The quantum internet would be a lot easier to build if we could use existing telecommuni­cations techno­logies and infra­structure. Over the past few years, researchers have discovered defects in silicon that could be used to send and store quantum information over widely-used telecommuni­cations wavelengths. Could these defects in silicon be the best choice among all the promising candidates to host qubits for quantum communi­cations? “It’s still a Wild West out there,” said Evelyn Hu from the Harvard John A. Paulson School of Engi­neering and Applied Sciences (SEAS). “Even though new candidate defects are a promising quantum memory platform, there is often almost nothing 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 fine-tune their behavior so they exhibit identical charac­teristics? If we are ever to make a technology out of this wide 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 with and control these potentially powerful quantum systems. The device uses a simple electric diode 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, tune its wavelength within the telecommuni­cations band and even turn it on and off. “One of the most exciting things about having these defects in silicon is that you can use well-understood devices like diodes in this familiar material to understand a whole new quantum system and do something new with it,” said Aaron Day, a Ph.D. candidate at SEAS. Day co-led the work with Madison Sutula, a research fellow at Harvard. While the research team used this approach to charac­terize defects in silicon, it could be used as a diagnostic and control tool for defects in other material systems.

Quantum defects – color centers or quantum emitters – are imperfections in otherwise perfect crystal lattices that can trap single electrons. When those electrons are hit with a laser, they emit photons in 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 trap electrons, the electrons emit photons in the O-band, which is widely used in telecommuni­cations. In this research, the team focused on G-center defects. The first thing they needed to figure out was how to make them. Unlike other types of defects, in which an atom is removed from a crystal lattice, G-center defects are made by adding atoms to the lattice, speci­fically carbon. But Hu, Day and the rest of the research team found that adding hydrogen atoms is also critical to consistently forming the defect.

Next, the researchers fabri­cated electrical diodes using a new approach which optimally sandwiches the defect at the center of every device without degrading the performance of either the defect or the diode. The fabrication method can create hundreds of devices with embedded defects across a commercial wafer. Hooking the whole device up to apply a voltage, or electric field, the team found that when a negative voltage was applied across the device, the defects turned off and went dark. “Under­standing when a change in environment leads to a loss of signal is important for engineering stable systems in networking appli­cations,” said Day.

The researchers also found that by using a local electric field, they could tune the wavelengths being emitted by the defect, which is important for quantum networking when disparate 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. “We found that the way we’re modifying the electric environment for the defects has a spatial profile, and we can image it directly by seeing the changes in the intensity of light being emitted by the defects,” said Day.

“By using so many emitters and getting statistics on their performance, we now have a good under­standing of how defects respond to changes in their environ­ment. We can use that information to inform how to build the best environments for these defects in future devices. We have a better understanding of what makes these defects happy and unhappy,” said Day. Next, the team aims to use the same techniques to understand the T-center defects in silicon. (Source: Harvard SEAS)

Reference: A. M. Day et al.: Electrical manipulation of telecom color centers in silicon, Nat. Commun. 15, 4722 (2024); DOI: 10.1038/s41467-024-48968-w

Link: Light-Matter Interaction, John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, USA