Though attractive from scalability aspects, optical approaches to quantum computing are highly prone to decoherence and rapid population loss due to nonradiative processes such as vibrational redistribution. We show that such effects can be reduced by adiabatic coherent control, in which quantum interference between multiple excitation pathways is used to cancel coupling to the unwanted, non-radiative channels. We focus on experimentally demonstrated adiabatic controlled population transfer experiments wherein the details on the coherence aspects are yet to be explored theoretically but are important for quantum computation. Such quantum computing schemes also form a back-action connection to coherent control developments.
A great deal of effort has been applied to understanding population dynamics within a variety of coherent excitation schemes. The goal in such studies has been to understand the conditions necessary for efficient transfer of population from one state to another. While many theoretical treatments include the effects of natural lifetimes that are present in any given system, some neglect this important aspect when considering specific cases. Adiabatic approximations are also widely made. Additionally, it is often difficult to envision how the different parameters controlling efficient population transfer are interrelated or even which parameters are the most critical, especially when the decay lifetimes are taken into account. This work describes a numerical study of coherent excitation applied to a
87Rb ladder system where spontaneous decay rates are included, and adiabaticity is not assumed. A useful method is introduced to explore the interdependence of various excitation parameters. The efficiency of population transfer as a function of several experimentally controllable parameters is explored, and other general trends are summarized.
Many schemes to realize quantum state transfer in spin chains are not robust to random fluctuations in the spin-spin coupling strength. In efforts to achieve robust quantum state transfer, an adiabatic quantum population transfer scheme is proposed in this study. The proposed scheme makes use of a slowly moving external parabolic potential and is qualitatively explained in terms of the adiabatic following of a quantum state with a moving separatrix structure in the classical phase space of a pendulum analogy. Detailed aspects of our adiabatic population transfer scheme, including its robustness, is studied computationally. Applications of our adiabatic scheme in quantum information transfer are also discussed, with emphasis placed on the usage of a dual spin chain to encode quantum phases. The results should also be useful for the control of electron tunneling in an array of quantum dots.