Bei-Lok HU, University of Maryland

Topic: "Understanding Macroscopic Quantum Phenomena*"  (Video)

Abstract: Macroscopic quantum phenomena refer to quantum features in objects of `large' sizes, systems with many components or degrees of freedom, organized in ways where they can be identified as `macroscopic’ objects. This emerging field is ushered in by several categories of definitive experiments in superconductivity, electromechanical systems, Bose-Einstein condensates and others. Yet this new field which is rich in open issues at the foundation of quantum and statistical physics remains little explored theoretically (with the notable exception of Leggett [0]). This talk summarizes our thoughts while attempting a systematic investigation into some key foundational issues of quantum macroscopic phenomena with the goal of ultimately revealing or building a viable theoretical framework.

Several key issues in the consideration of macro on one hand and quantum on the other are explored in three recent essays: the large N expansion [1], the correlation hierarchy [2] and quantum entanglement [3]. We give a sketch of the first two themes and then discuss the third. A) For macro, recognition that there exist many levels of structure in a composite body and only by judicious choice of an appropriate set of collective variables can one give the best description of the dynamics of a specific level of structure. Capturing the quantum features of a macroscopic object is greatly facilitated by the existence and functioning of these collective variables; B) For quantum, we can use entanglement as a measure of quantumness. Quantum entanglement, an exclusively quantum feature, is known to persist to high temperatures and large scales under certain conditions, and may actually decrease with increased connectivity in a quantum network. We mention these somewhat counter-intuitive examples to show that there are blind spots worthy of our attention and issues beyond what meet our eyes. Our purpose is to try to remove the stigma that quantum only pertains to micro, in order to make way for deeper probes into the conditions whereby quantum features of macroscopic systems may manifest.

* Based on [3][4] below. References in [1][2] have earlier work and reviews on MQP experiments.
[0] A J Leggett, Testing the limits of quantum mechanics: motivation, state of play, prospects. J. Phys.: Condens. Matter 14 (2002) R415-R451
[1] C H Chou, B L Hu, Y Subasi, Macroscopic quantum phenomena from the large N perspective.
J. Phys.: Conf. Ser. 306, 012002 (2011) [arXiv:1106.0556]
[2] C H Chou, B L Hu, Y Subasi, Macroscopic Quantum Phenomena from the Correlation, Coupling and Criticality Perspectives, J. Phys.: Conf. Ser. 330, 012003 (2011) [arXiv:1107.3008]
[3] B L Hu, Y Subasi, Pathways toward understanding Macroscopic Quantum Phenomena, [arXiv:1304.7839] Proceedings of DICE 2012 meeting in J. Physics (Conf. Series) 442, 012010 (2013)
[4]C H Chou, B L Hu, Y Subasi, Macroscopic Quantum Phenomena from the Coupling Pattern and Entanglement Structure Perspective [arXiv:1308.4225] submitted to Physica A.