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Massachusetts Institute of Technology
Physicists and engineers have long been interested in creating new forms of matter, which usually do not exist in nature. Such materials may one day be used in, for example, new computer chips. In addition to applications, they also reveal elusive insights about the basic workings of the universe. Recent work by MIT has created and characterized new quantum systems that exhibit dynamic symmetry—specific types of periodically repetitive behaviors, such as shapes that fold and reflect over time.
"We need to solve two problems," said Changhao Li, a graduate student in the laboratory of Paola Cappellaro, professor of nuclear science and engineering. Li recently published this work in "Physics Review Letters" with Cappellaro and graduate student Wang Guoqing. "The first question is that we need to design such a system. Second, how do we characterize it? How do we observe this symmetry?"
Specifically, the quantum system consists of diamond crystals about one millimeter in diameter. The crystal contains many defects caused by nitrogen atoms near the lattice gaps-so-called nitrogen vacancy centers. Just like electrons, each center has a quantum property called spin, with two discrete energy levels. Because the system is a quantum system, the spin can be found not only in one energy level, but also in a combination of two energy levels. Like Schrödinger’s theoretical cat, it can live and die at the same time.
The energy level of the system is defined by its Hamiltonian, and the researchers designed its periodic time dependence through microwave control. If the Hamiltonian of the system is not only the same after each time period t, but also the same after each t/2 or t/3, the system is said to have dynamic symmetry, such as folding a piece of paper in half or in half Three times so that no part protrudes. Georg Engelhardt, a postdoctoral fellow at the Beijing Institute of Computational Science, did not participate in this work, but his own theoretical work was used as a basis for comparing symmetry to guitar overtones, in which one string may vibrate at frequencies of 100 Hz and 50 Hz.
In order to induce and observe this dynamic symmetry, the MIT team first initialized the system with laser pulses. Then they directed microwave radiation of various selected frequencies at it and let it evolve, allowing it to absorb and emit energy. "The amazing thing is that when you add such a driver, it can show some very strange phenomena," Li said. "It will have some periodic vibrations." Finally, they fired another laser pulse at it and measured the fluorescent visible light it emits to measure its state. The measurement is just a snapshot, so they repeated the experiment many times and pieced together a flipbook describing its behavior over time.
"It is impressive that they can prove that they have this incredible control over quantum systems," Engelhart said. "Solving equations is easy, but it is very difficult to achieve this in experiments."
Crucially, the researchers observed that the dynamic symmetry of the Hamiltonian—the harmonics of the energy levels of the system—determines the possible transitions between one state and another. "The novelty of this work," Wang said, "is also that we have introduced a tool that can be used to characterize any quantum information platform, not just the nitrogen vacancy centers in diamonds. It has a wide range of applicability. Li pointed out that their technique is simpler than the previous method, which requires a constant laser pulse to drive and measure the periodic motion of the system.
An engineering application is in a quantum computer, that is, a system for manipulating qubits. These bits can be not only 0 or 1, but also a combination of 0 and 1. The spin of a diamond can encode a qubit in its two energy levels.
Qubits are subtle: they are easily broken down into simple bits, 1 or 0. Or the qubit may become the wrong combination of 0 and 1. "These tools for measuring dynamic symmetry," Engelhardt said, "can be used as a sanity check to see if your experiment has been adjusted correctly-and with very high accuracy." He noticed the problem of external disturbances in quantum computers. , He likened it to a detuned guitar. By adjusting the tension of the string-adjusting the microwave radiation-so that the harmonics meet some theoretical symmetry requirements, it is possible to ensure that the experiment is perfectly calibrated.
The MIT team has set its sights on the expansion of this work. "The next step is to apply our method to more complex systems and study more interesting physics," Li said. Their goal is more than two energy levels-three, 10 or more. With more energy levels, they can represent more qubits. "When you have more qubits, you have more complex symmetry," Li said. "You can use our method here to characterize them." Further explore how flawed diamonds "cause" a perfect quantum network. More information: Guoqing Wang et al. Observations on the selection rule of symmetry protection in periodic driving quantum systems, Physics Commentary Express (2021). DOI: 10.1103/PhysRevLett.127.140604 Journal information: Physics Review Letters
Provided by MIT
This story was reposted by MIT News (web.mit.edu/newsoffice/), a popular website covering MIT research, innovation, and teaching news.
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