A special class of sensors leverages quantum properties to measure tiny signals at levels that would be impossible using classical sensors alone. Such quantum sensors are currently being used to study the inner workings of cells and the outer depths of our universe.

Particularly promising are solid-state quantum sensors, which can operate at room temperature. Unfortunately, most solid-state quantum sensors today only measure one physical quantity at a time – such as the magnetic field, temperature, or strain in a material. Trying to measure both the magnetic field and temperature of a material at the same time causes their signals to get mixed up and measurements to become unreliable. 

Researchers have created a quantum sensor that can measure multiple physical quantities at high-resolution. The sensor is made from so-called nitrogen-vacancy centres in diamonds, where a carbon atom in the diamond’s crystal lattice is replaced by a nitrogen atom and a neighbouring atom is missing, creating an electronic spin that is sensitive to external effects. Image: Takuya Isogawa.

Now, MIT researchers have created a way to simultaneously measure multiple physical quantities with a solid-state quantum sensor. They achieved this by exploiting entanglement, where particles become correlated into a single quantum state.

In a new paper, the team demonstrated its approach in a commonly used quantum sensor at room temperature, measuring the amplitude, frequency, and phase of a microwave field in a single measurement. They also showed the approach works better than sequentially measuring each property or using traditional sensors.

The researchers say the approach could enable quantum sensors that can deepen our understanding of the behaviour of atoms and electrons inside materials and living systems like cancer cells.

“Quantum multiparameter estimation has been mostly theoretical to date,” says co-lead author of the paper Takuya Isogawa, a graduate student in nuclear science and engineering. “There have been very few experiments that actually demonstrate it, and that work focused on photons. We wanted to demonstrate multiparameter estimation in a more application-oriented setup: a solid-state quantum sensor in use today.”

Joining Isogawa on the paper are co-lead authors Guoqing Wang PhD ’23 and MIT PhD candidate Boning Li. The other authors on the paper are former MIT visiting students Zhiyao Hu and Ayumi Kanamoto; University of Tokyo PhD candidate Shunsuke Nishimura; Chinese University of Hong Kong Professor Haidong Yuan; and Paola Cappellaro, MIT’s Ford Professor of Engineering, a professor of nuclear science and engineering and of physics, and a member of the Research Laboratory of Electronics.

Quantum effects for measurement

Quantum sensors exploit quantum effects like entanglement, spin states, and superposition to measure changes in magnetic fields, electric fields, gravity, acceleration, and more. As such, they can be used to measure the activity of single molecules in ways that are useful for understanding biology and space, like tracking the activity of metabolites or enzymes inside cells. 

A photo of the experimental setup. The NV centres are located inside the diamond that is glowing under the green laser. The diamond sits at the centre of the green plate, which serves as the antenna used to apply the microwave/RF field. Photo: Courtesy of the researchers.

One particularly useful sensor in biology leverages what is known as nitrogen-vacancy (NV) centres in diamonds, a defect where a carbon atom in the diamond’s crystal lattice is replaced by a nitrogen atom, and a neighbouring lattice site is missing, or vacant.

The defect hosts an electronic spin whose transition frequencies can be read out optically. The NV centre’s spin state is extremely sensitive to external effects, such as magnetic fields and temperature, which can shift the spin state in ways that can be measured at extremely high resolution.

Unfortunately, different external effects change the energy resonances of the spin in similar ways, making it difficult to measure multiple effects at once. The result is that most solid-state quantum sensor applications measure a single physical quantity at one time.

“If you can only measure one quantity at a time, you have to repeat experiments to measure quantities one by one,” says Isogawa. “That takes more time, which means less sensitivity. It also makes experiments more susceptible to errors.”

For their experiment, the researchers used NV centres inside of a five-square-millimetre diamond. They pointed a laser into the diamond and studied its fluorescence to make their measurements, a common approach for such sensors. To study the electronic spin of the NV centre, they used a microwave antenna. To study the spin of the nitrogen atom they used a radio frequency field.

“We used those two spins as two qubits,” says Isogawa, referring to the building blocks of quantum computing systems. “If you have only one qubit, you can only measure one outcome: basically, 0 or 1. It’s the probability that it spins up or down. Think of it like a coin toss, with the probability of getting heads or tails. With two qubits, we increased the parameters that we could extract.”

The system worked because the spins of the sensor qubit and auxiliary qubit were entangled, a quantum property where the state of one particle is dependent on another. With one qubit, you get a binary outcome. With two, you get four possible outcomes with a total of three possible parameters.

The two qubits allowed researchers to measure those three quantities simultaneously using a technique known as the Bell state measurement.

Other researchers had used the Bell state measurement at extremely low temperatures before, but the MIT researchers developed a new technique to perform the measurement at room temperature. That technique was first proposed by Wang, who was previously a graduate student in Prof Cappellaro’s lab.

The researchers used the approach to simultaneously measure the amplitude, detuning, and phase of a microwave magnetic field. The researchers also say the approach could be used to measure electric fields, temperature, pressure, and strain.

“Measuring these parameters simultaneously can help us explore spin waves in materials, which is an important topic in condensed matter physics,” says Isogawa. “NV centre sensors have extremely high spatial resolution and versatility. It can measure a lot of different physical quantities.”

More practical quantum sensing

The researchers say this work is an important step towards using solid-state quantum sensors to more fully characterise systems in biomedical research and materials characterisation. That is because multiparameter estimation had never been achieved in realistic settings or in widely used quantum sensors.

“What makes the NV centre quantum sensors so special is they can operate at room temperature,” says Isogawa. “It’s very suitable for biological measurements or condensed matter physics experiments.”

Although the researchers say their sensor did not measure each quantity at the highest possible precision, in future work they plan to explore if their approach can achieve higher precision for each parameter.

They also plan to explore how their approach works to characterise heterogenous materials.

“In an extremely uniform environment, you could use many different classical and quantum sensors and measure each physical quantity at the same time,” says Isogawa. “But if the physical quantities change at different locations, you need high spatial sensors, and you need a sensor that can measure multiple physical quantities. This approach has major advantages in such situations.”