Appreciating the classical elegance of time crystals | 7/31/2019 | Staff
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Structures known as time crystals, which repeat in time the way conventional crystals repeat in space, have recently captured the interest and imagination of researchers across disciplines. The concept has emerged from the context of quantum many-body systems, but ETH physicists have now developed a versatile framework that clarifies connections to classical works dating back nearly two centuries, thus providing a unifying platform to explore seemingly dissimilar phenomena.

In a crystal, atoms are highly ordered, occupying well-defined locations that form spatial patterns. Seven years ago, the 2004 Physics Nobel laureate Frank Wilczek pondered the possibility of a time analogue of crystalline spatial order—systems that display sustained periodic temporal modulations in their lowest-energy state. The concept of such structures with an oscillating ground state is highly intriguing. Alas, not long after the idea was published, it was proven that such time crystals are not possible without breaking fundamental laws of physics. However, subsequent theory work suggested that when quantum many-body systems are periodically driven, new persistent time correlations emerge that are evocative of Wilczek's time crystals. These driven systems were dubbed discrete time crystals, and in 2017, the first experimental realizations of such states were reported in ensembles of coupled particles (ions, electrons and nuclei) that display quantum-mechanical properties.

Observers - Similarities - Time - Crystals - Systems

Before long, astute observers spotted distinct similarities between discrete time crystals in quantum systems and so-called parametric resonators, a concept in classical physics reaching back to work by Michael Faraday in 1831. The connection between these two bodies of work remained, however, opaque. Now, theorists have developed a new framework goes a long way toward lifting the ambiguities surrounding the similarities between periodically driven classical and quantum systems.

Writing in an article published today in the journal Physical Review Letters, Toni Heugel, a Ph.D. student in the Department of Physics at ETH Zurich, and Matthias...
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