Quantum: Ultrastable heterodyne quantum information

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The proposal "Ultrastable heterodyne quantum information" is a renewal request to support theactivity of the Quantum Optics and Quantum Information (QOQI) group in the Physics Department of the University of Virginia (UVa).The foundation of quantum information is the use of the mathematical axioms of quantum mechanics to process, store, and transmit information. One expects several benefits from such an approach. First and foremost, quantum computing can yield an exponential speedup over classical computation for certain problems, such as predicted by Feynman for simulating quantum systems and by Shor for factoring integers. Moreover, quantum key distribution also brings complete security against eavesdropping to cryptography.Daunting challenges face attempts at experimentally realizing a quantum computer. On the one hand, one needs scalability, i.e. a large number of quantum logic units ("qubits" if binary ones or "qudits" if multi-state ones), all individually addressable and able to interact pairwise to become entangled. On the other hand, one needs this interaction to be strictly controlled and limited to the qubits or qudits, in order to avoid decoherence, which is the measurement-like, irreversible random evolution that results from interaction of a quantum register with the environment, a reservoir of quantum systems. Spontaneous emission is an example of decoherence for atoms.This project is dedicated to all-optical implementation of quantum information. Light has remarkable resistance to decoherence, due to its extremely weak (photon-photon) interaction. This, is turn, may present a difficulty for generating quantum entanglement (aforementioned pairwise interactions) but this problem is solved by the use of the mature and ever more sophisticated techniques of nonlinear optics and, in our case, of the laser-like, highly spatially and temporally coherent optical beams emitted by the optical parametric oscillator (OPO). The potential of OPO's for quantum information and, in particular, quantum communication, is very well known but is still far from having been fully exploited. The experimental approach of the proposal is centered on marrying the principles and techniques of ultra-high resolution laser spectroscopy and time-frequency metrology to those of nonlinear and quantum optics. This proposal aims at extending classical signal processing techniques into the quantum domain, in particular by using the frequency domain to encode the qudits and realize "quantum multiplexing," a quantum optical version of FM versus AM radio signals. This will be realized with state-of-the-art phase- and frequency-stabilized OPO's and will subsequently enable the transfer of quantum information, by teleportation or direct entanglement, between different types of physical qudits, such as alkali atoms (quantum memory) or photons in optical fibers (quantum bus). Applications of qudit-based dense coding to ultrasensitive optical measurements impossible with qubits are also discussed.Broader impacts of the proposed work comprise an active contribution to the UVa Physics graduate program, with the direct research advising of five students, of several Departmental seminars per year, and of a new advanced course "Quantum Optics and Quantum Information" (Phys 888), which was created in the spring of 2005 by the P.I. In addition, undergraduate students are periodically joining in the research effort at various levels, including graduate research. Also included are collaborative efforts with UVa Engineering faculty to foster cross-Departmental research. Finally, broader dissemination of research results includes the organization of interdisciplinary conferences at UVa, in association with the Department of Mathematics. The P.I. co-organized one such conference, "Coding Theory and Quantum Computing," in May 2003 and has plans to reiterate in the near future.