A Valleytronics route towards future reversible computers

(Nanowerk Spotlight) Valleytronics is an emerging field exploiting electron's valley degree of freedom for device applications. This novel concept is based on utilizing the wave quantum number of an electron in a crystalline material (read more: "Valleytronics – an alternative electronics concept").
Simply put, 'valleys' are maxima and minima of electron energies in a crystalline solid. Being able to control electrons in different valleys could lead to faster and more efficient computer logic systems and data storage chips.
Despite much anticipations of valleytronics as a candidate for ‘beyond CMOS’ technology and as concept to extend Moore's Law, its progress is severely hindered by the lack of practical designs of valleytronic-based information processing unit.
"One major challenge in valleytronic-based electronics is the lack of all-electrical-controlled valley filter, a device that produces valley-polarized current via electrical controlling knob and serves as a fundamental building block of valleytronics," Dr. Yee Sin Ang, a Research Fellow at SUTD-MIT International Design Center at Singapore University of Technology and Design, tells Nanowerk.
In a paper in Physical Review B ("Valleytronics in merging Dirac cones: All-electric-controlled valley filter, valve, and universal reversible logic gate"), first-authored by Ang, the authors propose a versatile all-electric-controlled valley filter and demonstrate, for the first time, a concrete working design of valleytronic-based logic gate capable of performing all 16 types of Boolean logics.
They do this by harnessing the unusual physical properties of two-dimensional (2D) nanomaterials such as few-layer black phosphorus and topological Weyl/Dirac semi-metal thin films,
"Most importantly, this valleytronic-based logic gate can perform logically-reversible computing, which has broad applications including cryptography, signal and image processing, quantum computing, and is ultimately required to improve the energy efficiency of classical computer beyond the Landauer's limit – an irreducible waste-heat generation due to logically-irreversible computation," Ang points out.
(a) Schematic drawing of the valleytronic logic gate. (b) The operation of the valleytronic logic gate is analogous to the optical polarizer/analyzer setup. (c-e) Valley-dependent output conductance characteristics of the valleytronic logic gate. (f) Traditional Toffoli’s reversible gate which inevitably generates wasteful ancilla inputs and garbage outputs in maintaining logical reversibility. (g) Valleytronic-based reversible gate encodes information in the valley polarization of the computational outputs. (Image: Dr. Ang, Singapore University of Technology and Design) (click on image to enlarge)
Traditional reversible computation relies on complex circuitry which inevitably generates large quantity of wasteful bits, known as 'garbage' and 'ancilla' bits, in maintaining logical-reversibility.
The valleytronic universal reversible logic gate proposed by Ang and his co-authors encodes extra bits of information in the valley polarization of the computational output to preserve logical-reversibility.
"This valleytronic approach significantly reduces the complexity of reversible circuitry and generates only minimal amount of wasteful bits," concludes Ang. "Processing information using valleytronics may provide a new pathway towards ultimately energy-efficient reversible computer."
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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