College Professor Explains One Concept in 5 Levels of Difficulty | WIRED

hi i'm sheikha ramanathan i'm a professor at dartmouth college and today i've been challenged to explain a topic at five levels of difficulty so what is quantum sensing we're looking at the rules of microscopic world which is quantum mechanics and using those tools to help us build the ultimate sensors which means that they're as precise and as accurate as the laws of physics allows [Music] what's your name namina our topic today is quantum sensing so quantum is about the study of stuff that's really really really small and sensing is about measuring so the word sensing comes from kind of like our senses so do you know what your five senses are seeing hearing um tasting and smelling mm-hmm yeah it touches touch exactly so it's kind of really important for us to be able to have these senses so we know what's happening in the world around us right we're doing quantum sensing is we're trying to measure things that can be hard to see let me show you can you see inside it with your eyes no okay can you bounce it for me do you know what makes it bounce i think like inside of it it's foam that's fluffy but my second answer is i think it's very soft that's a great description can we cut one open and see what it looks like yeah i think that's a good idea here's a bowl that's been cut right in half you look inside it's hard this what gives it a certain texture kind of like the texture of like the top of a crayon but you were right that it was like foam it would be really cool if we could see inside the ball without cutting it open right but you could use a magnifying glass and then look at the ball but with the magnifying glass you'd only be able to see it's right near the surface right just you won't be able to see into the middle if you had the right tools you could start thinking about ways to look inside the ball without cutting it open that would then you'd still have your ball we could still play with it yeah it would be cool if we like use something like an x-ray we built an x-ray that was only made for both and you could see everything inside it every single detail you could zoom in and out in it and you could draw it print it out that's exactly the type of thing that we're doing we're sensing is we're trying to measure what's inside with right and do it without destroying the ball yeah for example we want to go inside let's say the human body and see what's happening sometimes we can look under the surface of the earth and see what's underneath it we can make really really precise clocks that will tell us that can measure time really really accurately and we can make very very fine measurements that'll tell us about the rules of science and how the world works around us but we need to build better tools that allow us to do that [Music] our topic today is going to be quantum sensing have you ever heard of it before no no what do you think it might mean if you just break down the words something on a very small scale because of the word quantum the sentencing part i'm not sure so sensing is really just about measuring stuff okay and at some level there's a different set of rules that seem to come into play as you can have particles at very microscopic scales seem to do really strange things but one of the quests of quantum sensing is to harvest some of these unique properties at the micro scale we are really interested in quantum sensors because we think they can give us the ultimate limit of sensitivity so they're really really sensitive to small changes but they're also going to be really reliable every time i make the measurement they'll always get the same results okay measurements on like what kinds of things could be in almost anything you want have you ever had broken a bone i fractured something though okay do you remember having an x-ray yeah x-ray and i have had a few mris before you've had an mri mris before and so both of those are in some ways a form of sensing and they rely on different types of sensing do you know what this image is maybe an mri exactly do you know what an mri how an mri works no i don't and i feel like i should because i've gotten them millions of times and what the mri scanner is doing is it's measuring the signal from all the water molecules that are present and specifically the hydrogen atom in our bodies we have these hydrogen atoms are essentially spinning around magnetic fields all the time and we just don't know them so in some sense you've already used a quantum sensor yeah so our mri is essentially more detailed x-rays they're not so they're giving us different types of information okay so this is an x-ray you don't see any of the soft tissue the x-ray gave us information about the bone yeah whereas mri is giving us information about things like the softer tissues yeah and in fact we don't see the bone very well and yeah so there are slightly different reasons why you would choose a two different things suppose i could have a higher resolution what would you think i would be able to see the different atoms and the structures of the particles start to see the different cells and yeah the different chemicals in the cells if you look at the mri images you can see that they give you the broad features of what the tissue looks like but if you want to zoom in a little bit more and see what's actually happening inside a tissue or inside a cell and you need a different type of sensor it's going to be more sensitive and for something like that you're going to need a quantum sensor are there different types of