So it's a real pleasure to
welcome today's speaker

Professor Dragomir Davitt a bitch
from the School of Physics.

Dragomir got his bachelor's in physics at

grad faculty of Sciences
before getting his Ph D.

at Johns Hopkins and then did a postdoc
post doctoral work at Harvard and

came directly to Georgia Tech after
that where he is currently an associate

professor in physics and
I will turn it over to you thank you thank

you hello.

Good.

Thank you for the introduction and
I'm glad to be here.

So I am right across the street from the
physics department and in today's talk.

We'll talk more about what has been going
on in my lab over the last couple of years

and in a broader context.

So we work our work is closely
related to the field of spintronic So

give us introductions of the field
focusing mainly on industrial applications

and then I will talk about our
particular research goals.

Which focus from understanding
the basic physics

in the dynamic of magnetic switching and
path to new project in physics.

And that includes the study for the
magnets in extremely small limits of size.

OK And also we are trying to observe
microscopic quantum phenomena and

i will they now mix in magmas of
the highly they even out of equilibrium.

And the next goal will be to move on
to Quantum Information Science to start

study multipart I think.

Between single electron spins
inside these nano magnets.

So.

OK Something went wrong here but our.

OK So what is spintronic So I call it
the electronical spin so it is a very

at this time it's a myth or science has
been around for twenty plus years and

therefore it is best explained by
its industrial applications and

this is the most famous one and if you
have a hard if you buy a hard disk for

example this is from e Bay and
open it up this is what is inside

this heart of this drive and
what you see here is a.

Rotating magnet the pins very fast.

And there is an actor can move left and

right and then there is a head and
this head here the very tip it is so

small you cannot see it
there is a reader and

the writer that will read
the information that is stored

on this disk so let's go a little further.

To explain that So
how does that either and

right to work so
here is the subject of similar disk so

the spinning there is a track and
this had floats just over the.

Area of the track and

on this track there are a lot of little
domains where the spins going to be up or

down or left and the right depend
on the when was this made.

And read had shown here this is a sensor
of a magnetic field based on dry and

magnitude resistance effect so

it turns out this if a zoom into
here you see here the domains and

here we are sending current
through this sensor and

the current is very strong with a bend in
the magnetic field and that's why it's

called the giant me to resistance and
therefore we're measuring the voltage drop

across this sensor with therefore see
whether the over the domain is up or

down and
that's how you read the memory OK so

now the context of that our work focuses
in the out the middle emitter of

storage than city in very small particles
which I'll talk a little bit later.

There are two varieties of these
sensors I forgot to mention one

more thing you can also see there
is a coil here there's an inductor

this causes you to write to split
the domain into the desired direction so

there's both or either here and
there right or just over the domain

coming to wear out is there lateral
systems based on G M R need to resistance

but more recently we are switching
to tunneling junctions or

funneling money to resistance sensors
with a vertical transport and

they have much higher sensitivity
than their giant meters distance.

So let's talk a little bit about typical
M.P.G. magnetic induction says everything

most of the things we do in
lab involve these junctions.

So here is a typical this is a high
level relatively high level magnetic

junction and it has to fire a magnets
in the middle and this is the.

Boron and the magnesium oxide in between.

So it turns out that there is
the tunneling resistance between these

two fire Magnus depends very strongly
on the interaction of these.

Small nano magnets so this is not the
sensor in the genome or in the hard disk.

Tools.

OK so this family matter is this is the
ratio of how much of the current changes

when the domain changes direction when so

it is given proportional
the polarization of spins in these.

Magnets So

what is the most what is the highest value
of the people relation every quarter is

only zero point seven That's not very high
that's state of the art of producible.

Spin polarization that can be made and

important just to give an example if
you want to do quantum information in

the system like perform the Alice
in Bob Ballard experiment P.

needs to be larger than zero point nine So
this gives you an idea how difficult

it would be to do quantum information
science in the solid state.

Now let's go a little bit further so
why is there so just explain what causes

a problem with resistance what is caused
by the band structure of magnets so for

example if you are called Bob Coble
has spin up and spin down at different

energies so the majority electrons have
the firm surface like in copper so

there is a normal model but
the minority terms intersect with the band

have a very complicated Fermi surface
with a very high density of states so

if I plugged the of states of minority and
majority

there would be an imbalance at the Fermi
level there's going to be a lot more.

