Four atomic quantum dots are coupled to form a “cell” for containing electrons. The cell is filled with just two electrons. Control charges are placed along a diagonal to direct the two electrons to reside at just two of the four quantum dots comprising the cell. This new level of control of electrons points to new computation schemes that require extremely low power to operate. Such a device is expected to require about 1,000 times less power and will be about 1,000 times smaller than today’s transistors. Credit: Robert A. Wolkow
Often referred to as artificial atoms, quantum dots have previously ranged in size from 2-10 nanometers in diameter. While typically composed of several thousand atoms, all the atoms pool their electrons to “sing with one voice”, that is, the electrons are shared and coordinated as if there is only one atomic nucleus at the centre. That property enables numerous revolutionary schemes for electronic devices.
The silicon atom dangling bond (DB) state serves as a quantum dot. Coulomb repulsion causes DBs separated by < » 2 nm to exhibit reduced localized charge enabling electron tunnel-coupling of DBs. Scanning tunneling microscopy measurements and theoretical modeling reveal that fabrication geometry of multi-DB assemblies determines net occupation and tunnel-coupling strength among dots. Electron occupation of DB assemblies can be controlled at room temperature. Electrostatic control over charge distribution within assemblies is demonstrated. QDs (Quantum Dots) have been pursued in the context of nanoelectronics applications (transistors, logic gates, spin devices, etc.), light emitting diodes and lasers, solar cells, ultra-dense memories, among other areas. Moreover, controlled coupling of the electronic states of QDs have been investigated as a basis for alternative computing approaches, such as quantum computing, and quantum cellular automata (QCA) schemes. However, current miniaturization of QD is far from reaching its limit.
Present QD fabrication techniques render QD assemblies of a scale that requires cryogenic conditions to attain energy level spacings greater than kBT – a key condition for enabling controlled electronic properties.
We report for the first time an experimentally observable tunnel coupling between zero-dimensional entities of atomic size: Si atom dangling bonds (DB) on an otherwise hydrogen terminated silicon crystal surface. Such DBs can serve as quantum dots and due to their strong charge localization, circumvent key problems associated with QD charging. Indeed, we show here that the charging and the tunnel coupling behavior within DB assemblies can be controlled even at room temperature. In addition, due to the fundamental similarities with semiconductor QDs and to the common Si-based fabrication platform, our approach can bring important advances to many of the above applications.
The robustness of the atomic system described here results from the relatively great energy level spacing of bound states. For this same reason, the coupling and controlled electron filling of assemblies of coupled DBs is achieved at room temperature rather than requiring cryogenic conditions. We assert that such DB states hold the prospect of a novel route to advancement in nano-electronics and computing devices, offering extreme miniaturization and a well-understood route to fabrication.
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A proposal for implementing classical cellular automata by systems designed with quantum dots has been proposed under the name “quantum cellular automata” by Paul Tougaw and Craig Lent, as a replacement for classical computation using CMOS technology. In order to better differentiate between this proposal and models of cellular automata which perform quantum computation, many authors working on this subject now refer to this as a quantum dot cellular automaton.
Complementary metal-oxide semiconductor (CMOS) technology has been the industry standard for implementing Very Large Scale Integrated (VLSI) devices for the last two decades, and for very good reasons – mainly due to the consequences of miniaturization of such devices (i.e. increasing switching speeds, increasing complexity and decreasing power consumption). Quantum Cellular Automata (QCA) is only one of the many alternative technologies proposed as a replacement solution to the fundamental limits CMOS technology will impose in the years to come.
Although QCA solves most of the limitations CMOS technology, it also brings its own. Optimistic assumptions suggest that intrinsic switching time of a QCA cell is in the order of terahertz, however, as mentioned earlier, switching speed is not limited by a cell’s intrinsic switching speed but by the proper quasi-adiabatic clock switching frequency setting. “Comparative analysis of circuit performance of QCA and CMOS against a representative computer task, suggests that realistic circuits of solid state QCA will have the maximum operating frequency of several megahertz. Small circuits of hypothetical molecular QCA might have the maximum operating frequency of several GHz, however, as the circuit size increases, capacitive loading effects will reduce the speed.” Moreover, solid-state QCA devices cannot operate at room temperature [until this new work]. The only alternative to this temperature limitation is the recently proposed “Molecular QCA” which theoretically has an inter-dot distance of 2 nm and an inter-cell distance of 6 nm. Molecular QCA is also considered to be the only feasible implementation method for mass production of QCA devices. QCA technology resolves, in principle, the problems of current CMOS technology, and it is only limited by the availability of its practical fabrication methods.