Physicists at the University of Regensburg coupled the vibration of macromolecular carbon nanotubes into a microwave cavity and created a novel and highly miniaturized optical mechanical system. Dr. Andreas K. Hurtl's team achieved this by using the quantization of charge, that is, the charge is carried by a single electron as a powerful amplification mechanism, and its research results were published in the journal Nature Communication. They represent an important step to combine completely different quantum technologies in one device (such as electron spin qubits and superconducting qubits).

In general, it is very difficult to couple the vibration of macromolecules such as carbon nanotubes with microwaves. Why? Because the electromagnetic wavelength used by quantum computing or cavity quantum electrodynamics equipment works at GHz frequency, in the millimeter range. Typical nanotube devices can not only capture electrons with known quantum states, but also be used as vibration resonators, with the length less than 1 micron and the amplitude less than 1 nanometer. Due to the size mismatch, the movement of nanotubes will not have much influence on the electromagnetic field of microwave cavity, and the coupling predicted by standard photodynamic theory is the smallest.

However, in many aspects, it is an attractive idea to realize this coupling and control it without driving the nanotubes to a large vibration amplitude. Nanotube is an excellent string resonator, which can store energy for a long time. Its vibration can be used to transform quantum information between fundamentally different degrees of freedom. Single-well electrons and superconducting microwave circuits are popular candidates for quantum computing architecture. The research shows that the interaction between vibration and electromagnetic field can be amplified to 10000 times compared with simple geometric prediction.

This is achieved by using so-called quantum capacitors: the current is carried by discrete electrons, which means that the charging of very small capacitors (such as nanotubes) does not occur continuously, but in steps. By selecting the working point on the step curve, the controllability of the optical-mechanical coupling is realized, and it can be turned on and off quickly. Dr Hurtl, who is currently engaged in research at Aalto University in Finland, said: We have realized a so-called dispersion-coupled optical mechanical system. On the one hand, the system is novel and exciting because of the miniaturization and single electron effect of the mechanical part.

On the other hand, as we all know, there have been a lot of theoretical and experimental studies on large-scale (even macro-scale) optical mechanical systems. Optical-mechanical interaction can be used to cool vibration, detect vibration in a highly sensitive way, amplify signals, and even prepare quantum states at will. The results show that in the near future, quantum control of the vibration of linear nanotubes can be realized. This makes it very attractive as a quantum switch, which combines very different quantum phenomena.