ATAP researchers have used powerful laser pulses to create color centers in silicon, promising a new way of forming the building blocks for a range of quantum technologies.
An international team of scientists led by ATAP researchers has used intense ion pules to form tiny artificial defects in silicon crystals that could allow information to be encoded in quantum bits. The work could lay the foundations for new devices for applications in quantum information science (QIS), an emerging field that promises to transform security, computing, and communications.
To realize the promise of QIS, a practical and reliable way of storing and transmitting information using many interconnected quantum bits (qubits)—the fundamental building blocks of quantum technologies—must be found. However, designing qubits with the desired properties is very challenging.
“While there are many types of qubits being investigated, ranging from superconducting qubits to trapped-ion qubits,” explains Thomas Schenkel, a Senior Scientist who heads ATAP’s Fusion Science & Ion Beam Technology Program and led the research, “defects in silicon crystals are emerging as one of the most promising candidates for making qubits.”
These microscopic defects, called color centers, have long been known to form when silicon crystals are exposed to high-energy particles. Photon-emitting defects are attracting considerable attention because of their ability to connect photons and the spins states of electrons and nuclei. This ability makes them ideal candidates for quantum applications with single photon sources and for use in quantum networking—communication networks that securely interconnect quantum devices and systems.
Schenkel says that color centers in silicon offer significant advantages over other potential platforms for qubits. “For instance, they emit photons in the telecommunications band, which enables high-speed, low-loss optical transmission, and they offer high spectral stability and long coherence times, which are essential properties for applications in QIS and technology with increasingly complex numbers of qubits and interconnects.”
Silicon, he adds, is also the mainstream material of choice for standard complementary metal-oxide semiconductor (CMOS) manufacturing processes, which are very widely used in the semiconductor industry for making integrated circuits. This, he says, makes “it amenable to the large-scale integration required for fabricating quantum devices made from many (potentially millions) of interconnected qubits, such as quantum information processors and integrated quantum repeaters.”
Tailoring ion beams for creating color centers
Color centers can take different forms, such as W-centers, which are intrinsic defects in the crystal lattice comprised of three silicon interstitial atoms, G-centers, in which implanted pairs of carbon atoms bind to a silicon interstitial atom, and many others.
To create these color centers in silicon crystals, the researchers used the Berkeley Lab Laser Accelerator (BELLA) Center’s petawatt laser to form intense, high-powered laser pulses quadrillionths (a million-billionths) of a second in duration. They aimed these pulses at a microns-thick foil target, creating a dense plasma from the foil that released low-energy atoms. These atoms, initially implanted near the surface of the silicon, can diffuse into the silicon and form G-centers and other color centers following pre-heating by high-energy ions also emitted from the same laser-ion pulse.
“The intense ion pulses produced by the laser accelerator allow for simultaneous heating, doping, and color center formation,” explains Schenkel, “enabling new directions for creating and optimizing color centers with properties tailored for use in selected applications, such as quantum repeaters and quantum networking.”
He adds that this approach links to a US Department of Energy initiative to identify research that addresses the relationship of fusion energy sciences to QIS. In 2018, a roundtable on the subject identified six priority research opportunities (PROs), which included PRO 4 – High energy density laboratory plasmas science (HEDLP) for novel quantum materials and PRO 5 – Relativistic plasma science for qubit control and quantum communication.
Schenkel says that without access to a high-quality laser like the one at the BELLA Center, which can deliver petawatt-level pulses at repetition rates up to 1 Hz, it would not have been possible to generate a series of many low- and high-energy ions pulses that are needed to reliably form and place the color centers in the crystalline lattice. The BELLA laser, he noted, “is an amazing resource for the secondary radiation formed for laser-plasma interaction, and is also being explored for use in a range of other applications, including inertial fusion energy, nuclear physics, and radiation biology, and to study the effects of radiation on materials.”
The team then used a suite of materials analysis techniques, including electron and helium ion microscopy to study the surface morphology of the silicon, secondary ion mass spectrometry, nuclear reaction analysis, and channeling Rutherford backscattering to measure the atomic composition and depth profile in the samples, as well as low-temperature photoluminescence to observe the optical properties of the color centers.
These techniques “allowed us to characterize the resulting changes in the surface structure of the crystals, and to correlate the beam properties with the microstructure evolution of the color centers following exposure to the ion beam,” explains Schenkel.
The work demonstrates a novel and effective method of “laser-ion doping” of materials and for creating color centers using ion pulses from a laser accelerator for direct local defect engineering and high-flux doping of semiconductors like silicon. He says these color centers in silicon “can allow for the strong coupling of a single electron spin and a single photon and could pave the way for the development of spin-photon qubits for applications in quantum computing, communications, sensing, and imaging technologies.”
He added that the collaboration with Boubacar Kanté, Associate Professor of Electrical Engineering and Computer Sciences at the University of California, Berkeley, and Liang Tan, a Staff Scientist from the Molecular Foundry at Berkeley Lab, “enabled us to address critical aspects of color center theory, synthesis, and characterization, and was essential to completing the study.”
Having successfully demonstrated the technique using carbon ions, Schenkel says they are now working with other dopants, such as boron or transition metals. “We are using predictions from theory to investigate how the properties of color centers can be tailored for applications in QIS.”
ATAP Division Director Cameron Geddes said the research “is an excellent example of how researchers at Berkeley Lab, working with colleagues from renowned national and international institutions, are spearheading innovative and novel techniques that could usher in new quantum technologies and lead to advances in energy and nuclear research and materials science.”
Learn More
- Redjem, W., Amsellem, A. J., Allen, F. I., Benndorf, G., Bin. J., Bulanov, S., Esarey, E., Feldman, L. C., Fernandez, J. F., Lopez, J. G., Geulig, L., Geddes, C. R., Hijazi, H., Ji, Q., Ivanov, V., Kante, B., Gonsalves, A., Meijer, J., Nakamura, K., Persaud, A., Pong, I., Obst-Huebl, L., Seidl, P. A., Simoni, J., Schroeder, C., Steinke, S., Tan, L. Z., Wunderlich, R., Wynne, B., and Schenkel, T. “Defect engineering of silicon with ion pulses from laser acceleration,” Commun. Mater. 4, 22 (2023), https://doi.org/10.1038/s43246-023-00349-4
- Fusion Energy Sciences on Quantum Information Science 2018 roundtable report
