Flat-optics generation of broadband photon pairs with tunable polarization entanglement

Abstract

The concept of “flat optics” is quickly conquering different fields of photonics, but its implementation in quantum optics is still in its infancy. In particular, polarization entanglement, strongly required in quantum photonics, is so far not realized on “flat” platforms. Meanwhile, relaxed phase matching of “flat” nonlinear optical sources enables enormous freedom in tailoring their polarization properties. Here we use this freedom to generate photon pairs with tunable polarization entanglement via spontaneous parametric downconversion (SPDC) in a 400-nm GaP film. By changing the pump polarization, we tune the polarization state of photon pairs from maximally entangled to almost disentangled, which is impossible in a single bulk SPDC source. Polarization entanglement, together with the broadband frequency spectrum, results in an ultranarrow (12 fs) Hong–Ou–Mandel effect and promises extensions to hyperentanglement.

Lithium Niobate Meta-Optics

“…After the first success of photon-pair generation, a lot of efforts have been dedicated to demonstrating entanglement. Recently, polarization entanglement in a subwavelength film, 167 momentum entanglement in a metasurface, 42 and approaches for cluster-state generation through spectral entanglement were reported. 165 4.2.2.…”

Photon pairs bi-directionally emitted from a resonant metasurface

“…3–5 Used at first just for the transformation and shaping of quantum light 3 and for the generation of single photons, 4,6 various nanoscale platforms are now producing entangled photons, through spontaneous four-wave mixing 7 and spontaneous parametric down-conversion (SPDC). 8–14 In particular, resonances in metasurfaces enable the enhancement of SPDC efficiency and shaping the spectrum of the emitted photon pairs. 10,14 At the same time, an important degree of freedom for photon pairs – namely, the direction of emission – remains almost unexplored.…”

“…[3][4][5] Used at first just for the transformation and shaping of quantum light 3 and for the generation of single photons, 4,6 various nanoscale platforms are now producing entangled photons, through spontaneous four-wave mixing 7 and spontaneous parametric down-conversion (SPDC). [8][9][10][11][12][13][14] In particular, resonances in metasurfaces enable the enhancement of SPDC efficiency and shaping the spectrum of the emitted photon pairs. 10,14 At the same time, an important degree of freedom for photon pairsnamely, the direction of emissionremains almost unexplored.…”

“…10,14 At the same time, an important degree of freedom for photon pairsnamely, the direction of emissionremains almost unexplored. 15 So far, photon pairs have been generated from nanoscale sources either (nearly) collinearly with the pumping radiation, 9,[11][12][13][14] or in the backward direction. 8,10 Meanwhile, careful engineering of such sources could offer a possibility of bi-directional emission of photons.…”

Background-Free Near-Infrared Biphoton Emission from Single GaAs Nanowires

“…Miniaturization to the micrometer or submicrometer scale, to sizes smaller than the coherence length, relaxes the phase-matching condition and reduces the footprint of the source . SPDC with no phase-matching requirements has recently been demonstrated in thin films − and metasurfaces, − for example by taking advantage of resonances such as bound states in the continuum or Mie resonances. A single nanoantenna does not need phase matching either, and arrays of nanoantennas can be collectively excited for an increased generation rate.…”