(Nanowerk Spotlight) Quantum physics holds great promise for advancing imaging technology beyond what traditional optics can achieve. At its core, quantum imaging uses pairs of photons – particles of light – that share a special connection called entanglement. This quantum connection allows imaging techniques that surpass classical limits, offering better resolution and the ability to detect extremely faint objects.
Scientists typically generate entangled photon pairs by shining laser light through special crystals. These crystals split some incoming photons into two lower-energy photons that remain quantum mechanically linked. However, this method has significant limitations. Traditional nonlinear crystals must be several millimeters thick to produce enough entangled photons, restricting the angles at which photons emerge. This constraint limits both resolution and field of view while also making integration into compact devices challenging.
Recent advances in nanotechnology offer a new path forward. Scientists can now precisely pattern materials at scales smaller than the wavelength of light, creating metasurfaces – ultra-thin structures that manipulate light in ways impossible with natural materials. These engineered surfaces, often just hundreds of nanometers thick, provide unprecedented control over light behavior.
A research team led by scientists at the Australian National University has demonstrated how metasurfaces can revolutionize quantum imaging. Their device consists of a precisely patterned silica (glass) structure atop an ultrathin lithium niobate film, measuring just 300 nanometers thick.
This metasurface generates entangled photon pairs when illuminated by a laser, but unlike traditional crystals, it allows precise control over their emission direction. By simply adjusting the laser’s wavelength, the researchers can steer the emitted photons across a wide range of angles, enabling an innovative imaging approach that combines two powerful techniques: ghost imaging and optical scanning.
Quantum imaging protocol with photon pairs from a nonlinear metasurface. The metasurface, incorporating a silica grating atop a lithium niobate thin film, produces spatially entangled signal and idler photons where their emission direction along y-direction is tunable via pump laser wavelength. Signal photons passing through the object are collected using a bucket single-photon detector (SPD), while idler photons without interacting with the object are captured by a z-oriented 1D SPD array. We perform quantum ghost imaging and all-optical scanning in the z- and y-direction respectively, achieving 2D imaging of the object. The inset on the bottom left shows the mode profiles of one unit cell of the metasurface for ky = 0 and ky = 0.1 rad/μm. (Image: Reprinted from DOI:10.1186/s43593-024-00080-8, CC BY 4.0)
In ghost imaging, one photon from an entangled pair interacts with the object being imaged while its partner photon is detected separately. The object’s image can be reconstructed even though the photon that touched it is never directly measured. The metasurface enables ghost imaging along one axis while allowing optical scanning along the perpendicular axis by tuning the laser wavelength. This combination allows the system to capture detailed two-dimensional images using only a simple one-dimensional row of detectors, significantly reducing the complexity of the setup.
Experimental results show that this approach can provide up to four orders of magnitude more resolution points than traditional nonlinear crystal-based systems. The metasurface enables photon emission at a much broader range of angles, greatly enhancing both resolution and field of view.
Beyond basic imaging, the researchers suggest their design could be adapted for multi-color imaging—for example, using infrared light to illuminate objects while detecting visible light. This could enable advanced night vision or medical imaging applications.
Additionally, the ability to steer quantum light beams by adjusting the laser wavelength could lead to quantum LIDAR for high-precision 3D mapping and even quantum object tracking for following moving targets with entangled photons.
Some challenges remain before practical implementation. The current system produces fewer photon pairs than traditional crystal-based sources, but the researchers identified ways to enhance efficiency through improved materials and metasurface designs. The prototype also required careful alignment in a controlled laboratory setting, though future refinements could improve robustness.
By providing fine-tuned control over quantum light in an ultra-compact package, metasurfaces offer a compelling path forward for quantum imaging. With continued development, this technology could transform fields such as medicine, security, and remote sensing, enabling us to see what was previously unseeable.
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