Quantum nano­photonics is an active research field with emerging appli­cations that range from quantum computing to imaging and tele­communications. This has motivated scientists and engineers to develop sources for entangled photons that can be integrated into nano-scale photonic circuits. Practical appli­cation of nanoscale devices requires a high photon-pair generation rate, room-temperature operation, and entangled photons emitted at telecommuni­cations wavelengths in a directional manner.

The most common way to create entangled photons is by spontaneous para­metric down conversion (SPDC) which involves a single photon being split into two entangled photons of lower frequencies, known as the signal and idler. Conven­tional approaches for SPDC rely on bulky devices that are up to several centi­meters in length and are not optimal for photonic circuit inte­gration. Conversely, at the nanoscale, the efficiency of the SPDC process is hindered by the small volume of the resonators, and the direc­tionality of the emitted photons is challenging to control. Dielectric meta­surfaces offer a promising route to enhance and tailor SPDC photon emission. To date, however, meta­surfaces have used relatively low quality-factor Mie resonances and have an accordingly broad emission spectrum, which restricts the spectral bright­ness of photons. New research reveals that extended bound states in the continuum (BIC) resonances make it possible to harness modes in the meta­surface that have very high quality factors.

This in turn means that the photon-pair generation inside the resonators is enhanced by many orders of magnitude and the wavelength of the photons will have a very narrow bandwidth. This results in a very high spectral brightness, which is beneficial for quantum network appli­cations. An inter­national team of researchers from the Australian National University (Matthew Parry, Dragomir N. Neshev, and Andrey A. Sukhorukov), Politecnico di Milano (Andrea Mazzanti and Giuseppe Della Valle) and ITMO University of St. Petersburg (Alexander Poddubny) recently demonstrated enhanced generation of non­degenerate photon pairs in nonlinear meta­surfaces. In a series of comprehensive simulations, they used separate BICs at slightly different wavelengths for the signal and idler photons in SPDC, which enabled them to enhance the brightness of entangled photons by five orders of magnitude over that of an unpatterned thin film of nonlinear material. They attribute this enhancement largely to the novel pheno­menon of hyperbolic transverse phase matching, which facilitates efficient photon genera­tion across a broad range of photon momentums.

Not only does their proposed method enable the generation of photon pairs that are quantum-entangled, but by simply changing the linear polari­zation of the pump laser it is possible to tune the polari­zation entanglement of the photons from full to none. This is an easily implemented way to control the entanglement, so that it meets the requirements of prospective appli­cations. The proposed platform is also highly confi­gurable with respect to both the wavelength of the signal and idler photons as well as the BICs used, which opens up the potential for engineering the direction in which photons are emitted. The researchers, whose work is supported by the Australian Research Council and by the European Commission's Horizon 2020 program, say that their advance is an important step towards minia­turized quantum devices for everyday appli­cations. (Source: SPIE)

Reference: M. Parry et al.: Enhanced generation of nondegenerate photon pairs in nonlinear metasurfaces, Adv. Phot. 3, 055001 (2021); DOI: 10.1117/1.AP.3.5.055001

Link: Nonlinear Physics Centre, Australian National University, Canberra, Australia