Experimental Demonstration of a Hybrid-Quantum-Emitter Producing Individual Entangled Photon Pairs in the Telecom Band

Abstract

Quantum emitters generating individual entangled photon pairs (IEPP) have significant fundamental advantages over schemes that suffer from multiple photon emission, or schemes that require post-selection techniques or the use of photon-number discriminating detectors. Quantum dots embedded within nanowires (QD-NWs) represent one of the most promising candidate for quantum emitters that provide a high collection efficiency of photons. However, a quantum emitter that generates IEPP in the telecom band is still an issue demanding a prompt solution. Here, we demonstrate in principle that IEPPs in the telecom band can be created by combining a single QD-NW and a nonlinear crystal waveguide. The QD-NW system serves as the single photon source, and the emitted visible single photons are split into IEPPs at approximately 1.55 μm through the process of spontaneous parametric down conversion (SPDC) in a periodically poled lithium niobate (PPLN) waveguide. The compatibility of the QD-PPLN interface is the determinant factor in constructing this novel hybrid-quantum-emitter (HQE). Benefiting from the desirable optical properties of QD-NWs and the extremely high nonlinear conversion efficiency of PPLN waveguides, we successfully generate IEPPs in the telecom band with the polarization degree of freedom. The entanglement of the generated photon pairs is confirmed by the entanglement witness. Our experiment paves the way to producing HQEs inheriting the advantages of multiple systems.

Figure 1. The optical characteristics of the QD-NW sample.

(a) The PL spectrum of a QD assemble on one certain NW. (b) The PL spectrum of four candidate QD-NWs, the measuring temperature is 15 K. (c) The temperature dependence of the PL intensity and linewidth of four candidate QD-NWs. (d) HBT measurement result of QD4 emission using continuous wave above-band gap excitation. The error bars are smaller than the dimension of the markers and are not shown.

Figure 2. Experimental test results of the PPLN waveguide.

(a) The degenerate down-conversion temperature points for varying pumping wavelength. (b) The wavelength-temperature dependence relationship of the telecom band photon pairs with 776.2 nm CW laser pumping. The error bars are smaller than the dimension of the markers and are not shown.

Figure 3. Experimental setup for the generation of IEPP of telecom band.

The single QD-NW in the cryostat is excited by the pulsed ultraviolet (UV) beam and generates visible single photons with wavelengths of approximately 775 nm. The polarization Sagnac interferometer (PSI) apparatus with a PPLN waveguide at the center is used to generate polarization entanglement. The single photons coupled into the PPLN waveguide have a probability of splitting into IEPP, which are detected by near-infrared single-photon detectors (NIR-SPDs). The four NIR-SPDs are gated by the electrical pulses from the photo-multiple tube (PMT). SMF - single mode fiber, HWP - half wave plate, QWP - quarter wave plate, DM - dichroic mirror, PBS - polarized beam splitter.

Figure 4

(a) Real and (b) Imaginary parts of the reconstructed density matrix of the PSI outcome state when the PPLN waveguide is pumped with a 776.2 nm continuous laser.

Figure 5. Experimental expectation values for every correlation function of the entanglement witness for the generated state.

The results are derived by twofold coincidence measurements along three complementary common bases: linear(H/V), diagonal(+/−), and circular(R/L), conditioned by the pulsed signals from PMT. The blue bars are results after eliminating coincidence from the NIR-SPDs’ dark count, whereas the red bars include this noise.