Researchers are taking a step toward new quantum simulators

Newswise – Some of the most exciting topics in modern physics, such as high-temperature superconductors and other proposals for quantum computers, are about the amazing things that happen when these systems fly. between two quantum states.

Unfortunately, understanding what happens at those points, known as quantum dots, has been difficult. The calculations are often very difficult to solve, and today’s computers are not always capable of simulating what happens, 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 the initiative to build another method, known as a quantum simulator. Although the new device, for now, simulates interactions between two quantum objects, the researchers argue in a paper published on January 30. Natural Physics that it can be lifted easily. If so, researchers can use it to simulate more complex processes and begin to answer some of the most exciting questions in physics.

“We always make mathematical models that we hope will capture the essence of the phenomena we are interested in, but even if we believe they are correct, they are often not solved in sufficient time” with current methods, said David Goldhaber -Gordon, professor of physics at Stanford and researcher at the Stanford Institute for Materials and Energy Sciences (SIMES). On the way to the quantum simulator, he said, “we have these switches that nobody has had before.”

Islands in a sea of ​​electrons

The basic idea of ​​a quantum simulator, Goldhaber-Gordon said, is like a model of a planetary system, where someone turns on the noise, and the connected gears rotate to represent the motion of the moon and planets. It is thought that such an “error” discovered in a shipwreck more than 2000 years ago, has produced many estimates of the timing of eclipses and the positions of the planets in the sky, and similar machines have been used until the late century for mathematical calculations that were too difficult for the then advanced digital computers.

Like the designers of a planetary system model, researchers creating quantum simulations must ensure that their simulations match the quantum states they are intended to simulate.

For Goldhaber-Gordon and his colleagues, most of the systems they are interested in—systems with critical properties such as certain superconductors—can be thought of as atoms of a single, organized substance. with a temporary layer embedded in a pool of moving electrons. The lattice atoms in such materials are all the same, and they all interact with the sea of ​​electrons around them.

To model such materials in a quantum simulator, the simulator needs to have stand-ins for the atoms of the lattice that are nearly identical, and these need to interact strongly with the surrounding pool of electrons. The system also needs to be manipulated in a certain way, so that the experimenters can change different test conditions to gain comparative insight.

Most quantum simulation solutions don’t meet all of those requirements at once, said Winston Pouse, a graduate student in Goldhaber-Gordon’s lab and first author of the book. Natural Physics paper. “At a higher level, there are ultracold atoms, where the atoms are very similar, but implementing strong coupling in the pool is difficult. Then there are simulators based on quantum dots, where “We can achieve a strong connection, but the areas are there. It’s not the same,” said Pouse.

Goldhaber-Gordon says a possible solution emerged from the work of French physicist Frédéric Pierre, who was studying nanoscale devices where an island of metal was sandwiched between specially designed pools of electrons. known as two-dimensional electron gases. Electrically controlled gates controlled the flow of electrons between the reservoirs and the metal island.

While studying the work of Pierre and his lab, Pouse, Goldhaber-Gordon and their colleagues realized that these materials could meet their criteria. These islands – stand-ins for lattice atoms – interact strongly with the electrons surrounding them, and if one Pierre island were to be expanded to a group of two or more islands they would interact strongly with each other. Metal islands also contain a greater number of electronic states compared to other materials, which has the effect of rendering any significant difference between two different blocks of metal invisible as one – to make them exactly the same. Finally, the system was implemented with electrical cables that regulate voltages.

A simple simulator

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

One of the hardest parts, it turned out, was actually building the equipment. First, the basic circuit elements must be nanoscopically arranged to become semiconductors. After that, someone has to insert and melt a small amount of metal on the substructure to make each metal island.

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

Despite those challenges, the team, whose work is part of a broader quantum science effort at Stanford and SLAC, was able to build a device consisting of two metal islands and analyze how the electrons have moved in it under different conditions. Their results are consistent with calculations that took weeks on a supercomputer – suggesting they may have found a way to investigate more important phenomena than before.

Andrew Mitchell, a physicist at the University of Dublin’s Center for Quantum Engineering, Science and Technology (C) Andrew Mitchell said: -QuEST) and co-author of the paper, “we can now create analogue devices bespoke ones with quantum features that can solve specific problems in quantum physics.”

Ultimately, Goldhaber-Gordon said, the team hopes to build materials with more islands, so they can mimic larger lattices of atoms, capturing the important properties of really.

First, however, they hope to improve the design of their two-island machine. Another goal is to reduce the size of the iron islands, which can make them work better at the temperatures reached: “refrigerators” of very low temperatures can reach temperatures of up to fifty degrees below zero, but that was not cold enough. for an experiment the researchers just completed. One is to develop a more reliable method of forming islands than dropping molten metal pieces onto a semiconductor.

But once things like this are resolved, the researchers believe their work could lay the groundwork for major advances in physicists’ understanding of other types of superconductors and perhaps even more surprising physics. pass, such as imaginary numbers that simulate fractions with only fractions. of electron charge.

“One thing that David and I share is the appreciation that doing such experiments was possible,” said Pouse, and for the future, “I’m really excited.”

The research was funded primarily by the DOE Office of Science, with the first phases supported by the Gordon and Betty Moore Foundation.

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