Collaborative Research: Optically created metastable mesoscopic nuclear spin states: Glassy transitions and properties beyond electron decoherence in quantum dots

University Of California-San Diego, La Jolla CA

Investigators

Abstract

Nontechnical Abstract Quantum dots, formed from semiconductors such as Indium Arsenide, act as artificial atoms and have seen widespread use in many current optoelectronic devices. However, their use for applications in quantum computing, communication and sensing has been limited by the relatively short lifetime of the excited state of the electrons in these dots. Recently, our team have discovered that polarizing the nuclear spins in these dots leads to a surprising and dramatic increase of the lifetime by many orders of magnitude opening the potential for using quantum dots in the next generation of quantum electronics. This project will explore the fundamental physics behind this dramatic increase in lifetime both experimentally and theoretically. The research will support the development of highly trained people, (including students from the NSF-Imes-Moore Bridge-program in Applied Physics) critical to the infrastructure of nano and quantum technology, at all levels of higher education including postdocs, graduate and undergraduate students. The results of this research will not only open up potential applications in quantum electronics but could provide advances in areas such as magnetic resonance imaging and the development of a path for transitioning from the Moore's law to the new paradigm of a post CMOS era. Technical Abstract In previous NSF supported work, we discovered dynamic nuclear spin quieting (DNSQ) in single and coupled InGaAs quantum dots (QDs) produced by optical coupling to the e-h spin that is accompanied by clear dynamical nuclear spin polarization (DNSP). The results show both local and nonlocal creation of mesoscopic metastable nuclear spin configurations characterized by DNSQ. The effect is long lived (>1sec) and reflects creation and locking of a metastable mesoscopic nuclear state involving >10,000 nuclei. The underlying physics is not clear but is obviously mediated by a nonlinear coupling between the optically driven e-h spins and the nuclei of the quantum dot through hyperfine coupling. The large exciton Bohr radius results in interaction of the electron-hole spins with nearly all the nuclei in the dot, unlike a simple atom with one nucleus. The results are highly significant for application of QDs or other structures to spin based quantum electro-photonic devices, such as quantum repeaters (e -spin serves as memory and the spontaneously emitted photons serves as the flying qubit) and for the potential use of the nuclear ensemble states for quantum metrology, classical memory and perhaps even improving MRIs. The importance of this research is rooted in that locked DNSQ increases e- -spin coherence time by more than three orders of magnitude , without dynamic intervention. The theoretical studies have, thus far, not provided a unified picture consistent with all the data, and the nature of the quiescent nuclear spin ensemble states remains a mystery. The proposed theory focuses on the properties of the quiescent nuclear states vis-a-vis the thermal nuclear states under optical control of the e-spin. Spin glass concepts and methodology will be adapted to the mesoscopic nature and the spin dynamics of the ensemble, to formulate a detailed theory for a comprehensive understanding of the physics. The results could lead to advances in electronic-photonic information processing on a long time scale, such as a quantum repeater or measurement processes. The nuclear quiescent state, analogous to spin glass, may also provide a laboratory for classical computer science and beyond. The proposal has two scientific objectives: 1. Measure and theoretically understand the dynamics of the interaction between the two distinct quantum systems (the nuclear ensemble spin and the e- -spin) leading the various mesoscopic metastable DNSQ states. This includes determining both longitudinal and transverse relaxation rates of the nuclear spin in these states including determining the level of quantum coherence in the nuclear spin states following switching; and 2. Measure and theoretically predict the e- -spin decoherence in single and coupled QDs as a function of the various mesoscopic nuclear states. This work will result in improving the understanding of the interaction between a non-equilibrium microscopic quantum system (a few e-- spins) and a mesoscopic quantum system (number of nuclear spins < thermodynamic limit). The complexity of the mechanism of optical control of the e --spin states stems from the back action of the resulting modified mesoscopic nuclear state on the optically controlled e- -spins . For quantum metrology, this is a paradigm system for measuring the properties of the mesoscopic system (nuclear) via the electron states or as a quantum measurement of the correlated electron systems under the influence of the nuclear ensembles as controllable environment. For potential applications, the broad bandwidth (THz) of the quantum physics enables high-speed optical control without connectivity problems and operates at 4-10 K.