Modern photonic quantum technologies require the on-demand generation of indistinguishable single photons with high brightness, together with flexible control of their emission across multiple degrees of freedom. Empowered by cavity quantum electrodynamics (cQED), cavity-enhanced epitaxial quantum dots (QDs) have set the benchmark for device performance, offering high single-photon purity, record source brightness, and near-unity photon indistinguishability. However, these sources typically operate in simple Gaussian-like modes, which limits their applications in high-dimensional quantum information processing. By contrast, metasurfaces provide a powerful route for tailoring photonic states, but have so far struggled to deliver strong Purcell enhancement and high indistinguishability because of weak light-matter interaction.
In a new paper published in eLight , a joint team of scientists, led by Professor Jin Liu and Professor Xuehua Wang of Sun Yat-sen University, China, Professor Liu Liu of Zhejiang University, China and Professor Yijie Shen of Nanyang Technological University, Singapore, has now developed a monolithic microcavity-metalens interface that brings together high-performance single-photon generation and flexible wavefront engineering on a single chip. The device consists of a QD-micropillar single-photon source on the front side and an ultra-thin metalens on the back side of a III-V compound semiconductor chip. This unique configuration allows independent optimization of photon generation and wavefront shaping. As the scientists explain, “We place the cavity-enhanced quantum dot source and the metalens on opposite sides of the same chip. In this way, the micropillar preserves the strong light-matter interaction needed for bright and indistinguishable single-photon emission, while the metalens independently reshapes the emitted photons in multiple degrees of freedom.”
They further explain the key operation principle: “The single photons emitted by the micropillar first propagate through a 350 μm-thick substrate, which expands the beam diameter from about 2 μm to about 103 μm. This propagation naturally matches the cavity output to a 200 μm metalens, making efficient and arbitrary wavefront engineering possible without compromising the cavity performance.” Based on this architecture, the integrated device delivers high single-photon purity with g (2) (0) = 0.070(1), a photon extraction efficiency of 35.7(5)%, and photon indistinguishability of 0.736(2), representing a record-performance source among existing high-dimensional single-photon sources based on quantum emitters integrated with metasurfaces.
Beyond source performance, the platform enables full and independent control over radiation divergence, emission directionality, polarization, and OAM of the emitted single photons. Specifically, the team demonstrated multifunctional quantum-state engineering through single-photon collimation, beam deflection, and spin-dependent OAM encoding for right- and left-handed circular polarization channels. The same platform further enabled the generation of polarization-OAM entanglement with a fidelity of 0.8914 ± 0.0023, as well as single-photon skyrmions with tunable skyrmion numbers. Notably, these topologically structured single photons maintained stable skyrmion numbers under atmospheric turbulence and showed greater robustness than conventional OAM-carrying single photons. “This platform brings together high-performance single-photon generation and multidimensional state engineering on one chip,” the scientists say. “It opens up new opportunities for resilient quantum entanglement, high-dimensional quantum communication, and integrated quantum meta-optics.”
eLight
High-performance sources of multidimensionally engineered quantum light based on monolithic microcavity-metalens interfaces