Well good afternoon everyone.
You're in for a treat because we are about
to hear from a dynamic young physicist
from Harvard.
Her name is Diana Prado Lopez Aude Craik
and she is originally from Brazil.
She came to Boston to study at MIT then she
went to England to get her PhD
from Oxford University and then came back to Boston to
continue her research
as a post-doctoral fellow at Harvard.
And we are so glad that she's here to share
with us her extraordinary experiences
trapping individual atoms for quantum computing.
So please help me welcome to the stage
Dr. Diana Aude Craik.
Thanks very much.
Okay.
I'm going to tell you guys today about how
to trap single atoms for quantum computing.
Okay, so I'm going to start off by showing you a photograph
that was taken with a regular camera.
And that little purple blob that you see in
the middle there is a single atom.
Now you may be thinking, hang on a minute.
A single atom is tiny.
Can you really photograph it with just
a regular camera?
Well actually what you're seeing there is
not the atom per se.
It is the fluorescence, the light emitted
by the atom when I shine a laser on it.
So when I shine a laser on this atom it sort
of glows and this glow can actually be captured
by a camera.
And, um, the first time I saw the glow from
a single atom like that
just suspended in a point in space,
I was really, really amazed
because everything around us is atoms, right.
I mean ev- the air is full of nitrogen atoms,
oxygen atoms.
We are made of atoms.
I mean can you really, how do you isolate
a single one, stop it like that,
and then just look at it emitting light?
So I'm going to tell you about the recipe
for how to do this, which I learned in grad school.
And first of all, let's start with the ingredients
that you need to trap an atom.
Okay, so first of all what you need is a bunch of atoms.
So, we have to first choose which type of
atom we'd like to trap.
I'm going to choose calcium atoms.
And this is a, a bunch of little blobs of
calcium atoms in a test tube that has had
all of the oxygen removed from it, which is
why it looks a little bit silver and shiny.
And you might expect calcium to look white because there's a lot of calcium in milk
for instance and your milk is white.
But that white only comes when you take it
out into the air and it can react with oxygen.
So that's why it looks shiny over there without oxygen.
Okay, so we've got the atom that we would like to trap.
We also need some kind of device that traps the atom.
And this type of device is called an ion trap.
We'll learn about why it's an ion trap
and not an atom trap a little bit later on.
But this device here that I'm showing
you, if, if we just zoom out a little bit
it looks a little bit like a computer chip, right.
And it's gold.
The electrodes in the middle there are gold.
And this one actually happens to be the one
that I made during my PhD.
Okay.
So we've got atoms, we've got the device that's
going to trap the atoms.
But how am I going to trap a single atom of
calcium with this device in air?
That's not going to work, right because if
I try to put this thing in air
what's going to happen is all of the atoms around are going to bump into my calcium atom, you know,
push it around and knock it out of the trap.
So what I have to do is take this ion trap and put it in what's called an ultrahigh vacuum system.
And this is just a metal can, okay, and it's
got a window, it's a circular window there,
which is why you can see the ion trap in there.
But basically it's a closed metal can and
it's connected to some very powerful pumps
that just suck out all of the air from inside there.
And to give you an idea of how little air,
how little gas there is in there, the pressure
in there is basically the same as the pressure on the moon.
Okay.
So we put our trap in vacuum and we take those
calcium atoms that we looked at the in beginning
and we actually put them in a little metallic
tube, which if you squint you can see over there.
And that little metallic tube, we call the
calcium oven.
Why is it an oven?
Doesn't really loo k like an oven, right?
Well it's an oven because we actually run
an electrical current through it
that we apply from outside the vacuum system.
And that electrical current heats up the calcium
that's inside, just like an oven would.
And when the calcium heats up it evaporates
into a gas and it gets sprayed over my ion trap.
Okay.
There's one more ingredient we need to trap
these atoms and that's lasers.
And that's all we need.
So I'm going to tell you how, using these
ingredients, we're going to trap a single calcium atom.
And to figure that out, we first need to look
at what an atom looks like inside.
And for the calcium atom we actually have
20 positively charged protons in the nucleus
and an equal number, 20, negative
electrons swishing around the nucleus.
Okay.
Now the total charge of this atom then is
plus 20 from the protons
in the nucleus minus 20 from the electrons, so it's zero.
It's what we call a neutral atom.
There's no charge, right.
So let's just for fun draw all of those 20
electrons on here.
And now I want you to focus just on the very
outer ones, the ones that are furthest away
from the nucleus.
So I'm going to hide all of the inner ones.
They're still there, I'm just not drawing
them.
And now I'm going to tell you that if I shine
a purple or blue laser onto this calcium atom,
I can actually pick out one of these outer electrons, get rid of it.
now the atom has one fewer electron.
So it actually has a positive charge.
And when an atom is charged we call it an ion.
Okay.
So this is a calcium ion, right.
So okay, so how does that help us?
