Introduction: State of the art in quantum light-matter interfacing
The task of faithfully combining quantum mechanics and information science has been the motivation behind important developments regarding our ability to coherently control individual quantum systems. Powerful new perspectives have been proposed in which such individual systems are combined and purposely-engineered for the realization of quantum information processing architectures. These quantum networks are the core of the physical implementation of quantum computing, communication and metrology. Given the recent success in the experimental realization of the individual blocks of such quantum networks, we are now in a stage in which possible elementary functionality can be achieved through the photon-assisted interfacing of the readily available state of the art matter-nodes. In this visualized elementary quantum architecture, individual nodes must perform several essential tasks (akin to the extended Di Vicenzo criteria) in order to achieve a universal platform for quantum information processing. Namely, individual matter nodes should be capable of receive, store and retrieve photonic qubits (quantum memory), allow non-demolition measurements of their quantum state (state detection) and mediate the engineered interaction of two qubits (quantum gates).
Decisive experimental progress has been made in order to achieve the aforementioned capabilities. However, the key feature of engineering a matter node in which deterministic two-qubit gates can be realized still remains an elusive goal for the quantum optics community. Experimental breakthroughs in this direction have been hampered by the enormous non-linearities needed to achieve a cross-phase modulation at the single-photon level. The generation of large non-linearities using electromagnetically-induced transparency (EIT) has provided means to, in principle, achieve the latter objective. Along these lines, the experimental realization of atomic four-level schemes using 87Rb ensembles has seen remarkable progress. Nonetheless, recent theoretical work and the latest experimental results suggest that current implementations using ensembles in free space might not be suitable to achieve the desired large non-linearities at the single-photon level.
In order to successfully achieve this objective, a new generation of experiments is needed in which a strong light-matter interaction, as provided by cavity quantum electrodynamics (QED), is merged with the Kerr enhancement effects provided by EIT. Pioneering experiments in this new line of research have very recently shown the feasibility of such powerful combination, for example by showing cavity EIT with single-atoms trapped inside high-finesse optical cavities , cavity-based all-optical switching using four-level schemes in warm atomic vapor or ion-crystals and vacuum induced transparency using cold atoms coupled to an optical cavity.
The aim of the proposed work is to build and explore the possibilities of a new architecture based upon a crossed-cavities QED system as a node to mediate strong interaction among quantum optical fields. The research program will pursue two intermediate objectives. First, the development of a new universal quantum node based upon a 87Rb ensemble embedded in a crossed-cavities setup. And secondly, the experimental realization of novel sources of quantum light tuned to 87Rb transitions (global estimated time 3 years).
The research objectives and milestones are described as follows:
Objective I: Experiments exploring novel uses of cavity electromagnetically-induced transparency in 87Rb ensembles.
Goal 1.1: Improving the performance of quantum memories and interfaces through the ideas of quantum impedance matching.
Goal 1.2: Building a novel scheme to detect single-photon level fields using the phenomenon of bistability in cavity EIT.
Goal 1.3: Exploring the possibilities of a cavity EIT system to operate as a quantum amplifier of Fock states.
Objective II: Building a universal quantum node based upon a 87Rb ensemble embedded in a doubly-resonant QED setup.
Goal 2.1: Measuring cross-phase modulation for two fields at the single-photon level using strong collective coupling for both modes.
Objective III: Bulding novel quantum light sources tuned to atomic transitions.
Goal 3.1: Implementing OPA-based single-photon sources at 780 and 795nm.
Goal 3.2: Creating Fock-state superpositions tuned to rubidium transitions using non-linear crystals and heralding detection.