(Nanowerk Spotlight) Quantum light sources are a fundamental building block for many quantum technologies, enabling secure communication, powerful computing, and precise sensing and imaging. Researchers have long sought to develop efficient, compact, and controllable sources of quantum light, such as entangled photon pairs and single photons. However, traditional approaches have relied on bulky nonlinear crystals or randomly distributed quantum dots and color centers, which limit scalability, flexibility, and device integration.
In recent years, the field of flat optics has emerged as a powerful tool for manipulating light at the nanoscale. Flat optical elements, such as metasurfaces, are surfaces patterned with subwavelength-scale nanostructures that can control the amplitude, phase, polarization, and other properties of light. By engineering the shape, size, and arrangement of these nanostructures, researchers can create ultra-thin, lightweight, and multifunctional optical components that surpass the capabilities of conventional bulky optics.
Now, a new wave of research is bringing together the fields of flat optics and quantum light sources, with the goal of creating enhanced, compact, and versatile sources of quantum light. In a comprehensive review published in Advanced Materials (“Engineering Quantum Light Sources with Flat Optics”), a team of researchers from Australia has surveyed the latest advancements in this exciting field, highlighting the potential of flat optics to revolutionize the generation and control of quantum light.
Sketch of the review. The review covers two types of flat-optics quantum light sources: i) entangled photon pairs from nonlinear films and metasurfaces (left column) and ii) single photon emission from an unstructured 2D material, a 2D material metasurface, and an epitaxial quantum dot (QD) metasurface (right column). The sources empower a variety of quantum applications (middle column), including quantum sensing, quantum communication and quantum imaging. (Image: Reprinted from DOI:10.1002/adma.202313589, CC BY) (click on image to enlarge)
The review focuses on two main types of quantum light sources: entangled photon pairs generated through spontaneous parametric down-conversion (SPDC) in nonlinear metasurfaces, and single photons emitted from quantum dots and color centers in materials such as gallium arsenide, diamond, and hexagonal boron nitride. In both cases, the researchers show how flat optical elements can enhance the efficiency, directionality, and functionality of these sources.
For instance, the researchers highlight a study where a lithium niobate metasurface was used to generate entangled photon pairs with a brightness enhancement of two orders of magnitude compared to an unstructured film. This metasurface, featuring carefully designed nanostructures, also enabled the generation of complex quantum states, such as spatially entangled modes and cluster states, which are essential for applications in quantum communication and quantum computing.
In another example, the integration of a single quantum dot with a circular Bragg grating metasurface led to a 20-fold increase in the collection efficiency of single photons by a low-numerical-aperture objective. This improvement was attributed to the metasurface’s ability to shape the emission into a Gaussian profile and enhance the emission rate through the Purcell effect.
The review also highlights the potential of emerging two-dimensional materials, such as transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN), as promising platforms for quantum light sources. TMDs like tungsten diselenide (WSe2) host optically active defects that can emit single photons, while hBN can host bright and photostable color centers.
The integration of these materials with flat optical elements, such as plasmonic nanocavities, has led to significant enhancements in the brightness and emission rate of the quantum emitters. For example, coupling WSe2 emitters to a plasmonic nanocavity resulted in a 15-fold reduction in the exciton lifetime, indicating strong Purcell enhancement.
Looking to the future, the researchers identify several key challenges and opportunities for the field. One important goal is to further increase the efficiency and brightness of metasurface-based quantum light sources, which currently lag behind their bulk counterparts due to the limited interaction length of the nanostructures. This could be addressed through advanced design strategies, such as employing high-quality-factor resonances with Q-factors exceeding 104 and optimizing the coupling between the emitters and the optical modes.
Another exciting prospect is the development of dynamically tunable and reconfigurable quantum light sources using active metasurfaces. By incorporating materials whose optical properties can be modulated by external stimuli such as electric fields or optical pulses, researchers could create quantum sources with unprecedented levels of control and adaptability.
Finally, the integration of flat optics-based quantum light sources with other photonic components, such as waveguides, detectors, and modulators, will be crucial for realizing complete quantum systems on a chip. This will require advances in nanofabrication, materials processing, and device design, but the payoff could be transformative for fields ranging from secure communication to biomedical imaging.
The convergence of flat optics and quantum light sources represents a major step forward in the quest for compact, efficient, and versatile quantum technologies. By harnessing the power of metasurfaces to generate, manipulate, and control quantum states of light at the nanoscale, researchers are opening up new frontiers in both fundamental science and practical applications. As the field continues to evolve, we can expect to see increasingly sophisticated quantum devices that leverage the unique capabilities of flat optics, bringing us closer to the long-sought goal of scalable, integrated quantum systems for computing, communication, and sensing.
While there are still many challenges to overcome, the progress highlighted in this review offers a tantalizing glimpse of the future of quantum technologies. With continued advances in materials science, nanofabrication, and optical design, flat optics-based quantum light sources could become a key enabling technology for a wide range of applications, from secure communication networks to ultra-sensitive biomedical imaging and beyond. As researchers continue to push the boundaries of what is possible with these innovative devices, we can look forward to a new era of quantum-enhanced technologies that will transform the way we process, transmit, and detect information at the nanoscale.
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