Transcript

Cats wanted dead and alive: quantum technologies | Professor Ben Murdin | TEDxSurreyUniversitySalon

thanks very much yes I want to talk to you about quantum physics and um we've heard a few people mention quantum physics already as something that we ought to be uh more aware of I suppose and understand a bit better and uh maybe I should just ask you first of all could you raise your hands if you think you understand quantum physics okay one one or two that's great okay it was sort of a trap really if I asked you that question because somebody much smarter than I did once said if you think you understand quantum physics then you actually haven't understood it properly um but I want to talk to you today about some quantum physics effects and the reason why I find it so fascinating is because of the fact that it's mindboggling um we aren't you really used in our everyday lives to encounter this uh um these kind of properties but at a small scale small things like atoms and electrons can be two places at once and do two different things at once and I don't know what your superpower would be but this is my friend Ruth she's um a a singer and musician and she would really love to be able to to to have her own band where she plays everything at once and I just um would like to sort of offer to you the concept that uh you could be really really powerful and you could do really amazing things if you could uh obey the laws of Quant physics yourself and we're trying to move towards that situation where we have big bigger and bigger things obey the principles of quantum physics go the next slide please okay so quantum physics is all around us actually and you will have encountered quantum physics um several times you'll have seen it yourself whenever you walk around under sodium Street lamps the thing that determines the color of those sodium Street lamps is quantum physics and here is another example this might seem like the physics of something really big but actually the the red stuff in this picture is uh hydrogen it's being stimulated by the hot blue things in the picture the Stars and the color that they emit is a bright red color astronomers call it the H Alpha line it's 656 nanometers if you're scientifically minded some some people often ask me what's your favorite color and I reply 488 nanometers but I don't know what your responsible be um the reason why the hydrogen atoms do this is because of uh the laws of quantum physics have the next slide please so here we show um some waves the reason why the hydrogen atom does that is because it's behaving like a wave and you're used to wave physics you understand um the way that musical instruments work I bet if you pluck a guitar string uh in the middle of the string it will do a motion uh the fundamental mode of the string uh and you can pluck it maybe a quarter of the way along and 3/4 of the way along and let go and it will play the first harmonic that's one octave higher and all that the electron that's orbiting around the hydrogen atom does is just a three-dimensional version of exact the same thing so there are specific oscillations that the that the hydrogen atom can can perform and when the atom jumps from one state to another it emits a a light of a very specific color uh corresponding to the uh energy jump between those two states and that what's what gives you the very characteristic lines and different atoms emit different lines that's why sodium emits uh this yellow light that you see in street lights and why hydrogen is the red uh that you saw on the previous on the previous picture Okay so a great thing about uh W waves are the thing that makes them different from our sort of everyday experience of of objects and particles is that they produce interference so if you add two waves together they can cancel each other out or they can amplify each other so here's a picture where I've shown uh a pair of uh light sources and they are producing uh an interference pattern uh and you can uh sort of understand that in terms of waves but if I said now what if particles and objects could do the same sort of thing supposing I took a pair of footballers and I put them uh at one end of the football pitch and I told them to shoot towards a goal it would be really surprising if I could set up the goal in some place at the far end of the field where it would be impossible for the footballers to get the balls in but actually atoms do exactly this we can make a pair of sources of atoms and we can fire the atoms out into space and we can put a detector in the back of the of the uh of the space where we're firing the atom and we can set it up so that um the atoms have no probability of going into the detector move the detector a little bit to one side and then the atoms go through so atoms and small scale things obey this wav likee property U we're just not used to seeing that on a large scale can I have the next click now you can also interfere waves in time as well if you add two waves together you can get new kinds of motion so if um you add the fundamental mode of your tar string and the uh and the one octave higher you can get a slightly jelly like wobbling motion and the same thing with the atoms you can put them into a a situation where they are doing two things at once they are both in the low frequency mode and they are in the high frequency mode at the same time so when a Quantum physicist tells you that something can do two things at once and it can be in two places at once I mean what what if I ask you this um first harmonic wave where is the wave you would say well it's sort of in two places it's at the top of the string and it's at the bottom of the string it's in both places at once so when as I say a Quantum