Quantum computing in the 21st Century – with David Jamieson

[Applause] well ladies and gentlemen thank you for coming to the lecture tonight it's a great pleasure to talk to you on one of my favorite topics um uh the contribution of Einstein to the foundation of a new field of Technology namely Quantum technology and the preeminent example of which is the quantum computer which I hope will be widespread in the mid 21st century I'm here uh today in the United Kingdom the Australian borders were closed for two years and I managed to as soon as they come it came down I was able to take up a fellowship that had been offered by the Royal Society a couple of years ago a Wolfson visiting fellow which has supported my visit and my activities here in the UK over the last few weeks but tonight's lecture Einstein's Revolution this is a field of research that I've been engaged with at a practical level for the last couple of decades and my aim tonight is not actually to present you with the blueprint of a 21st century quantum computer but to go through some of the physics that sits behind it and I should acknowledge that the University of Melbourne where I come from where I work is on the land of the cooler Nation the first uh discoverers and colonizer colonizers of the uh continent of Australia uh more than 60 000 years ago so this is indeed the University of Melbourne it's on a pleasant uh Green campus on the edge of the the city of Melbourne you can see that Blue Waters of Port Phillip Bay in the distance and the physics building is there in the corner and what's very important about the physics building is not that it's surrounded by numerous coffee shops where you can go to discuss physics anytime you like except when I'm giving lectures but also in the basement is my laboratory I have a large particle accelerator and a series of beam lines that we use for doing Material Science and in particular the material science uh directed towards our Quantum technology projects we also have a cryogenic system for measuring the properties of materials at very low temperatures particularly uh the the behavior of spins which I'll come on to later on and over here just down the hall from the big accelerator is a small accelerator which we use for insert pushing single atoms one at a time into crystals of silicon more on that later so here's the outline of my lecture tonight first of all I want to do a quick retrospective at the of the dawn of the computer age and what I call the first Quantum Revolution then I want to introduce you to the promise of quantum technology and the quantum atoms useful Spin and finally I'll wrap up with a quick review of some of the work we're doing on a building a large-scale device based on atom spins in Silicon so first a quick retrospective the first Quantum Revolution and the dawn of the computer age so just recently I made a major Milestone and I had occasion to reflect back over more than uh six decades on planet Earth so I just wanted to share with you some of those Reflections that brought me uh to where I am today so in 1958 okay I've given the game away the Americans launched their first satellite into orbit it was loaded with Geiger counters and it discovered the Van Allen radiation belts in the magnetic field of the earth charged particles particularly protons and electrons are trapped in the curving magnetic fields and they orbit backwards and forwards under the influence of magnetic forces these radiation belts can be hazardous to humans in orbit around the Earth and also to electronics as we'll see later on but if we fast forward to 1969 wow what a year that was with the Apollo missions and standing on the front porch of my home in Melbourne looking up at the moon and knowing there were a couple of people walking around on that moon was an extraordinary experience and I followed the Space Program very avidly now it was sometimes said that this was the dawn of the space age but actually it wasn't the dawn of the Space Age it was the dawn of the computer age because For the First Time humans had built machines which were too complicated for a human being to control and so the lunar module and the Command Module and the Saturn V rocket were all controlled by some Advanced for the day computer technology and in indeed the computer on the lunar lander here had a massive 2800 silicon chips each chip was crammed with six transistors and there was a whopping 70 kilobytes of core memory and this was responsible for getting the astronauts down safely onto the moon and safely back into orbit afterwards and if we fast forward to today uh there are now robotic explorers on uh the planet Mars This is the Curiosity Rover but the Martian atmosphere is very thin and radiation from space is able to reach the surface and that radiation consists of energetic ions protons the height nucleus of a hydrogen atom which is able to make it down through the thin atmosphere and pass through the electronics on the Rover and this inset here shows upsets in the memory of the computer every time a proton goes zapping through it and maybe changes a bit from a zero to a one or a one from to zero causes other upsets and the hardware and the software has to guard against those random perturbations from outside so that the computer continue continues to operate normally now 1958 was also a big year for Australia because For the First Time jet aircraft with a large capacity introduce Mass travel this is the Boeing 707. 