Erbium atoms in silicon: A prime candidate for quantum networks

A group of researchers at MPQ pioneered the incorporation of erbium atoms with special optical properties into a silicon crystal. Atoms can thus connect with light at a wavelength commonly used in telecommunications. This makes them ideal building blocks for future quantum networks that enable calculations with multiple quantum computers, as well as the secure exchange of data in a quantum internet. Since the new experimental results were achieved without sophisticated cooling and are based on established semiconductor fabrication methods, the method appears suitable for large networks.

When quantum computers are connected to a network, entirely new possibilities emerge — analogous to the internet of interconnected classical computers. Such a quantum network can be realized by entangling individual quantum information carriers, so-called qubits, with each other using light.

Qubits, in turn, can be made of individual atoms that are isolated from each other and embedded in a host crystal. A team of researchers at the Max Planck Institute for Quantum Optics (MPQ) in Garching and the Technical University of Munich has now demonstrated a feasible way to build a quantum network using atoms in a silicon crystal. This means that the same technology used in classical computers can also be used for quantum computers and their networks.

Their work is published in Physical Review X.

Low losses and strong cohesion

The new technology is based on erbium atoms implanted into the crystal lattice of silicon under very specific conditions. “We knew from previous experiments that erbium has good optical properties for such an application,” says Dr. Andreas Reiserer, head of the Otto Hahn Quantum Networks research group at MPQ: Atoms of this rare earth element emit infrared light at a wavelength about 1550 nanometers—the spectral range used to carry data over fiber optic cables. It exhibits only small propagation loss in a light-conducting fiber.

“Additionally, the light emitted by erbium has excellent coherence,” notes Reiserer. This means that the individual wave trains are in a stable phase relationship with each other – a prerequisite for the storage and transmission of quantum information. “These characteristics make erbium a prime candidate for realizing a quantum computer—or for use as an information carrier in a quantum network,” says Reiserer.

However, what may sound simple, posed a difficult technological challenge for the MPQ researchers. Among other things, the team had to embed individual atoms of the rare earth element into the crystal matrix in a targeted and reproducible way – and fix them in specific positions relative to the silicon atoms. “We chose silicon for this because it is already used for classical semiconductors that form the basis of our information society,” explains the physicist. “Established processes are available for the preparation of silicon crystals of high quality and purity.”

Moderate temperatures, narrow spectral lines

To incorporate erbium atoms into such a crystal—in technical terms, to impregnate it—they first had to be endowed with fine nanometer structures. They serve as photoconductive elements. The researchers then irradiated the silicon with a beam of erbium ions so that individual atoms penetrated and scattered at different points at high temperatures. “Unlike the usual process, we did not heat the chips to 1,000, but only to a maximum of 500 degrees Celsius,” says Andreas Gritsch, a PhD student in the team.

The consequence of the relatively moderate temperature was a particularly stable incorporation of individual erbium atoms into the crystal lattice, without larger numbers of atoms clustering together. “This manifested itself in unusually narrow spectral lines in the infrared emission from erbium,” reports Gritsch: at about 10 kilohertz, which is the smallest spectral line width measured in nanostructures to date. “This is also a favorable property for building a quantum network,” says the researcher.

And there is another feature that distinguishes the method optimized by the Garching researchers for doping the silicon crystal: The excellent optical properties of the introduced erbium atoms do not appear only in the immediate vicinity of absolute zero at minus 273 degrees Celsius as in previous experiments .

Conversely, they can also be observed at temperatures considered “high” for quantum effects around 8 Kelvin (degrees above absolute zero). “Such a temperature can be achieved by cooling in a cryostat with liquid helium,” says Andreas Reiserer. “This is technologically easy to do and paves the way for future applications.”

Different possible applications

The range of possible future applications for quantum networks is wide. From them, quantum computers could be built, in which a large number of separate processors are interconnected. With such computing machines, which use certain quantum mechanical effects, complex tasks that cannot be solved by conventional, classical systems can be mastered. Alternatively, quantum networks could be used to investigate the properties of new types of materials.

“Or they could be used to build a kind of quantum Internet in which previously inaccessible amounts of information could be transmitted – similar to the regular Internet, but securely encrypted using quantum cryptography,” says Reiserer.

The prerequisite for all these possible applications is the quantum mechanical entanglement of qubits in a network. “Showing that this is also possible based on erbium atoms on a silicon chip is our next task,” says Andreas Reiserer.

Together with his team, the physicist is already working to conquer this challenge. His goal: To show that circuits for powerful quantum networks can be produced similar to microchips for cell phones or laptops—but to open a wide field for new scientific findings and technical possibilities that are unimaginable today.