physicist tells you things can be in two place at once or it can do two things at once what they really mean is it's obeying wave physics so here's a little um guitar demonstration you can see if you click the button the move should start yes so you pluck the guitar string in the middle and you see it just does the fundamental mode of uh vibration if I played in slow motion we can um see that uh putting your uh finger on the 12 fret that's the one with the two spots that's exactly halfway along plays this uh fundamental mode but I can play the second harmonic uh the first the first uh excited say one octave Higher by plucking the string a quarter of the way along I can do something a bit more complicated I can play a harmonic chord both notes at once if I pluck the string a quarter of the way along but I lightly touch in the middle now you can see that the the wave motion is flowing back and forth along the the string it's actually playing both notes at once if I had recorded the sound of the guitar at the at the same time as making this movie you would actually be able to hear the difference in the first case you would hear a low note the second case you would hear a high note and then with the harmonic chord you would hear both notes at the same time now okay these wavelike properties uh what can they do for us so um one of the fathers of quantum physics win Schrodinger um thought of an experiment that you might be able to get us this small scale properties to the larger scale and he imagined um a situation what would happen if I took an atom and the atom I put it into this situation where it's doing both things at once it's both in the the relaxed State and it's in the excited state at the same time and supposing I take a detector which is capable of sensing which of the two states the atom is in then presumably the detector will be triggered and untriggered at the same time and supposing I collect connect it via some mechanism to a bottle of poison and the bottle of poison can be then both broken and unbroken at the same time and what if I put a cat next to the bottle of poison the cat can then be both poisoned and not poisoned and strange things about quantum physics are that if you don't look at the situation if you don't make a measurement then the objects can be in this two states at once so the cat ought to be both alive and dead if I put all this stuff into a box close the box if I put the atom into the excited State gradually the atom will relax as a function of time it's the probability of being in the excited state will will Decay but the atom is actually in both States at once and therefore the cat should be both alive and dead at once uh actually Shing has postulated this thought experiment in order to try to show how absurd quantum physics was he he actually didn't believe it he thought there was a fundamental problem with quantum physics actually we now believe that it it's true you can put large scale things in two states at once now why has nobody ever done this experiment well okay apart from being in inhumane it's actually a really really really hard experiment because of the number of atoms that are inside a cat I talked about the ability of very small objects to obey wav like properties um there are 10 to the^ of 26 atoms inside a cat that's a one with 26 zeros uh on it that's a lot of things to put into this uh uh very carefully controlled State and the other reason why why nobody's done it is because it's actually a really boring experiment the the idea that you would set up the atom in the excited state put everything into the box close the Box wait for the thing to be in two states at once and then eventually open the box and see what state the cat the cat is in it once you open it and look inside the cat will then decide either to be alive or dead and sometimes you'll measure alive and sometimes if you repeat the experiment you'll measure dead and the sum of those two probabilities will be 100% so what who cares but maybe we can add control supposing we can control whether or not the cat is alive or how much alive it is now we start to get to something really interesting and in fact um this kind of idea has actually been around for a long time I don't know if anybody here has ever had an MRI scan uh if you've had an MRI scan you will have been put into a a machine that actually puts all of the hydrogen atoms in your body into the state where they're both spinning clockwise and anticlockwise at the same time and when they do that when they're in this um superposition State they emit radio waves you can detect and therefore you can see how much hydrogen there is and where it is and that's what gives you the contrast on the MRI image it's telling you where the water is in your body and where there's water you see radio waves and when there isn't any water you don't see them and where the radio waves come from is just the hydrogen atoms that are in this superposition state so it gives us um some really interesting Technologies but there isn't any information technology that uses this idea yet uh Information Technology we're pretty familiar with it makes our silicon computer chips um and here's some some pictures of uh actually not quite the current smallest but this is a the small uh from a couple of years ago one of the smallest transistors that had ever been made 50 nanometers now we're down to about 17 nanometers being the smallest uh transistors that's about that's um uh 20 billionths of a meter it's extremely small and the information inside those transistors is carried by whether or not the transistor is on or off gives us the ones and zeros of binary information and we're getting to the point now where where we can um uh think of uh making