Demid Sychev, a postdoctoral researcher in Purdue University’s Elmore Family School of Electrical and Computer Engineering, is seen in the reflection of one of many mirrors in the nanophotonics laboratory of Vladimir Shalaev, Purdue’s Bob and Anne Burnett Distinguished Professor in Electrical and Computer Engineering. Sychev is the first author on a new paper demonstrating a ‘photonic transistor’. Credit: Purdue University/ Kevin Crisp.

Breakthrough achieves optical nonlinearity orders of magnitude higher than best-known materials, enabling terahertz-speed computers.

There are few technologies more fundamental to modern life than the ability to control light with precision. From fibre-optic communications to quantum sensors, the manipulation of photons underpins much of our digital infrastructure.

Yet one capability has remained frustratingly out of reach: controlling light with light itself at the most fundamental level using single photons to switch or modulate powerful optical beams.

Now, researchers at Purdue University have achieved this long-sought milestone, demonstrating what they call a ‘photonic transistor’ that operates at single-photon intensities.

Their findings report a nonlinear refractive index several orders of magnitude higher than the best-known materials, a leap that could finally make photonic computing practical.

Vladimir Shalaev, Purdue’s Bob and Anne Burnett Distinguished Professor in Electrical and Computer Engineering, said: “We demonstrated a way to realise a photonic transistor working at single-photon intensities.

“This was a long-standing problem, and we found a potential way of solving it.”

The achievement addresses a fundamental challenge in photonics: Traditional optical nonlinearity, where one beam of light affects another, requires enormous power levels.

First author of the paper, Demid Sychev, a postdoctoral researcher in Shalaev’s group in the Elmore Family School of Electrical and Computer Engineering, said: “Usually there is optical nonlinearity, which allows two beams to interact with each other, but typically, this interaction works only for macroscopic beams, for classical light, because the nonlinear refractive index is very small.

“This is a problem because this method cannot be used for single photons.”

Researchers at Purdue University have achieved a long-sought milestone, demonstrating what they call a ‘photonic transistor’ that operates at single-photon intensities. Credit: Purdue University/ Kevin Crisp.

Amplifying the quantum world

The solution came from an unexpected source: the avalanche multiplication process used in commercial single-photon detectors.

When a single photon strikes silicon and creates a single electron, that electron can trigger an avalanche that generates up to one million new electrons, a cascade that bridges the microscopic quantum world with macroscopic, measurable effects.

Sychev said: “This multiplication is a very powerful tool for connecting the microscopic quantum world with the macroscopic world.

“This principle was often used for single-photon detection, but what we did was apply this process to create a huge nonlinearity for optical beams, where one single-photon beam can control a huge macroscopic beam.”

Peigang Chen, a fourth-year PhD student in Shalaev’s group, noted the elegance of the approach: “When I first came to the group, I just thought this was a genius idea from Demid.

“In the future, we’re going to fabricate our own single-photon avalanche diodes (SPAD) for this specific design.

“But the easiest way for us to get this first result was to use a commercial SPAD.”

The device functions as an optical switch: a single photon in the control beam can modulate the properties of a much more powerful probe beam, effectively switching it on or off.

Three critical advantages

The Purdue team’s approach offers three key advantages over alternative methods that have been explored for single-photon nonlinearity.

First, it operates at room temperature. Sychev said: “Typically, what people use for single-photon nonlinearity these days are quantum systems where they use two-level systems, like a single-photon emitter coupled to a cavity.

“But this method is very sensitive to temperature; it cannot be applied at room temperature.”

Second, the technology is compatible with complementary metal-oxide-semiconductor, meaning it can be integrated into existing semiconductor manufacturing processes.

Chen said: “This is seamless and compact. For the others, it’s very different and complicated physics systems. This one is semiconductor, and it can always be fabricated on chip.”

Third, and perhaps most importantly, it operates at gigahertz speeds and could potentially reach hundreds of gigahertz, dramatically faster than existing approaches.

Sychev said: “Clock rates of such systems may go up to gigahertz, but with the methods we developed, in principle, it can be extended to hundreds of gigahertz.”

Applications: from quantum to classical

While the research has obvious applications in quantum computing, where it could increase the efficiency of generating single photons and enable faster quantum teleportation protocols, Sychev believes classical computing applications may be even more transformative.

He said: “The reason why a photonic computer is not realised is because the current approaches using photons are supposed to be much better. Photons consume less energy; they are faster.

“Ideally, from photons, you can get terahertz clock rates of CPUs, compared to currently existing 5 gigahertz in the best cases.

“But the problem is that there are no photonic switches like this. The needed interaction between photons typically requires high powers of optical light. With our method, in principle, you can do it with single photons.”

The implications extend beyond computing to data centres, optical communications and data transfer systems — anywhere that the speed and energy efficiency of photons could replace slower, more power-hungry electronics.

Chen said: “I feel like here I’m starting to change the world. This work really means a lot, not only for my career, but also because this device can make a difference in the industry and the science community.”

The findings are published in the journal Nature Nanotechnology.