Seeing the Universe Through Quantum Eyes

Public lecture presented by Cyndia Yu



so good evening and welcome to the return of the in-person slack public lectures

look i i thank you for being here um we've we've had lectures before where

this audience this auditorium was full uh today there's a lot of room for social distancing

but hopefully as the pandemic recedes we'll see crowds here again and really i thank

you folks for being the pioneers and coming i should say the speakers

are very enthusiastic about having real people in front of them so we thank you

very much for being here so today's lecture will actually be a really fascinating

one uh the lecturer is cindy yu who's a graduate student at stanford and

working with the kaipak institute here at slack and also with the um

the technical people who are working on quantum devices quantum sensors

and this is part of the 60th anniversary series of public

lectures where we want to emphasize new capabilities new technical capabilities

of slack and one of the important ones is using actually advanced devices

quantum sensors to understand the universe better than is possible

with those old-fashioned classical devices that one used to use to do astronomy

chemistry biology etc and cyndia is right in the middle of it

she's here doing her phd she was an undergraduate in arizona

she's come here to do her phd and she's building these things and she will tell you all about it so cynthia thank you

very much and we look forward to a very interesting watching thanks

all right um mike is good cool all right thank you all so much for being here and

for missing the warriors game to be here um so um

a bit about me uh the earliest memory i have of sciency

things was seeing the intel pentium fad so i was born and raised in chandler arizona um

which is mostly a suburb of phoenix so very hot and very dry and i think is best

known for being the site where intel does a lot of its fabrications so if you touched a personal computer

in the late 90s or early 2000s it probably had an intel pentium chip and

the first memory i have of vaguely sciency things was getting to actually do a tour of that um when i was i don't

know seven or so um but after arizona i moved out to

boston for college um where i learned what snow looks like and that i really thought physics was awesome

um after college i hung out in new york for a bit

studying mostly economics research and operations research and then i decided that physics was actually amazing

so i came out here to stanford where i proceeded to forget what snow is like um and i'm doing my phd now in the

physics department studying as michael said quantum sensors uh for studying the

universe and one of the cool things about being a grad student in this field

sort of pun intended is you can put your sensors in very interesting places so this is me

when i was installing some of my work at the south pole so

i am interested in measurement how do we know things about the universe we know things about the universe

because we see them we see them happening we even as early as people um

really can remember we observed things about the world around us and we found patterns we found

patterns in when the sun rises and sets when the seasons change when

um you know when the sky gets dark then suddenly

water falls out of the sky these sorts of things and we started getting better and better

at being systematic about making these observations um and

trying to use them to make predictions that allowed us to use those predictions to govern our lives to figure out based

on the movement of for example things in the sky when we should wake up when we should plant the crops

um and for the most part we've just been getting better and better at

these observations we've gotten devices to be able to help us see really small things and that allows us to look at

things like cells and try to understand microbiology we've gotten

things like telescopes that help us see really far away and we can see things like stars and planets

um and try to understand the movement of the celestial bodies in the solar system and beyond

and it turns out that we can also observe things that are not just directly in front of us in light

and we do that because we can use the pretty cool fact that magnets

repel and attract each other so you've probably encountered a fridge magnet you can push it against a fridge and it

attracts if you put two magnets together in one direction they attract each other in the other direction they repel each

other and similarly electric charge repels or attracts depending on its signs so um

here the balloon static uh is pulling this guy's hair up um

you can try this at home if you want but this means that we can now sense things like whether there's a magnetic field

present we can sense things like whether there is electric charge present and so we can start to un unlock phenomena like

we can understand that this like big bright flash in the sky has something to do with electricity and

that's pretty cool and in the 1800s people also started to

notice that it's not just that we can observe magnetic fields and we can observe

electric fields um but we can observe them changing because

moving charge so current makes magnetic fields if you just have

some current running and you put it near a compass the compass will shift and conversely moving magnetic fields

creates current um it creates changing charge so if you put these two things

together you have a changing electric field creates a magnetic field and then this magnetic field is changing and that

creates an electric field again and then you have these things the electric field and the magnetic field

propagating together as waves and they propagate at the speed

of light and that's awesome and it turns out that we

can now use this fact to send and receive information about um

light and about electricity and magnetism so at the end of the uh 1800s

hertz was playing around with a system where he just created these giant electric sparks

and then across the room another uh receiver that he had this antenna

on the right side of the screen um had also created sparks at the same rate

and so this was telling him that he was sending information about the electric field on one side of

the room to the other side of the room this electric field information the changing electric field

was turning into a magnetic field that was propagating um and then being received again as

electricity that he could see um they asked hertz what the use of this

invention was and he was like i don't know probably not much um

we now know that he was super wrong because this in some sense was the defining discovery of the 19

