Quantum information processing (QIP) is being intensively studied due to the promise of proposed quantum algorithms such as those for factoring large numbers, improving database searches, etc. The field of QIP is also likely to provide new ways of characterizing, understanding, and controlling the transition from the quantum world to the classical one we live in. Linear-optics based QIP protocols have been recently developed, in which ordinary optical elements are used in conjunction with high-efficiency single-photon detectors, that show the potential for scalability if the overall quantum efficiency of the system can be maintained nearly perfect and suitably entangled quantum states can be prepared. To date, such linear-optics based QIP has been implemented using free-space optical elements in conjunction with entangled states generated via spontaneous parametric down-conversion in bulk nonlinear-optical crystals. In free-space implementations of linear-optics based QIP, however, the transportation of quantum bits (qubits), subsequent coherent superposition of qubits, and the detection of qubits from different optical spatio-temporal modes, all suffer from a lack of scalability due to the use of bulk-optic nonlinear crystals. This is also because of optical diffraction, which limits the ability to transport qubits in free space. To avoid diffraction, one usually injects bulk-crystal generated entangled qubits into optical fibers. Such coupling of nonlinear-crystal generated entanglement to optical fibers for transportation is technically challenging, limiting the overall quantum efficiency and thus the achievable scalability. On the other hand, by implementing QIP functions directly in optical fiber, the problem of modal purity is solved as all involved optical modes have the very pure guided mode of an optical fiber. Recent work at Northwestern University has demonstrated the production of correlated-photon pairs in the telecom band of standard optical fibers, creation of polarization entanglement, and transmission of such fiber-generated entanglement for distances of up to 50 km. With such a source of entanglement, wide-area QIP can be envisioned as the low fiber loss in the telecom band can permit transmission of qubits for up to 50-100 km from one node to another in a distributed QIP environment. This aim of this project is to demonstrate prototype tools and functionalities that make progress towards realizing distributed QIP. A fiber-based quantum logic gate with advantages discussed above will be developed. Additionally, an all-optical single-photon switch will be demonstrated, which is expected to be a crucial technology for controlling the flow of qubits in a linear-optics based quantum processor. Such all-optical switching promises to eliminate the inefficiencies inherent in electro-optical switching. It will allow the detection of one photon to cause the storage of its twin in a fiber loop, or the nondestructive readout of a stored photon to occur for further processing. Together, these QIP functions will add significantly to the ability to transmit, store, detect, and otherwise control a linear-optics based quantum processor. Intellectual Merit of the Proposed Activity: The proposed activity will address a fundamental roadblock in the development of optics-based QIP, namely the need for optical-fiber based devices for manipulation and control of optical qubits. Theory and practice will need to go hand in hand for a successful outcome. The PI's research group brings a unique combination of theoretical and experimental expertise and skills to bear upon the problem. Broader impacts of the proposed activity: The students will get involved in cross-disciplinary work. They will be trained in the emerging field of quantum information in addition to the usual fiber optics and photonics in the curriculum. Outreach activities through the Center for Photonic Communication and Computing at Northwestern will be undertaken to have a much wider impact on our community at large. Full scale efforts will be made through all resources available at the University to engage underrepresented students in this project.
