Researchers at Max Planck Institute of Quantum Optics (MPQ) and Technical University of Munich (TUM) demonstrated a potential platform for large-scale quantum computing and communication networks. Secure quantum networks are of interest to financial institutions, medical facilities, government agencies, and other organizations that handle personal data and classified information due to their much higher level of security. To create an environment that supported quantum computing, the researchers excited individual atoms of the rare-earth metal erbium. The excitation process caused the erbium atoms to emit single photons with properties suitable for the construction of quantum networks. A single dopant in a nanophotonic silicon chip is used to generate photons at a wavelength that is compatible with existing optical fiber infrastructure. Courtesy of C. Hohmann/Munich Center for Quantum Science and Technology. The researchers built an optical resonator to control the way in which the erbium atoms emitted photons. “In this way, an interface for sending or receiving quantum information can be created,” researcher Andreas Gritsch said. Instead of using mirrors to construct the resonator, the researchers used crystalline silicon and placed nanometer-size holes, arranged in a consistent manner, in the silicon. The nanophotonic, silicon resonator measured only a few microns and contained just a few dozen erbium atoms. The researchers coupled the crystalline silicon resonator to an optical fiber to allow laser light to enter the resonator and excite the atoms. “In this way, we were able to accomplish the emission of individual photons with the desired characteristics,” Gritsch said. The erbium atoms that were doped into the silicon crystal lattice were positioned in a way that made it possible for them to emit light at a wavelength of 1536 nm. This wavelength is almost identical to the wavelength used for data transmission in classical telecommunications, and it exhibits relatively low loss. Nanophotonic resonator on a silicon chip. Courtesy of Max Planck Institute of Quantum Optics. “This gives the erbium atoms excellent optical properties,” professor Andreas Reiserer, who led the research, said. The generation of single photons in the main band of optical communication, where loss in optical fibers is minimal, will allow for quantum information to be transmitted over long distances. By integrating erbium dopants into the nanophotonic silicon resonator, the researchers achieved spin-resolved excitation of individual photons emitting with less than a 0.1-GHz spectral diffusion linewidth. Upon resonant driving, the researchers observed optical Rabi oscillations and single-photon emission with a 78-fold Purcell enhancement. One advantage of using silicon to construct the resonator is that the manufacturing techniques and processes required for crystalline silicon are technically mature and established in the semiconductor industry. “The fact that this is possible in crystalline silicon offers an additional opportunity for the realization of quantum networks, because this material has been used for decades to produce classic semiconductor elements, for example microchips for computers, smartphones, or navigation devices,” Reiserer said. “This means that for quantum technology applications, such as the construction of quantum networks, silicon crystals can also be produced in high quality and purity — and quite cheaply, too.” The system developed by MPQ and TUM scientists also offers advantages in the production, cooling, and range of data transmission of the network nodes. Thanks to a special preparation, the erbium atoms embedded in silicon exhibit excellent optical properties at not only absolute zero temperature at minus 273 °C, but also at up to 8 °C above this temperature mark. “And these few degrees make a big difference in practice, because such temperatures are technologically easy to achieve by cooling in a cryostat with liquid helium,” Reiserer said. Andreas Gritsch at the cryostat, in which the silicon doped with erbium atoms is cooled down to a few degrees above absolute zero. Courtesy of Thorsten Naeser. The generation of qubits for the transport of quantum information will provide the basis for extended networks that link quantum systems with each other, ultimately forming a quantum internet that will allow information to be exchanged via fiber optic networks with provable privacy and security. “All quantum technologies are based on so-called qubits, the elementary carriers of quantum information,” Reiserer said. “If qubits are connected with each other by light, a quantum network can be created, similar to what is done today in the classical internet.” The controlled use of quantum physical phenomena, such as the entanglement of particles, will enable the construction of highly sensitive quantum sensors and fast quantum computers. This new technology will enable quantum computers to quickly complete tasks that cannot be accomplished with conventional computers. Today, even the best encryption processes cannot guarantee complete security. In future quantum networks, as soon as an eavesdropper tries to intercept the information transmitted by prepared photons, the quantum properties of the photons will be lost and the data will become unusable. The research was published in Optica (www.doi.org/10.1364/OPTICA.486167).