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Jennifer Choy works in the lab
December 14, 2023

Ask an expert: Choy discusses qubits, atomic clocks, computing and more

Written By: Alex Holloway

A portal into the promising uber-tiny world of quantum technology.

Jennifer Choy is an assistant professor of electrical and computer engineering who studies quantum sensing. Quantum sensing is one of several growing research fields that use quantum mechanics for applications ranging from creating nanoscale optical sensors to the supercomputers of the future.

In this interview, Choy discusses the principles of quantum systems and some of the applications of quantum mechanics, including one we use every day.

Q: “Quantum” is a term we hear a lot across a variety of fields, often regarding things happening at a very small scale. But what, exactly, does quantum mean?

A. Quantum refers to the most fundamental unit of something. In a lot of applications, “quantum” means phenomena or physical systems for which the length scales are so small that you cannot describe their characteristics with classical physics.

The most accessible way to understand quantum behavior is to look at the atom itself. An excited atom (such as one in a gas tube that is heated) will emit light that is quantized (expressed in discrete values) in terms of its energy. Each individual atom, depending on the species, has a length scale on the order of about an angstrom (a hundred-millionth of a centimeter). At that scale, you can describe an atom both as a particle—as its own quantity—but also as a wave.

One of the most basic manifestations of quantum physics is this concept of wave-particle duality: Something has the properties of a wave and a particle at the same time. Through that duality, you can think of an atom as a little wave that is distributed in time and space. However, you also can say that when you’re measuring an atom, you have narrowed down its position at a particular location in space, and therefore it is behaving like a particle.

Q: How can quantum mechanics research benefit other fields, like quantum sensing?

A: The thing that excites me most about quantum mechanics is the ability to bring new functionality to existing technologies. In the case of sensing and metrology (the science of measurement), quantum technologies allow us to make measurements in a much more precise and accurate way.

One such quantum technology is the atomic clock. The way our world runs right now is reliant on using atoms to be able to keep time. Atomic clocks measure the quantized resonance frequency of atoms, often using the element cesium, to keep time with a very high degree of accuracy.

There are 24 GPS satellites orbiting Earth, and there are atomic clocks on each of them. As we receive signals from GPS satellites, they contain position information, along with a timestamp. Each one of these timestamps has data that comes from the measurement of atoms within the atomic clocks. That allows us to very accurately determine position, with very small degrees of uncertainty.

Leveraging the stability of atoms (and the ability to precisely measure their energy transition resonances), we can also very accurately measure gravity as well as other types of motional forces like acceleration or rotation. This concept can be applied to develop navigational sensors that are reliable even with interruptions in GPS signals.

Additionally, we can use quantized electron energies in atoms to develop ultra-sensitive magnetometers that are precise enough to measure electric currents that come from neurons firing in the brain. Similar concepts can be applied to make atomic-scale probes of electromagnetic fields in solid-state materials. For example, by introducing a defect in a diamond crystal, we can generate an artificial atom that is small and sensitive enough to study electromagnetic and temperature changes in cellular processes.

Q: Quantum computing is another area of quantum research that gets a lot of buzz. What are some advantages quantum mechanics brings to that field?

A: Quantum computing takes advantage of the quantum nature of devices at very small scales, and the ability to control and measure the states of quantum systems, to create quantum equivalents to classical bits like 0 and 1.

The state of a quantum system—for example, whether it is 0 or 1—is described probabilistically until it is measured. This enables the use of superposition, the idea that a quantum system probabilistically occupies multiple states at the same time until it’s measured.

Additionally, it is also possible to generate correlations in the measurements of multiple quantum systems through a unique quantum phenomenon called entanglement. This phenomenon occurs when, for a pair of quantum particles, measuring one determines the result of measuring the other even if the particles are separated. So in quantum computing, these quantum bit analogues, which are called “qubits,” may then have the properties of being 0 and 1 superimposed, and the states of the qubits correlated.

This can be handy for parallel computing. If we can scale up the number of qubits, we could solve problems that are considered too computationally intensive for classical computers. One example of this concept is being able to factor very large numbers, which has implications for security applications.

A quantum computer can also be used to simulate quantum behavior in materials. One example is to use such quantum simulators to study chemical processes on the individual molecule or atomic level and how those processes happen as a function of time. That could help further our understanding of chemical and biological processes and make it faster to perform synthesis of chemicals and develop new types of medicine.

Featured image caption: Jennifer Choy, an assistant professor of electrical and computer engineering, researches quantum sensing technologies. Her work includes using diamonds to create ultra-sensitive sensors. Credit: Sabrina Wu.