Nonlinear Interactions with an Ultrahigh Flux of Broadband Entangled Photons

We experimentally demonstrate sum-frequency generation with entangled photon pairs, generating as many as 40 000 photons per second, visible even to the naked eye. The nonclassical nature of the interaction is exhibited by a linear intensity dependence of the nonlinear process. The key element in our scheme is the generation of an ultrahigh flux of entangled photons while maintaining their nonclassical properties. This is made possible by generating the down-converted photons as broadband as possible, orders of magnitude wider than the pump. This approach can be applied to other nonlinear interactions, and may become useful for various quantum-measurement tasks.

Figure 1

Experimental layout. Entangled photons generated by down-conversion of a pump laser in one crystal are imaged through a set of four dispersion prisms onto a second crystal to generate the SFG photons. The entangled-photon beam is separated from the SFG photons by a harmonic-separator mirror and its power is measured by an InGaAs detector. The SFG photons are further filtered by 532 nm line filters and are counted with a single-photon counting module.

Figure 2

Power dependence of SFG with entangled photons. When the down-converted IR power is reduced by optical attenuators, the SFG rate drops quadratically: circles, experimental; line, quadratic fit, exactly like classical light sources. However, when the IR power is reduced by decreasing the pump-laser power, thus reducing the photon-pair production rate, the SFG rate decreases in a close-to-linear manner, as quantum mechanically predicted: squares, experimental; line, calculated $\alpha (n+n^2)$, with $\alpha$ chosen to fit. The log-log slope of these measurements varies from 1.01 at the lowest powers to 1.14 at the highest powers.