19th century because our entire modern society is built on the idea that we can catch

waves of light waves of light of all different lengths um in all different sizes

and turn it into electricity or magnetism and observe that again um

and light turns out to come in more than just what our eyes can see you can also

have light in the radio so these fields are really really spread apart um and so

we call that longer wavelength and those are very low energy fields and you can also have that the peaks and the crests

of these electric magnetic fields um are really really squished together

and so that's really high energy those are like x-rays or gamma rays and this is essentially like how your

car radio works is that some radio station is beaming out this information and your car antenna picks it up

turns it into an electric field that is vibrating well uh

that is vibrating a speaker and that's how you hear it as sound um you can also do something like wi-fi so

you know uh put your information into light waves at 2.4 gigahertz or 5

gigahertz um or whatever frequency your wi-fi band is

and then an antenna in your computer catches it and turns it back into the information that you uh can interact

with and we can also do this with telescopes catch big light waves from the universe and

turn that into electricity and use that to reconstruct what's happening um out in very distant galaxies

so what sorts of light might we want to measure you know things very far away far away galaxies um

astrophysical objects we might want to measure very small things take the light from microscopic or even like

subcellular objects and turn that into electricity or magnetism that we want to measure and

so we can start to ask questions like are there magnetic fields in you know

people's brains is electricity a part of um how human cells interact with each

other that's pretty cool but

um obviously i'm not going to send you home after 10 minutes people also started noticing this weird

observation so they knew that if you shine light on a piece of metal

you start getting charge you start getting something that you could like charge a battery with electrons start

coming out so units of charge and what do you expect to happen when you change this light

so if light is a wave you expect that a bigger wave so bigger amplitude like

um should kick out more energetic charge um and you can this sort of makes sense

right like if you have a bunch of beach balls on like a pool of water and you shake the

pool of water a lot then the beach balls have way more energy and if you shake the pool of water like not very much then the beach

balls have less energy um and if you have a bigger frequency so

you like shake it faster then you expect to kick out the beach balls um faster and so you might have like a faster

current um but when they did these experiments what they found was that

it was exactly the opposite so a bigger frequency so the faster you shake it um the more energetic the beach balls

and the bigger the amplitude um the

the more current the more um electric charge is coming out

and also they found that there's some cutoff frequency so some speed of shaking

at which you just get no current at all so what's happening

um we now call this the photoelectric effect and it's essentially this idea

that light is not just a wave but you can imagine it like a particle so the

difference between the left and the right here is the color is the frequency and if you have light that is too low of

frequency um it means that it's not energetic enough and so it can't kick electrons

out of the metal whereas if it's a higher frequency then it's kicking it's actually displacing

the electrons and if you have bigger amplitudes so

shaking it harder that means you're just throwing more light out and so you are pushing more um electrons out and you

get more current but you can't change um how energetic they are

and also as you turn down turn this down um you can't get half an electron so

there's a certain amplitude um that you can't get anything out and there's also a certain frequency that you can't get

anything out because eventually you get below one electron and eventually you

get not enough light and so i used to say that i said before that light is a wave um you know you

have this white light and it's made up of all of these different colors but also

light is a particle and this weird particle wave duality

is a key feature of quantum mechanics and so it gives us some pretty neat results so the photoelectric effect is

one of them another one is uh this idea of interference so you've

probably encountered interference if you for example throw a couple of rocks into a pond you see these ripples

and depending on how the ripples line up they might interfere constructively so

the waves come together and make a bigger wave or they might interfere destructively so

they cancel each other out so if you shoot light at this um what we

call a double slit so a wall with just two holes in it you can get bright parts and dark parts

depending on whether the light is interfering constructively or destructively

but what do you expect to happen if you throw an electron an electron is a particle it's an object

so you expect to just get a big pile behind each of the holes right

but when they do this experiment it turns out if you just shoot a beam of electrons at the screen

you also get an interference pattern so it's not just that light is both a particle in a wave

but things that we thought were particles also have wave-like behavior

and so this these pretty cool facts of quantum mechanics this particle wave duality

this idea that everything is quantized this idea that the electrons in some

sense look like they're taking both paths at once in order to create

this interference are key hallmarks of quantum mechanics and we can exploit these features

to create all sorts of cool quantum sensors all around uh the lab so here at stanford slack we

