Silicon Magic: Powering the Quantum Internet of the Future

The device uses a simple electrical diode to manipulate qubits inside a commercial silicon wafer. Credit: Second Bay Studios/Harvard SEAS

Using traditional semiconductor devices, researchers have unlocked new potential in quantum communication, pushing us closer to realizing the great potential of the quantum internet.

Building the quantum internet can be significantly simplified using existing telecommunications technologies and infrastructure. In recent years, researchers have identified defects in silicon—a widely used semiconductor material—that hold the potential for transmitting and storing quantum information across widespread telecommunications wavelengths. These silicon defects may just be the leading contenders to host qubits for efficient quantum communications.

Exploring quantum defects in silicon

“It’s still a Wild West out there,” said Evelyn Hu, the Tarr-Coyne Professor of Applied Physics and Electrical Engineering at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS). “Although new candidate defects are a promising quantum memory platform, often little is known about why certain recipes are used to create them and how you can rapidly characterize them and their interactions, even in ensembles. And ultimately, how can we adjust their behavior so that they exhibit identical characteristics? If we are ever going to make a technology out of this vast world of possibilities, we must have ways to characterize them better, faster, and more efficiently.”

Now, Hu and a team of researchers have developed a platform to probe, interact and control these potentially powerful quantum systems. The device uses a simple electrical diode, one of the most common components in semiconductor chips, to manipulate qubits inside a commercial silicon wafer. Using this device, the researchers were able to explore how the defect responds to changes in the electric field, adjust its wavelength within the telecommunications band, and even turn it on and off.

“If we’re ever going to make a technology out of this vast world of possibilities, we have to have ways to characterize them better, faster, and more efficiently.”

– Evelyn Hu, Tarr-Coyne Professor of Applied Physics and Electrical Engineering

Using defects for quantum communications

“One of the most exciting things about having these defects in silicon is that you can use well-understood devices like diodes in this known material to understand a whole new quantum system and do something new with it,” Aaron said. Day, a Ph. D. candidate at SEAS. Day co-led the work with Madison Sutula, a researcher at Harvard.

While the research team used this approach to characterize defects in silicon, it can be used as a diagnostic and screening tool for defects in other material systems.

The research was published in Nature Communications.

Quantum Emitters and Network Applications

Quantum defects, also known as color centers or quantum emitters, are imperfections in perfect crystal lattices that can trap single electrons. When those electrons are hit with a laser, they emit photons at specific wavelengths. The defects in silicon that researchers are most interested in for quantum communications are known as G-centers and T-centers. When these defects block electrons, the electrons emit photons at a wavelength called the O band, which is widely used in telecommunications.

In this research, the team focused on G-center defects. The first thing they had to figure out was how to make them. Unlike other types of defects, in which a atom removed from a crystal lattice, G-center defects are made by adding atoms to the lattice, especially carbon. But Hu, Day and the rest of the research team found that the addition of hydrogen atoms is also critical for the continued formation of the defect.

Development of tools for quantum networking

The researchers then fabricated electrical diodes using a new approach that optimally covers the defect at the center of each device without degrading the performance of either the defect or the diode. The fabrication method can create hundreds of devices with defects embedded on a commercial wafer. By connecting the entire device to apply a voltage or electric field, the team found that when a negative voltage was applied to the device, the defects turned off and darkened.

“Understanding when a change in the environment leads to a signal loss is important for engineering robust systems in networking applications,” said Day.

The researchers also found that by using a local electric field, they can tune the wavelengths emitted by the defect, which is important for quantum networks when different quantum systems need to be aligned.

The team also developed a diagnostic tool to image how the millions of defects embedded in the device change in space as the electric field is applied.

Future directions and commercial potential

“We found that the way we’re modifying the electrical environment for the defects has a spatial profile, and we can image that directly by looking at changes in the intensity of light emitted by the defects,” Day said. “By using so many emitters and getting statistics on their performance, we now have a good understanding of how bugs react to changes in their environment. We can use that information to inform how to build better environments for these bugs in future devices. We have a better understanding of what makes these bugs happy and unhappy.”

Next, the team aims to use the same techniques to understand T-center defects in silicon.

Reference: “Electrical Manipulation of Telecom Color Centers in Silicon” by Aaron M. Day, Madison Sutula, Jonathan R. Dietz, Alexander Raun, Denis D. Sukachev, Mihir K. Bhaskar, and Evelyn L. Hu, 3 June 2024, Nature Communications.
DOI: 10.1038/s41467-024-48968-w

The research was co-authored by Sutula, Jonathan R. Dietz, Alexander Raun of SEAS, and AWS research scientists Denis D. Sukachev and Mihir K. Bhaskar.

This work was supported by the AWS Center for Quantum Networking and the Harvard Quantum Initiative. Harvard’s Office of Technology Development has protected the intellectual property associated with this project and is pursuing commercialization opportunities.