New quantum light emitter that paves the way for the quantum internet

The prospect of a quantum Internet connecting quantum computers capable of transmitting data in a highly secure manner is tantalizing; however, there are significant challenges to achieving this goal. Indeed, transmitting quantum information requires the use of individual photons, rather than the light sources used in traditional fiber-optic networks.

To generate and manipulate individual photons, scientists are turning to quantum light emitters, also known as color centers. These atomic-level defects in semiconductor materials emit single photons of a fixed wavelength or color and allow the photons to interact with electron spin properties in a controlled manner.

Recently, a team of researchers at Lawrence Berkeley National Laboratory has deepened our understanding of the process of quantum emitter formation by demonstrating a more efficient technique for creating quantum emitters using pulsed ion beams. The work was led by Department of Energy Lawrence Berkeley National Laboratory (Berkeley Lab) researchers Thomas Schenkel, Liang Tan, and Boubacar Kanté, who is also an associate professor of electrical engineering and computer science at UC Berkeley.

The results were published in Physical Review Applied.

Schenkel, a senior scientist in Berkeley Lab's Accelerator Technology and Applied Physics (ATAP) division, explains, "The color cores we're fabricating are expected to be the backbone of the quantum Internet and a key resource for scalable quantum information processing. They can support connecting quantum computing nodes and enabling scalable quantum computing."

In this work, the team's goal is to create a specific type of color core in silicon that consists of two displaced carbon atoms and a slightly displaced silicon atom. The traditional way to produce these defects is to hit the silicon with a continuous beam of high-energy ions; however, the researchers found that a pulsed ion beam is significantly more efficient and produces more of the desired color centers.

Left: A model of the atomic structure of a quantum luminescent defect in silicon (gray), consisting of two displaced carbon atoms (black) and a silicon gap atom (pink). The size of the quantum emitter is about 1 nm (billionth of a meter). RIGHT: Spectra of the quantum emitter show that the silicon crystal emits more intense light when irradiated with strong pulses (black) of high-flux protons compared to the conventional long-duration, low-flux proton irradiation method (blue).

"We were surprised to find that pulsed ion beams are more likely to produce these defects; currently, continuous ion beams are predominantly used in industry and academia, but we have demonstrated a more efficient method." said Wei Liu, an ATAP postdoctoral fellow and first author of the paper.

The researchers believe that the transient excitation (rapid changes in temperature and system energy) produced by pulsed beams is the key to more efficient color-center formation, which they established through an earlier study using laser-driven gas pedal pulsed ion beams published in Communications Materials.

(a) Shift in ZPL transition energy versus the distance between the GCB and a perturbed defect, where the defect is either a vacancy (blue circle) or a silicon self-gap (red cross). (b), (c) Histograms of the ZPL transition due to (b) vacancies and (c) self-gaps in the green box-marked region in (a).

The team used a highly sensitive near-infrared detector to probe the optical signals of the color centers, which allowed them to determine the characteristics of the centers at low temperatures. They found that the intensity of the ion beams used to generate the color centers altered the optical properties of the photons they emitted.

Large-scale computer simulations performed on the Perlmutter system at the National Energy Research Scientific Computing Center (NERSC) further revealed this finding, finding that the wavelengths of the emitted photons are sensitive to strains in the lattice.

"First-principles electronic structure calculations have become the method of choice for understanding defect properties, and we have reached the point where we can predict defect behavior, even in complex environments." Vsevolod Ivanov, co-first author of the paper, added.

The findings also suggest new applications for quantum emitter color centers as radiation sensors.

Tan, a scientist at Berkeley Lab's Molecular Foundry, said, "This opens up new directions: we can form such color centers simply by hitting silicon with protons. We could potentially use this as a dark matter or neutrino detector with directionality, because we could see these different strain fields based on the direction of the radiation."

With a deeper understanding of the formation and properties of quantum emitters, the team continues to expand its exploration of color cores. Ongoing work includes generating a database of color centers predicted to exist in silicon, using computer simulations to determine the most suitable color centers for quantum computing and networking applications, and improving fabrication techniques to gain deterministic control over the creation of individual color centers.

Even more, the research team said, "We are working to realize a new design paradigm for quantum bits. Can we reliably create specific color cores that operate in the telecom band, are sufficiently bright, are not too difficult to fabricate, and have properties such as memory? We are exploring this and making some exciting progress."