have quantum sensors um using a variety of different

technologies that are all reliant on this fact that quantum mechanics is really weird and

that allows us to sense really really small signals

um and we're unified in general by this theme that we have to use quantum mechanics to get to fainter and fainter

signals like we have already gotten to the point that our classical

detectors just don't work well enough anymore to see the things that we want to be able to see

so what sorts of things are that um you might want to look at elementary particles so what is our entire universe

made out of um you might want to look at dark matter so we know that

the sorts of matter that you and i are used to the stuff that we're made out of is only

um about 20 of all of the matter that's in the universe all of the heaviness the

stuff and we have no idea what the rest of it is so we're trying to figure that out

um you might want to try to understand how chemistry works on

uh a better level in particular it's really hard nowaday

typically to watch chemical reactions happen um at the individual molecule

level and you want you might want to ask the question like how does this chemical process unfold

and can we watch it in real time you might want to understand biology so you might want to unders you ask the

question for example what is blood made out of like what is hemoglobin made out of if we understand better how

hemoglobin works then we can understand how to create artificial blood um but we it's currently extremely

difficult to measure the structure of the hemoglobin molecule

and create that artificially in the lab and we might also want to ask these huge questions so like where

where did the universe come from um lots more um

and i'm gonna be focused on this question right here of the birth of the universe

and so the thing that i am interested in measuring is what we call the cosmic microwave background or i'm going to say the cmb

and the cmb is a baby picture of the universe so if you think of our present day as you

know you look out into the night sky you see these stars in these galaxies if you look back farther in time you

start to see you know baby galaxies and then baby stars and the first stars started

turning on maybe 200 million years into the age of the universe

and before that we know that the universe had some hot done state there was some sort of big bang

and the cmb is the last light from that and so it's about 14 billion years old

and we've already learned a huge amount from it so it's from the cme that we know things like

what the universe is made out of we know the shape of the universe we know its age um we know a little bit about where

the universe came from it was the fact that the cmb exists at all um that there is this like afterglow of this hot dense

state that we knew that there has to be some notion of the universe having a beginning

but the experimental challenge is this thing is behind everything else in the universe so if we

want to detect it and we want to detect it here on earth because satellites are very expensive you have to look past the

clouds you have to look past the atmosphere past our solar system our milky way galaxy other galaxies all the

baby stars and stuff to this thing that is 14 billion years old um and very very cold it's about

three degrees above absolute zero so we use a uh quantum sensor to do this we

use superconducting sensors um and to understand what superconducting sensors

are we have to take a detour through what um non-superconducting things are

so resistors um taking a detour back out of the cmb

we are trying we use a technique called superconductivity which means that

resistance goes to zero so what does that mean i found this picture on the internet i

think it's very cute um but you can sort of think of moving current through materials as

having these three components so you have this yellow guy the volt who is

the size of the electric field difference so he is trying to push electrons through

and you have this guy the amp who looks very worried um probably rightfully so

because this is the amount of current so how much electrons are flowing and this guy in the cowboy hat um is the

resistance and he is essentially how difficult is it to push electrons

through a material so if there's a big resistance that means that he's really strong he's

pulling really hard and so there's very little flow of electrons

but if it's a very small resistance that means he's not squeezing very much and it's very easy to push electrons through it's very easy

to push current through this material so normally um you can think of some

things like insulators where it's very difficult to put push electrons

through the chemical structure is mostly that all of the

uh atoms in this material hold on to their electrons all of the molecules like to

hold on to their electrons and so there's not much room for other electrons to squeeze through this

material in metals what usually happens is there's this lattice

and electrons kind of flow around so there's this very easy

very neat grid and um when you try to force current through

they bounce around but there's like generally a path that they can get through and so we usually think of

metals as being pretty good conductors which means uh their resistance is not very high

um [Music] what happens though if you take this structure and you cool it down

so normally um temperature uh higher temperature corresponds to more movement so there's

just all of these uh atoms like kind of shaking around having dance party and the electrons are

trying to squeeze their way through and as you cool things down they start freezing a little bit

uh don't move as much and so the electrons have a little bit of an easier time going through and so usually

for materials um as the temperature drops the resistance also goes down so the dude with the

lasso is just pulling less hard and the electrons are having an easier time but at zero temperature this uh

generally comes to some finite resistance there is some amount

that the electrons still have to weave through all of this stuff that they have to travel through this

material in superconductors though at some temperature

the resistance just goes to zero so it's not even like the lasso is really big it's like he's not there at all um

and so all of the current flows through none of it gets lost to collisions none of it gets lost to heat

