Quantum Holograms Don’t Even Need to “See” Their Subject

A new quantum-mechanical holography technique can generate holograms of items without scientists ever directly capturing any light from those objects, a new study finds. This novel and surprising discovery already may have biomedical applications.

A hologram is an image that, when illuminated, acts much like a 2D window looking onto a 3D scene. Conventional holography creates holograms by using a laser beam to scan an object and encode its data onto a recording medium such as a film or plate.

Holography can have many uses beyond image displays. For instance, by helping reconstruct an object’s 3D shape and structure, holograms have been called a “progressive revolution in medicine”—with significant uses in many fields such as orthopedics, neurology, and others.

However, the light sensors employed in holography work best with visible wavelengths. Many biomedical applications for holography would benefit from using midinfrared light, which is more difficult to detect, says study senior author Markus Gräfe, a physicist at the Fraunhofer Institute for Applied Optics and Precision Engineering in Jena, Germany.

Now, with the help of the surreal nature of quantum physics, Gräfe and his colleagues have discovered a way to create holograms of items without ever detecting any light from them.

“The light that illuminates the object is never detected,” Gräfe says. “The light that is detected never interacted with the object.”

A key feature of quantum physics is that the universe becomes a fuzzy place at its very smallest levels. For example, atoms and other building blocks of the cosmos can exist in states of flux known as “superpositions,” meaning they can essentially be located in two or more places at once.

One consequence of quantum physics is entanglement, wherein multiple particles are linked and can influence each other instantly regardless of how far apart they are. One way to generate entangled photons is by shining a beam of light at a special so-called “nonlinear crystal” that can split each photon into two lower-energy, longer-wavelength photons (These resulting pairs are not necessarily both the same wavelength.)

In the new study, the researchers used a nonlinear crystal to split a violet laser beam into two beams, one far-red, the other near-infrared. They next used the far-red beam to illuminate a sample—a glass plate engraved with symbols—whereas they used a camera to record the near-infrared light. With the help of entanglement, they could use data from the near-infrared light to reconstruct a hologram based off the details of the object the far-red beam scanned.

“It is possible to carry out imaging and holography by having different light for illumination and detection by exploiting the quantum properties of light,” Gräfe says.

By tinkering with the way in which nonlinear crystals and other components manipulate light, this new “quantum holography” technique could use, say, a midinfrared beam to scan an object while using the partner visible light beam (which can then be detected by conventional, visible-light sensors) to generate the hologram.

“We can even go up to video-rate imaging,” Gräfe says. “The next steps are improving performance and building a scanning microscopic system for midinfrared microscopy with visible light for biomedical imaging.”

The scientists detailed their findings last month in the journal Science Advances.

This article appears in the April 2022 print issue as “Spooky Holography at a Distance.”