting, to find a violation of standard quantum mechanics when compared with the new theories' predictions. In this framework, a deeper understanding of the possible limits of validity of the quantum superposition principle is an interesting experimental challenge.

Organizers:
Angelo Bassi,  Univ. and INFN Trieste, Italy
Catalina Curceanu, LNF-INFN, Italy
Beattrih Hiesmayr, University of Vienna, Austria
Kristian Piscicchia, Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi Roma, and LNF-INFN Frascati (Roma)
Participants
• Alessandro Scordo
• Andrea Girardi
• Angelo Bassi
• Beatrix Hiesmayr
• Carlo Guaraldo
• Caslav Brukner
• Catalina Oana Curceanu
• Claudio Di Troia
• Danilo Domenici
• Davide Bufalini
• Diana Laura Sirghi
• Giulia Piumini
• Giulio Gasbarri
• Giuseppe Antonio Nistico'
• Giuseppina Modestino
• Hendrik Ulbricht
• Jerome Martin
• Johann Marton
• Kristian Piscicchia
• Kyrylo Simonov
• Lorenzo Piro
• Luca Curcuraci
• Luca Ferialdi
• Marco Miliucci
• Mario Hubert
• Matteo Brunelli
• Matteo Carlesso
• Matteo Morganti
• Maurizio Benfatto
• Mauro Paternostro
• Pawel Moskal
• Raffaele Del Grande
• Rainer Kaltenbaek
• Roderich Tumulka
• Samuel McMillen
• Simone La Cesa
• Stefano Bacchi
• Tommaso Mazzocchi
• Yaakov Fein
Support
• Wednesday, 24 May
• 09:40 10:10
Registration 30m
• 10:10 10:28
• 10:28 10:29
Chair: Matteo Morganti
• 10:30 11:00
The Wave-Function as a Field in Three-Dimensional Space 30m
It is generally argued that if the wave-function in the de Broglie--Bohm theory is a physical field, it needs to be a field in configuration space. I show, however, that it can be regarded as a physical field in three-dimensional space. Indeed, I propose a novel interpretation of the wave-function as a new type of physical field: a multi-field. The multi-field interpretation leads to a realistic understanding of the wave-function, while retaining the entire ontology in three dimensions.
Speaker: Mr Mario Hubert (University of Lausanne)
• 11:00 11:30
Coffee Break 30m
• 11:30 12:10
Consistency of value assignment by means of quantum correlations 40m
In Quantum Physics very rarely it happens that the value of an observable is obtained by a 'direct' measurement. In fact, in almost all cases, such a value is assigned by measuring another observable characterized by perfect correlation with the observable of interest, and then assigning it the value according to the correlation. The assignment of the value of the spin observable to a silver atom through a Stern and Gerlach apparatus is an effective example, where the value assignment, inferred from the exit localization (up or down), is often identified with an authentic measurement of the spin. Given the importance of the concept of measurements and of the role they play in the investigation about the foundational problems of Quantum Theory, such as the measurement problem, contextuality, locality, the tasks of precisely establishing the conceptual and formal status of these value assignments (we shall call 'evaluations') and of establishing how they are related to authentic measurement cannot be overlloked by Quantum Theory. In this talk, once demonstrated that evaluations cannot be identified with measurements, we single out the conditions under which an evaluation is a perfect simulation of the corresponding measurement. However, evaluations are allowed also when these conditions are notsatisfied. Then we identify the domains of circumstances, i.e. determined domains of observables, where assignment by evaluation is consistent with measurement.
Speaker: Giuseppe Antonio Nistico' (CS)
• 12:10 12:40
Non-Markovian Gaussian open system dynamics 30m
Non-Markovian dynamics describe general open quantum systems when no approximation is made. We provide the exact map for the class of Gaussian, completely positive, trace preserving, non-Markovian dynamics. We further characterize the class of stochastic Schroedinger equations that unravel this map. Moreover, by exploiting Wick's theorem, we derive the exact non-Markovian master equation for bosonic systems. We show that the master equation for non-Markovian quantum Brownian motion is a particular case of our general result.
