Testing the foundation of quantum physics in space via Interferometric and non-interferometric experiments with mesoscopic nanoparticles

PERSPECTIVE https://doi.org/10.1038/s42005-021-00656-7 OPEN Testing the foundation of quantum physics in space via Interferometric and non-interferometric experiments with mesoscopic nanoparticles 1234567890():,; Giulio Gasbarri 1,2,10 ✉, Alessio Belenchia 3,4,10 ✉, Matteo Carlesso Sandro Donadi6,7, Angelo Bassi 5,6, Rainer Kaltenbaek 8,9, Mauro Paternostro 4 & Hendrik Ulbricht2 4,5,6, Quantum technologies are opening novel avenues for applied and fundamental science at an impressive pace. In this perspective article, we focus on the promises coming from the combination of quantum technologies and space science to test the very foundations of quantum physics and, possibly, new physics. In particular, we survey the field of mesoscopic superpositions of nanoparticles and the potential of interferometric and non-interferometric experiments in space for the investigation of the superposition principle of quantum mechanics and the quantum-to-classical transition. We delve into the possibilities offered by the state-of-the-art of nanoparticle physics projected in the space environment and discuss the numerous challenges, and the corresponding potential advancements, that the space environment presents. In doing this, we also offer an ab-initio estimate of the potential of space-based interferometry with some of the largest systems ever considered and show that there is room for tests of quantum mechanics at an unprecedented level of detail. Q uantum mechanics is one of the most successful physical theories humankind has ever formulated. Nonetheless, its interpretation and range of validity elude our full grasping. One of the basic features of quantum physics is the superposition principle which, when applied to the macroscopic world, leads to counter-intuitive states akin to the celebrated Schödinger’s cat. While models beyond quantum mechanics, challenging some of its interpretational issues, have been formulated in their early days, testing the predictions of the theory when applied to the macroscopic world has proven to be a tall order. The main reason for this is the intrinsic difficulty in isolating large systems from their environment. Space offers a potentially attractive arena for such an endeavor, promising the possibility to create and verify the quantum properties of macroscopic superpositions far beyond current Earth-based capabilities1–4. In this work, we focus on the efforts to test the boundaries of quantum physics in space employing nanoparticles, which are one of the best-suited candidates for quantum superpositions of high-mass objects. It should be noticed that, while we will focus on testing quantum physics, large spatial superpositions of massive systems are bound to be 1 Física Teòrica: Informació i Fenòmens Quàntics, Department de Física, Universitat Autònoma de Barcelona, Bellaterra (Barcelona), Spain. 2 School of Physics and Astronomy, University of Southampton, Southampton, United Kingdom. 3 Institut für Theoretische Physik, Eberhard-Karls-Universität Tübingen, Tübingen, Germany. 4 Centre for Theoretical Atomic, Molecular, and Optical Physics, School of Mathematics and Physics, Queens University, Belfast, United Kingdom. 5 Department of Physics, University of Trieste, Trieste, Italy. 6 Istituto Nazionale di Fisica Nucleare, Trieste, Italy. 7 Frankfurt Institute for Advanced Studies (FIAS), Frankfurt am Main, Germany. 8 Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia. 9 Institute for Quantum Optics and Quantum Information, Vienna, Austria. 10These authors contributed equally: Giulio Gasbarri, Alessio Belenchia. ✉email: ; COMMUNICATIONS PHYSICS | (2021)4:155 | https://doi.org/10.1038/s42005-021-00656-7 | www.nature.com/commsphys 1 PERSPECTIVE COMMUNICATIONS PHYSICS | https://doi.org/10.1038/s42005-021-00656-7 sensitive probes for many other physical phenomena, from dark matter and dark energy searches5–13 to gravimetry and Earth observation applications14,15. In this perspective article, we delve into the possibilities offered by the state-of-the-art nanoparticle physics projected in the space environment. In doing so, we offer an ab-initio estimate of the potential of space-based interferometry with some of the largest systems ever considered and show that there is room for testing quantum mechanics at an unprecedented level of detail. In particular, after a brief introduction to the problem at hand and its relevance in fundamental physics, we discuss the advantages potentially offered by a space environment for quantum experiments based on large quantum superpositions of nanoparticles. We also give a self-contained overview of the current state-of-the-art for space-mission proposals and distinguish two classes of experiments that can be performed in space: noninterferometric and interferometric ones. The former does not require the creation of macroscopic superpositions and exploit the free-evolution spread of the position of a quantum particle. The latter, in contrast, require the creation and verification of large superpositions but also offer the benefit of a direct test of both the superposition principle of quantum mechanics and of competing theories. Both classes of experiments take advantage of the long free-fall times in space and can be used to cast stringent constraints on theoretical predictions. To showcase this last aspect, we present an ab-initio estimate of the constraints that can be expected from space-based interferometry with large nanoparticles. Superposition of macroscopic systems: the case for space The predictions of quantum physics have been confirmed with a high degree of precision in a multitude of experiments, from the sub-atomic scale up to matter-wave interferometry with tests masses of nearly 105 atomic mass units (amu)16. The basis for observing matter-wave interference is the quantum superposition principle, one of the pillars of quantum physics. While quantum physics does not pose any fundamental limitation to the size of quantum superposition states, the Gedankenexperiment of Schrödinger’s cat17 illustrates the controversies entailed by the superposition principle when extended to the macroscopic world. Many proposals have been formulated in an attempt to establish a mechanism that would lead to the emergence of a classical world at macroscopic scales. Among them, we find Bohmian mechanics18,19, decoherence histories20, the many-world interpretation21, and collapse models22,23 to name a few. The latter differs from the other proposals in the fact that they predict a phenomenology that deviates from one of standard quantum mechanics, albeit in a delicate fashion. In this sense, collapse models represent an alternative construction to standard quantum theory, more than an alternative interpretation recovering all the predictions of the latter. In light of the central role that they play in the experimental investigation of quantum macroscopicity24,25, in the following, we T (...truncated)