Joseph S. Friedman 

Reversible Skyrmion LogicReversible skyrmion logic leverages magnetic skyrmions in the first nanoscale realization of conservative logic, providing a vision for energyefficient computation. In this system, magnetic skyrmions propagate through a twodimensional ferromagnetic structure while performing reversible logic operations at the gate junctions. A simple global clock enables direct cascading with the potential for scalable highspeed lowpower reversible Boolean and quantum computing.Magnetic Skyrmion TrackMagnetic 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, +ydirected electrical current creates a spinHall force that pushes the skyrmion in the +ydirection, while also creating a skyrmionHall force that pushes the skyrmion in the xdirection. The skyrmion is placed within a track structure that suppresses the lateral skyrmionHall effect and forces the skyrmion to travel along the track.Reversible Logic GatesConservative logic envisions reversible computing with zero energy dissipation, and was originally proposed with an analogy to billiard balls. Though not dissipationfree, this reversible skyrmion logic system replaces the billiard balls with magnetic skyrmions in the first nanoscale realization of the conservative logic concept.The skyrmionHall effect and skyrmionskyrmion repulsion can be used together to perform reversible logic operations at the junction between two ferromagnetic tracks. The simplest conservative logic gate is the ANDOR gate, which conserves the magnetic skyrmions while computing the AND and OR functions of inputs A and B. The spinHall effect continuously pushes the skyrmions in the +ydirection, and the skyrmionHall 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, skyrmionskyrmion 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 INVCOPY gate which can generate any logical function when coupled with the ANDOR 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 ComputingTo 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 onebit full adder, in which seven notches are used to compute the sum and caryout signals in three clock cycles. Thanks to the synchronizing notches, this reversible logic circuit structure can be scaled up to large computing systems. The nonvolatility 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
