Quantum time crystals open up
Quantum time crystals open up
复制标题
量子时间晶体打开
DOI：
10.1038/s41563-021-01090-4
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发表时间：
2021
期刊：
影响因子：
41.2
通讯作者：
P. Ball
中科院分区：
文献类型：
--
作者：
P. Ball
Time crystals are a temporal analogue of ordinary ‘space’ crystals: many-body systems that are periodic not in space but in time. They evolve dynamically in a way that regularly returns to a particular configuration. After these curious entities were proposed by Wilczek1 in 2012, the question became: what manner of system might behave this way? On closer inspection, it became clear that such behaviour is not possible for the equilibrium ground state of any system2,3 — time crystals are possible only in driven, non-equilibrium systems; for example, with some periodic driving force. But if the driving is periodic in the first place, isn’t periodic dynamics inevitable? Yes — but it qualifies as time-crystal behaviour when that periodicity is self-organized at a different, lower frequency than the driving (that is, subharmonic). This response — a characteristic of so-called discrete time crystals — seems possible in principle, but then there’s another problem. Such a driven system would steadily absorb energy, getting hotter without limit until its dynamics are eventually swamped by thermal motion. So a time crystal like this would not be stable indefinitely. There’s an escape clause for quantum systems. The heating can be avoided if a certain kind of disorder in the arrangement of the component parts induces an effect called many-body localization, which inhibits energy exchange between their energy levels, preventing the spread and equilibration of heat. This effect stabilized the discrete quantum time crystals observed previously4,5. There’s another option, though, which is familiar enough for classical systems: simple dissipation of the added energy by radiating it into the environment. This implies that the system must be open — able to exchange energy with its surroundings. Such behaviour has already been seen in a classical analogue of a quantum time crystal, composed of two coupled, clamped vibrating strings6. But will it work for truly quantum systems? Yes, in theory — and several possible implementations in quantum optics have been proposed7–10. But only now has a dissipative quantum discrete time crystal been demonstrated experimentally. Keßler et al. have realized this state in a Bose–Einstein condensate of around 65,000 rubidium atoms held inside an optical cavity and pumped periodically using an optical laser11. In the Bose–Einstein state, the atoms all occupy the same quantum state and behave coherently. The characteristic signature of the time crystal here is an oscillation of ‘super-radiant’ light emission — a collective, coherent and laser-like emissive mode — at a subharmonic frequency. This behaviour appears above a critical intensity threshold of the pump laser, when the Bose–Einstein condensate oscillates subharmonically between two alternative ordered states in which the atoms occupy lattices like the black and white squares of a checkerboard. A characteristic of time-crystal behaviour is that this oscillation, which is intrinsic to the internal many-body dynamics, is stable against noise and perturbations in the driving force — just as Keßler et al. report. In this system, dissipation alone is the only defense against heating of the condensate. But although that seems sufficient to maintain the time crystal, it nevertheless decays on a timescale of milliseconds, because collisions between the rubidium atoms lead to their gradual ejection from the cavity. To hold onto this mercurial state of matter for longer, the researchers show that the atoms will need to feel one another’s presence less keenly. ❐
