A Harvard team has achieved a significant milestone in the pursuit of reliable, scalable quantum computing. The team has constructed a programmable, logical quantum processor capable of encoding up to 48 logical qubits and performing hundreds of logical gate operations for the first time. Their device is the first to demonstrate large-scale algorithm execution on an error-corrected quantum computer, signaling the arrival of early fault-tolerant, or consistently uninterrupted, quantum processing.
A quantum bit or “qubit” is one unit of information in quantum computing, similar to a binary bit in traditional computing. For more than two decades, scientists and technologists have demonstrated to the world that quantum computing is, in theory, achievable by manipulating quantum particles such as atoms, ions, or photons to generate physical qubits.
But successfully exploiting the weirdness of quantum mechanics for computation is more complicated than simply amassing a large-enough number of physical qubits, which are inherently unstable and prone to collapse out of their quantum states.
I think this is one of the moments in which it is clear that something very special is coming. Although there are still challenges ahead, we expect that this new advance will greatly accelerate the progress towards large-scale, useful quantum computers.
Mikhail Lukin
The real gold in usable quantum computing are logical qubits, which are bundles of redundant, error-corrected physical qubits that can store information for use in a quantum computation. Creating logical qubits as controlled units – like classical bits – has been a major barrier for the area, and it’s widely acknowledged that technologies won’t take off until quantum computers can run safely on logical qubits. To yet, the most advanced computing systems have proven one or two logical qubits and one quantum gate operation – equivalent to only one unit of code – between them.
A Harvard team led by Mikhail Lukin, the Joshua and Beth Friedman University Professor of Physics and co-director of the Harvard Quantum Initiative, has achieved a significant breakthrough in the pursuit of stable, scalable quantum computing. The team has constructed a programmable, logical quantum processor capable of encoding up to 48 logical qubits and performing hundreds of logical gate operations for the first time. Their device is the first to demonstrate large-scale algorithm execution on an error-corrected quantum computer, signaling the arrival of early fault-tolerant, or consistently uninterrupted, quantum processing.
Published in Nature, the work was performed in collaboration with Markus Greiner, the George Vasmer Leverett Professor of Physics; colleagues from MIT; and Boston-based QuEra Computing, a company founded on technology from Harvard labs. Harvard’s Office of Technology Development recently entered into a licensing agreement with QuEra for a patent portfolio based on innovations developed in Lukin’s group.
Lukin described the achievement as a possible inflection point akin to the early days in the field of artificial intelligence: the ideas of quantum error correction and fault tolerance, long theorized, are starting to bear fruit.
“I think this is one of the moments in which it is clear that something very special is coming,” Lukin said. “Although there are still challenges ahead, we expect that this new advance will greatly accelerate the progress towards large-scale, useful quantum computers.”
The breakthrough builds on several years of work on a quantum computing architecture known as a neutral atom array, pioneered in Lukin’s lab and now being commercialized by QuEra. The key components of the system are a block of ultra-cold, suspended rubidium atoms, in which the atoms – the system’s physical qubits — can move about and be connected into pairs – or “entangled” – mid-computation. Entangled pairs of atoms form gates, which are units of computing power. Previously, the team had demonstrated low error rates in their entangling operations, proving the reliability of their neutral atom array system.
“This breakthrough is a tour de force of quantum engineering and design,” said Denise Caldwell, acting assistant director of the National Science Foundation’s Mathematical and Physical Sciences Directorate, which provided funding for the research through the NSF’s Physics Frontiers Centers and Quantum Leap Challenge Institutes programs. “The team has not only accelerated the development of quantum information processing by using neutral atoms, but opened a new door to explorations of large-scale logical qubit devices which could enable transformative benefits for science and society as a whole.”
The researchers have now shown parallel, multiplexed control of a complete patch of logical qubits utilizing lasers using their logical quantum processor. This is more efficient and scalable than controlling 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 ones, and enabling a path toward larger devices,” said Dolev Bluvstein, a Griffin School of Arts and Sciences Ph.D. student in Lukin’s group.
The team will continue to work on showing new types of operations on its 48 logical qubits, as well as configuring their system to operate continuously rather than manually cycling as it currently does.
