Quantum entanglement — the phenomenon where two or more particles become inseparably linked such that the state of one instantaneously influences the state of another regardless of distance — sits at the heart of the immense promise quantum computers hold over classical counterparts. However, harnessing this phenomenon in practical devices requires not just maintaining coherence but also engineering precise interactions between qubits, the quantum analogues of classical bits. UNSW’s novel approach employs the nuclear spins of phosphorus atoms precisely implanted in silicon, a widely used semiconductor substrate, to store and process quantum information.

For over 15 years, the UNSW team, led by Scientia Professor Andrea Morello, has made persistent strides in harnessing phosphorus nuclear spins, which are renowned as some of the most isolated quantum objects in the solid state. The exceptionally long coherence times—on the order of 30 seconds—combined with the ability to perform quantum logic operations with fidelity surpassing 99%, position these nuclear spins as ideal qubit candidates. Yet, the intrinsic isolation that renders them so clean simultaneously impedes controlled interaction, making it challenging to engineer robust multi-qubit operations necessary for universal quantum computing.

Traditionally, entangling multiple nuclear spins required positioning them in immediate proximity so they could share the same resident electron, the quantum mediator enabling coherent coupling. Unfortunately, this proximity requirement severely limits device scalability and complicates individual qubit addressability. The new UNSW study circumvents this bottleneck by introducing an innovative mechanism whereby two nuclear spins, separated by about 20 nanometers — roughly one-thousandth the width of a human hair — become entangled through electron-mediated communication that does not necessitate their sharing the same electron.

This electron-mediated interaction can be thought of as a quantum telephone line between distant atomic nuclei. Rather than restricting qubits to a confined “room,” where interactions are limited and cannot extend beyond immediate neighbors, electrons serve as delocalized mediators capable of “reaching out” and coupling nuclear spins located in physically separated regions of the silicon lattice. The scientists demonstrated this by controlling electron exchange interactions that effectively act as quantum gates, generating entangled states even when nuclei are spatially separated beyond the reach of direct coupling.

Such a manipulation of electron wavefunctions to enable remote entanglement represents a leap forward because it aligns perfectly with current silicon fabrication technologies. The scale of 20 nanometers is directly compatible with the transistor dimensions used in modern commercial microchips, meaning this quantum architecture has the potential to be integrated within existing semiconductor manufacturing pipelines. This compatibility is crucial for transitioning quantum computing from isolated laboratory demonstrations to industrial-grade, scalable quantum processors.

The team’s approach also maintains the key advantage of phosphorus nuclear spin qubits: their exceptional coherence. Unlike other physical qubit systems prone to environmental noise and rapid decoherence, the nuclear spins in this system remain well-isolated from disruptive interactions. By leveraging electrons as controllable mediators that can be dynamically moved and shaped into elongated wavefunctions, the researchers have demonstrated fast, tunable quantum operations without sacrificing coherence, a balance that has eluded many alternative quantum platforms.

Lead researcher Dr. Holly Stemp elaborates that the electron-mediated entanglement scheme offers a powerful means to scale up quantum processors. The electron “telephones” can be switched on and off with precision, allowing selective gate operations between desired pairs of nuclei while preventing unwanted crosstalk. This flexibility paves the way not only for two-qubit entanglement but also for more complex multi-qubit architectures, by increasing the number of electrons and dynamically controlling their spatial distribution within the silicon crystal.

This scalable design also brings with it a remarkable robustness. Owing to the universal nature of electron wavefunctions and well-understood silicon fabrication processes, the architecture opens a clear route toward manufacturable large-scale quantum chips. Integrating ultra-pure silicon substrates from Japan’s Keio University and precisely implanting phosphorus atoms using advanced ion implantation techniques honed at the University of Melbourne, the study underscores the profound importance of interdisciplinary collaborations in turning quantum science into viable technology.

The implications of this research are profound. By overcoming the need for nuclear spins to be bound to a single electron and instead enabling long-distance entanglement mediated by electron exchange, the UNSW team effectively removes one of the most significant barriers to developing quantum devices scalable to millions of qubits. This breakthrough brings the vision of silicon-based quantum computers—leveraging decades of semiconductor industry expertise—much closer to reality.

