Collaborative Research: Optically Driven Quantum Dot Spins for Quantum Information: 2- and 3-Qubit Behavior with Nuclear Spin Narrowing

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

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.