UNSW engineers have achieved a monumental leap in the quest for quantum computing, successfully creating ‘quantum entangled states’—a phenomenon where two distinct particles become so profoundly linked they defy independent existence—by manipulating the spins of two atomic nuclei within silicon chips. This groundbreaking entanglement is the very cornerstone that bestows quantum computers with their extraordinary computational prowess, far surpassing that of conventional machines. Published on September 18th in the esteemed journal Science, this research marks a pivotal stride towards the realization of large-scale quantum computers, a pursuit that stands as one of the most captivating scientific and technological frontiers of the 21st century.

Dr. Holly Stemp, the lead author of the study, elucidated that this remarkable achievement democratizes the potential for constructing the future microchips essential for quantum computing, leveraging the very infrastructure and manufacturing processes already in place within the semiconductor industry. "We succeeded in making the cleanest, most isolated quantum objects talk to each other, at the scale at which standard silicon electronic devices are currently fabricated," she declared, highlighting the seamless integration of quantum innovation with existing technological paradigms.

The inherent challenge in quantum computer engineering has been a delicate balancing act: the imperative to shield the delicate quantum computing elements from external interference and debilitating noise, while simultaneously facilitating their interaction to execute complex computations. This intricate dilemma has fueled the diverse landscape of hardware approaches currently vying for dominance in the race to build the first operational quantum computer. Some systems excel at performing rapid operations but are plagued by susceptibility to noise, while others offer superior noise immunity but present formidable challenges in terms of operational complexity and scalability.

The UNSW team has strategically focused on a platform that, until this pivotal development, was largely categorized within the latter group. Their approach hinges on encoding quantum information within the nuclear spin of phosphorus atoms, meticulously implanted into a silicon chip. Scientia Professor Andrea Morello, from UNSW’s School of Electrical Engineering & Telecommunications, underscored the intrinsic superiority of this method, stating, "The spin of an atomic nucleus is the cleanest, most isolated quantum object one can find in the solid state."

Professor Morello further elaborated on the extensive groundwork laid by his group over the past decade and a half, which has propelled this technology to the forefront of the quantum computing race. "Over the last 15 years, our group has pioneered all the breakthroughs that made this technology a real contender in the quantum computing race. We already demonstrated that we could hold quantum information for over 30 seconds—an eternity, in the quantum world—and perform quantum logic operations with less than 1% errors." He acknowledged the initial limitations, "We were the first in the world to achieve this in a silicon device, but it all came at a price: the same isolation that makes atomic nuclei so clean, makes it hard to connect them together in a large-scale quantum processor."

Prior to this breakthrough, the primary method for orchestrating interactions between multiple atomic nuclei involved positioning them in extremely close proximity within a solid material and ensnaring them within the embrace of a single electron. Dr. Holly Stemp, who conducted this groundbreaking research at UNSW and is now a postdoctoral researcher at MIT, offered a vivid explanation of this phenomenon: "Most people think of an electron as the tiniest subatomic particle, but quantum physics tells us that it has the ability to ‘spread out’ in space, so that it can interact with multiple atomic nuclei." However, she cautioned, "Even so, the range over which the electron can spread is quite limited. Moreover, adding more nuclei to the same electron makes it very challenging to control each nucleus individually."

Making Atomic Nuclei Communicate Through Electronic ‘Telephones’

To encapsulate the essence of this advancement, Dr. Stemp employed a compelling metaphor: "By way of metaphor one could say that, until now, nuclei were like people placed in a sound-proof room. They can talk to each other as long as they are all in the same room, and the conversations are really clear. But they can’t hear anything from the outside, and there’s only so many people who can fit inside the room. This mode of conversation doesn’t ‘scale’." She then vividly contrasted this with the new paradigm: "With this breakthrough, it’s as if we gave people telephones to communicate to other rooms. All the rooms are still nice and quiet on the inside, but now we can have conversations between many more people, even if they are far away."

These metaphorical "telephones" are, in fact, electrons, acting as conduits for quantum communication. Mark van Blankenstein, another key author on the paper, delved into the intricate sub-atomic mechanics at play: "By their ability to spread out in space, two electrons can ‘touch’ each other at quite some distance. And if each electron is directly coupled to an atomic nucleus, the nuclei can communicate through that."

The question of spatial separation between the communicating nuclei is crucial. Dr. Stemp revealed, "The distance between our nuclei was about 20 nanometers—one thousandth of the width of a human hair." She then offered a striking analogy to convey the significance of this distance: "That doesn’t sound like much, but consider this: if we scaled each nucleus to the size of a person, the distance between the nuclei would be about the same as that between Sydney and Boston!" This comparison underscores the immense leap in controlling quantum interactions at relevant scales.

Crucially, Dr. Stemp emphasized the technological implications: "20 nanometers is the scale at which modern silicon computer chips are routinely manufactured to work in personal computers and mobile phones. You have billions of silicon transistors in your pocket or in your bag right now, each one about 20 nanometers in size. This is our real technological breakthrough: getting our cleanest and most isolated quantum objects talking to each other at the same scale as existing electronic devices. This means we can adapt the manufacturing processes developed by the trillion-dollar semiconductor industry, to the construction of quantum computers based on the spins of atomic nuclei."

A Scalable Pathway Forward

Despite the inherently exotic nature of quantum mechanics and these experiments, the researchers assert that the developed devices remain fundamentally compatible with the established manufacturing methodologies of all current computer chips. The precise implantation of phosphorus atoms into the silicon chip was a collaborative effort, undertaken by the distinguished team of Professor David Jamieson at the University of Melbourne, utilizing an ultra-pure silicon slab generously provided by Professor Kohei Itoh at Keio University in Japan.

By emancipating atomic nuclei from the constraint of being tethered to the same electron, the UNSW team has effectively dismantled the most significant impediment to the scalable development of silicon quantum computers that rely on atomic nuclei. Professor Morello expressed optimism regarding the future, stating, "Our method is remarkably robust and scalable. Here we just used two electrons, but in the future we can even add more electrons, and force them in an elongated shape, to spread out the nuclei even further." He concluded by highlighting the inherent advantages of this approach: "Electrons are easy to move around and to ‘massage’ into shape, which means the interactions can be switched on and off quickly and precisely. That’s exactly what is needed for a scalable quantum computer." This breakthrough signifies a profound step towards harnessing the full potential of quantum computing, paving the way for advancements that were once confined to the realm of theoretical possibility.