Department of Physics, City University of Hong Kong, Kowloon, Hong Kong SAR, China
Spin qubits in semiconductor quantum dots are promising candidates for quantum information processing due to their demonstrated long coherence time, reasonably high control fidelity, and prospects for scalability. Despite these successes, there remain great challenges to overcome, with the major one being decoherence, the process through which the qubit loses the information it carries while interacting with its environment. Various techniques are proposed to combat decoherence, including the dynamically corrected gates using composite pulse sequences that are able to cancel both nuclear and charge noises simultaneously while performing a quantum gate operation. In this talk, I will present recent developments on these noise-compensating composite pulses for spin qubits, including gate sequences which are simultaneously resilient to nuclear and charge noises at both single- and two-qubit level. I will also present our attempt to improve the control fidelity of capacitively coupled spin qubits by making simple modifications to control sequences that are already used in the laboratory. With time permitting, I will discuss the possibility to achieve the same level of robustness using analytically designed smooth pulse instead of the square pulse traditionally used. These results mark an important step forward in using semiconductor spin qubits as the platform for scalable, fault-tolerant quantum computation.
 X. Wang, L. S. Bishop, J. P. Kestner, E. Barnes, K. Sun, and S. Das Sarma, Composite pulses for robust universal control of singlet-triplet qubits, Nature Commun. 3, 997 (2012).
 J. P. Kestner, X. Wang, L. S. Bishop, E. Barnes, and S. Das Sarma, Noise-resistant control for a spin qubit array, Phys. Rev. Lett. 110, 140502 (2013).
 X. Wang, E. Barnes, and S. Das Sarma, Improving the gate fidelity of capacitively coupled spin qubits,
npj Quantum Information 1, 15003 (2015).
 E. Barnes, X. Wang, and S. Das Sarma, Robust quantum control using smooth pulses and topological winding, Sci. Rep. 5, 12685 (2015).
Brief CV of Prof. Xin (Sunny) Wang:
Prof. Xin (Sunny) Wang received B.S. from School of Physics, Peking University in 2005, and received his Ph.D. degree from Columbia University in 2010. His Ph.D. study was focused on the theory of strongly correlated materials, in particular the high-Tc superconductors. From 2010-2015, Dr. Wang was a Research Associate in Condensed Matter Theory Center at University of Maryland, College Park. Starting from 2015 he joined City University of Hong Kong as an Assistant Professor. His current research interests include the theory of quantum computation using electron spins and strongly correlated electrons. He has published more than 35 journal papers, including those in Nature Communications, npj Quantum Information, and Physical Review Letters.