transistors where the information instead of being zero or one the transistor being on or off we have transistors where the transistor is both on and off at the same time next slide please um and even better than that we can actually couple transistors together to get pairs of transistors imagine if I have a pair of atoms um there are there's now um four pieces of information can be stored in just two atoms because I need to keep track of the probability that both atoms are excited that uh the left one is excited and the right one is relaxed uh left relaxed and right excited or both relaxed that's four different probabilities I need to keep track of that's four pieces of information are in that system whereas if I have two ordinary transistors there's only two pieces of information it's uh the left one is on or off and the right one is on or off and you can imagine if I have three transistors then there' be eight pieces of information and so on it goes up exponentially so Quantum transistors and quantum computer registers would be really powerful for Quantum information technology and we're getting to the point where we can engineer single atoms inside silicon chips here is my friend Philip he's a research student um at University College London and we've overcome the challenge of being able to position an individual atom on the surface of a silicon chip now uh here you see a micrograph that shows a silicon crystal the surface of the silicon crystal is is Illustrated you see each individual silicon atom in the Crystal and he uh Philip has positioned on the surface a single atom single impurity atom and it's bulging out at the center it's also causing the neighboring silicon atoms to bulge out a little bit as well but there's just one single impurity there that is positioned with atomic precision and that's a really amazing engineering challenge it's a bit like um it's actually exactly the same scale factor as trying to manipulate individual ping pong balls on the surface of the Sun from Earth so it's a a really amazing feat of engineering uh and how do you pluck these atoms how do you cause them to go into this two states at once and the this strange light oscillation of being both excited and relaxed whether you need um some clever lasers uh use laser pulses to do the plucking of the string uh and here are some results I hope I don't get scooped we were talking we the previous speaker on the Ted conference was talking about being being scooped these are unpublished results so please don't tell anybody don't um but we've demonstrated now with these individual atoms that we can get these interferences so that the the data here shows clear oscillations and I've can I've shown here the the the spectrum of the oscillations showing that we get this very clear note uh lasting for long times after the original plucking of the string uh and this is evidence that we have um Quantum interference in these individual atoms so we're getting very close now to the point where we can make a big computer register and i' just like to end with a little bit of a movie to illustrate how you might use or imagine the power of a of a quantum [Music] computer in a complicated World there are always so many things to do imagine how efficient you'd be if you could split into multiple versions of yourself the ultimate in multitasking and what if you could then get your friends to do the work for you by getting them to split so that they could be in two places at once this magical behavior is exactly what goes on in the quantum World atoms can indeed be made to split into several versions of themselves we say they're in a superposition of different states they can also spread themselves over multiple locations and entangle with other atoms this is the basis of a quantum computer in which all possible computations can be carried out at once Okay so that was maybe a bit futuristic it's going to be a long long time before we can clone individual people because I say the the difference between a big human person and a small atom is quite large uh but actually we only need to get of the order of a 100 atoms into this two states at once in order to be able to make uh the most powerful computer that you could possibly imagine you in inside your ordinary mobile phone there are billions of transistors but I only need 100 Quantum transistors in order to be able to beat that that phone and so we're we're getting to the place where we can really imagine a revolutionary change in our Quantum Technologies but okay so I started this talk by talking about the mental Challenge and actually that's what really motivates me in doing this research is I find it just so strange the quantum ideas of being in two places at once and doing two different things at once and trying to understand uh what's going on when I do my experiments uh is uh sort of a surreal um it's a surreal uh idea and uh so academic research in this area I find a fascinating thing and I um so that's why I love to to work in this area and I should also say that um uh something that the previous speaker mentioned we should be trying to teach uh smaller children these kind of ideas I think um I've been well I've actually had an experience of this going on um SC going to science fairs like the big bang science fair and the raw Society summer science fair and meeting children and telling them these ideas being in two plac at once they don't bat an eyelid so the more we expose them to this kind of thing uh at an early age uh the better we'll all understand quantum physics so I'd just like to thank my sponsor um can you get to the next slide yes so my my sponsor epsrc and my friends who generated these these results and thanks very much for your attention [Applause]