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Professor Paola Cappellaro, “Design and control of quantum devices”, Caltech, May 30, 2024

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This presentation explores the concept of "Quantum Sensing," highlighting how exploiting quantum properties like superposition and entanglement in quantum systems can lead to sensors with significantly enhanced sensitivity beyond classical limits. It also discusses practical approaches to achieving these quantum advantages, even with limited entanglement, for near-term applications.

it's my great honor and pleasure to

introduce Professor Pao cap caparo from

MIT and Professor cap caparo is the

fourth professor of engineering at MIT

she is a professor of nuclear science

and engineering a professor of physics

and a member of research lab for

electronics where she leads the quantum

Engineering Group Professor caparo

received her PhD from MIT and joined

Harvard University as a postdoctoral

associate in The Institute for

theoretical Atomic molecular and Optical

physics before joining back before going

back to MIT as a faculty okay please

join me to welcome

Professor thank you and and frma you

should stop your sharing otherwise I

cannot start mine thank

okay can you hear me and uh see my

slides yes yes thank you

perfect uh thank you very much for the

invitation to present and uh also for

the uh very nice uh scheduling which had

me just after fre my presentation

because I'm sort of hopefully continuing

on the same topics and uh uh with

somehow a complimentary um view if you

want uh in which I'm I'm really trying

uh to sort of tease out uh what is

quantum in in Quantum sensing if you

want and uh why we want to do Quantum

sensing and what type of Advantage we

can have over a classical sensor uh when

we are exploiting quantum properties and

so the the first message that I really

want to convey is why are we interested

in using Quantum system uh for sensing

and this sort of goes back to uh the

very old thought experiment of shinger

the the famous shinger cut uh that can

be um a awake or or sleeping inside the

box and we don't know uh what it's doing

until we open the box but really uh when

we look at a real experiment well uh we

cannot really a superposition of a

Quantum cut or of if you want a

microscopic cut and that's is because

the cut even inside the box is still

interacting with its environment any

molecule of air any photons Inside the

Box will perur it and immediately

collapse it into other being awake or or

or sleeping and this is because Quantum

system are fragile because they interact

very strongly uh with uh the external

perturbation and this is probably bad if

you're trying to make a quantum computer

because you would like to keep this a

very large superp position but they also

make for very good sensor there a high

sensitive it uh to um any external field

and external perturbation makes Quant

system very good Quantum

sensor and uh of course the uh the goal

that we have is to reach sensitivity

which can be beyond what is possible

with classical sensor and this is a form

of quantum Advantage which sort of

mimics the advantage that you have in

quantum computer so in quantum computer

you have an exponential

Advantage for typical Quantum sensor I

mean there are many flavoring but the

most typical um scaling that we have is

that if you use quantum s systems as a

classical sensor or we just have n

classical sensor the signal to noise

ratio improves as a square root of the

number of sensor themselves however if

we use fanton sensor by exploiting there

a most fundamental property in

particular entanglement we can have a

skin in with the number of sensor which

is proportional to n and so this gives

this if you want polinomial improvement

with respect to classical systems uh

which is what gives the quantum

Advantage however reaching this Quantum

Advantage is uh relatively uh difficult

and so um when we were working actually

together with fredman and also Christian

Gan in trying to analyze and and provide

a review of what we mean by Quantum

sensing we came up with if you want

three classes of quantum sensors uh so

the first one is where we just use a

Quantum object uh uh to sense thing so

if we considering since we were just

discussing few minutes ago uh MRI of

course we're using spins in order to

detect for example the anatomy of brain

and Spins are intrinsically a Quantum

object but we can describe them really

as classical magnetic dipole so we don't

really use the all uh uh if you want

quantum mechanics to describe a very

Basics MRI experiment so we are using

Quantum object but not really exploiting

their Quantum property and this is a

sort of if you want a Quantum 1.0 type

of technology which we're not really uh

so interested in defining at least not

not today for me so what we really

interested or what we really mean when

we Define a Quantum sensor are cases in

which we are using for example at least

some Quantum property for example

coherence superposition in order to have

uh better

sensitivity and uh even more what we

would like to do as I said is to use

entanglement in order to achieve this

Quantum advantage and of course there

has been already some uh demonstration

of this type of sensors so we heard a

lot about V EnV Center so if you use a

single MV or if you use an ensemble but

without using if you want correlation