Moreover, the entanglement demonstrated in this work is not only a theoretical achievement but also experimentally verifiable, marking a critical step toward practical quantum error correction schemes and fault-tolerant quantum computing. As quantum processors grow in size and complexity, maintaining high-fidelity entanglement across well-isolated qubits at industrially relevant scales will be essential to realizing the full promise of quantum advantage across cryptography, simulation, and optimization.

Professor Morello emphasizes that while this result was obtained with a pair of nuclear spins, the principles underpinning the electron-mediated interactions readily scale to many more qubits. By shaping electrons into elongated wavefunctions—akin to quantum “fingers” reaching across the chip—it becomes feasible to network distant nuclei, achieving a coherent, controllable quantum processor architecture. This represents one of the most promising pathways to breaking the current quantum computing bottleneck.

Taken collectively, the UNSW team’s pioneering demonstration of scalable, electron-exchange-mediated nuclear spin entanglement marks a monumental stride forward on the quest for practical quantum computers. It not only showcases the power of silicon quantum devices but also highlights the elegant solutions that emerge at the confluence of fundamental physics, cutting-edge materials science, and innovative engineering. The future, it seems, increasingly belongs to the quantum revolution unfolding at the atomic scale inside everyday silicon chips.

Subject of Research: Quantum entanglement of nuclear spins mediated by electron exchange in silicon quantum devices

Article Title: Scalable entanglement of nuclear spins mediated by electron exchange

News Publication Date: 18-Sep-2025

References:
DOI: 10.1126/science.ady3799

Image Credits: Tony Melov / UNSW Sydney

Keywords: Quantum computing

Until now, one of the biggest roadblocks in quantum computing has been managing the interface between qubits and the classical electronics needed to control them. Traditional approaches employ wiring that connects room-temperature electronics to qubits maintained at sub-kelvin temperatures. This setup is increasingly impractical as the number of qubits rises, creating prohibitive heat loads, electrical noise, and latency issues. By engineering control electronics that operate in the millikelvin environment where the qubits reside, Professor Reilly’s team has successfully demonstrated a scalable path forward, ensuring that qubit fidelity and coherence are not compromised by proximity to control systems.

Central to this achievement is the use of spin qubits encoded in silicon, which offer inherent advantages due to their compatibility with standard complementary metal-oxide-semiconductor (CMOS) fabrication techniques. Unlike other qubit types, spin qubits hold promise for seamless integration with existing semiconductor manufacturing infrastructure, supporting possibilities for mass production. The control chip designed by the Sydney group employs these CMOS-compatible elements, enabling precise manipulation of electron spin states at temperatures just a fraction above absolute zero (-273.15°C). This marriage of mature semiconductor technology with delicate quantum systems is a pivotal advance.

Integrating control circuitry within the cryogenic environment entails formidable technical demands. Electronics must dissipate only minuscule amounts of power to avoid heating the qubits beyond their operational thresholds. The chip developed by the researchers consumes less than 10 microwatts, most of which powers digital logic, while analog components are optimized to dissipate merely nanowatts per megahertz. This ultra-low power consumption is critical, as any thermal disturbance risks decoherence of the fragile quantum states. The achievement reflects more than a decade of incremental design refinement to master low-noise, cryogenic operation of complex electronics.

Beyond power efficiency, the team tackled issues of electrical noise and interference, potential killers of qubit stability. The cryo-CMOS chip’s careful layout and shielding allow it to switch transistors in close proximity—less than a millimeter—from the qubits without measurable detriment to their coherence time or gate fidelity. Rigorous experimental benchmarking compared the chip’s performance against traditional room-temperature control systems connected via cables, finding negligible degradation in one- and two-qubit gate operations. This validates the feasibility of tightly integrated control-electronic architectures.

Industry cooperation played a vital role in realizing this milestone. The qubits themselves were furnished by Diraq, a spin-out from the University of New South Wales, under the leadership of Professor Andrew Dzurak. Meanwhile, the silicon-based control chip is being commercialized by Emergence Quantum, a company co-founded by Professor Reilly and Dr. Thomas Ohki that emerged explicitly to translate these laboratory advances into deployable technology. Such synergy between academic research and commercial ventures accelerates the journey from proof-of-concept demos to robust quantum hardware platforms.

