Calculating with Quantum Cascades

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Science  15 Nov 2002:
Vol. 298, Issue 5597, pp. 1295
DOI: 10.1126/science.298.5597.1295c

Cascades of hopping by CO molecules on a surface at cryogenic temperatures has been harnessed to perform one-time logic operations. Heinrich et al. (p. 1381; see the cover) used a scanning tunneling microscope (STM) to identify isotopically distinct CO molecules on a Cu(111) surface through their inelastic electron tunneling spectra. These molecules were then assembled with the STM into a chevron pattern. A CO molecule at one end was given a shove with the STM tip, and a cascade of induced hops could be observed over a period of a few seconds. Below 6 kelvin, the hopping rates were temperature independent but showed a pronounced isotope effect, both hallmarks of quantum tunneling. The authors used these molecular cascades to perform computation by assigning initial and final positions of CO molecules as 0 or 1, respectively. They created AND and OR gates, and intersecting patterns with multiple inputs could operate as a three-input sorter.

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