Abstract:

Speaker: Dr. Zhen Zhang

Paper 1:

Title: Localization in silicon nanophotonic slow-light waveguides

Journal reference: Nature Photon. 2, 90 (2008).

Abstract: Slowing down light on a chip can lead to the development of optical buffers, filters and memory elements4 useful for optical interconnects and for resonantly enhanced chip-based nonlinear optics. Several approaches to slow light rely on the phenomenon of light interference in a sequence of coupled resonators; however, light interference is also responsible, in disordered structures, for the localization of light, an effect particularly prominent in one-dimensional devices. Until now, the length of the waveguides investigated has been less than the localization length. Here we report the first observation of light localization in compact silicon nanophotonic slow-light waveguides consisting of long sequences of coupled resonators. Our results show that disorder limits how much light can be slowed, and that localization leads to spatially concentrated and locally trapped light in a quasi-one-dimensional waveguide at wavelengths near the band edge.

Paper 2:

Title: Robust optical delay lines with topological protection

Journal reference: Nature Physics, DOI: 10.1038/NPHYS2063

Abstract: Phenomena associated with the topological properties of physical systems can be naturally robust against perturbations. This robustness is exemplified by quantized conductance and edge state transport in the quantum Hall and quantum spin Hall effects. Here we show how exploiting topological properties of optical systems can be used to improve photonic devices. We demonstrate how quantum spin Hall Hamiltonians can be created with linear optical elements using a network of coupled resonator optical waveguides (CROW) in two dimensions. We find that key features of quantum Hall systems, including the characteristic Hofstadter butterfly and robust edge state transport, can be obtained in such systems. As a specific application, we show that topological protection can be used to improve the performance of optical delay lines and to overcome some limitations related to disorder in photonic technologies.

 

Speaker: Dr. Chao Shen

Paper 1:

Title: Electron-Mediated Nuclear-Spin Interactions between Distant Nitrogen-Vacancy Centers

Journal reference: Phys. Rev. Lett. 107, 150503 (2011)

Abstract: We propose a scheme enabling controlled quantum coherent interactions between separated nitrogenvacancy centers in diamond in the presence of strong magnetic fluctuations. The proposed scheme couples nuclear qubits employing the magnetic dipole-dipole interaction between the electron spins and, crucially, benefits from the suppression of the effect of environmental magnetic field fluctuations thanks to a strong microwave driving. This scheme provides a basic building block for a full-scale quantum-information processor or quantum simulator based on solid-state technology.

Paper 2:

Title: Robust Trapped-Ion Quantum Logic Gates by Microwave Dynamical Decoupling

Journal reference: arXiv:1110.1870

Abstract: We introduce a hybrid scheme that combines laser-driven phonon-mediated quantum logic gates in trapped ions with the benefits of microwave dynamical decoupling. We demonstrate theoretically that a strong driving of the qubit decouples it from the external magnetic noise, and thus enhances the fidelity of two-qubit quantum gates. Moreover, the scheme does not require ground-state cooling, is inherently robust to undesired ac-Stark shifts, and simplifies previous gate schemes thus decreasing the effort in their realization.

Paper 3: (Optional)

Title: Universal Digital Quantum Simulation with Trapped Ions

Journal reference: Science 334, 57 (2011)

Abstract: A digital quantum simulator is an envisioned quantum device that can be programmed to efficiently simulate any other local system. We demonstrate and investigate the digital approach to quantum simulation in a system of trapped ions. With sequences of up to 100 gates and 6 qubits, the full time dynamics of a range of spin systems are digitally simulated. Interactions beyond those naturally present in our simulator are accurately reproduced, and quantitative bounds are provided for the overall simulation quality. Our results demonstrate the key principles of digital quantum simulation and provide evidence that the level of control required for a full-scale device is within reach.

Short Bio: