College Professor Explains One Concept in 5 Levels of Difficulty

Hi, I'm Sekhar Ramanathan.

I'm a professor at Dartmouth College

and today, I've been challenged to explain a topic

at five levels of difficulty.

[upbeat suspensive music]

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.

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, tasting and smelling.

Mm-hmm.

Yeah, and touch. 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?

In 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, I don't think so. No? Okay.

Can you bounce it for me?

Mm-hmm.

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.

You think that's a good idea?

Here's a ball that's been cut straight in half

and you look inside.

It's hard. It is.

What gives it a certain texture?

It's like, the texture of like the top of a crayon.

Oh, 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

what's right near the surface, right?

Just. Yeah.

You wouldn't be able to see into the middle.

If you had the right tools,

you could start to think 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, yeah, it would be cool if we like

used something like an x-ray, we build an x-ray

Yeah. That was only made for balls

and you could see everything inside it,

every single detail, you could zoom in and out

Yeah. And you could draw it,

print it out.

That's exactly the type of thing that we're doing.

We are sensing, is we're trying to measure what's inside,

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.

[upbeat techno music]

Our topic today is gonna be quantum sensing.

Have you ever heard of it before?

No. No.

Okay, 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. Yeah.

The sensing part, I'm not sure.

So sensing is really just about measuring stuff.

Okay.

And at some level, there's different set of rules

that seem to come into play

'cause 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 gonna be really reliable.

Every time I make that measurement,

I'll always get the same results.

Okay, measurements on like what kinds of things?

Could be on almost anything you want.

Have you ever broken a bone?

Well, I fractured something though.

Okay, do you remember having an x-ray?

Yeah, x-ray and I have had a few MRIs before too.

You've had a few

MRIs before. Yeah.

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. Yeah.

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

that 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 are MRIs 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.

[Julia] Yeah.

Whereas MRIs giving us information

about things like the softer tissues.

Yeah. And in fact,

we don't see the bone very well

in the MRI. Yeah.

So there are slightly different reasons

why you would choose the two different things.

Suppose I could get a higher resolution.

Mm-hmm.

What do you think I would be able to see?

The different atoms and the structures of the particles.

Yeah. Start to see

the different cells

Yeah. And then 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 wanna 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

that's gonna be more sensitive and for something like that,

you're gonna 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

that are called nitrogen-vacancy centers

Okay. In a diamond

and people actually now make nano diamonds

that they can try to put inside the human body

to look at the chemistry inside the cells.

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't actually do it inside the body.

So right now, we're struggling to figure out

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 are 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.

They 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.

Yeah. What does that

help us learn?

What is that good for?

Well, if I wanna navigate, and I know what the structure

of the Earth's magnetic fields are,

in some ways, that's how birds navigate.

Okay. The avian compass.

Yeah. In fact, people think

of that as a quantum sensor.

Okay, so they've got

like built-in. A biological quantum sensor.

Yeah. They have a built-in sensor

and one of the ideas is that,

they're using quantum phenomena

Yeah. To figure out

what the direction of the Earth's

magnetic field is. Okay.

That's why they're able to be,

homing pigeons are able to come back

Yeah. To their original location.

Oh, that's cool. Yeah.

[upbeat synthwave music]

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. Yeah.

It is exactly using 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

over time too. 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 you doing?

Like, what are you studying?

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.

Okay. Which is a really nice

platform because 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

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 nuclei.

So you mentioned the diamonds that are used

to create the sensors. Right.

So how long does it take to make a sensor

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 idea of superposition?

Mm-hmm, 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 superposition 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 partially on and partially off.

But one of the challenges with quantum systems is that

these superpositions are really hard to maintain

because we don't see superpositions in the world around us.

In quantum computing, 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,

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 it's even more fragile.

So there's always this trade-off between being super fragile

and being super sensitive

at the same time. How does entanglement

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

that 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

when you have an entangled- And so, it's more precise.

It's more precise, If you have it entangled.

Absolutely. Okay.

Is an atomic clock a quantum sensor?

In some ways, it is

and you know, 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.

They're a set of satellites,

each of which has an atomic clock on board

and they send out a timestamp

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.

[upbeat music]

So what drew you into quantum computing?

I think what got me into material science

was actually making semiconductors

Okay. For solar panels.

Then, that drew me into new types of technology

that used semiconductors with the one

that's very popular now is quantum computings.

And what about you?

What got you interested in quantum sensing?

Yeah, I started out doing magnetic resonance,

studying 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 nitrogen-vacancy center

is a single defect.

So you can actually see a single electron.

In normal magnetic resonance,

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 in 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 a micro wave.

Okay, I see. Yeah.

And that's why you're able to do it

at room temperature as well.

What are some of the challenges you face

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?

So 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

'cause when I entangle something,

it's much more sensitive to de-coherence 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

that use hybrids, semiconductor, superconductor structures.

Yeah, semiconductors,

are you introducing new noise sources potentially

that might affect the coherence times?

Yeah, yeah, so the big one is charge noise,

'cause I guess a lot of the superconducting qubits,

they have made 'em in such a way that

they're insensitive to charge. Exactly.

So when you think of noise,

in what way is a noise bad for your system?

I usually think of it like,

well, we work with quantum systems.

[Sekhar] Yeah.

And those are very sensitive to fluctuations.

Yeah. 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 wanna 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.

Oh, yeah. 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

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 quantum 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 wanna 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 wanna measure,

which is the magnetic field outside the sample.

[upbeat music]

So our topic today is quantum sensing,

which you are an expert on.

Can you recap for us in your perspective,

what is quantum sensing?

[laughs] That's a million dollar or maybe a billion dollar

question. 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 quantum mechanics,

quantumness involved. Right.

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,

anything that uses superposition could be a 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 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. Right.

But I don't know that there's necessarily a limit.

I mean, 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 electron

and control 10 or 15 of them.

So do you think you can integrate multiple NV centers

or 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 electrically, instead of optically.

If you could do that,

then you could pack a whole lot more into a smaller space

using tiny electrodes. Right.

And you could possibly have them spaced

at nanometer scales instead of at micron skills

and I think the application there is clearly sensing.

Right. Right.

So you think they'd retain their coherence times

if you pack them in?

Yeah, what's limiting the coherence is really local.

Local, right. 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 defraction 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 NV centers

that are more than a couple tens of nanometers

away from each other, they just don't talk to each other.

Too much isolated, yeah. Yeah.

So from that point of view,

the technology could be really dense, right?

Which is why, you know, some companies or groups

are trying to make quantum computers

based on spins and semiconductors

'cause 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?

And it may not be.

If we think about quantum sensing in particular,

it really involves understanding materials,

solid state materials, chemicals, you know,

chemistry, biology, engineering, electrical engineering,

optics, photonics, I mean so many different areas.

And 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 gonna come up

with the breakthroughs of saying, oh wait,

I could design this molecule to do this thing.

Yeah.

And that I think is gonna make real breakthroughs

in the next 10 years, is the fact that

we're just having this much larger group

of scientists. Right.

People bring in very different perspectives

into what used to be a very niche field.

I remember in 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 a world of the 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.

[upbeat music]