Transcript of Distinguished Lecture by Distinguished Professor Arnan Mitchell
Held on Tuesday 17 May 2022, 2.00-3.00pm
Optical microcombs: measuring almost anything – from earthquakes and tsunamis to the gases in our atmosphere to planets of distant suns
WELCOME – Distinguished Professor Xinghuo Yu
Welcome everyone. I am Xinghuo Yu, the Chair of RMIT Professorial Academy, the host of this event.
First, I would like to acknowledge the people of the Kulin Nation on whose unceded lands we are meeting today. I respectively acknowledge their Elders, past and present.
Today we shall hear from Distinguished Professor Arnan Mitchell about Optical microcombs: measuring almost anything – from earthquakes and tsunamis to the gases in our atmosphere to planets of distant suns.
This is part of the activities hosted by the Professorial Academy in fulfilling its obligations as an ambassador, advocator and thought leader for RMIT.
Before we start, let’s go through some housekeeping matters. This is a Teams live event. You will not be able to directly ask any questions via microphones. Please post your questions in the Conversation section during the lecture. At the end of the lecture, I will pick up those popular questions to ask the presenter on your behalf.
Let’s start the Lecture by introducing the speaker. Distinguished Professor Arnan Mitchell is Director of RMIT's Micro Nano Research Facility and the Integrated Photonics and Applications Centre. He graduated with a PhD from RMIT in 2000 and has been with RMIT ever since. He has built RMIT's capability in photonics (the science of using and manipulating light) over his 20-year career through national and international collaboration.
Without further ado, please join me to welcome Sara to deliver her lecture. Over to you, Arnan.
LECTURE – Distinguished Professor Arnan Mitchell
Thanks Xing. Hopefully you can all hear me out here and also online, I'd also like to acknowledge the Wurundjeri Willim people of the Kulin nation on who's unceded land I am presenting from and I'd like to acknowledge their ancestors and elders, past, present, and future.
I'm also looking forward this weekend to voting for a Political party who's going take action on the statement from the heart.
So, I'm here to talk to you today about optical microcombs measure almost anything.
I'll finish off with some of the many things that we're talking about. I want to start with a bit of apology and for my last presentation which you can still find on YouTube. There I promised I would talk to you about precision measurement, positioning satellites and turbocharging internet. I pretty much just spoke about the Internet the entire time, so this time I will talk at least a little bit about positioning with satellites and maybe a little bit of precision measurement. So hopefully this will make some amends for my presentation in 2020. If you're interested in that presentation, you can still find it on YouTube.
I'm also on social media, so you can have a look at some of the things that we're doing on Twitter and particularly on LinkedIn. This seems to be quite a lot of activity on LinkedIn. I have some web pages as well. I'll talk about those towards the end.
[4:27]
So, onto the topic of the presentation, most of this presentation is about precision measurement and particularly navigation. So, knowing where you are. It’s important for ships particularly to know where they are, perhaps within hundreds of meters, certainly within kilometres if they're trying to go across the ocean from one country to another.
In order for me to get RMIT today, I had to use Google Maps. And so, you can see maybe you can see I've got the laser pointer on here. This is the route that I took to come in today and Google Maps can position me to maybe within 10 meters. So that's pretty good. People talk about self-driving cars; I've got a Tesla. It almost self-drives. I wouldn't really trust it to drive on its own, but I would hope that self-driving car could know its own position probably better than a meter. Otherwise, you might end up crashing into another car. But there are lots of other things that people are talking about wanting to do or to autonomously, for example sort of measuring civil infrastructure like railway lines and bridges, having drones and other vehicles sort of operating under the ocean. And people are even sort of looking at robotic surgery. I tried here to get the least disturbing picture of robotic surgery I could find, and Racheal tells me this is still fairly disturbing. So, but there's a lot of ideas about having sort of automated positioning systems ranging from 10s of meters to kilometres down to sub-millimetres, which is what I want for any robot performing surgery.
[6:22]
OK. Can we do that? Let's have a go. Alright and. That better, getting better? Alright.
Specifically, how did I find my way to RMIT today and that sort of goes then to a question of how does my phone know where I am to or where it is to within 10 meters?
The answer’s a bit technical and perhaps a little bit more technical than most people would be aware and I should mention that I've deliberately tried to make this lecture accessible and not really assuming that anybody has any particular technical insight. So, if I say something that you want more explanation on please do feel free to put something into the chat if you're not physically here or ask me a question at the end.