Electrons with.

Right so this has been this need
to be flipped by mistake but

you know a minority states have much
to states that the majority wants and

this paper is ation is
related to the imbalance

in the that's the of state and the PM R.

is given in terms of the speed in two
sides of the junction and by the jury or

formula shown here so if I have the best
possible junction today made in

Japan then we will get about
three hundred percent of it so

peak was to zero point seven produces
a three hundred percent ratio

between the para and
the barrel resistance.

OK so now there is another
breakthrough later on maybe ten this

enough fifteen years old no more almost
twenty years old this is called.

The Another school magnetic
random access memory.

So you see if it is hard this there is
a spinning there's a spindle there's actor

to this moving part of this is not very
practical for fast applications he said

we need to have a fast access memory which
is known as the random access memory

which has no moving parts and the idea of
that is that they would have a tunneling

drunken troll here would spin up and
spin up and down then if we sent

a large parent from one leads to
another lead if it is efficiently large

it can reorients the magnetic zation of
another lead so we don't need any moving

heads we just need a tunneling dungeon
that can do both reading and writing.

And here is the Act This is much more calm
we see that there is a little transistor

here this resistor is very important
because it turns out that this type of

switching which is known as a spin torque
transfer spin transfer torque switching

requires very high care and
that's in fact they're so

high that it's not practical for
many electronics supplications.

So this to Mr Here is used to
boost the current on a chip so

that there is enough current
capability to switch the magnet.

To kids very complicated there's
a lot of variance and here is ever

spin is one of the companies that makes
these trips that can be bought and

you can do this random access memory
has been placed in many devices like

specially of satellites or if you go to
outer space you have a more reliable these

magnets are much more reliable than Flash
or any of the other types of memories.

And just a very quick reminder
of what is there and.

Maxus memory of junctions containing
these bit lines in the word lines and

then by selecting a word light of the Beat
line I can write the magnetic information.

To be down or up as shown here.

So using that transistor I
can send a lot of current

between a particular junction
to write the state and

then later on we can read it by measuring
the tunneling when the resistance whether

remember is up or down and
clearly this will be much faster than the.

Tunnel when the speed but
it is because of all the power.

OK so this was the general overview.

So let me a little bit of what is our
work is all work related to this.

In fact we want to make a magnetic
random access memory for the Q.B.

three The out is a quantum computer so
all this problem with the power for

Congress get exponentially higher
if you want to do acuity doubt.

Which is much more sensitive to
temperature heating noise etc etc.

So in other words if we want to make
them what I am going to Mass to store

information from cubits and there are many
reasons this would be very good to have.

Then we have to solve the problem of the
damping because damping or dissipation is

the reason why we need to send too
much current to right information so

the point is that if we sense being
porous current through a magnet

it doesn't remain spin porous for
long time but it dissipates very fast so

the result is you need to send
a lot to flip the magnetic moment.

So we need to reduce the damping
significantly in order to reduce the power

requirements.

Enable Q.B. three though so
we are now doing basic research about that

about the damping in
very small structures.

OK And here is the expense
we're currently developing

so we want to study very fast magnetic
dynamics in there even magnets with a deer

to move to quantum information science and
study entanglement in these systems.

OK so.

This is just the quick schematic
of the device we're making so

making a three apply a layer because
this thing the hard magnet Cobalt.

And another hundred magnet Cobalt which is
shown here they play the role of the fixed

layer and a very thin and
wide perma Lloyd soft magnet particle

as shown there and this will be applied
it will apply microwaves to drive

the permalink particle out of equilibrium
and send it into an uncertain state.

So this means that if the system is very
strongly driven actually permanent or

will not be magnetic anymore the magnetic
moment was moved around too fast.

But at the same time the fixed
layers will stay pinned because they

have much higher and I saw therapy and
they will not respond to the microwaves so

we'll have a system
that's highly driven and

then the question that arises fundamental
question is what kind of uncertainty will

develop in this particle is going to be
the classical uncertainty which means that

the model is a string moves too fast to
be measured savage and out to zero or

is the true quantum uncertainty
from a quantum state.

And it turns out based on everything that
actually that I've the states actually all

have to be non-classical there will
be very strong quantum uncertainty

in these Magnan that's
what we want to study.

OK So let me now discuss discuss
a little bit the magnets so

ninety nine point nine percent of
spintronic C involves classical dynamics.

