Dr. Holly Stemp, the lead author of the study, highlighted the profound implications of this achievement, stating that it unlocks the potential to construct the next generation of microchips essential for quantum computing, leveraging existing, mature technologies and manufacturing processes. "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 explained, emphasizing the delicate balance required in quantum computing hardware.

The inherent challenge in building quantum computers lies in simultaneously satisfying two seemingly contradictory demands: the need to shield delicate quantum elements from environmental interference and noise, while simultaneously enabling them to interact precisely to perform complex computations. This intricate balancing act has led to a diverse array of hardware approaches vying for dominance in the quantum computing race. Some platforms excel at rapid operations but are susceptible to noise, while others offer superior noise isolation but present significant hurdles in terms of operational complexity and scalability.

The UNSW team has focused on a platform that, until this breakthrough, was primarily categorized in the latter group. Their approach utilizes the nuclear spin of phosphorus atoms, meticulously implanted into a silicon chip, as the medium for encoding quantum information. Scientia Professor Andrea Morello from the UNSW School of Electrical Engineering & Telecommunications elaborated on the fundamental advantage of this choice: "The spin of an atomic nucleus is the cleanest, most isolated quantum object one can find in the solid state."

Professor Morello further detailed the extensive groundwork laid by his group over the past 15 years, which has propelled this technology into serious contention in the quantum computing arena. "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 remarked. "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."

Previously, the only viable method for enabling multiple atomic nuclei to interact was to position them in very close proximity within a solid material, typically mediated by a single shared electron. Dr. Stemp, who conducted this research at UNSW and is now a postdoctoral researcher at MIT, explained the physics behind this limitation: "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 noted, "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 Talk Through Electronic ‘Telephones’

To better illustrate the significance of their 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 continued, "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 reality, electrons. Mark van Blankenstein, another key author on the paper, provided a more detailed explanation of the sub-atomic mechanism: "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 crucial question then becomes: how far apart were the nuclei involved in these groundbreaking experiments? Dr. Stemp revealed, "The distance between our nuclei was about 20 nanometers – one thousandth of the width of a human hair." While this distance might seem minuscule, she offered a striking analogy to convey its scale: "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 20-nanometer scale is precisely the dimension at which modern silicon computer chips are routinely fabricated for everyday electronic devices like 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 Way Forward

Despite the highly specialized and exotic nature of the quantum experiments, the UNSW researchers emphasize that their devices are fundamentally compatible with the established manufacturing paradigms of current computer chips. The precise implantation of phosphorus atoms into the silicon chip was a collaborative effort, involving the expertise of Professor David Jamieson’s team at the University of Melbourne and an ultra-pure silicon slab provided by Professor Kohei Itoh at Keio University in Japan.

By circumventing the necessity for atomic nuclei to be tethered to the same electron, the UNSW team has effectively removed the most significant impediment to the scalable development of silicon quantum computers that utilize atomic nuclei as their quantum bits. "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," stated Professor Morello. He further elaborated on the 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 pivotal step, bridging the gap between fundamental quantum phenomena and the practical realization of quantum computing on a scale that can leverage the vast infrastructure of the semiconductor industry.