Microwave Squid MUX, part of an imaging array.

Konrad Lehnert and his group investigate the basic physics of nanoelectronic devices to verify the consistency of basic electrical units, including the amp, the volt, and the ohm. As an initial step they want to better understand why nanoscale electronic devices based on conventional technology operate more and more slowly, the smaller they get. This happens because, at the nanometer scale, there's a disparity between two different laws of nature that control resistance: the impedence of free space and quantum resistance. The impedence of free space, which has a value of approximately 377 ohms, controls the propagation of electronmagnetic waves through a vacuum. Quantum resistance, which is a fundamental constant equal to 25.8 kiloohms, controls the resistance of electron flow through nanoscale devices. The mismatch between these two laws means that "conventional" nanoscale electronic devices will be such poor generators of electromagnetic waves that they won't be capable of generating measurable signals. It also means that further miniaturization of the modern transistor, which can now switch 10 times per nanosecond, could soon lead to ultrasmall transistors capable of switching once a century, which is clearly unacceptable. Consequently, Lehnert's group focuses on innovative nanoelectronic designs that circumvent the disparity problem. "In our lab, we're not interested in squeezing better performance out of existing technology," Lehnert says. "We'd rather invent new technology."

The researchers know that ultrasmall electronics devices will inevitably be slow unless their designers are clever. As a result, they are working to develop microwave techniques to overcome the slowness problem. It appears the group's new techniques will make it possible to design transistors that are not only a hundred times smaller, but also as fast as today's technology. The techniques will be the basis for future nanoelectronic devices that are smaller, faster, and ultralow power. The group is currently using its new approach to design ultralow power, superconducting electronics for future space-based observatories.

The Lehnert group recently designed and tested a displacement detector based on electrons tunneling across a vacuum from an atomic point contact (APC). The APC is formed by bringing an atomically sharp conducting point within one nanometer of another conducting object. A key design feature is that microwave frequencies are used to detect the APC's resistance. With the device, the researchers can both observe movement on time scales of less than 10 nanoseconds and measure the device's backaction, or recoil force. The device is sensitive enough to resolve submicrosecond, femtometer resonant motion of a nanomechanical beam driven by thermal noise at temperatures of less than 1 K.

In other work, the group is developing a single-electron counting ammeter. The purpose of this device is to be able measure electric current by counting individual charges. Because a nanoamp of current moves about six billion electrons per second, the device requires electronics that are sensitive enough to measure a flow of electrons during nanosecond time intervals.