Statistics of Narrow-Band Single Photons for Quantum Memories Generated by Ultrabright Cavity-Enhanced Parametric Down-Conversion

Narrow-band single photons represent an important resource for quantum memories due to their efficient interaction with atomic resonances. In this Letter, we report on the generation of photons with 3 MHz linewidth by cavity-enhanced parametric down-conversion and demonstrate direct proof of their single-photon character by detection of heralding idler photons. Compared to a Poissonian source, a suppression of higher-order photon numbers by nearly 2 orders of magnitude could be achieved. Moreover, the brightness of our source exceeds previous realizations by more than a factor of 100.

Figure 1

Schematic of the double-resonant OPO. Type-II SPDC is performed in a periodically poled KTP crystal (PPKTP). An additional KTP crystal, rotated by 90°, is used for compensation. The cavity is locked to the pump beam via the Hänsch-Couillaud (HC) method.

Figure 2

Temporal second-order cross-correlation function $G_{S/I}^{(2)}$ between signal and idler modes. Linewidths of 2.7 and 3.0 MHz for signal and idler can be derived from the exponential decays. The inset provides a zoom into the region around $\tau =0$ where the red line follows a theoretical model that takes into account the time resolution of the correlation setup of 620 ps.

Figure 3

Normalized heralded signal autocorrelation function $g_c^{(2)}$ as a function of signal-signal time delay. Antibunching of $g_c^{(2)}(0)\approx 0.2$ and $g_c^{(2)}(0)\approx 0.4$ has been achieved using coincidence windows ${\tau }_{\mathrm{Coin}}=40\mathrm{ns}$ (■) and ${\tau }_{\mathrm{Coin}}=200\mathrm{ns}$ (□), respectively. Solid lines show theoretical fits.

Figure 4

Normalized idler-triggered signal autocorrelation function $g_c^{(2)}(0)$. The solid line follows the theoretical model. This parameter characterizes the quality of our heralded single-photon source, and values down to $g_c^{(2)}(0)\approx 0.01$ could be achieved.

Figure 5

Suppression of higher photon number contributions. Measurements of the heralding efficiency $P(1)$ (■) and the relative suppression $\beta =P(n\ge 2)/P(1)$ (□) are plotted versus the heralding rate $R_H$. $P(1)$ is constant at 0.55 as expected in this regime (dashed line). $\beta$ follows a linear behavior with the solid line providing a fit according to theory [22].