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Physical Review A

Exploring metal nanoparticles for matter-wave interferometry

Sebastian Pedalino, Tomás Sousa, Yaakov Y. Fein, Stefan Gerlich, Markus Arndt

Published Aug 19, 2022• 7 citations
Publisher

Abstract

High-mass matter-wave interferometry opens interesting perspectives for testing fundamental physics while posing intriguing challenges in experimental technologies. Here we explore the source and detection techniques that will be the basis for the next generation of quantum experiments with objects at least one order of magnitude more massive than the current state of the art. Magnetron sputtering provides a means to produce a continuous metal cluster beam with a large variability in material composition, structure, temperature, velocity, electric, and magnetic properties. The mass-scalable nanoparticles come with high ionization yield and a detected beam brilliance of up to ${10}^{11} {\mathrm{sr}}^{\ensuremath{-}1}$ ${\mathrm{s}}^{\ensuremath{-}1}$ clusters. We find favorably slow and narrow velocity distributions for mass-filtered clusters up to 800 kDa, which can also be readily ionized, manipulated, and detected by 266 nm UV laser light. The source-detector combination that we demonstrate here fulfills the essential requirements for a next generation of nanoparticle interferometry.

DOI
10.1103/physreva.106.023312

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High-mass matter-wave interferometry opens interesting perspectives for testing fundamental physics while posing intriguing challenges in experimental technologies. Here we explore the source and detection techniques that will be the basis for the next generation of quantum experiments with objects at least one order of magnitude more massive than the current state of the art. Magnetron sputtering provides a means to produce a continuous metal cluster beam with a large variability in material composition, structure, temperature, velocity, electric, and magnetic properties. The mass-scalable nanoparticles come with high ionization yield and a detected beam brilliance of up to 1011sr\ensuremath−1{10}^{11} {\mathrm{sr}}^{\ensuremath{-}1}1011sr\ensuremath−1 s\ensuremath−1{\mathrm{s}}^{\ensuremath{-}1}s\ensuremath−1 clusters. We find favorably slow and narrow velocity distributions for mass-filtered clusters up to 800 kDa, which can also be readily ionized, manipulated, and detected by 266 nm UV laser light. The source-detector combination that we demonstrate here fulfills the essential requirements for a next generation of nanoparticle interferometry.
We discuss recent advances towards matter-wave interference experiments with free beams of metallic and dielectric nanoparticles. They require a brilliant source, an efficient detection scheme and a coherent method to divide the de Broglie waves associated with these clusters: We describe an approach based on a magnetron sputtering source which ejects an intense cluster beam with a wide mass dispersion but a small velocity spread of Δv/v < 10%. The source is universal as it can be used with all conducting and many semiconducting or even insulating materials. Here we focus on metals and dielectrics with a low work function of the bulk and thus a low cluster ionization energy. This allows us to realize photoionization gratings as coherent matter-wave beam splitters and also to realize an efficient ionization detection scheme. These new methods are now combined in an upgraded Talbot-Lau interferometer with three 266 nm depletion gratings. We here describe the experimental boundary conditions and how to realize them in the lab. This next generation of near-field interferometers shall allow us to soon push the limits of matter-wave interference to masses up to 106 amu.
Abstract The quantum superposition principle is a fundamental concept of physics 1 and the basis of numerous quantum technologies 2,3 . Yet, it is still often regarded counterintuitive because we do not observe its key features on the macroscopic scales of our daily lives. It is, therefore, interesting to ask how quantum properties persist or change as we increase the size and complexity of objects 4 . A model test for this question can be realized by matter-wave interferometry, in which the motion of individual massive particles becomes delocalized and needs to be described by a wave function that spans regions far larger than the particle itself 5 . Over the years, this has been explored with a series of objects of increasing mass and complexity 6–9 and a growing community aims at pushing this to ever larger limits. Here we present an experimental platform that extends matter-wave interference to large metal clusters, a qualitatively new material class for quantum experiments. We specifically demonstrate quantum interference of sodium nanoparticles, which can each contain more than 7,000 atoms at masses greater than 170,000 Da. They propagate in a Schrödinger cat state with a macroscopicity 10 of μ = 15.5, surpassing previous experiments 5,9,11 by an order of magnitude.
The wave-particle duality is a key concept of quantum physics and extending quantum superpositions to bodies of increasing mass and complexity has become a key goal to numerous research groups. Such experiments are motivated by the desire to probe and better understand the quantum-classical interface in advanced tests of spontaneous or gravity-induced wave function collapse and in the search for non-linear modifications of quantum theory. High mass interferometry has been proposed to serve in the search for light dark matter via weak recoil decoherence as well as in tests of the weak equivalence principle by comparing a large variety of quantum systems with very distinct internal composition, excitation, mass, shape or angular momentum. Here we discuss the experimental state of the art in high-mass matter-wave interferometry, as well as new perspectives from cluster and nanoparticle physics to push quantum superpositions by 3 to 4 orders of magnitude in mass or up to 6 orders of magnitude in 'macroscopicity'. We discuss different near-field interferometer configurations that shall become useful in near-future demonstrations of quantum phenomena with objects as diverse as proteins, metal clusters and dielectric nanoparticles.
Matter-wave interferometry has become a ubiquitous and reliable tool for studying fundamental forces and phenomena and for realizing sensitive measurements. Our recent experiments with metal clusters have allowed us to demonstrate the quantum-wave nature of nanoparticles exceeding 170 kDa. Interestingly, the same experiments also allow for the preparation of moiré fringes of objects in the megadalton range. In this work, we demonstrate such moiré fringes of large sodium nanoparticles and, owing to the platform's stability, resolve nanometer-scale fringe displacements between successive contrast revivals, directly observing the predicted π-phase flip as a half-period shift. We further discuss how this feature turns the interferometer into a device for advanced sensing, even when operating in the regime where the predictions of quantum and classical physics merge.
Creating quantum superposition states of bodies with increasing mass and complexity is an exciting and important challenge. Demonstrating such superpositions is vital for understanding how classical observations arise from the underlying quantum physics. Here, we discuss how recent progress in macromolecule interferometry can be combined with the state of the art in cluster physics to push the mass record for matter-wave interference with wide state separation by 3 to 4 orders of magnitude. We show how near-field interferometers in different configurations can achieve this goal for a wide range of particle materials with strongly varying properties. This universality will become important in advanced tests of wave function collapse and of other modifications of quantum mechanics, as well as in the search for light dark matter and in tests of gravity with composite quantum systems.
Quantum superposition is central to quantum theory but challenges our concepts of reality and spacetime when applied to macroscopic objects like Schrödinger's cat. For that reason, it has been a long-standing question whether quantum physics remains valid unmodified even for truly macroscopic objects. By now, the predictions of quantum theory have been confirmed via matter-wave interferometry for massive objects up to 104 10^4\,104 atomic mass units (amu). The rapid development of new technologies promises to soon allow tests of quantum theory for significantly higher test masses by using novel techniques of quantum optomechanics and high-mass matter-wave interferometry. Such experiments may yield novel insights into the foundations of quantum theory, pose stringent limits on alternative theoretical models or even uncover deviations from quantum physics. However, performing experiments of this type on Earth may soon face principal limitations due to requirements of long times of flight, ultra-low vibrations, and extremely high vacuum. Here, we present a short overview of recent developments towards the implementation of the proposed space-mission MAQRO, which promises to overcome those limitations and to perform matter-wave interferometry in a parameter regime orders of magnitude beyond state-of-the-art.