quantum sensors for different things so one of the quantum sensors that's related to the work that i do is based on these defects they're called nitrogen vacancy centers in diamond and people actually now make nanodiamonds that they can try to put inside the human body to look at the chemistry inside the cell so is that used for drug trials and when testing out new treatments we can do it on tissues right now or on the surface but we can actually do it inside the body so right now we're struggling for which scenarios can we use this to get better information and when can we not do it are there any other quantum sensors at the moment that aren't in the developmental stage anymore that we are using so there are quantum sensors that are sold for very specific applications one of them is a magnetometer and those can be really really sensitive to measure small variations in magnetic fields that are trying to develop sensors that are gravitational sensors right now we have no way of probing what's under the ground without digging into the ground you talked about a sensor measuring magnetic fields what does that help us learn what is that good for well if i want to navigate and i know what the structure of the earth's magnetic fields are in some ways that's how birds uh navigate the avian compass in fact people think of that as a quantum sensor okay so they've got like built quantum centers they have a built-in sensor and one of the ideas is that they're using quantum phenomena to figure out what the direction of the earth's magnetic field is that's why they're able to be the homing pigeons are able to come back yeah operational looking oh that's cool what year are you in i'm a senior i'm studying physics right now cool what do you think of when you hear the words quantum sensing i think that using some sort of quantum computing to sense some quantum level molecules or particles like interactions and stuff maybe is exactly using a quantum phenomena to sense and measure things and the idea is that if i can harness quantum phenomena and i can push the limits that are possible i can get something that's ultimately more precise and potentially more accurate okay how is it more precise we believe quantum mechanics tells us what the true laws of physics are and so a quantum sensor in that sense would reach the limits of what's attainable it would be the top tier it would be the top tier what are what are you doing like what are what are you so i study spins and so spins are one of the platforms that people have suggested is a useful platform for building quantum technologies and i study spins on the solid state and one of the platforms i work on is nitrogen vacancy centers in diamond which is a really nice platform because it the spins show their quantum properties even at room temperature so are you studying the spins of the electrons so in some sense the phenomena we're studying are essentially is nuclear magnetic resonance or electron spin resonance which is a very similar phenomena but uses the spin of the electron rather than the spin of the the nuclei so you mentioned the the diamonds that are used to create the sensors right so how long does it take to make a sensor and and to make that diamond is that created do you like put energy into it or so you can implant nitrogen into a diamond and then you bombard it with electrons to create the vacancies and then you heat it up and anneal it and then you get these nitrogen vacancy centers in your system so you mentioned quantum computing earlier so have you heard of the the idea of superposition yeah so that's in some ways the key to both quantum sensing as well as quantum computing it's the idea that you can take a system and put it in a superstition of two states normally we think of classically a bit can be a zero or a one so switch is either on or off whereas in a quantum system it can be in what's called a superposition so it can be in partially on and partially off but one of the challenges with quantum systems is that these superstitions are really hard to maintain because we don't see superpositions in the world around us at one computer you try really hard to isolate everything so that you can maintain this quantum property and the fact that it's actually going to lose its quantum properties as it interacts with the world also makes it a great sensor because now you're actually that you're using that fact that it's interacting with the world to say wait it's sensing something okay so it's like using like the quantum computer would be kind of like the base level and then like you take it out into the world and see how like it differs so rather than trying to build a lot of complex algorithms and gates with it what you do is you take these quantum bits and you take them out into the world and say what do you see what are you sensitive to so you can use an idea called entanglement to make an even more sensitive quantum sensor but that's even more fragile so there's always this trade-off between being super fragile and being super sensitive how does entanglement um work into it so entanglement is the idea that two particles are correlated they're essentially in the same quantum state so that you can't disturb one particle without disturbing the second particle and so if i have a large number of quantum sensors are entangled then they're all going to interact much more strongly than if i just had one of them interact at a time okay and so that gives you a boost in sensitivity it's more precise absolutely is an atomic clock a quantum sensor in some ways it is and atomic