1958 and of course if we fast forward uh to today we've got supersonic aircraft and you can go from Melbourne to London in just a few hours well unfortunately we didn't go down that particular timeline what we got instead was that and I I don't I think we went Australia there somewhere okay all right so never mind uh in 1958 if you wanted to make a telephone call you used a telephone for the benefit of the young people in the audience uh this is a device that has only a single app on it it's the telephone app you can't put the talk tick app on it for loading videos or anything I think that's what it's called on it and you can't take it with you when you go because it's Tethered to the wall by a cable today of course if you want to make a telephone call you almost certainly have one of those a mobile phone in 1958 one of the first single lens reflex cameras uh was when on the market the Minolta sr7 I have one of these in my vintage camera collection and this device employs some of the best quantum nanotechnology you've probably never heard of but unfortunately is now obsolete but I don't have time to say very much about the wonders of electrons in Quantum Wells inside silver halide crystals but it's a wonderful technology now sadly obsolete but fast forward to today needless to say if you're going to take a photo you'll use one of these in 1958 if you wanted to do a calculation you used a slide rule which exploited the properties of logarithms to convert multiplication into summation and if you fast forward to today you probably have a calculator app on one of those now let's take a big step back 60 000 years ago this is one of the first memory sticks that humans ever made made with an early form of lithography that is to say the data was scratched onto the memory stick with a sharp Rock a lithography process unfortunately what algorithm is used to interpret the nine bits that have been stored carefully on this memory stick has now been lost so we don't know the meaning of these nine bits but it's probably very important in human development if we fast forward to 1958 Jack Kilby discovered a way of integrating transistors onto single chips of silicon before that transistors came in individual packages and had to be soldered together but this idea of integrating many transistors onto one chip eventually won him the Nobel Prize in the year 2000 it took the Nobel committee a little while to figure out that this this invention conferred the greatest benefit on humankind that should be so uh yeah they Swedish committee Ponders very slowly and today of course if you want uh an updated memory stick you don't have to scratch marks on Bones you can buy a two terabyte uh USB drive that's an extraordinary amount of data that could be put onto one of these sticks now what's interesting uh that is that the technology to produce that sort of density of information is imperfect and this memory stick has on board special error correction mechanisms to cope with the fact that not every memory element works all the time and the onboard algorithms detect when memory elements begin to fail or didn't work in the first place and wall them off to prevent errors so the key takeaway from this technology is that it's possible to build large-scale devices from tiny tiny components with imperfect techniques and still have them work just like the Rover on Mars is imperfect because the radiation from space is constantly causing upsets in the memory so you have on boards on onboard algorithms to detect and correct those those errors this is an important concept which I'll come back to later on and today of course you can buy personal computer chips with uh billions of transistors integrated onto them and the cost per transistor is about 10 nano dollars that's an extraordinary number of human artifacts available at a very low price or about three Nano Euros a remarkable Advance since 1958. so all of these bits and pieces go together to what I call to make what I call the first Quantum Revolution the idea of quantum mechanics is the foundation on which all these gadgets computer chips Optical fibers mobile phones supercomputers solar cells implantable devices Medical Imaging systems all of these things rest on the foundations of quantum mechanics but most of these devices operate via classical principles the fundamental Quantum phenomena are buried down inside the chips inside the Silicon crystals which comprise the fundamentals of the of these gadgets and of course this has led to another a revolution in our society in the 20 20th century in the early 21st century and here we see the modern way of having a conversation the cafe using apps on the personal computers I don't know if that's an advance or not but this is the first Quantum Revolution so now I want to move on and talk about the seeds of a second quantum Revolution and to do that I want to illustrate some of the basic physics that have led uh to this SEC that is leading to this second quantum Revolution and so it's a great thrill to be speaking to you today in Thomas Young's lecture theater here at the Royal Institution where he surely did this experiment to an interested audience such as yourselves back in 1802 now this experiment revealed that light was clearly a wave passing through space in 1802 it wasn't clear what exactly was waiving but this was what Thomas Young did well not exactly this but close to this and and this is what it looks like in its modern form first of all you take a source of light um of course Thomas Young in 1802 didn't have an electric light bulb because it hadn't been invented yet so he used the light from a nearby star the sun obviously it passed it through a narrow collimator where upon it produced waves uh of Light which then it was incident on a barrier with two slits and the waves that pass through those two slits then created an interference pattern on the screen so I'd now like to demonstrate this in real time in the lecture theater so we've got our where you we've updated it with uh using a laser so it's a very bright source of light yeah so let's and um in uh in front of the laser there's this uh beautifully hand crafted uh two slits is that right Tom yep and you can see the interference pattern