Paul van Gerven
8 November 2021

A new type of optical switching device only needs a single photon to change state. Unfortunately, it also ignores most incoming photons.

It’s still a long way from a light-based computer, but researchers from IBM Research and Skolkovo Institute of Science and Technology have at least provided a glimpse of what such systems could look like. They developed an ‘optical transistor’ that switches up to a thousand times faster than CMOS transistors, requiring only the energy of a single photon to change state.

The device consists of a 35-nanometer-thin sheet of an organic semiconducting polymer, sandwiched between two inorganic ‘mirrors.’ Light introduced inside this microcavity gets trapped, maximizing its interaction with the polymer.

This strong coupling between light and the organic material forms the basis of the device’s operation. When photons couple strongly to excitons (electron-hole pairs) in the semiconductor, it gives rise to the formation of exciton-polaritons. Clusters of these quasi-particles, in turn, form a so-called Bose-Einstein condensate, meaning they start behaving collectively. It’s this condensate that can switch between two measurable states, ie the 0 and the 1.

Attojoule

The difference between the two states is the energy of the quasi-particles in the condensate. One state corresponds to a large number of quasi-particles in the ground energy state, and the other to none or just a few quasiparticles in this state.

The researchers switch between states using two lasers, the so-called “pump” laser and a weaker “seed” laser. Shining the pump laser creates a condensate consisting of quasi-particles that are predominantly in higher energy states. To accomplish a switch, a seed laser pulse bumps a few quasi-particles to the ground state, triggering an avalanche that pushes most quasi-particles to the ground state as well.

The switching process is extremely quick but also requires surprisingly little energy. “What makes the new device so energy efficient is that it only takes a few photons to switch. In fact, in our Skoltech labs, we achieved switching with just one photon at room temperature,” Pavlos Lagoudakis of Skolkovo Institute of Science and Technology commented.

With one or a few photons, the energy needed to flip states is about 1 attojoule (10-18 joule). Modern CMOS transistors require at least ten times more. In research, transistors that work at attojoule energy levels using single electrons have been developed, but those aren’t nearly as fast and most don’t work at room temperature.

Single shot

Still, it’s far too early to start dreaming about optical computers. As Sebastian Klembt of Julius Maximilians University of Würzburg points out in a commentary, working with only a few photons or less poses limitations. As the number of photons in a pulse decreases, it becomes harder to detect them with 100 percent reliability. With a single photon, the chance of it going undetected approaches 90 percent. For an all-optical transistor, that would mean that a pulse sent to flip the device wouldn’t have the intended effect most of the time. The researchers had to fire hundreds of pulses to prove that a switching event requires a single photon on average.

“It remains to be seen whether fundamental limitations will ultimately prevent the detection of individual photons in a single-shot experiment using the authors’ technique,” Klembt writes. He’s cautiously optimistic, however, that progress can be made soon. “Given that the field of organic quantum optics is relatively young, and technological advances and methodological improvements are happening frequently, improved switching capabilities will probably emerge soon.”

Main image credit: Skoltech