Researchers take step toward new quantum simulators — ScienceDaily

Some of the most exciting topics in modern physics, such as high-temperature superconductors and some proposals for quantum computers, come down to the exotic things that happen when these systems drift between two quantum states.

Unfortunately, understanding what happens at these points, known as quantum critical points, has proven challenging. The math is often too hard to solve, and today’s computers aren’t always up to the task of simulating what’s going on, especially in systems with any appreciable number of atoms involved.

Now, researchers at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory and their colleagues have taken a step toward building an alternative approach, known as a quantum simulator. Although the new device for now only simulates interactions between two quantum objects, the researchers claim in a paper published on January 30 in Physics of nature that it could be increased relatively easily. If so, researchers could use it to simulate more complicated systems and begin to answer some of the most interesting questions in physics.

“We’re always building mathematical models that we hope capture the essence of the phenomena we’re interested in, but even if we believe they’re accurate, they’re often not solvable in a reasonable amount of time” with current methods, said David Goldhaber-Gordon, a Stanford physics professor. and researcher at the Stanford Institute for Materials and Energy Sciences (SIMES). Along the way toward the quantum simulator, he said, “we have these buttons that no one has had before.”

Islands in a sea of ​​electrons

The basic idea of ​​the quantum simulator, Goldhaber-Gordon said, is in some ways similar to a mechanical model of the solar system, where someone turns a crank and interconnected gears turn to represent the motion of the moon and planets. Such an “orrery” discovered in a more than 2,000-year-old shipwreck is thought to have produced quantitative predictions of eclipse times and planetary positions in the sky, and analog machines were used as late as the 20th century for mathematical calculations that were too difficult for the most advanced digital computers at the time.

Like the designers of a mechanical model of the solar system, researchers building quantum simulators must be sure that their simulators are reasonably well aligned with the mathematical models they are supposed to simulate.

For Goldhaber-Gordon and his colleagues, many of the systems of interest — systems with quantum critical points such as some superconductors — can be thought of as atoms of a single element arranged in a periodic lattice embedded in a container of mobile electrons. All lattice atoms in such a material are identical and all interact with each other and with the sea of ​​electrons that surround them.

To model such materials with a quantum simulator, the simulator should have reserves for lattice atoms that are nearly identical to each other, and they must interact strongly with each other and with the surrounding electron reservoir. The system should also be configurable in some way, so that experimenters can change various parameters of the experiment to gain insight into the simulation.

Most quantum simulation proposals don’t meet all of these requirements at once, said Winston Pouse, a graduate student in the Goldhaber-Gordon lab and first author. Physics of nature paper. “At a high level, there are ultracold atoms, where the atoms are completely identical, but implementing strong coupling to the reservoir is difficult. Then there are simulators based on quantum dots, where we can achieve strong coupling, but the sites are not identical,” Pouse said.

Goldhaber-Gordon said a possible solution emerged in the work of French physicist Frédéric Pierre, who studied nanoscale devices in which an island of metal is sandwiched between specially designed pools of electrons known as two-dimensional electron gases. A voltage-controlled gate regulated the flow of electrons between the pool and the metal island.

By studying the work of Pierre and his lab, Pouse, Goldhaber-Gordon and their colleagues realized that these devices could meet their criteria. The islands — stand-ins for lattice atoms — interacted strongly with the electron gases around them, and if Pierre’s single island expanded into a cluster of two or more islands, they would also interact strongly. Metal islands also have a significantly larger number of electronic states compared to other materials, which has the effect of averaging out all the significant differences between two different invisibly tiny blocks of the same metal — making them effectively identical. Finally, the system could be adjusted using electrical lines that controlled the voltages.

A simple simulator

The team also realized that by pairing Pierre’s metal islands, they could create a simple system that should display something like the quantum critical phenomenon they were interested in.

It turns out that one of the hardest parts is actually building the device. First, basic circuit outlines must be nanoscopically etched into semiconductors. Next, someone must deposit and melt a tiny blob of metal onto the underlying structure to create each metal island.

“They’re very difficult to make,” Pouse said of the devices. “It’s not a super clean process and it’s important to make good contact” between the metal and the underlying semiconductor.

Despite these difficulties, the team, whose work is part of a broader quantum science effort at Stanford and SLAC, was able to build a device with two metal islands and test how electrons move through it under different conditions. Their results match calculations that took weeks on a supercomputer — hinting that they may have found a way to investigate quantum critical phenomena much more efficiently than before.

“Although we have not yet built a multipurpose programmable quantum computer with enough power to solve all the open problems in physics,” said Andrew Mitchell, a theoretical physicist at the University of Dublin’s Center for Quantum Engineering, Science and Technology (C-QuEST) and a co-author on the paper, “we can now create custom analog devices with quantum components that can solve specific problems in quantum physics.”

Eventually, Goldhaber-Gordon said, the team hopes to build devices with more and more islands, so they can simulate larger and larger lattices of atoms, capturing the essential behaviors of real materials.

However, they first hope to improve the design of their dual-island device. One goal is to reduce the size of the metal islands, which could allow them to work better at available temperatures: high-end ultra-low-temperature “coolers” can reach temperatures of up to fifty degrees above absolute zero, but that’s barely cold enough for the experiment the researchers just completed . Another is to develop a more reliable process for creating islands than dripping molten bits of metal onto a semiconductor.

But once such hurdles are cleared, the researchers believe, their work could lay the groundwork for significant advances in physics’ understanding of certain types of superconductors and perhaps even more exotic physics, such as hypothetical quantum states that mimic particles with only a fraction of the charge of an electron.

“One thing David and I share is an appreciation for the fact that doing such an experiment is even possible,” Pouse said, and for the future, “I’m certainly excited.”

The research was primarily funded by the DOE Office of Science, with early stage support from the Gordon and Betty Moore Foundation.

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