created by the light waves passing through the two slits where the peaks in one wave overlap with the peaks in the other way if we get a bright line and where the troughs of one wave overlap with the peaks of the other way if we get a dark line destructive interference so this is very different to what you would get if light was classical particles you could imagine throwing very small tennis balls at the barrier with the two slits and all you'd get on the screen here is the shadow of the two slits not this beautiful extended interference pattern that is a direct consequence of the wave-like nature of light thank you very much and I should have mentioned that the diagram down here on the lower right is in Thomas Young's own hand explaining uh why you get that row of uh into that interference pattern of the Brighton uh dark lines now um and here's one we prepared earlier in case it didn't work but it did of course it did it's physics always works if we now fast forward just over 100 years to 1905. Einstein had his

miraculous year where he published four new ideas in physics the consequences of which we are still unpacking even in the 21st century but the most radical idea that he published in 1905 seemed to contradict Young's conclusion from the two slit experiments that light was a wave Einstein wrote in the introduction to one of his 1905 papers according to the Assumption to be contemplated here in his paper when a light Ray is spreading from a point the energy is not spread distributed continuously over ever increasing spaces but consists of a finite number of energy quanta that are localized in points in space move without dividing and can be absorbed or generated only as a whole and this sentence in Einstein's 1905 paper is widely attributed to be the most revolutionary sentence written by a physicist in the 20th century so it's uh it's a tricky thing to demonstrate in the lecture theater but I want to give you an idea of how it works first of all I bought my Woolen jumper all the way from Australia for this demonstration for you and those of you who are admiring the painting out the front of this lecture theater will recognize this bit of apparatus that appears uh in the painting it's an electroscope and what I'm going to do is I'm going to charge up the electroscope by rubbing a few electrons off this plastic rod and bringing it into the electroscope grounding it and then moving it away I've just charged the electroscope by induction for those of you who are sitting up the back uh it might be a bit hard to see but um oh there we go so we can see the uh the two leaves of this electroscope have been deflected because they've been charged up now this electroscope has got a sheet of uh zinc on the top of it and I'm now going to shine a very bright light onto the zinc and knock electrons out of the surface of the zinc with this beautiful incandescent torch that's come out of the archives of the Royal Institution for me to use in this demonstration for you tonight so is anything happening nothing's happening of course nothing's happening because the photons in this torch are too feeble individually to knock an electron out of the surface of the zinc now the the zinc plate now we tried every other bright light in the possession of the Royal Institution this afternoon including their very dangerous ultraviolet light but unfortunately their ultraviolet light is not dangerous enough and the photons are just not energetic enough so ins so to show you this demonstration I'll have to show you a demonstration recorded in my laboratory in Melbourne but if you use photons that are individually sufficiently energetic to knock electrons out of the surface of the zinc you discharge the electroscope but this was Einstein's idea from 1905 to explain that phenomenon the light coming out of that torch or out of that laser or out of the very dangerous ultraviolet uh Light lamp that I'll show you in a moment consists of individual packets of energy that cannot be further subdivided so in other words you can't have half a photon you've got to take the whole Photon or not at all so this was a rather unusual idea in 1905 but it's been tested over and over again and found to be true and this is uh the photoelectric effect that Einstein eventually got the Nobel Prize for so here's the basic idea there's your piece of zinc which has got a whole lot of electrons embedded in the surface here's some waves of light approaching the surface of the zinc and here is the photon associated with those waves now here's where the strange idea comes in although that wave spreads itself out across the entire surface of that piece of zinc the photon can only be absorbed in its entirety or not at all so it's like the photon each Photon somehow tastes all the electrons on the surface of the zinc because it's a wave as it approaches it and finds one electron that it likes the taste of I'm I'm being metaphorical here that photons don't taste and it knocks one electron out of the surface so it's as if the photon in its wave-like manifestation is dispersed over a large area it's in multiple places at the same time but it's all absorbed finally in one place and gives all its energy to one photon so this is what um this is what we were ex what we can do in Melbourne with our very powerful light so I'm just going to fast forward this to two minutes we don't need the sound on this because I'll do the voiceover so here my trustee assistant Steve has got this very dangerous ultraviolet light he's going to turn off don't look at it Steve no then he brings it over the electroscope shines it on the zinc plate and you could see the uh the the leaf of the electroscope deflector so the charge is uh leaks away into the air in the room so that's what happens if your photons are sufficiently energetic to interact with the electrons and