Just for KICS!

Sensors capable of detecting individual photons of visible light have become essential for applications ranging from imaging faint galaxies to quantum computing and DNA sequencing. Researchers at the National Institute of Standards and Technology (NIST) have now devised a simpler and potentially more accurate system to read out measurements from large numbers of the most sensitive of these single-photon detectors.

Developed and perfected at NIST, these detectors, known as transition edge sensors (TES), consist of a tiny metal film kept at a temperature right at the edge of the transition between superconducting (zero resistance to current flow) and normal resistance. When radiation strikes a single TES detector, it raises the temperature and increases the resistance of the TES, which registers as a change in electric current. The more energy an individual photon carries, the larger the signal.

The change in current is so small that it must be amplified by a special read-out system. Typically, researchers have used a superconducting quantum interference device (SQUID), which converts the tiny current from a TES into an amplified magnetic signal.

Although SQUIDs have been used to read out hundreds to thousands of TES pixels sensitive to photons at radio and x-ray wavelengths, they operate too slowly to gather data from more than a handful of TES pixels that detect visible-light photons. In addition, SQUIDS are relatively bulky and would be difficult to employ in emerging applications that require thousands of tightly packed TES pixels.

To overcome these disadvantages Paul Szypryt of NIST and the University of Colorado Boulder and his colleagues replaced the SQUID with another readout system, known as a kinetic inductance current sensor (KICS). Each KICS features a superconductor that naturally resonates at a particular frequency. The device changes its resonant frequency when current from a TES pulses through the circuits. Importantly, these frequency changes are rapid enough that an array of KICS can simultaneously read the signals from thousands of visible-light TES pixels.

A KICS offers another advantage: It greatly reduces a key source of electronic noise. In order to maximize the sensitivity of a KICS circuit, a direct current, or bias current, must be applied to the device. If that current has to be continuously supplied, it can create a spurious electronic signal. However, because the KICS is superconducting, it traps and permanently retains the bias current within its resistance-free circuit. The current therefore only needs to be supplied once, reducing spurious signals and enabling the KICS to read out the TES current with great accuracy.

Szypryt and his colleagues, who include scientists from the University of Milano-Bicocca in Italy, reported their findings online Nov. 6 in Communications Engineering.

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Paper:

P. Szypryt, D.A. Bennett, I.F. Florang, J.W. Fowler, A. Giachero, R. Hummatov, A.E. Lita, J.A.B. Mates, S.W. Nam, G.C. O’Neil, D.S. Swetz, J.N. Ullom, M.R. Vissers, J. Wheeler, and J. Gao. Kinetic inductance current sensor for visible to near-infrared wavelength transition-edge sensor readout. Communications Engineering. DOI: https://doi.org/10.1038/s44172-024-00308-y