We consider two representative problems that deal with the
fluctuator-induced decoherence from two very different
perspectives-microscopic and macroscopic. In the first part, we consider an individual two-level system
inside a Josephson junction shunted by a resistor. If the TLS modulates the Josephson energy and/or is optically
active, it can be Rabi driven by the Josephson oscillation. The Rabi oscillations, in turn, translate into oscillations
of current and voltage which can be detected in noise measurements. This effect provides an option to
fully characterize the TLS inside Josephson junction and to find the TLS's contribution to the decoherence when
the junction is used as a qubit. In the second part, we study the contribution of an ensemble of non-stationary
glassy charge fluctuators on qubit decoherence. Low-temperature dynamics of insulating glasses is dominated
by a macroscopic concentration of tunneling two-level systems. Due to exponentially broad distribution of their
tunneling rates and the finite experimental manipulation timescales, some of the fluctuators are temporarily
stuck in high-energy non-thermal states. We find that at low enough temperatures, non-stationary contribution
due to these slow non-thermal fluctuators can dominate the stationary (thermal) one, and discuss how this effect
can be minimized.
A Quantum Computer (QC) is a device that utilizes the principles of Quantum Mechanics to perform computations. Such a machine would be capable of accomplishing tasks not achievable by means of any conventional digital computer, for instance factoring large numbers. Currently it appears that the QC architecture based on an array of spin quantum bits (qubits) embedded in a solid-state matrix is one of the most promising approaches to fabrication of a scalable QC. However, the fabrication and operation of a Solid State Quantum Computer (SSQC) presents very formidable challenges; primary amongst these are: (1) the characterization and control of the fabrication process of the device during its construction and (2) the readout of the computational result. Magnetic Resonance Force Microscopy (MRFM) - a novel scanning probe technique based on mechanical detection of magnetic resonance - provides an attractive means of addressing these requirements. The sensitivity of the MRFM significantly exceeds that of conventional magnetic resonance measurement methods, and it has the potential for single electron spin detection. Moreover, the MRFM is capable of true 3D subsurface imaging. These features will make MRFM an invaluable tool for the implementation of a spin-based QC. Here we present the general principles of MRFM operation, the current status of its development and indicate future directions for its improvement.