By Dr. Gavin Brennen, Director of BTQ AU
Director Macquarie Centre for Quantum Engineering (MQCQE), Chief Investigator ARC Centre for Excellence in Engineered Quantum Systems (EQUS), Executive Board Member Sydney Quantum Academy (SQA)

Google has unveiled its latest breakthrough in quantum computing with the Willow chip, a processor featuring 105 connected superconducting qubits. As the successor to the Bristlecone chip, which housed 75 qubits, Willow marks a major step forward in quantum hardware. This advanced processor was designed to address critical challenges in quantum computing, such as improving error correction and demonstrating the scalability of quantum systems. These achievements highlight Willow’s potential to push the boundaries of quantum technology and pave the way for practical applications.

Improved Error Correction with Larger Codes

One of the key achievements with Willow is its demonstration of significantly improved error correction using larger error-correcting codes. This is no small feat—larger codes require more qubits and occupy more space on the chip, which increases the likelihood of errors. Despite these challenges, Google demonstrated that a logical qubit encoded with error correction could retain information twice as long as an unprotected qubit.

During these experiments, researchers also discovered a new type of highly correlated error whose physical origin is not yet understood. This discovery was made possible because of Willow’s large, coherent processor, underscoring its value not just in solving problems but also in advancing our understanding of quantum systems.

Tackling the Sampling Problem

Another milestone achieved with Willow was solving a larger instance of the sampling problem, the same problem used to claim quantum supremacy in 2019. Although the sampling problem has no practical applications, it serves as a benchmark to demonstrate quantum computers' speed and power compared to classical systems.

Unlike the factoring problem, used to break public key encryption, where a solution can be checked simply by multiplying the factors, the solution of the sampling problem can’t be verified easily. In fact they were only able to perform statistical consistency checks that the output from the chip was correct. Nevertheless, the ability to solve such a problem with Willow showcases the capabilities of a fully connected and controllable quantum circuit at this scale.

Future Possibilities with Willow

The Willow chip opens the door to more advanced demonstrations of quantum computing, including:

• Logical gates across multiple protected qubits
• Scalable small quantum programs with practical applications
• Enhanced quantum error correction using advances in hardware and software

As qubit numbers grow, quantum processors will eventually outgrow single-chip designs, requiring modular approaches to connect multiple quantum units. The Willow chip represents a key step toward these modular architectures since it shows that all the control electronics works together within one.

Scaling Challenges and Parallel Efforts

Scaling quantum systems comes with challenges. As qubits increase, so do the types and sources of errors. Researchers can only address these issues by experimenting with ever-larger registers. In parallel with Google’s superconducting qubit approach, other technologies—such as photonic, atomic, ionic, and silicon-spin quantum processors—are also advancing, contributing to a diversified landscape of quantum computing research.

The Transformative Potential of Quantum Computing

The ultimate goal of quantum computing is transformative. For instance, a quantum computer with several hundred thousand qubits could theoretically crack 256-bit elliptic curve digital signatures within hours—a task that would take classical supercomputers longer than the age of the universe to complete by brute force. Achieving this scale will require continued breakthroughs like those demonstrated by Willow.

Google’s Willow chip represents not just a technological milestone but also a glimpse into the future of quantum computing. With advancements in error correction, scalability, and processor coherence, the Willow chip highlights the progress being made toward practical quantum systems. While challenges remain—such as increasing qubit counts and connecting modular systems—each step forward brings us closer to realizing the transformative potential of quantum computing. As research continues and technologies mature, breakthroughs like Willow will pave the way for applications that were once considered impossible.