The ability to instantaneously “teleport” information over long distances is one of the strange phenomena that could make quantum computers uniquely powerful. Researchers at Quantinuum have now shown they can achieve the feat with a “logical qubit,” which they say is a major milestone towards large-scale quantum computers.
Teleportation relies on another quirk of quantum mechanics know as entanglement, which can link physical systems together so that they share a quantum state. This connection makes it possible to quickly transfer quantum information between two entangled particles even when they’re a significant distance apart from each other.
Implementing this in a quantum computer is challenging though. The qubits used to encode quantum information are inherently noisy and unreliable, so researchers have created quantum error-correction codes that spread information across many physical qubits to create a more stable “logical qubit.” These are more resilient to errors and are seen as critical for achieving the holy grail of a fault-tolerant quantum computer. But they also make teleporting quantum information considerably more complicated due to the larger number of qubits involved.
In a paper published in Science earlier this month, a team from Quantinuum, in Broomfield, Colo., described how they reliably teleported logical qubits using the company’s trapped-ion quantum computer. David Hayes, principal theorist at Quantinuum, says the breakthrough could be crucial for building bigger quantum computers by speeding the transfer of information within machines. “This allows you to move [quantum information] from one side of the computer to the other in a flash,” he says.
The work was motivated by a challenge set by the U.S. Intelligence Advance Research Projects Activity (IARPA) in 2022 to demonstrate the ability to teleport logical qubits with a fidelity of over 95 percent. The group carried out their experiments on Quantinuum’s H2 processor, which features 32 trapped-ion qubits that shuttle around a tiny racetrack.
The protocol for teleporting a single qubit was developed back in 1993. First, two qubits are entangled, with qubit A remaining in the same place and qubit B transported to another location. Another qubit encoding the information that needs to be transmitted, qubit C, is then entangled with qubit A. A measurement is taken to compare the quantum states of qubit A and qubit C, which results in two bits of classical information that encode the relationship between their quantum states.
These bits are then transmitted by conventional means to qubit B. Because qubit A and B have identical quantum states, this information can be used to transform the quantum state of qubit B so that it becomes identical to qubit C. The quantum information in qubit C is thereby teleported to qubit B without being directly transmitted.
The H2 processor is housed in an ultrahigh-vacuum chamberQuantinuum
The protocol gets much more complicated when working with logical qubits, though, says Hayes, because it requires you to entangle groups of qubits rather than individual ones. This was made slightly easier on Quantinuum’s hardware because the ability to physically move the qubits along the company’s racetrack makes it possible to connect them arbitrarily. This allowed the group to use so-called “transversal gates” to entangle their logical qubits, which were each made up of seven physical qubits.
“You can almost imagine the logical qubits sitting right on top of one another and interacting directly,” says Hayes. “Qubit one interacts with the other qubit one, qubit two interacts with the other qubit two, and all seven of them do that.”
This simplicity allowed them to teleport logical qubits with a fidelity of 97.5 percent, significantly exceeding the target set by IARPA. Not all quantum computers can use transversal gates, however, as many architectures feature qubits that are fixed in place, including those that rely on superconducting qubits. So the team also tested another approach known as “lattice surgery,” which makes it possible to entangle logical qubits without direct interaction between all of their physical qubits. However, the approach is more complicated and involves considerably more operations, says Hayes, so they were able to only achieve fidelities of 85.1 percent.
Being able to reliably teleport logical qubits could prove very useful for building larger trapped-ion quantum computers, says Hayes, and potentially even distributed quantum computing systems. While the ability to move qubits around is one of the architecture’s major selling points, it is a relatively slow process, which could become unwieldy at larger scales. A working teleportation protocol that doesn’t require physically moving qubits would make it possible to transfer quantum information just as quickly as a classical computer can, says Hayes, because all that needs to be transmitted is the two bits of classical information. “You can move information around really fast just through electrical signals and wires, almost at the speed of light,” he says.
The most exciting thing about the new results, says Stephen Bartlett, a professor of physics at the University of Sydney, is that they demonstrate it’s possible to actually compute on an error-corrected quantum computer. Previous experiments have shown that it’s possible to use error correction to “keep quantum information alive,” but not that this information could be manipulated. Using teleportation to move logical qubits around inside a processor shows that its “it’s starting to work as a computer and not just as a memory,” says Bartlett.