um and in fact you can the current in principle can go forever there are people who have

started um current going in super conducting loops and it's been going for decades um and

you know there's no reason that it should stop so what's going on here so if we go back to this picture of the

metal atoms arranged in this lattice and when you try to push current through the

electrons are zooming around and they might bump into other things in the lattice

as you cool stuff down um the atoms shake less and so now you have this

kind of ordered grade and now you have this electron that wants to come through

and remember that i said that um charge like charges either repel or attract depending on their sign so an electron

is negative sign and these metal cores are uh when it comes through the metal cores

are positive and they want to come a little bit closer to the electron they prefer to be closer to the electron than

anywhere else and so creates these little pockets where things are a little bit more

positive than they otherwise are in that um in this lattice and so when an another electron comes through it

prefers to join it and so they actually become friends um and they travel together we call these uh friendships

cooper pairs and they move around much more easily than individual electrons

but the magic of quantum mechanics isn't just that they move around more easily or that their friendships or their

friends friendship is magical but it's not a quantum effect um but it's that

they move around so easily that the resistance goes to zero because cooper pairs they don't travel like particles

anymore now they move like a fluid they move like a wave and they travel through this material without really even seeing

that all of these metal uh atomic cores are there

so superconductivity we want to use it to measure the cosmic

microwave background we're trying to measure the leftover light from the big bang this is the most

recent all-sky picture from the european space agency planck satellite it's very old and it's very very cold

it's about three degrees above the coldest temperature possible in the universe so how do we

measure it um it's a good approximation when you want to measure a thing first you have to

catch it so we take this light that's traveled to us and we put it in a bucket um

we catch it and now we want to understand what is this light and the main

thing that we want to understand about this light is how hot or cold is it what are these

slightly hotter and slightly colder patches that you see in this picture and so if you want to measure how hot or

cold this light in our bucket is you use a very good thermometer so

where do we get a very good thermometer we can use superconductors because it turns out

that this transition to zero resistance here is not just instantaneous

but it's this very very very sharp transition um

where the resistance versus the temperature changes a lot but

not instantaneously and so if we get light coming in from the cosmic microwave background it comes

in it heats up our bucket a little bit it heats up our material a tiny bit we will see a huge change in resistance

so these are the sensors that my experiment uses they are fabricated by nasa jet

propulsion laboratory and um what you can see here is you can see this

giant piece of metal and that's essentially our bucket that's the thing

that we're using to collect all the all of the light after we've gotten it uh into our telescope

and we are now trying to measure a big change in resistance that came

from a small change in temperature and these transition edge sensors as we call them

um are so sensitive that they can measure single photons they can see a single particle of light come in because

it's such a big change in resistance so now we have this change in resistance

it's quite large and that allows us to read things out as a big change in current because you

can't just measure resistance you have to see how much it suddenly allows the

electrons the green guy to flow through your material how do we measure a big change in

current we use a squared okay not a sea creature a squid is a super conducting quantum

interference device um and this is the circuit diagram and i'll walk you through it

um but i first wanted to show you a pretty picture of the squids that we use these ones were

made at the national institute of standards and technology out in boulder

um and it's this really cool kind of uh cloverleaf pattern that you have here

so what is this doing remember how i said that um

sorry so remember how i said that uh current

you have a giant change in current and this current wants to go through this if you have a current going through

a superconducting loop it could go forever but instead what we have are these x's

here so these x's are very very small breaks in the loop um

and electrons will go across because they're a wave also right they're not just a particle so when they hit a wall they

don't necessarily stop with some probability because they're a wave they'll go to the other side they'll

tunnel through quantum mechanics is amazing um they'll tunnel through and they'll make

it to the other side of the loop so um in this version i have

uh two different breaks sometimes you can have one depending on the kind of squid that you're using but now you have some scenario where

current can actually get through this loop and come out the other end but

um what's amazing is that the path length so how long the different arms are of

this loop depends on the magnetic field that's going through the middle

so um you can sort of now think of this as

another version of our double slit experiment because the current could go through one arm the current could go

through the other arm but depending on which path length is longer shorter you suddenly can create

an interference pattern you can have that the two currents interfere

constructively so it gets really big current on the other end or you could have that they interfere destructively

and you get no current out on the other end um and so you get huge changes in how much

current comes out of this device depending on the magnetic field inside the loop and what's pretty amazing about this is

that um the spacing in between these things so

how much magnetic field creates one of these fringes is insanely small so

um squids are the best magnetometers in the world um they can measure something like