So, in order for my phone to know where it is, it's actually receiving messages from satellites that are orbiting the Earth and it needs to get 4 messages from four different satellites to work out where I am to within to within 10 meters. It needs three spatial dimensions, so 3 satellites, measurements from 3 satellites give it three spatial dimensions latitude, longitude, and altitude. So, X, Y, Z perhaps. But I also need to know time because especially if I'm moving, I need to know whether I'm still where the satellites were telling me I was before.
So, this process is sort of illustrated here. This is a picture that I pilfered from the Smithsonian Institute. So, I think it's worth before diving into all of the technical details here to just sort of back up a little bit and go through the history of navigation and how did this come about?
[8:33]
So how do we get here now? A very, very brief history of navigation.
So, going back three hundred years, people really couldn't navigate very well. They didn't know where they were. This illustration is an illustration of what's called the Skilly Naval Disaster, and this is where a whole fleet of warships ran aground on the Scilly Islands in 1707. Four of the ships were destroyed, 2000 people were killed and it's all because they didn't know where they were. And they didn't know where they were to tens to maybe even hundreds of kilometres, so that so they just ran aground. And so, if they had better navigation, this disaster could have been mitigated.
[9:24]
So, the question then is how were they navigating? What was, I mean, there were, they were out in their ships of selling around the world. They must have had some form of navigation so I'm going to say approximately how this navigation was done. So, you needed to find latitude and longitude, altitude probably wasn't that important then it was most of it was happening at sea level. So, they needed to find latitude and longitude.
[9:55]
The latitude is relatively easy, so if you can see where the sun is in the sky and you know what date it is, then you can pretty much work out where you are between the North and South pole on the planet. And so, here's an illustration of somebody using what's called a sextant, which you can use to sort of measure the angle of things. So, you measure the angle between the sun and the horizon. And then you can use that along with the dates to work out where you are. So, for example in this illustration there's a location on the globe where the sun would be directly above your head at 90 degrees and then if your further north, then there's a slighter angle. So, you can work out based on understanding of how the Earth's tilted and some basic geometry where you are in terms of latitude.
[10:53]
But longitudinal is a much more difficult problem, so longitude is how far along the circumference of the world. So back then back three hundred, four hundred years ago, maybe even more people used to navigate using celestial bodies. The stars, the planet, the moon, the planets, the moon.
So, what they did was they had models of the where they expected the planets to be, and these weren't bad back then. So, people had a reasonably good understanding of how the how the planets moved around the sun and back then.
[11:30]
And also, the means to observe where they the planets are, and so this is actually a recent picture this happened about a month ago. All of the planets, this is a picture over New York, all of the planets lined up, lined up and aligned. So, you can see Saturn, Mars, Venus, Jupiter. So, if you could predict where the planets should be and then measure where you see them in the sky with a sextant, [12:02] the same tool that was being used to sort of measure the angle with the sun, then you could use that along with the model to determine where you were.
[12:12]
But not many sailors at the time had the astronomical knowledge to be able to work that out, so they had to use tables. And so, they're basically had monthly tables of where all the celestial bodies would be at particular times and on particular dates and what positions they corresponded to.
[12:34]
So that was workable, but it meant that you needed to have pretty skilled people even to look up the tables, you needed to sort of make all these measurements. You need to have a lot of skill to make up the make the measurements. But you're particularly needed access to these accurate astronomical models, and this is a picture of Isaac Newton who was one of the astronomers and spent most of his life coming up with models for how the planets moved around the sun.
But one of the things motivating him was there was a lot of money to be made in selling these this information to sales so they could navigate accurately.
[13:23]
So, when so when the Skilly disaster happened this motivated Newton and other astronomers to convince the British government to actually set a prize for a better system of navigation. And particularly, I think Newton was hoping that this would pump a lot of money into physics and astronomy. And it did. There was, there was a lot of people working on better astronomical models and improving the system but basically the prize was set at 20,000 British pounds at the time for half a degree of accuracy around the globe, which corresponds to about 5,000,000 Australian dollars today for about 50 kilometres of accuracy. So, it was it wasn't possible to determine where you were within 50 kilometres at the time that was, that was worthy of what was described as a King's ransom at the time.
