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46 DESIGN007 MAGAZINE I JUNE 2018 Research groups from Purdue University, the Techno- logical University of Delft, Netherlands and the University of Wisconsin-Madison have discovered that silicon has unique spin-orbit interactions that can enable the manip- ulation of qubits using electric fields, without the need for any artificial agents. "Qubits encoded in the spins of electrons are especially long-lived in silicon, but they are difficult to control by electric fields. Spin-orbit interaction is an important knob for the design of qubits that was thought to be small in this material, traditionally," said Rajib Rahman, research assistant professor in Purdue's School of Electrical and Computer Engineering. The strength of spin-orbit interaction, which is the interaction of an electron's spin with its motion, is an important factor for the quality of a qubit. The researchers found more prominent spin-orbit interaction than usual at the surface of silicon where qubits are located in the form of so-called quantum dots—electrons confined in three dimensions. Rahman's lab identified that this spin- orbit interaction is anisotropic in nature, meaning that it is dependent on the angle of an external magnetic field, and strongly affected by atomic details of the surface. The researchers published their findings on June 5 in Nature Partner Journals – Quantum Information. The Wisconsin-Madison team fabricated the silicon device, the Delft team performed the experiments and the Purdue team led the theoretical investigation of the experimental observations. Upcoming work in Rahman's lab will focus on taking advantage of the anisotropic nature of spin-orbit interactions to further enhance the coherence and control of qubits, and, therefore, the scaling up of quan- tum computer chips. Silicon Provides Means to Control Quantum Bits for Faster Algorithms the memory device pins and the parallel termination placed at the end of the transmission line. • Stub capacitance, along with the parasitic input capacitance of the receiver pin, creates an imperfection in the termination network. • There are no reflections when the passing address signal trace goes directly to the receiver pins with no stub and the termi- nation is at the very end of the line. • Stubs create reflections and have a detrimental impact on the address signal in the fly-by topology. • Simulations suggest that the stub must be a maximum of 200mil to alleviate the impact of the reflections. References: 1. Barry Olney's Beyond Design: DDR3/4 Fly- by vs T-topology Routing, Impedance Match- ing: Terminations. 2. High-Speed Signal Propagation, by How- ard Johnson. Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board- level simulation. The company developed the iCD Design Integrity software incorporating the iCD Stackup, PDN and CPW Planner. The software can be downloaded from www.icd.com.au. To contact Olney, or read past columns, click here.