dislodge them one by one and none of these other Heights had an energetic enough photons another way of looking at this which is easier perhaps to understand is imagine you're surfing I guess this doesn't work so well in the UK as it does in Australia but imagine you're surfing and you see this wave approaching and there's a whole bunch of surfers lined up ready to Catch the Wave in Einstein's idea only one Surfer can catch the wave so it's like the other two Surfers left and right are left flat in the water and just the surfer guy in the middle catches the entire wave and so in some ways the wave the function that describes the wave collapses just to one point when it has to interact this is a fundamental mystery of quantum mechanics the other day under the terms of the fellowship I to go around giving lectures at different universities and I went down to the University of Surrey caught the train down on one of the non-strike days I emphasize and I saw this sign at Guildford Station and as a physicist this makes me very nervous because they're asking me to spread out along the platform like a Quantum particle but I'm too big I'm a classical particle so I couldn't obey that sign or maybe just reading the sign caused me to decohere into this one place so I could read the sign I don't know I think it's a very disturbing sign and there should be an asterisk do not take literally let's fast forward to 1911 when Ernest Rutherford reverse engineered the atomic nucleus discovered the nucleus surrounded by its cloud of electrons the nucleus is like the size of a dust particle floating inside a cathedral representing the cloud of electrons something very tiny and very dense and then his postdoc Niels Bohr just a few years later discovered the quantum atom he discovered how the electrons that orbit that nucleus that his supervisor had discovered how they orbit and they orbit not by classical uh trajectories like you have with planets orbiting the Sun but they orbit in quantum mechanical standing waves now what's remarkable about that idea is that quantum mechanics is starting to be manifest in more and more places not just with Einstein's photoelectric effect the other remarkable thing about Boar's paper both Rutherford and boar were working at the University of Manchester at the time of these two discoveries where I'm based at the moment was that Boar's paper was read for the first time at a conference in my physics department at the University of Melbourne I wasn't there in 1914 when it was read but the British Association for the advancement of science had its meeting in Australia in 1914 it rotated around the Commonwealth mostly it was within the UK but once it went to Canada and this time it went to Australia Rutherford was the president of the British Association his postdoc Niels Bohr wasn't there he was on holidays so Rutherford read the paper and it's remarkable that this this original idea of the quantum atom uh should have been uh uh heard at a conference for the first time in the old physics building at the University of Melbourne um the we have detailed records of the meeting and the Chemists in the audience thought that physicists should stick to physics and not meddle in atomic theory because everyone knew atom Square but they reluctantly conceded that there seemed to be some grain of Truth in these ideas even though they were quite radical but we now know that we can describe the quantum mechanical nature of matter using Schrodinger's equation now just in case someone stops you on the tube on the way home and asks you did you go to that Einstein lecture at the Royal Institution and did you see Schrodinger's equation you can say yes we did but Schrodinger's equation there's nothing mysterious about it it's simply what we're already familiar in the classical world that the energy of a particle such as this cricket ball propagating through space or this Moon orbiting that planet is just given by the sum of the energy due to motion the kinetic energy plus the energy due to any interactions between the two objects such as gravity or electrostatic forces but for quantum mechanics we need to be able to describe things that are in many places at the same time a wave and so we just translate these classical ideas the kinetic energy and the potential energy into functions so that we can find the shape of the wave represented here by the Greek letter PSI but it's exactly the same concept we're just replacing scalar quantities with function quantities and that then even for a simple system like an electron orbiting a proton in the hydrogen atom leads to these beautiful three-dimensional shapes that describe the nature of the electron Quantum mechanically speaking in its orbit around its proton nucleus in the hydrogen atom and this can be applied to many other elements in the periodic table including phosphorus atoms in the silicon crystal which I'll come to later on so the next step in our quantum mechanical Journey is the discovery of spin now it's worth spending just a few moments to talk about spin because it's actually a rather strange concept first of all I have a child's toy here a very nice top it's got a sharp Point here which means ah you just can't balance it you know just won't balance but look what happens if you give it a bit of spin it does balance it doesn't fall over like it does what would happen if it wasn't spinning but you can see in the gravitational field of the earth gravity is trying to tip that over and for reasons to do with the velocity Vector constantly going around in circles of the um rim of the top it processes around the gravitational field Direction which is vertical and as it slows down due to friction with the table that procession or that wobble becomes more and more extreme but this is oh until finally it succumbs to gravity as it always will but this