A sub-femtojoule electrical spin switch based on liquid light

Abstract

Light travels fast, which is why nowadays all of our communications involve optical fibers. But our computations are based on matter, specifically, electrons that move inside wires and transistors. The problem is that electrons interact with matter, thus causing heat. To minimize heat and to squeeze more transistors onto chips, we have made them smaller and smaller to keep up with Moore’s law (the observation that the number of transistors in CPUs—central processing units—doubles every two years). It seems, however, that that we are about to hit a hard wall. When we make the wires very thin and our transistors very small, quantum mechanical interference ruins the signals. Consequently, large technology companies like Intel and IBM are trying new ways of using optical interconnects between separate chips or even integrated inside chips. The idea here is that light does not produce as much heat as electronics do, and it can be 100 times faster. The bottleneck is the conversion between electronics and optics. The Holy Grail for optical computing is a switch that can convert electrical signals to optical signals quickly and efficiently and can be integrated inside chips. Our group has recently demonstrated an ultra-low-energy spin switch based on a ‘liquid-light’ exciton-polariton condensate.1 These condensates are half-matter, half-light. Using their matter properties, we can electronically control them and take advantage of their fast dynamics (because they are half-light). It turns out that, similar to field-effect transistors (FETs), we can switch the polarization of liquid lights with minuscule amounts of energy and, because they are micrometer size, they can be integrated into chips as well. Exciton-polaritons (polaritons) are a superposition of photons in a Fabry-Pérot microcavity and confined excitons (typically in 2D quantum wells).2 They are very light (100,000 times lighter than electrons) and very fast (>100GHz) thanks to their photonic component, but they can also strongly interact with each Figure 1. (a) Electrically controlled polariton spin switch. (b) Trapped condensate of polaritons (yellow emission) forms by nonresonant excitation (with blue lasers) of the microcavity. V ̇: Voltage.