The persistent challenge in realizing useful quantum computers lies in the ephemeral nature of information stored in qubits; they tend to lose their quantum state very rapidly. "The real challenge, the thing that stops us from having useful quantum computers today, is that you build a qubit and the information just doesn’t last very long," stated Andrew Houck, leader of a federally funded national quantum research center, Princeton’s dean of engineering, and co-principal investigator on the seminal paper. "This is the next big jump forward." Published on November 5th in the prestigious journal Nature, the Princeton team’s research details a qubit that maintains its quantum coherence for over 1 millisecond, a threefold increase over the longest previously documented lifetime in laboratory experiments and a staggering fifteenfold improvement compared to the standard employed in industrial quantum processors. Crucially, the team validated their breakthrough by constructing a functional quantum chip, demonstrating the design’s capacity for error correction and its scalability towards larger, more complex quantum systems.

This new qubit design is remarkably compatible with the established architectures utilized by industry giants like Google and IBM. Preliminary analyses suggest that integrating Princeton’s innovation into Google’s Willow processor could yield a thousandfold increase in performance. Houck further emphasized that the benefits of this design amplify exponentially as the number of qubits in a system increases.

The Critical Importance of Enhanced Qubit Stability in Quantum Computing

Quantum computers hold immense promise for tackling problems intractable for even the most powerful classical computers. However, their current capabilities are significantly hampered by the short coherence times of qubits, which often lose their information before complex computations can be completed. Extending coherence time is therefore paramount for the development of practical quantum hardware. Princeton’s achievement represents the most significant single leap in qubit coherence time seen in over a decade.

While diverse qubit technologies are being explored globally, Princeton’s innovation builds upon the well-established transmon qubit, a superconducting circuit operating at extremely low temperatures. Transmons are favored for their inherent resistance to environmental noise and their compatibility with contemporary manufacturing techniques. Despite these advantages, extending the coherence time of transmons has been an exceptionally difficult hurdle, with recent findings from Google identifying material defects as the primary obstacle in their most advanced processors.

A Novel Materials Strategy: The Synergy of Tantalum and Silicon

The Princeton researchers devised a two-pronged materials strategy to overcome these long-standing challenges. Their approach first involved the incorporation of tantalum, a metal renowned for its ability to retain energy within delicate circuits. Secondly, they substituted the conventional sapphire substrate with high-purity silicon, a material fundamental to the entire computing industry. Fabricating tantalum directly onto silicon presented a complex set of technical hurdles related to material interactions, but the researchers successfully navigated these complexities, uncovering significant advantages in the process.

Nathalie de Leon, co-director of Princeton’s Quantum Initiative and a co-principal investigator on the project, highlighted that their tantalum-silicon design not only surpasses previous approaches in performance but is also more amenable to large-scale manufacturing. "Our results are really pushing the state of the art," she asserted.

Michel Devoret, chief scientist for hardware at Google Quantum AI, which provided partial funding, underscored the formidable difficulty of extending the operational lifetime of quantum circuits, describing the field as a "graveyard" of attempted solutions. He lauded de Leon’s determination: "Nathalie really had the guts to pursue this strategy and make it work," he remarked, referencing Devoret’s own upcoming Nobel Prize in Physics for 2025.

The project received its primary financial backing from the U.S. Department of Energy National Quantum Information Science Research Centers and the Co-design Center for Quantum Advantage (C2QA). Houck directed C2QA from 2021 to 2025 and currently serves as its chief scientist. Postdoctoral researcher Faranak Bahrami and graduate student Matthew P. Bland are credited as co-lead authors of the groundbreaking paper.

Unlocking Qubit Stability: The Role of Tantalum

Houck, who also holds the Anthony H.P. Lee ’79 P11 P14 Professorship of Electrical and Computer Engineering, elucidated the two key determinants of a quantum computer’s power: the sheer number of interconnected qubits and the number of operations each qubit can perform before accumulating errors. Enhancing the robustness of individual qubits positively impacts both these factors, with longer coherence times directly enabling greater scalability and more reliable error correction.

Energy loss is the most prevalent failure mechanism in these sensitive systems. Microscopic surface defects within the metallic components can trap energy, disrupting qubit operations during calculations. These disruptions are amplified as more qubits are integrated. Tantalum offers a distinct advantage due to its inherently lower concentration of such defects compared to materials like aluminum, leading to fewer errors and simplifying the subsequent error correction processes.

Houck and de Leon initially proposed tantalum for superconducting chips in 2021, a concept that gained traction with the assistance of Princeton chemist Robert Cava, the Russell Wellman Moore Professor of Chemistry. Cava, a specialist in superconducting materials, became intrigued by the problem after attending one of de Leon’s presentations, eventually suggesting tantalum as a promising candidate. "Then she went and did it," Cava remarked with admiration. "That’s the amazing part."

Subsequent research across three different laboratory groups explored this idea, culminating in the construction of a tantalum-based superconducting circuit on a sapphire substrate. This initial development yielded a significant improvement in coherence time, nearing the previous world record.

Bahrami pointed out tantalum’s exceptional durability, noting its ability to withstand the rigorous cleaning processes essential for removing contamination during fabrication. "You can put tantalum in acid, and still the properties don’t change," she stated.

Upon meticulously removing contaminants, the team investigated the remaining sources of energy loss, identifying the sapphire substrate as the primary culprit. The transition to high-purity silicon effectively eliminated this issue. The synergistic combination of tantalum and silicon, coupled with refined fabrication techniques, resulted in one of the most substantial advancements ever achieved in transmon qubit technology. Houck described the outcome as "a major breakthrough on the path to enabling useful quantum computing." He further projected that, given the exponential scaling benefits, replacing current industry-leading qubits with Princeton’s design could theoretically enhance the effectiveness of a 1,000-qubit computer by a factor of one billion.

A Silicon-Based Design for Industry-Scale Advancement

This transformative project draws upon the distinct expertise of three research groups. Houck’s team excels in the design and optimization of superconducting circuits. De Leon’s laboratory focuses on quantum metrology, along with the critical materials and fabrication methodologies that dictate qubit performance. Cava’s group brings decades of experience in developing superconducting materials. This interdisciplinary collaboration allowed for achievements unattainable by any single group, and their success has already garnered significant attention from the quantum industry.

Devoret emphasized the indispensable role of collaborations between academic institutions and industry in propelling advanced technologies forward. "There is a rather harmonious relationship between industry and academic research," he observed, explaining that university researchers can explore the fundamental limits of quantum performance, while industry partners can then translate these discoveries into large-scale applications.

"We’ve shown that it’s possible in silicon," de Leon concluded. "The fact that we’ve shown what the critical steps are, and the important underlying characteristics that will enable these kinds of coherence times, now makes it pretty easy for anyone who’s working on scaled processors to adopt."

The paper, "Millisecond lifetimes and coherence times in 2D transmon qubits," was published in Nature on November 5th. The author list includes Jeronimo G.C. Martinez, Paal H. Prestegaard, Basil M. Smitham, Atharv Joshi, Elizabeth Hedrick, Alex Pakpour-Tabrizi, Shashwat Kumar, Apoorv Jindal, Ray D. Chang, Ambrose Yang, Guangming Cheng, and Nan Yao, in addition to de Leon, Houck, Cava, Bahrami, and Bland. This groundbreaking research received primary funding from the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, and the Co-design Center for Quantum Advantage (C2QA), with additional support from Google Quantum AI.