A groundbreaking advancement in the quest for powerful quantum computers has been achieved by engineers at UNSW, who have successfully created ‘quantum entangled states’ between two separate atomic nuclei embedded within silicon chips. This remarkable feat, where two particles become so profoundly linked that they transcend independent behavior, represents a pivotal step towards harnessing the immense computational power that quantum entanglement promises, offering a significant edge over conventional computing. The research, published on September 18th in the prestigious journal Science, heralds a new era for quantum computing, potentially enabling the construction of large-scale quantum machines—one of the most exhilarating scientific and technological frontiers of the 21st century.

Dr. Holly Stemp, the lead author of the study, emphasized the transformative potential of this breakthrough, stating that it unlocks the ability to build the essential microchips for quantum computing using established, existing 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. This achievement directly addresses a fundamental challenge in quantum computing: the delicate balancing act between shielding sensitive quantum elements from disruptive external interference and noise, while simultaneously facilitating their interaction to perform complex computations. The diverse landscape of quantum hardware currently being explored reflects this challenge; some platforms excel at rapid operations but are susceptible to noise, while others offer superior noise resistance but prove difficult to manipulate and scale.

The UNSW team has strategically focused on a platform that, until this development, was primarily categorized in the latter group. Their approach encodes quantum information using the spin of phosphorus atoms, meticulously implanted within a silicon chip. Scientia Professor Andrea Morello, from UNSW’s School of Electrical Engineering & Telecommunications, elaborated on the inherent advantages of this methodology: "The spin of an atomic nucleus is the cleanest, most isolated quantum object one can find in the solid state." He further highlighted the extensive progress his group has made over the past 15 years, pioneering breakthroughs that have positioned this technology as a serious 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," Professor Morello stated. He acknowledged that their earlier successes, including being the first to achieve these feats in a silicon device, came at a cost: the very isolation that made atomic nuclei so pristine also made connecting them for large-scale processing a significant hurdle.

Previously, the only known method to orchestrate interactions between multiple atomic nuclei involved placing them in extremely close proximity within a solid material and ensnaring them within the influence of a single electron. Dr. Stemp, who conducted this pivotal research at UNSW and is now a postdoctoral researcher at MIT, explained the quantum mechanical behavior of electrons: "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 that the extent of this spatial spread is limited, and the presence of additional nuclei interacting with the same electron complicates individual control.

To convey the essence of their breakthrough, 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 introduced the transformative aspect of their new method: "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, acting as intermediaries. Mark van Blankenstein, another key author on the paper, provided a more technical explanation of the sub-atomic mechanics: "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 researchers quantified the distances involved in their experiments: "The distance between our nuclei was about 20 nanometers—one thousandth of the width of a human hair," Dr. Stemp revealed. To put this into perspective, she offered another striking analogy: "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 scale of 20 nanometers is highly significant because it aligns precisely with the dimensions at which modern silicon computer chips are routinely fabricated for 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."

The implications of this research extend to the very scalability of silicon quantum computers. Despite the highly specialized and exotic nature of the quantum experiments, the UNSW team asserts that their devices remain fundamentally compatible with the established manufacturing paradigms of current computer chips. The crucial step of introducing the phosphorus atoms into the chip was facilitated by the expertise of Professor David Jamieson’s team at the University of Melbourne, using an ultra-pure silicon substrate provided by Professor Kohei Itoh at Keio University in Japan.

By circumventing the requirement for atomic nuclei to be tethered to the same electron, the UNSW team has effectively dismantled the most significant obstacle to scaling up silicon quantum computers that utilize atomic nuclei as their fundamental units of information. Professor Morello concluded with an optimistic outlook on the future: "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 emphasized the practical 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 monumental leap forward, paving a practical and scalable path toward realizing the immense potential of quantum computing.