a hundred million million times smaller than a fridge magnet in magnetic field

um and so they are sensitive to you know even the smallest changes in

magnetic field from exam for example neurons firing in someone's brain so

people use these things for magnetic imaging among other things

and at this point we can just connect these superconducting devices which need to be

kept super super cold um and so we have to put them in these refrigerators to keep them super

conducting we can now connect them to room temperature electronics so these are ordinary in the sense that

you don't need to keep them in a fridge but they're still super sophisticated lots of people here at slack spend all

of their time building different versions of these so this is one of the ones that i work with um we keep it in an electronics rack for

those of you that like playing with electronics you might recognize a spectrum analyzer and a vector network

analyzer um and we keep this in a corner of our um

in this picture fairly clean lab um we try to keep it cleaner um

so let's put this all together so we have this uh transition edge sensor ours is made

um by nasa but lots of people make them and we are using it to catch the light from

the cmb or whatever very small signal you're trying to measure and

this light comes in it gives you a very big change in resistance and you

want to measure that as a big change in current so what we do is we take a lot of these sensors and we connect them up in these

arrays or cameras full of these things and we

can script a very friendly postdoc named ari in this picture to connect them

together so he is using a technique called wire bonding it's kind of like soldering

for those of you that play with electronics at home but instead of connecting the two wires

in front of you use a microscope and ultrasonic power to bond um the two ends of your

microelectronic devices together and so now that it's connected to the squids we can hook all of that up

to wires and put all of this into a telescope

so here at slack here is um a team that includes ziesh who's sitting in the back

who uh put these things together into different experiments and we take these experiments

and we take them to in our case the south pole um

and from there we let it look at the sky and measure the early universe

but the south pole and measuring the early universe are not the only things that you can do with quantum sensors so

slack quantum sensors also go down into mine so they're shielded by

hundreds of meters of rock to get away from all the stuff in the atmosphere in order to go to the very quietest places on

earth to try to hear things like um whether dark matter is passing through um

or whether neutrinos these super super super tiny um neutral charged particles what are they

doing um we also try to put these sensors for example at

in the high mountain tops of chile to try to measure early galaxies to try to measure

um early stars and also maybe even echoes of the big bang we also

just other building that way we'll put these quantum sensors in our

beam lines here at slack so we have these um ultra high energy

electron beams x-ray beams and we collide them with various samples and

in order to try to image them and we can use these quantum sensors to make better and more sensitive images of

all sorts of chemical processes all sorts of biological processes so

where is quantum sensing going um there are a few frontiers in quantum sensing that uh

people in the community are involved with especially here at slack so one of them is new detector

concepts so a couple of things that i'm excited about so um

backing up if you uh this is a circuit called

an inductor capacitor resonance so inductors and capacitors are essentially different ways of

storing electromagnetic energy so inductors store them in the form of a magnetic field and capacitors store them

in the form of an electric field and so if you hook them up together you'll find that they actually ring

because the energy is just sloshing back and forth between being stored in the electric field and being stored in the

magnetic field and how fast it rings so its frequency of ringing depends on how big the inductor

how big the capacitor are and so what we can do is we can build a device that gets an

inductance from these cooper pairs so these um pairs of electrons that are

friends and what happens is when light comes in a photon comes in it can actually break a cooper pair destroys

the friendship it's very sad um and the inductance changes and so you

suddenly get that this bell that was ringing at one frequency starts ringing at a different frequency

and so then you m you know that light must have come in somewhere another cool concept that um

relies on quantum mechanics is using qubits so you might have heard

of quantum computing um and how it is uh the hot new thing and essentially how

quantum computing works is in classical computing you have these bits in your computer

they're either zero or one but in quantum computing in the same way that light is both a particle and a wave

the current travels through both arms of the squid loop um you can have that the state of the bit

is somewhere in between zero and one it can be anywhere on the sphere um and

qubits make great detectors because now you're not just sensitive to signals that flip you between a zero and a one

you're sensitive to anything that moves you anywhere on the sphere so you can have something um you can now have a

detector that gets you like slightly from the top of the sphere to like halfway and that is

information that you can actually read out as different and that wasn't something that was available to us before

um and so here's actually a cool picture of all three of the sensing technologies i

just told you about the um inductor capacitor one the transition edge sensor and the qubit all in one

fridge um another way that we are pushing the frontiers of quantum sensing

is this notion of multiplexing so for a lot of our experiments we want to

have many detectors these detectors must be superconducting so they have to be cold they have to be low enough

temperature that you can actually see the transition and so that's usually below minus 270

degrees celsius our electronics that we are hooking up to the thing that actually stores the

data to the hard drives that um we then analyze is at room temperature so 25

degrees celsius and so you have to run all of these wires between something very cold and very hot

and wires uh also bring heat with them and also they are kind of a pain

sometimes just to manage and so multiplexing is a way of

reducing both just the pain of putting these things together and the temperature difference uh so how

much you're connecting the hot thing to the very cold thing by changing by putting many many

detector signals on as few wires as possible so this is a picture that is reading out about 15

