New approach may help break down barriers to large-scale quantum computing

Building an airplane by flying it isn’t usually a goal for most, but for a team of physicists led by Harvard, this general idea could be the key to eventually building large-scale quantum computers.

Described in a new article by NatureThe research team, which includes collaborators from QuEra Computing, MIT and the University of Innsbruck, has developed a novel approach to quantum information processing that allows them to dynamically change the arrangement of atoms in their system by moving them and connecting them to each other in the middle of the calculation.

This ability to mix qubits (the fundamental building blocks of quantum computers and the source of their massive processing power) during the computational process while preserving their quantum state greatly extends processing capabilities and enables self-correction of errors. Overcoming this hurdle marks a major step towards building large-scale machines that take advantage of bizarre features of quantum mechanics and promise to deliver real-world breakthroughs in materials science, communications technology , finance and many other fields.

“The reason building large-scale quantum computers is difficult is that you end up getting errors,” said Mikhail Lukin, George Vasmer Leverett physics professor, co-director of the Harvard Quantum Initiative and one of the leading study authors. “One way to reduce these errors is to simply upgrade your qubits more and more, but another more systematic and ultimately practical way is to do something called quantum error correction. This means that even if you have errors, you can correct these errors during your calculation process with redundancy.”

In classical computing, error correction is done by simply copying the information of a single digit or binary bit so that it is clear when and where it failed. For example, a single bit of 0 can be copied three times to read 000. Suddenly, when it reads 001, it is clear where the error is and can be corrected. A fundamental limitation of quantum mechanics is that information cannot be copied, which makes it difficult to correct errors.

The workaround implemented by the researchers creates a sort of backup system for the atoms and their information called a quantum error correction code. The researchers use their new technique to create many of these correction codes, including what’s called a toric code, and distribute them throughout the system.

“The key idea is that we want to take a single qubit of information and spread it as non-locally as possible across many qubits, so that if one of those qubits fails, it doesn’t affect the qubit as much. ‘entire state,’ Dolev said. Bluvstein, a graduate student in the physics department of the Lukin Group who led this work.

What makes this approach possible is that the team has developed a new method where any qubit can connect to any other qubit on demand. It happens by entanglement or what Einstein called “frightening action at a distance”. In this context, two atoms become linked and able to exchange information regardless of their distance. This phenomenon is what makes quantum computers so powerful.

“This tangle can store and process an exponential amount of information,” Bluvstein said.

The new work builds on the programmable quantum simulator that the lab has been developing since 2017. The researchers have added new capabilities to it to allow them to move entangled atoms without losing their quantum state and while they are working.

Previous research on quantum systems has shown that once the computational process is started, the atoms, or qubits, are locked in their positions and only interact with nearby qubits, limiting the types of computations and simulations quantums that can be performed between them.

The key is that researchers can create and store information in what are called hyperfine qubits. The quantum state of these more robust qubits lasts much longer than ordinary qubits in their system (several seconds versus microseconds). This gives them the time they need to tangle with other qubits, even distant ones, so they can create complex states of entangled atoms.

The whole process looks like this: the researchers perform an initial pairing of qubits, fire a global laser from their system to create a quantum gate that entangles those pairs, and then stores the pair’s information in the hyperfine qubits. Then, using a two-dimensional array of individually focused laser beams called optical tweezers, they move these qubits into new pairs with other atoms in the system to entangle them as well. They repeat the steps according to the pattern they want to create different types of quantum circuits to run different algorithms. Eventually, the atoms all become connected in a so-called cluster state and are spread out enough that they can act as backups for each other in the event of an error.

Already, Bluvstein and his colleagues have used this architecture to generate a programmable error-correcting quantum computer running at 24 qubits and plan to scale from there. The system became the basis for their vision of a quantum processor.

“In the very short term, we can basically start using this new method as a kind of sandbox where we really start developing practical methods for error correction and exploring quantum algorithms,” Lukin said. “Right now [in terms of getting to large-scale, useful quantum computers]i would say we have climbed the mountain enough to see where the peak is and can now see a path from where we are to the highest peak.

Sherry J. Basler