Well it's very important that the atom is
charged because if you have a charged thing,
you can actually use other charges to push it around, right.
And if we can push the atom around with other charges, we can maybe confine
it to one point in space.
So let's see how that works.
First let's remember that if I have two opposite
charges, a positive one and a negative one,
what happens?
They actually attract each other, right?
And if I have charges that are the same, what
happens is the opposite.
They don't like to be next to each other.
They repel, right?
So using these forces of re- of attraction
and repulsion I'm going to trap this ion.
Now how might we do this?
Well if we're going to apply all of these
charges around the, the ion, first of all
we remember that it's positive.
We better have something to apply the charges
to, right.
And what that something is, is just a bunch
of metallic electrodes that we can apply voltages to
and charge up.
okay let's put two electrodes on, two
pointy electrodes on here.
And these electrodes, these pointy ones, we
nowadays call the end cap electrodes.
And okay let's, let's put some more electrodes on.
Let's put two big ones here and another two
the other way around.
And basically now we've surrounded the ion
with electrodes and these big flat ones we
call the blade electrodes because they kind
of look like part of the blade of a knife.
And you might recognize this if I look at
this electrode geometry sideways on,
it's actually that picture that I showed you right
at the beginning of the talk.
So the pointy electrodes are there, the end
cap electrodes there
above and below the ion you see the blade electrodes, two of the blade electrodes
and the ion in the middle.
Okay.
So if I zoom in here first, my
ion is positive, right?
So I want to keep it in the middle.
So one thing I can think of doing is I can
make those two end cap electrodes positive too.
So it won't want to go towards one, it won't
want to go towards the other.
It will just stay in the middle, right?
Well yeah but it can still kind of escape
up and down through those blade electrodes,
right.
So we have to do something with the blade
electrodes too.
So one thing we could think of is we already
made these end cap ones positive.
So maybe I can make this one positive, make
this one positive, I could make this one positive,
I could make this one positive and now it's all positive.
It doesn't want to go near any of them.
It will just stay in the middle right?
Well unfortunately that doesn't work.
That configuration of charges is unstable
and if you do that, the ion was always going
to find some way of wriggling out and escaping the trap.
So you actually have to do something a little bit fancier.
And here's how it goes.
What you do is the end caps are still positive.
I make two of my blade electrodes positive.
But I make the other two negative and now
you think, okay hang on a minute.
But negative attracts the, the positive ion, right?
Yeah and if the ion isn't right in the middle
of the trap it's probably going to get attracted
to the nearest negative blade, right?
Yeah.
And if it gets attracted to that, it's just
going to hit that and that's no good.
So what we do is we don't keep it like that.
We flip the charges around so that the blades
that were positive not become negative and
now they push the ion away, okay.
And now I flip it again and now the ion gets
pushed to the other side again
and it's just moving back and forth like that, right.
And now I want you to do a little bit
of a jump
and think about the ion now as a marble.
And think about the positive electrodes as
hills because the marble doesn't want to go
up the hill, okay.
And think about the negative electrodes as valleys.
The marble wants to go towards the negative
electrodes, down the valley, okay.
And if you think about it like that, you can
actually visualize exactly what this configuration
of charges looks like to the ion.
And it looks like this.
It looks like a flipping saddle, okay.
The positive electrodes are the hills, the
negative electrodes are the valleys
and you flip the charges, so it goes up and down.
And if you flip this saddle quickly enough,
then the ion can't escape.
So that's the basic principle of operation of an ion trap.
Okay.
Great.
So let's just zoom out from that picture of
the single ion and this is a, a, a regular
ion trap that has that geometry that we've
been talking about that whole time, okay.
And the size of this, if you're wondering,
is about a ping pong ball.
But what about that little gold chip that
I showed you at the start and I told you that
was an ion trap?
That, that looks nothing like that, right?
Yeah so that's actually quite exciting because
about a decade ago scientists figure out that
you actually don't have to surround the ion with electrodes.
What you can do is you can take those electrodes,
you can project them onto a plane,
you can put them all on a flat surface like the surface of an, a chip, okay.
And actually if you apply a similar configuration
of charges to those flat electrodes,
just above the surface of that chip you can still
trap the ion.
And when I say just above, it's about, you
know, the diameter of a hair above the chip,
100 microns above the chip.
And that's really exciting because these chips
are much smaller, first of all, about the
size of a dime.
And you can fabricate them very easily just
like you make computer chips.
You can basically stamp them out, print them
out, make loads of them and make many electrodes
on a single chip so that instead of just making
one ion trap, you can make many on a little
piece of gold like that, okay.
So let me just show you if you zoomed into
some ions trapped in the center region
above this chip, you would see something like this, a string of four ions trapped there.
All right.
So now we've trapped an atom.
That's very cool and all right, but what can
we do with it?
Can actually use these single atoms to build
the basic building blocks of a quantum computer.
