Reaching the quantum ground state of a nanomechanical oscillator consisting of millions of atoms would allow to study the weirdness of quantum mechanics in a new regime. Suspended carbon nanotubes can oscillate at different frequencies similar to a guitar string. In this paper, we propose a novel way to cool such a system towards the absolute zero of temperature, when these modes are in their quantum ground state with the minimal possible energy. Just like in an ordinary refrigerator electric current is used to extract the energy. By attaching one normal (N) and one superconducting (S) lead to the nanotube the electron transport process couples the hot environment to the zero-temperature reservoir of Cooper pairs in the superconductor allowing to achieve an unprecedented cooling efficiency. In this manner several mechanical modes can be brought into the quantum ground state, which was not possible in previous proposals usually limited to one particular mode. Our method paves the way to sophisticated quantum manipulation of carbon nanotube oscillators like a coherent superposition of modes which might be used as quantum limited sensor of weight or motion.
Pascal Stadler, Wolfgang Belzig, and Gianluca Rastelli
Phys. Rev. Lett. 117, 197202 (2016)