Next step towards quantum “mechanical” networks


Future communication networks will be increasingly based on the principles of quantum physics. The long-term vision is a “quantum internet” with radically enhanced security, efficiency and computational capabilities. One of its technological cornerstones is the ability to reliably establish quantum entanglement over long distances. Researchers at the University of Vienna and the TU Delft have now shown such distributed entanglement in an architecture that is based solely on circuits of light and mechanical motion. This is the first purely chip-based backbone for realistic quantum networks that is fully compatible with the telecom-standard of silicon photonics. Their findings are reported in this week’s issue of Nature.

In an ever more interconnected world, we are constantly required to improve the handling and processing of our data. This ranges from secure communication and cloud storage, over large bandwidth requirements, to high-performance distributed computing. The principles of quantum physics allow in principle for a drastic improvement in all of these domains, which is why a large effort worldwide is currently underway to improve on the development of the necessary quantum technologies. One crucial technical requirement is to establish links that allow to create and store quantum entanglement, i.e. highly non-classical correlations, between distant sites. Ideally, such quantum links can be interfaced with present high-speed communication technology, which is based on low-loss glass fibers and silicon photonics that are operating in the telecom frequency band (at a wavelength of around 1550 nanometers). A team of researchers at the University of Vienna and the TU Delft have now succeed in employing the same technology basis, silicon photonics in the telecom regime, to establish a quantum link. They generated quantum entanglement between two mechanical memory elements on remote silicon photonics chips. Relying on telecommunication lasers for the distribution, their method is compatible with currently deployed fiber optics infrastructure. Their work provides a compelling route towards a large area quantum network based on silicon photonics.

In September 2017, the team of researchers at the TU Delft and the University of Vienna already demonstrated a new level of quantum control over mechanical memory elements. They used laser pulses to kick a micromechanical tuning fork that could store a quantum of energy in its vibration. Now they have taken an important next step: they have created entanglement between two such micromechanical resonators mediated by 'telecom' photons. Entanglement is famously known as the spooky action at a distance between two objects that can only be described with quantum theory. "Entanglement is a crucial resource for quantum communication networks", says prof. Simon Gröblacher of the Kavli Institute of Nanoscience at TU Delft. "Particularly important is the ability to distribute entanglement between remote quantum memories. Previous realizations have utilized systems like atoms embedded in cavities, but here we introduce a purely nanofabricated solid-state platform in the form of chip-based microresonators - little silicon beams that simultaneously confine light and vibrations. By extending the control of single mechanical quanta to multiple devices, we demonstrate entanglement between such micromechanical devices on two chips that are separated by 20 cm."

The devices used consist of micrometer-sized silicon beams. They are patterned in such a way that their vibrations can be 'written' onto laser pulses traveling through them and vice versa. The vibrating beams consist of eight billion atoms each, have the size of a small cell and can therefore easily be seen in a magnifying glass or microscope. "Nanomachined optomechanical devices are a very promising platform for integrated quantum information processing with phonons, as the parameters of the system, like optical conversion wavelength and quantum memory times, can be freely tailored through the design. For example, we deliberately chose the optical wavelength of the device to be in the telecommunication band, which is typically used in the distribution of high bandwidth internet. Thereby we show that quantum networks could be constructed just using conventional fiber optics in combination with our devices", says Dr. Sungkun Hong from the University of Vienna. Another key advantage is that their devices can be integrated on a chip together with other solid-state quantum systems. The authors, for instance, expect that their devices could potentially be interfaced with superconducting quantum circuits and used as quantum 'ethernet ports' that transfer quantum information between the circuits and optical signals.

"The next step will be to build a network consisting of more beams and working over hundreds of meters, maybe even several kilometers, getting us closer to realizing a system than can be used for real quantum applications", says Prof. Gröblacher. "We see no fundamental obstacles in taking these steps in the next few years."





R. Riedinger, A. Wallucks, I.r Marinković, C. Löschnauer, M. Aspelmeyer, S. Hong, and S. Gröblacher, Remote quantum entanglement between two micromechanical oscillators, Nature 556, 473–477 (2018).

Artist’s impression of two mechanical oscillators that are brought into a quantum entangled state through a light field inside an optical interferometer. The two systems exhibit so-called entanglement, stronger correlations than classically possible, often referred to as spooky action at a distance. This demonstration of entanglement between engineered systems could help to directly realize a quantum network. © Moritz Forsch. Kavli Institute of Nanoscience, Delft University of Technology