clocks are remarkable devices and being able to measure time that precisely has really important consequences in fact our old gps system is based on the accuracy of atomic clocks certain set of satellites each of which has an atomic clock on board and they send out a time stamp and so once it gets a signal from three different satellites it can triangulate and figure out exactly where you are now if you could make those clocks even more precise you could actually accurately position where you are even more accurately okay that's really cool so some ways you know when atomic clocks were designed and built we didn't necessarily think of gps but technology often works that way is that there are new discoveries and then someone else comes along and says hey this is a great tool for some other application [Music] so what drew you into quantum computing i think what got me into material science was actually making semiconductors for solar panels then that drew me into new types of technology that used semiconductors with the one that's very popular now as quantum computing and what about you what got you interested in quantum sensing i started out doing magnetic resonance study things like bone and biomedical magnetic resonance ended up playing with spins for a long time and the physics of spins just fascinated me so what do you think is a big difference between imaging large biological objects versus sensing very small quantum objects i guess in a way it's part of the same continuum what you're doing is changing the technological platform and you're actually able to probe it more sensitively the resolution you're able to get is much higher so you can see smaller signals in a much smaller volume how is the resolution higher so it's because the the nitrogen vacancy center is a single defect so you can actually see a single electron in normal magnetic resonance you it's you don't have the sensitivity in order to be sensitive to like a single electron do you have to be really close to it you have to be close to it you can detect it optically because if we tried to detect the magnetic moment of the electron we wouldn't be able to do that because there the energy is too low compared to thermal energies but what the diamond system gives you is a natural up conversion energy so you can couple into an optical photon which is then much easier to detect a single optical photon than it is to detect microwave okay i see yeah and that's why it's able you're able to do it at room temperature as well what are some of the challenges you've faced when trying to do quantum sensing with this platform one of the key challenges i think for all any quantum technology is really understanding what limits your coherence times and then the next question that comes up often is how do we make this better if i take a single qubit or a single spin there's a certain limit up to its sensitivity but if i can take entangled spins in principle i could make the system much more sensitive but it usually comes at a cost because when i entangle something it's much more sensitive to decoherence as well in a similar way but maybe even in the opposite way where we want to figure out how to be as resilient from noise and all the kinds of noise sources exactly okay what are you studying i'm studying superconducting qubits they use hybrid semiconductor superconductor structures the semiconductors are you introducing new noise sources potentially that might affect the coherence times yeah yeah so the big one is charge noise because i guess a lot of the superconducting qubits they have made them in such a way that they're insensitive to charge exactly so when you think of noise in what way it is is a noise bad for your system i usually think of it like well we work with quantum systems yeah and those are very sensitive to fluctuations i guess any fluctuations can kick your quantum system either out of the state that it's in to another state i think as you said you know anything that interferes with my signal is noise but it can come from different sources in some ways the operation of the quantum system itself as it's sensitive to different physical phenomena the ones that i don't like i call noise the ones i do like i call signal and that's an artificial definition that i'm making when i choose to build a sensor one of the challenges we have is we're trying to figure out if i want to control it where is it coming from i remember we had experiments running in our lab one day and we were running these experiments about 100 megahertz all of a sudden we saw these big spikes coming in and we realized we're picking up the local fm stations and that was a source of noise like it's completely random but it is still there and then the other form is very much what is intrinsically within your experiment itself because some of the materials that you have have defects that are coupling into your sensor into your quantum system and are also producing noise but yeah the interesting stuff really is where you're picking up the quantum noise from intrinsically from whatever right it could give you information if you read it out about what's happening or you have to find smart ways to suppress it so that you can focus on what you really do care about so what are the kind of noise and fluctuations that you're worried about so one of the things that we're interested in is looking at suppose i want to build an entangled quantity sensor when i put a number of spins together in addition to being sensitive to an external field they're sensitive to each other and they start talking to each