In a milestone that brings quantum computing tangibly closer to large-scale practical use, scientists at Oxford University's Department of Physics have demonstrated the first instance of distributed quantum computing. Using a photonic network interface, they successfully linked two separate quantum processors to form a single, fully connected quantum computer, paving the way to tackling computational challenges previously out of reach. The results have been published in Nature.
The breakthrough addresses quantum's 'scalability problem': a quantum computer powerful enough to be industry-disrupting would have to be capable of processing millions of qubits. Packing all these processors in a single device, however, would require a machine of an immense size. In this new approach, small quantum devices are linked together, enabling computations to be distributed across the network. In theory, there is no limit to the number of processors that could be in the network.
By interconnecting the modules using photonic links, our system gains valuable flexibility, allowing modules to be upgraded or swapped out without disrupting the entire architecture.
Dougal Main, Department of Physics, University of Oxford
The scalable architecture is based on modules which each contain only a small number of trapped-ion qubits (atomic-scale carriers of quantum information). These are linked together using optical fibres, and use light (photons) rather than electrical signals to transmit data between them. These photonic links enable qubits in separate modules to be entangled*, allowing quantum logic to be performed across the modules using quantum teleportation.**
Although quantum teleportation of states has been achieved previously, this study is the first demonstration of quantum teleportation of logical gates (the minimum components of an algorithm) across a network link. According to the researchers, this could lay the groundwork for a future 'quantum internet,' where distant processors could form an ultra-secure network for communication, computation and sensing.
Study lead Dougal Main (Department of Physics) said: 'Previous demonstrations of quantum teleportation have focused on transferring quantum states between physically separated systems. In our study, we use quantum teleportation to create interactions between these distant systems. By carefully tailoring these interactions, we can perform logical quantum gates - the fundamental operations of quantum computing - between qubits housed in separate quantum computers. This breakthrough enables us to effectively 'wire together' distinct quantum processors into a single, fully-connected quantum computer.'
Dougal Main and Beth Nichol working on the distributed quantum computer. Image credit: John Cairns.The concept is similar to how traditional supercomputers work. These are made up of smaller computers linked together to achieve capabilities that are greater than those of each separate unit. This strategy circumvents many of the engineering obstacles associated with packing ever larger numbers of qubits into a single device, while preserving the delicate quantum properties needed for accurate and robust computations.
Professor David Lucas. Image credit: Martin Small.The researchers demonstrated the effectiveness of the method by executing Grover's search algorithm. This quantum method searches for a particular item in a large, unstructured dataset much faster than a regular computer can, using the quantum phenomena of superposition and entanglement to explore many possibilities in parallel. Its successful demonstration underscores how a distributed approach can extend quantum capabilities beyond the limits of a single device, setting the stage for scalable, high-performance quantum computers powerful enough to run calculations in hours that today's supercomputers would take many years to solve.
Professor David Lucas, principal investigator of the research team and lead scientist for the UK Quantum Computing and Simulation Hub, led from the Department of Physics, said: 'Our experiment demonstrates that network-distributed quantum information processing is feasible with current technology. 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.'
The study 'Distributed Quantum Computing across an Optical Network Link,' has been published in Nature. Principal funding for this research was provided by UKRI EPSRC, via the UK Quantum Computing and Simulation (QCS) Hub, part of the UK National Quantum Technologies Programme.
*Quantum entanglement: Where two particles, such as a pair of photons, remain correlated even when separated by vast distances. This allows them to share information without having to travel physically.
** Quantum teleportation: The transfer of quantum information over long distances almost instantly, using entanglement.