Left so we have here a particle
way the spin as fifty and.

This the magnitude of the magnet is
a sure spin doesn't change with time and

the spin can only rotate
as the basic idea.

Of it so
the magnetic eyes up the piano just so

the energy of the magnet depends
on the direction of this.

And it is given by and I saw trippy and

the magnetic field which is a month and
drawn here.

And the law of motion of the spin vector
is known as the lander Lifshitz equation

or sometimes land Aleutians Gilbert
equation which is basically similar to

it's a vector type equation for
spin of actor that

has a cross product as cross and
then there is another vector here so

it moves like a torque so easily this is a
torque type equation look if you have you

know if something is spinning that's how
it moves and you see the magnetic field

is the applied field and the internal and
I is author of the field shown here.

We need to also add them ping and noise
terms to properly describe the NAM mix but

here it gives you roughly an idea how
the magnetic the how the spin moves

in a magnet this is idealized case so
if this is the easy access or

the magnetic field axis than
the spin would precess around the Z.

axis so in this particular case this spin
will be processing as shown by the laser.

Or on the Z.

axis.

But eventually this perception die out
of the system is not driven because of

the damping because the energy of the
station is lost to the heat the bat and

that's what we are now what about
those non-classical States if you have

a non-classical say then
this completely fails so

forth happens is that we
have a value of the spin.

And the magnitude of
spin can be much smaller.

Than the value of the spin school.

Words So this is not quantum mechanics.

So if you remember quantum courses
that angular momentum has.

Al squared so
when I say out square is plus one.

So that's what is manned
by the magnitude of spin so

the magazine has been given still very
loud but the average value of the spin

vector has negligible amplitude
compared to the magnitude of the spin.

Of the so this is a quantum
uncertainty of the spit so

in order to describe such a process we
need to use a magnetic coming of Tonia So

we are quantized the classical and
energy operators shown here and

then we have to solve the highs and very
question of motion for the spin operator

who would have to forget about this
completely and it's very similar.

Also very different.

We also have to add the coupling to
the bizarre Nick he the bath and

one example probably only example
of quantum dynamics in magnets and

started to feel today these microscopic
quantum tunnelling of magnetization

this will be done extensively
in the late ninety's.

OK those are extremely different
difficult experiments and there's not but

two of them now on that.

OK So now how do we how do we make these
magnets quantum from plastic and magnets

so what we need to do is we need to
create states which are non-classical and

in particular the uncertainty of these
states has to be comparable to as squared.

By comparison the answer
to quantum uncertainty or

classical states in the Elysium So now we
have to have something very different.

So we'll do this in a few steps first we
have to do squeezing which means that

we have to create extremely high earners
out there but that is to say the wheel.

Make these particles about one hundred
nanometers in the amateur the extremely

thin and this has been that regularly
with thermal You can make three nanometer

per Moloi particle with four
hundred nine to me today amateur

you know slightly ellipsoidal shape.

Which is represented by the following and
I was out there be Hamiltonian as shown

here this ratio of point zero zero
two is similar to the aspect ratio H.

over the.

So as you see here the energy is very
large if the spin goes out of the plane

and the spin wants to stay in the plane
with a very you know we can as after being

inside the plane and so even if we do this
in eigenstates and I give as is that and

find the ground state it was to be
classical with extremely small uncertainty

OK So this is not going to do
the trick but it will get us closer.

So let's go then how do we what do we
do next while we have to the next step

when they are going to lies this
Hamiltonian that I showed you in

the previous page to see where
these non-classical States located.

They agonize when you have to solve the
eigenvalue problem if allowed to Matrix

represented by the Hamiltonian So

we use than thousand spin than thousand
and twenty thousand electrons bins.

And this is what we find this with him so
the results of the simulation that.

What you see on the right is the energy
surface of the magnet this is the function

energy versus direction of the spin.

Of this particle.

And the theta is the angle
with respect to the Z.

axis.

So they want to be at PI half which means
that the speed of also being the plane and

then fees the as a myth and you see
that the classical surface in energy

as a function of the direction has a
saddle point which is very well known but

this subtle point here is very shallow and
below the saddle point and

gets very steep when it's
above distinguishing nature.

OK so now I will also find the energy
levels of the spin Hamiltonian

which are shown by the blue line as
shown here so these blue line is these

are the dots the ten thousand dots here
one for each quantum level of the magnet.

