World’s First Logical Quantum Processor Unveiled by Harvard

A major landmark in quantum computing has been achieved by researchers at Harvard, who have developed a logical quantum processor that is programmable, capable of encoding 48 logical qubits, and performing hundreds of logical gate operations. The first true demonstration of a large-scale algorithm running on a quantum computer free from error, this development is being praised as potentially game-changing for the field.

The new logical quantum processor from Harvard holds 48 logical qubits and successfully runs large-scale algorithms on a system corrected for errors. Led by Mikhail Lukin, this development is a significant stride in the direction of practical, fault-tolerant quantum computers.

A unit of information in quantum computing is identified as a "qubit" or quantum bit, akin to a binary bit in traditional computing. For over 20 years, the feasibility of quantum computing has been demonstrated by physicists and engineers who have manipulated quantum particles, such as atoms, ions, or photons, to generate physical qubits.

However, the process of exploiting quantum mechanics' quirks for computation is more complex than merely amassing a large quantity of physical qubits, which are inherently unstable and prone to shifting out of their quantum states.

Truly functional quantum computing relies on what are known as logical qubits. These are error-corrected, redundant clusters of physical qubits that can store information for usage in a quantum algorithm. Creating logical qubits that can be controlled like classical bits has been a major challenge for the field, with it widely agreed that technologies will not be able to take off until logical qubits can be reliably utilised in quantum computers. Presently, the most advanced computing systems have achieved one or two logical qubits, and a single quantum gate operation which is similar to a single code unit.

A team of Harvard researchers led by quantum expert Mikhail Lukin has made great strides in quantum computing. As the first author on the paper, Dolev Bluvstein, a Ph.D. student in Lukin's lab, was a crucial contributor to the breakthrough.

The team has now pioneered a significant milestone towards achieving stable, scalable quantum computing by way of a programmed, logical quantum processor that is capable of encoding up to 48 logical qubits and performing hundreds of logical gate operations. The onset of early fault-tolerant, or consistently uninterrupted, quantum computation is signaled by their system, marking the first demonstration of large-scale algorithm execution on a quantum computer free from error.

The research was carried out in partnership with Markus Greiner, who is the George Vasmer Leverett Professor of Physics, MIT colleagues, and QuEra Computing, a Boston-based company. The innovations engineered in Lukin's group have led to a licensing agreement with QuEra, under the Harvard Office of Technology Development. The quantum error correction and fault tolerance concepts long-theorized are beginning to demonstrate success, leading to a possible turning point akin to the early stages in artificial intelligence.

Lukin believes this is a defining moment in quantum computing that could herald very special developments in the field. Despite ongoing challenges, this latest achievement is expected to spur on the progress of large-scale, practical quantum computers.

The landmark achievement comes after years of work on the quantum computing architecture known as neutral atom array. Pioneered in Lukin’s lab, this architecture is now being commercialized by QuEra. It relies on a block of ultra-cold, suspended rubidium atoms, with the atoms – which serve as the system's physical qubits – being capable of moving and pairing mid-computation. These entangled pairs of atoms form gates and represent units of computing power. Previously, the team demonstrated low error rates in their entangling operations, cementing the reliability of their neutral atom array system.

According to Denise Caldwell, acting assistant director of the National Science Foundation's Mathematical and Physical Sciences Directorate, the breakthrough is a testament to the skillfulness of the team's quantum engineering and design capabilities. Caldwell believes the development will not only hasten progress in quantum information processing using neutral atoms but will also help explore large-scale logical qubit devices that hold transformative potential benefits for science and society as a whole.

With their logical quantum processor, the researchers now demonstrate parallel, multiplexed control of an entire patch 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 ones, and enabling a path toward larger devices,” said paper first author Dolev Bluvstein, a Griffin School of Arts and Sciences Ph.D. student in Lukin’s lab.

The team will continue to work toward demonstrating more types of operations on their 48 logical qubits, and to configure their system to run continuously, as opposed to manual cycling as it does now.