000 detectors and technology that we're working on here at

slack can reduce that to just these uh four-ish pairs of wires in this

fridge um and so yeah the wires that you see here are capable of reading out probably 10

000 sensors at a time and so that's dramatic reduction in the number of wires and also in how difficult this

thing was to put together and similarly when you're trying we're talking about scaling up superconducting

sensing it's not only just the wiring um the multiplexing but we're also just trying

to make ways to make more of them so it used to be kind of a pain because you'd

have to make every single superconducting sensor and hook it up and that's a lot of work um

and so borrowing from the advances that um a lot of them actually came out of

silicon valley here of uh fabrication techniques for computers

for computer chips you can actually lithograph um entire arrays full of sensors now so

this one is out of argon national lab these are um a bunch of transition edge sensors that have now just been printed

onto a silicon wafer and it gets put through many of the exact same processes that the computer

chips in your phones or in your personal computers are made out of um and so we're working on this here at

slack this is a picture i took of myself in one of these very shiny wafers last week

and one of the cool things about making these things is you can put your own logos on them um and create art

and finally um in addition to uh

developing many of the techniques to do this at slack we're actually in just the next

building over building a brand new superconducting nano fabrication

facility so most nano fabrication facilities um use a bunch of different metals depending on

whether people are trying to make regular computer chips or what other whatever other

devices that they're trying to make but superconductors don't play nicely with

normal metals they get contaminated and then they don't superconduct anymore um and so having a dedicated facility

for just superconducting devices is really important to making high quality devices and also making them at scale so

currently the clean room is mostly just an empty room but um it's a very clean big empty

room that is being filled with instruments in the coming like this summer um i've

started seeing crates for instruments come in and so we're going to be mass producing

wafers for all sorts of experiments in the next couple of years

so as i've alluded to a bunch of times slack really is a new leader in this field of quantum sensing we have people

from doing everything from coming up with these new device concepts and testing them

figuring out how to take this idea and turn it into like how to actually get information out

of it and how to actually connect it to a real experiment with all the other wiring people who are figuring out how to do

this more than just once people who actually run the experiments put this

thing all together get new data out of it and try to understand what other things do we want to do what

other experiments do we want to run what new capabilities do we want to have and then we have people who also come up

with new goals for where we can put these quantum sensors now that we have quantum

sensors what sorts of things can we measure now that we weren't able to measure before um

and people are working all over this it's not really so much a circle as it is a giant interconnected

web um but i couldn't figure out how to make powerpoint give that to me very easily so now it's a circle for you

um i mostly focus on the electronics and system integration side as well as running experiments and analyzing data

from them um my friend katie here who is also happy to answer questions um after this

uh actually works on designing and simulating new devices and characterizing them and she also runs

her own experiment hers looks for dark matter a different a type of

dark matter particle that people have thought of and if you search stanford q farm

quantum fundamentals architectures and machines this is a new initiative between stanford and slack that is

aiming to bridge all of the different quantum efforts um between stanford and slack so this is

quantum sensing but also quantum simulation quantum computing um all of these new frontiers that

are unlocked by quantum mechanics so um in summary quantum sensing enables

this brand new sensitivity across a huge range of science goals we can now study

things like biological processes things that previously were too sensitive or too

faint for us to be able to study classically we can study

dark matter we can study fundamental particles we can study like we can make movies of chemical reactions happening

these are all things um that quantum sensing has unlocked for us and there's a huge number of these super

interesting but super weak signals that are worth searching for that are worth people spending time on

and the last thing i'll just say is that this field is an exciting one there are lots of new detector concepts emerging

and there are lots of new experiments to run so with that i'll take questions thank you

so much for having me

thank you very much um so this is a nice room for questions all

of you have in front of you a microphone um if you push the red button in the

center you can talk what i'd like to do is have you raise your hands and i'll call on someone so

we only have one microphone going at a time so questions please

yes hi thank you so much for your motivation um

um whoa speaking of power um

what kind of bandwidth can that combo work at that's a great question so the question

um i think zoom probably heard the mic anyways but the transition edge sensor squid

