Speaker: Seogjoo Jang (Queens College of the City University of New York)

Title: Theory of coherent resonance energy transfer for coherent initial condition:

A theory of coherent resonance energy transfer developed recently [Journal of Chemical Physics 129, 101104 (2008)] is extended for coherent initial condition. For the general situation where the initial excitation can be arbitrary linear combination of donor and acceptor excitations, a second order time local and polaron transformed quantum master equation is derived. The inhomogeneous terms in the resulting equation have contributions not only from the initial donor and acceptor populations but also from their coherence terms. Numerical tests are performed for general super Ohmic spectral densities where the bath coupled to the donor excitation can be correlated with that coupled to the acceptor excitation. The results show the sensitivity of the early nonstationary population dynamics on the relative phase of initial donor and acceptor excitations. It is also shown that the contribution of inhomogeneous terms is more significant for coherent initial excitations than for initial excitation localized in the donor only. The overall numerical results demonstrate the importance of including all the competing effects such as nonequilibrium, nonMarkovian, and quantum coherence for quantitative modeling of population dynamics of resonance energy transfer.

--------------------------------------------------------------------------------

Speaker: Ran Zhao (Georgia Institute of Technology)

Title: Long-lived Quantum Memories

Coauthors: Y. O. Dudin, S. D. Jenkins, C. J. Campbell, D. N. Matsukevich, A. Kuzmich, and T. A. B. Kennedy

A memory based on hyperfine atomic coherences (spin waves) which can be read out optically at the single photon level, when classical noise sources have been eliminated, is a quantum memory. The spin waves are generally sensitive to ambient magnetic fields that limit their storage time to tens of microseconds. By optical pumping of the atoms and use of the clock coherence sensitivity to magnetic fields can be greatly reduced. Even in ultra-cold atomic samples motional dephasing becomes important on a scale of hundreds of microseconds. We present results of our work which circumvent both of these difficulties to achieve an atomic memory with a lifetime of several milliseconds. We will discuss various applications of the long-lived atomic memory, including deterministic single photon sources, matter qubit rotations, and matter-light entanglement.

--------------------------------------------------------------------------------

Speaker: Susan Clark (Stanford University)

Title: Ultrafast optical spin echo for electron spins in semiconductors

Coauthors: Kai-Mei C. Fu, Qiang Zhang, Thaddeus D. Ladd, Colin Stanley, Yoshihisa Yamamoto

Spin-based quantum computing and magnetic resonance techniques rely on the ability to measure the coherence time, T2, of a spin system. We report on the experimental implementation of all-optical spin echo to determine the T2 time of a semiconductor electron-spin system. We use three ultrafast optical pulses to rotate spins an arbitrary angle and measure an echo signal as the time between pulses is lengthened. Unlike previous spin-echo techniques using microwaves, ultrafast optical pulses allow clean T2 measurements of systems with dephasing times (T2*) fast in comparison to the timescale for microwave control. This demonstration provides a step toward ultrafast optical dynamic decoupling of spin-based qubits. Such a scheme could be used to extend the spin-memory time of a spin-based quantum computer and can be integrated into quantum bus schemes for quantum computing.