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REM Atoms and Nanophotonic Resonator Offer Path to Quantum Networks

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GARCHING, Germany, June 29, 2023 — 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, MCQST.
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 silicium (silicon) chip. Courtesy of the Max Planck Institute of Quantum Optics.
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.
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).

Published: June 2023
Glossary
quantum
The term quantum refers to the fundamental unit or discrete amount of a physical quantity involved in interactions at the atomic and subatomic scales. It originates from quantum theory, a branch of physics that emerged in the early 20th century to explain phenomena observed on very small scales, where classical physics fails to provide accurate explanations. In the context of quantum theory, several key concepts are associated with the term quantum: Quantum mechanics: This is the branch of...
nano
An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
nanophotonics
Nanophotonics is a branch of science and technology that explores the behavior of light on the nanometer scale, typically at dimensions smaller than the wavelength of light. It involves the study and manipulation of light using nanoscale structures and materials, often at dimensions comparable to or smaller than the wavelength of the light being manipulated. Aspects and applications of nanophotonics include: Nanoscale optical components: Nanophotonics involves the design and fabrication of...
quantum entanglement
Quantum entanglement is a phenomenon in quantum mechanics where two or more particles become correlated to such an extent that the state of one particle instantly influences the state of the other(s), regardless of the distance separating them. This means that the properties of each particle, such as position, momentum, spin, or polarization, are interdependent in a way that classical physics cannot explain. When particles become entangled, their individual quantum states become inseparable,...
integrated photonics
Integrated photonics is a field of study and technology that involves the integration of optical components, such as lasers, modulators, detectors, and waveguides, on a single chip or substrate. The goal of integrated photonics is to miniaturize and consolidate optical elements in a manner similar to the integration of electronic components on a microchip in traditional integrated circuits. Key aspects of integrated photonics include: Miniaturization: Integrated photonics aims to...
nanopositioning
Nanopositioning refers to the precise and controlled movement or manipulation of objects or components at the nanometer scale. This technology enables the positioning of objects with extremely high accuracy and resolution, typically in the range of nanometers or even sub-nanometer levels. Nanopositioning systems are employed in various scientific, industrial, and research applications where ultra-precise positioning is required. Key features and aspects of nanopositioning include: Small...
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