Quantum entanglement, the invisible connection between particles that entwines them in such a way that they act as one, has fascinated scientists for decades. It is also one of the most important ingredients for technologies to come—quantum computers, secure networks, and sensors with never-before-seen accuracy. But turning that theoretical concept into working devices was not easy.
Researchers at UNSW Sydney have now shown an unprecedented way to entangle atomic nuclei in silicon, creating quantum states across record distances. The study is a step toward making scalable quantum machines to eventually power industries and change everyday life.
A New Way to Link Atoms
The group managed to link the nuclear spins of two phosphorus atoms embedded in a silicon chip. The two atoms had each had an electron attached, and those electrons were tricked into communicating with each other. Essentially, the electrons had worked like subatomic telephones, relaying messages between nuclei that were not otherwise able to talk to each other.
Lead author Dr. Holly Stemp described the breakthrough in simple terms: nuclei had been like people in soundproof rooms. They could talk to others in the same room, but not to people outside. “With this breakthrough, it’s as if we gave people telephones,” she said. “Now, people can talk between rooms that were essentially shut off.”
Dr Mark van Blankenstein, Dr Holly Stemp and Prof. Andrea Morello. (CREDIT: UNSW Sydney)
The “telephones” in this case are electrons that naturally fan out in space, allowing them to come into contact with each other over surprisingly great distances. When the electrons are hooked up, they enable the phosphorus nuclei to get “entangled,” even if they are 20 nanometers apart. To put that in perspective, that’s roughly one-thousandth the width of a human hair.
Dr. Stemp explained that if you make the nuclei about the same size as human beings, their separation would put one in Sydney and the other in Boston.
Why Nuclear Spins Are Important
Nuclear spins—tiny magnetic orientations deep inside atoms—are precious in quantum science because they’re so stable. Unlike electrons or photons, which degrade rapidly, nuclei are able to maintain quantum information for scores of seconds. In quantum physics, that’s nearly a lifetime.
But stability has come at a cost. Nuclei barely communicate with one another. The inherent magnetic interaction between two phosphorus nuclei is only some 10 hertz, even when they are a mere nanometer away from one another. That’s far too weak to be useful for quantum computing.
Previous efforts tried to use one electron as the mediator, linking numerous nuclei in proximity. That only worked with the atoms being squeezed into a few nanometers. Add more nuclei and the signals would become hopelessly entangled, similar to voices on the same broadcast frequency. That problem made the technique unscaleable.
Two electrons “spread out in space”. On the right we highlight the centres of their probability distributions, which are 20 nm apart; on the left, we use a different color scale to highlight that the two distributions “touch each other”. This is the mechanism by which the two nuclear qubits can communicate. (CREDIT: UNSW Sydney)
UNSW researchers solved the problem by allocating one electron to each nucleus and making the electrons interact. This gave a neat, tunable bridge across much greater distances.
Building the Silicon Device
To build the system, the team implanted two phosphorus donor atoms into an ultra-high-purity silicon-28 crystal lattice. Each of the donor atoms had one nucleus and one electron. Inside each donor, the nucleus and electron were tightly bound, with hyperfine coupling at about 111 and 113 megahertz in the two donors.
The trick was really making the two electrons talk. With a highly controlled process called Heisenberg exchange, the electrons paired up with a force of around 12 megahertz. That was strong enough to carry information between nuclei 16 to 20 nanometers apart—distances previously beyond reach.
Professor Andrea Morello, who has led the field at UNSW for more than 15 years, said this leap was essential. “The spin of an atomic nucleus is the cleanest, most isolated quantum object one can find in the solid state,” he said. “But the same isolation that makes them so clean makes it hard to connect them. We’ve now shown how to make them talk.”
The CZ Gate
The trick was to create a controlled-Z, or CZ, gate. This operation is a quantum logic building block that gives a 180-degree phase shift to one particle depending on the state of another. Put simply, it is an entanglement basic ingredient.
By preparing both nuclei, and then passing them exact pulses of electrons, the researchers conducted the CZ gate in just two microseconds. That’s quick because nuclear operations are generally slow. Running the gate on electron times permitted entanglement to be built much faster and with more consistency.
The team confirmed that they had achieved a Bell state, the hallmark of quantum entanglement. The measurements were 76 percent fidelity and 0.67 concurrence, both high metrics that real entanglement had been achieved.
Taming Errors and Challenges
Despite the success, there was nothing absolutely perfect. The largest source of error was in initializing the electrons. Nuclei were readable more than 99 percent of the time, but placing electrons in their ground state worked only 86 percent. This limited the overall fidelity of the nuclear entanglement.
Long-range entanglement between two nuclei. Circuit diagram depicting the pulse sequence used to generate the Bell state. (CREDIT: Science)
The researchers explain that this hurdle can be overcome with more effective cooling and better electronics. Techniques demonstrated in other labs already achieve electron fidelities greater than 98 percent. With further refinement, the UNSW process might reach entangled state fidelity greater than 99 percent, the threshold for useful quantum computers.
Yet another challenge is placing donor atoms precisely in the silicon crystal lattice. Minor differences in spacings will alter the depth of the interaction of electrons. With the help of deterministic single-ion implantation, scientists hope to implant atoms with an accuracy to a pinpoint, ensuring consistent performance.
A Scalable Path to Quantum Machines
The most exciting aspect of the work is that it is scalable. The nuclei of phosphorus were entangled to the same 20-nanometer scale used in the semiconductor industry to make billions of transistors on computer chips. That means that the manufacturing processes of today being used on smartphones and laptops can be reused in quantum processors of tomorrow.
Dr. Stemp noted this connection: “This is our true technological progress—getting the cleanest quantum objects talk to each other at the same scale as today’s electronic devices.”
The phosphorus donors used in the experiment were implanted by a group at the University of Melbourne, and the ultra-pure silicon came from Japan’s Keio University. The multi-disciplinary collaboration indicates the degree to which the pursuit of quantum computing has gone global.
Looking Ahead
The breakthrough is an implementation of the 1998 theoretical vision for a silicon quantum computer by Bruce Kane. His proposal utilized electrons as nuclear spin linkers, and UNSW scientists have now demonstrated that it is feasible.
Forward looking entails expanding entanglement even more with the help of elongated electron shapes, large “jellybean” quantum dots, or superconducting resonators. These can link nuclei separated by as much as hundreds of nanometers. On a bright note, vast networks of nuclear qubits can ultimately cross silicon chips.
For now, the study is proof that nuclear spins, long considered to be too far apart, can become linked at technologically significant scales. It’s a milestone that advances the science from promise to actual, scalable devices.
Practical Applications of the Study
This research creates a roadmap to building full-scale quantum computers on the same silicon platform used in electronics today. By making nuclear spins talk across distances the size of today’s microchips, the research closes the gap between the strange quantum realm and real-world devices.
The advance could bring nearer the day that quantum computers can be constructed that can perform tasks beyond the capabilities of conventional computers—creating new medicines, modeling the climate system, or creating secure communication networks. With further development, the method can result in low-cost, mass-market quantum processors.
Research findings are available online in the journal Science.
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