Superconducting sensors collecting X-ray or gamma-ray photons that are generated to characterize materials. Amplification using the microwave multiplexer and a new quantum-based amplifier enhances the resolution of the signals without introducing background noise (Image: Courtesy of the National Institute of Standards and Technology)

Understanding how energy moves in materials is fundamental to the study of quantum phenomena, catalytic reactions, and complex proteins. Measuring how energy moves involves shining special X-ray light onto a sample to start a reaction. Detectors then collect the radiation emitted by the reaction. However, conventional sensors usually lack the sensitivity needed for these studies.

One solution is to use superconducting sensors. But amplifying the signals from these sensors is a major challenge. Building on advances from quantum computing, researchers from the National Institute of Standards and Technology (NIST) added a special type of amplifier — a superconducting traveling-wave parametric amplifier. While most amplifiers add noise to the measurement, these are almost noiseless. In a major advance, researchers recently showed that the amplifiers can operate at 4 Kelvin, which is considered to be a relatively high operating temperature.

Reducing the noise that is added during signal processing can improve a sensor’s performance. Amplification allows each sensor to operate faster and be more sensitive. Recent experiments have shown that parametric amplifiers can potentially analyze signals from many superconducting sensors at the same time. Superconducting sensors work at very low temperatures and at these temperatures, parametric amplifiers have very good noise performance, close to the limit of quantum mechanics. The advance paves the way to integrate such amplifiers with a variety of sensor technologies.

A superconducting sensor consists of a superconducting thermometer and an absorber. When X-rays are stopped in the absorber, they change the superconducting state of the sensor. This generates a small current in an electrical circuit. To make the detector more sensitive, many sensors are arranged into an array, like in a digital camera. Since superconducting sensors operate at very cold temperatures (approximately 0.09 Kelvin), they require specialized readout electronics and amplifiers. These amplifiers need to combine the signals from multiple sensors on a single readout line. One efficient way to do this is to couple each sensor in an array to a resonator. The resonators are then coupled to a single output line (combining signals is known as multiplexing). The current produced by an absorbed photon shifts the resonant frequency in a unique way for each sensor.

Because these resonators work at microwave frequencies, the electronic chip that contains all the resonators as well as the output feedline is called the microwave multiplexer. Researchers are preparing to measure the signals from an array of sensors and a microwave multiplexer with a readout chain whose first amplifier is a kinetic-inductance traveling-wave parametric amplifier instead of a conventional semiconductor amplifier. Using the parametric amplifier will reduce readout noise and enable larger arrays of faster sensors.