Developing Radio Telescope Receivers
Chris - The precursor telescopes to the Square Kilometre Array are pioneering new receivers to detect astronomical radio waves. Earlier, Ben Valsler spoke to a PhD student called Sarah Thompson from the Cavendish Laboratory at Cambridge University about her research into the physics of receivers to see how detectors like those used on the MRO can be made as sensitive as possible.
Sarah - My research is focused on a thing called Kinetic Inductance Detectors (KIDs). These are superconducting detectors that would be used for cosmological or astrophysics observations from the ground or also in space. They're a bit like cameras, but they're not really used for visible light. They're more used for other wavelengths. What I actually research on is we make these devices and we test them in the lab to see how they behave as you change certain elements of their environment. The idea is that we'll get a better understanding of the physics of these devices and then you can make better devices.
Ben - So, rather than building a new telescope, your research will enable us to build better telescopes in the future.
Sarah - Exactly, yes.
Ben - I know that I can pick up radio waves very easily using a bit of old coat hanger that's stuck in my radio. So, what is a superconducting material and why do we need these materials with interesting properties?
Sarah - A superconducting material is simply a material which has a temperature at which there's a sudden change in the material. So above this temperature it acts as a conductor or even sometimes an insulator, but below that temperature the properties of the material change so that suddenly electricity flows through it with zero resistance. Basically, in terms of building a device or a detector, you instantly get a lot less noise in the device. Superconducting detectors tend to be a lot more sensitive than non-superconducting detectors and in a world in astrophysics where we want to be able to detect more and more distant galaxies and stars and with things like the cosmic microwave background radiation which is very old and has been traveling since the beginning of the universe, there's not much of that light coming to Earth. So you're only talking about a very small amount of light and you need to have a very sensitive device to be able to pick up that light and give you an accurate image.
Ben - With optical telescopes, of course, the way that we've done that is to get bigger, to get bigger mirrors, bigger lenses, larger areas to collect more light. Is that the same in microwaves and in radio waves? Are we just getting bigger?
Sarah - Yes. If you're talking about the total amount of light collected then a larger array will collect more light. If you're talking about an individual pixel, that's one single detector and if you want to be able to capture a single photon on that single pixel then you need to make sure that a single photon will trigger a significant enough response from the detector that the signal will stand out clearly from the noise and the background of the detector. So, not only do you need a large array to capture more total light, you need more sensitive detectors to be able to actually detect that light when it hits the array.
Ben - So, we are talking about detecting radiation from the birth of the universe.
Sarah - Yes.
Ben - To me, I think that would need a very large bit of kit, but you've brought a few things in that are so small that you seem to need tweezers to actually look at them. So, what is it that you've got here?
Sarah - One of the main reasons for being interested in kinetic inductance detectors at all is they're very simple devices. You can attach many of the devices to the same piece of readout equipment which is quite rare in superconducting detectors. Often, you need a single readout piece of equipment per detector. If you want to take a large image with high resolution, you need hundreds or thousands of pixels or even tens of thousands of pixels. Now, that's tens of thousands of devices. If each of those devices have their own piece of readout line, their own piece of readout equipment then the wiring, simply, the wiring by itself becomes an unmanageable task! Now, there are lots of different ways of getting around this, but KIDs originally gathered interest because you can attach many of them to the same piece of readout equipment and the same readout line. So technically, it should be very easy to make large arrays of these devices. The reason that my devices require tweezers is because KIDs are very small. They are on the order of millimetres so even a very large array of them is going to be very small. Secondly of course, this is only a test device, so this doesn't have hundreds of thousands of KIDs. This in fact only has 10 KIDs. So, it's only about 8mm long and 3mm wide.
Ben - Well, it looks like a piece of perfectly glass about a quarter of a size of a postage stamp. So, that has got multiple receivers that can detect radiation from the birth of the universe.
Sarah - Exactly, yes.
Ben - That's quite astounding. How does that actually work to pick up that radiation?
Sarah - With the telescope, you have the aperture which actually sees the sky and then you'll have perhaps a series of lenses which will take the light that was collected by the telescope and focus it on one point called your focal plane. Now at that point, that's where you put your detectors. The KIDs themselves don't actually absorb light particularly well. You have to attach them to an antenna or an absorber depending on which wavelength of light you want to look at and that absorbs the light and transmits it into the KID itself. The KID itself is a really simple device. It's called a microwave resonator. It acts a bit like a notch filter. So, if you look at it over range of frequencies, then what you'll see is an almost flat perfect transmission from the input to your output except at one particular frequency called the resonant frequency of the device. At this frequency, all of the energy that you put in, what we call the probe signal, goes into the device, gets reflected back and never comes to the output.
So, when you actually want to read out from a KID array, what you do is you have a series of devices with different resonant frequencies so that your transmission across the entire range almost looks like zero. But then if light hits one of your detectors, its electromagnetic properties change and all of a sudden, its transmission at that resonant frequency goes from zero to very high. And that's how you know that you've detected something and how much the transition changes by and over what kind of timescale, that gives you information about how much energy the light had.
Chris - Sarah Thompson who's completing her PhD at the Cavendish Laboratory at Cambridge University.