And they are plotted as a function of the
direction of average spin quantum others.

And it's very interesting that we find all
the levels located just above the valleys

and below the ridges of this surface so
these are the quantum states

so then further on we can all
calculated these we can calculate.

Spin the expectation value
of spin versus these levels

at zero there is a subtle point energy
just ignore the top graph you don't need

that just look at the bottom graph that
shows a subtle point this is not working.

It's work.

I see.

It's a little they are.

So this is the point so

I plot the expectation value of
sex asked why and there's a Z.

So these are three
components of the spend.

Versus source energy and

what you see here the saddle point
the average value goes to zero.

And over the saddle point
because of the steepness

the spin is very strongly suppressed
compared two hundred percent so

this is the regime of very high
quantum uncertainty of the magnet.

So now we actually know what we need to
do we need to basically drive pump this

magnet into the city tour of energies
at slightly above the saddle point and

then we will study its been
polarized dynamics in this magnet so

that's the that's what
we have now microwaves.

So how do we create a how do
we populate the states over.

How do we populate these states here and
along this.

This is your this is a ridge.

Well this is that by
making a time Crystal so

time crystals of type of crystals
of the in the time domain so

if you apply a periodic perturbation
on to the system that's shown here.

By microwave fields and then we're
modulating the bad Hamiltonian of

the system so this particular example we
have a classical surface this movie left

and right as shown here so
if you have a magnetization direction

initially at one minimum it will
tunnel towards the other many

By the time it arrives there the shape
of the potential changes and

this is still at the first minute so
basically what happens is there's

a full the localization of
magnetization taking place classically.

And in not just the localize it
disintegrates completely there's no more

spin vector that doesn't exist
anymore it goes to zero.

Ok saw how the we if there's other is
a very nice theory of this time crystals

how do we solve these equations this is
not the same has a very question except we

have a microwave field there is
a flow which is very similar to

the blocks theorem which is the foundation
of contests about the physics

that the function the way
function is a function of time

has similar characteristics as
block wave so that qualify and

edges and the periodic functions
of time so this is how they look.

And these absolute and new unknown is the
cause energy levels so they play a very

important role in time to stalls so this
is a very nice picture it turns out so

we need to find the distribution of the
particle states when it is highly driven.

You can no longer use the classical
Boltzmann distribution the big

they become different so you have to have
user density matrix which is shown here

I don't want to get into the details so
the diagonal of the matrix should be

the Boltzmann distribution but
if law and they are going all elements.

But it's more intuitive
to go to the right so

if we Now I plot the average speed
versus time what you see here is

that this goes if I start with
a particle from the ground states and

turn the microwave on the magnet
ization rapidly the case to zero

because of the process
of decomposition and

look the vocalisation and this is a very
long time scale indeed you see the quantum

average of the spin will reach zero but
it will have some noise around it.

So this is how we access.

Our plan to access these states OK And
how do we now get so

this is now the motivation I we
haven't really reached that point yet

we are working my students
are working on this very hard.

And here I am going to talk to one
facet of this project saw first.

That is how do we measure
fast Moneta dynamics.

OK So this is the project done
by Jason dark in my lab so

we want to learn how to measure
the switching very fast clearly we

want them better rapidly because we want
to study the dynamics of these states.

So the challenge here is that we
think because it was very high

resistance samples about
one hundred people.

And as if you're familiar with high
frequency electronics it's extremely

difficult to measure fast signals from
high resistance samples especially at

the low temperature so here is
a device that we make in our lab so

in this case you just have
a simple one that the.

Shown here there's no island yet
between two leads

the junction is shown there and these
are some these are evaporation shadows.

I'm not sure if you this is a man
of five types I mean I'm not sure

if anyone here uses the shadow
the position technique is probably not so

this is something that physicists like to
do it's a very prototype small scale fab

that you can do without clean room but
you can make samples so

let me let me show you how we do that.

So we use a standard lithographic we do
a by a layer spin coat followed by E.B.M.

with our graffiti it is in-house
development evaporation the position and

lift off so I show you how does
this shadow the position work

well we created bridges we make three
dimensional structures of B M M A.

For example here is a bridge
it is supported along the Z.

axis and then we deposit one material.

For example be permanently.

