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Ultrasensitive and Ulstrastable Devices

Ultrasensitive Motion Detection

The Konrad Lehnert group is working on an ultrasensitive device that can precisely measure the vibrations of a tiny thin aluminum drum only 100-nm thick. The superconducting drum has a diameter of 15 µm. It forms the upper plate of a 50-nm capacitor, which is connected to a spiral inductor. Together they create a resonant circuit that can receive or transmit signals to other (classical) electronic setups in the laboratory. When the superconducting drum and the spiral inductor are cooled with a combination of refrigeration and laser-like cooling to milli-Kelvin temperatures, the entire resonant circuit behaves quantum mechanically. The resonant circuit is big enough to see with a low-power microscope, making it one of the largest objects ever cooled to its quantum mechanical ground state.

In 2013, the group figured out how to use its new precision measurement device to store quantum information. It encoded a quantum state onto an electric circuit, then transported this information from the circuit into the tiny mechanical drum, where it was stored. Then, the researchers retrieved the information by reconverting it into an electrical signal. With this demonstration, it now appears that tiny drums could be used for memory storage in quantum computers. The demonstration also opens the door to using the drums as intermediaries in systems that convert quantum information from one physical system, such as a microwave field, into another physical system such as a laser light field.

To ensure the highest precision in their innovative measurements, the group invented and now routinely uses a virtually noiseless amplifier known as a Josephson Parametric Amplifier, or JPA for short. The JPA employs hundreds of Josephson junctions. Josephson junctions are superfast switches made of thin layers of insulating material sandwiched between layers of superconducting material. The amplifier 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, even at very cold temperatures. As a result, the JPA is an ideal measurement device for the Lehnert group’s investigation of cold resonant circuits.

The researchers are using JPAs in a collaboration with the Cindy Regal group on coupling a resonant circuit to an optical cavity by arranging for a drum-like mechanical oscillator to couple to both the circuit and the cavity. With this arrangement, a microwave signal can be stored in the drum and then retrieved as a visible light signal from the optical cavity. This coupling allows researchers from both groups to take advantage of the ability of microwaves to create and manipulate quantum information and the ability of light to transmit information over long distances.

The Lehnert group’s long-term goal is to figure out how to combine advanced electronics with advanced optical communications.

Ultrastable Atomic Force Microscopy

The Thomas Perkins group has developed an ultrastable atomic force microscope (AFM) for probing biological molecules in real-world conditions such as in air or liquid. Because real-world conditions are dynamic, uncontrolled drift between the microscope tip and the sample limits the utility of AFM in biophysics studies. To address this problem, the Perkins group has figured out how to use lasers to simultaneously measure the positions of both the AFM probe and the sample.

This instrument uses backscattered laser light to measure and control a microscope cover slip in three dimensions to distances of less than 1 Å (0.1 nm). A second laser is scattered off the back of a commercial AFM tip, making it possible to control the tip position to less than 0.4 Å (0.04 nm). These two measurements occur within a few micrometers of one another. This method eliminates the environmentally induced drift that occurs in other AFM systems.

The group’s ultrastable AFM is a hundredfold more stable than what was previously available for AFM-based biological research. It can operate in a wide range of environments for long periods. The researchers can use their ultrastable instrument to slowly scan a sample while averaging the mechanical response of the AFM arm (cantilever) in real time. Its precision now rivals that of AFM instruments operated at cryogenic temperatures (below −238 °F).

Since biological molecules don’t behave normally at such low temperatures, the new AFM is allowing the group to image and mechanically unfold such molecules as membrane proteins. The group is also pursuing applications of the ultrastable AFM to nanotechnology.

In addition to new investigations made possible by its ultrastable microscope, the group continues to work to improve the AFM itself. In 2012, for example, the group improved the force precision of its AFM in liquid by removing the gold coating from the AFM cantilevers with a quick chemical process. Both the short- and long-term force precision improved significantly. Enhanced precision was also observed under different experimental conditions and occurs with a minimum amount of settling time (~30 minutes). In 2014, the group made further improvements to the cantilevers that resulted in another 25-fold improvement to its AFM technology.

These advances have given the group a powerful new tool for exploring the physics of biomolecules in liquid.