Imroatul Nasiyah
In a groundbreaking leap for quantum computing, engineers at the University of New South Wales (UNSW) have successfully engineered ‘quantum entangled states’ between two atomic nuclei within silicon chips. This pivotal achievement signifies a major stride towards realizing the dream of large-scale quantum computers, a pursuit that stands as one of the most electrifying scientific and technological frontiers of the 21st century. The research, meticulously detailed in the prestigious journal Science on September 18th, unlocks the immense potential for fabricating the intricate microchips required for quantum computation by leveraging existing, well-established industrial technology and manufacturing processes.
Dr. Holly Stemp, the lead author of the study, articulated the profound implications of this breakthrough, emphasizing its role in paving the way for future quantum computing hardware. "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 stated, highlighting the remarkable precision achieved.
The core challenge that has long plagued quantum computer engineers is the delicate balancing act between two seemingly contradictory requirements: ensuring that the delicate quantum computing elements are shielded from the pervasive external interference and noise that can corrupt quantum information, while simultaneously enabling these elements to interact effectively to perform complex computations. This inherent difficulty has fostered a diverse landscape of hardware approaches vying for dominance in the race to build the first operational quantum computer. Some architectures excel at executing rapid quantum operations but are susceptible to noise, while others offer robust noise immunity but present significant hurdles in terms of operational complexity and scalability.
The UNSW team has, until this breakthrough, been operating within the latter category, focusing on a platform that prioritizes isolation and stability. Their approach ingeniously encodes quantum information using the nuclear spin of phosphorus atoms, meticulously implanted into a silicon chip. Scientia Professor Andrea Morello, from the UNSW School of Electrical Engineering & Telecommunications, elaborated on the inherent advantages of this method. "The spin of an atomic nucleus is the cleanest, most isolated quantum object one can find in the solid state," he explained, underscoring the purity of the quantum information carriers.
Professor Morello further recounted the significant progress his group has made over the past fifteen years, pioneering breakthroughs that have propelled this silicon-based technology to the forefront of 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 proudly noted. This demonstrated longevity and fidelity of quantum information, achieved for the first time in a silicon device, came with a significant caveat: the very isolation that rendered atomic nuclei so pristine also made it exceedingly difficult to connect them in the intricate architectures required for large-scale quantum processors.
Historically, the only viable method for enabling multiple atomic nuclei to interact and perform computations involved placing them in extremely close proximity within a solid material, often sharing the same surrounding electron. Dr. Stemp clarified the quantum mechanical underpinnings 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," she explained. However, the range of this electron-induced interaction is inherently limited. Furthermore, as more nuclei are integrated into the same electron’s influence, the ability to individually control each nucleus becomes increasingly compromised, posing a significant impediment to scaling.
This is where the UNSW team’s revolutionary breakthrough, metaphorically described as enabling atomic nuclei to communicate via "electronic telephones," fundamentally alters the landscape. Dr. Stemp vividly illustrated the previous limitation: "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’."
The paradigm shift achieved by the UNSW researchers, as Dr. Stemp continued, is akin to equipping these individuals with telephones: "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."
The "telephones" in this quantum context are, in fact, electrons. Mark van Blankenstein, another key contributor to the research, provided a more technical explanation of the underlying physics. "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." This mediated interaction allows for communication between nuclei that are not necessarily in immediate physical contact.
The distances involved in these experiments, while seemingly small in absolute terms, represent a monumental achievement in the context of quantum computing. The nuclei in the UNSW experiments were separated by approximately 20 nanometers – a distance that is a mere one-thousandth of the width of a human hair. To put this into perspective, Dr. Stemp offered a 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!"
Crucially, Dr. Stemp highlighted the profound technological significance of this 20-nanometer separation: "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." This direct compatibility with existing semiconductor manufacturing infrastructure is a game-changer, promising a more rapid and cost-effective path to scaling quantum computers.
Despite the sophisticated and often counter-intuitive nature of quantum phenomena, the researchers emphasize that the devices they have developed remain fundamentally compatible with the established methods of building conventional computer chips. The precise implantation of phosphorus atoms into the silicon substrate was a collaborative effort, involving the expertise of Professor David Jamieson’s team at the University of Melbourne and utilizing an ultra-pure silicon slab supplied by Professor Kohei Itoh at Keio University in Japan.
By liberating atomic nuclei from their previous dependence on being tethered to the same electron, the UNSW team has effectively dismantled one of the most significant roadblocks to scaling silicon quantum computers based on atomic nuclei. Professor Morello expressed optimism about the future scalability of their approach. "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 stated. The inherent malleability and ease of manipulation of electrons are key advantages. "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 ability to precisely control and switch interactions is paramount for building complex quantum algorithms and achieving fault-tolerant quantum computation. The breakthrough signifies not just an advancement in quantum entanglement, but a pragmatic and industrially compatible pathway towards realizing the transformative potential of quantum computing.
