Quantum information systems derive their power from controllable interactions that produce quantum entanglement. Building scalable quantum information systems requires programmable operations between desired qubits within a quantum processor. In most advanced approaches, qubits interact locally, constrained by the connectivity associated with their fixed spatial arrangement.
In a new study, scientists demonstrated a quantum processor in which qubits are transported coherently and highly parallel across two spatial dimensions. Also, the processor has dynamic, non-local connectivity.
This new approach to processing quantum information allows scientists to dynamically change the layout of atoms by moving and connecting them during computation.
The ability to shuffle qubits while maintaining a quantum state throughout the computational process drastically improves processing capabilities. It also allows self-correction of errors.
Overcoming this hurdle marks a significant step toward building large machines that harness the bizarre properties of quantum mechanics and promise real breakthroughs in materials science, communications technologies, finance, and many other fields.
Mikhail Lukin, the George Vasmer Leverett Professor of Physics, co-director of the Harvard Quantum Initiative and one of the lead authors of the study, said: “The reason building large quantum computers is difficult is because eventually you have bugs. One way to reduce these errors is to keep making your qubits better. Another systematic and ultimately practical way is the so-called quantum error correction. Even if you have some errors, you can correct these errors with redundancy during your calculation process.”
For this work, the scientists created a backup system for the atoms and their information, called the quantum error correction code. They used a new technique to generate these codes, including a toric code.
Dolev Bluvstein, a graduate student in the Lukin group’s physics department who led this work, said: “The key idea is that we want to take a single qubit of information and spread it across many qubits as non-locally as possible, so that the loss of a single one of those qubits doesn’t affect the overall state as much.”
This approach is made possible thanks to a newly developed method, in which each qubit can connect to any other qubit if necessary. This happens due to “creepy action at a distance”.
In this context, two atoms are connected and can exchange information no matter how far apart they are. This phenomenon makes quantum computers so powerful.
Bluvstein said “This entanglement can store and process an exponentially large amount of information.”
The highlight: the researchers can generate and store information in so-called hyperfine qubits. The quantum state of these more robust qubits lasts significantly longer than regular qubits in their system (several seconds versus microseconds). It gives them the time they need to entangle with other qubits, even distant ones, so they can create complex states of entangled atoms.
Scientists first pair qubits and then pulse a global laser from their system to create a quantum gate that entangles the pairs and stores the pair’s information in hyperfine qubits. They then entangle these qubits by moving them into new pairs with other atoms in the system using a two-dimensional array of individually focused laser beams known as optical tweezers. They repeat the processes in any order to make different types of quantum circuits that can run different algorithms. The atoms are eventually connected in a clustered state, where they are sufficiently spaced apart to serve as backups for each other in the event of failure.
With this architecture, scientists could create a programmable, error-correcting quantum computer that works with 24 qubits. The system has become the basis of their vision of a quantum processor.
luke said “In the short term, we can basically use this new method as a kind of sandbox in which we will start developing practical error correction methods and exploring quantum algorithms.” At the moment [in terms of getting to large-scale, useful quantum computers]I would say we have climbed far enough up the mountain to see where the top is and can now actually see a path from where we are to the highest peak.”
The system is being built by the research team, which includes collaborators from QuEra Computing, MIT and the University of Innsbruck.
- Bluvstein D, Levine H, Semeghini G et al. A quantum processor based on the coherent transport of entangled atomic arrays. Nature 604, 451–456 (2022). DOI: 10.1038/s41586-022-04592-6