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Ultrasensitive and Ultrastable Devices
JILA’s experimental physicists with expertise in precision measurement are developing ultrasensitive and ultrastable devices to enable laboratory studies of the quantum fluctuations of nanomechanical oscillators (ultrasensitive motion detection) and biomolecules in ambient conditions (ultrastable atomic force microscopy). The work on ultrasensitive devices also includes ultralow power electronics for future space-based observatories.
Konrad Lehnert and his students are working on ultrasensitive devices to observe and measure wires so small that they behave quantum mechanically. To do this, they’ve tucked a really thin aluminum wire (also called a nanomechanical beam) inside a tiny resonant microwave cavity made of lightweight superconducting aluminum. The nanomechanical beam and the cavity are assembled on a silicon chip. This design ensures that small forces will cause large detectable motion. The microwave cavity used in these experiments is analogous to the much larger optical cavities in use throughout JILA.
Initially, the new device had excellent force sensitivity (3aN/√Hz) and could detect tiny heat-induced motions at milliKelvin temperatures. Its displacement imprecision was 30 times the standard quantum limit of ***. Plus, the device was within striking distance of being able to detect purely quantum fluctuations.
Inspired by these results, the Lehnert group set out to improve their measurement system by making it colder and designing a better amplifier. To make the system colder, the group is working on a technique analogous to laser cooling. Starting with a microwave generator (like those found in cell phones), the group plans to arrange its detection system so that the nanomechanical beam slightly increases the frequency of the microwaves inside the cavity, a process that will also cool the beam.
To help make even better precision measurements of beam motions inside the microwave cavity, the Lehnert group invented a virtually noiseless amplifier. The tunable device operates in frequencies ranging from 4 to 8 GHz. The device also has the lowest system noise ever measured for an amplifier. For instance, it produces 80 times less noise than the best commercial amplifier.
The amplifier is a parametric device employing Josephson junctions, which are superfast switches made of thin layers of insulating material sandwiched between layers of superconducting material. It consists of a series array of 960 Josephson junctions split into parallel pairs that form SQUIDS (Superconducting Quantum Interference Devices). The SQUIDS are exquisitely sensitive to tiny changes in electric current. The SQUID array forms a composite metamaterial that gains its properties from its structure, rather than directly from its chemical composition. The entire structure is embedded in the same microwave cavity that houses the nanomechanical beam.
The amplifier was a key addition to the Lehnert group’s efforts to measure the position of nanomechanical oscillators. The group is using its new device to amplify signals generated by nanoscale motion of the aluminum wire inside the microwave cavity and transmit them to a larger, more traditional amplifier, which sends out signals that can be read with ordinary electronics.
The Tom Perkins group has combined atomic force microscopy with detection technology from optical trapping microscopy to produce an atomic force microscope (AFM) that is a hundred times more stable than other state-of-the-art AFMs in real-world (ambient) conditions. The Perkins group is now pursuing biological applications of this ultrastable AFM.
Atomic force microscopes (AFMs) are a major enabling tool underlying nanoscience research and development. These robust, yet atomically sensitive, tools are increasingly being used on a wide variety of room-temperature systems, ranging from nanotechnology to biomedical sciences. However, current generation AFMs cannot return to the same feature with sub-nanometer precision; nor can they hover over an individual protein to measure its conformational dynamics over time. These and other tasks are prevented because real-world environmental fluctuations cause the probe tip and sample to move uncontrollably with respect one another.
Historically, the AFM community has focused on developing sharper tips and higher-sensitivity force-detection schemes for improved microscope performance. Precise atomic-scale control has been limited to rarefied environments (e.g, cryogenic temperatures and ultrahigh vacuum) that are not compatible with many biological (or industrial) settings.
To address this compatibility issue, Perkins and his co-workers developed a novel strategy to directly measure the tip and the sample positions in three dimensions with lasers. They used local detection to eliminate the unmeasured, environmentally induced drift that occurs in other instruments. In proof-of-concept experiment, they imaged 5-nm gold spheres using this ultrastable AFM. The resulting drift was less than 0.4 nm over one hour of imaging.
The Perkins group is currently focusing on developing this ultrastable AFM to study the structure and dynamics of important biological molecules, such as membrane proteins. Applications of the ultrastable AFM to nanotechnology are also being pursued.