Demonstration of coherent time-frequency Schmidt mode selection using dispersion-engineered frequency conversion

Time-frequency Schmidt (TFS) modes of ultrafast quantum states are naturally compatible with high-bit-rate integrated quantum communication networks. Thus they offer an attractive alternative for the realization of high-dimensional quantum optics. Here, we present a quantum pulse gate based on dispersion-engineered ultrafast frequency conversion in a nonlinear optical waveguide, which is a key element for harnessing the potential of TFS modes. We experimentally retrieve the modal spectral-temporal structure of our device and demonstrate a single-mode operation fidelity of 80%, which is limited by experimental shortcomings. In addition, we retrieve a conversion efficiency of 87.7% with a high signal-to-noise ratio of 8.8 when operating the quantum pulse gate at the single-photon level.

Figure 1

Illustration of a QPG. A single TFS mode $\widehat{A}$ from a multimode input state is selected and converted to an output mode ${\widehat{C}}^{†}$ at a different frequency. The remaining modes are simply transmitted. For more information see the text.

Figure 2

Experimental setup. PS, pulse shaper; OPO, optical parametric oscillator; BPF, bandpass filter; ND, neutral density filter; HWP, half-wave plate; DM, dichroic mirror; AL, aspheric lens; WG, waveguide sample; MMF, multimode fiber; APD, avalanche photodiode; CCD, single-photon sensitive CCD spectrometer. For more information see the text.

Figure 3

Measurement results for the reconstruction of the QPG TFS modes. In (a), we plot chosen recorded output spectra for the indicated pump wavelengths, whereas in (b) we show the converted output intensity $I_{\mathrm{out}}$ as a function of the pump wavelength ${\lambda }_{\mathrm{p}}$. Note that error bars in (b) are smaller than the symbols. For more information see the text.

Figure 4

Measurement results for the adaption of the QPG TFS modes to an unknown input. For more details see the text.

Figure 5

Performance benchmarks of our QPG. From the spectral suppression in (a), we obtain a mode-selectivity of 80%, whereas the efficiency measurement in (b) reveals an internal conversion efficiency of 87.7%. In both plots, error bars are smaller than the symbols. For more information see the text.