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Quantum Computing

Physicists Develop Special Quantum Computer With 256 Qubits

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Image: Rose Lincoln/Harvard Staff Photographer

In what is a major advancement in quantum computing, a team of physicists from the Harvard-MIT Center for Ultracold Atoms and other universities have created a special type of quantum computer. This system is called a programmable quantum simulator, and it can operate with 256 quantum bits, or “qubits.” Qubits are fundamental to the operation of quantum computers, and they are the source of their processing power.

The new development brings us closer to achieving large-scale quantum machines, which could be used to gain deep insight into complex quantum processes. They could also have major implications in fields like material science, communication technologies, finance, and various others that are currently facing barriers in research.

The research was published back on July 9 in Nature

Pushing the Field Forward

Mikhail Lukin is the George Vasmer Leverett Professor of Physics and co-director of the Harvard Quantum Initiative. He is also one of the senior authors of the study.

“This moves the field into a new domain where no one has ever been to thus far,” said Lukin. “We are entering a completely new part of the quantum world.”

Sepehr Ebadi is a physics student in the Graduate School of Arts and Sciences and the study’s lead author. 

According to Ebadi, the system’s greatest features are its size and programmability, which makes it one of the top systems around. It can harness the properties of matter at extremely small scales, which enable it to advance processing power. An increase in qubits can help the system store and process exponentially more information than classical bits, which standard computers rely on.

“The number of quantum states that are possible with only 256 qubits exceeds the number of atoms in the solar system,” Ebadi said.

The simulator has enabled researchers to observe exotic quantum states of matter, as well as perform a quantum phase transition study, which was extremely precise and demonstrated how magnetism works at the quantum level.

According to the researchers, these experiments could help scientists learn how to design new materials with exotic properties.

The New System

The project relies on a platform developed in 2017 by the researchers, but it was significantly upgraded this time around. It was capable of reaching a size of 51 qubits in the past, and it enabled the researchers to capture ultra-cold rubidium atoms and arrange them in a specific order through the use of a one-dimensional array of individually focused laser beams. 

This system allows atoms to be assembled in two-dimensional arrays of optical tweezers, which is the name for the laser beams. This enables the achievable system size to increase from 51 to 256 qubits. The researchers can then use the tweezers to arrange the atoms in defect-free patterns and create programmable shapes, which enables different interactions between the qubits.

“The workhorse of this new platform is a device called the spatial light modulator, which is used to shape an optical wavefront to produce hundreds of individually focused optical tweezer beams,” said Ebadi. “These devices are essentially the same as what is used inside a computer projector to display images on a screen, but we have adapted them to be a critical component of our quantum simulator.”

The atoms are first loaded into the optical tweezers randomly before the researchers move the atoms around and arrange them in target geometries. A second set of moving optical tweezers is then used to drag the atoms to their desired locations, which eliminates the initial randomness. The lasers allow the researchers to take full control over the positioning of the atomic qubits and their coherent quantum manipulation.

Tout Wang is a research associate in physics at Harvard and one of the authors of the paper.

“Our work is part of a really intense, high-visibility global race to build bigger and better quantum computers,” said Wang. “The overall effort [beyond our own] has top academic research institutions involved and major private-sector investment from Google, IBM, Amazon, and many others.”

The team is now working to improve the system by improving laser control over qubits, as well as making the system more programmable. According to the researchers, possible applications include probing exotic forms of quantum matter and solving real-world problems that can be naturally encoded on the qubits.

“This work enables a vast number of new scientific directions,” Ebadi said. “We are nowhere near the limits of what can be done with these systems.”