Sunday, March 03, 2019

Michelle Simmons and Si-based quantum computing

A last tidbit before the March Meeting.

Earlier this week, Prof. Michelle Simmons came to Rice for our Chapman Lecture series and gave a great talk about her team's project to develop quantum computing in a silicon platform, with individual phosphorus donor atoms as the qubits.   This idea goes back more than twenty years to this proposal by Bruce Kane.   Actually implementing this approach requires overcoming many technical challenges, including positioning individual phosphorus atoms inside single-crystal Si with nearly atomic precision, and similarly fabricating control and read-out electrodes in registry with those.  

Prof. Simmons' group has made truly remarkable progress in this direction.  The key enabling technique is using a scanning tunneling microscope (STM) as a lithography tool.  Single-crystal Si surfaces are prepared in ultrahigh vacuum and terminated with a hydrogen.  The STM tip is then used to strip off the hydrogen atoms with atomic precision.  (This is a serial technique, and so scaling up to the production numbers of the present-day Si industry would require something different, but for now it's fine.)  Phosphine gas decomposes in a particular way when exposed to the dangling bonds left behind by stripping the hydrogen, placing P atoms in particular locations.  This approach can also be used to make highly conductive wires and gates by doping, enabling transport measurements through single dopant atoms.   Growing more single-crystal Si on top of the dopants without having the dopants move around is another success story, making possible 3D fabrication schemes.  With isotopically pure Si, encapsulating the donors can give long coherence times.

There are many competing platforms for possible quantum computer implementations, and this approach is undoubtedly difficult.  In terms of technical achievement, though, this effort has shown the power of sustained support - progress has been truly impressive.

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