among the spins this will fall into this

category another good example are atomic

clock so atomic clock are sort of like

in between sometime people complain to

me that they should be in the force

category uh but really we had a lot of

advances for example brought by Juni and

an Maria re in which you're using

Quantum logic to improve your ions at

atomic clocks you're using Ensemble and

correlation to make them even better so

it's sort of like bridging that Gap to

achieve these really exploiting Quantum

properties and uh even ligo in its first

ination was if you want in the second

category like just interferometer but

more recently they've been using a

squeeze light injecting squeeze light in

order to improve the detection of

gravitational W so we already have these

um examples of achieving Quantum

Advantage with this type of systems and

really this is the type things that we

are also trying to do uh to improve

Quantum sensing really to bring the

advantages of quantum sensing uh to real

applications um same as fredman my

favorite Quantum sensor is the EnV

Center in diamond and I think that like

after restock I don't need to really

motivate why uh this system is awesome

uh it can really sort of bridge a lot of

different um modality of sensing and

different applications so uh looking for

example at magnetism and the Nanos scale

looking at effects from Super conductors

effect from ferromagnetic materials uh

but with a spal resolution which are

really down to the nanoc scale um for

example through Vis scanning tip it can

look at biological effects uh for

example some work done uh to detect uh

HIV RNA uh in um like a a bios uh type

of setup and also you can look at just

the Dynamics of other Quantum system for

example correlation of quantum spin

Associated either with the diamond or on

the surface or the Diel so it's really a

very powerful sensor with many different

applications and uh well my manx slide

probably it's a bit superflu since we

already had an introduction of NV Center

much better than mine and by now I keep

it really down to just one slide uh

again these are just optically active

defect uh so we will illuminate with

green light and we see uh the red light

emitted and uh to put in a plug for

tlaps which was the two talks ago they

actually s sell a Quantum diamonds with

Envy defect so you can also purchase

that and start doing uh your own

experiment and of course uh we combine

typically with Optical uh polarization

so if you want cool down by Optical

means and um I used the conventional

Optical rout for MV centers and we

combine that with

microwave control uh where of course

your Envy Center acts a little bit like

the compass needle uh pointing up or

down in response to an external magnetic

field and you can have magnetic resonant

driving OD MV and where you detect a lot

of light if it's pointing up and uh less

light when it's pointing down and you

can drop this very nice coherent

superposition and this is work going

back to a 2006 B children but people

have been uh observing uh even better

coherence time since that time so how

can we achieve this type of quantum

Advantage with this type of defects one

way uh that indeed we propose even back

then uh in some of the first few

proposal is to use large ensembles of NV

centers and trying to exploit it

interaction among them however the force

me that appears when you're having large

ensembles with very very dense and

Venter is that the interaction among the

spin V cells uh sort of make your

coherence time decrease and so you have

less time to do the detection and so the

sensitivity so if you want the minimum

field that you can detect uh is not so

good and you cannot go for very long

sense in time because the coherence

kicks in and you lose your signal so

what you can try to do is to try to uh

if you wanted to couple or cancel out

this interaction and uh some uh very uh

clever people in Misha looking group

they uh devise very powerful sequences

which were able to improve dramatically

how well you can detect your magnetic

field with very high uh dense system

however this does not give you yet um if

you want um the quantum Advantage per se

so uh if you were able to change the

number of mvs uh inside the sample uh

which uh like in this experiment we were

not doing uh you would see that the

sensitivity would just improve as the

one with the square root of the number

of sensor at

most however if we could indeed exploit

instead interaction among the spin and

achieve a type of correlation for

example in the form of a a squeeze state

of the spin then the sensitivity could

improve even

more um we proposed that back in 2009

and we have not really uh been able to

demonstrate an advantage we have been

able to demonstrate a small Advantage

based on some small entanglement but

this powerful scaling with this uh which

improves over the classical level as a

square root of n has not been able to

demonstrate yet so um of course uh I've

been trying to do something else uh in

between and the idea is that well maybe

there is something in between these two

uh limits so we might not just use um

coherence of single uh Spin and we might

not look for the full entanglement uh

enance Metrology but there is maybe some

regime in between The Chew where we use

some small um Quantum system uh some

small amount of entanglement to achieve

some task that a single Cubit sensor a