is a classical object it turns out down inside the atom the electrons that are orbiting um the nucleus also have spin like this and so we represent them by objects that look like this so and this is just one I made myself from the stuff I got from the local craft shop and a very nice tube of self-adhesive glitter think of this as as a an electron or an atomic nucleus and um it's spinning about its axis and we represent the direction of the spin using the right hand rule where my thumb points in the direction of the the vector representing the direction of the spin now what was discovered in 1921 was that inner silver atom which was known to have no spin except for the last electron added to make up the whole atom which was which was all by itself in the outer shell so the whole spin of the silver atom was basically carried by that last electron and when you boil silver atoms in a vacuum out of a furnace and pass them through a very strong magnetic field you find that the beam of silver atoms splits in two when it passes through the magnetic field in the same way we saw the wobbles of this top due to the presence of the gravitational field but now we're talking about a quantum mechanical spin in a magnetic field but the physical principles are very similar and when this experiment was done first of all this is the result of a beam of silicon atoms hitting this postcard without the magnetic field you just see a line of silver atoms piling into the card then when you turn on the magnetic field you find the line splits in two they're not two parallel lines because you need to use a very strong inhomogenous field but this discovery was immediately reported to Niels Bohr who by then had left Manchester and come back to Copenhagen to head up a new Research Institute because it was so exciting so unexpected that down inside the atom there were objects we now know are the electrons that were spinning and they process in a magnetic field in the same way this top processes in a gravitational field and what was mysterious was that the spin could only have two possible orientations reflected by the fact there are only two lines here so it was concluded that the quantum mechanical attribute of an electron was such that it could either be spin up or spin down but nothing in between space was found to be quantized it was like this top could only wobble like that or like that nothing in between which of course is not the case for the real top because it's a huge macroscopic object and quantum mechanics is hidden so this was a further Quantum attribute of matter that was discovered in 1922 now you can see the same effect by taking light from the sun and passing it through this a slit and into a spectrometer like dispersing the light through all the colors of the rainbow and here is this black line represents the slit of a spectrometer and it passes across a sunspot where the magnetic field is very strong and sure enough when you look at the the colors of the light coming from the surface of the Sun you find that away from the Sunspot it's a single color and then as the light comes from the Sunspot in this case it splits into three not two three and this was later found to be due to the fact that the spin had a different quantum number an integer rather than for an electron the spin is always one half so this uncovered a lot about the internal Machinery of how electrons orbited uh the atomic nucleus so all of this stuff was boiled down into the first iteration of quantum mechanics of which Paul Dirac here in the UK was one of the leading exponents but in 1928 he made the somewhat depressing observation the underlying physical laws necessary for the mathematical Theory Schrodinger's equation of a large part of physics and the whole of chemistry athas completely known I don't know if the chemists would have agreed but that's the physicist View and the difficulty is only that the exact application of these laws leads to equations much more much too complicated to be soluble so let's go back to the caffeine molecule what a wonderful molecule it docks with all the physics receptors in the brain and makes them go around faster and you get new ideas and you it's great to talk about quantum mechanics and spin and directs depressing observation but the caffeine molecule has got 100 of spinning electrons orbiting the carbon the hydrogen and the oxygen that make up the molecule that's a little electron I made as well which can only have two orientations in space um and the number of electron electron interactions across those 100 outer shell electrons in the molecule are governed by Schrodinger's equation but there are so many interactions that even the world's best supercomputer is powerless to calculate the structure of the caffeine molecule it's just too complicated for a classical computer to solve even the most powerful supercomputers today can only do about 30 electrons and every time you add another electron you get more and more electron electron interactions and the problem becomes exponentially difficult so there it sat for 70 or 80 years but in 1959 Feynman gave this remarkable lecture there's plenty of room at the bottom where he put forward this idea for a new type of calculation not just using classical circuits but a system involving the quantized energy level or the interaction of quantized spins in other words to build a computer that employs Quantum spins as its bits instead of classical bits that would allow you to use a Quantum system to model another quantum system but there was no technology in 1959 to do anything with that idea but we are trying to do something with that idea and this is a phosphorus atom that goes into a silicon crystal beautifully and we can see it's got a nuclear spin inside the phosphorus nucleus and there's a single electron orbiting around it which also has a spin