CTQM Seminar "When quantum-information scrambling met quasiprobabilities"

Nicole Yunger Halpern, Harvard-Smithsonian ATAMP & Harvard University

Abstract: Two topics have been gaining momentum in distinct subfields of physics: At the intersection of quantum information, condensed matter, and high-energy physics lies the out-of-time-ordered correlator (OTOC). The OTOC reflects quantum many-body thermalization; chaos; and scrambling, the spread of quantum information through many-body entanglement. In quantum optics and foundations, quasiprobabilities resemble probabilities but can become negative and nonreal. Such nonclassical values can signal nonclassical physics, such as the capacity for superclassical computation. I unite these two topics, showing that the OTOC equals an average over a quasiprobability distribution. The distribution, a set of numbers, contains more information than the OTOC, one number that follows from coarse-graining over the distribution. Aside from providing insight into the OTOC’s fundamental nature, the OTOC quasiprobability has several applications: Experimentally, the quasiprobability points to a scheme for measuring the OTOC (via weak measurements, which refrain from disturbing the measured system much). The quasiprobability also signals false positives in attempts to measure scrambling of open systems. Theoretically, the quasiprobability links the OTOC to uncertainty relations, to nonequilibrium statistical mechanics, and more strongly to chaos.

Friday, January 25, 2019 at 12:00pm

Duane Physics and Astrophysics, G126
2000 Colorado Avenue, Boulder, CO 80309

Event Type


College, School & Unit

Arts & Sciences

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