Quantum bits, or qubits, are notoriously fragile, susceptible to spontaneous errors that have long posed a barrier to the practical realization of reliable quantum machines. While error correction codes help mitigate this problem by encoding a single logical qubit into multiple physical qubits, the exponential increase in physical qubit resources—termed hardware overhead—has remained a daunting engineering challenge. The GKP codes present a theoretical framework to significantly alleviate this overhead by encoding logical qubits in continuous quantum variables, effectively translating the analog nature of quantum states into discrete, digital-like patterns that facilitate error detection and correction.

Until now, GKP codes have existed largely as a theoretical promise rather than an experimentally viable solution. The team led by Dr. Tingrei Tan has not only materialized these codes in the lab but has also engineered a universal set of quantum gates acting on GKP-encoded qubits. This universal set is fundamental because it means researchers can perform any quantum operation necessary for computation using qubits stored within the same physical quantum system. Their approach leverages exquisite control over the harmonic oscillations of a single trapped ytterbium ion, manipulating its motion in quantized vibrational modes to represent two logical qubits simultaneously.

Achieving entanglement between these logical qubits was a pivotal milestone. Entanglement, a uniquely quantum phenomenon where the state of one qubit instantaneously correlates with another regardless of distance, underpins the enhanced computational capabilities of quantum computers. Here, rather than entangling separate physical qubits, the researchers ingeniously entangled two distinct quantum vibrational modes—akin to quantized oscillations—within a single atom. This “quantum plumbing” not only conserves physical resources but also simplifies the traditionally complex architecture of quantum processors.

The logic gate constructed operates by precisely tuning the trapped ion’s quantum vibrations using advanced quantum control software developed by Q-CTRL, a spin-off from the laboratory itself. This control software employs physics-based models to minimize deleterious distortions to the delicate GKP code states throughout quantum operations. Maintaining the GKP code’s intricate structure during gate execution is paramount, as any degradation could negate the error-correcting advantages these codes provide. The experimental fidelity achieved in these control protocols demonstrates a crucial proof of concept for high-quality logical qubit manipulation within a practicable physical system.

A central experimental tool in this research is the Paul trap, a sophisticated device that confines charged ions using oscillating electric fields generated by precisely arranged electrodes. Unlike many quantum platforms requiring ultracold conditions, this trap operates at room temperature while maintaining stable control over the ion’s complex vibrational dynamics in three dimensions. By isolating and manipulating two specific motional modes of the ytterbium ion, the team effectively harnessed the continuous quantum variables necessary for GKP encoding, merging the mechanical quantum properties of the ion with state-of-the-art quantum error correction techniques.

This accomplishment represents more than a novel method; it acts as a blueprint for dramatically scaling quantum computers while overcoming one of their most formidable limitations: the physical qubit resource overhead. By embedding two error-correctable logical qubits within a single atom and demonstrating entanglement gates between them, the researchers have significantly lowered the barrier to hardware-efficient quantum computation. This efficiency is critical as the quantum computing community races to build devices with millions of logical qubits, which until now required unimaginably complex arrays of physical qubits.

The implications extend beyond mere hardware efficiency. The logical gates realized in this work establish a path toward more robust quantum information processing that leverages continuous-variable quantum systems. Unlike conventional qubits, which are two-state systems, continuous-variable qubits stored in harmonic oscillators enable richer encoding schemes and naturally integrate with quantum error correction protocols such as the GKP code. This hybrid approach effectively combines the benefits of discrete and continuous quantum systems, broadening the technological toolbox for quantum engineers.

Collaborator and lead author Vassili Matsos emphasizes the collaboration between theoretical and experimental quantum control, highlighting how the precisely engineered gate designs were made possible by integrating quantum control algorithms with physical modeling of GKP states. This synergy not only realized the first universal logical gate set for GKP qubits but also points toward a future in which complex quantum algorithms can be executed with unprecedented fidelity using these codes, reducing error rates that have historically limited the scale and reliability of quantum processors.

Looking forward, the researchers aim to expand their methods to entangle multiple logical qubits and integrate these gates into larger quantum circuits, laying foundational work for scalable, fault-tolerant quantum computers. As quantum devices continue to grow in both size and complexity, innovations like this will be instrumental in overcoming the resource bottleneck. They bring the theoretical promises of quantum error correction and continuous-variable qubits into tangible, programmable architectures, accelerating the timeline for quantum technologies capable of transformative applications in cryptography, materials science, and beyond.

This breakthrough, published in Nature Physics, not only validates longstanding theoretical models but raises compelling questions about the future architectures of quantum machines. By turning the abstract mathematical symmetries of the GKP code into a practical engineering reality, the University of Sydney researchers demonstrate how fundamental physics insights can revolutionize computing paradigms. Their achievement underscores the intricate dance between quantum theory, precision engineering, and control algorithms necessary to unlock quantum computing’s full potential.

The team acknowledges the critical role of international and interdisciplinary support, including funding from the Australian Research Council, the US Office of Naval Research, the US Army Research Office, the US Air Force Office of Scientific Research, Lockheed Martin, Sydney Quantum Academy, and private benefactors. Such collaboration and resources are essential as quantum research ventures deeper into uncharted scientific and technological territory, where each milestone requires the convergence of expertise and innovation at the highest level.

Media enquiries regarding this research can be directed to Marcus Strom at the University of Sydney. The detailed paper titled “Universal quantum gate set for Gottesman-Kitaev-Preskill logical qubits” offers comprehensive experimental data and technical discourse for those wishing to dive deeper into the mechanisms underpinning this advance.

Subject of Research: Not applicable

Article Title: Universal quantum gate set for Gottesman-Kitaev-Preskill logical qubits

News Publication Date: 21-Aug-2025

Web References:
– Quantum Control Laboratory: https://quantum.sydney.edu.au/research/quantum-control-laboratory/
– University of Sydney Nano Institute: https://www.sydney.edu.au/nano/
– Original article: https://www.nature.com/nphys/

References:
Matsos, V. et al ‘Universal quantum gate set for Gottesman-Kitaev-Preskill logical qubits’ (Nature Physics 2025) DOI: 10.1038/s41567-025-03002-8

Image Credits: Fiona Wolf/University of Sydney

Keywords

Quantum computing, quantum error correction, Gottesman-Kitaev-Preskill (GKP) code, logical qubits, trapped ion, quantum entanglement, Paul trap, quantum logic gates, harmonic oscillations, quantum control, continuous-variable quantum systems, scalable 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