And that we deposit change the angle of
the position by rotating the stage in

the posit another layer
following the oxide.

Another layer of P.

which is perma law.

And here we made a thousand dungeon
is very simple we don't have any

sputtering or

any We don't need high vacuum so I mean we
have a pretty high vacuum with a vacuum.

Yes.

Or so this is governing there is either
the directions or some other show you.

OK So you see the bridge was here.

OK so the bridge was supported.

OK my students know better than me but
that I have to now

I have to because they're in the process
so the bridge was a basically.

So essentially this is one lead and
this is another and

these shadows that this is is always
more complicated than sketch but

these shadows cannot
touch any of these two.

So we do careful designed to make sure
that you know when you make shadowing

that there's no touches this very
important is a very important point.

So in this particular case so

the bridge was shown so
there were two wires I could have and

I reconstructed it so
there's one wire here and

then one wire here and
the bridge was in between in the city.

OK So
this is what you see from the top of the.

There is a wire here and
a wire here that's all you see and

then there is a bridge which is nothing
underneath except the bridge so

then basically deposit first one here and

then out of the same wire here and
then we change the angle

actually the other way and
then I'll bet that we change the angle.

OK So we first and then we change
the angle and then we deposit

another wire here and another one here
OK So basically First we did this and

that and then we change the angle
of the deposit this and

that is very soon sort
of there with simple.

OK yeah I saw this have you have to
there's a lot of designing going on in

this deciding how this will work and
we use a dye by Larry.

And am a way to maybe able to make
these bridges everything is digitally

thank you let's now go to the next
step saw the cooling system so

we have a liquid human problem so
we have found.

A scrap.

That nobody used anymore it
was waiting to be trashed and

they rebuilt it in my lab and
refurbished it so

this is a seven Calvin car and
they're really made a small magnet.

So this is basically a glorified.

That goes to six and a half gallon that.

And my students have machines a lot of
these things including the magnet so

for spintronic The fortunate thing
about spintronic is on the much field

the switching fields
a hundred military most so

that's easy to do We don't need to be
experts on magnets we don't need fourteen

Tesla that's the thing you do there's
no way you can do this at six Kelly.

OK so it's up it's a vacuum and
it has maybe one hour maybe

three hour sample time to switch
the samples but you cannot continuously.

OK And here are some of the.

Resistant tunnelling the resistance is so
when you measure.

The resistance or resistance of
the drunks in the verses time.

And we get history says looks so and up.

Leads up arrow on the top of
the bottom leads up arrow then

we have a low distance and then the and
the peril they switch to hide is the stay.

On them so basically the way it
works I feel the field down and

then switches up and then switches and
then I feel the up and

then switches up and
the power stayed in this which is that.

So then here is another
sample I don't understand

why the student did not go further but
OK you can ask him later.

But I have to show the sample because
this is a sample where most of the stuff

was done so there's a problem.

This otherwise OK so
here you have to perm a lot so

the first is there peril and
they could be.

OK So now let's go to.

The next layer so we have partnered with
engineers electrical engineer with a group

of the class learn the students.

So the next step is a study dynamics so
we use silicon germanium transistors

to that end and the idea is to
integrate the junction with the silicon

germanium resistor shown here so
we get this resistors for

the customers group and
they make the junctions and.

Then they would have a year.

Which.

P.D.F. would be better but no this is
a look at this is five I think it's.

So on the right you have the something
known as a gamble Skirt three sticks of

these three sisters that we use so

basically you have a basic meter voltage
and you measure the collector current and

there's a usual exponential dependence
on the base of the to voltage

so you don't need to understand is got the
point is that this does this is work down

to very low temperatures the Dumfries
out so we can use for John experiments.

OK And here is the so this is for now
how things look they're not yet optimal.

Here's a magnetic junction
that we made in the lab and

here's a silicon germanium through SR This
is about one millimeter as you see and

then we just connected on tape.

And so the idea is we would mess
with the empathy right here

of the signal on a hundred kilo and
junction and

send it up through the leads and
measure the dynamics.

So the surface is very simple so.

We have tried of more complicated
amplifier but in the end what really works

is you just put the sample to the base and
use it to bias the base of the transistor

and then measure the collector current
shown here as the output of the signal.

OK So here's the schematic of the circuit
OK so here is how it works so

here's one example in that sample that did
not have that part in the original figure

so this is an example of the results
of this is biased to the very high.

