Researchers create first programmable, logical quantum processor

Individual physical qubits, such as rubidium atoms, are notoriously fragile and easily perturbed by interactions with their surroundings. To minimize errors, the researchers bundled the atoms together to form a “logical qubit, which can be combined with other logical qubits into a fault-tolerant quantum circuit. Credit: S. Kelley/NIST

Harvard researchers have realized a significant milestone in the quest for stable, scalable quantum computing, an ultra-high-speed technology that could enable game-changing advances in a variety of fields, including medicine, science, and finance.

The team, led by Mikhail Lukin, the Joshua and Beth Friedman University Professor of physics and co-director of the Harvard Quantum Initiative, created the first programmable, logical quantum processor, capable of encoding up to 48 logical qubits, and implementation of hundreds of logical gate operations, a vast improvement over previous efforts.

Published in NATURE, the work was done in collaboration with Markus Greiner, the George Vasmer Leverett Professor of Physics; colleagues from MIT; and QuEra Computing, a Boston company built on technology from Harvard labs.

The system is the first demonstration of large-scale algorithmic implementation of an error-corrected quantum computer, heralding the arrival of early fault-tolerant, or reliably uninterrupted, quantum computation.

Lukin describes the breakthrough as a possible inflection point similar to the early days of the field of artificial intelligence: the ideas of quantum error correction and fault tolerance, long theorized, began to bear fruit.

“I think it’s one of those moments where it’s clear that something very special is coming,” Lukin said. “Although there are challenges ahead, we expect this new development to greatly accelerate progress toward large, useful quantum computers.”

Denise Caldwell of the National Science Foundation agrees.

“This breakthrough is a tour de force in quantum engineering and design,” said Caldwell, acting assistant director of the Mathematical and Physical Sciences Directorate, which supports the research through NSF’s Physics Frontiers Centers and Quantum Leap Challenge Institutes programs. “The team not only accelerates the development of quantum information processing by using neutral atoms, but opens a new door to the exploration of large-scale logical qubit devices, which can transformative benefits for science and society in general.”

It’s a long, complicated road.

In quantum computing, a quantum bit or “qubit” is a unit of information, like a binary bit in classical computing. For more than two decades, physicists and engineers have shown that quantum computing is, in principle, possible by manipulating quantum particles such as atoms, ions, or photons to create physical qubits. .

But successfully exploiting the strangeness of quantum mechanics for computation is more complicated than simply assembling a large enough number of qubits, which are inherently unstable and prone to collapse from their states. in quantum.

The real coins of the realm are the so-called logical qubits: sets of redundant, error-correcting physical qubits, which can store information for use in a quantum algorithm. Creating logical qubits as controllable units like classical bits is a fundamental hurdle for the field, and it is generally accepted that until quantum computers can reliably run on logical qubits, the technology never works.

To date, the best computing systems have featured one or two logical qubits, and a quantum gate operation in just one unit of code between them.

The Harvard team’s breakthrough builds on several years of work on a quantum computing architecture known as a neutral atom array, pioneered by Lukin’s lab. It is now being commercialized by QuEra, which recently entered into a licensing agreement with Harvard’s Office of Technology Advancement for a patent portfolio based on innovations made by Lukin’s group.

The key part of the system is a block of ultra-cold, suspended rubidium atoms, in which atoms the physical qubits of the system can move and are connected into pairs or “linked” in the middle compute.

Linked pairs of atoms form gates, which are units of computing power. Previously, the team demonstrated a low error rate in their operations, proving the reliability of their neutral atom array system.

With their logical quantum processor, researchers have now demonstrated parallel, multiplexed control of an entire patch of logical qubits, using lasers. This result is more efficient and scalable than controlling individual physical qubits.

“We are trying to mark a transition in the field, towards starting to test algorithms with error-corrected qubits instead of physical ones, and moving a path towards larger devices,” said the paper’s first author Dolev Bluvstein, a Griffin School of Arts and Sciences Ph.D. student in Lukin’s lab.

The team will continue to work towards demonstrating many types of operations on their 48 logical qubits and to configure their system to keep running, as opposed to manual cycling as is currently done.

More information:
Dolev Bluvstein et al, Logical quantum processor based on reconfigurable atom arrays, NATURE (2023). DOI: 10.1038/s41586-023-06927-3

Given by Harvard University

This story was published courtesy of the Harvard Gazette, the official newspaper of Harvard University. For more university news, visit

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