Time crystals, as proposed by Frank Wilczek in 2012, are temporal analogs of conventional space crystals . Just as conventional crystals require the breaking of space translation symmetry, time crystals require the breaking of time translation symmetry (see Viewpoint: Crystals of Time). These exotic, dynamical phases of matter have been realized on various experimental platforms, but in all cases, the time crystal phases have been accommodated by closed systems that are subject to coherent manipulations (see Viewpoint: How to Create a Time Crystal) . Hans Keßler from the University of Hamburg, Germany, and his colleagues have now reported the first observation of time-crystalline behaviors in an open quantum system .
Time crystals can break continuous symmetry when realized in a time-independent (energy-conserving) system and break discrete time translation when realized in a periodically driven (Floquet) system. The former, as conceived by Wilczek, was proven to be impossible to achieve in ground states or thermal-equilibrium states of short-range interacting systems . But the latter, in which the constituents adopt a recurring spatial configuration with a period that is a multiple (typically double) of the driving period, has been demonstrated in some closed spin systems with strong disorder and interactions . To distinguish these “discrete time crystals” from other dynamical phenomena such as Rabi oscillations, this period multiplication must exhibit “rigidity”; that is, it must be robust against small perturbations in system parameters or driving protocols. A widely used recipe to create discrete time crystals with the necessary rigidity is to drive a symmetry-broken system to switch from one symmetry-broken state to another per driving period.
All experimental demonstrations of discrete time crystals so far have used closed systems, leaving open the question of whether they can be achieved in the presence of dissipation and decoherence. This fundamental question has practical importance, since real systems can never be completely isolated from their surroundings. While dissipation generally destroys a time crystal’s order, there are situations where the order is retained if the system-environment coupling can be tailored appropriately. In quantum computing and quantum-state engineering, such tailoring already allows dissipation to be harnessed as a useful resource . Recent developments in experimental atomic, molecular, and optical physics have now made it possible to use this approach to realize novel dynamical order in open quantum systems .
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