And so, Newton thought that the solution was going to be better astronomical models, but there was another alternative.
[14:35]
So, if you had a really good way of measuring time, you could also navigate using the time and the way this worked was is illustrated in this picture. I've also pilfered this from the Smithsonian Institute. So, what you do is you set your time at your home port. So, in this case it's Greenwich and you can know that when it's 12 noon because the sun is at its at its peak and so you wait for the sun to be at its peak. You set that as 12 noon where you are, then you set off and you keep the clock at the time of where you left, and you measure when 12 noon is at your current location. And the time that the if you go halfway around the globe, it'll be night time on one side of the globe when it's 12 noon on the other side of the globe, so it'll be different time for noon as you go around in longitude if you note when it's 12 noon where you are and look at the clock that was keeping time from where you left. The time difference tells you your longitude. So, one hour is about 15 degrees and longitude 24 hours is 360 degrees, not a surprise. So, one minute of time difference is about quarter of a degree, which is about 25 kilometres. So, if you had a clock that could maintain time, in over a year in the harsh environment on a ship to within a minute, then you had an accurate enough navigation tool to win the longitude prize.
[16:25]
So now remember this is 25 kilometres of accuracy. My mobile phone is able to tell where I am within 10 meters, so more than more than 25,000 times more accurate. Sorry. 2,500 times more accurate. So, things have come a long way since then.
[16:48]
The contender for this longitude prize was invented by a celebrated engineer John Harrison. The story is described in this book, it’s one of my favourite books. It pitches John Harrison, the engineer as the hero and Newton and all the astronomers and physicists as the evil empire. It’s sort of a really ripping yarn. And it talks about not only. The history of science and technology, but a lot of the story about the personalities of the time as well. So, I heartily recommend you recommend this book to you.
This is the clock that he made the first prototype clock that he made that was actually sent on a ship and got close, it got close to sort of winning the winning the longitude prize.
[17:50]
So, with a really good clock, you can measure where you are fairly accurately, within 25 kilometres at noon each day. But it you might want to know where you are at other times so you can check in at noon each day where you are, but it might be important you can travel a fair distance in 24 hours, and it might be important to know to know where you are at other times. \
So, to do that you use what's called inertial navigation. So, you predict where you are based on the direction you're traveling in and the speed.
[18:23]
So, if you know the direction you're traveling and you can use a compass to determine what direction you are, where you are relative to the magnetic north. For example, with this magnetic compass.
[18:35]
And if you can measure your speed and interestingly the way that they measured speed at that time was, you can see illustrated in this picture here that, you basically had a rope, you had a log attached to that rope. The rope had knots tied in it. You threw the log overboard, it stayed with the water. And as the ship moved and pulled the rope, you counted the how many knots in a certain amount of time. And then you wrote the knots in a logbook. So, to record how the log was going. So, this is where logs are knots come from. And that gave you the speed of the ship.
[19:15]
So, with the speed, the direction and how long you've been traveling you could then work out what distance you had travelled and chart that on a map and you could check in every 24 hours how accurate it was and updated. There are systems today that still use inertial navigation so if you need more accuracy than you can get with GPS if I need more than 10 meters of accuracy, you can use initial navigation between readings to give you a little bit more accuracy.
[19:49]
Hopefully this video will work. This is a video actually of that prototype at the Greenwich Museum in London and I've visited, and the clock has been running pretty much for 300 years, so it's still running today. It's quite remarkable. You can see that it has pendulums, so it's got four actually, so there's, I can't see my mouse on top of this, but it's got basically pendulum's top and bottom that are counterweighted, and I'll just start that again, if I can. And let me just start that again.
The other point I wanted to make about this was the oscillations are happening about once a second, so this is what gives it its accuracy. It's got a very stable oscillator that's happening relatively quickly. So, the cycle time, the back-and-forth motion of those pendular about once a second.
So, if we compare that, so the question then comes up, OK. So why is that more accurate than navigating by the stars?
[21:15]
So, if we think about the solar system as a giant oscillator, so basically this is like a pendulum, the planets go around the sun and they do that in a cyclic repeatable fashion. So, the cycle time for the earth going around the sun takes a year of defining a year. Other planets can take quite a lot longer. So, the reference scale here is about a year.
