Individual physical qubits, like rubidium atoms, are notoriously fragile and easily disrupted by interactions with their environment. To minimize errors, researchers entangle atoms to form a single “logical qubit,” which can be combined with other logical qubits into a fault-tolerant quantum circuit. Credit: S. Kelley/NIST
Harvard researchers have reached a key milestone in the quest for stable and scalable quantum computing, a lightning-fast technology that will enable revolutionary advances in diverse fields, including medicine, science and finance.
The team, led by Mikhail Lukin, professor of physics at Joshua and Beth Friedman University and co-director of the Harvard Quantum Initiative, created the first logical and programmable quantum processor, capable of encoding up to 48 logical qubits and run hundreds of logical qubits. logic gate operations, a big improvement over previous efforts.
Published in Nature, the work was carried out in collaboration with Markus Greiner, 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 algorithm execution on an error-correcting quantum computer, heralding the advent of early, fault-tolerant or reliable, uninterrupted quantum computing.
Lukin described this achievement as a possible inflection point similar to the early days in the field of artificial intelligence: the long-theorized ideas of quantum error correction and fault tolerance are beginning to bear fruit.
“I think this is one of the moments where it’s clear that something very special is brewing,” Lukin said. “Although there are still challenges ahead, we hope that this new advance will significantly accelerate progress toward useful large-scale 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 deputy director of the Mathematical and Physical Sciences Directorate, which supported the research through the Physics Frontiers Centers and Quantum Leap Challenge Institutes of the NSF. “The team not only accelerated the development of quantum information processing using neutral atoms, but also opened a new door to the exploration of large-scale logical qubit devices, which could bring transformative benefits to science and society as a whole.”
It has been a long and complex journey.
In quantum computing, a quantum bit or “qubit” is a unit of information, just like a binary bit in classical computing. For more than two decades, physicists and engineers have shown the world that quantum computing is, in principle, possible by manipulating quantum particles (whether atoms, ions, or photons) to create physical qubits.
But successfully exploiting the quirks of quantum mechanics for computational purposes is more complicated than simply accumulating a sufficiently large number of qubits, which are inherently unstable and prone to collapsing out of their quantum state.
The real pieces of the kingdom are what are called logical qubits: sets of redundant, error-corrected physical qubits, which can store information for use in a quantum algorithm. Creating logical qubits as controllable units, like classical bits, has been a fundamental obstacle in this field, and it is generally accepted that until quantum computers can reliably operate on logical qubits, the technology won’t really be able to take off.
To date, the best computing systems have demonstrated one or two logical qubits and a quantum gate operation (akin to a single unit of code) between them.
The Harvard team’s breakthrough builds on several years of work on a quantum computing architecture known as the neutral atom lattice, pioneered in Lukin’s lab. It is now marketed by QuEra, which recently entered into a licensing agreement with Harvard’s Office of Technology Development for a patent portfolio based on innovations developed by the Lukin Group.
The key component of the system is a block of ultra-cold, suspended rubidium atoms, in which the atoms – the physical qubits of the system – can move around and be connected in pairs – or “entangled” – during calculation.
Pairs of entangled atoms form gates, which are units of computing power. Previously, the team demonstrated low error rates in their entanglement operations, proving the reliability of their neutral atom lattice system.
With their logical quantum processor, the researchers now demonstrate the parallel and multiplexed control of a complete set of logical qubits, using lasers. This result is more efficient and scalable than having to control individual physical qubits.
“We are trying to mark a transition in the field, toward starting to test algorithms with error-corrected qubits instead of physical qubits, and pave the way for larger devices,” said Dolev Bluvstein, principal author of the article, from the Griffin School of Arts and Sciences. student in Lukin’s lab.
The team will continue to work to demonstrate more types of operations on its 48 logical qubits and to configure its system to operate continuously, as opposed to manually cycling as is currently the case.
More information:
Dolev Bluvstein et al, Logical quantum processor based on reconfigurable atom networks, Nature (2023). DOI: 10.1038/s41586-023-06927-3
Provided by Harvard University
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