Focus: Ratchets Learn to Embrace Logic

Phys. Rev. Focus 11, 28
An electronic ratchet could turn magnetic superconductor vortices into the elements of a simple computer.
Figure caption
M. Hastings/Los Alamos National Lab
Little bitty bits. Magnetic vortices moving back and forth inside depressions on a superconducting surface could serve as single-particle bits for a nanoscale computer. A new technique can manipulate them for calculations. (See video below.)

In the race to cram more elements onto a single computer chip, computers may eventually use particles rather than voltages to store the 1’s and 0’s of digital bits. A so-called ratchet mechanism could produce single-particle digital operations with little waste heat, according to theory and simulations reported in the 20 June PRL. The particles would be magnetic vortices–narrow tubes of magnetic field that penetrate superconductors.

A magnetic field can penetrate a superconductor through many small, non-superconducting patches surrounded by circulating currents of superconducting electrons. The magnetic fields of these vortices cause them to repel one another just as like-oriented magnets do.

Vortices prefer to penetrate as little material as possible, so depressions etched into the surface of a superconductor can isolate and trap them. A vortex in a long and narrow ditch-shaped depression can represent a 0 or 1 by sliding to either end. But for computing, this value must then be transmitted from one part of the computer to another, and it must be usable for so-called logic operations that are the basic bit-wise manipulations in all computer calculations. These two problems have the same solution, according to Matt Hastings of the Los Alamos National Laboratory and his colleagues.

Video courtesy of M. Hastings, Los Alamos National Lab.
Vortex-based Computer Operation. A “fan out” operation starts with one copy of a bit value at the left and creates two copies at the right. The ratchet mechanism allows the values to propagate across.

To transmit the value of a bit, imagine a line-up of parallel ditches side by side, each containing a vortex. Because of their mutual repulsion, each vortex would be pushed to the opposite end from its two neighbors, making an up-down-up-down pattern. Now imagine that the first “bit” is switched and held down, setting it to the value “1”. The next vortex would then tend to shift up because of the repulsion. The idea is to switch the bits sequentially, ultimately putting the new value at the far end. The problem is that with this arrangement, a vortex can get stuck in the middle of its ditch, equally repulsed by its two neighbors.

Hastings and his colleagues report that a combination of different sized depressions and a “ratchet” mechanism could solve the problem by creating narrow gaps when needed to influence a neighboring bit and wider gaps to prevent a bit from stalling. The ratchet also leads to digital switches that allow simple “and” and “or” operations on bits.

Each normal depression would be followed by two wider ones, each of which is not quite flat, but sloped upward toward its wider neighbor and downward toward its narrower one. In this arrangement, the vortices in the first and second ditches will start out closer together than those of the second and third. Hastings and his colleagues propose applying a three-stage cyclic electric field which moves all the vortices in the wider depressions first left, then right, then releases them (see video below).

The result is that the gap between vortices moves back and forth in just the right way to allow each vortex to switch its downstream neighbor. A simulation of 144 vortices indicated the mechanism would indeed propagate a bit flip at a frequency set by the cyclic field. Using two rows of bits that can influence one another, the team devised logical “and” and “or” gates.

The researchers calculate that the superconductor approach would dissipate less heat than semiconductor designs, because relatively few electrons are in the resistive, non-superconducting state. They also say the ratchet approach could be applied to other proposed single-particle systems, such as quantum dots.

Ratchets are a “clever” solution to the stalled vortex problem, says George Crabtree of Argonne National Laboratory in Illinois. But constructing a device along these lines still may take years, he says.

–JR Minkel

JR Minkel is a freelance science writer in New York City.

More Information

  • More on vortex ratchets, including video of an “and” gate, at the authors’ web site.

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