The moon is also used for measuring time, so the moon takes about a month and the moon and month of the same word root. So about 28 days for the moon to go through a cycle. So, if you use the planets and the moon, you're comparing a year time frame reference to a month time frame reference and so that gives you sort of days of accuracy.
[22:15]
However, as the earth is rotating, that takes precisely a day 24 hours. The clock made by John Harrison oscillates every second. So, if you're referencing a one second oscillator to a 24-hour oscillator, then you get the precision that is required to sort of locate a ship within about 25 kilometres, and so this is significantly more accurate than navigating by the celestial bodies, the moon, and the planets, particularly because of the scale of the speed of oscillation.
[22:55]
I'm comparing all of those here. You can see that, OK, the clock is oscillating at a second. The earth rotation a day is 86,000 seconds. A lunar cycle, 28 days is about 2,000,000 seconds and the orbit of the earth around the sun is about 31 million seconds and other planets can take 10s of years to go around and so that's can be significantly longer. So, in principle using the Harrison clock and the day night cycle of the Earth as a reference should be about a million times more accurate than using the stars using, this the same sort of reckoning.
[23:32]
The clock thinks in seconds, so it’s oscillating in seconds, but we want to read it off in minutes and hours and days and so most of the clock the pendulums, important the oscillator that that's actually the pendulum's important, but another important aspect of the clock is what's called the clockwork. This is what converts that the one second ticking of the pendulum into a much slower movement of the hands on the face that actually gives us a readout in time that we can understand. And so, I'll come back to that later, this is really important with optical frequency combs and we'll get back to lasers as soon.
[24:22]
So, what happened next? Well, by the end of his life, John Harrison had significantly improved the clock. So basically, this is his third prototype there was a fourth prototype as well, but this is the third prototype that was actually presented to the King of England at the time to collect the longitude prize. But actually, James Harrison was never formally awarded this prize, Newton and all his cronies made sure that he wasn't able to collect this prize. And again, I encourage you read the book to see all the ends and outs of what happened there. He was eventually awarded all the money, but not the prize at age 79.
For more than 100 years, so this is 1773, this is what clocks look like then, looks a bit like a pocket watch, it is a bit bigger, it's about 13 centimetres across. This is what pocket watches look like in the 1900s, so not hugely different.
[25:30]
It wasn't really till 1969 that with electronics that things got significantly better. It was always even, even with the sort of modern clocks, it was just faster and faster and more and more precise pendula, but there was still ticking at maybe five or six times a second, not more than that.
So the next major breakthrough really was the quartz oscillator, and you can see one here. This is an oscillator circuit. It looks like a tuning fork that basically the arms oscillate but it's but instead of being read out mechanically with cogs, it's actually read out electronically and so it really took the advent of electronics to for this innovation to take flight. It oscillates, the original one oscillated at about 8000 times a second, that then ramped up to about 30,000 ticks per second. And that was enough for most watches to stay accurate to within about a second a day. So, this is about 30,000 times more accurate than the John Harrison clock that's ticking at once a second. This is 32,000 times a second. And this is for example is what drive, drive digital watches and I think what drives most quartz watches these days. So that's accurate enough to maybe get you within kilometres, or hundreds of meters, but many applications you want 10 meters.
My Google Maps, or if I want robotic surgery, I want a hell of a lot better than 10 metres for me. My self-driving car and maybe millimetres or submillimetre for robotic surgery.
[27:30]
So, the most precise clocks basically went away from artificial oscillators and went back to natural oscillators, so the natural oscillators that we're using previously was the planets rotating around the sun, the earth rotating on its on its axis, the moon going around the earth. But instead of looking out to the stars and the planets in the middle of the last century, people looked into to the atoms, and found that the caesium atom actually was a pretty good reference. So, the caesium atom has an electron that sort of has an electron orbit that if you ping it will oscillate very, very stably at about 9 gigs, about 9.1 1 gigahertz or 9000 million ticks a second. And so, this is 9000 million times potentially more accurate than the John Harrison clock.
[28:33]
And this is what the original prototype looked like in 1959, and there's been a sequence of atomic clocks developed since then that have been getting more and more and more accurate.
These are very similar to the sorts of atomic clocks that are actually used, or the sorts of clocks that are used in satellites.
