This post is co-written by FQXi members Catalina Curceanu (LNF-INFN, Italy) and Angelo Bassi (Univ. of Trieste and INFN, Italy):
In May, we organised an FQXi-sponsored workshop dedicated to Quantum Foundations at the Laboratori Nazionali di Frascati, LNF-INFN, in Italy, on "'Events' as we see them: experimental test of the collapse models as a solution of the measurement problem." (The event was co-organised with our colleague Kristian Piscicchia, of the Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi Roma, and LNF-INFN Frascati, Italy.)
The aim of the workshop was to discuss the possible limits of the validity of "standard quantum mechanics" and, related to this, collapse models and, more generally, theories which go beyond standard quantum theory and the experiments aiming to test them. In this context, the role which gravity might play was vividly discussed.
From the theoretical point of view, since the almost 100 years old Einstein-Bohr debate, quantum mechanics never stopped raising questions about its interpretation and possible limits. 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 properties (the so-called "measurement problem"), continues to puzzle (at least part of) the scientific community. For this reason theories/models beyond the standard quantum formulation are explored.
From the experimental point of view quantum theory is certainly the best verified available theory. It is therefore a very compelling challenge to look for possible small violations predicted by alternative theories/models. The aim of ambitious experiments is either to put stronger observational bounds on the new models, i.e. on the models parameters, or, much more exciting, to find violations of standard quantum mechanical predictions. In this framework, a deeper understanding of the possible limits of the validity of the quantum superposition principle is a real experimental challenge.
40 experts and young scientists, theoreticians and experimentalists, and also some philosophers, took part to the workshop and had vivid discussions.
In what follows, we present some of the items discussed during the workshop: Schrödinger's cat meowing.
How well can we find out whether a wave function has collapsed? asks Roderich Tumulka of Eberhard-Karls University. If the GRW (Ghirardi-Rimini-Weber) theory were true, then how could we measure the number of collapses that have occurred for a given physical system in a given time interval? Roderich provided 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.
Matteo Morganti of University of Roma Tre, Roma, discussed 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, he moved back to the original 'relative-state' proposal made by Everett, and assessed to what extent it really qualifies as a many-world formulation of quantum mechanics. In the process, he considered, albeit briefly, some important issues concerning probabilities, empirical adequacy, decoherence and the philosophical status of the theory (or theories) in question.
Experimental bounds on collapse models from gravitational wave detectors were illustrated by Matteo Carlesso, Univ. of Trieste, Italy. Wave function collapse models postulate a fundamental breakdown of the quantum superposition principle at the macroscale. Upper bounds on the collapse parameters, which can be inferred by the gravitational wave detectors LIGO, LISA Pathfinder and AURIGA were shown in the framework of the Continuous Spontaneous Localization (CSL) model. These experiments exclude a large portion of the CSL parameter space at high correlation length, or rc, values.
Kristian Piscicchia has shown that for low values of rc, including that originally proposed by GRW, the best constraints come from the measurement of the spontaneous radiation. The interaction with the collapsing stochastic "noise" causes the emission of electromagnetic radiation for charged particles, which is not predicted by standard quantum mechanics, an effect known as spontaneous radiation emission. Comparing the X-ray emitted spectrum measured with ultra-pure Germanium detectors with the expected spontaneous radiation prediction allows to obtain the most stringent limit on the lambda collapse parameter for values of rc below the micron range, and in the near future orders of magnitude better limits are reachable.
An interesting presentation about Cosmic Inflation and Quantum Mechanics was held by Jerome Martin of CNRS, France. 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 a phenomenologically very appealing theory but it is also an ideal playground to discuss deep questions in a cosmological context. Jerome reviewed and discussed those quantum-mechanical aspects of inflation. He explained why inflationary quantum perturbations represent a system which is very similar to systems found in quantum optics. He also pointed out the limitations of this approach and investigated whether the 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.
Hendrik Ulbricht of the University of Southampton, UK, presented recent results on manipulation of levitated optomechanics for tests of fundamental physics, in particular the trapping and cooling experiments of optically levitated nanoparticles. 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 were reported, together with 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 an experimental test of both the quantum superposition principle and the interplay between gravity and quantum mechanics.
Towards a platform for macroscopic quantum experiments in space, was the subject discussed by Rainer Kaltenbaek of University of Vienna, Vienna Center for Quantum Science and Technology, Faculty of Physics, Austria. Recent developments 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. Rainer presented 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.
Yaakov Fein of the University of Vienna, Austria, discussed the progress at LUMI: the Long Baseline Universal Matter-Wave. At LUMI a Kapitza-Dirac-Talbot-Lau interferometer scheme with a one-meter grating separation is exploited. The aim is to detect interference at a mass scale beyond 100,000 amu, as well as to investigate massive and complex biomolecules, including bounds which can be placed on (certain) spontaneous collapse models.
Mauro Paternostro of CTAMOP, Queen's University Belfast, Ireland, introduced the entanglement between masses as a probe of the quantum nature of gravity. 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. Mauro first showed that the states of two micron dimension test masses in adjacent matter-wave interferometers could be detectably entangled solely through their mutual gravitational interaction. Then he argued 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.
The workshop testifies that we are moving from a fruitful present in Quantum Foundation towards an even more exciting future: not only for a better understanding of the quantum universe we live in, but also to set the basis for future quantum technologies on earth and in space.
More information, including the files of the various presentations, can be found on the workshop dedicated web-page.
