Tunnel Vision

A key challenge in developing new nanotechnologies is figuring out a fast, low-noise technique for translating small mechanical motions into reasonable electronic signals. Solving this problem will one day make it possible to build electronic signal processing devices that are much more compact than their purely electronic counterparts. Much sooner, it will enable the design of advanced scanning tunneling microscopes that operate hundreds to thousands of times faster than current models.

As exciting as these possibilities are, they aren't the primary driver for research by graduate student Nathan Flowers-Jacobs, former research associate Dan Schmidt, and Fellow Konrad Lehnert on the properties of an atomic point contact displacement detector they designed and built. The researchers want to understand the fundamental physics of the device, which operates in accordance with the laws of quantum mechanics. To accomplish this, they've developed a microwave-based technique to efficiently measure its performance.

To make the displacement detector, the researchers used lithography tools to create a gold structure with a freely suspended nanomechanical beam (100 nm thick) connected initially to an atomic point contact, as shown below. Then the researchers ran a current through the point, creating an electron wind that shoved atoms aside, creating a tiny gap between the point and the beam. During this process, they measured the resistance of the point contact, looking for a characteristic increase in resistance indicating that electrons had started to hop (tunnel) across the gap. An artist's conception of the atom-sized gap between the atomic point contact and the freely suspended nanomechanical beam is shown at the right.

Once the device was up and running, the researchers monitored the size of the atom-sized gap by measuring the current that flowed through the atomic point contact. However, there was also shot noise in the current. This noise arises from the fact that current across the gap is composed of individual, discreet tunneling electrons subject to the laws of quantum mechanics, i.e., tunneling is a random probabilistic event. Even though the researchers could easily determine the average current, the nature of a tunneling current meant there would always be fluctuations around this average, or shot noise. The shot noise limited how accurately the position of the freely suspended beam could be determined.

However, when more electrons traveled across the gap per second, less noise affected the displacement measurement. If shot noise were the only issue in measuring tunneling current, then Flowers-Jacobs and his colleagues could simply have increased the average current through their device until the displacement noise became as small as desired. Unfortunately, there was also a random force acting on the freely suspended beam due to the measurement itself. Called backaction, this random force could noticeably shake the beam and move it more than the measurement uncertainty due to shot noise. Naturally, the backaction got worse as the average current increased, creating a tradeoff between backaction and imprecision in the measurements. The researchers decided their best strategy was to control the current through the atomic point contact such that the effects of the shot noise and backaction were approximately equal. Their measurement was so sensitive it could detect the beam's random thermal motion at temperatures as low as 250 mK and the precision was only 40-fold worse than the quantum mechanical limit.

To achieve this precision, Flowers-Jacobs and his colleagues used a very clever microwave measurement technique that was fast enough to detect the freely suspended beam's resonant motion. To make the displacement measurement, the researchers hooked up their device to a microwave-frequency tank circuit, which is a resonance circuit that stores electrical energy. They determined the size of the atomic point contact gap by measuring how quickly electrical energy leaked out of the tank circuit. The rapid measurements possible with this system allowed them to "see" the freely suspended beam wiggle more than 40 million times per second and quantify both the shot-noise-limited imprecision (2.3 fm/vHz) and the backaction force (78 aN/vHz) .   - Julie Phillips