Quantum ‘ghost imaging’ reveals plants’ dark side

Quantum ‘ghost imaging’ reveals plants’ dark side

Rachel Berkowitz

Imagine taking a time-lapse video of your garden throughout the year. You’d see the details of the flowers as they change from day to night and season to season. Scientists want to see similar changes at the molecular level, but the intense light used to take microscopic photos of plants interferes with the processes biologists want to observe, especially at night. Writing in a journal Optica, Physicist Duncan Ryan of the Los Alamos National Laboratory (LANL) and his colleagues recently demonstrated a tool for imaging living plant tissue while exposing it to less light than it would receive under a starry sky.

First demonstrated in 1995, a technique called ghost imaging involves splitting a light source to produce photons of two different wavelengths at exactly the same time and place. The photons are entangled (a quantum phenomenon) and researchers can infer information about one particle in the pair by measuring the other. A sample can thus be probed at one wavelength and imaged at another.

In the case of plants, this means that researchers can image objects with visible light photons and gain knowledge about infrared photons that interact with water-rich molecules that are important for biological functions. In the new study, to do this, the team aimed a stream of infrared photons at a plant in a transparent box with a photon counter behind it, and simultaneously aimed the visible counterparts of those particles at an empty box at the same distance with a camera behind it. Each visible photon aimed at the empty box hit a pixel and was detected at its exact location. This was a much more precise measurement than an infrared camera could achieve. Meanwhile, infrared photons traveled to the plant’s box, but not all of them were counted. The plant absorbed a certain percentage of the photons at a specific location. The computer recorded the location of a pixel only if a photon hit both the camera and the counter at the same time. In this way, the researchers were able to create an image of the plant’s leaves using photons that did not touch the leaves at all, essentially forming an infrared image with the visible camera. “It’s like playing the game Battleships,” says Ryan.

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Ghost imaging has proven effective at taking pictures of simpler test designs. But for samples with low light transmission, such as plants, the absorption of microstructures often varies by only a few percent. The key lies in highly sensitive detectors developed at LANL that track the arrival of each infrared photon with an accuracy of a trillionth of a second. This allows them to map leaf tissue and peek into the nocturnal activity of living plants. “We saw the plants respond to the darkness by closing their stomata (called stomata),” Ryan says.

Ghost imaging “creates the possibility of long-time-scale dynamic imaging that does not damage living samples,” said Audrey Eshun, a laser spectroscopy and quantum optics researcher at Lawrence Livermore National Laboratory, who called the new research “truly groundbreaking work.”

These observations allow researchers to track how plants use water and sunlight throughout their circadian cycles. “We’re watching plants respond to their environment, not reacting to what we observe,” Ryan says.