Efficient generation of entangled photons in the telecommunications range using nonlinear metasurfaces integrated with ScAlN/GaN heterostructures

Abstract

Entangled photons provide non-classical correlations that enable measurement sensitivities beyond classical limits, scalable fault-tolerant quantum computation, and fundamentally secure quantum communication, making them a foundational necessity for next-generation quantum technologies. Here we propose and analyze a novel source of entangled photons based on ScAlN/GaN quantum wells integrated with dielectric metasurfaces. Giant second-order intersubband nonlinearity of the GaN quantum wells with strain-compensated delta-doped ScAlN barriers caused by strong built-in electric fields combined with superior mode-coupling performance of metasurfaces optimized by inverse design give rise to efficient parametric down-conversion and generation of entangled photons in the telecom range. We develop a rigorous Heisenberg-Langevin formalism which includes field quantization, dissipation and fluctuations for all fields, parametric amplification of thermal noise and zero-point fluctuations, and other relevant effects. Our proposed approach of employing the emergent photonic material ScAlN promises high biphoton generation rate over $10^{10}$ s$^{-1}$ from a compact integrated structure that is only 0.5 $μ$m thick while mitigating strain-related issues that have so far impeded progress of nitride-based heterostructures for quantum photonic applications into the infrared and visible wavelengths. Our result therefore is relevant for numerous applications ranging from quantum sensing, quantum information, and computing.

Figures (4)

• Figure 1: Schematic of the process of entangled-photon generation from an inverse designed metasurface integrated with GaN/ScAlN quantum wells. A pump beam at frequency $\omega_p$ (pump, blue) excites a resonant mode of the metasurface, producing a strongly enhanced local field in the QW layer. The large second-order nonlinearity of the QWs then mediates spontaneous parametric down-conversion of the pump into a pair of lower-frequency photons at $\omega_s$ (signal, red) and $\omega_i$ (idler, orange). The inset shows the conduction-band profile of a single GaN/ScAlN QW period together with the probability density of the confined subband states used to realize the intersubband nonlinearity.
• Figure 2: 8-band k.p band structure calculations of conduction band edge profiles of a single heterostructure period and the confined energy levels involved in SPDC in (a) a single QW consisting of 7 monolayers of GaN QW with 6 nm Sc$_{0.14}$Al$_{0.86}$N barriers, and (b) a double QW composed of 4 monolayers of GaN, 3 monolayers of AlN, and 7 monolayers of GaN, also confined by 6 nm Sc$_{0.14}$Al$_{0.86}$N barriers. An N-type delta-doping layer with a sheet density $2.4\times 10^{13}$ cm$^{-2}$ is introduced 1 nm away from the GaN quantum well, as indicated by the vertical purple dashed line.
• Figure 3: The spectrum of $|\chi_s^{(2)}|$ as a function of the signal field wavelength. Panel (a): for the structure in Fig. 2(a); Panel (b): for the structure in Fig. 2(b). In both (a) and (b) an N-type delta-doping layer with a sheet density $2.4\times 10^{13}$ cm$^{-2}$ is introduced 1 nm away from the GaN quantum well.
• Figure 4: (a, b) In-plane (x-y) spatial profile of one period of the inverse-designed metasurface (a) and a dielectric grating (b). The black region corresponds to presence of material (silicon) and white region corresponds to free space. The metasurface and gratings are assumed to be an infinite periodic array of the structures shown in (a) and (b). (c, d) Numerically calculated absolute value of the z-component of the electric field in the central x-y plane of the nonlinear multi-QW region and y-z plane at the center of the unit cell for the modes at the pump (2$\omega$) and signal frequencies ($\omega$) for the inverse designed metasurface (c) and dielectric grating (d). The corresponding quality factors (Q) of the modes for both the structures are mentioned in the figures. (e,f) Numerically calculated absolute value of the z-component of the electric field normalized with respect to the incident field $E_{inc}$ in the y-z plane at the center of the unit cell for planewave excitation at (2$\omega$) and signal frequencies ($\omega$) for the inverse-designed metasurface (a) and dielectric grating (b). The inverse-designed structure couples significantly better to the modes shown in (c) compared to the dielectric gratings (d).