Building a plane while it flies isn’t usually a goal for most, but for a team of physicists led by Harvard, this general idea could be a key to finally building large quantum computers.
Described in a new article in Naturethe research team, which includes collaborators from QuEra Computing, MIT and the University of Innsbruck, developed a new approach to processing quantum information that allows them to dynamically change the layout of atoms in their system by moving and connecting them together in the middle Calculate.
This ability to shuffle the qubits (the fundamental building blocks of quantum computers and the source of their tremendous processing power) during the computational process while preserving their quantum state dramatically expands processing capabilities and allows for self-correction of errors. Overcoming this hurdle marks a major step toward building large machines that harness the bizarre properties of quantum mechanics and promise real-world breakthroughs in materials science, communications technologies, finance, and many other fields.
“The reason building large quantum computers is difficult is that you will eventually have bugs,” said Mikhail Lukin, George Vasmer Leverett Professor of Physics, co-director of the Harvard Quantum Initiative and one of the lead authors of the study. “One way to reduce these errors is to just keep making your qubits better and better, but another, more systematic and ultimately practical way is to do something called quantum error correction. This means that even if you have some errors, you can correct those errors with redundancy during your calculation process.”
In classical data processing, error correction is done by simply copying information from a single binary digit or bit, making it clear when and where an error occurred. For example, a single bit of 0 can be copied three times to read 000. Suddenly, when it reads 001, it’s clear where the mistake is and can be corrected. A fundamental limitation of quantum mechanics is that information cannot be copied, making error correction difficult.
The workaround implemented by the researchers creates a kind 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 they distribute them throughout the system.
“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 failure of a single one of those qubits doesn’t affect the entire state as much,” said Dolev 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 when needed. This occurs through entanglement, or what Einstein called “spooky 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.
“This entanglement can store and process an exponentially large amount of information,” Bluvstein said.
The new work builds on the programmable quantum simulator that the lab has been developing since 2017. The researchers added new abilities to it, allowing them to move entangled atoms without losing their quantum state and while working.
Previous research on quantum systems has shown that once the computational process begins, the atoms or qubits are stuck in their positions and only interact with nearby qubits, limiting the types of quantum computations and simulations that can be performed between them.
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.
The whole process goes like this: the researchers perform an initial pairing of qubits, pulse a global laser out of their system to create a quantum gate that entangles those pairs, and then store the pair’s information in the hyperfine qubits. Then, using a two-dimensional array of individually focused laser beams called optical tweezers, they move those qubits into new pairs with other atoms in the system to entangle them as well. You repeat the steps in any pattern to create different types of quantum circuits to run different algorithms. Finally, the atoms all connect in what is known as a cluster state and are spread out so widely that they can serve as backups for one another in the event of a failure.
Bluvstein and his colleagues have already used this architecture to generate a programmable, error-correcting quantum computer that operates at 24 qubits and plan to scale from there. The system has become the basis of their vision of a quantum processor.
“In the short term, we can basically use this new method as a kind of
Sandbox, where we’re really going to start developing practical error correction methods and exploring quantum algorithms,” Lukin said. “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.”
A quantum processor based on the coherent transport of entangled atomic arrays
Article publication date
April 20, 2022
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