Cold atoms at space probe’s heart
There are things you can do with atoms that will be technological game-changers. One such project, emerging from the UK’s Quantum Technology Hubs, could revolutionise navigation and mapping systems.
One of the problems with quantum technologies is that they exist in an intellectual plane that hovers some distance above the vast majority of us. Where else would you find a beast such as Schrödinger’s cat that is both dead and alive at the same time? As Douglas Adams might observe, presumably Schrödinger himself disappeared in a puff of logic at some point.
Quantum’s other problem is that it is perceived to be some distance over the technological horizon in terms of being useful. But this is a perception that is not shared by everyone.
Dave Wilson, vice president of product marketing for software, academia and customer education at National Instruments, said at the recent NI Days conference: “We are working with the University of Aarhus on quantum computing. One of the points they are making is that it won’t let you stream high-def video faster, that’s not the kind of computing you will get out of it. But if you rely on brute force to work on a billion different iterations of something, then quantum will come in and do it in just a few cycles. That is the kind of place where it will help tremendously.”
There are more immediate possibilities, but even then they need to be developed with one eye kept on doors opening down different application and technology routes. Teledyne e2v, which includes imaging sensors within its portfolio and runs one of the few wafer fabs in the UK, is such a company that is already looking at the application end of quantum technology. Steve Maddox, systems engineer with the company, says: “We’ve had to try to keep a lot of organisation on what we’re doing: trying to follow a really rigorous engineering approach but keeping things flexible enough, because technology’s emerging and we don’t want to squash innovation. It’s quite a difficult balance to achieve and the science is all new. We’re working with lots of academics and it’s a really exciting place to be, but there are challenges.”
‘We’re very good at bringing together diverse technologies that all have to work in a very confined space and challenging environment. I’d say this probably has been five times harder than most of the things we’ve tried so far.’
Originally the English Electric Valve Company (hence the e2v), Teledyne has been around for over 70 years and is an established supplier of imaging sensors on Nasa and European Space Agency (ESA) missions. It is exploring quantum technologies for use in space with a view to using that technology for ground-based activities as well. It’s happened before – really high-end space imaging has spun off into top-of-the-range machine vision for production lines. One example is measuring individual pixels on TVs as they speed down a conveyor.
Cold atoms move slowly and behave in a uniform way, which makes them easier to measure than warm atoms that are typically full of energy and buzzing around in all sorts of directions. At an atomic level, if atoms can be trapped and all movement (even the tiniest vibration) can be stopped, it has the effect of cooling those atoms. If the atom can be rendered completely static then all its energy is removed and its temperature plummets to within milliKelvin of absolute zero (-273°C). When atoms are this cold they have the potential to be very good sensors.
The University of Birmingham, which is a partner on the cold atom project with Teledyne e2v, has developed a magneto-optical trap. This is a vacuum chamber into which rubidium chromate is released and heated to turn it into a vapour consisting only of rubidium atoms. A combination of laser beams and magnetic fields are used to slow the atoms down and trap them in a confined space in the middle – a coherent cloud that can then be measured.
Rubidium is the atom of choice for this simply because its energy levels match easily accessible lasers. Rubidium needs 780nm lasers, which matches those used on CD players and so were both cheap and plentiful. Although this is less the case now, telecoms lasers with a frequency doubler can just as easily be used.
Studies of gravity have moved on from measuring the time it takes an apple to drop from a tree but the principles remain the same. Maddox takes up the story: “People make extremely precise cubes and they drop them inside vacuum chambers and using lasers measure how quickly it takes that mass to fall and hit the chamber. And we’re doing the same with atoms.
“The thing that’s good about atoms is every atom is the same. The problem with those cubes that you drop [is that] if you make ten of them, they’ll be ten different cubes, even with today’s manufacturing technologies, and when you drop one, it wears slightly. With the atoms, every single atom is exactly the same. We don’t need to catch the same atoms again; we can just capture more rubidium atoms and know they’re the same as the other atoms we used.”
As the atoms are released a laser shines on them and effectively stamps a time on them. After they have fallen for a certain length of time another laser imprints another time. A photodiode picks up the readings – the brightness of the cloud – and the difference between the two can be used to calculate gravity very accurately.
Gravity is not a tidy 9.81m/s2.as some of us might imagine. “When you get to the ninth or tenth digit, it changes a little bit depending on where you are on the Earth and depending on what’s underneath you,” explains Maddox. “Mass gravity is really just one mass being attracted to another mass, so we’re measuring how much the atoms are attracted to the Earth. But if there’s a big hole in the Earth underneath you, you would measure slightly less gravity, and if there was something slightly more dense underneath you, it measures slightly more. If you take a measure of gravity here and then take a measure of gravity somewhere else, gradually you can survey an area. Basically, you make a map of where gravity is stronger, where it’s weaker, and that correlates to changes in the structure of the Earth underneath.”
There are endless applications for such technology. It can look for pipes or infrastructure underground, like tunnels – people tunnelling under borders or other defence applications such as looking for bunkers. It is a technique that can be used to detect small things close up or large things far away. Early stages of mineral exploration, for example, are ideally done from an airborne platform which allows large areas to be surveyed quickly, looking for signature rock structures that might merit closer inspection.
