#Metasurface enabled quantum edge detection

Metasurfaces provide unique platforms to realize exotic phenomena including negative refraction, achromatic focusing, and electromagnetic cloaking due to the engineered dielectric or metallic architectures. The intersection of metasurfaces and quantum optics can lead to significant opportunities that remain to be explored. In a new report now published on Science Advances, Junxiao Zhou, Shikai Liu and a research team in quantum information, nano-optoelectronic devices and computer engineering in China and the U.S. proposed and demonstrated a polarization-entangled photon source. They used the source to switch the optical edge mode in an imaging system to ON or OFF states based on a highly dielectric metasurface. The experiment enriched the fields of quantum optics and metamaterials as a promising direction toward quantum edge detection and image processing with a remarkable signal-to-noise ratio.

Combining quantum entanglement and edge detection

Photonic metasurfaces are two-dimensional (2-D) ultrathin arrays of engineered metallic or dielectric structures that can facilitate electromagnetic field manipulation of the local phase, amplitude and polarization. Researchers generally develop such capabilities for a variety of applications in classical optics. Quantum entanglement is essential in quantum optics for many applications including quantum cryptography, teleportation, superresolving metrology and quantum imaging. Recent efforts show a trend to combine the metasurface with entangled photons for potential applications in quantum optics. Edge detection is another factor that contributes to image processing to define the boundaries between regions in an image. It is a basic tool in computer vision to pre-process automations in medical imaging and forms a critical component of autonomous vehicles. Metasurface-enabled edge detection can be used in quantum optics to offer possibilities of remote-controlled image processing and cryptography. In this work, Zhou et al. therefore realized a polarization-entangled photon source and high-efficiency metasurface enabled switchable optical edge detection method. The combined strategy showed a high signal-to-noise (SNR) ratio at the same photon flux level (the number of photons per second per unit area).

Using the “Schrödinger’s cat” concept

Zhou et al. used the Schrödinger’s cat concept to illustrate the expected performance of the switchable quantum edge detection scheme. They reviewed the basic principle of edge detection based on classical continuous wave (CW) light-illumination. In the experimental setup, the edge detection imaging arm was independent of the entangled source and the heralding arm, as well as the coincidence measurement components. When the incident photons achieved a horizontal polarization state, the beam of illuminated light passed through a cat-shaped aperture and an engineered metasurface to separate into a left- and right-handed overlapped polarized image with a horizontal shift. The overlapped components then passed through a horizontally oriented analyzer to form a ‘solid cat’ image. If, however, the incident photons were vertically polarized, the overlapped components recombined to a linear polarized component that is completely blocked by the analyzer to only form an outline of a cat. The researchers therefore used polarization-entangled photons as a source of illumination to develop quantum switchable edge detection in this way.

The experimental setup and polarization-entangled photon pairs

The researchers generated polarization entangled photons using a spontaneous parametric down-conversion process in a 20-mm-long type II phase-matched periodically poled potassium titanyl phosphate (KTiOPO₄/PPKTP) crystal embedded in a Sagnac interferometer. They set the temperature of the crystal to 17 degrees Celsius and used two broadband dielectric mirrors and a dual-wavelength polarization beam splitter to form the self-stable Sagnac interferometer. They then used a continuous wave single-frequency diode laser at 405 nm to generate the pump beam focused by a pair of lenses with optimized focal lengths to attain a beam waist approximating 40 microns at the center of the crystal. To balance the power in the clockwise and counter-clockwise-directions, Zhou et al. used a quarter-wave plate (QWP) and a half-wave plate (HWP) in front to the Sagnac loop.