Max Planck and TU Munich researchers say development looks ideal for quantum-encrypted optical networking.

Researchers in southern Germany made a novel optical resonator that they say looks ideal for the construction of quantum networks.

The collaboration featuring the Max Planck Institute of Quantum Optics (MPQ) in Garching and Technical University of Munich (TUM) fabricated an erbium-doped silicon crystal, and found that it could emit single photons at a wavelength of 1536 nm.

Single-photon emitters are seen as a key component for quantum-encrypted optical networking, because the physical properties of a photon will always be altered if the link is intercepted – so any hack of the link will be seen.

They could also be used for future quantum networks, enabling calculations between multiple quantum computers.

Nanophotonic resonator
The development follows earlier work by the same team to embed individual erbium atoms in crystalline silicon, using a relatively low temperature of 500°C to ensure that large numbers of erbium atoms do not cluster together in the silicon lattice.

Detailing the latest experimental results in the journal Optica, lead author Andreas Reiserer and colleagues wrote that although the promise of individual erbium dopants in silicon has been recognized for quantum networking previously, their integration into optical resonators had not been demonstrated before.

“We have [now] demonstrated that single erbium dopants in silicon can be resolved, and that their emission can be enhanced using a nanophotonic resonator,” stated the team in its summary. “This offers great promise for the implementation of quantum networks over long distances.”

Together with known methods for achieving quantum entanglement, they believe that the latest advance would establish erbium dopants in silicon as a prime candidate for large-scale quantum computing and communication networks.

Laser excitation
In a release from MPQ, Reiserer and colleagues explained that their erbium-doped resonator was unlike conventional designs, in that it did not feature any mirrors.

Instead of mirrors, a regular pattern of nanometer-scale holes in the crystalline silicon are used. It means that the entire resonator measures only a few microns in length, and contains only a few dozen erbium atoms.

That nanophotonic structure was then coupled to an optical fiber to allow laser excitation of the ebrium atoms. Andreas Gritsch from the team explained: “In this way, we were able to accomplish the emission of individual photons with the desired characteristics.”

Reiserer commented: “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.

“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.”

Practical implementation
A further advantage of the new design is that it operates not just at absolute zero, but at the relatively high temperature – at least in the quantum world – of 8 K.

“And these few degrees make a big difference in practice,” Reiserer says. “Because such temperatures are technologically easy to achieve by cooling in a cryostat with liquid helium.”

That characteristic is expected to help pave the way towards real applications of the nanophotonic system. The team expects it to be of interest to financial institutions, medical facilities, or government agencies, where sensitive personal data or classified information is handled.

They explain: “While even the best encryption cannot guarantee complete security, a quantum network would offer perfect data protection: as soon as an eavesdropper tried to intercept the information transmitted by prepared photons, their quantum properties would be lost and the data would become unusable.”