Two-Photon Coincident-Frequency Entanglement via Extended Phase Matching

We demonstrate a new class of frequency-entangled states generated via spontaneous parametric down-conversion under extended phase-matching conditions. Biphoton entanglement with coincident signal and idler frequencies is observed over a broad bandwidth in periodically poled ${\mathrm{K}\mathrm{T}\mathrm{i}\mathrm{O}\mathrm{P}\mathrm{O}}_4$. We demonstrate high visibility in Hong-Ou-Mandel interferometric measurements under pulsed pumping without spectral filtering, which indicates excellent frequency indistinguishability between the down-converted photons. The coincident-frequency entanglement source is useful for quantum information processing and quantum measurement applications.

Figure 1

Experimental setup showing the entanglement source and the fiber HOM interferometer used to test the coincidence-frequency-entangled state. DM: dichroic mirror, HWP: half-wave plate, FPBS: fiber polarization beam splitter.

Figure 2

HOM interferometric measurements under cw pumping at 792 nm. The experimental data are fitted with a triangular-dip HOM function (solid line). Base-to-base width of the HOM dip is $l_c=0.85\pm 0.16\mathrm{m}\mathrm{m}$ corresponding to a biphoton coherence time ${\tau }_c=1.42\pm 0.26\mathrm{p}\mathrm{s}$. HOM dip visibility is $95\pm 5\%$.

Figure 3

HOM interferometric measurements for a pulsed pump centered at 790 nm with a 3 dB bandwidth of 6 nm. The experimental data are fitted with a triangular-dip HOM function (solid line). Base-to-base width of the HOM dip is $l_c=0.8\pm 0.2\mathrm{m}\mathrm{m}$ corresponding to a biphoton coherence time ${\tau }_c=1.3\pm 0.3\mathrm{p}\mathrm{s}$. HOM dip visibility is $85\pm 7\%$.