Base a meter voltage.

So then we measure both of the we
measured both the base current and

the collector current and the collector
current is not amplified by seventeen

thousand this is a you know enormous
number so that you can see that

while we are measuring the collector
current is very noisy and

small than this this is the base
current the collector the magnified or

amplified collector current is much larger
and that's what we see that is done slowly

so this gain seventy thousand will enable
us to measure it faster because the R.C.

time constants go down
by the amplification so

here so now we have trying to measure
rapid switching So here is a.

Prime trace of time is magnetic
field here by the way so

as I see it change the time
I change the magnetic field.

And here is a switch it's very basically
there's a lot more noise because

it's very high bandwidth now but
it's a mistake about it it's right there.

We reduce the gain we start to lose
the signal at these rapid time places.

So if you take a closer look there's about
a million points here this is a lot of

data saw let's not look at
a high gain situation so

that event will next we obtain this is
how it looks at a short time scale.

And.

This is finally when we zoom in and then
we get that I was of six microseconds.

So this is very this it out in the fast to
give you my this is one hundred kilometers

is that in the system this is very
fast this time constant of six like

it is that ice time of six microseconds is
actually limited by I wouldn't have a trap

with fire because we use a commercial
trance to be the sample of fire to measure

these signals and they have a time
cost of about eight microsecond so

basically we are a band with limited now
by their own temperature electronics so

my son is a now trying to measure this
faster from using room temperature

but another thing that we already probably
see this is about my students thing that

actually there is an excess noise I knew
who was switching which is shown here and

there and these are two data
places that were taken so

the rad shows more noise right
proud to the switching so

we already think there's even already even
though this has not been optimized yet

we are seeing some signatures
of which in magnetic dynamics

these time scales so
that's a good science and I'll be.

Trying to make improvements and

there are a lot more traces that show that
unfortunately they were not stored but

they remembered it's going
to throw them on for

so we need to get hundreds of thousands
of these and then do the statistical

analysis to prove that there and
to find the correlation times it's either.

OK So that's one project hope it gives
with another interesting thing about

this technique is that I can find
the spectral density of the noise

of the collector current and essentially
we are either around here at the corner

where there's a one over noise below
about one hundred kilo Hertz and

the noise becomes splat and

basically it is the shutter noise limit of
measurement so we are right around here.

So basically we are in the fundamental
limits of measurement already

even with such a simple setup you
know with indium dots and sort of.

Commercial amplifiers of the temperature.

So I'll talk now but it's about the second
person coming with them to him.

So this is the title of my the we also
interested in the problem is a very small

particles This is grants project so
this is the ultimate limit of magnetism So

here is an image of nickel particles
on the surface of aluminum oxide.

And they're like two.

Nanometers in the AM with the crystal
you can barely see these images with

in the material science department.

They're not permanent magnets by the way
the room temperature there's no magnetism

in fact you need to go below ten
Calvin for them to become magnetic.

That's so it's very small but even though
they're not magnetic there is a surprising

amount of applications of these back
tickles this particular ones by

the drug delivery they're huge in
medical science cancer treatment and

we are more interested in letting storage
so they see the impact I have done some

research about the impact of nanoparticles
in medical sciences is probably comparable

to the impact of dry and we need
the resistance in our sciences like and

so but we just don't do biology
here in our department OK.

So let's not go further how do we measure
one of these particles we use the same

technique as before.

You can use microswitch that's way too
difficult there's only one group that it

does well in the world and they're not
doing it anymore really so it is very hard

to do so we will do electron
transport that's much easier to do so

basically we use the same
technique as before and

then we put particles in the tunnel
induction between two aluminum leads

using the fad of the position to keep and
here is the image of one of the samples so

let me give you more idea so
what is going on so

these are the leads that we make by shadow
the position there's a small overlap.

And these are particles so you see.

If you have to reduce these by
Factor five to get here and

then you reduce it by another factor
of ten you get this tiny square OK so

this tiny square is this whole
square shown here OK So this is how

we are trying to do it so that you want to
just measure one so the you can do is by

random chance you cannot make this using
standard the target field so this is the.

Articles will be a better by a factor of
planted than the cutting edge technology

of publication which is about.

Twenty nanometers it's actually less but
but

how do we do it to really make them using
the massive chancel the student has

to make hundred samples and then they have
to measure them all the screening and

it's a lot of work you know and
many of them may not work so.

