A team of scientists from the University of Maryland (UMD) and the National Institute of Standards and Technology (NIST) have created a quantum simulator using 53 interacting atomic qubits to mimic magnetic quantum matter. Prior to this breakthrough, leading researchers had only created quantum simulators of 20 qubits or less.
Quantum simulators are a restricted type of quantum computer that use qubits to mimic complex quantum matter. By deploying 53 individual ytterbium ions—charged atoms trapped in place by gold-coated and razor-sharp electrodes—the UMD-NIST quantum simulator is on the cusp of exploring physics that is unreachable by even the fastest modern supercomputers.
The building of qubit simulators is a key step in efforts to build a full-fledged quantum computer capable of tackling any complex computational problem. And, according to the UMD-NIST team, adding even more qubits is just a matter of lassoing more atoms into the mix.
“We are continuing to refine our system, and we think that soon, we will be able to control 100 ion qubits, or more,” said Jiehang Zhang, a postdoctoral researcher in the UMD Department of Physics, and the lead author of a paper about the team’s 53 qubit quantum simulator that appears in this week’s issue of the journal Nature. “At that point, we can potentially explore difficult problems in quantum chemistry or materials design.”
The UMD-NIST paper appears in Nature together with a complementary paper on a previously announced 51 qubit quantum simulator designed by Harvard and MIT researchers that uses rubidium atoms confined by an array of laser beams.
“Each ion qubit is a stable atomic clock that can be perfectly replicated,” said UMD team lead Christopher Monroe, a Distinguished University Professor of Physics and Bice Sechi-Zorn Professor at UMD, and co-founder and chief scientist of IonQ Inc., a UMD-based quantum computing startup company. “They are effectively wired together with external laser beams. This means that the same device can be reprogrammed and reconfigured, from the outside, to adapt to any type of quantum simulation or future quantum computer application that comes up.”
Monroe, who is also a fellow of the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science, has been one of the early pioneers in quantum computing and the UMD-NIST quantum simulator is part of a blueprint for a general-purpose quantum computer.
Quantum hardware for a quantum problem
While modern, transistor-driven computers are great for crunching their way through many problems, they can screech to a halt when dealing with more than 20 interacting quantum objects. That’s certainly the case for quantum magnetism, in which the interactions can lead to magnetic alignment or to a jumble of competing interests at the quantum scale.
“What makes this problem hard is that each magnet interacts with all the other magnets,” said UMD research scientist Zhexuan Gong, lead theorist and a co-author of the study. “With the 53 interacting quantum magnets in this experiment, there are over a quadrillion possible magnet configurations, and this number doubles with each additional magnet. Simulating this large-scale problem on a conventional computer is extremely challenging, if at all possible.”
When these calculations hit a wall, a quantum simulator may help scientists push the envelope on difficult problems. Qubits are isolated and well-controlled quantum systems that can be in a combination of two or more states at once. Qubits come in different forms, and atoms—the versatile building blocks of everything—are one of the leading choices for making qubits. In recent years, scientists have controlled 10 to 20 atomic qubits in small-scale quantum simulations.
Currently, tech industry behemoths, startups and university researchers are in a fierce race to build prototype quantum computers that can control even more qubits. But qubits are delicate and must stay isolated from the environment to protect the device’s quantum nature. With each added qubit, this protection becomes more difficult, especially if qubits are not identical from the start, as is the case with fabricated circuits. This is one reason that atoms are an attractive choice that can dramatically simplify the process of scaling up to large-scale quantum machinery.
An atomic advantage
Unlike the integrated circuitry of modern computers, atomic qubits reside inside of a room-temperature vacuum chamber that maintains a pressure similar to outer space. This isolation is necessary to keep the destructive environment at bay, and it allows the scientists to precisely control the atomic qubits with a highly engineered network of lasers, lenses, mirrors, optical fibers and electrical circuitry.
“The principles of quantum computing differ radically from those of conventional computing, so there’s no reason to expect that these two technologies will look anything alike,” said Monroe.
“Quantum simulations are widely believed to be one of the first useful applications of quantum computers. After perfecting these quantum simulators, we can then implement quantum circuits and eventually quantum-connect many such ion chains together to build a full-scale quantum computer with a much wider domain of applications,” said study co-author Alexey Gorshkov, a NIST theoretical physicist, JQI and QuICS fellow, and adjunct assistant professor in the UMD Department of Physics.