which can be harnessed for technological applications so let me explain the power of a Quantum bit instead of a classical bit first of all let's take this Abacus used for thousands of years and the same principles behind that Abacus is also at heart at the heart of the classical processing that's going on in my mobile phone here now the way an abacus works for those of you who need a quick refresher is that the value of the bit is determined by its position on the wire in the frame of the Abacus so that bit on the top right there is in the down position that's called zero but if you flip this the bit and put it to the up position that represents a one so effectively what you've done is you've replaced the bit with a an arrow which we can call a vector and by flipping the vector through 180 degrees you change the value of the classical bit but they're the only two possible values that classical bit can have it's either down zero or up one now imagine grabbing the wire out of the frame of the Abacus and allowing it to occupy any orientation in space this is what a Quantum bit is a Quantum bit exists as a superposition of being in two places at the same time like Einstein's Photon being a wave and it can be in a the sum of the zero State and the one state which determines its orientation in space there's only one catch you can exploit this remarkable uh uh large versatility providing you don't observe the qubit because as soon as you observe it it'll instantly Deco here just like the photon changes from a wave to a particle when it interacts with just one electron into its classical values of zero or one so you can do massive computations to exploit this enormous phase space as we say in technical speak providing the quantum computer exploiting this technology doesn't interact with the outside world until it finishes the calculation that's a challenging thing to do nevertheless in 1998 young postdoc at the University of New South Wales Bruce Kane published this single author paper in nature the journal a silicon-based nuclear spin quantum computer based on phosphorus atoms embedded in a silicon crystal lattice this paper has been cited thousands of times and triggered a reappraisal of the idea of qubits in an engineered device today with my colleagues in the center for Quantum computation and communication technology in Australia we have refined Bruce Kane's architecture into something we believe we can build based on single phosphorus atoms embedded in Silicon Each of which is individually controlled by electrodes integrated into the surface of the chip in the same way there are myriads of electrodes integrated into the um the classical devices inside this phone so basically we're using the standard tools of the semiconductor industry perfected now over 60 or 70 years to enter the quantum domain so this is now the second quantum Revolution just emerging and why do we want to do this well getting back to feynman's idea of using a Quantum system to model another quantum system one day the next fact the next pandemic may be fought not with test tubes and Petri dishes in the traditional way but by reverse engineering the spike protein in the next virus and then Engineering in Silicon the shape of a molecule needed to gum up the works and act as a vaccine this is a dream uh which would have enormous benefit and today quantum computers are already being built with tens or even hundreds of qubits where the first tentative steps in that direction are being taken but if you were going to divide devise a vaccine you're going to have to do massive calculations you will need millions or even billions of qubits and what's the cheapest qubit it's possible to make a qubit based on a single atom atoms are really cheap there are other Quantum attributes including entanglement barrier tunneling coherence D coherence Schrodinger's Cat and teleportation that could also be incorporated into new Quantum Technologies but I don't have enough time to say very much about them so in the last part of my talk I want to talk I want to describe some of the work we're doing in building a large-scale device by using iron implantation to insert single phosphorus atom qubits into silicon crystals and this cartoon shows the way it might work we see a whole array of phosphorus atoms just below the surface of a silicon crystal and each atom is controlled by an electrode integrated on the surface and by changing the bias voltages on the electrodes we can cause the qubits to interact without doing any measurement so that they retain their delicate Quantum states of being in two places at the same time so this brings me to the classical Quantum divide after Young's two-sleep interference experiment the question was asked okay it works beautifully with photons does it work with matter can a particle of matter like an electron being two places at the same time and this is a diagram taken from feynman's famous lectures in physics I'm putting this up because I once sat next to Feynman at a colloquium when I was a postdoc at Caltech and I'll tell you the Charisma wow it was amazing what a remarkable guy not a little dangerous perhaps but um a brilliant physicist but in his uh famous textbook he uh described the way Mata behaves using Young's to Slit interference experiment he said you can take an electron gun emitting particles of matter electrons you can direct them to a wall that contains two slits and over here on the backstop there's a detector for measuring the position of all the electrons that pass from the electron gun through the barrier to the detector and when you do this he said you will end up with a Young's two-slit interference pattern not the classical shadow of these two slits but he comments this experiment has never been done in this way because when these lectures were published in the early 60s there was