MKIDs

Microwave kinetic inductance detectors

MKIDs are devices on a chip for measuring light. The camera in your phone has a sensor chip that also senses light, but they are terrible for measuring very faint levels of light, and very very crude for measuring the frequency of light. MKIDs are one of the first devices that measure both the intensity and frequency of light at the same time. In the past, complicated devices that need gratings or prisms were used to measure the frequency of light, or sets of filters, and made these measurements very slowly.  MKIDs do this all at the same time.

How do they work?

There is a whole lot of physics in MKIDs. You may have heard of superconductors – materials that act like they have zero resistance when they are very cold. Superconducting materials actually have a very abrupt transition in behavior when reaching their “critical” temperature, T_c.  This means that they change behavior very drastically if you warm them up just the tiniest bit from below to above T_c.  What does that mean when something radically changes behavior from just a small change in conditions? That means it is very sensitive – a perfect detector. When an MKIDs interacts with a single particle of light, a photon, the tiny, tiny energy in that photon is enough to warm the superconducting material, greatly increase its resistance, and this is easy to detect – the tiny energy in a single photon becomes easy to detect.

What is this all for?

ECL’s MKIDs team, led by Professor Mehdi Shafiee, is producing and testing novel designs of MKIDs for the purpose of making practical detectors for optical astrophysics cameras, for measuring the cosmic microwave background, and eventually, for medical and nuclear safety applications. The efficiency of detection, the frequency resolution, and the number of pixels in a device all need to be improved before these devices can make the breakthroughs we seek.

At ECL we have a special cryogenic research facility, with refrigerators that can run these devices at near-absolute-zero temperatures where superconductivity occurs. We assemble our devices in our clean room facility, and we are working with the NU physics department (Tikhonov group and others) leveraging their expertise and device fabrication facilities for this work as well.  We have plans to use these devices in astronomical cameras to help in the study of transients including gamma-ray bursts, on the ECL NUTTelA-TAO [link] telescope. Currently, the telescope is taxed by the great weight of the three-camera and dichroic optic system of the BSTI instrument.  This could all be replaced by a lighter, simpler instrument, and the frequency resolution could be improved by factors of ~ 10 by the replacement with an MKIDs camera. An MKIDs camera for use in galaxy surveys would likewise reduce the number of cameras required, and increase survey speeds. An MKIDs camera for the cosmic microwave background would revolutionize these experiments, an excellent fit to the requirements for the next set of future experiments, called “Stage 4”, leading to the discovery of primordial gravitational waves, fundamental physics, and much more. The increase in detector density and frequency resolution would also revolutionize medical imaging, replacing the detectors used in CT devices today; the same for detection of nuclear materials or X-ray imaging for security.

More

There is much more to make MKIDs special. Today we need a completely different kind of device and camera to measure light we can see, UV light, X-ray light, microwave light, etc.  MKIDs can be manufactured to be sensitive in *any* of those bands.  Another advantage of MKIDs is that sensors have millions of pixels, and are often surrounded by connections of thousands of wires to read them out. An MKIDs has only one wire, allowing simpler devices and tiling many more closer together. As mentioned above, MKIDs are run at very low, superconducting temperatures.  This means that the random, false signals called, “thermal noise”, are eliminated in these devices, making them inherently extremely sensitive.

Some Technical Details

MKIDs are actually a simple resonant L-R-C circuit. We detect photons by detecting the changes in the resonant behavior of the circuit; absorption of photons changes the kinetic inductance, the “L” in L-R-C. Detecting photons actually achieved by monitoring for changes in the resonance.  Pictures below show the way MKIDs work.

– Photons with energy hν are absorbed in a superconducting film, producing a number of excitation, called quasi-particles ( a and b).

– To sensitively measure these quasiparticles, the film is placed in a high frequency planar resonant circuit. The change in the surface impedance of the film following a photon absorption event pushes the resonance to lower frequency and changes the amplitude and phase of the transmission through the circuit (c and d).