single Quantum sensor could not achieve

and this is like a very fruitful regime

which sort of uh is the intersection

where engineering um advances come

together with the fundamental quantum

mechanical advances to bring um some

real practical application and this is

what I want to talk in in the rest of my

talk and there are many different things

that we have looked uh over the years

for example one can use some additional

um Cubit sensor to do Quantum eror

correction or to use VMS Quantum memory

in order to both improve the sensing

time and so the coherence time or to

look at correlation what I'm going to

focus today is how to use some

additional uh control and some

additional uh level other some virtual

degrees of freedom or some real degrees

of freedom associated with other spin in

the system in order to achieve frequency

up conversion in order to achieve Vector

sensing and also to enhance

coherency so the first thing that I want

to talk is this vectorial sensing uh

which is achieved uh by doing some modulated

modulated

driving so um before that I want just to

step uh back and sort of describe some

very simple way in which you could do uh

vectorial um sensing so first of all you

could do the typical Ramsey magnetometry

to detect field which are in the Z

direction if you want aligned with the

NV axis with the MV quantization

axis the idea here is to prepare

superposition state of your MV Center

let it evolve for some time under an

external magnetic field which is aligned

in the Z Direction and then detect out

the phase which has been

accumulated uh and this will tell you

how strong of a magnetic field uh it was there

there

if you want to instead detect a

transverse field if you do this you get

some perturbation but it's very small so

it's very insensitive way of detecting

transverse field so what you can instead

do is to detect field which are rapidly

oscillating at the resonance frequency

of your uh spin of your MV Center and

this will uh indeed induce a a pression

uh of the spin now in the um Z let's say

zy plane but this can only detect things

which are uh oscillating at the

resonance frequency or the MV Center

itself so these are gigahertz um for uh

the Z field you could uh indeed also

measure oscillating field okay for

example by using dynamical decoupling

sequences but the typical frequency here

on the order of megaherz while here on

the order of gigahertz so um it's

difficult to bridge between uh these two

uh modalities and also the experiments

are quite different so if you have

system I if you have experimental error

they will be different in the two

experiments and so it's a bit difficult

to really get out some very nicely

consistent way of of detecting SP so how

can we uh Bridge uh these two and find

the single protocol to do indeed Vector

magnetometry so before I explain that I

just want to take a little bit in aide

so if you do want to do this rabi

magnetometry how can we understand that

well we could have this oscillating

field as I said like a transverse field

that we might want to measure or use to

control the spin if you have this

oscillating field you can always

consider it as the Su or two counterpropagating

counterpropagating

field and then if you jump into the

frame rotating with the red uh field uh

this will become of course static in

that rotating frame and the other field

will be rotating very fast so we can

actually ignore it the static field will

then cause a tipping of your Spin and

you can detect how fast it is indeed

rotating and with that you do rabby

magnetometry indeed uh we we did that

actually with my colleague Danielle

Breer of linol lab and achieve some very

good um picotesla level sensitivity for

this fast oscillating field uh by using

their expertise in achieving uh the best

overall sensitivity in Ensemble like

large Ensemble magnetometry so this is

the current sensitivity limit if you

want for this high frequency field

detection now going back to our

oscillating field if the oscillating

field is along the Z Direction and again

you jump into a rotating frame in the XY

Direction well okay nothing really

happens so it's a it's a bit boring but

we can use this fact to achieve this

Vector magnetometry let's say it now I

have an oscillating field okay which is

in some um arbitrary Direction in the uh

in in any direction in Des XY space and

in the lab frame this is that this

frequency Omega s but if I look at it in

the rotating frame rotating at the

frequency of my apply microwave for

example then this splits the two

components so the component which is

longitudinal will stay at its original

frequency but the one which is in the

transverse plane will have a shift of

frequency by the rotating frame uh uh

frequency itself Omega and so one can

sort of detect The Chew in different

ways and indeed this is what we do we

detected transverse component uh by uh

applying for example uh by by looking at

this Shi the frequency while we detect

the longitudinal component at the uh

original frequency and we can indeed see

them uh in this particular case we see

that we were applying a field at about

28 megahertz and this uh can be detected

the longitudinal component can be

detected at the original frequency while

the transverse one is shifted by the NV

frequency and so we are able to use

basically the same uh setup uh the same

NV and the same um experimental um