no technology for detecting single electrons at a time but folks let's look at the shocking truth of the way this experiment works an electron a particle of matter emerges from the electron gun and passes through both slits the electron a particle of matter is in two places at the same time because somehow it turns from a particle when it emerges from the electron gun into a wave passes through both slits and then converts itself back into a particle to make the detector go ping and it all goes to one place just like Einstein's Photon in the photoelectric effect Humanity finds this rather shocking the idea that a particle can be in two places at the same time so when we teach this in undergraduate physics we bury it under a thick layer of mathematics to conceal from the students the shocking truth of what's actually happening here and if they ask questions about what's actually happening here we just say shut up and calculate I've been doing it for 20 years but I reveal the shocking truth so here we have the electron behaving like a Quantum wave as it passes through both slits at the same time and then here we have the electron behaving like a particle when it all converges on the detector and makes the detector go ping so when you move the detector up and down along the backstop you record the intensity of the electrons forming the interference pattern which you saw in the lecture theater at the beginning and so today this is the Quantum classical divide we need a classical readout device for our quantum mechanical operations that we've engineered into our device and I was very taken with this paper published in 2013 where the authors are very cheeky they copy feynman's diagram from his book and say actually we have done that experiment they did take an electron gun they did pass the electrons through the two slits and they did have a detector and they recorded the the interference pattern as the electrons interfere because of their Quantum attributes the more a group in Austria has done the same experiment with a bucky ball 60 carbon atoms in this giant molecule fired at a grating and the molecule can be in two or three places at the same time and make a Young's two-slit interference pattern I find that very spooky but a wonderful aspect of the real world and today this uh classical Quantum divide seen here where we have Quantum attributes is engineered into quantum computer devices the prototypes these are superconducting qubits consisting of little magnetic circuits where the magnet magnetic field can be up or down or in a superposition of up and down and each of these devices now has a classical readout device to report the results to the outside world our architecture consists of a single phosphorus atom in a block of silicon with control circuitry the spin of the phosphorus atom is our Quantum particle it can be spinning in both orientations at the same time represented by the Double Arrow here and then there's a single electron transistor formed by these electrodes which is connected to an oscilloscope and all these guys are looking at the oscilloscope to Deco here the nucleus into either the spin up or the spin down orientation The Observer has to be in the loop it seems and the first time we got this to work I was in my office in Melbourne I got the call from Sydney where this the measurement is done after we implanted the phosphorus atoms and they said it's working after all these years of work finally we've got a device that is displaying the quantum attributes we've been eagerly seeking so I said don't touch anything don't touch anything I'm coming right now I went to the airport I got on the next available flight you know how much Qantas charges if you go to the airport and say put me on the next flight to Sydney it was extortionate but I was in the lab a couple of hours later and I saw with my own eyes something I'd been teaching for two decades actually happening in the laboratory we could see the electron spin orientation we could see the nuclear spin orientation we could see that nature was digital at the fundamental level so this is the heart of our device a silicon crystal lattice consisting of silicon atoms and one of which we replace with a phosphorus atom using iron implantation now silicon is not perfect there is a rare isotope naturally occurring called silicon 29 about five percent of natural silicon and it has a nucleus which is a bit peculiar it's got an extra Neutron and that in new extra Neutron confers spin on the nucleus whereas all the other silicon Isotopes have no spin in the nucleus because all the nucleons are beautifully paired up together so that's like having a little bar magnet inside our chip which is interacting with our phosphorus atoms and destroying the quantum coherence so step one is to remove the Silicon 29s and replace them with silicon 28 the zero spin isotope and this is like a vacuum with nothing going on except what we've engineered into it with our phosphorus atom it's the next best thing to a real vacuum for doing quantum mechanics we then use radio frequency pulses to flip the spin orientations from the zero state to the one state or indeed to put them in the quantum superposition of being spin up and spin down at the same time this is what the device looks like this is about um uh I guess about a million times actual size on the screen in the lecture theater right in the middle is a small area where we implant a number of phosphorus atoms and then we use these control gates to tune in on just one of them and that's where we do our Quantum experiments and this is what it looks like in a cartoon a single electron is brought across onto the phosphorus atom then we do a radio frequency pulse to put it in the quantum superposition this is all at 100 Milli Kelvin it has to be very cold and then by controlling the position of the