# Sisi Zhou: Measuring the Microscopic with Quantum Metrology

Pushing the boundaries of precision measurement is an aspiration that Perimeter faculty member Sisi Zhou takes seriously. It has led her on a journey across countries and across disciplines. For her, the spark of interest began in high school, where she competed in physics and math Olympiads – competitions designed to test the skills of young people at the top of their game. It inspired a twin passion for math and physics in her, leading her to pursue advanced degrees in physics, studying at Yale University, and completing a postdoc at Caltech before coming to Perimeter in 2023.

Now, she is at the forefront of a rapidly growing field of physics known as quantum metrology. Quantum metrology is a form of measurement, but it isn’t about kilometers in a marathon or teaspoons in your baking: it’s about measuring the world at the scale of particles.

Quantum metrology has a lot of potential. It takes advantage of quantum effects like entanglement and uncertainty, to enable new diagnostic tools in medicine, and to create more accurate atomic clocks. It’s even being used in the groundbreaking study of gravitational waves, where it can improve the sensitivity of detectors at the Laser Interferometer Gravitational-Wave Observatory (LIGO).

Zhou wants to see how far the field – and the applications – can take us.

“Quantum metrology has been overlooked compared to quantum computing and quantum communication, which are the main applications of quantum technology that people have in mind nowadays,” she says. “But they're all subfields of quantum information science, which is under rapid development recently. Quantum metrology is going to become more and more useful alongside the development of quantum computing and communication.”

**Quantum Metrology Meets Quantum Error Correction**

To make quantum metrology work, researchers must first overcome a foe that quantum metrology shares with quantum computing: ‘noise’. Quantum systems are extremely sensitive to interference from the outside world, making them vulnerable to information loss. And you can’t simply make a copy of that information, like you can in classical computing.

“It becomes really difficult to scale up a system,” says Zhou. “We can build a larger and larger quantum computer or quantum sensor, but it becomes more and more difficult to track the individual qubits and more and more difficult to control each of them.”

That’s where quantum error correction comes in. By entangling together many bits of quantum information, data can be protected from loss. Zhou has been exploring ways that quantum error correction can improve the sensitivity of quantum metrology techniques.

“You can use quantum error correction to enhance quantum metrology. If the quantum error correction code is good enough, you can reach a very high sensitivity in a metrology scenario,” she says.

But there are ultimate limits, or boundaries, that nature puts on how sensitive a measurement can ultimately be. Zhou is researching how we might use these limits to describe the properties of error correction codes, and ultimately improve them. It’s a virtuous cycle: studying the theoretical limits of metrology can improve error correction, and error correction in turn improves the practical capabilities of metrology.

**Towards a Quantum Advantage**

The central challenge in quantum metrology, then, is not to make it work: It does already. The challenge is making it work better than classical measurement does: achieving a so-called ‘quantum advantage’.

Achieving a practical quantum advantage may actually be closer to reality in quantum metrology than it is in quantum computing. This is because a quantum computer doing a calculation will either get an answer right or wrong. You can make a lot of progress by improving an error correction code, but if it isn’t perfect, even if it is just barely below the threshold for success, it will still fail. With quantum metrology on the other hand, success is measured on a continuum – as scientists improve the sensitivity and precision of their measurements, each successive bit of progress will be reflected in the results.

This has Zhou excited about the prospects for the future of her field.

“As long as the performance of the highest sensitivity you can get is better than the classical case, then you have shown the advantage of quantum. This perspective puts demonstrating quantum advantage in metrology in a favorable position,” she says.

The prospects are exciting, and Zhou is gearing up to take the next step.

**Curious about Perimeter’s researchers or the advances in theoretical physics that are happening now? Stay in the know with our ****monthly newsletter****, or watch this space for new researcher profiles and science highlights.**