Nagaoka ferromagnetism observed in a quantum dot plaquette

Abstract

Engineered, highly-controllable quantum systems hold promise as simulators of emergent physics beyond the capabilities of classical computers [1]. An important problem in many-body physics is itinerant magnetism, which originates purely from long-range interactions of free electrons and whose existence in real systems has been subject to debate for decades [2,3]. Here we use a quantum simulator consisting of a four-site square plaquette of quantum dots [4] to demonstrate Nagaoka ferromagnetism [5]. This form of itinerant magnetism has been rigorously studied theoretically [6][7][8][9] but has remained unattainable in experiment. We load the plaquette with three electrons and demonstrate the predicted emergence of spontaneous ferromagnetic correlations through pairwise measurements of spin. We find the ferromagnetic ground state is remarkably robust to engineered disorder in the on-site potentials and can induce a transition to the low-spin state by changing the plaquette topology to an open chain. This demonstration of Nagaoka ferromagnetism highlights that quantum simulators can be used to study physical phenomena that have not yet been observed in any system before. The work also constitutes an important step towards large-scale quantum dot simulators of correlated electron systems.The potential impact of discovering and understanding exotic forms of magnetism and superconductivity is one of the largest motivations for research in condensedmatter physics. These quantum mechanically governed effects result from the strong correlations that arise between interacting electrons. Modelling and simulating such systems can in some instances only be achieved through the use of engineered, controllable systems that operate in the quantum regime [1]. Efforts to build quantum simulators have already demonstrated great promise at this early stage [10], mainly led by the ultracold atom * These authors contributed equally to this work. † e-mail: l.m.k.vandersypen@tudelft.nl community [11-17]. More broadly, quantum simulations of many-body fermionic systems have been carried out in a range of experimental systems such as quantum dot lattices [18], dopant atoms [19], superconducting circuits [20] and trapped ions [21]. Electrostatically defined semiconductor quantum dots [22][23][24] have been proposed as excellent candidates for quantum simulations [25][26][27]. Their ability to reach thermal energies far below the hopping and on-site interaction energies enable access to previously unexplored material phases. Quantum dot systems have already achieved success in realising simulations of Mott-insulator physics in linear arrays [28]. Additionally, the feasibility to extend these systems into 2D lattices has recently been demonstrated [4,[29][30][31][32], including the ability to perform measurements of spin correlations [4]. As a result, quantum dot systems are now prime candidates for exploring how superconductivity and magnetism emerge in strongly-correlated electron systems [33][34][35].The emergence of magnetism in purely i...

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