sequence to detect two components of the

magnetic field so the X component and

the Z

component the limitation here was that

as you can see the difference between

the two is only about 6 mahz and this

was what the frequency of MV we needed

to set and that uh if you know the

technicality that means that you need to

work at a very specific external

magnetic field and this is the if you

want the constraint that you have in

order to be able to detect both of them

simultaneously if we make this a field

frequency with signal frequency much

much higher then it's not really

possible to autod detect the transverse

component with the same type of sequence

so this is sort of a a bit of a

limitation but the advantage is that you

have the same experimental errors and

you can use a single MV uh to do uh both

vectorial magnetometry with all

different axis and so you can also of

course apply this to an ensemble without

needing to use different classes of MV

for different

direction So to avoid this sort of

bandwidth limitation that we had in this

sample uh we decide to use the MV Center

as a Quantum frequency mixer so what you

would need is of course sort of like to

upcon convert or that convert your

frequency to a frequency which your MV

is particularly sensitive to and this

what we wanted indeed to do so again

like if I have H just an arbitrary um

field which is oscillating uh I have

that they are sort of like splitter the

two components are splited and one is

has a frequency shift but this frequency

shift might not be at the frequency

Target which I like and so what we can

do is to add some additional bias field

at an intermediate frequency so that I

will have an effective signal uh which

is now nicely situated at a frequency

typically on the order of a TENS of

megahertz which my MV can detect very

very easily with common uh U um

detection uh um protocols that people

have sort of like deoy them many many

times and indeed we do this and now we

are able to detect for example signal

which are at around 200 150 megahertz

which is typically frequency difficult

for the mvy center but just using a very simple

simple

protocols so how does this work well we

can analyze this by using a FL formalism

which uh What uh it tells us that really

what is happening is that we are

creating some additional virtual degrees

of Freedom additional uh energy levels

uh thanks to uh the drive so thanks to

the drive of our bias field in addition

to the signal field so we are

engineering some additional degrees of

freedom which allows us to do this

better sensing

task and the advantage again is that we

can then use typical um rabi or cpmg

sensing sequence to detect fields which

otherwise we will not at the gigahertz

that we will have needed for Rabbi or

low meah hortz that we would have needed

for cpmg the disadvantages that we have

a heat in the sensitivity uh but of

course if you cannot sens this sort of

like 100 of megahertz or or maybe 100 of

gigahertz frequency in other ways well

you you can sort of survive um to uh to

vis a reduction in

sensitivity so we demonstrated that and

we demonstrate that we can achieve the

same level of sensitivity well we we use

a rabi sensing protocol or a cpmg

sensing protocol cpng is where you apply

periodic pip polies which match uh the

frequency of your target signal and we

can indeed

achieve very good sensitivity with this

produ so this is an example in which we

just use control to engineer if you want

this vition additional energy level but

what we can also do is actually to use

additional degrees of freedom inside the

diamond in particular associated with

the uh nuclear spin in order to uh

improve the sensing task and actually

use the electronic spin now just as a

controller so before I introduce uh the

Envy Center um and also I think that in

the freedom introduction we focus on the

electronic spin but of course we also

have a nuclear spin in particular the

nitrogen 14 nuclear spin which is a spin

one and this has both and hyperfine

interaction which couples the nuclear

spin to the NV and also a quadrupolar

interaction which gives if you want a

local magnetic field on the the nuclear

spin itself and we can control also the

nuclear spin in the same way that we do

the electronic Spin and again by mapping

the nucleus

spin uh um state to the MV Center also

read it out so this is a much slower RAB

sorry this is I think rabi illation it's

better label of the EnV center of the

nitrogen 14 of the MV Center so we can

in principle also use this as a a

magnetic field sensor but it's not very

sensitive and so what people have been

using the um nitrogen spin is for uh

this idea of building a nuclear spin

gyroscope associated with the NV centers

because uh nuclear spin are as sensitive

as electronic spin to rotation and we

can use the electronic spin to mediate

the polariz ation without a

control so uh we uh indeed were looking

at the nitogen 14 and we wanted to test

the fact that nuclear spin in principle

should be uh very very uh uh robust to

any external perturbation and so I have

very long coherence time instead we

found relatively long coherence time

this is almost millisecond time but also

some puzzling behavior in particular if

the MV Center is in the minus one state

or plus one state the coherence time was

much shorter on the order of 2

millisecond instead of almost 1

millisecond so what we figure out very

