For the first time, researchers were able to strongly couple quantum systems with each other over a greater distance. This was achieved with a new method in which a laser light loop connects the systems. In this way, they can exchange information and interact with one another almost without loss. The new method could be used in quantum networks and quantum sensors, physicists from the Universities of Basel and Hanover report in the journal Science.

Quantum technology is currently one of the most active research areas worldwide. It takes advantage of the special properties of the quantum mechanical states of atoms, light or nanostructures, for example to develop novel sensors for medicine and navigation, networks for information processing or more powerful simulators for materials science. Generating these quantum states usually requires a strong interaction between the systems involved, for example between several atoms or nanostructures.

So far, however, a sufficiently strong interaction has been limited to short distances: For this purpose, two systems were placed as close as possible to one another at low temperatures on the same chip or in the same vacuum chamber, where they can interact with one another through electro- or magnetostatic forces. However, coupling over larger distances is a prerequisite for various applications such as quantum networks or certain types of sensors.

Physicists around Prof. Dr. Philipp Treutlein from the Department of Physics at the University of Basel and the Swiss Nanoscience Institute (SNI) has now succeeded for the first time in creating a strong coupling of two systems over a greater distance and through a room temperature environment. In their experiment, they used laser light to strongly couple the vibrations of a 100 nanometer thin membrane and the movement of the spin of atoms over a distance of one meter. As a result, each vibration of the membrane also sets the spin of the atoms in motion and vice versa.

A loop of light acts as a mechanical spring

The experiment is based on a concept that the researchers worked with the theoretical physicist Prof. Dr. Klemens Hammerer from the University of Hanover. A laser beam is sent back and forth between the two systems several times. “The light then behaves like a mechanical spring that is stretched between the atoms and the membrane and transmits forces between them,” says Dr. Thomas Karg, who carried out the experiments as part of his doctoral thesis at the University of Basel. In this loop of laser light, the properties of the light can be set so that no information about the movement of the two systems gets outside, and their quantum mechanical interaction remains undisturbed.

The researchers were able to implement this concept for the first time in an experiment and use it for a number of experiments. “Coupling quantum systems with light is very flexible and versatile,” says research director Treutlein. “We can control the light beam between the systems and thus generate a wide variety of interactions that are of interest, for example, for quantum sensors.”

A new tool for quantum technology

In addition to the coupling of atoms with nanomechanical membranes, the new method could also be used in many other systems, for example in the coupling of superconducting quantum bits or spin systems in solids, which are being researched for quantum computing. The new process for light-induced coupling could link such systems to quantum networks for information processing and simulation. Treutlein is convinced: “Here we have a new and very useful tool in the kit of quantum technology.”

The Basel researchers’ experiments were funded by the European Research Council as part of the “MODULAR” project and by the SNI doctoral school.

Reference: Thomas M. Karg, Baptiste Gouraud, Chun Tat Ngai, Gian-Luca Schmid, Klemens Hammerer, and Philipp Treutlein
Light-mediated strong coupling between a mechanical oscillator and atomic spins one meter apart Science (2020), doi: 10.1126 / science.abb0328