Lu Jeu Sham - US grants

9421966 Sham This project is a theoretical study of the time evolution of the nonlinear optical processes in solid state systems. The theory requires taking into account simultaneously the interaction of electrons with intense electromagnetic fields, the interaction among the electrons, and the interaction of the electron with its host environment. Special attention is paid to the interplay between the polarization of light and the spin dynamics of the electrons. The following systems will be studied: III-V nonmagnetic semiconductor heterostructures (promising systems for optoelectronic devices), II-VI magnetic semiconductor heterostructures (important for visible optical and magneto-optical devices), and light element compounds (exhibit properties of strongly correlated electrons). The approach adopted is field theoretic, with a diagrammatic representation. The results are expected to be a unified explanation for a wide range of observed nonlinear optical phenomena in each class of these solid state systems, predictions of new behavior, and a deeper understanding of strong electron correlation through optical processes which can test electron theories. %%% This work involves modeling the interaction of light with several solid systems, including layered (composite) semiconducting systems and light element compounds. This interaction provides a means of measuring the electronic properties in these systems and forms a basis for making ultrafast optical devices. The light interacts with the electronic properties in certain classes to be studied and with magnetic properties in others. The results are expected to be a unified explanation for a wide range of observed nonlinear optical phenomena in three classes of solid state systems, predictions of new behavior, and a deeper understanding of the electron-electron interactions, which are a key to the theory behind many materials. ***

This award supports Professor Lu Sham and a junior associate of the University of California at San Diego to collaborate with Professor Friedhelm Bechstedt of the Friedrich Schiller University of Jena, Germany. They are studying coherence effects in strongly-correlated insulators and metals. In particular, they will investigate the possible detectability of coherence through the nonlinear optical response of the system. The collaborating research groups bring complementary expertise in many body theory and in optical properties of condensed matter systems. Strong electron-electron correlation plays a pivotal role in the electronic properties of modern metals and insulators such as the cuprate compounds and the heavy fermion compounds. If optically excited coherence effects can be shown to exist in theory, this will provide an operational definition for strong correlation in a large number of metals and insulators. In turn, this would lead to an optical means for experimentally observing strongly correlated behavior and an effective optical test for predictions of strong-correlation theories.

This Information Technology Research (ITR) medium program will develop ultrafast optical methods for controlling electronic, magnetic, vibrational, and excitonic properties of semiconductors for fast information processing. Successful manipulation of quantum states and processes in solids will be a necessary breakthrough for implementing the emerging technologies of spintronics and quantum information science. Optical control, as opposed to electrical control, has the advantage of performing quantum control on femtosecond time scales, which will be explored in the following specific contexts: (1) Optical control of ferromagnetism in magnetic III-V semiconductors, (2) optical control of band structure via the dynamic Franz-Keldysh effect, (3) optical control of electric fields in GaN/InGaN strain superlattices, and (4) optical control of excitons in coupled quantum wells. The proposed research work will train student researchers for future employment in high technology fields such as nanoscience and quantum information science and to produce scientists and engineers with a strong background in spectroscopy, optics, photonics, solid state theory, many-body theory, and quantum information theory. New courses on nanotechnology and quantum information science will be developed in the Applied Physics curricula at Rice University, the University of Florida, and the University of California at San Diego.

This Information Technology Research (ITR) medium program is focused on fundamental studies of control of quantum electron dynamics. It will develop 'designer' electronic processes for applications in the emerging fields of spin-based electronics and quantum information processing. The passive paradigm of studying natural processes is replaced by the active one of designing controls of electron and crystal motion at the quantum level. Advanced lasers are used to control motion at the time scale between a trillionth and a quadrillionth of a second. Molecular beam fabrication of nanostructures is used to confine the electrons to a spatial dimension around a billionth of a meter. Such a space-time regime between that of the smallest macroscopic device extant and the microscopic regime of the elementary particles may be advantageous for quantum control and will be explored to utilize the electron spin as an extra dimension for information processing and to produce desirable magnetic, electrical and transport properties at will. The program contains a strong effort in providing interdisciplinary education and research experience in nanoscience and quantum optics. It prepares students for future employment in the increasingly quantum-oriented world of high technology.

The ability to control spin dynamics is critical for enabling the operation of a host of magnetic devices. Exciting opportunities for manipulate spins are offered by directly using optical fields, as was recently demonstrated in experiments. This program introduces a comprehensive theoretical and computational framework for the characterization, modeling, and design of magnetic devices involving optical control of spin dynamics.

The proposed research has a theoretical physics, computational physics, engineering, technological, and educational component. The research provides fundamental understanding of optics driven spin dynamics, such as near Curie-point switching, all-optical switching, combined optical-spin transfer torque phenomena, and optics-induced spin waves. The program identifies new spin dynamics phenomena and structures that exhibit novel functionalities and the associated fluctuations and noise reduction particularly prevalent on the quantum scale. The research creates new methods and efficient simulators, including high-performance atomistic, mesoscale, and multiscale (hybrid atomistic-mesoscale) solvers. The research spawns new opportunities for modeling magnetic and optical devices and systems. The introduced multi-scale framework is used to address critical technological problems involving magnetic materials and devices employing optical control, such as energy assisted magnetic recording. The program includes an essential component of education of undergraduates and graduates, through courses and mentoring, in application of quantum mechanics to modern technology, particularly, micro- and nano-magnetics and electromagnetics. The work involves graduate and undergraduate students in research, and contributes to diversity.

