Multicolor Quantum Metrology with Entangled Photons

Entangled photons can be used to make measurements with an accuracy beyond that possible with classical light. While most implementations of quantum metrology have used states made up of a single color of photons, we show that entangled states of two colors can show supersensitivity to optical phase and path length by using a photonic crystal fiber source of photon pairs inside an interferometer. This setup is relatively simple and robust to experimental imperfections. We demonstrate sensitivity beyond the standard quantum limit and show superresolved interference fringes using entangled states of two, four, and six photons.

Figure 1

Generation of entangled states for enhanced measurement of $\theta$ (a) by HOM interference between two single photons, (b) by a bright coherent state $\alpha$, which pumps two identical PCF pair-photon sources inside the interferometer. The two methods result in equivalent states, except that in (b) the photon pairs do not need to be degenerate in wavelength. (c) Experimental setup using one fiber source of nondegenerate photon pairs pumped in two directions. BS: beam splitter; PBS: polarizing beam splitter; DM: dichroic mirror; S-B: Soleil-Babinet compensator.

Figure 2

(a) Classical interference with the pump beam at 720 nm as the position of the Soleil-Babinet compensator is varied and (b) two photon interference with half the period (red upward pointing triangles: bunched coincidences; blue downward triangles: antibunched). Both with sinusoidal fit lines.

Figure 3

Fourfold counts for (a) the $|11{⟩}_s|11{⟩}_i$ output, (b) the $|20{⟩}_s|11{⟩}_i$ output, and (c) the $|20{⟩}_s|02{⟩}_i$ output. Fit lines are based on calculation [19].

Figure 4

Sixfold coincidences in the $|21{⟩}_s|21{⟩}_i$ output with a fit line based on theory.