Indistinguishable Heralded Photon Pairs from a Whispering Gallery Resonator

Whispering Gallery Mode Resonators (WGMRs) made of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\chi^{2}$</tex> nonlinear dielectric materials have been shown to be a versatile source of quantum states [1]–[4]. Here, we investigate this type of source and demonstrate a Hong-Ou-Mandel (HOM) interference between heralded photon pairs that are produced in two spontaneous parametric downconversion (SPDC) processes within a single WGMR [5]. The WGMR is pumped by a continuous-wave laser with only 100 nW in total from the clockwise (CW) and counterclockwise (CCW) directions in two degen-erate counter-propagating modes. Hence, the heralded photon pairs from the two directions are indistinguishable in frequency. The indistinguishability of spatial and polarization modes is achieved by using standard optical elements. We measure the Glauber correlation function, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$g_{si}^{(2)}(t)$</tex>, from both directions, and find that the two functions are almost identical. As a result, the photon pairs generated from both directions are highly indistinguishable in the temporal domain (See Fig. 1(a)). The signal photons emitted in both directions are combined on a beam splitter, while the idler photons are coupled directly into detectors. The HOM interference fringe is extracted by detecting four-fold coincidence counts. The highest measured HOM visibilities reach a value above 74% (See Fig. 1(b)). We also show that the visibilities change when the value of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$g_{si}^{(2)}(0)$</tex> varies, and it agrees well with the theoretical prediction provided by Ref. [6]. (See Fig. 1(c)). Finally, we measure the Glauber correlation function from two different WGMRs (See Fig. 1(d)) showing that the two functions are almost identical. This indicates that it is possible to match the temporal mode of heralded photon pairs from different WGMRs. Together with the extremely low pump power requirement, WGMRs could be a potential platform for large-scale quantum applications.