UNSW engineers have achieved a groundbreaking feat in quantum computing, successfully creating ‘quantum entangled states’ between two separate atomic nuclei within silicon chips. This signifies a crucial leap forward in the quest to build powerful quantum computers, as entanglement is the fundamental resource that imbues these machines with their extraordinary processing capabilities over conventional ones. The research, published on September 18th in the prestigious journal Science, represents a pivotal step towards realizing the ambitious vision of large-scale quantum computers, a paramount scientific and technological challenge of the 21st century.

Dr. Holly Stemp, the lead author of the study, emphasized the profound implications of this achievement. "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. This breakthrough unlocks the potential to leverage existing silicon chip manufacturing technology and processes for the construction of future quantum computing microchips.

The core challenge in quantum computing hardware development has always been a delicate balancing act: isolating the delicate quantum bits (qubits) from environmental noise and interference, while simultaneously enabling them to interact for performing complex computations. This inherent tension has led to a diverse landscape of competing quantum computing architectures, each with its own strengths and weaknesses. Some platforms excel at rapid operations but are susceptible to noise, while others offer superior noise immunity but face significant hurdles in terms of operational complexity and scalability.

The UNSW team has historically focused on a platform that, until this latest breakthrough, was primarily characterized by its isolation. They have ingeniously employed the nuclear spin of phosphorus atoms, precisely implanted into a silicon chip, as the fundamental unit for encoding quantum information. Scientia Professor Andrea Morello, from the UNSW School of Electrical Engineering & Telecommunications, elaborated on this choice: "The spin of an atomic nucleus is the cleanest, most isolated quantum object one can find in the solid state."

Professor Morello highlighted the extensive groundwork laid by his group over the past decade and a half. "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. 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 advancement, the only viable method for enabling interactions between multiple atomic nuclei was to place them in very close proximity, typically within the same solid material and mediated by a shared electron. Dr. Stemp 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. 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."

Bridging the Gap: Atomic Nuclei Communicate via Electronic ‘Telephones’

To illustrate the significance of their achievement, Dr. Stemp employed a compelling analogy. "By way of metaphor, one could say that, until now, nuclei were like people placed in a sound-proof room," she explained. "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 fact, precisely controlled electrons. Mark van Blankenstein, another key contributor to the research, elaborated on the underlying 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 remarkable feat lies in the distance over which this entanglement was achieved. In the experiments, the nuclei were separated by approximately 20 nanometers. To put this into perspective, Dr. Stemp noted that this distance is about one-thousandth the width of a human hair. She further elaborated on the 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!"

Crucially, this 20-nanometer separation is precisely the scale at which modern silicon computer chips, found in personal computers and mobile phones, are routinely manufactured. "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 Path Forward for Silicon Quantum Computing

Despite the cutting-edge and seemingly exotic nature of the experiments, the researchers underscore that these quantum devices are fundamentally compatible with existing silicon chip manufacturing paradigms. The precision implantation of phosphorus atoms into the ultra-pure silicon substrate was a collaborative effort, with the team of Professor David Jamieson at the University of Melbourne providing their expertise, and Professor Kohei Itoh at Keio University in Japan supplying the high-quality silicon material.

By circumventing the necessity 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 qubits. 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 further emphasized 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 represents a significant stride towards building powerful and practical quantum computers by harnessing the established infrastructure and precision of the semiconductor industry.