Normally, you have to collect particles that come from the object to image it, says Anton Zeilinger, a physicist at the Austrian Academy of Sciences in Vienna who led the work. “Now, for the first time, you don’t have to do that."
One advantage of this imaging technique is that the two photons need not be of the same energy, Zeilinger says, meaning that the light that touches the object can be of a different colour than the light that is detected. For example, a quantum imager could probe delicate biological samples by sending low-energy photons through them while building up the image using visible-range photons and a conventional camera. (!)
According to the laws of quantum physics, if no one detects which path a photon took, the particle effectively has taken both routes, and a photon pair is created in each path at once, says Gabriela Barreto Lemos, a physicist at Austrian Academy of Sciences and a co-author on the latest paper.
In the first path, one photon in the pair passes through the object to be imaged, and the other does not. The photon that passed through the object is then recombined with its other ‘possible self’ — which travelled down the second path and not through the object — and is thrown away. The remaining photon from the second path is also reunited with itself from the first path and directed towards a camera, where it is used to build the image, despite having never interacted with the object.
The researchers imaged a cut-out of a cat, a few millimetres wide, as well as other shapes etched into silicon. The team probed the cat cut-out using a wavelength of light which they knew could not be detected by their camera. "That's important, it's the proof that it's working," says Zeilinger.
Information is central to quantum mechanics. In particular, quantum interference occurs only if there exists no information to distinguish between the superposed states. The mere possibility of obtaining information that could distinguish between overlapping states inhibits quantum interference. Here we introduce and experimentally demonstrate a quantum imaging concept based on induced coherence without induced emission.
The experiment is fundamentally different from previous quantum imaging techniques, such as interaction-free imaging or ghost imaging, because now the photons used to illuminate the object do not have to be detected at all and no coincidence detection is necessary. This enables the probe wavelength to be chosen in a range for which suitable detectors are not available. To illustrate this, we show images of objects that are either opaque or invisible to the detected photons.