Speaker: Luca Ferialdi (TS)
• 12:40 13:20
How well can we find out whether a wave function has collapsed? 40m
In this talk, I focus on a question that is slightly different from testing collapse models: We know that "quantum measurements" are not measurements in the usual meaning of the word. So how about measurements, in the usual meaning of the word, in a collapse model? For example, if GRW theory were true, then how could we measure the number of collapses that have occurred in a given physical system in a given time interval? I provide a mathematical analysis of some simple cases. It turns out that there are limitations to knowledge---that is, that some well-defined quantities cannot be reliably measured empirically. Such a situation may seem philosophically problematical, but I will argue that it is not. This is joint work with Charles Wesley Cowan
Speaker: Roderich Tumulka (Eberhard-Karls University)
• 13:30 14:30
Lunch 1h
• 14:43 14:44
Chair: Hendrik Ulbricht 1m
• 14:45 15:15
An introduction to the spontaneous wave function collapse models 30m
Collapse models are phenomenological models, proposed to solve the measurement problem. In these models the Schroedinger equation is modified and the state vectors evolve according to a non-linear and stochastic dynamics. The effect of these non-linear terms is to induce the wave function collapse in space. The dynamics is built in such a way that the deviations from the linear Schroedinger dynamics are very small for microscopic systems (e.g. particles and atoms) while they become more and more relevant when the system's size increases, explaining the quantum to classical transition. The models make predictions different from quantum mechanics hence, they can be tested in experiments. In this talk, I will give a general introduction to the most relevant collapse models and their properties. Then, I will present a summary of the current bounds set by different kind of experiments (macromolecule interference, radiation emission, cantilevers, etc..) on the parameters of these models.
• 15:15 15:45
Non-commutative kinematics for a point-like particle 30m
I will present a toy model which exhibit interesting properties from the point of view of the foundations of quantum mechanics and probability theory. The model describes a point-like particle which moves by random jumps on a discrete random space, that is described by a collection of random walks. The particle at a certain time t is assumed to be completely described by two quantities: its position and its velocity, which are represented by two random variables. Another random variable is the configuration of the space at the same time t. The model is not deterministic, as such its description is done in terms of the probability distributions of these random variables. It is possible to prove that the position and the velocity of the particle at the time t fulfil an entropic uncertainty relation, after conditioning with respect to the configuration space at the same time (operation that in some sense “removes the space from the model”). Entropic uncertainty relations are additive uncertainty relations between the Shannon’s entropies of two non-commuting operators. This implies that, if we want to describe the position and the velocity random variables in this model, without any reference on the space, we have to use non-commuting operators over some Hilbert space. During the talk an intuitive proof of these facts will be provided.
Speaker: Luca Curcuraci (T)
• 15:45 16:10
Coffee Break 25m
• 16:10 16:50
Many worlds quantum mechanics and the measurement problem 40m
In this talk, I will discuss the attempt(s) to solve the measurement problem by making quantum mechanics a ‘many-world theory’. Starting from the naïve idea that measurement events literally cause the universe to branch, I will then move back to the original ‘relative-state’ proposal made by Everett, and assess to what extent it really qualifies as a many-world formulation of quantum mechanics. In the process, I will consider, albeit briefly, some important issues concerning probabilities, empirical adequacy, decoherence and the philosophical status of the theory (or theories) in question.
Speaker: Dr Matteo Morganti (University of Rome TRE)
• 16:50 17:30
Studies of discrete symmetries in Nature by means of the J-PET tomograph 40m
If the Nature was utterly symmetric the matter would not exist. Yet, processes driven by the gravitational, electromagnetic and strong interactions seem to be symmetric with respect to reflection in space (P), reversal in time (T) and charge conjugation (C). So far violations of these symmetries were observed only in processes governed by the weak interaction. Interestingly, though the matter which we know is made of quarks and leptons, violation of CP and T symmetries have been observed only for systems including quarks, and it has not yet been discovered in any processes involving purely leptonic matter. Positronium is the lightest purely leptonic object decaying into photons. As an atom bound by a central potential, it is a parity eigenstate, and as an atom built out of an electron and an anti-electron, it is an eigenstate of the charge conjugation operator. The newly constructed Jagiellonian Positron Emission Tomograph (J-PET) is a first PET tomograph built from plastic scintillators [1] and it enables to study the decays of positronium atoms [2]. Specifically it enables to perform tests of discrete symmetries in the leptonic sector via the determination of the expectation values of the discrete-symmetries-odd operators, which may be constructed from the spin of ortho-positronium atom and the momenta and polarization vectors of photons originating from its annihilation. We will present the potential of the J-PET detector to test the C, CP, T and CPT symmetries in the decays of positronium atoms and report on the first data-taking campaigns. [1] J-PET: P. Moskal et al., Nucl. Instrum. Meth. A764 (2014) 317. J-PET: P. Moskal et al., Nucl. Instrum. Meth. A775 (2015) 54. J-PET: P. Moskal et al., Phys. Med. Biol. 61 (2016) 2025. J-PET: L. Raczyński et al, Phys. Med. Biol. (2017) in print.
