Electric Diodes Unlock Control of Promising Silicon Qubits

In a recent study published in the journal Nature, researchers from Harvard John A. Paulson School of Engineering and Applied Sciences revealed defects in silicon, a common semiconductor material, that could be used to transmit and store quantum information over widely used telecommunications wavelengths.

Among all the potential choices to host qubits for quantum communications, are these silicon imperfections the most advantageous option?

It is still a Wild West out there, 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.

Evelyn Hu, Tarr-Coyne Professor, Applied Physics and Electrical Engineering, Harvard John A. Paulson School of Engineering and Applied Sciences

Hu and his colleagues have created a framework allowing them to study, work with, and manipulate these potentially strong quantum systems. The gadget manipulates qubits inside a commercial silicon wafer using a basic electric diode, one of the most widely used parts in semiconductor devices.

Using this device, the researchers were able to tailor the defect's wavelength within the telecoms band, investigate how it reacts to variations in the electric field, 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.

Aaron Day, Ph.D. Candidate, Harvard John A. Paulson School of Engineering and Applied Sciences

Day co-led the work with Madison Sutula, a research fellow at Harvard.

Although the study team employed this method to describe faults in silicon, it could also be applied as a control and diagnostic method for defects in other material systems.

Known by several names, such as color centers or quantum emitters, quantum defects are flaws in otherwise flawless crystal lattices that can capture individual electrons. When a laser strikes them, those electrons release certain wavelengths of photons.

G-centers and T-centers are the two types of silicon defects researchers are most interested in studying for quantum communications. The O-band, a wavelength frequently utilized in telecommunications, is produced when these imperfections trap electrons.

In this study, the group concentrated on G-center faults. The first thing they had to figure out was how to make them. G-center defects are created by adding atoms, specifically carbon, to the crystal lattice, as opposed to other forms of defects that need an atom to be removed. However, Hu, Day, and the other researchers discovered that the consistent formation of the defect also depends on the addition of hydrogen atoms.

Subsequently, the researchers used a novel technique to create electrical diodes that optimally sandwich the defect in the middle of each device without compromising the diode's or defect's functionality.

The production process can produce hundreds of devices containing embedded defects across a commercial wafer. After connecting the entire apparatus to generate an electric field or voltage, the researchers discovered that the flaws disappeared when a negative voltage was introduced across the apparatus.

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

Additionally, the researchers discovered that they could adjust the wavelengths that the defect was emitting by applying a local electric field. This discovery is significant for quantum networking since it requires the alignment of dispersed quantum systems.

The team also created a diagnostic tool to visualize how the millions of defects incorporated into the device change in space when an electric field is applied.

“We found that the way we are 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.

Aaron Day, Ph.D. Candidate, Harvard John A. Paulson School of Engineering and Applied Sciences

The team's next goal is to apply the same methods to comprehend silicon's T-center flaws.

Additional authors include Sutula, Jonathan R. Dietz, Alexander Raun from SEAS, and AWS research scientists Denis D. Sukachev and Mihir K. Bhaskar.

The research was funded 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.

Journal Reference:

Day, A. M., et al. (2024) Electrical manipulation of telecom color centers in silicon. Nature Communications. doi.org/10.1038/s41467-024-48968-w.