Nonclassical two-photon interference between independent telecommunication light pulses converted by difference-frequency generation

We experimentally demonstrated the Hong-Ou-Mandel (HOM) interference between two photons after visible-to-telecommunication wavelength conversion. In the experiment, we prepared a heralded single photon by using spontaneous parametric down conversion (SPDC) and the other photon from a weak laser source at 780 nm. We converted the wavelength of both photons to the telecommunication wavelength of 1522 nm by using difference-frequency generation in a periodically poled lithium niobate, and then observed the HOM interference between the photons. The observed visibility is $0.76\pm 0.12$ which clearly shows the nonclassical interference of the two photons. The high-visibility interference is an important step for fiber-based quantum communications of photons generated from visible-photon emitters.

Figure 1

An elementary link of quantum repeaters with the quantum interface. Each quantum memory has two excitation modes coupled to two modes of a photon such as polarization. Visible photons entangled with quantum memories are converted to the telecommunication bands, and pass through optical fibers. The Bell measurement is performed on the two photons for establishing entanglement between the quantum memories.

Figure 2

Concept of our experimental setup. A single photon is prepared by measuring one of the photon pair from the SPDC. The coherent light pulse is prepared by the laser source.

Figure 3

(a) The experimental setup for the HOM interference after the wavelength conversion. The single PPLN waveguide is used for the wavelength conversion of the heralded single photon and the coherent light pulse. (b) Two-fold coincidence counts between ${\mathrm{D}}_{\mathrm{V}}$ and ${\mathrm{D}}_{\mathrm{T}1}$, and between ${\mathrm{D}}_{\mathrm{V}}$ and ${\mathrm{D}}_{\mathrm{T}2}$. In each histogram, three peaks of the arrival time of the stop signal are observed. For the threefold coincidence events among ${\mathrm{D}}_{\mathrm{V}}$, ${\mathrm{D}}_{\mathrm{T}1}$, and ${\mathrm{D}}_{\mathrm{T}2}$, we postselect the 300 ps time windows of the two central peaks.

Figure 4

Observed HOM interference between two light pulses. The threefold coincidence counts have been observed in the 300 ps time windows. The solid curve is the Gaussian fit to the experimental counts. The dashed curve is obtained by Eq. (4) with experimental parameters. The dashed horizontal line describes the minimum values of the dips in the classical wave theory.