quickly is that actually variation in

the hyperfine and quadrupolar

interaction were reducing the coherence

time so one can sort of use some Eon on

the Envy Center to correct for

this but we were also interested in

trying to find some other system which

hopefully didn't have this problem at

all and so what we did was to turn to

N15 so N15 is a spin 1 half isotope so

it does not have any quadripolar

interaction so well it should have a

better coherence time and there is no

hyper fine if the NV is in the MS equal

zero state where this spin is inde zero

and so what we expected was that as we

were going from the minus

one state for the EnV Center and looked

in at the coherence time of the nuclear

spin so if we move to the MS equal Z we

should expect much longer coherence time

instead it was a bit longer but not as

much as we were

expecting so again this was a brief

puzzle until we of course figure out if

I have some water I don't have water we

figure out that actually there is some

hyper fine because we have some

transverse component of the hyper fine

and as soon as you have a little bit of

external transverse magnetic

field with this coupled in in a very

very effective

way because that I need to find water

otherwise I'm not going to be able to continue

sorry about that let's see if it is

not so what we found was that transverse

field couple into the

hyperfine giving rise to a shift in the

resonance frequency and if the hyper

fine is in homogeneous then you have

again some Decay and this shift can be

quite large especially around uh the

level The Crossing for the nucleus Spin

and uh and of the electronic nucleus

spin so what can we do well again it

becomes better actually to uh sort of

alternate between the minus one and plus

one states of the electronic Spin and

this will actually improve the coherence

time of the nuclear spin itself because

this will sort of refocus the hyperfine

neogenes that we overwise obsorb and

indeed if you look again at the nuclear

spin coherence Time by doing this

protection scheme we can uh improve the

coherence Time by almost an order of

magnitude so the last uh thing that I

want to say is that we can actually use

the electronic spin also as a Quantum

amplifier again I told you that we have

this announcement of a transverse

magnetic field uh because of the

coupling with the hyper fine between the

nuclear and electronic spin so the

nuclear spin itself will see a much enan

uh uh transverse magnetic field and this

can be used to detect a better a

rotating magnetic field or if you want

if the diamond itself is rotating with

respect to a static magnetic field uh

the nuclear spin will see a much larger

rotation than the electronic spin itself

so what we can do is then to exploit

this fact to have an anouncement in the

sensitivity to rotation sensing so as

you can see the rotation of the diamond

is very slow but the nuclear spin see a

a much larger change and uh its Dynamics

can just be a still like AA following of

its initial state so it's very robust to

additional noise but it can give you a

very sharp advantage in the

sensitivity so what I show you that we

can use the electronic spin uh both as a

Quantum mixer as a a Quantum controller

and as a Quantum amplifier so all type

of quantum Advantage which we can get

from our Quantum sensor we really

needing to go for very large quantum

entanglement so this tells us that

indeed we can already get some Quantum

advantages although of a slightly

different time for hopefully near term

technology so even beyond the isber

limit even if you have some small amount

of entanglement or some additional

degrees of freedom we can make Quantum

and un sensing much more practical so

for example we can have a frequency

conversion of this cerence protection or

amplification of some external

perturbation so what are other

challenges of opportunity for Quantum

sensor I think that what PR describ in

the previous uh uh talk that was

perfectly spot on uh what we want to do

is really to uh try to bring all these

devices from our Labs out in the world

and there is still a lot of work to be

done there in particular in trying to

make uh sensor which are much more

robust so that they can

work in um the condition that people are

really interested in and I think the

other thing which is quite interesting

nowadays is to try to integrate Quantum

sensor in the larger Quantum ecosystem

so really uh integrate Quantum sensor

into Quantum networks integrate Quantum

sensor with quantum computer so that we

can uh really uh both exploit for

example distributed sensing and also uh

try to use a small Quantum compitation

to improve the Sens in task itself with

this I want to thank uh the people who

have uh done uh the work in particularly

minty and Goin Wang who not in this

picture because he already graduated and

uh thank you for your attention and

over one second sorry this is very s you can

can

take okay can can you hear

it uh do we have any questions for Professor

question we are little we they can uh we

can answer the there's a some question

in the Q&A but

maybe Professor can can answer those

offline yes because we a bit behind Okay

and we can start thank you so much thank

you so much I'll stop my sharing thank

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YouTube Transcript:Professor Paola Cappellaro, “Design and control of quantum devices”, Caltech, May 30, 2024