Toward quantum frequency combs: Boosting the generation of highly nonclassical light states by cavity-enhanced parametric down-conversion at high repetition rates

We demonstrate the generation of multiphoton quantum states of light by cavity-enhanced parametric down-conversion in the high-repetition-rate pulsed regime. An external enhancement cavity resonant with the spectral comb of modes of a mode-locked pump laser provides a coherent buildup of the pump intensity and greatly enhances the parametric gain without sacrificing its high repetition rate and comb structure. We probe the parametric gain enhancement by the conditional generation and tomographic analysis of two-photon Fock states. Besides its potential impact for efficiently generating highly nonclassical or entangled multiphoton states in many existing experimental setups, this scheme opens exciting perspectives toward the combination of quantum and comb technologies for enhanced measurements and advanced quantum computation protocols.

Figure 1

(Color online) Experimental setup. The pump enhancement cavity (a simplified version is shown here for clarity) is built around the SPDC crystal [β–barium borate (BBO)]. Along the idler channel two on-off detectors are connected to a $3\mathrm{dB}$ single-mode fiber coupler after a series of spectral and spatial filters (F). The high-transmission beam splitter (HT-BS) is used to extract a small portion of the main laser source to be used as a reference beam (local oscillator) in the balanced homodyne detector (BHD) after mode-matching optics (not shown). The phase between the local oscillator and the analyzed state is adjusted by a mirror mounted on a piezoelectric stage (PZT). Once one of the two (or both) on-off detectors clicks, the homodyne signal is acquired by a digital scope and then analyzed.

Figure 2

(Color online) Experimentally reconstructed density matrix diagonal elements and corresponding Wigner function for the two-photon Fock state generated by cavity-enhanced SPDC.

Figure 3

(Color online) Cavity-enhanced production rates for the single- [red (lower) curves] and two-photon [blue (upper) curves] Fock states. The horizontal axis represents the overall effective reflectivity $R_m$ of the cavity, and two curves are calculated for different reflectivities $R_i$ of the input coupler ($R_i=99\%$, dashed curves; $R_i=90\%$, solid curves). Current experimentally measured enhancement factors are indicated by solid circles corresponding to $R_m=0.93$ and $R_i=90\%$. The expected enhancements for an optimized, impedance-matched cavity with $R_i=R_m=0.99$ are indicated by stars.