By harnessing virtual particles that constantly blink in and out of existence, a new type of sensor can detect infinitesimal vibrations to identify molecules. The novel device may one day help identify diseases and detect trace levels of contaminants in factories and the environment, researchers say.
The way in which atoms move within a molecule can supply details about the kinds of bonds that connect these atoms. By shining light onto molecules to analyze these vibrations, techniques such as infrared spectroscopy or Raman spectroscopy can identify these molecules. Such insights have a wide range of applications, such as revealing the presence of diseases ranging from infections to cancer.
However, conventional techniques for analyzing molecular vibrations are limited by weak interactions between the light they use and the matter they are probing. This leads to signals that are often faint, easily drowned out by background noise, and difficult to isolate in complex biological environments such as blood or tissue.
In a new study, researchers aimed to create strong interactions between light and molecular vibrations. They started with highly reflective gold mirrors about 12 nanometers thick to create optical cavities about 6 micrometers large. To understand why that’s helpful, we have to peer into the quantum world of atoms.
Harnessing Quantum Physics
The strange nature of quantum physics suggests that the universe is inherently fuzzy. For instance, you can never know a subatomic particle’s momentum and position at the same time. A consequence of this uncertainty is that space—such as the area within an optical cavity—is never completely empty but instead buzzes with so-called virtual particles that constantly pop in and out of existence.
The optical cavities forced virtual photons, or particles of light, to reflect back and forth, helping them couple with the vibrations of molecules that were also enclosed within the receptacles. The virtual photons and the molecular vibrations became so intertwined, they formed a new kind of hybrid quantum state, a quasiparticle called a vibropolariton. The researchers could then use infrared light to analyze these vibropolaritons.
“This advance required three ingredients—precise nanophotonic engineering to confine light strongly enough to couple with vibrations, theoretical advances in understanding quantum hybrid states, and modern spectroscopic tools capable of resolving very small shifts in molecular signals,” says Peng Zheng, an associate research scientist in the department of mechanical engineering at Johns Hopkins University, in Baltimore, who worked on the project. “Only recently have these technologies matured to the point where all three could be combined.”
In experiments, by analyzing the spectral features of these vibropolaritons, the new quantum sensor was able to identify an organic molecule known as 4-mercaptobenzonitrile dissolved in an organic solvent.
“Quantum hybrid light-matter states, something often thought of as highly abstract, can actually make molecules easier to detect in practical conditions,” says Ishan Barman, a professor of mechanical engineering at Johns Hopkins. “By tapping into these states, we found a way to amplify molecular sensitivity beyond what classical optics can do.”
A Path to Real-World Applications
These experiments achieved this feat under ambient, real-world conditions, without the need for the kind of high-vacuum, cryogenic, or other extreme environments typically required to preserve fragile quantum states.
“We now have a pathway toward molecular detection using quantum states in practical conditions,” Barman says. “The big-picture message is that quantum physics isn’t just a curiosity here; it can be harnessed to build real-world sensors for health, safety, and the environment.”
Ultimately, Barman envisions compact, microchip-scale quantum sensors. Potential applications include medical diagnostics that can detect trace levels of disease-linked molecules at the very early stages of a condition, real-time analysis in drug or vaccine production, and environmental monitoring to detect harmful chemicals at extremely low levels where one molecule matters, Zheng adds.
Future research needs to show these quantum sensors can work in real-world clinically relevant conditions. “We want to integrate these sensors into portable, point-of-care devices,” Barman says. “That will take clever materials engineering and smart device design.
The scientists detailed their findings 15 August in the journal Science Advances.
