The photograph above is of an invisible cat, achieved through the process of quantum entanglement, the same phenomenon that might one day allow for a sort of teleportation.
Confused? Let’s backtrack. First, there is sadly, no macroscopic invisible cat, only a stencil of a cat. And the stencil is only invisible at certain wavelengths, having been made from silicon transparent to infrared light. The silicon blocks red light however, and Dr. Gabriela Baretto Lemos of the Austrian Academy of Sciences entangled red and infrared photons so that the red photons matched the behavior of infrared ones.
Quantum entanglement is the process by which two or more subatomic particles are linked together so that changes to one inevitably affect the others. The choice of stencil shape is a homage to the famous Shrödinger’s cat thought experiment, which was an attempt to demonstrate what would happen if entanglement was extended to the macroscopic world.
The experiment, reported in Nature, was conducted by shining a laser on crystals that produce entangled photons of different wavelengths. "In the experiment, the laser illuminates two separate crystals, creating one pair of twin photons (consisting of one infrared photon and a "sister" red photon) in either crystal," Lemos says. "The object [stencil] is placed in between the two crystals. The arrangement is such that if a photon pair is created in the first crystal only the infrared photon passes through the imaged object. Its path then goes through the second crystal where it fully combines with any infrared photons that would be created there."
A camera detecting the photons cannot tell which crystal they come from. Moreover, the authors note, "There is now no information in the infrared photon about the object." However, the information the infrared photons obtained about the stencil has been passed on to the red photons, so that when the recombined beams reach a camera, patterns of light and dark form an exact image of the object. To ensure the experiment was working as intended, the camera they used could not detect infrared light, demonstrating the pattern was formed by red photons that had never interacted with the stencil.
Nature. Schematic of the experiment. Confusingly, the infrared beam is shown in red, and the red in yellow.
"For the first time, an image has been obtained without ever detecting the light that was used to illuminate the imaged object, while the light revealing the image never touches the imaged object," the authors write.
In 2009, researchers demonstrated something similar known as ghost imaging. However, this was done using light of the same wavelength, so even though the photons that formed the image never interacted with the object being photographed, they were identical to the ones that did interact. Ghost imaging relied on coincidence detection, while this experiment uses single photon interference.
As Lemos puts it, “The photons used to illuminate the object do not have to be detected at all.” Lemos adds, “This enables the probe wavelength to be chosen in a range for which suitable detectors are not available.”
Some medical experiments are best done with wavelengths that are hard to photograph, particularly the mid-infrared. Image quality may be improved by entangling the observation beam with easier to detect wavelengths. Another suggestion is a more direct version of “false color” images, which are actually produced in the ultraviolet or infrared.
