The underlying approach towards few photon optical nonlinearities we consider relies on the collective coupling of light to ultracold ensembles of strongly interacting so-called Rydberg atoms. Here, the general idea is to map photons onto such atomic states and exploit the resulting particle-particle interactions to mediate an effective non-local interaction between photons. The required mapping is achieved by means of electromagnetically induced transparency (EIT), which establishes a strong yet coherent coupling between photons and atomic Rydberg states. Under such conditions, single photons are permitted to propagate without loss in the form of Rydberg polariton states: quasi-particles formed from the superposition of a photon and a collective Rydberg excitation. However, the strong Rydberg interactions result in a blockade phenomenon that prevents the simultaneous excitation of multiple Rydberg states within a mesoscopic volume. The effect of this therefore prevents the simultaneous formation of two Rydberg polariton states in close proximity, and thus results in a nonlinear optical response that strongly depends on the number of photons in the system.
Within Qcool, we explore such Rydberg-mediated photon interactions on both a theoretical and experimental level. Some of the research directions we currently pursue are detailed below.
With conventional approaches to nonlinear optics based on Rydberg-EIT, the strong atomic interactions are exploited to break EIT conditions, resulting in strong nonlinear photon absorption. However, despite the utility of such a dissipative photon interaction, the accompanying decoherence fundamentally prevents the realization of many applications reliant on quantum coherence. Within Qcool, we consider new approaches to coherent few-photon optical nonlinearities based on interacting polaritons in Rydberg-EIT media that aim to overcome this issue. One idea we are exploring is based on an interaction-controlled switching between fundamentally distinct polaritons, rather than blocking their formation altogether. This naturally circumvents the typical road-blocks of currently employed blockade mechanisms, thereby holding potential for crucial breakthroughs in quantum nonlinear optics.
Based on the novel forms of photon interaction mechanisms explored within Qcool, we aim to utilize the emergent nonlinear optical effects to realize new quantum devices and applications. In particular, reflective photon interactions can be realized with the polariton switching nonlinearity discussed above that we envisage to utilize for robust single photon routing. A photon router is a quantum device that empowers a single photon to redistribute the flow of a large number of other photons. It can operate similarly to classical routers that manage data traffic through the internet but can handle quantum information, and can be employed as the central building block of an optical quantum computer.
We develop new theoretical methods for describing the quantum dynamics of many interacting photons within the Qcool experimental setup. Based on these insights, we then design novel control techniques for engineering complex quantum states of light that feature strong entanglement or non-classical spatio-temporal correlations between photons. In particular, by engineering a reflective optical nonlinearity, one can employ the Qcool setup as a nonlinear multiphoton filter that converts a classical input into multi-photon quantum states, such as single-photons or photon crystals. Such states will be a vital resource for numerous potential applications in optical quantum information information processing.
