Traditional imaging techniques, like an x-ray or even a simple photograph, involve capturing light that bounces off an object. But quantum imaging takes advantage of the “spooky” (the term Albert Einstein used to describe the phenomenon) action of quantum entanglement. When two photons are entangled, anything that happens to one photon immediately affects the state of its entangled partner, even if those two photons are separated in distance and direction. In quantum imaging, an “idler” photon is used to actually scan the target object, but a second “signal” photon — which is entangled with the idler — is detected to create the image.
In this new approach, the researchers used a non-linear crystal to create photons with infrared wavelengths as idlers, each entangled with signal photons in the visible range. There are significant advantages, the researchers say, to using infrared light for illumination and visible light for rendering images.
“Infrared wavelengths are preferred for biological imaging because they can penetrate skin and are safe for delicate structures, but they require expensive infrared detectors for imaging,” said Liu, a Brown senior concentrating in engineering physics and applied math. “The advantage of our approach is that we can use infrared for probing an object, but the light we use for detection is in visible range. So we can use standard, inexpensive silicon detectors.”
The key advance is bringing quantum imaging into the 3D realm by dealing with a problem known as “phase wrapping.”
Phase wrapping occurs in any imaging technique that uses the phase of light waves — the peaks and valleys of the waveform — to determine the depth of contours of an object. If the depth of those contours is greater than the length of the wave, a “wrap-around” effect emerges that makes deeper features indistinguishable from shallower ones that happen to fall in the same part of the wave cycle. For example, the phase wrapping problem makes a contour that’s a half wavelength deep indistinguishable from one that’s one- or two-and-a-half wavelengths deep.
To overcome that problem, the Brown team used two sets of idler and signal photon wavelengths that are slightly different. The difference in the wavelengths greatly expands the depth features that technique can accurately measure and image.
“By using two slightly different wavelengths, we effectively create a much longer synthetic wavelength — about 25 times longer than the originals,” Liu said. “That gives us a much larger measurable range that’s more applicable to cells and other biological materials.”
The team showed that they could successfully create a holographic image of a test object — a roughly 1.5 millimeter letter ‘B’ (an homage to Brown University) fashioned from metal. The researchers say this is a strong proof-of-concept for creating high-fidelity 3D images using quantum entanglement.
Both Liu and Zhang said they relished the opportunity to present at an international scientific conference.
“We had been reading papers by pioneers in this field, so it was great to be able to attend the conference and meet some of them in person,” Zhang said. “It’s really an amazing opportunity.”
Liu’s work on this project and his related senior thesis were recognized by the School of Engineering with the Ionata Award, which recognizes a senior who demonstrates an unusual degree creativity and imagination in an independent study project.
The research was funded by the Department of Defense (W911NF-24-1-0138, FA2386-24-14068, FA9550-19-1-0355) and the National Science Foundation (2231901).