Most people don’t set out to build a plane while they are already in flight, but a group of scientists lead by Harvard believe that this broad concept could be the key to developing large-scale quantum computers.
The research team, which includes members from QuEra Computing, MIT, and the University of Innsbruck, created a novel method for processing quantum information that enables them to dynamically alter the arrangement of their system’s atoms by moving and connecting them while performing calculations, as described in a recent paper in Nature.
The ability to shuffle the qubits the essential components of quantum computers and the source of their tremendous processing power while maintaining their quantum state throughout computation significantly increases processing capacity and enables error correction.
When this obstacle is overcome, it will be much easier to create massive machines that take use of quantum mechanics’ peculiar properties and promise to lead to advancements in a variety of sectors, including material science, communication technology, finance, and many others.
“The reason why building large scale quantum computers is hard is because eventually you have errors,” said Mikhail Lukin, the George Vasmer Leverett Professor of Physics, co-director of the Harvard Quantum Initiative, and one of the senior authors of the study.
“One way to reduce these errors is to just make your qubits better and better, but another more systematic and ultimately practical way is to do something which is called quantum error correction. That means that even if you have some errors, you can correct these errors during your computation process with redundancy.”
In traditional computing, error correction is accomplished by merely copying data from a single binary digit or bit, making it obvious when and where the error occurred. One bit of 0 can be cloned three times to read 000, for instance.
When it displays 001, it becomes immediately apparent where the mistake is and how to fix it. Information cannot be copied, which is a fundamental restriction of quantum mechanics, making error correction challenging.
The researchers’ workaround develops a quantum error correcting code, which functions as a sort of backup system for the atoms and their data. Many of these correction codes, including the so-called toric code, are created by the researchers using their novel method, which also disperses them throughout the system.
In the very near term, we basically can start using this new method as a kind of sandbox where we will really start developing practical methods for error correction and exploring quantum algorithms. 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 top is and can now actually see a path from where we are to the highest top.
Mikhail Lukin
“The key idea is we want to take a single qubit of information and spread it as nonlocally as possible across many qubits, so that if any single one of these qubits fails it doesn’t actually affect the entire state that much,” said Dolev Bluvstein, a graduate student in the physics department from the Lukin group who led this work.
This strategy is made possible by the novel technique the team created, which enables every qubit to instantly connect to any other qubit. Entanglement, or what Einstein referred to as “spooky action at a distance,” is what causes this.
In this situation, no matter how far away two atoms are, they connect up and can communicate with one another. The strength of quantum computers is due to this phenomena.
“This entanglement can store and process an exponentially large amount of information,” Bluvstein said.
The brand-new research expands on the programmable quantum simulator that the group has been creating since 2017. In order to relocate entangled atoms while they are operating and without losing their quantum state, the researchers gave it new powers.
Previous studies on quantum systems demonstrated that the atoms, or qubits, are fixed in their places once the calculation process begins and only interact with qubits nearby, restricting the kind of quantum computations and simulations that may be done between them.
The ability of the researchers to produce and store data in so-called hyperfine qubits is crucial. These stronger qubits’ quantum states persist for a lot longer in their system than do conventional qubits (several seconds versus microseconds). It provides them the time to form complicated states of entangled atoms by entangling them with other qubits, even those located far away.
The full procedure appears as follows: Qubits are first paired by the researchers, who then pulse a global laser from their apparatus to produce a quantum gate that entangles the pairs, storing the pair’s information in the hyperfine qubits.
Then, to entangle additional atoms in the system, scientists transfer these qubits into new pairings with other atoms in the system using a two-dimensional array of individually focused laser beams known as optical tweezers. They carry out the procedures repeatedly in whichever order they choose, resulting in several quantum circuit types that can execute various algorithms.
Eventually, the atoms are sufficiently dispersed and connected to one another to serve as backups for one another in the event of an error, a state known as the cluster state.
Bluvstein and his coworkers have already created a programmable, error-correcting quantum computer using this design, and they intend to grow it up from there. Their idea of a quantum processor is now based on the system.
“In the very near term, we basically can start using this new method as a kind of sandbox where we will 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 top is and can now actually see a path from where we are to the highest top.”
