Physicists have created a new form of state of matter, which may be the key to the development of new quantum machines.
Harvard University
Translation Yuan Zhujun (Peking University)
Editing Ding Jiaqi
Prof. Zhou Hengyi (Ph.D. student and writer of Harvard University Physics)
This view of symmetry breaking makes us have a profound understanding of the structure of the crystal. We can even use the lower symmetry of crystals to make a complete classification of all possible crystal forms. For example, in the three-dimensional space, we have only 230 different crystal forms.
Liquids and crystals have high symmetry in time, and they do not change at any time during time shift. In other words, both liquids and crystals are stable over time. Then do we have a matter of time, which is the translation of the symmetry-breaking matter state in time? (The so-called time crystals are spontaneous oscillations in time.)
The answer is yes, and time crystals (spontaneous oscillations in time) are a very common phenomenon. After an old-fashioned pendulum clock and a clockwork, the pendulum will periodically oscillate by itself and produce spontaneous oscillations. This is a time crystal. In any electronic watch or smart phone, there is an electronic oscillator. The electronic oscillator is also a time crystal.
Why is this article calling time crystals new discoveries? In fact, this article is about a timeless crystal. Spontaneous oscillation is not a new phenomenon, and no fever is a new phenomenon. Many online articles introducing time crystals only emphasize the old phenomenon of spontaneous oscillation, but they do not (or rarely) emphasize the new phenomenon of no fever.
The so-called no heat, it does not consume energy. Usually, everyone thinks that any exercise will generate friction and it will heat up and consume energy. Therefore, a spontaneous oscillation that does not generate friction is indeed a surprising discovery. In a sense, a crystal with no fever can be seen as a spontaneous perpetual motion perpetual motion. In addition, no heat, that is, does not destroy the quantum coherence. This operation that does not destroy quantum coherence may have many uses in quantum computing.
This article describes the work of a Harvard research team. Several other research groups had similar findings.
- Wen Xiaogang
Ordinary quartz crystal (Source: pixabay)
The traditional crystal refers to the simple periodic arrangement of atoms on a three-dimensional grid. Salt, sugar, and even diamonds belong to this category. The time crystals are based on the spatial periodicity of the atoms and the periodicity of the fourth dimension. This means that under certain conditions, these atoms in the material will also have periodic structures in time.
The team led by Mikhail Lukin and Eugene Demler, professors of physics at Harvard University, created a quantum system made of a small piece of diamond with millions of atomic-scale impurities—nitrogen—vacancy. NV) color center. Then, they used a microwave pulse to force the system out of balance, causing the spins of the NV color centers to flip at very precise intervals - one of the key signs of time crystals. This work was published on the March 8 issue of Nature.
The author of the article, Lukin, said: "The importance of creating a crystal for time is not only to prove that a material that was previously theoretical only is achievable, but also because it opens up a tempting physicist. Window to help us study the behavior of non-equilibrium systems."
“Today, there is a lot of prospects for the physics of non-equilibrium quantum systems. There is a lot of work going on,†Lukin said. “This is an area that is related to a variety of quantum technologies, because a quantum computer basically It can be described as a quantum system that is far from equilibrium. This is very frontier research... and we have just touched the surface of some of the fur."
Understanding this non-equilibrium system will not only help researchers take the quantum computing path in the long run, but also create the technology behind the crystal may also bring many recent applications.
“We think that this technology may come in handy in one area. This is the field of precision measurement. This is one of the first motivations for our work,†Lukin said. "Later we discovered that using the spin of NV Color Center can make magnetic field detectors," he said. "So, the non-equilibrium substances we create are likely to come in handy."
However, in the beginning, many people suspected that such systems could not be manufactured at all. In fact, several scholars (Patrick Bruno, Haruki Watanabe, Masaki Oshikawa) have even proven that a quantum system cannot be time-crystal in equilibrium.
"Most things in our lives are balanced," Lukin explained. "This means that if you put a hot object and a cold object together, their temperature will eventually tend to be equal. But Not all systems are like this."
One of the most common non-equilibrium materials is what many people wear daily – diamonds. As a crystal form of carbon formed under extremely high temperature and pressure conditions, diamond is not so common because it is metastable. This means that once it forms this crystal form it will remain in this state even after removing the high temperature and pressure conditions.
Lukin said that until recently, researchers have come to realize that non-equilibrium systems, especially those that are “driving†(ie, systems that researchers can “drive†with periodic energy pulses), can demonstrate the characteristics of time crystals. . One of these qualities is that the crystal's response to disturbances is robust in time—that is, it has a tendency to resist disturbances.
"A solid crystal is hard, if you press it, maybe the distance between atoms has changed a little, but the crystal itself can survive," he said. "The idea of ​​time crystals is this order in the time domain. Sex, and must be robust to disturbances."
"Another important feature of Time Crystal is that, usually, if you force a system to stay away from equilibrium, it will begin to heat up, but such systems can resist this heat," Lukin added. "It turns out that time crystal effects This idea is very close: the system is excited, but it does not absorb energy."
In order to prepare such a system, Lukin and his colleagues started with a small piece of diamond that has a very large number of NV color centers embedded so that it does not look transparent but black.
"We put this diamond under the microwave pulse, which can change the direction of NV color center spin," Lukin explained. "This can affect almost all spin up, let them turn down, then the next The pulses then make them all face up.†To test the robustness of the system, Lukin and his colleagues changed the timing of the pulse to see if the material could keep the time-crystal-like response.
"If you can't turn all the spins up and down all at once, the system will soon become completely random," Lukin said. "But, the interaction between the NV color centers stabilizes the system's response: they force the system to The same cycle response, which is the behavior of the time crystal."
Such systems may ultimately play a decisive role in the development of quantum computers and quantum detectors, Lukin said, because they allow us to see the two decisive qualities of longer quantum memory time and very high qubit density, Can coexist. "In many applications, we all want to have both of the above properties at the same time," he said. "But the requirements of these two natures are usually contradictory... This is a very common problem. The current work shows that We can accomplish both at the same time. Although there is still a lot of work to be done, we believe that these effects may help us create a new generation of quantum detectors, and in the long run, this may also bring other applications. , such as atomic clocks, etc."
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