Oxford researchers pull off quantum first with distributed gate teleportation

Einstein’s spooky action at a distance just got an upgrade

Oxford University researchers have taken a significant step toward large-scale distributed quantum computing by demonstrating the first successful quantum teleportation of a controlled quantum gate between two modules.

Published in Nature, the study doesn't claim to be the first to achieve quantum teleportation - after all, scientists have been teleporting quantum states for years. What is new is the deterministic (eg, once entanglement is established, the teleportation always succeeds) and repeatable teleportation of a quantum logic gate.

"Previous demonstrations of quantum teleportation have focused on transferring quantum states between physically separated systems," study lead Dougal Main, a graduate student at Oxford's physics department, said in a university announcement. "In our study, we use quantum teleportation to create interactions between these distant systems."

Quantum gates - the building blocks of quantum computers - manipulate qubit states. The Oxford team claims to have deterministically teleported a fundamental two-qubit quantum gate across two metres of optical fiber, linking two separate quantum modules.

"One of the key aspects of our result is that the interactions between the two separated qubits can be done deterministically, even if the photonic link connecting the two modules is lossy," Main explained in an emailed comment. "This is particularly important for quantum computing, since if these interactions were probabilistic [ie, prone to failure], then for large computations, the probability of successfully completing a computation without any failures becomes exponentially small."

"We deterministically teleport a controlled-Z (CZ) gate between two circuit qubits in separate modules, achieving 86 percent fidelity," the group reported in their paper. Their work marks "the first implementation of a distributed quantum algorithm comprising several non-local, two-qubit gates." 

This breakthrough enables us to effectively 'wire together' distinct quantum processors into a single, fully-connected quantum computer

What makes the achievement important is its potential to tackle quantum computing's scalability challenge. According to Oxford, a practical quantum computer that is powerful enough to be industry-disrupting would need to be capable of processing millions of qubits, making it prohibitively large and complex. The approach demonstrated by Main's team shows how distributing quantum operations across smaller, interconnected devices - each handling only a few qubits - could provide a scalable path forward for building large-scale quantum systems.

"This breakthrough enables us to effectively 'wire together' distinct quantum processors into a single, fully-connected quantum computer," Main said. 

Not only did the team achieve quantum gate teleportation, but they also demonstrated Grover's algorithm - a quantum algorithm designed to speed up searches through unstructured data - using fewer queries as part of their experiment.

"In the two-qubit case, there are four items to search through. Classically, the item a could be identified with, on average, two queries," the paper authors said. "Using the quantum circuit … the same task can be accomplished with only one query." 

The team achieved a 71 percent success rate with Grover's, and also demonstrated distributed iSWAP and SWAP gates. 

A far-from-perfect first

"In our experiment, we linked two trapped-ion quantum computers located two meters apart," Main told The Register in an email. "There's no fundamental limit to how far apart they can be," Main added, though he did say that longer distances cause signal loss that can slow down operations. One potential solution is quantum repeaters, which have made significant progress in recent years.

"A key advantage, though, is that these losses don't destroy any quantum information," Main added. "With gate teleportation, you only carry out the final step of the process once entanglement is successfully established." 

But at two meters apart, and just 86 percent fidelity, Main's team's work isn't exactly ready to build usable distributed quantum computers quite yet. 

"Typically, you'd want entangling gates with fidelities above 99.9 percent," Main told us. 

That said, Main noted that his work is a proof of concept that shows a distributed quantum computing approach works - now we just need time, money and more research to improve the fidelity. 

"The very first quantum computers had relatively low fidelities, but have improved greatly over the past two decades," Main said. "With the increasing commercial investment in quantum technologies, we anticipate rapid progress for distributed quantum computing. Some companies are already building toward this kind of architecture." 

It's not clear how long it'll take to get that improved fidelity. The research team hopes their work will lead to distributed quantum computing, a quantum internet, improved cryptography, physics, and more.

Beyond improving the error rate, quantum information processing across a network is achievable using current technology, noted Professor David Lucas, principal investigator on Main's research team.

"Scaling up quantum computers remains a formidable technical challenge that will likely require new physics insights as well as intensive engineering effort over the coming years," said Lucas, lead scientist of the UK quantum computing and simulation hub at Oxford. But "our experiment demonstrates that network-distributed quantum information processing is feasible with current technology." ®