OK so here is the example when
things work very well so we measure

them in the dilution refrigerator which
has several cooling stages that we go to.

And here's a Cobalt but it will be
a perfectly perfect staircase so the I.V.

curve at low temperatures throw these
steps as a function of voltage so

these steps occur because
they're quantized levels so

our current can only go to
the particle if energy level

is below the firm level of the lead so
as you sweep the voltage across

in different quantum levels and
we observe these steps.

And then we take the derivative of the
curve and get this called the tunneling

spectra the ID versus bias voltage and
then we can study

quantum mechanics of these particles by
subserving the properties of the spectrum.

And he says in all data for
just a given idea so

does an aluminum particle and
these black spots the quantum levels.

The field and
the we see that these levels split.

The Zaman degeneracy and the G.

factor here is one point nine So

as a short these are the definitely
quantum levels they also have avoided

crossings you can probably see here
that there is not very well but

there's definitely a minimum between and
they will they are bad and.

Void crossings are also shown so they can
only happen if you have quantum says.

You cannot classical systems
will have crossings or ways.

We can also study magnetism using
that typical example is here Cobalt

particle This was also five years ago so
if you measure current versus field it

will show he said he slope from
single particle and we see here

as we change the temperature this is from
sixty Miller Calvin to fork out when

the switching few drops significantly and
attend.

That too you cannot see any more history
says that's because the particle is so

small that switching field memory
disappears because of the temperature.

Effects OK so now I want to talk about how
they measure the damping in these systems

so how can electron transport provide
information about the mechanism that being

and other dynamics as I said earlier
we want to reduce them in the system so

for that through the noise
spin on spectroscopy

in fact noise is a very powerful method to
measure the response function of a system.

So how can you measure for example a
resistor of a sample how they measure this

there's two ways the standard
way is to apply voltage across

an unknown sample and
then measure current using the ammeter.

And then that existence will be by
then this gets Campbell a little bit

the resistance will be v are equal to
VO over I this is the usual method.

But we don't need to do this we can
actually stay we don't need to apply ball

to visual thirty really measure the
voltage fluctuations on top of this it is

that OK so the output of the amplifier
will be the time dependent current.

Which is noise which is the out of
R F which is the feedback resistor.

OK and then using the dance on the noise
formula which is an example of a.

Very general fluctuation
dissipation Theorem one can relate

the noise in current to the response
function in this case it is those are.

So the noise per hertz of bandwidth
is given by this equation so

if you know the temperature you can
measure the resistance by measuring

current noise so this is different way

this is actually better because there
is no you don't need any voltages.

So this is how we will measure
the damping in magnetic particles

This is actually the.

I should have done the P.D.F. but.

So how do we now do the how do we
then measure the magnetic motion so

we would read out process so.

There is a method to convert
the spin motion into a charge motion

this is known spin to
charge conversion and

this is been mainly invented mainly for
quantum computing for the.

Silicon cubits.

So the idea is in that case for
the if you want to read out of.

Cubit in quantum dot like silicon so there
is a single electron cubit which is shown

here so this is a single electron
cube it can be occupied with

one electron only which can be
either spin down or spin up.

And there is a nearby single
electron transistor connected

to the source and the rain.

OK so now if the electron is on the dot

then this single electron
resistor will send the charge.

Of that dot and
it will know that there is an electron.

But if it is not on the doubt then
it doesn't says that charge OK so

the single like them to this can tell
whether there is an electron in the OR

NOT so
that's how you can actually measure spin.

Because it turns out that if
election results with spin down.

OK then the selector can move into
the lot and then move back out and

then one can see a transiency goal which
is that either out of the spin state and

they comes in a variety of ways.

So we have something
actually similar going on so

we have a spin on the dot on
the magnetic particle that can move.

Because of the microwaves or
it can move because of the transport.

Because an ironic really believe facts and

then there is a spin orbit coupling where
basically the energy of the magma depends

as I showed you earlier on
the direction of the magnetization.

So then the voltages vary just the near
the threshold of the book eight.

Or those steps to the show before then the
current becomes a very sensitive function

of the direction of the magnet is ation So

in the particular for some directions
there is going to be no current and for

Sun directions there will be current So
basically the noise

in the current of that we measure is
dissuaded lated to fish is related to the.

