The first demonstrations of Quantum Computers employing trapped ions, cavity QED, and NMR inspired a somewhat optimistic prognosis for the advancement of this field. Proof of principle demonstrations were shown, but little else. Apart from being able to perform basic quantum operations with high reliability and a sufficient coherence time to complete a meaningful calculation, the physical principles of operation of a quantum computer should allow miniaturization and integration of the individual devices as well as high clock frequency. It is commonly believed that these goals can be achieved using a solid-state based quantum computer (QC). On the other hand, quasiparticles in the solid-state are normally subject to strong relaxation processes; this makes creation of the solid-state QC a challenging problem. In recent years, a number of the QC concepts based on different solid-state quantum phenomena have been proposed. These are summarized in our section on Quantum Computing. From all these opportunities, we believe that the design being investigated in our research (shown at right), based on the electron spin degree of freedom in gated Si or SiC quantum dots, provides one of the more promising candidates.
Electron spin can potentially be used as a capacious information storage cell, be involved in the transfer of information, and be integrated with electric-charge counterparts in combined designs. Our focus is to develop new spin device concepts through comprehensive understanding of electron spin dependent electronic and optical properties; and build an advanced modeling and simulation capability for spin dynamics in the nanoscale. Specifically, the issues relating to spin relaxation and spin polarized transport are examined in the nonlinear and/or hot-carrier regimes by utilizing advanced modeling and simulation tools such as the Monte Carlo method. A photon repeater (shown conceptually at left) that preserves the phase information of the received photon during storage, processing and retransmission is being investigated. Another specific new device concept also under development is a ballistic T-shaped spin filter (based on the spin-orbit coupling) that can filter the spin-up and spin-down fluxes of an electron current with very high efficiency without an external magnetic field.
The nitrides with their unique material properties are expected to exhibit some unusual characteristics for potential device application. One such phenomenon is the so-called low-field runaway effect, which provides sustained high drift velocities at pre-threshold fields. The focus of the research in the nitride electronic devices is to explore the possibility of "carrier distribution function engineering" through the runaway effect for high-speed (sub-picosecond) and high-power applications (e.g., HBTs, IBTs, HETs). In addition, the investigation examines the ways to utilize the effect in tailoring the negative differential resistance for efficient microwave generation. The effort on nitride optoelectronic devices primarily concerns UV emitters and detectors. Particularly the emphasis is on providing solutions for: (1) large hole concentration in the active media (i.e., p doping), (2) high quantum efficiency, and (3) appropriate designs of active optical elements for laser/detector manipulations. Very recently we proposed two innovative device structures that can increase the hole density by more than an order of magnitude compared to the existing schemes. Detailed analyses of these devices as well as solutions for the other problems are being pursued.
The aim is to identify and analyze the structures in which the phonons and the coupling of phonons to carriers lead to enhanced device performance. Low-dimensional nanostructures provide the opportunity for external modification of phonon properties (i.e., "phonon engineering"). Currently the activity centers on electrical generation of coherent acoustic and optical phonons as well as its feasibility as a coherent phonon source (including the phonon mirror). Potential applications include THz modulation of electrical/optical signal, phonon active control of electron transport, phonon-induced photo-transition in indirect gap semiconductors, heat removal, nondestructive testing, etc.
Collective interactions lead to such interesting phenomena as superconductivity and zero point Bose-Einstein quantum effects. Artificially structured nanoscale devices can modify the local interactions of atomic structures, leading to dramatic modification of local collective interactions among the particles involved. Such a situation clearly exists in single-walled carbon nanotube systems "stuffed" with binary compounds. Speculation exists that nanostructure systems like these could exhibit superconductivity. Models to develop the theory to this possibility are being explored.