electron we move it back into this single electron transistor there's a flow of current and that's connected to an oscilloscope and we can see the quantum state of our electron and then we reinitialize the device and it starts all over again so here's the quite spooky Quantum bit in our device Einstein in 1916 said imagine you've got a Quantum object which can be in two energy states a low energy State and a high energy State and that is indeed what happens when you put a spin in a magnetic field it's like putting two bar magnets together when the North and South poles are together they stick together beautifully but if you try and push two North Poles together it doesn't work it it's in a higher energy state will spontaneously flip now in a spin which can be up the low energy state or down the high energy State you can flip the spin by hitting it with a photon there's a lot of photons going on here but this is a microwave photon now Einstein said if you're in the high energy state after the low energy State absorbs a photon and goes to the high energy State another Photon of the same energy will cause it to Decay back to the ground state so while the photons are on they are both lifting the spin into the high energy State and inducing it to Decay back into the ground state so when you turn the photons off the spin is in this Quantum superposition of being up plus down and because I've been lecturing to undergraduates I have to put you know this is the spooky Quantum bit of the device sorry about that um and then when you read it out you decoherit the detector goes ping either in the spin down State high energy or spin up State low energy and you repeat that over and over again with increasing time intervals after you turn the photons off and if this was a classical particle 50 of the time when you do the measurements you'd find it's in the high energy State and 50 of the time it would be in the low energy state it would just be random and indeed Galileo and Newton would predict this is the result you would get for this experiment but when we do the experiment because this spin is a Quantum entity we find it's not a probability of 50 up and fifty percent down it actually oscillates between the up and down configurations because of its Quantum nature the wave-like nature of the spin and the longer those oscillations go on the more Quantum calculations you can do when it's in that superposition of quantum States and we found that the atomic nucleus can last for more than 30 seconds in this superposition of up and down because of the duration of these oscillations and 30 seconds is an eternity for a Quantum state and so we were very pleased to get this expected Quantum state so this is our device we remove the Silicon 29s we implant a few phosphorus atoms we tune in on just one and we do all these experiments but this is not good enough for a large scale device so just to conclude let me explain how we're going to take the results we've obtained just on our single phosphorus atoms and take it to the tens or even hundreds this is a silicon lattice we implant phosphorus atoms into it using the standard techniques of the semiconductor industry there's probably 15 or 20 different Iron implantation steps that have gone to make the Silicon chip in here then we heat up the Crystal and we push the implanted phosphorus atoms onto the crystal lattice site but the way that's done in industry it's like raindrops on a window the raindrops fall at random points on the window this is not good enough for making a large-scale device we have to have order and so what we want to do is make this deterministic non-stochastic we want to put the atoms in an orderly array so they can be coupled to the surface control electrodes and we can do large-scale quantum computations and it better not take too long so we have re-engineered an atomic Force microscope that has the Precision to localize the implants using this tiny Nano aperture so the iron beam Rains Down on the cantilever there's the little tip of the atomic Force microscope and every time a single phosphorus atom passes through the Nano aperture and implants we hear a click and so here folks is the sound of single phosphorus atoms being implanted into a silicon chip let me just get the sound going here [Music] is it uncertain that one oh another one another one you can it's like rain on a tin roof and every time we hear one of those pings we move the cantilever to a new location and wait for another one to come down this soundtrack was featured on the Australian broadcasting commission's sounds of science so if you Google it up you could download it and use it if you as your ringtone if you wish so we are now in the process of building a large-scale device using this technology we have already built a three qubit device just by luck we found two phosphorus atoms which were close enough together that we could couple them and this is the artist's impression of those two phosphorus atoms with a single electron in orbit around both of them this is an artist's impression but you've got to have fancy artist impression if you want to get on the cover of nature as we did back in January with this discovery and this device worked beautifully we were able to maintain the spooky Quantum state for a very long time and we made a very high fidelity gate set something that is the building block of a large-scale machine and this was very satisfying even though we made it only one device out of many we made work like this because we were just implanting the atoms at random but now we're going to impose order on our device and build large-scale devices of single phosphorus atoms in our Ultra Pure silicon and so I hope you'll stay tuned for more developments in this area in the years to come thank you very much for your attention [Applause]

2022-10-01 03:28

Other news