[28:55]
Here's an example of the original Global Positioning System, actually is a US based satellite system. There's the Galileo system and that I've got illustrated here. Here's a picture of people assembling the Galileo system. This is actually a pair of these atomic clocks, so you can see they're relatively large but small enough to go into a satellite. And so, there are dozens of these satellites in orbit around the Earth now and are available for providing positioning systems, and so the advertised positioning is premium service 1 meter. You know general public 5 meters, so pretty good, pretty good.
[29:45]
So, this is the technology that actually enabled me to get to RMIT today.
[29:53]
So from once upon a time when people couldn't navigate, is this happily ever after?
I pulled out this new story from 2017 that was reporting on the alarming rate of the clocks failing in the Galileo satellites. So they're smallish about this big. They’re pretty complex and can be fairly failure prone and it's quite hard to maintain things once you've launched them into space and so there's still a way to go. Also 1 meter accuracy good enough to get me here to RMIT, probably not good enough for other automation that you might want, so there's still a way to go.
[30:40]
Now I still haven't touched on lasers and that's actually my area. So, I did promise you lasers.
[30:46]
The most accurate atomic clock demonstrated to date isn't based on caesium, it's based on strontium. Similar idea, it's not a microwave transition, it's an optical transition. So there are, you hit different atoms, they ring at different frequencies like tuning forks. But the strontium atom has a transition that is 429 9 terahertz, which actually is an optical frequency. You can see that you can see this with your eyes, it's sort of red colour.
But you can't measure the ticks directly. There are 429 million million ticks per second, so it's 420 million million times more accurate than the original Harrison clock. And this should be 40,000 times more accurate than the microwave atomic clock that was in the Galileo satellite. This should be enough to give you submillimeter accuracy positioning with something like GPS.
But the question then remains how do you ping this atom and how do you measure its frequency?
[32:02]
Lasers were developed in the sort of 1960s and 70s along with these atomic clocks and are the most precise way of generating an optical frequency and also measuring them and so, for example, this is a red laser, roundabout 500 terahertz and I've just sort of illustrated that as, OK, if you have optical power it's all concentrated at a particular frequency. This is one of the unique properties of lasers, they put all of their power at one frequency.
But you can't actually measure those oscillations directly. Nothing can really go as fast as the as the direct oscillations of the optical wave. We can see the red light, but we can't see the electric fields moving at the very, very fast rate that they're moving.
[32:53]
So, remember, this was a similar problem that John Harrison had with the one second oscillation and trying to turn that into something slower that actually meant something to us into turn the second text every second into minutes and hours and days that we could read off the face of the clock. And so, we need something like a clockwork for lasers.
[33:18]
So hopefully this works, and I might just turn the sound up on my laptop.
[video demonstrates hitting tuning forks to hear ‘beats’]
You should be able to hear exactly the same tone for both of these oscillators. And what the person's going to do now is actually detune one of them so it's slightly different.
Did you hear anything different? Should be able to hear the beating.
And now it's much slower.
[34:32]
So maybe that was a bit subtle. Let me just turn my sound down again. So, I'm not distracting myself.
If you have two tuning forks, and they're exactly in tune. If you hit them both, you don't hear anything, you just hear the sound. But if you slightly detune them, then what you can hear is beating between them so you can hear the difference in the frequency between them, so they can both be very high frequency. In the case of the tuning forks, there were around about 440 Hertz, so 440 oscillations per second. You can't actually hear those individually, but when there were detuned, you could sort of hear something that was maybe 5 or 10 beats per second, you could sort of hear [imitates sound] sort of sound that you could actually count off.
That's what the trick with the laser is. So, if you take two lasers that are pretty close together and listen to the beat between them.
[35:28]
So, we have one laser one frequency. If you have a comb of frequencies, so the lasers themselves are about 600 terahertz, but the spacing between the lasers is about 10 gigahertz, so about 50 or 60,000 times lower in frequency between them. Then you can actually measure the beating in between the lines using a piece of electronics.
[35:53]
And so, this is essentially a clockwork for light. So, it allows you to turn the direct oscillation of a laser beam into something you can measure using electronics, and so this was a revolutionary advancement for atomic clocks. It earned John Hall and Theodore Hansch the 2005 Nobel Prize. The work that they were doing in publishing was around about 2000 and were awarded the 2005 Nobel Prize. Theodor Hansch started the company Menlo, that actually commercializes this system. And so, you can see this is an optical frequency comb system to about as big as I am. And this is you can buy system from them today and there's a number of these in various laboratories around the world. they cost over $1,000,000. And this is John Halls team at NIST went on to keep breaking the world record for the best atomic clocks and so this is the strontium clock that I mentioned before, which I think still holds the world record. But you can see this is an enormous tangle of optical components on an optical bench. So, a very, very. delicate instrument.
