TDespite all advances, the existing quantum computers are usually not yet of use for practical applications. One of the reasons for this is that the arithmetic operations cannot yet be adequately controlled and varied. “Before quantum computers become truly universal calculating machines, they can serve as an experimental platform to investigate fundamental questions of quantum dynamics,” predicts the solid-state physicist Roderich Moessner, Director at the Max Planck Institute for the Physics of Complex Systems in Dresden. Now an international research group, which Moessner also belongs to, such an experiment succeeded. Like the scientists in the specialist magazine Nature messages, they programmed the Sycamore quantum computer from Google in such a way that its elementary arithmetic units, the quantum bits, jumped back and forth between different quantum states in an orderly manner. That sounds banal at first. The amazing thing about it, however, was that this periodic movement consumed almost no energy – and ideally would have lasted forever. Physicists call this strange phenomenon a time crystal.
For a long time a time crystal seemed to be an impossibility. Because periodic movements that persist indefinitely without a regular external impulse cannot really exist. A swing, for example, has to be bumped repeatedly in order for it to swing. Even an ideal, frictionless pendulum only moves infinitely long when viewed as a one-dimensional system. In reality, however, it is a many-particle system in which the kinetic energy is gradually distributed over the internal degrees of freedom of its molecules. So the pendulum would warm up over time and swing slower and slower. This even distribution of energy over all degrees of freedom follows from the second law of thermodynamics, which says that entropy – a measure of disorder – always increases. For a long time, physicists believed that time crystals would violate this physical law.
Time crystal oscillates in a diamond
The phenomenon of a time crystal was first described by the physicist and Nobel Prize winner Frank Wilczek from Massachusetts Institute of Technology in 2012. “It’s like a crystal, but with regard to time,” Wilczek is said to have explained to his wife about the idea. She then suggested the catchy name “Time Crystals”, says Wilczek. An essential property of a conventional crystal – a diamond, an ice or salt crystal, for example – is the regular lattice structure of the atoms, ions or molecules that make it up. It is similar in time crystals: Here the structures are not repeated in space, but in time. Wilzcek argued much more theoretically: Crystals break the symmetry of space. Let’s take ice cream. When water freezes, a small fluctuation at the beginning decides where the first molecule is located. From now on, the molecules no longer have a free choice of place because they have a fixed position in the crystal lattice. The rule that any point in space is equally available is broken – and with it the so-called translational symmetry of space.
Similarly, a symmetry breaking could exist in time, Wilczek suspected. Because despite the obvious differences between the fourth dimension and the three spatial dimensions, they have a few things in common: The four dimensions are relative, they can be compressed and expanded. So why shouldn’t there also be a system, according to Wilczek, in which not all points in time are “equal”? I.In 2016, Roderich Moessner and three other researchers found a physical system in which Wilczek’s ideas are evidently realized: in a so-called many-body localization. Here the individual particles of the collective are to a certain extent trapped in a strongly fissured energy landscape, so that the system never reaches thermodynamic equilibrium. Instead, in the state of lowest energy, it periodically oscillates back and forth between different configurations. It is in an excited but at the same time stable state – in a time crystal. This conclusion was reached by chance, says Moessner.