Speaker: Pawel Moskal (Jagiellonian University)
• 17:30 18:00
Experimental bounds on collapse models from gravitational wave detectors 30m
Wave function collapse models postulate a fundamental breakdown of the quantum superposition principle at the macroscale. We compute the upper bounds on the col- lapse parameters, which can be inferred by the gravitational wave detectors LIGO, LISA Pathfinder and AURIGA. We consider the most widely used collapse model, the Continuous Spontaneous Localization (CSL) model. We show that these experiments exclude a huge portion of the CSL parameter space, the strongest bound being set by the recently launched space mission LISA Pathfinder.
Speaker: Matteo Carlesso (TS)
• Thursday, 25 May
• 09:29 09:30
Chair: Angelo Bassi 1m
• 09:30 10:10
Manipulation of levitated optomechanics for tests of fundamental physics 40m
We will discuss our trapping and cooling experiments of optically levitated nanoparticles at Southampton. We will report on the cooling of all translational motional degrees of freedom of a single trapped silica particle to ~1mK simultaneously at vacuum of 10-5 mbar using a parabolic mirror to form the optical trap. We will further report on the squeezing of a thermal motional state of the trapped particle by rapid switch of the trap frequency. Such experiments are relevant to pave the way towards experimental test of both the quantum superposition principle and the interplay between gravity and quantum mechanics.
Speaker: Dr Hendrik Ulbricht (University of Southampton)
• 10:10 10:50
Testing CSL model with the spontaneous radiation emission process 40m
Collapse models are phenomenological models introduced to solve the measurement problem of quantum mechanics and describe the transition from the quantum to the classical regime; among them, one of the most relevant and well studied is known as Continuous Spontaneous Localization (CSL). According to this model, the linear and unitary Schroedinger dynamics is modified by adding a non-linear term and the interaction with a stochastic noise field. These modifications account for the localization of the macroscopic objects, leaving unchanged the dynamics at microscopic level, but also require the introduction of two phenomenological parameters, a collapse rate parameter ($\lambda$) and a correlation length ($r_C$). The interaction with the stochastic field causes an extra emission of electromagnetic radiation for charged particles, which is not predicted by the standard quantum mechanics, known as spontaneous radiation emission. We will show that comparing the X-ray emission measured with ultra-pure Germanium detectors with the expected spontaneous radiation spectrum allows to obtain the most stringent limits on $\lambda$ and $r_C$ on a broad range of the parameters space.
Speaker: Mr Kristian Piscicchia (LNF)
• 10:50 11:20
Coffee Break 30m
• 11:20 12:00
Quantum formulation of the Einstein Equivalence Principle 40m
The Einstein Equivalence Principle (EEP) enables understanding gravity as space-time geometry. The principle requires equivalence between the rest, inertial and gravitational mass-energies. However, in quantum mechanics, even non-relativistic, quantized internal energies contribute to the total mass-energies. In the talk I will introduce a “quantum formulation of the weak EEP” as the equivalence between the rest, inertial and gravitational internal energy operators. Importantly, validity of the classical EEP does not imply the validity of its quantum formulation, which requires an independent experimental verification. I will explore the ways to test the quantum EEP.
Speaker: Prof. Caslav Brukner (University of Vienna)
• 12:00 12:40
Entanglement Between Masses as a Probe of the Quantum Nature of Gravity 40m
Interactions between two material objects are mediated by fields. If quantum entanglement is created between two such objects due to their interaction, then it follows that the "mediating" field must have been a quantum entity. In this talk I first show that the states of two micrometer dimension test masses in adjacent matter-wave interferometers could be detectably entangled solely through their mutual gravitational interaction. I then argue that the purely gravitational mechanism for this entanglement implies that witnessing it is equivalent to certifying the quantum nature of the gravitational field that mediates the entanglement.
