Figure 4: Entangled photonic gears. | Nature Communications

Figure 4: Entangled photonic gears.

From: Photonic polarization gears for ultra-sensitive angular measurements

Figure 4

(a) Experimental setup. An entangled photon pair in the polarization state |ψ› is generated by type-II spontaneous parametric down conversion. The state can be converted to the |› state by inserting a HWP in the path of Bob’s photon. Each photon is then converted into SAM−OAM hybrid states by the q-plates qA and qB and a HWP, as before, and is sent to a different rotation stage for the analysis. (b) Normalized experimental correlations (blue points), with ππ=HH, obtained with the |ψ› state by measuring the two-fold coincidences in the H-polarization bases on both modes for different values of the rotation angles θA and θB. We observe the gear enhancement with respect to the polarization-only case (red surface, theory) in the oscillation frequencies in both directions θA (with mA=2qA−1=2) and θB (with mB=2qB+1=11). (c) Normalized experimental correlations again with |ψ› (blue points) but for mA=2qA+1=7 and mB=2qB+1=11. (d) Normalized experimental correlations obtained with the |ψ› and |› states when rotating the two stages by the same angle θA=θB=θ, for mA=2qA+1=7 and mB=2qB+1=11. The polarization correlations (blue points: data for |ψ›, red points: data for |›) now present an oscillation pattern with a periodicity enhancement of (mA+mB) for |› and mAmB for |ψ›, due to quantum entanglement combined with the gear effect. The theoretical polarization-only HH correlation (without the gear enhancement) is also shown, for reference, as a red solid curve in the |› state case, oscillating as 2θ, and as a blue solid curve in the |ψ› state case, which is constant and vanishing. Yellow points: experimental data for single-photon gear with m=(mA+mB)/2=9, oscillating at half the frequency of . Dashed curves: best fit of the experimental data. The visibility of the pattern for |› state is . In all plots, error bars in the correlations are due to the Poissonian statistics of the recorded events, whereas error bars in the set value of the angle θ are due to the mechanical resolution of the rotation stage.

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