Terahertz-frequency electrical pulses will be fired at tiny semiconductor structures to watch how single electrons behave in real time—a million times faster than current methods allow. This matters because the physics of quantum-confined electronic systems—the sort found in quantum dots, layered semiconductors, and nanowires—has been studied mostly with slow direct-current measurements or with optical light. Neither approach can capture the rapid, picosecond-scale dynamics of electrons as they shift between quantum states. The terahertz frequency range (roughly 1 trillion cycles per second) matches the natural energy scale of these systems, but generating and detecting such signals next to a cryogenically cooled sample has been technically impossible until now. If successful, the project will create a generic toolkit for mesoscopic physics: guided-wave techniques that deliver single-cycle terahertz pulses directly to the device, bypassing the bandwidth limits of conventional wiring. Researchers could then observe the birth or change of a quantum state on its actual timescale. This is fundamental science with no immediate practical application. But similar work on quantum-confined systems has already underpinned transistors, lasers, and quantum computing components. A deeper understanding of picosecond electron dynamics could eventually improve high-speed electronics, terahertz imaging, or quantum information processing.
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Over the last 20 years, the study of mesoscopic quantum-confined electronic systems has revealed a wealth of exciting and fundamental physics. These studies show no sign of abating as advances in device fabrication and measurement techniques enable ever more intricate structures and more sophisticated experiments to be made. The characteristic energy scale in many important mesoscopic devices such as two-dimensional electron systems, layered semiconductor structures, semiconductor quantum dots, and laterally-confined wires, dots, and other geometries, corresponds to the terahertz (THz) frequency range (1 THz = 1x10^12 Hz = 4.1 meV), which until recently has been difficult to access. Furthermore, although the majority of studies of mesoscopic systems use dc transport or optical (near-infrared) techniques, invaluable information on the states and dynamics of carriers in condensed matter systems, not obtainable by dc transport methods, can potentially be accessed though the dynamic (high frequency) electronic response. Our vision is to create a step-change in the study of mesoscopic electronic systems by developing and exploiting THz fre-quency technology, and in particular, guided-wave techniques, to probe the THz frequency / picosecond response of quantum-confined electronic systems. We will develop quasi-optical techniques to generate (and detect) single-cycle THz / picosecond electronic pulses adjacent to the mesoscopic system in the cryostat, avoiding the RC bandwidth-limiting problems inherent in previous high frequency (up to the gigahertz range) electrical measurements. We will also develop the methodology to perform picosecond-resolution measurements capable of monitoring the spatial position of single electrons three orders-of-magnitude faster than achieved previously; this will provide a generic technology for the field of mesoscopic physics where the onset of, or change in, a quantum state occurs on a picosecond time scale. This programme, which comprises the symbiotic development of THz frequency science and technology in quantum con-fined electronic systems, will be unique internationally and will open an important new direction for mesoscopic physics.
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