Magnetic noise and

then says noise can measure damping
that's how we can study them.

Anyway so.

I have to be a little bit faster but

the the point here what we observe this
is only a nickel particle we observe

a very strong magnetic field
the presence of the tragedy.

And here is an example of
these are the I.V. curves.

And.

I think the derivative
which is the spectra and

then what you see here is that
there the signal the conduct

those verses voltages quiet at
low field and high field but

there's a narrow field range where
there's a lot of noise just shown here.

So the fact that this noise is
very concentrated emetic field

is the magnetic origin of noise that
this is not a charge of noise and

we have other criteria
that we use to eliminate.

Tiredness is a typical noise source in
these type of devices which is a plague of

quantum computing and
everything in single economies but it

is very clear these are the noise that it
is it's very sound constriction of noise.

So you see the other samples
Here's a particularly surround and

it is this is a high field is
a narrow concentration of noise.

In current at particular voltages and
particular fields

OK so
what what causes this what causes this.

Minimum for this was nice so if you have.

So I have to go back so what's happening
at the very low magnetic fields.

There's nothing to hold that modernisation
and there's electrons that flow

these electrons that flow starts
to move the magnetic moment and

that's been factored in all possible
directions as I have shown.

Here.

OK.

So if most McGrath moan moves
too fast then it moves slower.

If I apply this week with I think feel
this magnetic field start to concentrate

this back to.

The axis of the magnetic field and finally
the very strong feel of the decision

doesn't move at all OK so
it's been to the magnetic field and

that's why if you measure
the current noise there's a peak

in the noise is in two samples this is
the are mass current versus in that field.

So what happens here at low feels it moves
too fast so if it moves too fast you don't

see the noise it's averaged out to zero
by the time constant of the amplifier and

magnetic field doesn't move because
it's been by them and I think feel and

in this decision region hear
the noise it is a peak.

Here.

So now the next step is to get the damping
time using the peak value of the current

So there is an elaborate procedure which
I don't have the time to get into.

A calibration which can be done so
the bottom line is we don't need

to use any theoretical models there is
no Hamiltonians not know what ammeters

we can use a very fundamental way
a fundamental logical way to extract

the damping time from the time constants
and I don't have the time for that.

Just to give you an idea how
do we do the calibration.

Of the system that's very important.

So the calibration of the system
can be done using spectra So

by measuring the amount of these level
shift which I showed here we can obtain.

The chemical potential change
of the particle as the magnetic

field is changing so in any case.

Just to finish up we find the following
is that we actually find thirteen in five

milliseconds in two samples is
extremely long damping times so

just you know you know bulk it's about

the people in clean samples in the bulk
is about one point one nano second for

perma like this is extremely long but
it's not surprising it is so

long this is a two nanometer particle
the supergroup the cubits quantum dots and

silicon have got sation times of
six seconds five to six seconds so

that's why there are few bits and these
particles here have about two thousand

spins so it's about having
thousand of these cubits so.

This is not a reasonable in fact so
basically the limit of very low

damping and
we have some ideas why it is so.

It was a recently published
rapid communications.

OK So future.

We'll probably skip most of this but

by driving these magnets
around the state shown here.

You know I think this is
probably I would not get anywhere

if I tried to explain that the bottom line
is these states of the spin multiple that.

When they expressed in terms
of single electron state and

tens of thousands of them.

There.

Is a known as a big quantum optics.

So this.

Moment and
you people if you are familiar with the.

Moment which is in belts
they which is a spin single.

So this type experiment.

So now we don't have that we actually
have something much bigger OK so.

The properties of these magnets in
these type of states around the.

Strongest suppressed is that small
number of electrons are not the bangle

they do not violate ballin the qualities
However if you have a very large number of

electrons they become extremely
strong to be bad as a collective.

So that's what we are after so so

basically we want to basically the
experiments to have two of these particles

that I've been with micro phase and
then start the transfer of angular

momentum in that state from one to another
that is the project we are working.

So thank you.

So I have.

Overview of experiments and

theory in the lab we have performed single
shot measurements of the magnetic switch.

From.

The shuttle.

And we have found a very long damping
times in the particles by spin noise but.

And now we're working on this
thread to call development of time

in there no magnets for
future use of entanglement.

And this was done by students.

And collaborators Victor.

From Georgia Tech thank you.

Thank you very much.