High-fidelity conversion of photonic quantum information to telecommunication wavelength with superconducting single-photon detectors

We experimentally demonstrate a high-fidelity visible-to-telecommunication wavelength conversion of a photon by using a solid-state-based difference frequency generation. In the experiment, one half of a picosecond visible entangled photon pair at 780 nm is converted to a 1522-nm photon, resulting in the entangled photon pair between 780 and 1522 nm. Using superconducting single-photon detectors with low dark count rates and small timing jitters, we selectively observed well-defined temporal modes containing the two photons. We achieved a fidelity of $0.93\pm 0.04$ after the wavelength conversion, indicating that our solid-state-based scheme can be used for faithful frequency down conversion of visible photons emitted from quantum memories composed of various media.

Figure 1

The experimental setup for frequency down conversion of one half of the visible entangled photon pairs.

Figure 2

The dependencies of the conversion efficiency and the rate of the background noises on the pump power. The former has been measured by using a picosecond coherent light in Ref. [7]. The latter was measured by using the SSPD for $D_T$ without the UV pulse. The first vertical axis is the conversion efficiency. A maximum conversion efficiency is achieved at a pump power of 700 mW. The second vertical axis is the detection rate of the background noises. We fitted the experimental data by a function of $bP+d.b$ and $d$ are estimated as 80 Hz/mW and 266 Hz, respectively.

Figure 3

The coincidence counts of the photon pairs recorded by the TDC with the timing resolution of 100 ps when we used rhombuses: the pair of the silicon APDs for $D_V$ and InGaAs/InP APD for $D_T$; squares: the silicon APD for $D_V$ and the SSPD for $D_T$; and circles: the SSPDs for both the detectors. Each result of the coincidence counts was fitted by the Gaussian function after the subtraction of the counts from the background noises.

Figure 4

The real parts of the reconstructed density matrices. (a) The initial entangled photon pair ${\rho }_{AB}$ prepared by the BBO crystal. (b) The two-photon state ${\rho }_{A{C}^{\prime }}^{AS}$ after the wavelength conversion when we used the silicon APD for $D_V$ and the SSPD for $D_T$. (c) The two-photon state ${\rho }_{A{C}^{\prime }}^{SS}$ after the wavelength conversion when we used the SSPDs for both detectors $D_V$ and $D_T$.