combination bandwidth so um for those unfamiliar bandwidth is

essentially how fast of a signal can you see um like how quickly can you see differences

and there's a bunch of different things that set the bandwidth of the system

usually we prefer to run so that the transition edge sensor is the dominant thing that controls the

bandwidth you don't want the squid to control your bandwidth because then there might be a detector

signal that you can't see anymore because the squid isn't fast enough so the transition edge sensor the bandwidth

is set by um essentially how good this bucket is at changing temperature um

so how much the resistor actually responds to incoming light and um depending on the

application that sets how fast you want these things to run so um for the cosmic microwave background

we don't actually want these things to run very fast so we tend to run in you know the hundreds of hertz sort

of range but for people doing um pulses they're trying to actually measure like the

energy of a single pulse like an x-ray pulse that's coming from the x-ray beam line here at slack they

will run up at like several gigahertz not gigahertz sorry several kilohertz up

to several megahertz of bandwidth

yeah i'd be fascinated to know a bit more about how this technology could transform people's

lives quantum detection development

your example about artificial blood sounded quite interesting are there any more examples of what this

development could mean for the ordinary man and woman on the street yeah that's a great question um

i mean anything that requires measuring an incredibly small signal is

interesting so one of the first applications of the technology that i worked on on this

multiplexing technology was trying to understand how to do better

security so if you want to for example detect whether or not people are

violating nuclear nonproliferation treaties you're trying to figure out whether very very small amounts of nuclear

material is getting into for example the atmosphere or the water around some place and these are really really really

small amounts of some nuclear material and so it's very hard to detect with

classical methods but if you have a huge number of sensors looking for them these very faint

signals then that enables you to detect much more easily and much earlier

whether new nuclear materials are coming out the artificial blood example um

is a former office mate of katie in mind uh who worked on that so trying to understand the structure of hemoglobin

like how it actually binds to oxygen usually it's basically impossible to

actually image the structure of hemoglobin in the laboratory because any sample of

blood that you create you usually have to either freeze it or dry it out to put it through

the x-ray beam line here but then you're destroying how the hemoglobin is actually working

so um trying to figure out ways and then you're also like bombarding it

with x-rays and so trying to figure out ways to much more sensitively measure this thing um with much higher precision

allows you to figure out whether the iron is binding to the oxygen in these different ways and therefore how

to um possibly create artificial ones um

yeah essentially anything that you can think of that you would want a very sensitive sensor for and especially a

non-invasive one so because all of these things are collecting light collecting information

and not sending out signals in order to get information it's also hugely useful for

just like non-invasive imaging

for the transition edge sensors what is the resolution for those sensors rather

what's the minimum step size for discerning different signal levels

um that's a great question the resolution of a tes depends on the material that you're using so some of

them can see single photons and some of them obviously

need to have several photons in order to be able to see them it really depends on the application

that you're looking at but once you're making for the same

single microwave um the ones that we're making for the cmb uh can see

microwave photons but we usually don't uh try to resolve individual photons

so uh do you make any progress on the dark matter discovery

yeah katie where's dark matter um on the dark matter

yeah so i guess i can speak uh very briefly about the dark matter side um so

basically all the physicists know about dark matter right now is that we don't know what it is um so there's a lot of like wide unexplored parameter space and

with experiments even ones that are really sensitive like the ones that we're building with these quantum sensors you still have like a lot of

places to search and you actually have to search kind of very slowly because you're waiting and integrating and like building up these small signals

um and so while like there are actually like there are many new experiments coming out for

different types of um sort of dark matter that they're looking for so the ones that i particularly work with are

looking for a type of particle called the axion um there's we're still like very much in

the in the stages of like a lot of these experiments like coming online and like and like collecting data um and so while

there haven't been any like you know groundbreaking like oh we found dark matter um there's a lot of like

parameter space that's being sort of like uh probed and like swept through and um

you know i guess just a lot of progress is being made but it's like it's easy to look at sort of uh the range of like possible dark matter

candidates that like we haven't rolled out yet and say like how are you ever gonna find it um but i would say that there's like a lot of new experiments

coming online that are that are doing very cool things i think the other thing forward though

yeah so yeah so in the case of the axion which is the the experiment or the type of particle that the experiments that i

work on uh look for um there's actually uh sort of theories proposed that there's

this incredibly uh small interaction between um the dark matter particle and um standard model like electromagnetism

which is like uh you know cyndia talked a lot about um you know electromagnetism and how that's something that we can measure with these

