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Optics and photonics

Optics and photonics

New optical transistor uses quasiparticle condensate to switch rapidly

28 Sep 2021
Inside the the Hybrid Photonics Labs at Skoltech
Fast switching: the Hybrid Photonics Labs at Skoltech where the new optical transistor was created. (Courtesy: Skoltech)

A new optical transistor has been designed by researchers in Russia, Switzerland, and Germany. The team, led by Anton Zasedatelev at Skoltech in Moscow, used a combination of laser beams, an optical cavity, and a specialized organic polymer to trigger sudden switching between two distinct quantum states in their device. The transistor could be a promising step towards advanced optical computers, which have the potential to outperform their electronic counterparts.

The transistors at the heart of modern technology work by switching currents of electrons on and off. As electrons flow through circuits, they dissipate heat. Getting rid of this waste heat is a significant challenge in modern chips containing vast numbers of tightly-packed transistors.

A more energy efficient avenue involves quantum optics technologies. These replace the role of electrons with photons, which dissipate much less heat than electrons. In their study, Zasedatelev’s team developed a new concept for an optical transistor, This features an organic semiconducting polymer, sandwiched between the highly reflective walls of a light-trapping microcavity. The researchers direct two pulsed laser beams at the material: a bright “pump” laser, and a far weaker “seed” laser, which delivers just a few photons per pulse.

Light–matter hybrids

As the pump pulses bounced repeatedly between the cavity walls, their intensity is boosted by a factor of up to 23,000. This results in a strong coupling between the laser photons and the polymer’s organic molecules, generating groups of quasiparticles called exciton-polaritons – quantum particles that are a hybrid of light and matter.

When the seed pulses are switched on, it stimulates the exciton-polaritons to make a sudden transition from a quantum state with the same energy as the pump beam, to a Bose-Einstein condensate. The latter is an exotic state of matter comprising many identical exciton-polaritons in a collective ground state.

By measuring the difference between the number of exciton-polaritons in their ground state both with and without the seed beam, Zasedatelev and colleagues could reliably detect light at the single-photon level. As a result, their device could rapidly and efficiently switch between two possible logic states – making for an ideal optical transistor.

Faster and less power

Compared with the latest electrical transistors, the device exhibited numerous advantages: it requires 10,000 times less power and operates at room temperatures. In addition, it could perform some 1 trillion operations per second – at least 100 times faster than the most advanced electrical transistors available today.

Although the commercial rollout of the technology is still some way off, Zasedatelev’s team expect that further improvements could soon be made by replacing the organic polymer with perovskite crystals, which enhance the coupling between light and matter. This could enable the use of less intense pump lasers, further reducing power consumption. More broadly, the researchers hope that their transistor could become part of a growing toolkit of optical components, suitable for a new generation of vastly superior optical computers.

The research is described in Nature.

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