At the heart of the scientific accomplishment lies a series of complex experiments meticulously conducted by PhD researcher and lead author Dr. Sam Bartee. These experiments characterized quantum gate operations controlled by the milli-kelvin CMOS chiplet, confirming resilience of quantum coherence and gate fidelity in an environment previously deemed too hostile for integrated electronics. Dr. Bartee’s work exemplifies the emerging generation of quantum engineers propelling the field into new frontiers of scale and practicality.

The implications of this cryogenic control platform extend well beyond computing. As Professor Reilly emphasizes, the capacity to integrate sophisticated electronics at ultra-low temperatures can revolutionize quantum sensors, which demand similar environmental conditions to exploit quantum-enhanced sensitivity. Moreover, data centers of the future, grappling with escalating energy consumption, might benefit from quantum devices designed with tightly coupled cryogenic control electronics, yielding systems that compute more efficiently and with greater precision.

Complementing the technical ingenuity is an astute commercial vision. Professor Dzurak, CEO of Diraq, highlights how integrating silicon qubits with classical control electronics in compact packages aims to make quantum computers more affordable and energy-efficient. This evolution could democratize access to quantum computational power, accelerating innovation in fields from cryptography to materials science. The advances bring us closer to realizing the long-envisioned potential of quantum technologies impacting everyday life.

Technically, the design challenges were immense, encompassing noise mitigation, thermal budget restrictions, and precise gate implementation. The cryo-CMOS chip’s architecture balances digital and analog domains, enabling precise pulse-shaping and qubit control sequences. Notably, the analog part dissipates an exceptionally low power which allows the system’s scalability up to millions of qubits without a corresponding jump in cooling requirements. This shifts the paradigm from bulky and unwieldy quantum systems toward compact, integrable quantum processors.

Professor Reilly’s assertion that fragile spin qubits "hardly notice" the presence of switching transistors nearby encapsulates the finesse achieved. This subtlety speaks to the meticulous electrical engineering that shields quantum states from decoherence while maintaining fast, accurate control signals. Such a breakthrough disrupts prior assumptions and validates decades of theoretical modeling about the viability of cryogenic integration.

Furthermore, the tight collaboration between multiple institutions and spin-out companies signifies a maturing ecosystem that can sustain the quantum computing revolution. Combining materials science, electrical engineering, quantum physics, and entrepreneurship, this research serves as a model for future interdisciplinary innovation. The shared expertise, resources, and strategic partnerships substantially reduce the time from discovery to real-world application.

Looking toward the future, this cryogenic control system opens avenues for broader exploration of hybrid quantum-classical architectures. As algorithms become more sophisticated and qubit counts swell, integrated control at operating temperatures eases architectural constraints. The work performed at the University of Sydney sets a powerful precedent and offers a foundation for global quantum technology efforts to build upon.

In summary, the “Spin-qubit control with a milli-kelvin CMOS chip” reported in Nature is a defining advancement that transforms our approach to quantum control electronics. By demonstrating ultra-low power, low-interference integrated circuits operating within the qubit cooling environment, Professor David Reilly and his team have paved the way for quantum computers operating at scales previously deemed impossible. Their contribution marks a turning point from experimental curiosity toward scalable quantum computing infrastructure, signaling a new era of technological potential.

Subject of Research: Quantum computing, spin qubits, and cryogenic control electronics integration for scalable quantum processors.

Article Title: Spin-qubit control with a milli-kelvin CMOS chip

News Publication Date: 25 June 2025

Web References:

• https://www.nature.com/
• https://www.sydney.edu.au/science/about/our-people/academic-staff/david-reilly.html
• https://emergencequantum.com/
• https://diraq.com/

References:
Bartee, S. et al. ‘Spin-qubit control with a milli-Kelvin CMOS chip’ (Nature 2025) DOI: 10.1038/s41586-025-09157-x

Image Credits: Fiona Wolf/University of Sydney

Keywords:
Quantum computing, Quantum processors, Electrical engineering, Quantum information, Quantum information processing, Quantum information science