Other applications include archaeology, including our buried industrial heritage – particularly mineshafts. Records of these in some areas of the UK are historically poor. Recently it emerged that there may have been an old mineshaft under a recently built playground in a Black Country school.
Maddox explains what happened next. “They discovered a record that a mineshaft was in a particular place where now they’ve built a school playground. So, I went on a survey there actually using a gravity sensor – basically, a mass on a spring – to try to locate where on the school playground that mineshaft was.
“The quantum sensors aren’t quite ready yet to do that,” he explains. “They’re being developed and they’re showing a lot of promise, but they’re not ready to go out.
“To do that school playing field, it took a week to survey an area that’s not that large. We think that is a critical selling point of this new technology. It is not necessarily greater sensitivity, although we could push that in some places, but it’s being able to use the technology much more quickly. You could condense that survey time from a week down to a day essentially, and if you could do that, a lot of people would use it. We’d begin to compete with some of the other technologies, like penetrating radar.”
Penetrating radar has its limitations. If it is wet then it can only penetrate a couple of centimetres beneath the surface. Such a shielding effect will not make any difference to gravity readings. These quantum sensors therefore have the possibility of providing new capabilities from space, allowing surveys to be done on a much larger scale. Current space surveys are based on observation and can therefore only see what is at ground level.
Quantum sensors, says Maddox, could do much more: “We could look at the height of the water table. You could tell if the water table is higher or lower than it should be and that’s very useful for predicting droughts, predicting floods, on a reasonably large scale: on a continental or country scale. We are hoping to get the resolution down so it’s much more localised as well.”
Other applications could include measurement of the polar ice caps, ocean currents and sea level and also for use in deep space navigation.
Two dovetailed projects are in progress. The one in tandem with University of Birmingham is sponsored by the Defence Science and Technology Laboratory (Dstl) and is focused on designing the prototype.
The second is an Innovate UK project that will bring that prototype into reality in the form of CASPA (Cold Atom Space Payload). It also involves additional technology partners XCAM, Coversion, the University of Southampton, Gooch & Housego and Clyde Space, which is responsible for the CubeSat satellite that will house the quantum sensor.
The projects finish early in 2019, so it is hoped that CASPA will be ready for flight later that year. In its initial form it will be a 6U CubeSat (i.e. six units of 10x10x10cm), but a fully functional follow-up flight would have to be larger. The intention is for CASPA to prove that it is a viable platform for cold atom technology in space – the first time it will have been achieved on a free-flying satellite. On the inaugural flight it will not actually be an instrument to measure anything, just a demonstrator to prove it can be done.
“It has been proposed for a number of missions already, but the ESA has previously said no, that the technology’s not mature enough,” says Maddox. “This is our approach to build up the maturity of the technology so that it can then be accepted for a proper mission in space. So this is a stepping stone, but it’s an important stepping stone to try to position ourselves and the UK, because we’ve got lots of different UK partners as part of the supply chain, for future missions in space. We actually took this to the European Space Agency in the Netherlands and they were really, really keen on it; they’re now very interested in flying it actually.”
The project team appreciates the objections regarding heritage equipment. It costs a lot of money to get a satellite into space and so it can’t afford to fail. It is therefore no surprise that it is working with Clyde Space, which itself followed the route of flying a demonstrator module – the UK Space Agency’s UKube-1. Having gained this and subsequent space heritage it now represents standard CubeSat hardware and is low risk – particularly important in such an experimental project as CASPA.
X-Cam, which makes the imaging board, also flew on UKube-1. This time, however, the camera will not be looking at the ground but inside the chamber to monitor the cold atom process. X-Cam is also building the control electronics boards to create the constant-current sources needed for pairing the coils and for dispensing the rubidium, and general control for the whole experiment.
There is plenty of development work to do before the project ends. Getting the right frequency of light is one focus area. Maddox continues: “A lot of the work is around making sure that it’s all going to work through the shock and the vibration profiles that we’ve got. Basically it’s got to be robust enough to survive the launch. And there’s a lot of work on the autonomy as well, making sure it will work autonomously without human interaction.”
Dr Michael Holynski, innovation fellow at the University of Birmingham, adds: “Radiation environment is a problem. I think in the orbital this is going into, radiation is not so much of a concern, but we are also involved in development of amplifiers for optical communications in space where they experience much higher radiation dosage, so a lot of our components and the fibres have been tested for radiation purposes.”
He concludes: “You don’t need to understand what quantum physics is. The important bit is the fact that it’s going to be useful. No one understands how an Intel microchip works, does anyone worry about that? No, but everyone uses a computer. In 20 years’ time will everyone be using a quantum sensor? We hope so, but they won’t understand quantum physics!”
An alternative is for inertial navigation. With a known starting point it could be used to accurately measure the position based on the movements taken. Rather than take measurements of the acceleration vertically, as in the gravity sensor, it would mean measuring acceleration horizontally.
“However,” Maddox comments, “that’s all a bit further away.”
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