So, it's way, way more accurate. These tools are extremely accurate. But there's still big there's still complex and remember the story about the atomic clocks failing on the Galileo satellites? There are some optical clocks on those satellites and they're also still fairly failure prone because they're made out of all of these discrete components.
So, we need something that's smaller, cheaper, and more robust really to really to make an impact.
[37:53]
Our vision is that you should be able to take the frequency combs system, for example, like the one that Menlo is working on here, but integrated onto an optical chip using integrated optics. And this is the area that y team works on is basically sort of printing photonic tools onto the surface of chips using similar technology to electronics.
And if you can do that, you should be able to make optical frequency comb systems that are as robust and cheap and compact as you would find in a piece of consumer electronics.
[38:30]
We showed two years ago, in May 2020, that we could use these sorts of chips for doing ultra-high-speed transmission and this is what my previous talk was about. Since then, we've done a lot of work on trying to use this for optical neural networks.
[38:45]
But we've also set up a collaboration with Andre Luiten at Adelaide University, who set up a company QuantX, to do atomic systems for measuring time, but also measuring other things, magnetic fields, and movement. And so, we've just been awarded a project with Andre to explore trying to integrate some of the components of his system which you can see in the background there onto a photonic chip.
[39:21]
So, enough about clocks. What else could microcombs measure well?
[39:30]
This is a little bit of a boring slide. I got it from the NIST website, but it I put it up here for a reason. These are all the standard things that you can measure and all the other things that you can measure can be made up from one of these things. So, you can measure length and weight and time, and you know electrical current.
The second here is actually defined in terms of oscillations of that caesium atom. So, this is the definition of time as it stands at the moment. It's not just a measure of time, it actually defines time.
[40:04]
What's interesting is that the oscillation of caesium atoms actually turns up in all of the other measurements as well, so this atomic oscillation that you can measure with a laser is actually you can measure anything with this tool in principle.
[40:29]
If you can measure the atoms oscillating, then in principle you can measure anything.
So just before I finish and I'm just about done, so there should be some time for some questions. Here's a few things that we've got in the pipeline.
[40:45]
One of the things we're doing with optical frequency Combs is we're working with some researchers at the University of Technology Sydney. Irena Kabakova is pictured here. She uses lasers and frequency combs to measure the mechanical properties of materials and actually living tissue as well. So, she uses laser light to excite vibrations in things like the cells of fish here and you can actually by measuring the different acoustic frequencies that are bounce back, you can actually determine what the mechanical properties are, how hard or rubbery the different parts are, and so you can essentially see the different parts of the fish here. But you can also measure things like if tissue has become cancerous, it changes its mechanical properties and becomes stiffer for example.
[41:44]
We're doing a lot of work with Advanced Navigation. They're in Australian company that sells navigation solutions to all of the automotive manufacturers and companies like Google and the thing that I particularly excel in is that inertial navigation. If you can work out what direction you're going in and what speed you're going at and how long you've been going there for, you can sort of chart a course of where you're going. There are situations where you don't have GPS, for example under the water, the satellite signals don't penetrate the water and go under the water. So, if you want to have positioning systems under water, you need to use some other mechanism.
In space, there's no GPS either, so if you want to have spaceships docking onto other spaceships, then you're going to need to have probably better than meter scale accuracy in terms of being able to position them. And again, there's no GPS to let you know where you're going so. We're working on tools with them to try and miniaturize their navigation systems and make them more accurate and more robust so that they can be used in these sorts of applications.
[42:57]
We're also working with astronomers. I can work with astronomers, like Jean Brodie, who has strong connections to the Keck Observatory in Hawaii. Using that observatory if you can calibrate tools for measuring the spectra you can actually measure stars which are Suns in distant galaxies, or stars in our own galaxy. And if there are planets going around those stars, the stars will actually wobble in response to the planets going around them. And as the stars are wobbling, you actually get Doppler shift, so the frequency changes ever so slightly and you can detect the presence of that planet from the shift and then you, there's a planet there, you can time your measurement. So as the planet goes in front of the sun, you can then actually do a spectral measurement of the atmosphere of the planet and so these are very, very small numbers of photons coming from these standards and very, very tiny shifts in spectral, so you really need a very, very highly calibrated source to measure this. And this is one of the things we're using our frequency comes for.
