Since the double-slit experiment, physicists have understood that light can manifest as either a wave or a collection of particles. In typical imaging, the wave-like properties of light dominate, with receptors capturing the information within these waves to perceive a scene.
Recently, Chloé Vernière and Hugo Defienne at Sorbonne University's Paris Institute of Nanoscience have employed quantum correlations to encode an image into light that only reveals itself when its particles (photons) are detected by a camera sensitive to single photons; otherwise, the image remains hidden.
Utilizing Quantum Correlations for Encoding
In their research highlighted in Physical Review Letters, Vernière and Defienne managed to conceal a cat image from standard light detection devices by encoding the image using quantum entangled photons, or photon-pair correlations. They achieved this by shaping spatial correlations between entangled photons, using arbitrary amplitude and phase objects to encode image information within these correlations. Due to this method, only an electron-multiplied charge coupled device (EMCCD) camera, capable of detecting single photons, can reveal the hidden image, as conventional measurement tools cannot detect it.
"Quantum entanglement, a fascinating aspect of quantum mechanics and central to many quantum technologies, is a crucial concept in our research," notes Defienne. "In previous studies, we discovered that quantum correlations between photons can be more resilient to disturbances like noise or optical scattering than classical light. This led us to explore using these correlations as a foundation, or 'canvas', for imprinting images, which is what we've done here."
Techniques for Hiding an Image
To create entangled photons, the researchers applied spontaneous parametric down-conversion (SPDC), a technique common in quantum optics experiments, which involves using a nonlinear crystal to split a single high-energy photon into two lower energy entangled photons, with the properties of these photons determined by the characteristics of the crystal and the pump beam.
In their experiment, a continuous-wave laser emitting horizontally polarized 405 nm light was used to illuminate a cat-shaped mask, which was then imaged onto a nonlinear crystal using a lens. The process produced spatially entangled photons at near-infrared 810 nm wavelengths, which were detected by another lens and the EMCCD, resulting in an encoded image of the cat that is invisible with regular cameras and only visible when photon by photon detection is applied.
"It's remarkably intriguing that an image can be completely hidden when viewed with a standard camera, yet becomes visible when observed 'quantumly' by counting individual photons and examining their correlations," says Vernière, a PhD student involved in the project. "This represents a novel approach to optical imaging, and I anticipate many significant applications arising from this method."
Future Prospects
Building on past research, Defienne indicates that the next objective is to demonstrate practical applications for this new imaging technique beyond its scientific novelty. "Given that images encoded in quantum correlations are less susceptible to external disturbances like noise or scattering than classical light, we plan to exploit this robustness to enhance image resolution in scattering media."
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