A team of researchers at the MPQ has pioneered the integration of erbium atoms with specific optical characteristics into a silicon crystal. The atoms may therefore be joined by light at a wavelength that is routinely utilised in telecommunications. This makes them perfect building elements for future quantum networks that allow computations with many quantum computers, as well as the safe flow of data in a quantum internet. Since the new experimental findings were accomplished without complex cooling and are based on proven techniques of semiconductor fabrication, the technology looks appropriate for huge networks.
When quantum computers are joined to a network, totally new possibilities arise—analogous to the internet consisting of interconnected classical computers. Such a quantum network may be constructed by entangling individual carriers of quantum information, so-called qubits, with each other using light.
The qubits, in turn, may be created from individual atoms that are segregated from one another and placed in a host crystal. A team of researchers from the Max Planck Institute of Quantum Optics (MPQ) in Garching and the Technical University of Munich has now established a possible approach to create a quantum network utilising atoms in a silicon crystal. This implies that the same technology utilised in classical computers may also be used for quantum computers and their networks.
Low losses and good coherence
The new approach depends on erbium atoms that are inserted into the crystal lattice of silicon under highly precise circumstances. “We knew from past investigations that erbium had excellent optical qualities for such an application,” explains Dr Andreas Reiserer, leader of the Otto Hahn Quantum Networks research group at MPQ: The atoms of this rare-earth metal produce infrared light at a wavelength of roughly 1550 nanometers—the spectral range employed for data transit in optical fibre cables. It displays only little loss during propagation in a light-conducting fibre.
“In addition, the light produced by erbium has outstanding coherence,” Reiserer says. This implies that individual wave trains are in a stable phase connection to one another—a necessity for the storing and transmission of quantum information. “These properties make erbium an ideal option for constructing a quantum computer—or for being employed as an information carrier in a quantum network,” Reiserer explains.
However, what may appear easy, was a tough technical barrier for the MPQ team. Among other things, the team had to embed individual atoms of the rare-earth element in the crystalline matrix in a targeted and predictable manner—and set them in certain places relative to the silicon atoms. “We picked silicon for this since it is already used for classical semiconductors that constitute the backbone of our information society,” says the researcher. “Established techniques are available for the preparation of silicon crystals of high grade and purity.”
Moderate temperatures, thin spectral lines
To incorporate erbium atoms into such a crystal—in technical lingo, to dope it—it first had to be supplied with nanometer-fine structures. They act as light-conducting elements. Then, the researchers bombarded the silicon with a stream of erbium ions such that individual atoms penetrated and scattered to various regions at high temperatures. “In contrast to the customary process, we did not heat the chips to 1,000, but only to a maximum of 500 degrees Celsius,” explains Andreas Gritsch, a doctorate student in the team.
The upshot of the comparably modest temperature was a very stable integration of individual erbium atoms in the crystal lattice, without a higher number of atoms grouping together. “This expressed itself as extraordinarily thin spectral lines in the emission of infrared light by the erbium,” writes Gritsch: at roughly 10 kilohertz, which is the shortest spectral linewidth reported in nanostructures to date. “This is also a positive characteristic for the creation of a quantum network,” the researcher notes. And there is another feature that distinguishes the method optimised by the Garching researchers for doping the silicon crystal: The excellent optical properties of the introduced erbium atoms do not only show up near absolute zero at minus 273 degrees Celsius as in previous experiments.
Instead, they may also be detected at temperatures deemed “high” for quantum phenomena of roughly 8 Kelvin (degrees above absolute zero) (degrees above absolute zero). “Such a temperature may be attained by chilling in a cryostat using liquid helium,” explains Andreas Reiserer. “This is technologically straightforward to accomplish and opens the path towards future uses.”
Diverse possible uses
The variety of probable future uses for quantum networks is extensive. Quantum computers might be developed from them, in which a vast number of independent processors are linked. With such computing devices, which employ specific quantum mechanical processes, complicated tasks may be mastered that cannot be accomplished with ordinary, classical systems. Alternatively, quantum networks might be utilised to examine the characteristics of new kinds of materials.
“Or they may be used to establish a type of quantum internet in which heretofore unreachable volumes of information could be transmitted—similar to the standard internet, but secured securely using quantum cryptography,” explains Reiserer.
The precondition for all these prospective uses is to quantum-mechanically entangle qubits in a network. “To prove that this is also doable based on erbium atoms in silicon chips is our next job,” says Andreas Reiserer.
Together with his team, the physicist is already working on conquering this issue. His goal: To prove that the circuits for strong quantum networks may be created similarly to microchips for mobile phones or laptop computers—but clear a vast field for new scientific breakthroughs and technological possibilities that are inconceivable today.
