Studying mechanical motion on the quantum scale is a topic that is not only of fundamental interest, but one that may lead to developments in quantum measurements and quantum computing. Opto- and electromechanical cavities allow such motion to be studied, but the key to success lies in ensuring the optical/electrical impulse couples strongly to the mechanical oscillation. A team from the National Institute of Standards and Technology has recently made significant progress in this area, with the demonstration of an electromechanical system in which the electrical and mechanical oscillations are more strongly coupled than in previous attempts, by greater than two orders of magnitude [Teufel et al., Nature (2011) 471, 204].
The system uses an aluminum membrane suspended above an electrode to produce a capacitor that vibrates, and looks, like a drum. The membrane is 100 nm thick and 15 µm wide, and separated from the base electrode by 50 nm. The two plates are then connected by a coiled wire resonator circuit. The electric cavity possessed a resonant frequency of 7.5 GHz, while the mechanical drum has a natural frequency close to 11 MHz. By applying a microwave drive signal at the frequency difference between the two, the researchers were able to push the coupling to the maximum, smashing the previous record.
Dr John Teufel spoke to Materials Today about the device, “The major improvement over previous electromechanical circuits is the vastly improved interaction, or coupling, between the microwave signals and the mechanical motion. The drum responds to even the weakest microwave signals. Likewise, the amplitude of the vibrations that we are detecting are extremely small, approximately [several] femtometers. We benefit greatly from the fact that both the drum and the electrical microwave resonant circuit both have very high quality factors. The drum, for example, continues to ring for roughly 5 milliseconds (360 000 periods of oscillation) after the drive is turned off.”
The authors of the study propose several simple methods for improving the system further; including, using a thinner drum membrane, reducing the spacing between the membrane and the base electrode, and examining higher order modes. Teufel explained the value of such a system, as “circuits like these could have many practical applications, including extremely sensitive detectors of force, displacement, or even mass.” However, the team is also hopeful the device could be used on a more fundamental level, to study vibration on a quantum level, “While quantum effect are routinely observed in microscopic systems like atoms or molecules, seeing these effects in relatively large objects like this drum is an exciting and active area of research.”


Stewart Bland