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structure that repeats in time, as well as space; a kind of non-equilibrium matter
A time crystal or space-time crystal is a state of matter that repeats in time, as well as in space. Normal three-dimensional crystals have a repeating pattern in space, but remain unchanged as time passes. Time crystals repeat themselves in time as well, leading the crystal to change from moment to moment.
If a discrete time translation symmetry is broken (which may be realized in periodically driven systems), then the system is referred to as a discrete time crystal. A discrete time crystal never reaches thermal equilibrium, as it is a type of non-equilibrium matter, a form of matter proposed in 2012, and first observed in 2017.
The idea of a quantized time crystal was first described by Nobel laureateFrank Wilczek in 2012. In 2014 Krzysztof Sacha predicted the behavior of discrete time crystals in a periodically-driven many-body system and in 2016, Norman Yao et al. proposed a different way to create discrete time crystals in spin systems. From there, Christopher Monroe and Mikhail Lukin independently confirmed this in their labs. Both experiments were published in Nature in 2017. In 2019 it was theoretically proven that a quantum time crystal can be realized in isolated systems with long range multi-particle interactions.
In response to Wilczek and Zhang, Patrick Bruno, a theorist at the European Synchrotron Radiation Facility in Grenoble, France, published several articles in 2013 claiming to show that space-time crystals were impossible. Also later Masaki Oshikawa from the University of Tokyo showed that time crystals would be impossible at their ground state; moreover, he implied that any matter cannot exist in non-equilibrium in its ground state.
Several realizations of time crystals, which avoid the equilibrium no-go arguments, were later proposed. Krzysztof Sacha at Jagiellonian University in Krakow predicted the behaviour of discrete time crystals in a periodically driven system of ultracold atoms. Later works suggested that periodically driven quantum spin systems could show similar behaviour.
In 2019 physicists Valerii Kozin and Oleksandr Kyriienko proved that, in theory, a permanent quantum time crystal can exist as an isolated system, if the system contains unusual long-range multiparticle interactions. The original "no-go" argument only holds in the presence of typical short-range fields that decay as quickly as r-α for some α>0. Kozin and Kyriienko instead analyzed a spin-1/2 many-body Hamiltonian with long-range multispin interactions, and showed it broke continuous time-translational symmetry. Certain spin correlations in the system oscillate in time, despite the system being closed and in a ground energy state. However, demonstrating such a system in practice might be prohibitively difficult, and concerns about the physicality of the long-range nature of the model have been raised.
Time translation symmetry
Symmetries in nature lead directly to conservation laws, something which is precisely formulated by the Noether theorem.
The basic idea of time-translation symmetry is that a translation in time has no effect on physical laws, i.e. that the laws of nature that apply today were the same in the past and will be the same in the future. This symmetry implies the conservation of energy.
Broken symmetry in normal crystals
Normal process (N-process) and Umklapp process (U-process). While the N-process conserves total phonon momentum, the U-process changes phonon momentum.
Normal crystals exhibit broken translation symmetry: they have repeated patterns in space and are not invariant under arbitrary translations or rotations. The laws of physics are unchanged by arbitrary translations and rotations. However, if we hold fixed the atoms of a crystal, the dynamics of an electron or other particle in the crystal depend on how it moves relative to the crystal, and particle momentum can change by interacting with the atoms of a crystal -- for example in Umklapp processes.Quasimomentum, however, is conserved in a perfect crystal.
Time crystals show a broken symmetry analogous to a discrete space-translation symmetry breaking. For example, the molecules of a liquid freezing on the surface of a crystal can align with the molecules of the crystal, but with a pattern less symmetric than the crystal: it breaks the initial symmetry. This broken symmetry exhibits three important characteristics:
the system has a lower symmetry than the underlying arrangement of the crystal,
the system exhibits spatial and temporal long-range order (unlike a local and intermittent order in a liquid near the surface of a crystal),
it is the result of interactions between the constituents of the system, which align themselves relative to each other.
