As quantum technologies continue to develop there is a growing desire to explore high-dimensional entanglement and multiphoton states for information processing. However, these tasks demand new forms of large-scale, controllable and stable quantum light sources.
Now, Lin Li and co-workers from China, Hong Kong and Taiwan have demonstrated a 100-path spontaneous parametric down-conversion (SPDC) photon source by combining a 10 × 10 metalens array with a thin (0.5 mm thick) nonlinear crystal of β-barium borate (BBO) (Science 368, 1487–1490; 2020).
The metalens array is composed of a dense pattern of GaN nanopillars fabricated by electron-beam lithography, dry etching and resist removal. Each metalens in the 10 × 10 array has an area of 100 μm by 100 μm and is designed to have a focal length of 1.1 mm at a working wavelength of 404 nm. The nanopillars are 800 nm high and spaced by 200 nm (pictured; scale bar, 1 μm).
In order to control quantum states by phase of light, the metalenses in the array are designed to have different initial phases. There are four phase differences of Δϕ = 0, π/2, π and 3π/2 between the adjacent metalenses.
When a diode laser (404 nm, 100 mW) was incident on the metalens array, a 10 × 10 array of focal spots was formed inside the BBO crystal. Each of the spots triggered a SPDC process and generated pairs of entangled photons in a probabilistic manner. Two-dimensional (2D), 3D and 4D entanglement states were thus generated by two, three and four metalenses, respectively, as verified by reconstructions and quantum state tomography measurements.
The fidelities of the pure 2D, 3D and 4D entangled states were estimated to be 0.984, 0.966 and 0.955, respectively, close to the perfect value of 1.
Furthermore, the multiphoton source based on the metalens array system was characterized with a 415-nm femtosecond pulsed laser. The quadratic and cubic pump-power dependencies confirmed the four- and six-photon generation from two and three adjacent metalenses, respectively. The purity and indistinguishability of four-photon generation was experimentally confirmed by the Hong–Ou–Mandel interference.
“Metasurface structures can provide a route for the generation and control of complex quantum states, not only increasing the quantum system dimensionality but also allowing for the coherent control of multiple photons”, Li said.