Robert Wolkow and his team reviews their recent efforts in building atom-scale quantum-dot cellular automata circuits on a silicon surface. Note – published papers are usually summarizing work that is 2-3 years old. I saw Robert Wolkow summarize this work at the Foresight Technical conference early this year but could not report upon it until the paper was released just days ago. BTW that is a reason to go the next Foresight conference in 2014. Get the latest news in nanotechnology before it can be publicly released.
* yields of a tiny fraction of 1% have jumped to 80%
* they were producing 100 atomscale quantum dot structures in 1 minute
* In a day with continuous operation 100,000 atomscale quantum dot structures could be built
* they also lay out the case why their dangling bond approach is better than Michelle Simmons’ phosphor atom qubits (it is the 21st century so there are competing atom scale quantum dot and atom scale qubit approaches)
Our building block consists of silicon dangling bond on a H-Si(OO1) surface, which has been shown to act as a quantum dot. First the fabrication, experimental imaging, and charging character of the dangling bond are discussed. We then show how precise assemblies of such dots can be created to form artificial molecules. Such complex structures can be used as systems with custom optical properties, circuit elements for quantum-dot cellular automata, and quantum computing. Considerations on macro—to—atom connections are discussed.
There are two broad problems facing any prospective nano-scale electronic device building block. It must have an attractive property such as to switch, store or conduct information, but also, there must be an established architecture in which the new entity can be deployed and wherein it will function in concert with other elements. Nanoscale electronic device research has in few instances so far led to functional blocks that are ready for insertion into existing device designs. In this work we discuss a range of atom-based device concepts which, while requiring further development before commercial products can emerge, have the great advantage that an overall architecture is well established that calls for exactly the type of building block we have developed.
The atomic silicon quantum dot (ASiQD) described here fits within ultra low power schemes for beyond CMOS electronics based upon quantum dots that have been refined over the past 2 decades. The well known quantum dot cellular automata (QCA) scheme due to Lent and co-workers achieves classical binary logic functions without the use of conventional current-based technology.
Instrumentation and Custom Lithography to Make Prototype QCA Circuitry
State of the art instrumentation is required to make advances in this area. Years of ordinary STM investigations were hampered by at least two problems. One is non-ideal scanning and fabrication control something we will refer again to in the next section. The other aspect is a lack of a bridge between the atom sale and the macro scale.
Our first approach to making sufficiently fine lithographic features to controllably interact with atom scale structures began nearly 20 years ago. Titanium silicide contacts were prepared using a normal optical lithography and lift-off approach. When examined at the atomic scale, lithographic features prepared in this way are unacceptably rough and crudely defined for our purposes.
Going forward, combined lithographic approaches will allow for suitably small and high quality lithographic features. Multiple contacts will be connected and active while a device is in the STM fabrication and inspection tool allowing prototyping methods and device testing to advance substantially over what has been available to date.
In parallel with the development of nano-lithographic methods, a multi-probe STM has been developed to allow nano-scale electrical characterization that has until now been out of reach. The instrument shown in Figure 18 has three independently scanable tips, watched over by a scanning electron microscope. Each tip can be quickly redeployed as a scanned probe for imaging or touched down as a current source or as a voltage probe.
Recently, our reevaluation of non-idealities inherent to the scanned probe fabrication process and in the character of the scanned probe tip itself have led to a large improvement. Yields of a tiny fraction of 1% have jumped to 80%. The various refinements will not be discussed here but the results can be seen.
We see good overall pattern fidelity and a clear demonstration that we have broken free of the 4 atom limit of a few years ago and may soon be able to make 100 atom structures with excellent fidelity. The circuits in Figure 19 required about 1 minute to fabricate and were automatically made by computer upon input of pattern required.
Advantages of Dangling Bonds (DB) over Phosphor atom (P Atom) qubits
The DB appears to be a far more attractive electronspin qubit than the implanted P atom in silicon. That is our belief. But this writing is the first to our knowledge to point out the many advantages. The main disadvantage of the DB route to spin-based quantum computing is that passivation or encapsulation is required, but that seems a surmountable problem. The advantages to using DBs are many, unlike P atom insertion through a multistep process, DBs can be made instantly. While P atoms cannot be placed exactly and reproducibly with respect to other P atoms, any number of ASiQDs can be perfectly juxtaposed, just as designed. Some of the latest strategies for achieving
robust qubits by combining multiple physical qubits into one logical qubit are greatly aided by this precise multi qubit fabrication facility