Physicists in Aalto University, Finland, have shown how a nanomechanical oscillator can be used for detection and amplification of feeble radio waves or microwaves. A measurement using such a tiny device, resembling a miniaturized guitar string, can be performed with the least possible disturbance. The results were recently published in the British journal Nature.

The researchers cooled the nanomechanical oscillator, thousand times thinner than a human hair, down to a low temperature near the absolute zero at -273 centigrade. Under such extreme conditions, even nearly macroscopic sized objects follow the laws of quantum physics which often contradict common sense. In the Low Temperature Laboratory experiments, the nearly billion atoms comprising the nanomechanical resonator were oscillating in pace in their shared quantum state.

The scientists had fabricated the device in contact with a superconducting cavity resonator, which exchanges energy with the nanomechanical resonator. This allowed amplification of their resonant motion. This is very similar to what happens in a guitar, where the string and the echo chamber resonate at the same frequency. Instead of the musician playing the guitar string, the energy source was provided by a microwave laser.

Microwaves get amplified by interaction of quantum oscillations

Researchers from the Low Temperature Laboratory, Aalto University, have shown how to detect and amplify electromagnetic signals almost noiselessly using a guitar-string like mechanical vibrating wire. In the ideal case the method adds only the minimum amount of noise required by quantum mechanics.

The presently used semiconductor transistor amplifiers are complicated and noisy devices, and operate far away from a fundamental disturbance limit set by quantum physics. The Low Temperature Laboratory scientists showed that by taking advantage of the quantum resonant motion, injected microwave radiation can be amplified with little disturbance. The principle hence allows for detecting much weaker signals than usually.

"Any measurement method or device always adds some disturbance. Ideally, all the noise is due vacuum fluctuations predicted by quantum mechanics. In theory, our principle reaches this fundamental limit. In the experiment, we got very close to this limit," says Dr. Francesco Massel.

"The discovery was actually quite unexpected. We were aiming to cool the nanomechanical resonator down to its quantum ground state. The cooling should manifest as a weakening of a probing signal, which we observed. But when we slightly changed the frequency of the microwave laser, we saw the probing signal to strengthen enormously. We had created a nearly quantum limited microwave amplifier," says Academy Research Fellow Mika Sillanpää who planned the project and made the measurements.

Certain real-life applications will benefit from the better amplifier based on the new Aalto method, but reaching this stage requires more research effort. Most likely, the mechanical microwave amplifier will be first applied in related basic research, which will further expand our knowledge of the borderline between the everyday world and the quantum realm.

According to Academy Research Fellow Tero Heikkilä, the beauty of the amplifier is in its simplicity: it consists of two coupled oscillators. Therefore, the same method can be realized in basically any media. By using a different structure of the cavity, one could detect terahertz radiation which would also be a major application.

The research was carried out in the Low Temperature Laboratory, which belongs to the Aalto University School of Science, and is part of the Centre of Excellence in Low Temperature Quantum Phenomena and Devices of the Finnish Academy. The devices used in the measurements were fabricated by VTT Nanotechnologies and microsystems. The research was funded by the Finnish Academy, European Research Council ERC, and the European Union.