In an advancement toward a single photon source for use in quantum computing and communications devices, researchers at the Massachusetts Institute of Technology (MIT) and ETH Zurich showed that individual perovskite quantum dots could be used as a source for individual photons with precisely known and consistent properties, including wavelength. Each photon produced would be indistinguishable from the others, allowing a pair of photons to interact in a phenomenon known as quantum interference. A scanning transmission electron microscope image (STEM) of single perovskite quantum dots. A new study shows that single perovskite quantum dots could be a fundamental building block for quantum-photonic technologies for computing and communications. Courtesy of H. Utzat et al./MIT and ETH Zurich. “This quantum interference between different indistinguishable single photons is the basis of many optical quantum information technologies using single photons as information carriers,” said researcher Hendrik Utzat. “But it only works if the photons are coherent, meaning they preserve their quantum states for a sufficiently long time.” The team demonstrated that individual colloidal lead halide perovskite quantum dots (PQDs) could display highly efficient single-photon emission with optical coherence times as long as 80 picoseconds (ps). The researchers found that the perovskites could emit photons very quickly after being stimulated by a laser beam, and that the perovskite quantum dots had very little interaction with their surroundings, enhancing their coherence properties and stability. Previous chemically made colloidal quantum dot materials had impractically short coherence times, but this team found that making the quantum dots from perovskites produced coherence levels that were more than 1000 times better than previous versions. According to the researchers, the coherence properties of these colloidal perovskite quantum dots can approach the levels of established emitters, such as atom-like defects in diamond or quantum dots grown using gas-phase beam epitaxy. “Without having a source of coherent single photons, you can’t use any of the quantum effects that are the foundation of optical quantum information manipulation,” said professor Moungi Bawendi. Another important quantum effect that can be harnessed by having coherent photons, Bawendi said, is entanglement, in which two photons essentially behave as if they were one. The next step for the researchers will be to work on optimizing and improving the performance of their quantum dots to make them scalable and practical. Specifically, they will work toward achieving 100 percent indistinguishability in the photons produced by the quantum dots. So far, they have reached 20 percent. “Perovskite quantum dots still have a long way to go until they become applicable in real applications,” Utzat said, “but this is a new materials system available for quantum photonics that can now be optimized and potentially integrated with devices.” These results could be a starting point for the design of lead halide perovskite-based quantum emitters with fast emission, wide spectral tunability, and scalable production. “Our study is very fundamental,” Bawendi said. “However, it’s a big step toward developing a new material platform that is promising.” The research was published in Science (https://doi.org/10.1126/science.aau7392).