Photons are superior information carriers. The ability of manipulating photons with high fidelity is central to the realization of quantum communications and quantum computations. Among a variety of experimental schemes, the all-optical switching (AOS) and cross-phase modulation (XPM), which were based on the idea of electromagnetically induced transparency (EIT), have provided a workable way for manipulating photonic information. AOS and XPM are optical processes of nonlinear nature, in which the photons can interact with each other via the photon-atom interaction sustained by an EIT medium. The efficiency of a non-linear optical process is proportional to the interaction time. In order to achieve the adequate nonlinearity so that two light pulses can interact with each other for a long duration in a medium, one pulse can be stopped as stationary light pulse (SLP), while the other is stopped as stored light. A sufficiently high optical density (OD) is required to make a light pulse stationary and hence, prolong the interaction time. By considering cold atomic media, we provide the theoretical studies and experiments to realize the enhanced nonlinearity. Theoretical analysis shows that our approach can achieve efficiency below a single photon level per atomic absorption cross section. Comparing to the existing approach by using moving light pulses, in which 2 photons per atomic absorption cross section were observed in the best situation, our experimental results by employing an SLP and a stored light pulse significantly improve the efficiency, as only 0.56 photons are needed. Moreover, the simulation results also confirm that the efficiency could be further improved by increasing the optical density of the medium without any upper limit.
The existence of four-wave mixing (FWM) process in a general four-level AOS or XPM system would greatly degrade the nonlinearity. We propose a new idea and a comprehensive investigation by considering the phase mismatch condition to make AOS or XPM able to completely intact even under the influence of FWM. The experimental data demonstrated this idea. Our work makes the single-photon modulation, e.g. one photon switched or phase-modulated by another photon, not far from reality with the current scheme and advances the technology in quantum information manipulation utilizing photons.

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