Imroatul Nasiyah
UNSW engineers have achieved a monumental breakthrough in the quest for quantum computing by successfully creating "quantum entangled states" within silicon chips, a pivotal development that allows two separate atomic nuclei to become so profoundly linked that they transcend independent behavior. This deep entanglement is the very essence of quantum computing’s power, providing the fundamental advantage over conventional computers. This groundbreaking research, published on September 18th in the prestigious journal Science, marks a significant stride towards the realization of large-scale quantum computers, a challenge at the forefront of scientific and technological ambition in the 21st century.
Dr. Holly Stemp, the lead author of the study, emphasized the profound implications of this achievement, stating that it unlocks the potential to construct the future microchips necessary for quantum computing by leveraging existing, established 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, highlighting the crucial convergence of quantum precision with industrial-scale fabrication.
The inherent challenge in quantum computing engineering lies in a delicate balancing act: shielding the delicate quantum computing elements from the pervasive interference and noise of the external environment while simultaneously enabling them to interact precisely enough to perform complex computations. This intricate trade-off has fueled a diverse landscape of hardware approaches vying for dominance in the race to build the first operational quantum computer. Some platforms excel at rapid operations but are susceptible to noise, while others offer superior noise immunity but present significant hurdles in terms of operational complexity and scalability.
The UNSW team has strategically invested in a platform that, until this recent breakthrough, was primarily categorized in the latter group. Their innovative approach involves encoding quantum information using the nuclear spin of phosphorus atoms, meticulously implanted within a silicon chip. "The spin of an atomic nucleus is the cleanest, most isolated quantum object one can find in the solid state," explained Scientia Professor Andrea Morello of the UNSW School of Electrical Engineering & Telecommunications.
Professor Morello elaborated on the UNSW group’s extensive pioneering work, stating, "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 further underscored the significance of their prior achievements, noting, "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."
Historically, the only viable method for operating multiple atomic nuclei involved positioning them in very close proximity within a solid matrix and having them share a single electron. Dr. Stemp, who conducted this pivotal research at UNSW and is now a postdoctoral researcher at MIT, provided further insight: "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 the inherent limitations of this approach: "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 vividly illustrate the paradigm shift, 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 "telephones," in the realm of quantum physics, are in fact electrons. Mark van Blankenstein, another co-author on the paper, offered a more technical explanation of the underlying 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 question of the distance between the nuclei involved in these experiments is crucial to understanding the scalability of this approach. Dr. Stemp revealed, "The distance between our nuclei was about 20 nanometers – one thousandth of the width of a human hair." She then provided 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!"
Her statement underscored the profound technological alignment: "She adds that 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 entanglement experiments, the UNSW researchers emphasized that their devices remain fundamentally compatible with the established manufacturing paradigms of all current computer chips. The crucial implantation of phosphorus atoms into the silicon chip was meticulously executed by the esteemed 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 liberating atomic nuclei from the constraint of being tethered to the same electron, the UNSW team has effectively dismantled the most significant impediment to the large-scale fabrication of silicon quantum computers based on atomic nuclei. "Our method is remarkably robust and scalable," affirmed Professor Morello. "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 further elaborated on the advantages of this electron-mediated communication: "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 critical leap towards harnessing the immense power of quantum mechanics for practical, scalable computing applications.