other you don't just see the external spins you see the fluctuations of all the other spins in your system so what you want to do is make sure that they don't interact with each other but they still stay sensitive to everything else and there you could think about the local interactions the magnetic interactions between the spins as a form of noise in some ways it's interfering with what you want to measure which is the magnetic field outside the sample so our topic today is quantum sensing which you're an expert on can you recap for us in your perspective what is quantum sensing that's that's a million dollar or maybe a billion dollar question exactly yeah i think a lot of people in the field have different definitions for it absolutely what would you want to be like the smoking gun of a quantum sensor depends on who i'm talking to right you know trying to talk to students and get them excited or you know try to talk about the elements of quantum mechanics i think maybe we could agree that you know things that use superposition have a certain degree of of quantum mechanics quantum yes involved maybe they should be using elements of quantum computation so i don't have a strong view on it but i do think it's an interesting question i would tend to agree that i think in some sense all anything that uses superposition could be quantum sensor but then spectroscopy uses superpositions and has been around for 60 70 years i think what excites me most now is the idea that can we push the boundaries of how sensitive one can make this technique how improving sensitivity specificity what other limits and as we define it better are there fundamental physical limits that's where the excitement lies is when we really start to leverage having you know access to individual quantum degrees of freedom whether that's a single photon or a single spin and in principle then you could also imagine entangling it and you know doing some quantum computations on it in order to make it an even better sensor so do you think there's a maximum number of spins you can have if i think about a single nv as a register right i mean people have thought about this it's an interesting question you can think about you have the electron and it's surrounded by some nuclei and you could change the density of those nuclei and so if it's a lot more dense then you have a lot more that are strongly coupled yeah but you also have a lot more noise but i i don't know that there's necessarily a limit i mean it's been it keeps expanding i mean i think that there are some groups that are able to identify you know 30 40 individual nuclear spins around a single a single electron and control 10 or 15 of them so do you think you can integrate multiple nv centers on multiple optical sensors so are there ways in which you can overcome this question of there's a spot size and that limits how many nvs i can pack into a certain region that's another great question a couple of groups actually that are working on trying to read out the spin state of nv centers uh electrically instead of optically if you could do that then you could pack a whole lot more into a smaller space using tiny electrodes and you could possibly have them spaced at nanometer scales instead of at micron scales and i think the application there is clearly sensing right right do you think they'd retain their coherence times if you pack them in yeah what's limiting the coherence is really local right you know nanometer scale but it happens to be that most of the time when we try to read them out with light well then the trouble is that the diffraction limit of light is you know hundreds of nanometers and so then we need them to be apart but you know if you have two envy centers that are more than a couple tens of nanometers away from each other they just don't talk to each other yeah so from that point of view the technology could be really dense right which is why you know some you know companies or groups are trying to make quantum computers based on spins and semiconductors because they could be really densely integrated using modern technology but the question for a sensor is as you say how do you address it how do you initialize it how do you read it out and is optics the best way to go it may not be if we think about uh quantum sensing in particular it really involves understanding materials um solid-state materials chemicals you know chemistry biology engineering electrical engineering optics photonics i mean so many different areas i think that that's one of the most exciting things about that is the degree to which it's engaging a much larger cross-section of scientists they're the ones that i think are going to come up with the breakthroughs of saying oh wait i could design this molecule to do this thing yeah and that i think is going to make real breakthroughs in the next 10 years is the fact that we're just having this much larger group of scientists people bring in very different perspectives into what used to be a very niche feeling i remember physics you'd only talk to people in your subfield and now we're picking up the phone and talking to people in the different departments completely different areas and we're forced to learn different languages the quantum world is essentially the world of very small but one of the quests of quantum sensing is to harvest some of these unique properties at the micro scale and with these tools we will be able to have new technologies and new measurements that we are unable to make today

2022-08-01 09:11

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