Joseph S. Friedman


Reversible Skyrmion Logic

Reversible skyrmion logic leverages magnetic skyrmions in the first nanoscale realization of conservative logic, providing a vision for energy-efficient computation. In this system, magnetic skyrmions propagate through a two-dimensional ferromagnetic structure while performing reversible logic operations at the gate junctions. A simple global clock enables direct cascading with the potential for scalable high-speed low-power reversible Boolean and quantum computing.

Magnetic Skyrmion Track

Magnetic skyrmions are stable regions of a ferromagnet in which the magnetization of the skyrmion core is oriented in the opposite direction of the rest of the ferromagnet. These skyrmions can be moved with an electrical current that transports the magnetic spin information (no physical particles are actually transported). As shown in the figure, +y-directed electrical current creates a spin-Hall force that pushes the skyrmion in the +y-direction, while also creating a skyrmion-Hall force that pushes the skyrmion in the -x-direction. The skyrmion is placed within a track structure that suppresses the lateral skyrmion-Hall effect and forces the skyrmion to travel along the track.

Reversible Logic Gates

Conservative logic envisions reversible computing with zero energy dissipation, and was originally proposed with an analogy to billiard balls. Though not dissipation-free, this reversible skyrmion logic system replaces the billiard balls with magnetic skyrmions in the first nanoscale realization of the conservative logic concept.

The skyrmion-Hall effect and skyrmion-skyrmion repulsion can be used together to perform reversible logic operations at the junction between two ferromagnetic tracks. The simplest conservative logic gate is the AND-OR gate, which conserves the magnetic skyrmions while computing the AND and OR functions of inputs A and B. The spin-Hall effect continuously pushes the skyrmions in the +y-direction, and the skyrmion-Hall effect pushes the skyrmions from the right track to the left track whenever possible. However, when a skyrmion is already present in the left track, skyrmion-skyrmion repulsion prevents the skyrmion on the right from switching to the left track. Other logic gates have also been demonstrated via micromagnetic simulation, notably the INV-COPY gate which can generate any logical function when coupled with the AND-OR gate.

This reversible logic paradigm also enables a fundamental element of quantum computing, the Fredkin gate, which performs a reversible logic function.

Skyrmion Synchronization, Cascaded Logic Gates, & Pipelined Computing

To ensure proper functionality within a large system, it is critical that the skyrmions enter the logic gate junctions simultaneously. The skyrmions are therefore synchronized with global clock pulses that provide short current spikes to the entire skyrmion logic circuit that shrink the skyrmions and enable them to simultaneously pass notches within the tracks.

This procedure is illustrated for the one-bit full adder, in which seven notches are used to compute the sum and cary-out signals in three clock cycles. Thanks to the synchronizing notches, this reversible logic circuit structure can be scaled up to large computing systems.

The non-volatility of this system, in concert with the clocked data propagation, enables pipelined logical operations. Whereas the latency of logical operations may be multiple clock cycles, pipelining enables each logic circuit to provide a throughput of one logical result per clock cycle.

Related Publications

  1. X. Hu, M. Chauwin, F. Garcia-Sanchez, N. Betrabet, A. Paler, C. Moutafis, J. S. Friedman, Skyrmion Logic System for Large-Scale Reversible Computing, IEEE International Conference on Rebooting Computing, Dec. 2020.
  2. J. S. Friedman,Scalable Reversible Computing with Skyrmion Billiard Balls, CCC Workshop on Physics & Engineering Issues in Adiabatic/Reversible Classical Computing, Oct. 2020.
  3. M. Chauwin*, X. Hu*, F. Garcia-Sanchez, N. Betrabet, A. Paler, C. Moutafis, J. S. Friedman, Skyrmion Logic System for Large-Scale Reversible Computation, Physical Review Applied 12:6, 064053 (2019).
  4.   • Featured as Editors' Suggestion
  5. X. Hu, M. Chauwin, F. Garcia-Sanchez, N. Betrabet, C. Moutafis, J. S. Friedman, Cascaded Skyrmion Logic System Inspired by Conservative Logic, SPIE Spintronics, Aug. 2019 (invited).
  6. X. Hu, M. Chauwin, F. Garcia-Sanchez, N. Betrabet, C. Moutafis, J. S. Friedman, Cascaded Skyrmion Logic System Inspired by Conservative Logic, Joint IEEE International Magnetics Conference & Conference on Magnetism and Magnetic Materials, Jan. 2019.