Heralding efficiency and correlated-mode coupling of near-IR fiber-coupled photon pairs

We report on a systematic experimental study of the heralding efficiency and generation rate of telecom-band infrared photon pairs generated by spontaneous parametric down-conversion and coupled to single-mode optical fibers. We define the correlated-mode coupling efficiency, an inherent source efficiency, and explain its relation to heralding efficiency. For our experiment, we developed a reconfigurable computer-controlled pump-beam and collection-mode optical apparatus which we used to measure the generation rate and correlated-mode coupling efficiency. The use of low-noise, high-efficiency superconducting nanowire single-photon detectors in this setup allowed us to explore focus configurations with low overall photon flux. The measured data agree well with theory, and we demonstrated a correlated-mode coupling efficiency of $97\%\pm 2\%$, which is the highest efficiency yet achieved for this type of system. These results confirm theoretical treatments and demonstrate that very high overall heralding efficiencies can, in principle, be achieved in quantum optical systems. It is expected that these results and techniques will be widely incorporated into future systems that require, or benefit from, a high heralding efficiency.

Figure 1

Correlated-mode coupling behavior and relative photon-pair emission rate as a function of collection-mode focal parameter ${\xi }_a$ for a range of pump-beam focal parameters ${\xi }_p$ are shown. (a) shows the behavior for ${\xi }_p=2.84$; all photon-pair rates in this figure are normalized to the maximum pair rate achieved for this focusing configuration. (b) shows the behavior for ${\xi }_p=0.284$, and (c) shows the behavior for ${\xi }_p=0.0284$.

Figure 2

The experimental configurations (a) for the single-detector apparatus and (b) for the two-detector apparatuses are shown. For both configurations the pump beam is launched into free space, sent through a variable zoom lens and computer-controlled steering mirrors, and then focused to the center of the PPKTP crystal. The crystal generates photon pairs which pass through a collimation lens, reflect off four dichroic mirrors, pass through a computer-controlled zoom lens, and are then coupled into a single-mode optical fiber. For the configuration in (a), the photons pass through a fiber polarization controller and into the SNSPD apparatus. For the configuration in (b), the orthogonally polarized photon pairs pass through a fiber polarization controller, are separated at a fiber polarizing beam splitter, and are sent through fiber polarization controllers and into the SNSPD apparatuses. For each configuration, the detector outputs are sent to timing and counting hardware and recorded on a computer.

Figure 3

Single-detector-configuration data are shown as a function of collection-mode focal parameter ${\xi }_a$. (a) shows total photon emission rates in counts per second (cps) per milliwatt of pump power as a function of collection-mode focal parameter ${\xi }_a$ for three pump-mode focal parameters ${\xi }_p$. Measured data, calculated from Eq. (18), are shown as points, and predicted behavior from Eq. (13) is shown as solid curves. (b) shows coincident photon-pair emission rates in cps per milliwatt of pump power as a function of collection-mode focal parameter ${\xi }_a$ for three pump-mode focal parameters ${\xi }_p$. Measured data, calculated from Eq. (17), are shown as points, and predicted behavior from Eq. (8) is shown as solid curves. (c) shows correlated-mode coupling efficiencies as a function of collection-mode focal parameter ${\xi }_a$ for three pump-mode focal parameters ${\xi }_p$. Measured data, calculated from Eq. (19), are shown as points, and predicted behavior from Eq. (14) is shown as solid curves.

Figure 4

Dual-detector-configuration data are shown as a function of collection-mode focal parameter ${\xi }_a$. (a) shows total photon rates and coincident photon-pair rates in counts per second (cps) per milliwatt of pump power for pump-mode focal parameter ${\xi }_p=0.0243$. Measured data, calculated from Eqs. (18) and (25), are shown as points, and predicted behaviors from Eqs. (13) and (8) are shown as solid curves. (b) shows correlated-mode coupling efficiencies as a function of collection-mode focal parameter ${\xi }_a$ for pump-mode focal parameter ${\xi }_p=0.0243$. Measured data, calculated from Eq. (26), are shown as points, and predicted behavior from Eq. (14) is shown as a solid curve.