Broken symmetry in discrete time crystals
Time crystals seem to break time-translation symmetry and have repeated patterns in time even if the laws of the system are invariant by translation of time. Actually, studied time crystals show discrete time-translation symmetry breaking: they are periodically driven systems oscillating at a fraction of the frequency of the driving force. The initial symmetry is already a discrete time-translation symmetry (), not a continuous one (), which are instead described by magnetic space groups.
However, Floquet time crystals are unique in that they follow a strict definition of discrete time-translation symmetry breaking:
it is a broken symmetry – the system shows oscillations with a period longer than the driving force,
the system is in crypto-equilibrium – these oscillations generate no entropy, and a time-dependant frame can be found in which the system is indistinguishable from an equilibrium when measured stroboscopically (which is not the case of convection cells, oscillating chemical reactions and aerodynamic flutter),
the system exhibits long-range order – the oscillations are in phase (synchronized) over arbitrarily long distances and time.
Moreover, the broken symmetry in time crystals is the result of many-body interactions: the order is the consequence of a collective process, just like in spatial crystals. This is not the case for NMR spin echos.
Fields or particles may change their energy by interacting with a time crystal, just as they can change their momentum by interacting with a spatial crystal.
These characteristics makes time crystals analogous to spatial crystals as described above.
Time crystals do not violate the laws of thermodynamics: energy in the overall system is conserved, such a crystal does not spontaneously convert thermal energy into mechanical work, and it cannot serve as a perpetual store of work. But it may change perpetually in a fixed pattern in time for as long as the system can be maintained. They possess "motion without energy"--their apparent motion does not represent conventional kinetic energy.
It has been proven that a time crystal cannot exist in thermal equilibrium. Recent years have seen more studies of non-equilibrium quantum fluctuations.
In October 2016, Christopher Monroe at the University of Maryland claimed to have created the world's first discrete time crystal. Using the idea from Yao's proposal, his team trapped a chain of 171Yb+ ions in a Paul trap, confined by radio-frequency electromagnetic fields. One of the two spin states was selected by a pair of laser beams. The lasers were pulsed, with the shape of the pulse controlled by an acousto-optic modulator, using the Tukey window to avoid too much energy at the wrong optical frequency. The hyperfine electron states in that setup, 2S1/2 and , have very close energy levels, separated by 12.642831 GHz. Ten Doppler-cooled ions were placed in a line 0.025 mm long and coupled together.
The researchers observed a subharmonic oscillation of the drive. The experiment showed "rigidity" of the time crystal, where the oscillation frequency remained unchanged even when the time crystal was perturbed, and that it gained a frequency of its own and vibrated according to it (rather than only the frequency of the drive). However, once the perturbation or frequency of vibration grew too strong, the time crystal "melted" and lost this subharmonic oscillation, and it returned to the same state as before where it moved only with the induced frequency.
Later in 2016, Mikhail Lukin at Harvard also reported the creation of a driven time crystal. His group used a diamond crystal doped with a high concentration of nitrogen-vacancy centers, which have strong dipole-dipole coupling and relatively long-lived spin coherence. This strongly interacting dipolar spin system was driven with microwave fields, and the ensemble spin state was determined with an optical (laser) field. It was observed that the spin polarization evolved at half the frequency of the microwave drive. The oscillations persisted for over 100 cycles. This subharmonic response to the drive frequency is seen as a signature of time-crystalline order.
On August 17, 2020 Nature Materials published a letter from Aalto University saying that for the first time they were able to observe interactions and the flow of constituent particles between two time crystals in a Helium-3superfluid cooled to within one ten thousandth of a degree from absolute zero (0.0001K or -273.15°C)
A similar idea called a choreographic crystal has been proposed.
By relaxing additional restrictions on the definition of time crystals, continuous time-translation symmetry breaking can be achieved in exceptional cases. For instance, if one allows the system to be open to an environment, but undriven, many-body systems with the appropriate algebraic structure can be time crystals . Likewise, if one drops the requirement of long-range order in space, purely time-translation symmetry breaking is possible .