Ginzton lab researchers help build new quantum matter for use in simulations
BY RACHEL TOMPA
Some quantum problems are too complex to be solved with numbers and computers. Scientists can use math to predict the behavior of single particles or pairs of particles with relative ease, but the behavior of multiple particles can cause them to throw up their hands in despair, computationally speaking. So researchers have to turn to other ways of making accurate predictions about the quantum behavior of a group of many particles. Now, researchers at Stanford have constructed a special kind of quantum matter that could be a steppingstone to simulate the previously unpredictable behavior of multiple-particle systems.
A team of researchers from the Ginzton Laboratory at Stanford and the National Institute of Informatics and the NTT basic research laboratory in Japan, led by Yoshihisa Yamamoto, Stanford professor of applied physics and electrical engineering, have succeeded in manipulating a state analogous to a Bose-Einstein condensate—a bizarre quantum phenomenon where particles fuse to form one "superparticle"—in ways previously thought to be impossible. Their results were published in the Nov. 22 issue of Nature.
The scientists hope that these manipulations will lead to better predictions of other murky quantum phenomena such as high-temperature superconductivity, a state where certain materials show zero electrical resistance at higher temperatures than can be explained by current knowledge. Physicists have proposed models to explain these phenomena, but "you cannot solve these models analytically," Yamamoto said.
Chih-Wei Lai, a visiting scholar in the Ginzton Lab and first author of the study, said that some predictions are just too hard to make without concrete physical examples. "There's no way you could predict the existence of life if you only knew about atoms," he said. "Similarly, we know the basics of how electrons move, but we can't predict whether a certain material will be a superconductor or not based only on these basic laws."
Much as engineers build small-scale models of complicated airplanes before tackling the real thing, the team of researchers constructed an artificial system that could lead to better understanding of complicated natural systems. By building on the quantum principles that certain particles at very high densities or very cold temperatures can form a supercohesive Bose-Einstein condensate, Yamamoto's group hoped to create a system with tunable parameters to simulate the environment where electrons move in a solid, thus gaining more insight into the function of natural many-particle systems.
The scientists fabricated a quantum double-decker sandwich, where multiple thin sheets of semiconductor material represent the cold cuts and tiny mirrors are the bread. Photons absorbed into the sandwich cause electrons to move, creating "quasiparticles" consisting of a dislocated electron and the hole it once occupied in the semiconductor sheet. Weak attractive forces between the homeless electron and its now empty home keep the pair in a semistable state.
Like star-crossed lovers, the electron and the hole can't stay away from each other for long and the quasiparticle eventually collapses back to its original state, releasing the originally absorbed photon. The photon bounces off the mirror and back into the semiconductor, creating a new electron-hole quasiparticle. This self-perpetuating loop creates a sort of quasi-quasiparticle, half electron-hole and half photon, termed a polariton. Because the only part of this polariton with any mass is the single electron, the entire particle is very light for its size, about one ten-thousandth of the free electron mass. The light mass of the airy polaritons leads to formation of a superfluid-like state at relatively high temperatures—about 100 Kelvin (about negative 279.67 F) versus 1 micro-Kelvin (negative 459.6699982 F, close to absolute zero) required for similar experiments done with gases.
The key next step in their experiment, and the one that Lai said some scientists were skeptical could be achieved, was to show that they could manipulate their "superfluid" quantum state in a solid. That is, that they could change some parameters of the system and read out corresponding changes in its behavior. This manipulation can simulate other quantum many-particle systems.
They loaded the polaritons onto an ordered array and found that a new state emerged. Specifically, some of the particles settled into a low-energy ground state, which the scientists expected based on results from previous superfluid studies, but other particles settled into a higher energy state. This result was unexpected, and Lai said it points to the importance of exploring states such as superfluidity in multiple systems, since it had not been observed in previous studies using superfluid gases.
Lai and Yamamoto think this artificial quantum simulation could potentially yield powerful predictions about physical phenomena related to Bose-Einstein condensation in general, but Lai stressed that many steps remain to reach that goal. But by changing the parameters of their "superfluid" state and observing the corresponding changes in the particles, they hope to learn more about how these quirky quantum states behave and eventually extend these observations to make predictions about other currently unexplained physical phenomena.
"That's the ultimate goal," Lai said. "Eventually we want to be able to predict not just many-particle systems but other quantum effects."
Funding for the study was provided by the JST/SORST program and by Special Coordination Funds for Promoting Science and Technology in Japan.
Also contributing to the research were Na Young Kim, Shoko Utsunomiya, Georgios Roumpos, Hui Deng, Michael Fraser, Tim Byrnes, Patrick Recher, Norio Kumada and Toshimasa Fujisawa.
Rachel Tompa is a science-writing intern at the Stanford News Service.
