Entangled photons from subwavelength nonlinear films

Abstract

Miniaturized entangled photon sources, in particular based on subwavelength metasurfaces, are highly demanded for the development of integrated quantum photonics. Here, as a first step towards the development of quantum optical metasurfaces (QOMs), we demonstrate generation of entangled photons via spontaneous parametric down-conversion (SPDC) from subwavelength films. We achieve photon pair generation with a high coincidence-to-accidental ratio in lithium niobate and gallium phosphide nanofilms. By implementing the fiber spectroscopy of SPDC in nanofilms, we measure a spectrum with a bandwidth of 500 nm, limited only by the overall detection efficiency. The spectrum reveals vacuum field enhancement due to a Fabry–Perot resonance inside the nonlinear films. It also suggests a strategy for observing SPDC from QOM. Our experiments lay the groundwork for future development of flat SPDC sources, including QOM.

Photon Pairs from Resonant Metasurfaces

“…The coincidences-to-accidentals (peak-to-background) ratio (CAR) considerably exceeds 2 in both cases, which proves the generation of photon pairs in each QOM. 15 The maximal obtained CAR is 361. The rates of real coincidences, found from the total number of coincidences after subtraction of the accidental coincidences, are 5.4 ± 0.1 Hz and 1.8 ± 0.1 Hz for QOMs A and B, respectively.…”

“…Here, we observed a much larger bandwidth, limited only by the spectral sensitivity of our setup. The spectrum of the LN film is broad, because the phase matching is relaxed 15 and the vacuum field seeds SPDC uniformly over the spectrum. 16 …”

Enhanced generation of nondegenerate photon pairs in nonlinear metasurfaces

“…16 At the subwavelength scale, generation of photon pairs was reported experimentally from a single AlGaAs nanoresonator 17 and lithium niobate MSs, 18 and studies were also conducted on monolayers of MoS 2 , 19 carbon nanotubes, 20 and directional emission from nanoresonators. 21 Importantly, SPDC in ultrathin nonlinear layers [22][23][24] can give rise to strong spatial correlations and allow quantum state engineering without the constraints of longitudinal phase matching. It has been proposed that a so-called "accidental" BIC at the pump frequency can increase the photon rate at a single nanoresonator, 25 while a photonic crystal slab with a BIC resonance can enhance SPDC in a monolayer of WS 2 , 26 although the theoretically estimated rate was still much lower than with conventional sources.…”

Quantum photonics based on metasurfaces

“…Of course, this method can be expanded to any other types of nanostructures too. Experimentally, some material, such as crystalline AlGaAs, is able to combine resonance enhancement with high nonlinear susceptibility together 85 , thus bridging the gap between nonlinear optics and quantum optics. Over the past few decades, there has been so many works concerning how to augment and control nonlinear frequency conversion, and these works provide enormous and rich knowledge for manipulation of quantum source.…”