Purdue’s single-photon switch promises ‘terahertz-speed’ photonic computing...

25 Nov 2025

...and KAIST’s efficient quantum process tomography enables scalable optical quantum computing.

There are few technologies more fundamental to modern life than the ability to control light with precision. From fiber-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, published in a paper in Nature Nanotechnology, describe a nonlinear refractive index several orders of magnitude higher than the best-known materials, a leap that could finally make photonic computing practical.

“We demonstrated a way to realize a photonic transistor working at single-photon intensities,” said Vladimir Shalaev, Purdue’s Bob and Anne Burnett Distinguished Professor in Electrical and Computer Engineering. “This was a long-standing problem, and we found a potential way of solving it.”

“Usually there is optical nonlinearity, which allows two beams to interact with each other,” said Demid Sychev, a postdoctoral researcher in Shalaev’s group in the Elmore Family School of Electrical and Computer Engineering and the first author of this paper. “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.”

Amplifying 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 1 million new electrons, a cascade that bridges the microscopic quantum world with macroscopic, measurable effects.

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

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. “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,” Sychev said. “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. “This is seamless and compact,” said Peigang Chen, a fourth-year PhD student working in the group. “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. “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,” Sychev said.

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

“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 centers, optical communications and data transfer systems — anywhere that the speed and energy efficiency of photons could replace slower, more power-hungry electronics. “I feel like here I’m starting to change the world,” Chen said. “This work really means a lot because this device can make a difference in the industry and the science community.”

Quantum tomography for scalable optical quantum computing

Optical quantum computers are gaining attention as a next-generation computing technology with high speed and scalability. However, accurately characterizing complex optical processes, where multiple optical modes interact to generate quantum entanglement, has been considered an extremely challenging task.

Now a research team at KAIST (Korea Advanced Institute of Science & Technology) has overcome this limitation, developing a highly efficient technique that enables complete characterization of complex multimode quantum operations in experiment. This technology, which can analyze large-scale operations with less data, represents an important step toward scalable quantum computing and quantum communication technologies.

KAIST announced on November 17th that a research team led by Professor Young-Sik Ra from the Department of Physics has developed a Multimode Quantum Process Tomography technique capable of efficiently identifying the characteristics of second-order nonlinear optical quantum processes that are essential for optical quantum computing. The work is described in a paper in Nature Photonics.

New mathematical framework

Inside a quantum computer, multiple optical modes interact in a highly complex and entangled way. The research team has introduced a new mathematical framework that precisely describes multimode second-order nonlinear optical quantum processes.

This method analyzes how input states change under a given operation using two key components: the 'Amplification matrix,' which describes how the mean fields of light are transformed, and the 'Noise matrix,' which captures the noise or loss introduced through environmental interactions.

Together, these components create a 'quantum state map' that enables accurate and simultaneous observation of both the ideal quantum evolution of light and the unavoidable noise present in real devices. This leads to a much more realistic characterization of how an optical quantum computer actually operates.

To determine how a quantum operation works, the research team input several types of quantum states and observed how the outputs changed. They then applied a statistical method known as Maximum Likelihood Estimation to reconstruct the internal operation that most accurately explains the collected data while satisfying the necessary physical conditions.

Using this approach, the research team dramatically reduced the amount of measurement data required. Whereas existing methods quickly become impractical—requiring enormous datasets even for systems with slightly more than a few modes and typically limiting analysis to about five modes—the new technique overcomes this bottleneck. The team successfully performed the world’s first experimental characterization of a large-scale optical quantum operation involving 16 modes, an unprecedented milestone in the field.

Professor Young-Sik Ra said, “This research significantly increases the efficiency of Quantum Process Tomography, a foundational technology essential for quantum computing. The acquired technology will greatly contribute to enhancing the scalability and reliability of various quantum technologies, including quantum computing, quantum communication, and quantum sensing.”