Maximally entangled and gigahertz-clocked on-demand photon pair source

We present a 1 GHz-clocked, maximally entangled and on-demand photon pair source based on droplet etched GaAs quantum dots using two-photon excitation. By employing these GaP microlens-enhanced devices in conjunction with their substantial brightness, raw entanglement fidelities of up to $0.95\pm 0.01$ and postselected photon indistinguishabilities of up to $0.93\pm 0.01$, the suitability for quantum repeater based long range quantum entanglement distribution schemes is shown. Comprehensive investigations of a complete set of polarization selective two-photon correlations facilitate an innovative method to determine the extraction and excitation efficiencies directly, as opposed to commonly employed indirect techniques. Additionally, time-resolved analysis of Hong-Ou-Mandel interference traces reveal an alternative approach to the investigation of pure photon dephasing.

Figure 1

1.0 GHz-clocked photon pair source using two-photon excitation (TPE). (a) High-resolution photoluminescence spectrum at $\pi$-pulse TPE with average power of $11\mu \mathrm{W}$. Basic QD transitions are annotated. (b) Superconducting single-photon detector (SSPD) count rates as a function of excitation power. (c) Schematic illustration of the $\text{XX}\text{−}\text{X}$ cascade. Two-photon (d) auto- and (e),(f) cross-correlation traces in detection polarization bases $H$ and $V$.

Figure 2

Matrix of polarization-dependent two-photon correlation traces of detected time delay differences $\delta \tau ={\tau }_{\text{X}}^I-{\tau }_{\text{XX}}^J$. Polarization combinations ($IJ$) of $\text{XX}$ and $\text{X}$, respectively, are annotated. Experimental data (1 ps binning) taken with four detectors in TTTR mode, fitted model, and theoretical curves are shown. Details are given in the text.

Figure 3

Two-photon entanglement extracted from polarization-resolved correlation traces using 8 ps binning. (a) Exemplary representations of the two-photon density matrix elements $\left|\rho \right|$ for selected time delays and correlation modes. (b) Entanglement negativity $N\left(\rho \right)$ as a function of time delay difference $\delta \tau$ for fitted model as well as for two- and four-detector based correlation matrices utilizing histogram and TTTR measurement modes, respectively. (c) $N\left(\rho \right)$ plotted against the correlation integration time window $\delta t$.

Figure 4

Photon coherence investigations using (a) Michelson (MI) and (b),(c) Hong-Ou-Mandel (HOM) interferometers with and without employing an etalon (Etn) of 2 GHz bandwidth. In the former the $T_2$ values for $\text{X}$ and $\text{XX}$ are determined by modeling of the MI visibility of the respective experimental data to Lorentzian and Gaussian product line shapes. (b) Co- and cross-polarized ($\parallel$ and $\perp$) HOM two-photon interference traces. (c) Closeup of the central HOM interference manifold as well as the corresponding fitted model functions (see text) plotted against the two-photon interference time delay. The corresponding non-time-convoluted (NC) theory functions are drawn as dotted lines.

Figure 5

Schematic illustration of the experimental setup. The following abbreviations are used: notch filter (NF), beam splitter (BS), polarization projection (PP), bandpass (BP), polarizing beam splitter (PBS), superconducting single-photon nanowire detector (SSPD), and time-correlated single-photon counting (TCSPC).