quantum sensors um and so you're looking for this like incredibly incredibly small electromagnetic signal um out in

for the axon at a particular frequency that's related to the mass of the particle

so um another question related to dark matter is there a reason why um

i guess many scientists and physicists believe that we should prefer the idea of dark matter over something like

modified newtonian dynamics

sure so that's a great question um modified newtonian dynamics

is essentially the idea that um our law of gravity

doesn't actually work um and there's just a very small change to it and that should give you

um dark matter but it turns out that um as far as we can tell dark matter

so first of all the cmb tells us that there is something massive um some

that actually exists um that we can't detect um

and it's actually incredibly difficult to reproduce the cmb using just modified

gravity so i usually say i don't believe a modified gravity scenario unless it also predicts the cmb

looking the way it does and essentially none of them do the other answer is just modified gravity

doesn't really solve the dark matter problem because you need to explain why it works only on certain scales and for

every single scale that we have ever after the law of gravity it exactly matches what we expect and so

if dark matter is just modified newtonian gravity like why is it only interacting at these scales that we

can't detect and how does that scale map onto the thing that we observe which is this very large phenomenon

can i understand the modified determinant of gravity was originally introduced to explain

the rotation of galaxies so you introduce a parameter that works with the scale of galaxy

but now we have a very large number of probes with dark matter so we see it in galaxy

we see it in clusters of galaxies

and the simple model says it's a particle it interacts under gravity

it produces all the phenomena that we see in nature and the modified gravity it only

explains a tiny part of that so it's just a much more compelling theory at this moment

of course you know it would be nice to discover the particle but uh yeah um unfortunately the all these theories

work whether the particle weighs um essentially the mass of an electron

or about 10 000 times smaller or

a million times larger and so there's an enormous range to search

and you know i'm a theoretical physicist every theorist has their own favorite value within that range

so most of us will be disappointed but eventually i think we'll find it

yeah please

thanks very much for your presentation um so if we were to say that at the origins of

the universe different points in space quote unquote update at different times

would we be able to detect that with quantum sensors of this resolution

that's a great question so um

it's true that um different parts of like what we are

seeing when we see when we talk about the cosmic microwave background is we're seeing the light

that came off of the um hot dense state of the universe

that is being released as the universe cools down and so that light

um is constantly changing because as time evolves the part of that hot dense

state that's cooling down that you can see is going a little bit further back

and unfortunately that really happens on time scales that

are much much longer than a human lifetime um so if we were to start an experiment and

just run it forever and ever and ever and compare the map that we get now

with a map that we get in like you know a few million years it will be a different map um

but the points that we try to make in um when we study the cosmic microwave background

is usually that we don't care like what that specific part of the sky says

but we care on average so cosmology is trying to understand the universe as a whole

um and so we're trying to understand like on average what does the sky look like

what did what is it made out of and what does that tell us

let's take one more question yes please

what is so special about the 300 000 years of the cmb um

the earliest light that we received from the cme was 300 000 years right yeah um

so that's a great question the cmb uh we say is about 300 000 years into the age

of the universe and that we know is um how long it took for the universe to

actually cool off enough so that the light got released so the early universe was super hot and super

dense um so it was kind of on fire everywhere like you should imagine the early universe is like the

sun it was just on fire and it was so hot that um photons so particles of light couldn't

actually escape it they would just like bounce and run into a charged particle

and um get absorbed and then re-emitted and it like um a crazy fact is the

a photon and particle of light that is born in the center of the sun it will take about a million years to get all

the way out um and so the early universe was the same way and then it was cooling

cooling cooling and eventually it got so cold that

it became neutral so it was no longer charged particles but instead

neutral hydrogen mostly and photons don't like to bounce off of

um don't like to scatter off of neutral particles so instead of scattering off of

the neutral hydrogen it instead just prefers to stream out into the universe and so that's what we call the cmb

is the thing that bounced off that last charged particle and it's now scattering towards us and

so that 300 thousand years is approximately how long it took for the universe to cool off enough that that was possible

well it's india thank you very much very illuminating lecture

okay we will be back here in person in early august or the next of these public

lectures uh please look for the announcement um those of you who are not on our

mailing list uh talk to me or talk to carmen here to get in our

mailing list and you'll get the announcement and again thank you very much for coming

we'll we're restarting the tradition of having these lectures on a regular basis every couple months

and we hope to see you again so thank you very much

oh i should say one more thing which is that uh cynthia and katie will be out in the

lobby uh once we get everything cleaned up and we can answer more questions for a little while so if you want to wait

out there we'll be out there soon

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