This award supports an international collaboration for research and education among Harvard University and University of Florida (UF) in the USA, University of Basel and ETH (Zurich) in Switzerland, and University of Regensburg in Germany. The main goal of the project is to understand, control, and utilize the hyperfine interaction of electron spins, confined in low-dimensional condensed matter systems, with the nuclear spins of the host lattice. Four emerging themes in condensed matter physics draw particular attention to this topic, making a Network-scale activity timely. Those themes are: i) advances in nanoscale control of matter, e.g. , quantum dots, where electrons interact with far fewer nuclear spins thereby greatly enhancing the effectiveness of hyperfine coupling; ii) emergence of spintronics devices employing the electron's spin rather than charge for future prospects of quantum repeaters, quantum computers, and quantum memory for secure communication and enhanced computation, iii) engineered interactions in novel materials, such as gating and tailoring of band structure, and iv) availability of high-quality fabrication facilities. There are three main thrusts in this Network: i) control of spin and electron-nuclear interaction in III-V semiconductor quantum dots [experiment: Harvard; theory: Harvard, Basel]; ii) coupling of nuclear spins via itinerant carriers (RKKY interaction) and nuclear magnetism in p-doped heterostructures [experiment: ETH, Regensburg; theory: UF, Basel, Harvard]; and iii) electron-nuclear interactions in 13C-enriched nanotubes [experiment: Harvard, theory: Harvard, Basel, UF]. By investigating the interface between fundamental and applied problems in an international environment, this Network contributes to the kind of cross-training of graduate students that is needed for the next generation of device engineers and scientists, perhaps working with quantum-coherent devices. Exchange of students between experimental groups at Harvard and ETH and between theoretical groups at Harvard, Basel, and UF will take place over the course of the project
Nanoscale semiconducor devices, containing a very small number of conduction electrons, show promise for several novel applications, as sensors and as possible elements of a quantum computer. In many applications, the degrees of freedom related to the electron spin play a crucial role: device operation depends on the control and manipulation of the electron spins. Semiconductor devices alsocontain a very large number of nuclear spins, however, which interact weakly with the electron spins, but can alter their behavior in ways that may be very harmful, or in some cases helpful to the operation of a device. The electon-nuclear interaction can also be used to produce a net nuclear polarization through sequential manipulation of the electrons via voltage pulses appied to metal gates on the semiconductor device. In one aspect of our work we have considered a structure consisting of two adjacent electron traps (a "double quantum dot") with two conduction electrons that can be shuttled between them, and we have shown how propertly tailored protocols can produce a desired polarization difference between the nucleii in two dots. We have shown how the degree of polarization may be influenced by the ratio between the cycle time of the gate-voltage protocol and the "Larmor precession time" for a nuclear spin to rotate in the applied magnetic field. We have also studied how nuclear polarization effects may saturate after a large number of gate cycles. A second aspect of our work has focused on triple-dot structures which are designed so that desired manipulations of the electron spins can be performed without need for a diffenence in nuclear polarization between the dots, and where unwanted affects of randomly oriented nuclear spins may be minimized by rapid manipulation of the electron spins. Here, members of our group have participated in the design and analysis of experiments at the University of Copenhagen that demonstrated successful operation of a triple-dot device. Research projects supported by this grant have furthered the education of several postdoctoral fellows and graduate students at Harvard and at the University of Copenhagen.
