Engineers at the University of New South Wales (UNSW) have achieved a significant breakthrough in quantum computing by creating so-called "quantum entangled states." In these states, two separate particles become so closely linked that they no longer behave independently. To accomplish this, the researchers used the nuclear spins of two atoms—a key resource that gives quantum computers their advantage over traditional ones.

The study, published on September 18 in the journal Science, marks an important step toward building scalable quantum computers—one of the most ambitious scientific and technological challenges of the 21st century. This achievement opens the door to developing future microchips for quantum computing using existing technologies and manufacturing processes. As the lead author notes, the team managed to make the purest and most isolated quantum objects interact with each other at the same scale as today’s standard silicon electronic devices.

Balancing Isolation and Interaction

Quantum computer engineers face the challenge of finding the right balance between shielding computational elements from external interference and noise, and allowing them to interact for calculations. This is why there are many different hardware platforms vying to become the first working quantum computer: some offer fast operations but are susceptible to noise, while others are well-protected but difficult to scale. The UNSW team chose a platform previously considered part of the latter group, using the nuclear spins of phosphorus atoms embedded in a silicon wafer to encode quantum information.

Professor Andrea Morello from the UNSW School of Electrical Engineering and Telecommunications explains: “The spin of an atomic nucleus is the purest and most isolated quantum object you can find in solid matter.” Over the past 15 years, the group has made all the key breakthroughs that have turned this technology into a real contender in the quantum computing race. It has already been shown that quantum information can be stored for over 30 seconds—which is an eternity in the quantum world—and logical operations can be performed with less than 1% error. However, the high degree of isolation that makes atomic nuclei so pure also makes it difficult to combine them into a scalable quantum processor.

A New Approach to Scaling

Until recently, the only way to control multiple atomic nuclei was to place them very close together inside a solid and surround them with the same electron. While an electron can “spread out” in space and interact with several nuclei, this range is limited. Moreover, increasing the number of nuclei linked to a single electron complicates individual control of each nucleus.

A researcher explains this with a metaphor: “Previously, nuclei were like people in a soundproof room—they could talk to each other, but couldn’t hear anything outside, and only a limited number of people could fit in the room. This way of communicating doesn’t scale.” Thanks to the new breakthrough, the “people” now have telephones to connect with other rooms, and the role of these “telephones” is played by electrons.

At the subatomic level, two electrons can “touch” each other over significant distances thanks to their ability to spread out in space. If each electron is directly linked to an atomic nucleus, the nuclei can interact with each other through this electronic connection.

Experiment Scale and Future Prospects

In the experiment, the distance between the nuclei was about 20 nanometers—roughly one-thousandth the width of a human hair. If you imagine each nucleus enlarged to the size of a person, the distance between them would be comparable to that between Sydney and Boston. This scale matches the size of modern silicon microchips used in personal computers and mobile phones. As the researchers point out, the billions of silicon transistors in your devices are about 20 nanometers in size. This is a true technological breakthrough: the team managed to make the purest and most isolated quantum objects interact at the same scale as existing electronic devices. This means that manufacturing processes developed for the semiconductor industry can be adapted to create quantum computers based on atomic nuclear spins.

Technological Compatibility and the Future

Despite the exotic nature of the experiments, the researchers emphasize that these devices are compatible with modern computer chip manufacturing technologies. The phosphorus atoms were embedded in the chip using ultra-pure silicon provided by Keio University in Japan, and the chip itself was fabricated by a team from the University of Melbourne.

By eliminating the need to link atomic nuclei to a single electron, the UNSW team has overcome the main obstacle to scaling up silicon-based quantum computers using atomic nuclei. According to Professor Morello, their method is exceptionally reliable and scalable. The current experiment used only two electrons, but in the future, more electrons can be added and arranged in elongated shapes to further increase the distance between nuclei. Electrons can be easily moved and “shaped” as needed, allowing for fast and precise switching of interactions—exactly what is required for a scalable quantum computer.