[44:14]
You can also use the same technique if you can precisely measure caesium atoms, you can precisely measure other atoms. For example, you can measure transitions in methane or carbon dioxide, and actually do measurements of the atmosphere. And so, we're looking at ways of monitoring emissions you know methane emissions, for example, from cows, and also improving agriculture. So, for example using drones to monitor the emissions from fruit trees to see how healthy they are and whether the nutrients you're using are being effective and whether there are disease trees. And so, we have a collaborative project with the Food Agility CRC here on that.
[45:03]
And last but not least, we you can use the optical fibre infrastructure that's used for telecommunications to actually measure vibrations and we’ve been using some of our frequency time systems with the optical fibre network that's attached to our RMIT to measure vibrations and this is me and out in the field with Sim who's here in the audience and Megan Miller from a new and Voon Lai, who both are seismologists from ANU. We're basically calibrating the fibres by tapping very lightly on the ground here to see the vibrations turning up back in our lab, this is reported on the Internet. You can use this for measuring things like cars going past you can the cars going past and trams, but you can also measure earthquakes, so they've picked up a number of earthquakes through the system in our lab over the last few months. And you can also actually just use the sort of background noise and hum of the city to illuminate the bedrock and actually measure the shape of the bedrock around Melbourne and something a little bit like ultrasound, but on a on a city scale. So, you can use this for monitoring the integrity of tectonic plates, but also you for mining safety and mineral exploration.
[46:48]
We've just set up a website to describe some of the things that we're doing. You can now find this at this web address here and there's more information about that story with the seismologist there.
[46:53]
And you can also feel free to ask me questions now or if you don't want to ask me now, feel free to send me an email or follow me on Twitter or LinkedIn.
Thank you.
Q&A – Distinguished Professor Xinghuo Yu
Alright. Thank you very much for the fascinating discussion, fascinating lecture. Any question.
[question from audience - inaudible]
Response – Distinguished Professor Arnan Mitchell
I think this is sort of coming to the heart of the problem with the oscillator is it's OK. It's very easy to count oscillations like I can tell whether it's today or tomorrow. But it, but you're right, it's hard to pick exactly the noon day sun. I'm sure they had techniques for doing it. It's very similar to the astronomical problem of saying oh, now how far away are these two planets from each other? It's very easy to see whether it's summer or winter. You can probably you can come up with ways of doing it, but it's just not as absolute as counting ticks, yeah.
Question– Distinguished Professor Xinghuo Yu
So, one question from the from online.
With the increased ability to detect / measure time on a smaller scale, have there been any unexpected ways that "time" doesn't match our expectations or assumptions of how time 'flows' or moves?
Response – Distinguished Professor Arnan Mitchell
So that that's a really, a really good question. So just to repeat the question is given accuracy. Are there any surprises so that is there any is there any anything that's turned up the that we don't expect?
I think the probably the answer is no, but the perhaps it is surprising that you can measure things like relativistic effects. So, Einstein predicted also proved Newton wrong. Einstein predicted that the only thing that was constant was the speed of light. That everything else changed the there's space could expand and contract depending on how fast you were going and time itself would, it would expand and contract. So, the Galileo satellites, the clocks on the satellites are accurate enough to actually be able to tell the difference in the passage of time not the measurement of time. The actual passage of time is different for the satellite as it is orbiting around the Earth than it is for the base station that's talking to it. So, you've got to constantly be correcting for who's time are we talking about? Are we talking about the time that the satellites experience or we're experiencing on Earth? This is all predictable like this is all, this all fits with relativistic mechanics, but even some of the tools we work on with advanced navigation, the way they work is as you rotate you have a coil of fibre, if you rotate it and even rotating it quite slowly, you can actually measure the difference in time it takes for the light to go one way around the coil then it does the other. And the time, the space itself has actually contracted in one in one dimension, which is, I mean you hear it when I heard about relativity and you thought ‘yes, that's good for black holes and the centre of the galaxy’, but to have it actually happening in your lab and being able to measure it is quite something.
