Fig. 1: Dynamic control of entangled quantum states using a nonlinear metasurface.

a The polarization-entangled two-qubit state is described by \(\left| {{{\mathrm{{\Psi}}}}} \right\rangle = \frac{1}{{\sqrt {\left| \alpha \right|^2 + \left| \beta \right|^2} }}\left( {\alpha \left| {V_S} \right\rangle \left| {V_I} \right\rangle + \beta \left| {H_S} \right\rangle \left| {H_I} \right\rangle } \right)\). The metasurface is acting on only one of the photons. The nonlinear modulation of the quantum state \(\left| {{{\mathrm{{\Psi}}}}} \right\rangle\) is accomplished by external control light with power \(P_{{{\mathrm{C}}}}\) (green beam). \(\left| {{{\mathrm{{\Psi}}}}} \right\rangle\) is not maximally entangled when \(\alpha /\beta \ne 1\). The state distillation corresponds to the case of recovering \(\alpha /\beta\) to 1. b Schematic of a single unit cell. Gold film with 45 nm thickness was sputtered onto a fused silica substrate. An array of anisotropic structured ASRs was patterned by FIB milling. A nonlinear switching layer of ethyl red polymer film was spin-coated on top. The designed geometric parameters are the period \(P = 300\,{{{\mathrm{nm}}}}\), the slit length \(l = 250\,{{{\mathrm{nm}}}}\), the slit width \(w = 45\,{{{\mathrm{nm}}}}\), and the gap between “C” arm and “I” slit \(d = 95\,{{{\mathrm{nm}}}}\). Representative SEM image of the sample is given in bottom panel. Vertical (V) and horizontal (H) directions are defined as along y- and x-axes, respectively. c The ethyl red molecule isomerizes from trans state to cis state by green-light irradiation, and then recovers to the original state through thermal relaxation in dark. Such reversible structural transformation would change the optical response of the metasurface, hence the value of \(\alpha /\beta\)