Researchers have found that isotopic purity is the 'missing link' for making silicon a world-class quantum material.
In the strange world of quantum physics, even the tiniest tweak can unlock outsized rewards. In a new study, scientists have shown that simply swapping one type of hydrogen atom for a slightly heavier version inside silicon can make it dramatically better at producing single photons.
This may sound like a minor chemical adjustment, but it could have major consequences for quantum computers and ultra-secure communication networks. “Efficient single-photon emitters are desirable for quantum technologies, including quantum networks and photonic quantum computers,” the study authors note.The study challenges the long-held belief that silicon is an inefficient host for quantum light sources. Instead, it shows that silicon, which is already the backbone of modern electronics, may also power the quantum internet of the future.Creating the perfect defectAt the center of this discovery is a tiny imperfection in silicon known as the T center. A color center is a small defect in a crystal lattice—in this case, two carbon atoms and one hydrogen atom embedded inside silicon. When energized, this defect can emit a single photon, which is exactly what quantum technologies need. The T center is particularly attractive because it emits light in the same wavelength band used by fiber-optic internet cables . This means it could connect directly to today’s communication infrastructure. However, there has been a problem. The T center sometimes loses its energy without emitting light. Instead of releasing a photon, it dissipates energy as vibration—a process called nonradiative decay. Scientists know this happens, but they don’t understand why or how to stop it. The researchers decided to find an answer. Their study began with isotopes. “The T center, which consists of two carbon atoms and a hydrogen atom in the silicon lattice, can be produced in different isotopic forms. For example, the hydrogen can be either the common, lighter isotope or the rarer, heavier isotope ,” Moein Kazemi, one of the lead researchers, told Phys.org.Since deuterium is heavier, it changes how atoms vibrate inside the crystal. To investigate this effect carefully, the study authors first needed exceptionally pure silicon. Their collaborators in Germany grew high-purity silicon crystals originally developed for the Avogadro project, which aimed to redefine the kilogram using nearly perfect silicon spheres. These ultra-clean samples were ideal for studying delicate quantum properties.The researchers then created T centers by irradiating the silicon with high-energy particles. After irradiation, they carefully heated and cooled the samples to allow the defects to form correctly.They prepared three types of samples. One with natural hydrogen , the second deliberately infused with deuterium, so the heavier isotope dominated, and a third one enriched with carbon-13, creating different carbon isotope configurations.To clearly observe subtle differences between these variants, the samples were cooled to below 4 Kelvin using liquid helium. At such low temperatures, atomic vibrations slow dramatically, making quantum effects easier to measure.Watching vibrations steal lightWith the samples prepared, the team used photoluminescence spectroscopy and a Fourier transform infrared spectrometer to identify each isotopic variant’s emission lines. These measurements allowed them to directly observe vibrational modes inside the defect.They found that replacing hydrogen with deuterium lowered the energy of the carbon-hydrogen bond vibration. This seemingly small change turned out to be crucial. Lower vibrational energy suppresses the unwanted decay pathway that drains energy without producing light.To measure how long each T center remained excited before emitting a photon, the team used pulsed resonant laser excitation. By tuning the laser precisely, they could target one isotopic variant at a time. Photon arrival times were recorded using time-resolved single-photon detectors.The results were interesting. The excited-state lifetime of the deuterated T center was 5.4 times longer than that of the common protium version. In fact, its lifetime was nearly what one would expect if nonradiative decay did not occur at all. Moreover, Initial estimates suggest the deuterated T center could exceed 90% efficiency—and possibly even reach above 98%. This enormous difference revealed what the researchers call a giant isotope effect. It showed that energy loss is strongly linked to vibrations of the local C–H bond. “Our collaborators from the U.S. Naval Research Lab, Mark Turiansky and John Lyons, modeled this decay process and found that the standard ‘accepting mode’ approach for modeling vibrational decay completely fails in this case,” Daniel Higginbottom, one of the study authors, said.“We show that a very simple alternative ansatz, considering only the C-H stretch mode, matches the experiment quite well and reproduces the strong isotope dependence,” Higginbottom added. A heavier atom, a lighter path to the quantum internetThe heavier isotope also improved what is known as optical cyclicity—the number of times the system can be excited and emit light before it must be reset. For instance, the study authors estimate that the deuterated T center can be optically cycled roughly 300 times more than the protium version. This makes “single-shot readout of the electron spin feasible and could speed up quantum operations on T centers,” Higginbottom said.For many years, silicon color centers were largely overlooked because they were thought to be inefficient compared to defects in materials like diamond. This study provides some of the strongest evidence yet that silicon can host highly efficient single-photon emitters. Since T centers naturally emit in the telecom O-band, they are well-suited for distributing quantum information over tens of kilometers of existing optical fiber. Interestingly, Photonic Inc, a quantum technology company that was also involved in the research, has already begun incorporating the deuterated T center into its development pipeline, demonstrating how quickly fundamental research can move toward practical technology.However, this doesn’t mean the research part is over. “As a next step, we are carrying out a comprehensive study of the fundamental vibrational modes across all possible isotopic variants of the T center. These measurements will allow us to more precisely understand how the color center’s vibrational structure affects its optical properties,” Kazemi saidThe study is published in the journal Physical Review Letters.
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