Question– Distinguished Professor Xinghuo Yu
[question from Prof Xu - inaudible]
Response – Distinguished Professor Arnan Mitchell
So, we work with a collaborator of ours that you UCSB who's an expert in photonic integration, this is John Bowers. He has been showing photonic chip lasers, and so these are the sorts of things that you can just print out in their millions that are costs of cents. But they have sort of millihertz stability, so they are incredibly rate, like the most accurate. Accurate enough to sort of make these sorts of measurements on atomic clock, so the lasers themselves can be quite practical. This is only really happened in the last year or two that you've been able to do this in sort of manufacturing facilities, so the lasers are practical.
One of the big challenges is interfacing the lasers to the caesium atoms or the rubidium atoms. And so, this is a project that we're working on at the moment with Andre Luiten is OK how, if you're going to integrate all this stuff together in some cheap printed circuit, how do you, how do you get the atoms in there?
Question– Distinguished Professor Xinghuo Yu
[51:59]
[question from audience - inaudible]
Response – Distinguished Professor Arnan Mitchell
[52:19]
So, I guess science and technology again the engineers versus the physicists, I think there are some science opportunities like discovering habitable planets in other solar systems. That's a that's a big opportunity. Being able to measure precisely spectra using these sorts of tools, there's one particular piece of science called the sand gauge test, which is basically measuring has physics always been the same? So, there's a there's a fine constant that sort of measures how physics works and by looking at very, very distant stars, you can sort of measure the beginnings of the universe and basically look at whether physics from that time. So, we're working with astronomers to do that. So that's some of the science that's enabled by this.
Coming it back at the other end in terms of actually making these frequency combs, there's some quite sophisticated science and technology in actually generating the laser light in a way that is stable. Basically, making those oscillators that are a stable as atoms. The atoms come ready made. You've got to actually physically make an oscillator on a chip that sort of competes with it so that you can use the two as a reference from each other, and there's some quite interesting nonlinear physics in the way the light goes round the on the chips to that actually sort of generates the frequency comb.
Question– Distinguished Professor Xinghuo Yu
[54:01]
[question from audience - inaudible]
Response – Distinguished Professor Arnan Mitchell
[55:08]
I mean, I must say I'm just talking of the top of my head, but I would think they would know the timing of that to easily within milliseconds. These are all very, very predictable systems. I mean, I should mention that there's an assumption built in that these things are cyclic. That basically the sorbet around the sun and the moon's orbit around the earth is cyclic. It's not quite, each cycle is a little bit different there are wobbles and other things that go on, but they know about these and are sort of building them into their models as well. So, my off the top of my head, I would be surprised to discover it was, it wasn't milliseconds that they would know when particular things like the eclipse were happening. In terms of the different websites, I imagine they're targeting different locations.
Closing – Distinguished Professor Xinghuo Yu
[56:02]
Okay, please join me in thanking Arnan for an excellent lecture. Thank you.
Closing – Distinguished Professor Arnan Mitchell
Thank you very much Xing. I'll be around for a few more minutes if you want to just come and have a chat with me, I'd love to do that.
Closing – Distinguished Professor Xinghuo Yu
Thank you everyone for participating and I look forward to seeing you at another distinguished lecture.
END
17 May 2022, presented by Distinguished Professor Arnan Mitchell
Video blurb: From accurately tracking and estimating our Google Maps journeys to using biomedical imaging to gain detailed images inside our bodies, being able to measure things precisely underpins almost everything we do.
In 2005, two physicists were awarded the Nobel Prize for developing an approach – the optical frequency comb – to measure almost anything with unprecedented precision. This approach gave us the GPS we use on a day-to-day basis, however, it was also expected to change the way we measure many other things, from the gases in our atmosphere to the discovery of earth-like planets in distant solar systems.
Seventeen years on, the world-changing potential of optical frequency combs remains largely untapped, mainly due to their large size and complexity. Photonic chip technology – technology that can miniaturise entire lab benches onto a chip the size of a fingernail – may hold the answer. Distinguished Professor Arnan Mitchell discusses how photonic chip optical frequency combs could lead to 3D analysis of living organisms, map and monitor the geological structure of our lands and oceans, and allow brain-like machine learning to transform the behaviour of autonomous drones and satellites.