Image: Gabriela Barreto Lemos / University of Vienna
The famous "picture of Schrödinger's cat" is not a photo of a cat that is both alive and dead. It is an image of a small cat-shaped stencil, made in 2014 by physicists in Vienna using a trick of quantum entanglement: the light that actually passed through the stencil was never detected, and the light that reached the camera never touched the stencil. The cat outline was a playful nod to the most famous thought experiment in physics, but the real story is a new way of taking pictures.
Below, we look at what the experiment did, why it worked, how it relates to Schrödinger's original idea, and why scientists were excited about a method that can "see" with light it never measures.
What the picture actually shows
The image comes from a paper titled "Quantum imaging with undetected photons", published in Nature in August 2014. Its authors were Gabriela Barreto Lemos, Victoria Borish, Garrett D. Cole, Sven Ramelow, Radek Lapkiewicz and Anton Zeilinger, working at the Institute for Quantum Optics and Quantum Information (IQOQI), the Vienna Center for Quantum Science and Technology and the University of Vienna.
The object in the picture is a tiny cutout shaped like a cat's head, etched into a thin silicon plate. The team also imaged other test shapes, but the cat was the one that made headlines, because a cat in a quantum experiment is almost guaranteed to be linked with Erwin Schrödinger. The bright and dark areas in the final picture are interference patterns, not a conventional shadow, and they reveal where the stencil blocked light and where it let light through.
How you can image something with light you never detect
The setup sounds like a magic trick, so it helps to take it one step at a time.
- A laser feeds two crystals. A laser beam is directed at two separate non-linear crystals. Occasionally, a crystal splits one laser photon into a pair of new photons: one in the infrared and one at a visible red wavelength. The two members of each pair are entangled, which means their properties are linked and described by a single shared quantum state.
- Only the infrared photons meet the object. The stencil sits between the two crystals, in the path of the infrared light. Silicon is opaque or transparent depending on wavelength, so the object interacts with the infrared beam but would be practically invisible to the red light.
- The paths are made indistinguishable. The infrared beam from the first crystal is lined up so precisely with the infrared beam from the second crystal that, in principle, nobody could tell which crystal produced a given pair. The infrared light is then thrown away and never measured.
- The red photons carry the picture. The red photons from both crystals are combined and sent to a camera that cannot register infrared light at all. Because of the quantum link with their partners, the red light forms bright and dark interference patterns that trace the shape of the stencil.
In the University of Vienna's own words, the object "is illuminated with light that remains undetected". The camera only ever sees red photons that never went near the cat.
Why this is not Schrödinger's cat (and why the name stuck)
Schrödinger's cat is a thought experiment that Austrian physicist Erwin Schrödinger published in 1935. He imagined a cat sealed in a box with a radioactive atom, a Geiger counter and a vial of poison. If the atom decays, the poison is released. Under a strict reading of quantum theory, until someone looks, the atom is in a superposition of decayed and not decayed, so the cat would seem to be both alive and dead at once.
Schrödinger did not think a real cat could be half dead. He designed the scenario to show how strange it is to apply the quantum description of tiny particles to everyday objects, and to criticise the interpretation popular at the time. In the same period he gave entanglement its name, the German word Verschränkung.
The Vienna experiment shares two things with that story: a cat and entanglement. That is where the resemblance ends. No animal was involved, nothing was in a superposition of alive and dead, and the stencil was a fixed piece of silicon. Headlines such as "Schrödinger's cat caught on camera" were fun, but they described the decoration rather than the science.
The physics behind the trick
The method builds on an effect first demonstrated in 1991 by Xin Yu Zou, Li Jun Wang and Leonard Mandel, sometimes called induced coherence without induced emission. When two sources can each produce the same photon pair and the paths are perfectly overlapped, the red photons from the two sources interfere with each other. Anything that disturbs the infrared path, such as an object partly blocking it, changes how strongly the red light interferes. That change is what the camera records.
It is worth clearing up one common misunderstanding. The experiment does not send information faster than light, and it is not "seeing without any light at all". Light does pass through the object; it just is not the light that gets measured. Entanglement links the outcomes, but the image only appears once the red light is collected and processed in the lab.
It is also different from classic "ghost imaging", which has existed since the mid-1990s. Ghost imaging uses two detectors and compares their signals; here, the photons that touch the object are never detected at all.
Why scientists cared: practical uses
The appeal is freedom of wavelength. Some wavelengths are excellent for probing a sample but are hard or expensive to detect, especially in the mid-infrared, where sensitive low-light cameras are scarce. With this method, a sample can be illuminated at a wavelength that suits it, while the image is captured at a wavelength that cheap, sensitive cameras handle well.
The researchers suggested the technique could be extended into the mid-infrared and might help with low-light imaging in biology and medicine, where delicate samples are easily damaged by strong illumination. Since 2014, other groups have built on the idea for microscopy, spectroscopy and optical coherence tomography using undetected photons, though most of this work is still at the laboratory and prototype stage rather than in everyday clinical use.
What to take away
- The image is a real 2014 laboratory result, published in Nature by a team linked to Anton Zeilinger, who later shared the 2022 Nobel Prize in Physics for work on entanglement.
- The cat is a silicon stencil chosen for fun, not a cat in a quantum superposition.
- The breakthrough is imaging an object with photons that are never detected, letting scientists separate the wavelength that probes a sample from the wavelength that forms the picture.
- Quantum physics is strange, but this experiment is a careful, measurable demonstration rather than a mystery: the weirdness lives in how the information is shared between entangled photons.
Sources: Image by Gabriela Barreto Lemos and colleagues, University of Vienna. Lemos et al., Nature 512, 409 (2014); arXiv preprint: Quantum imaging with undetected photons; ScienceDaily / University of Vienna, 28 August 2014; Wikipedia: Schrödinger's cat; Wikipedia: Quantum imaging; originally reported by New Scientist and Gizmodo.
COMMENTS