****TECHNICAL ABSTRACT**** The project aims to design and manipulate quantum states of electrons in one-dimensional quantum dots fabricated from carbon nanotubes. Ultrashort carbon nanotubes are experimental representations of a one-dimensional potential well, in which the allowed energy level spacing depends strongly on the length of the nanotube. By coupling a short section of a nanotube to ferromagnetic or superconducting electrodes via tunnel junctions, one can achieve fine control of the electronic quantum states on the nanotube. These quantum states will be studied through conductance and current shot noise measurements, which will depend strongly on the spins of the electrons. This project will support the education of graduate and undergraduate students, who will receive hands-on training in cutting-edge nanofabrication and low-temperature measurement techniques. In addition to exploring the fundamental issues of custom-designing and controlling quantum states of electrons, the results will serve as the basis for development of novel spin-based quantum devices. In particular, the possibility of creating and observing elusive Majorana fermions would have an enormous impact on both the fundamental understanding of physics and the future topological quantum computing technology.

Nontechnical Abstract

Electrons are elementary particles with an intrinsic quantum property called spin, which can be either "up" or "down". In quantum mechanics, electrons have all the properties of particles with spin, but they also behave as waves. As a consequence of their wave nature, the energies of electrons in a one-dimensional wire can only take certain allowed values - just like the note played on a guitar string depends on the length of the string, the energy of an electron in a quantum wire depends on the length of the wire. This project aims to design and manipulate the energy states and the spins of electrons in short sections of carbon nanotubes. This will be achieved via electrical measurements that will use spin-sensitive configurations of ferromagnetic and superconducting electrodes. This project will support the education of graduate and undergraduate students, who will receive hands-on training in cutting-edge nanofabrication and measurement techniques at temperatures close to absolute zero. In addition to exploring the fundamental physics of custom-designing and controlling quantum states of electrons, the results will serve as the basis for development of novel spin-based quantum devices. The project offers a unique opportunity to create and observe a new state of matter, which would enable the development of fundamentally fault-tolerant quantum computing technology.

Project Report

Nanoscale world is ruled by quantum physics, which allows electrons to exist only in well-defined quantum states. The ability to create and control these quantum states in a controlled and reproducible way is a necessary requirement for development of transformative technologies based on quantum effects. This award focused on controlling quantum states in one-dimensional systems, namely carbon nanotubes and superconducting nanowires. It has lead to development of new nanofabrication methods, design of new measurement equipment, and experimental discovery of novel quantum effects. Short carbon nanotubes are an experimental realization of a model system that appears in introductory quantum physics textbooks: the particle in a box. In particular, the spacing between the allowed quantum states becomes larger in shorter nanotubes, allowing us to resolve these states by monitoring the current through the nanotube. Using ferromagnetic and superconducting materials for the electrical leads, we demonstrated the ability to manipulate the properties of the quantum states and the spin of the electrons on the nanotube. One aspect of the project required a narrow superconducting wire to serve as the source of the electrons. For this purpose, we have developed a new fabrication method, which yields ultra-narrow uniform nanowires with precisely controlled cross-sections. The perfect conductivity in superconductors is often disrupted in a presence of a magnet, which creates small vortices of supercurrent that can move and cause resistance. These vortices can be pinned at the edges of a sample, but it is more difficult to pin them in the bulk of the material. The width of our nanowires is such that only one vortex row can fit in the nanowire - with an edge on each side, the vortices are trapped. We found that we can deliberately manipulate individual vortices by tuning the strength of the magnet. We can observe this by measuring the current through the nanowire, which reaches a maximum when each additional vortex enters the nanowire. The observed behavior of vortices in nanowires is another example of a particle-in-a-box effect. It is analogous to the behavior of electrons in short carbon nanotubes, but has not been previously demonstrated for vortices, which are ten million times larger than electrons. The ability to control the vortices will greatly improve the operation of superconducting quantum circuits, in which movement of vortices causes errors. As part of the effort to control the electrical contact between the leads and the carbon nanotubes, we have used graphene, which is chemically similar to carbon nanotubes. We found experimental evidence that electrons and holes in graphene are intricately connected, and that they scatter differently on charged impurities. To probe these effects, we used quantum noise measurements, which is uniquely sensitive to fundamental symmetries that describe the charge carriers in graphene. In order to obtain good insulating layers required for gate electrodes, we have built an atomic layer deposition system, which is now part of the research infrastructure at our institution. The award has supported the work of one postdoctoral researcher, two graduate students and four undergraduate students over the last three years, and has so far produced fifteen publications in refereed journals and one patent application. The award has also provided research opportunities for three high school students. Two publications were co-authored by undergraduate students, and the patent application was co-authored by a high school student and an undergraduate student. Four students were female and one student was from an underrepresented minority group. All students are continuing to actively pursue careers in science. Besides advancing our understanding of the fundamental physics of quantum systems, this project has provided means for custom-designing and controlling quantum states, laying groundwork for development of novel quantum devices and creating exciting opportunities for future research.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Guebre X. Tessema
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Johns Hopkins University
United States
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