Searching for Unorthodox Improvements to Quantum Systems

Computers that leverage the principles of quantum mechanics stand to far exceed the capabilities of classical computers — but there are still many hurdles for the technology to overcome. 

For example, researchers still have to reduce error rates and figure out how to fix the mistakes they can’t prevent. They have to improve connectivity within these systems while scaling them orders of magnitude larger. And they have to refine and develop hardware that can work at incredibly high levels of precision on extremely small scales. 

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At Columbia Engineering, Xueyue (Sherry) Zhang, an assistant professor of applied physics and applied mathematics, is leading a new research group that’s investigating overlooked and underappreciated ideas for addressing some of the most important outstanding problems in quantum computing. 

We sat down with Zhang to learn more about the research happening in her Photonic Quantum Integrated Systems Lab. 

What’s the overarching goal of your research program?

We’re focused on building quantum devices and hardware that can potentially outperform classical computers by harnessing quantum mechanics. Our goal is to find practical pathways towards realizing quantum computing — something that many researchers consider a holy grail. However, there are substantial challenges to overcome, especially concerning the interactions among qubits, the quantum bits fundamental to these systems.

What makes your approach unique?

We are addressing core quantum computing challenges from a fresh perspective. Rather than incrementally advancing existing technologies, we’re exploring unconventional methods and architectures that have previously been overlooked or dismissed. Specifically, we're rethinking how quantum bits communicate with each other, moving beyond traditional short-range interactions.

Can you give an example?

Our current project, funded by Amazon, focuses on superconducting qubits. These are among the most advanced qubit technologies today and are already widely adopted by major companies like Google, IBM, and Amazon. Superconducting qubits are solid-state devices made on silicon or sapphire substrates using semiconductor manufacturing techniques, offering exceptional controllability and reliability.

However, traditional superconducting quantum processors arrange qubits in two-dimensional grids. While effective for simpler quantum circuits, this setup becomes inefficient when scaling up to hundreds or thousands of qubits, especially when performing complex quantum error corrections required for practical quantum computing. The limitation arises because each qubit is connected only to its immediate neighbors, forcing information to take inefficient, roundabout paths.

How is your research addressing this issue?

Our solution involves enabling long-range interactions between qubits through waveguides, which are communication channels for quantum signals. They’re similar to a fiber-optic cable for quantum signals, guiding microwave photons across the processor. These photons act as carriers of quantum information, linking qubits that are physically distant from each other. This significantly enhances connectivity without needing extensive physical wiring or complicated layering.

Could you explain more about the waveguides you're using?

In our experiments, waveguides are fabricated alongside the superconducting qubits on the same silicon chip. They form a pathway that photons can use to travel freely, allowing multiple qubits to interact efficiently across greater distances. This architecture reduces the complexity and increases the effectiveness of the quantum processor, potentially making error correction and quantum computing operations more efficient.

What’s the status of this project today?

We're working on building prototypes to demonstrate the viability of these concepts. Initially, our lab will create smaller-scale setups of around ten qubits to show proof of concept. Demonstrations at this level are the first step toward larger quantum processors comprising hundreds of qubits.

How will this project impact the future of quantum computing?

Our waveguide-based architecture would significantly accelerate quantum computing's progress by simplifying the pathway to more scalable, practical quantum processors. This approach addresses fundamental bottlenecks in quantum information processing and could become a key technology enabling more efficient quantum error correction and larger quantum systems.

Why is Columbia Engineering an ideal place for this research?

Columbia Engineering is uniquely positioned due to its strong emphasis on innovative and interdisciplinary approaches. The culture here supports taking risks on new ideas that might initially seem unconventional but have the potential to address significant scientific and technological challenges. The strong partnership with industry leaders like Amazon also provides essential resources and a direct link to real-world quantum computing applications.

What's next for your research?

Beyond the current project, we continue exploring novel architectures and coupling strategies for quantum processors. We're actively pursuing additional avenues that can further enhance quantum performance and scalability, including entirely different types of qubits and advanced control techniques.

Ultimately, our goal is to bring quantum computing closer to reality, transforming this powerful theoretical technology into practical, usable devices that can tackle computational challenges far beyond the capabilities of today’s classical computers.

Lead Photo Caption: A CAD rendering of a quantum chip 
Lead Photo Credit: Zhang Lab