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Macroscopic Quantum Systems

Paraphrasing a famous comment by Richard Feynman, one could say that the only mystery of quantum mechanics is the phenomenon of entanglement, and the mystery is deepest when one considers entanglement in many-body or macroscopic systems.

Entanglement is a mind-boggling feature of the superposition of quantum states. By creating inextricable links between the states of physical systems, entanglement allows us to perform tasks that would be impossible classically such as teleportation, quantum dense coding and, most impressive of all, quantum computation.

Scientists first explored entanglement in simple systems, such as between pairs of particles or photons. However, our macroscopic world is made of objects containing vast numbers of particles, i.e. many-body systems. That makes research on many-body systems interesting for reasons ranging from the purely scientific desire to understand their rich physical behaviour to the need to answer practical questions encountered when developing new materials and technologies.

Although many-body systems have been extensively studied in condensed matter physics, thermodynamics and materials science, our ability to model many-body systems, through techniques such as matrix product states is connected with the amount of entanglement they have. As a result, there are many unexplained phenomena related to highly entangled many-body systems. One such unexplained phenomenon is high-temperature superconductivity. The unique perspective provided by quantum information may yield new insights into the mechanism behind this. Moreover, it is still unclear how entanglement relates to other key properties of large physical systems. We know that entanglement is necessary in quantum computations, but what role does it play in other many-body phenomena that are regularly observed in experiments, such as quantum phase transitions and Bose-Einstein condensation?

Studying these systems also provides insight into foundational questions relating to the nature of reality. Classical physics is a macroscopic description of the world around us, which must arise out of the microscopic theory, quantum mechanics, and yet these two, on the surface, appear incompatible. It is the nature of entanglement which separates them. Our recent studies have started to show that it is also entanglement that is responsible for resolving the contradictions, demonstrating how the classical phenomena that we are familiar with at large scales emerge from a microscopically entangled quantum system.

There are hints that entanglement can play a role in many more aspects of many-body systems behaviour, and researchers at CQT are involved in the search for rigorous evidence.