The program has a range of broader impacts. The fundamental quantum mechanical models for magnetics contributes to the general field of quantum mechanics and its relation to practical devices. Various components of the high-performance methods can be used in other fields, such as biophysics, chemistry, and astrophysics. The proposed multi-physics solvers can be hybridized with solvers in other physical domains such as electrical/mechanical/fluidic modeling frameworks. The magnetic recording applications contributes to the development of data storage devices which are pervasive in modern society. The effort also develops a set of educational materials covering the proposed theory, simulation framework, and device study.

Gordon Moore, the founder of Intel, was the first to note that the number of transistors per chip roughly doubled every two years (this is called Moore's Law). The corresponding volume per transistor had to decrease exponentially with time. Miniaturization would eventually lead to such a small number of electrons per transistor that the discriminable charging effect of the electrons in a transistor fails. The point where miniaturization destroys the transistor function is known as Moore's limit. This limit is imminent within the next generation of scientists and engineers. New avenues of progress beyond current technology must be sought. This research follows the approach of quantum technology to continue the evolution of information processing beyond Moore's limit. It works to develop the necessary foundational knowledge for an approach to devices, where the behavior of these devices is governed not by the usual rules that govern the aggregate properties of a large number of particles, but by the laws of quantum mechanics, which governs electrons in the atomic limit. Specifically, this project focuses on the study of optically controlled semiconductor quantum dots. A dot effectively traps one electron, which is manipulated by ultrafast pulses of light that enable terahertz control speeds without the complexities of metal contacts. The studies emphasize the isolation of the electron from the influence of the other electrons (known as the valence electrons) and the nuclei, in particular, which constitute the dot, and the networking of the target electrons in separate dots. Light is used to excite the valence electrons to control a single target electron and to entangle two target electrons, while shielding them from the effects of nuclear fluctuations. The primary intellectual merit is based on the scientific objectives of this three-year research program; produce quantum entangled states between two electron spins separated by a large distance, demonstrate a multi-quantum-bit high-speed logic gate, and explore the feasibility of using optical control of nuclear states to store and or process information. The broader impact includes developing the basic knowledge to advance the technology of information processing beyond Moore's limit via a highly interdisciplinary and collaborative research program, training and preparing the next generation of scientists and engineers for new challenges they will be facing, and facilitating efforts by both our universities to inform and educate the public and students about the importance of this quantum research to society's well-being, including how this research is training students in STEM areas in order to maintain a competitive work force.

This research focuses on quantum information processing by ultrafast optical control of electron spins trapped individually in structurally defined semiconductor quantum dots (QDs). Numerous advances made possible by previous NSF support, have helped meet many of the milestones of quantum operations required by the Di Vincenzo criteria. Recent advances such as demonstration of the flying qubit and nuclear spin fluctuation freezing to extend the electron-spin coherence time and creation of high optical quality laterally positioned dots are fundamental to the quantum network approach to scaling up the system. The critical discovery of nuclear spin fluctuation freezing enables lengthening the coherence time of the qubit by over 2-3 orders of magnitude for time scales lasting longer than 1 second. This allows the time for a complete sequence of computational steps with the ultrafast control including error correction. The method consists in freezing out the fluctuations of the nuclear spins unavoidably present in QDs and temporally separates the decoherence abatement from the control operations, in contrast to the common but more limited and complex method of dynamic decoupling of the qubits being processed at the same time. Building on these achievements five experimental objectives to advance the frontier of scalable quantum information processing based on optical control of the spins will be accomplished: demonstrate a two-bit controlled not-gate and of a simple algorithm using a quantum dot molecule; demonstrate teleportation of information in a single photon qubit to a QD using spontaneous parametric (singe-photon) down conversion (SPDC) to produce the single photon source; produce an entangled state between a quantum dot spin and a spontaneously emitted photon (at 960nm) and convert it to the telecom wavelength around 1.55 microns; demonstrate heralded entanglement of two QD spins using photons.; and extend the work on nuclear spin fluctuation freezing to using optically detected NMR to more completely understand the underlying physics state of nuclei in the dot.

The work is an interdisciplinary research effort in the physics of semiconductor nano-structures, high-precision coherent optical control, and spectroscopy of QDs. These studies are paralleled by the coherent transient time domain studies consistent with device applications and scalable architectures. The amelioration of the environmental problem (decoherence) lies in the coordinated theoretical and experimental treatment of the quantum correlated dynamics of the optically controlled electron spin and the nuclear spins in the QD through the hyperfine interaction without additional stochastic assumptions.The intellectual merit arises from the increasingly sophisticated understanding of the interaction effects between a microscopic system (the electron spins as the qubits) and a macroscopic system (the dot environment, control or measurement) within quantum theory. The broad bandwidth control of the quantum physics makes the QD system the high speed processing unit partner to the quantum memory device of trapped ions. The operating temperature of 4-10 K is above the milli-K required by gated dots and superconducting circuits and optical control avoids some of the connection problems.

The broader impact is first in the development of highly trained people critical to the infrastructure of quantum technology, at all levels of higher education: postdocs, graduate and undergraduate students. The second broader impact is that the optical approach positions our research to play a key role in the transition from the electronic devices that currently drive the Moore?s law to the new paradigm of post CMOS era. The further miniaturization of the current devices that process classical information will reach the quantum barrier where devices operating on quantum information will take over. The future lies in a hybrid structure of classical devices for interface with human and quantum devices for information processing. The optical approach to quantum devices, based on the established III-V material with a large industrial infrastructure and optical sources and gating based on telecom technology, may provide a smooth transition to the hybrid structure.

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.