Speaker: Prof. Mauro Paternostro (CTAMOP, Queen's University Belfast)
• 12:40 13:20
Progress at LUMI: the Long Baseline Universal Matter-Wave Interferometer 40m
Molecular interferometry has evolved into a rich field that addresses topics ranging from precise metrology to fundamental quantum properties. At LUMI we exploit a Kapitza-Dirac-Talbot-Lau interferometer scheme with a one-meter grating separation. We aim to detect interference at a mass scale beyond 100,000 amu, as well as to investigate massive and complex biomolecules. In this talk, I will present the current status of the experiment as well as some of the challenges inherent to a molecule interferometer of this scale. I will also discuss the outlook of the experiment, and the bounds it can place on certain spontaneous collapse models.
Speaker: Yaakov Fein (University of Vienna)
• 13:30 14:30
Lunch 1h
• 14:44 14:45
Chair: Mauro Paternostro 1m
• 14:45 15:15
Quantum-limited estimation of continuous spontaneous localization 30m
In this talk I will apply the formalism of quantum estimation theory to extract information about potential collapse mechanisms of the continuous spontaneous localisation (CSL) form. The strength with which the field responsible for the CSL mechanism couples to massive systems is estimated through the optomechanical interaction between a mechanical resonator and a cavity field. In particular I will focus on all-optical measurements, such as homodyne and heterodyne measurements, given their practical feasibility. The performances of such strategies are also compared with those of a spin-assisted optomechanical system, where the estimation of the CSL parameter is performed through time-gated spin-like measurements.
Speaker: Mr Matteo Brunelli (Queen's University Belfast)
• 15:15 15:55
Can a Pauli-forbidden atomic transition happen? 40m
The Pauli Exclusion Principle (PEP) and its connection to the spin-statistics theorem is a empirically very well proved in fermionic systems. However, from theoretical considerations there are speculations about possible violations in the lepton sector, i.e. for neutrinos, which would have severe consequences for cosmology. In an experiment at the Gran Sasso Laboratory (LNGS-INFN) we are searching for the limit of the validity of PEP for leptons by searching PEP-forbidden electron transitions in a high-sensitivity experiment VIP2. The underlying concept of this experiment, preliminary results and an outlook for the next steps will be given.
Speaker: Dr Johann Marton (Stefan Meyer Institute)
• 15:55 16:25
Coffee Break 30m
• 16:25 17:05
Cosmic Inflation and Quantum Mechanics 40m
According to cosmic inflation, the inhomogeneities in our universe are of quantum mechanical origin. This scenario was recently spectacularly confirmed by the data obtained by the European Space Agency (ESA) Planck satellite. In fact, cosmic inflation represents the unique situation in Physics where quantum mechanics and general relativity are needed to establish the predictions of the theory and where, at the same time, we have high accuracy data at our disposal to test the resulting framework. So inflation is not only aphenomenologically very appealing theory but it is also an ideal playground to discuss deep questions in a cosmological context. In this talk, I review and discuss those quantum-mechanical aspects of inflation. In particular, I explain why inflationary quantum perturbations represent a system which is very similar to systems found in quantum optics. But I also point out the limitation of this approach and investigate whetherthe large squeezing of the perturbations can allow us to observe a genuine observational signature in the sky of the quantum origin of the cosmological fluctuations.
Speaker: Mr Jerome Martin (CNRS)
• 20:00 21:15
Workshop Dinner (Belvedere Restaurant Frascati) 1h 15m
• Friday, 26 May
• 09:29 09:30
Chair: Hans Marton 1m
• 09:30 10:10
Description of the electron configuration using a gravity model 40m
Starting from a reinterpretation of the inverse quadratic law that spatially describes a gravitational field, we come to formulas compatible and easily testable with celestial mechanics. The same formulas apply a similar hypothesis to a system of Bohr radius size dimensions, obtaining a stable structure, while not introducing quantum mechanics. The validity of this hypothesis is all to be demonstrated, and for this a more accurate comparison is required, both with experimental measurements and just with quantum mechanics theory
Speaker: Giuseppina Modestino (LNF)
• 10:10 10:50
Towards a platform for macroscopic quantum experiments in space 40m
Tremendous progress has been made in space technology over the last decades. This technological heritage promises enabling applications of quantum technology in space already now or in the near future. Heritage in laser and optical technologies from LISA Pathfinder comprises core technologies required for quantum optical experiments. Low-noise micro-thruster technology from GAIA allows achieving impressive microgravity levels, and passive as well as active cryogenic cooling has been harnessed in a harnessed in missions ranging from Planck to the James Webb Space Telescope and may be adapted for the requirements of quantum experiments. Developments like these have rendered space an increasingly attractive platform for quantum-enhanced sensing and for fundamental tests of physics using quantum technology. In particular, there already have been significant efforts towards realizing atom interferometry and atomic clocks in space as well as efforts to harness space as an environment for fundamental tests of physics using quantum optomechanics and high-mass matter-wave interferometry. Here, we will present recent efforts in mission planning, spacecraft design and technology development towards this latter goal in the context of the mission proposal MAQRO and ESA's recent call for New Science Ideas.
