Quantum entanglement, a phenomenon famously dubbed ‘spooky action at a distance’ by Albert Einstein, has historically captivated the imagination and perplexed even the most experienced scientists.
However, for those working in quantum physics today, entanglement is a fundamental and crucial connection between particles, serving as the defining characteristic of quantum computers.
While these advanced computing devices are still in their early stages, it’s this unique entangled connection that will empower them to perform tasks impossible for classical computers. This includes the ability to simulate complex natural quantum systems, from intricate molecules and new pharmaceuticals to powerful catalysts, with unprecedented accuracy.
In a new study recently published in the prestigious journal *Science*, my team and I successfully demonstrated quantum entanglement between two atomic nuclei, even when they were separated by approximately 20 nanometers.
Although 20 nanometers might seem like a small distance, the technique we employed represents both a practical and conceptual leap forward. It could be instrumental in building quantum computers utilizing one of the most accurate and stable systems known for preserving quantum information.
The Delicate Balance: Controlling Quantum Systems Amidst Noise
Quantum computer engineers face a significant hurdle: how to effectively balance two inherently contradictory requirements.
On one hand, the extremely delicate computing elements must be rigorously protected from any outside interference or noise. On the other, we need a reliable way to interact with these elements to perform valuable computations.
This fundamental dilemma explains why numerous hardware approaches are currently competing to develop the first truly functional quantum computer.
Some existing designs excel at rapid operations but are highly susceptible to noise. Conversely, others offer excellent shielding from noise but prove challenging to operate and expand for larger systems.
Enabling Atomic Nuclei to Communicate
My team has dedicated its efforts to a quantum computing platform that, until recently, fit squarely into the latter category (noise-resistant but hard to scale). Our method involves implanting phosphorus atoms into silicon chips and leveraging the spin of their atomic nuclei to encode quantum information.
For a quantum computer to be truly useful, it must be able to handle many atomic nuclei simultaneously. Previously, the only way to achieve this was by positioning multiple nuclei extremely close together within a solid material, all sharing the influence of a single electron.
While we typically visualize electrons as being much smaller than an atom’s nucleus, quantum mechanics reveals that an electron can ‘spread out’ across space, allowing it to interact with several atomic nuclei simultaneously.
Despite this ‘spreading’ ability, the range of a single electron remains significantly constrained. Furthermore, increasing the number of nuclei interacting with the same electron makes it incredibly difficult to precisely control each individual nucleus.
Introducing Electronic ‘Telephones’ for Quantum Communication
Imagine this: until now, our atomic nuclei were like individuals confined to soundproof rooms. They could converse clearly, but only if they were all present in the very same room.
They couldn’t hear anything from the outside world, and the room had limited capacity. This ‘room-based’ conversation method simply wasn’t scalable for larger quantum systems.
Our recent research is akin to providing these individuals with telephones, allowing them to communicate with people in other rooms. Each room maintains its peaceful isolation, but now, extensive conversations can occur between many more individuals, even across greater distances.
In this analogy, the ‘telephones’ are actually electrons. Because of their quantum nature, electrons can delocalize or ‘spread out’ in space, enabling two electrons to interact even when physically separated by a considerable distance.
Crucially, if each electron is directly linked to an atomic nucleus, the nuclei can then communicate indirectly through the interaction of their respective electrons.
We leveraged this electron-mediated channel to establish quantum entanglement between the nuclei. This was achieved using a technique known as the ‘geometric gate,’ a method we previously employed for high-precision quantum operations with silicon-based atoms.
Remarkably, we have now demonstrated—for the first time in a silicon platform—that this method can be scaled beyond just pairs of nuclei connected by a single electron.
Seamless Integration with Existing Integrated Circuits
During our experiment, the phosphorus nuclei were positioned 20 nanometers apart. While this might still appear to be a minuscule separation, it’s worth noting that this distance encompasses fewer than 40 silicon atoms.
Crucially, this 20-nanometer scale is precisely the dimension at which conventional silicon transistors are manufactured. Achieving quantum entanglement at this level implies that our durable, well-protected nuclear spin qubits can be integrated directly into the established architecture of standard silicon chips, much like those found in our everyday smartphones and computers.
Looking ahead, we aim to extend the entanglement distance even further. This is possible because electrons can be physically manipulated, or ‘squeezed’ into more elongated configurations, enhancing their reach.
This recent breakthrough signifies that advancements in electron-based quantum devices can now directly contribute to building quantum computers that leverage robust, long-lived nuclear spins for highly reliable computations.
Andrea Morello, a professor of quantum nanosystems at UNSW Sydney, is the author of this article, which was originally published by The Conversation.