In addition to direct power saving, the switch requires no cooling and operates at 1 trillion operations per second, it is between 100 and 1,000 times faster than today’s commercial transistors.
The device relies on two lasers to set its state to “0” or “1” and to switch between them. A very weak control laser beam is used to turn another, brighter laser beam on or off. It only takes a few photons in the control beam, hence the device’s high efficiency.
The switching occurs inside a microcavity — a 35-nanometer thin organic semiconducting polymer sandwiched between highly reflective inorganic structures. The microcavity is built in such a way as to keep incoming light trapped inside for as long as possible to favor its coupling with the cavity’s material.
This light-matter coupling forms the basis of the new device. When photons couple strongly to bound electron-hole pairs — aka excitons — in the cavity’s material, this gives rise to short-lived entities called exciton-polaritons, which are a kind of quasiparticles at the heart of the switch’s operation.
When the pump laser — the brighter one of the two — shines on the switch, this creates thousands of identical quasiparticles in the same location, forming so-called Bose-Einstein condensate, which encodes the “0” and “1” logic states of the device.
To switch between the two levels of the device, the team used a control laser pulse seeding the condensate shortly before the arrival of the pump laser pulse. As a result, it stimulates energy conversion from the pump laser, boosting the amount of quasiparticles at the condensate. The high amount of particles in there corresponds to the “1” state of the device.
The researchers used several tweaks to ensure low power consumption: First, efficient switching was aided by the vibrations of the semiconducting polymer’s molecules.
The trick was to match the energy gap between the pumped states and the condensate state to the energy of one particular molecular vibration in the polymer.
Second, the team managed to find the optimal wavelength to tune their laser to and implemented a new measurement scheme enabling single-shot condensate detection.
Third, the control laser seeding the condensate and its detection scheme were matched in a way that suppressed the noise from the device’s “background” emission.
These measures maximized the signal-to-noise level of the device and prevented an excess of energy from being absorbed by the microcavity, which would only serve to heat it up through molecular vibrations.