Speaker: Dr Rainer Kaltenbaek (University of Vienna, Vienna Center for Quantum Science and Technology, Faculty of Physics)
• 10:50 11:20
Coffee Break 30m
• 11:20 11:50
On a correspondence between Trace Dynamics and Quantum Theories 30m
Abstract. Trace Dynamics (TD) is a pre-quantum theory, which declares to recover standard Quantum Mechanics (QM) by a thermodynamic-like procedure as explained in [Adl04]. TD is an underlying theory for spontaneous collapse models [BLS+13], which at present are phenomenological models for solving the quantum measurement problem and the absence of macroscopic superposition. TD is a very young theory, with several crucial open problems both from the theoretical physics as well as mathematical physics point of view, we explain here the main issues. In order to obtain QM from TD, one needs an average procedure on the Grassmann algebraic space over which TD lies; so we specify the mathematical details of this procedure. We evolve the space of TD into a super Hilbert space and we proof the existence of a well-defined relation between the super Hilbert space of TD and a standard Hilbert space. Moreover we make explicit what are the constraints in order to derive a rigorous correspondence between TD and Quantum Field Theory. References. [Adl04] S.L. Adler. Quantum Theory as an Emergent Phenomenon: The Statistical Mechanics of Matrix Models as the Precursor of Quantum Field Theory. Cambridge University Press, 2004. [BLS+13] Angelo Bassi, Kinjalk Lochan, Seema Satin, Tejinder P. Singh, and Hendrik Ulbricht. Models of wave-function collapse, underlying theories, and experimental tests. Rev. Mod. Phys., 85:471–527, Apr 2013.
Speaker: Stefano Bacchi (T)
• 11:50 12:20
Probing collapse models at high energy scale: New Aspects? 30m
The measurement problem reveals one of the conceptual difficulties the quantum theory meets despite its extreme success. Why do we not observe a table or a cat in two places at the same time? A microscopic system is in a superposition: in which way is it broken up while we perform a measurement on the system? What is a measurement apparatus? So-called models of spontaneous collapse (collapse models) claim to provide an answer to these questions through a powerful and mathematically complete approach which models the collapse of the wave function as an objective physical process. These models feature an important property, namely they can be excluded in plethora of experiments, for example, tests by X-rays, optomechanical systems, cold-atom experiments and others. In this talk we present a study of the two popular models of spontaneous collapse at high energy scale, i.e. within flavor oscillating neutral mesons which are superpositions of two different mass-eigenstates. We show how these systems contribute to the testing of collapse models constraining the possible collapse scenarios by experimental data and propose a new interpretation of the decay mechanism of neutral mesons via spontaneous collapse.
Speaker: Mr Kyrylo Simonov (University of Vienna)
• 12:20 12:50
Hypothesis testing of Continuous Spontaneous Localisation 30m
We apply the formalism of hypothesis testing to the detection of the continuous spontaneous localisation (CSL) model. The testbed consists of a cavity optomechanical system: the collapse mechanism acts as an additional heating of the mechanical resonator, thereby indirectly affecting the cavity field on which we can perform measurements. Initialresults are based on discrimination between two squeezed thermal states with different temperatures. We calculate the asymptotic error probabilities associated with a classical statistical test, using all-optical homodyne measurements, and compare the results with the quantum Chernoff bound.
Speaker: Mr Samuel McMillen (Queen's University Belfast)
• 12:50 13:30
Final discussions, farewell 40m