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Physics Colloquium – “Donor Quantum-Dot Coupled Qubits”

September 11 @ 4:00 pm

Malcolm Carroll

Sandia National Laboratory

Abstract:  There are numerous efforts to develop quantum bits (qubits) for quantum computing.  Silicon is recognized as an appealing material system because, for example, it offers a low decoherence environment for spin qubits combined with its historical foundation in device and computer chip fabrication.  Donors in silicon have been proposed as qubits and the combination of the electron and nucleus introduce a rich variety of qubit device architectural ideas as indicated in the literature.  Donor qubits also promise very high fidelity operation, uniformity (i.e., every donor is the same) and extraordinarily well protected idle/memory (i.e., nuclear spins).  However, placement, read-out and coherent control of single donors has been notoriously challenging.  To date, it has been demonstrated in only two groups in the world, one of which is the Sandia National Lab quantum information, science and technology (QIST) group.  At this time a central question is how to couple donor-based-qubits.

In this talk I will discuss the first experimental demonstration of coherent singlet-triplet (S-T) rotations in a MOS quantum dot and a single donor system.  This represents both a new qubit configuration as well as a central demonstration of coherently coupling a buried donor spin with the oxide-silicon interface.  This is a key step towards the realization of a long sought general goal of many donor device architectures, to mediate entanglement between two donors using the interface.  Using a donor as one of the wells of the canonical double quantum dot S-T qubit configuration furthermore introduces an efficient, potentially very fast and potentially much higher fidelity gradient-field through the contact hyperfine interaction of the single 31P nucleus.  That is, this resolves a challenge for S-T implementations in enriched silicon that otherwise need to introduce an external gradient field (e.g., micro-magnets) or add further complexity to achieve all-electrical control through addition of a third quantum dot.  Instead this approach reduces lay-out complexity because it relies on only a single MOS quantum dot, instead of two, and provides a reliably repeatable gradient field specific to the donor species without the need for continuous pumping and monitoring of the background nuclear spin bath as is done in GaAs. 

Berthoud Hall 241

Bio:  Malcolm S. Carroll worked on simulation and measurements of phonon imaging with Prof. J. Wolfe at the University of Illinois as an undergraduate.  He completed a Bachelor’s degree in Engineering Physics from the University of Illinois.  From 1994 to 1995 he was a Fulbright fellow at the Johannes-Guttenberg University of Mainz, Germany, working on Monte-Carlo simulation of spin phase transitions.  In 2001 he received a Ph.D. in Electrical Engineering from Princeton University working for Prof. J. C. Sturm.  The thesis work was on scaling of silicon nanostructures specifically SiGeC heterojunction bipolar transistors.  He joined the semiconductor division of Bell Labs/Lucent Technologies at Murray Hill, NJ, which subsequently became Agere Systems in 2002. Part of this research resulted in a patent defining approach to integrate germanium detectors with CMOS electronics, which was later used by a start-up company called Noble Peak Vision.  He is now a distinguished member of the technical staff at Sandia National Laboratories. His most recent research has centered on materials, device physics and cryogenic circuits for quantum information science (QIS).  This includes a world first demonstration of coherent spin manipulation of a MOSFET quantum-dot qubit with a single-donor-atom qubit.  He is presently the senior scientist for silicon quantum computing at Sandia National Laboratories.  Dr. Carroll has been a first- or co-author on over 85 peer reviewed articles with an estimated 950 citations.  He is also a coauthor of 6 patents.  He is on the international technical committee for ISTDM.  He is a founder of the International Silicon Quantum Electronics Workshop (http://www.sandia.gov/QIST/workshops.html) & the International Adiabatic Quantum Computing Workshop series (http://www.isi.edu/events/aqc2014/home). He has acted as an international advisor for the Australian Centre for Quantum Computing and Communication Technologies (CQC2T), the Canadian Foundation for Innovation and the Princeton Center for Complex Materials a NSF-MRSEC.


September 11
4:00 pm
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Berthoud Hall
1516 Illinois St.
Golden, CO 80401 United States
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