Session Z1: Coherent Optical Manipulation of Electron and Nuclear Spin in Artificial Atomic and Molecular Systems in Solids

Ultrafast optical spin echo for electron spins in semiconductors

Spin-based quantum computing and magnetic resonance techniques rely on the ability to measure the coherence time, T2, of a spin system. We report on the experimental implementation of all-optical spin echo to determine the T2 time of a semiconductor electron-spin system. We use three ultrafast optical pulses to rotate spins an arbitrary angle and measure an echo signal as the time between pulses is lengthened. Unlike previous spin-echo techniques using microwaves, ultrafast optical pulses allow clean T2 measurements of systems with dephasing times (T2*) fast in comparison to the timescale for microwave control. This demonstration provides a step toward ultrafast optical dynamic decoupling of spin-based qubits.   

Increasing the electron spin coherence time by coherent optical control of the nuclear spin fluctuations

A single electron spin plays a central role for spin-based quantum information science and electronic devices. One crucial requirement for the future success is to have a long quantum coherence time. It has been demonstrated that in III-V materials, the electron spin coherence time deteriorates rapidly due to the hyperfine coupling with the nuclear environment. Here, we report the increase of the electron spin coherence time by optical controlled suppression of nuclear spin fluctuations through coherent dark-state spectroscopy. The experiment is performed in a single negatively charged InAs self assembled quantum dot (SAQD). The dynamic nuclear spin polarization manifests itself as a hysteresis in the probe absorption spectrum and in the spectral position of the dark state as a function of the frequency scanning direction of the probe field. We demonstrated that the nuclear field can be locked to the maximum trion excitation by observing a flat-top of the trion absorption lineshape, and the switching of the nuclei from unstable to stable configurations by fixing the laser frequencies and monitoring the coherent optical response as a function of time. The optically controlled locking of the nuclear field leads to an enhancement of the electron spin coherence time, which is measured through dark state spectroscopy. The suppression of the nuclear field fluctuations result from a hole spin assisted dynamic nuclear spin polarization feed-back process. We further demonstrated the electron spin coherence enhancement by a three-beam measurement, where two-pump beams lock the nuclear field and the third probe measures the coherence time through the dark state. The inferred spin coherence time is increased by nearly 3 orders of magnitude compared to its thermal value. Our work lays the groundwork for the reproducible preparation of the nuclear spin environment for repetitive control and measurement of a single spin with minimal statistical broadening.   

Tunable spin interactions in self-assembled semiconductor quantum dot molecules

Carrier spins in coupled semiconductor quantum dots have been proposed as logic elements for quantum information processing. The spatial localization of spins in quantum dots has two obvious advantages: common dephasing mechanisms are suppressed and the spins can be spatially selected using a focused laser beam. Here we describe a very important but less obvious advantage. In a cluster of tunnel-coupled dots, Coulomb interactions are substantial, and state energies are very sensitive to the position of each carrier. Through the Pauli principle, small shifts in position can be used to induce large changes in spin energies. This provides a high degree of flexibility and is particularly useful in an optically controlled system, where exciton dipoles produce additional energy shifts. We have developed two-spin quantum dot molecules where one or more electrons or holes can tunnel between two quantum dots [1]. Combining applied magnetic and electric fields, kinetic spin exchange [2] and Zeeman splittings can be used to generate new spin mixings that are not easily obtained in single quantum dots. The mixing results from small asymmetric exchange interactions and produces optical selection rules that can be used for spin initialization, rotation, and measurement [3]. Tunable exchange energies also provide an important level of control over the two-spin resident carrier states that are a model for two-qubit gates and spin entanglement in a semiconductor system. \\[4pt] [1] E. Stinaff, et al., 311, 636 (2006). \\[0pt] [2] M. Scheibner, et al., Phys. Rev. B 75, 245318 (2007). \\[0pt] [3] D. Kim, et al., Phys. Rev. Lett. 101, 236804 (2008).