BQ2: What principles govern strongly interacting matter?

Many quantum systems in nature are composed of strongly interacting constituents.  Duke physicists develop novel mathematical and experimental techniques to prepare, probe, and theoretically study strongly interacting quantum systems, in order to uncover the principles that govern such phenomena.   Examples include nucleons, atomic nuclei, ultra-cold atoms, quantum magnets, topological systems, and nanomaterials; exotic phases of matter such as the quark-gluon plasma, high-temperature superconductors, and quantum spin liquids; and molecules forced into nonclassical spin states. What is common among all these systems is that while they are described by the same basic equations of quantum mechanics, the strong interactions between their constituent particles result in the emergence of unexpected behaviors and phenomena. Traditional physical intuition is usually based around localized effects (treating one electron or atom at a time); in contrast, strongly coupled systems exhibit collective phenomena which are exceptionally complex.

Strong coupling-based phenomena often force modifications of traditional concepts and lead to the discovery of new principles. This has already significantly advanced material design and synthesis, while pushing the boundaries of experimental techniques and laying the groundwork for future technological and industrial innovations. Such systems also pose new computational challenges since naively performing a calculation without approximations requires exponentially large resources.

Recent research also points to surprising connections to other fields. For example, dual descriptions of physical systems, in which non-perturbative phenomena in one formulation are captured by weak coupling physics in the other, have been found in a variety of quantum field theories and in string theory models, mainly in supersymmetric models.  Investigating these duality relations is a promising way to develop our understanding of strongly coupled systems, as well as our understanding of quantum gravity.  In fact, although traditionally it was believed that quantum mechanics and classical gravity are at odds with each other, recent work is beginning to reveal that the two fields may not be that different in the presence of strong interactions. Using this connection between traditionally two different fields it was recently discovered that strongly interacting quantum systems can form perfect fluids with minimal viscosity. The quark gluon plasma, created in experiments by colliding large nuclei is believed to be such a perfect fluid. Similarly, new paradigms are being proposed in order to understand the properties of materials such as high Tc superconductors, quantum magnets and topological insulators. A common thread behind these new proposals is the notion that quantum mechanics can create collective phenomena due to the feature referred to as quantum entanglement, which includes exotic phenomena such as fractional quantum numbers and emergent topological excitations. In most systems quantum entanglement can be destroyed easily through thermal effects; an interesting exception is para-hydrogen, stable for months at room temperature and an interesting source of spin order. More generally, though, to study the exotic features of entanglement experimentalists are trying to constructs microscopic quantum units called qubits and create entanglement among them. Quantum entanglement in many-body systems remains an exciting frontier of research across multiple fields, from condensed matter and materials science to particle physics and theories of fundamental forces.

Using high-energy electron microscopes and intense gamma-rays, we explore the emergence of the three-dimensional structure of the nucleon and nuclei from the underlying QCD dynamics. We are particularly interested in how these quantum systems with strongly interacting constituents interact while respecting the fundamental symmetries of nature. Theoretically, we investigate the properties of the quark-gluon plasma by developing new computational models, and we use effective field theories to understand hadrons, nuclei, and atomic systems. On the condensed matter physics side, we are also interested in understanding ground state and finite temperature properties of strongly correlated systems and materials that may be naturally occurring or experimentally designed. In particular, we strive to design, synthesize and characterize quantum materials to study theoretically predicted exotic states of matter such as quantum spin liquids. Quantum spin liquids are theoretical realizations of quantum entanglement over long distances that may explain the origin of high-temperature superconductivity. We also study the new features that arise when strongly interacting quantum systems evolve in time. To that end, experimentally we utilize state-of-the-art synthesis and crystal growth methods as well as advanced characterization techniques like neutron and x-ray scattering, while theoretically we use methods like quantum Monte Carlo, Density Matrix Renormalization Group and tensor network states.

There are many future opportunities in this field. The worldwide QCD community is preparing for the construction of an Electron Ion Collider (EIC) in the US. This machine will provide us with the ability to study nuclei with a resolution that makes the ‘sea’ of quantum fluctuations and gluon interactions visible, thus allowing us to understand the origin of the most fundamental properties of visible matter such as mass and confinement into hadrons. Duke physicists are ideally positioned to take advantage of this once-in-a-generation project and playing leadership roles certain to provide transformational insight into the nature of visible matter. Technological advances in laser Compton gamma-ray beam sources made during the last two decades are providing researchers with beam capabilities that enable unprecedented high precision measurements of dipole excitations in nuclei and of nucleon properties associated with the collective responses of its constituents. Recent advances in accelerator and laser technologies make possible a new generation of laser Compton gamma-ray sources with orders of magnitude higher beam intensities and lower beam spread than available at current facilities. The next generation gamma-ray beam facilities will create new research opportunities for exploring the QCD foundations of the collective spin responses of nucleons, of charge-symmetry breaking in the strong interaction, and of weak interactions in hadronic systems. The Duke nuclear physics experiment and theory groups are positioned to lead in the development of the next generation laser Compton gamma-ray source and its application to studying nucleons at the confinement distance scale.

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