But first before I explain to you how we can
build a quantum computer with atoms,
we first need to understand how we build a regular
computer.
Now okay, a regular computer we're used to
talking about how computers store information
in zeros and ones.
And what that means is just that the computer
can represent any information you would like
to represent as a string of zeros and ones.
So for instance, if I want to write down a number
I can represent it as a string of zeros and ones.
One to, one through four is up there on the board.
I can also represent a letter as a string
of zeros and ones using a code that translates
the letter to a string of eight of these
zeros and ones called ASCII.
I can write the letter A like that and, you
know, at the start of the talk you might have
noticed that my name is quite long.
People often tell me this.
But in binary. it's even longer.
It's that long.
And one of these zeros and ones is actually
what we call a binary digit, which if I abbreviate
might remind you of something you have heard
of a lot, a bit.
And this is one unit of information for a
normal computer.
Okay, so these bits, these zero and ones,
how, what are they?
What are they physically right?
Well they're actually switches and the way
we implement these switches
in our normal computers is we use what's called a transistor.
A transistor is just a switch.
It looks a bit like that weird thing over
there that has three wires coming out.
And that one over there is off and so we say
when it's off it's representing a zero.
To turn it on, all you have to do is 
apply a little bit of electrical charge,
to that middle wire over there to turn the switch on.
And when it's on we say it's representing a one.
Okay. It turns out that if you know how to
store information in bits, if you know how
to flip the bit between zero and one which
we just talked about, it's just changing the
switch from on to off, you only need one more
ingredient really to be able to do any type
of calculation you would like on your computer
and that ingredient is a logic gate.
What's a logic gate?
Well a logic gate is basically a little machine.
It takes in two bits, the state of two bits.
And I'll label these two as input
A and input B. And it outputs another bit.
Now, okay.
Okay.
Now these gates in normal computing are actually
very aptly named.
Their named, uh, names are very informative.
So I'm going to show you an example gate which
is the And gate.
And the And gate, the way it works is very
simple.
The output is only one if both input A and input B are one.
If they're anything else, the output is zero.
Okay.
And just to, uh, go through how it works,
okay, say input A and input B are zero.
I get zero out.
If one of them is one, the other one of them
is zero, I get zero out.
But if both of them are one, I get one out.
Okay.
So basically it's just a little machine that
depending on what the input states of the
two bits are, I get a different output state.
And just with the logic gate and with the ability to flip
the bit,
you can do any calculation you might want to do on your computer.
It's very simple.
Right, so we know how to make a normal computer.
What's the big difference between that and
a quantum computer?
Well this bit that we've been talking about
in the regular computer can only ever be in
one of two states.
It can either be zero or it can be one.
But a quantum bit has this funky property
of being able to kind of be zero and one at the same time.
It's actually what's called quantum superposition.
The state of the quantum bit can be in the
superposition of zero and one at the same time.
Okay, but why is that useful?
Well to see why that's useful let's think
about having eight bits, eight normal bits.
We usually call that a bite, okay.
Now how many states can this bite store at
a given time?
Well, there are two to the power of eight
possible configurations of this bite.
But at any given time it can only be in one state.
Okay?
What's the state that it's in right now?
Well it's storing one, one, zero, one, zero,
one, zero, zero.
That's what it's in.
Okay, what about if I have, if I have more
bits?
Well say I have 80 bits.
Well that's got two to the 80 possible configurations
but again, at any given time
the number of states it's storing is one.
Now let's make these 80 bits into quantum bits.
Well if I have 80 quantum bits, then at any
given time I can actually have them all in
a superposition of all possible two to the 80 states.
And how big is two to the 80?
Well it's that big, but that doesn't tell
you much, okay.
To give you an idea of how big it is, it is bigger than the number of atoms in the observable universe.
So if we think about this for a second,
if I take every single atom in the observable universe and I make that into a regular computer bit,
I couldn’t store as much information with those bits as I can store with 80 quantum bits.
Now that sounds pretty good.
But after all of this I still haven't told you how I can make quantum bits with the trapped  atoms, right.
So let's get to that now.
Okay.
So to see how I can make a quantum bit with the atom
I need to take you back to what that atom looks like inside.
So remember that we had kicked out one of
the outer electrons with the laser and so
we left that poor, lonely one out there.
The single outer electron there.
So if we, we just think about that extra electron for a second,
it's got an extra property that I haven't told you
about yet called spin.
Now one way you can think about spin is you
can think about the electron as being also
like a little bar magnet, okay.
And this bar magnet can point up, north/south.
And when it does, I'm going to say that the
ion is storing a one.
Or it can point down and when it's pointing
down I'm going to say the ion is storing a zero.
And to go between zero and one, what I can
do is actually apply some microwaves,
just blast my ion with a little bit of microwave
radiation, and that microwave radiation actually
rotates that spin, that little magnet that
the electron is associated with.
And the length of time that I leave my microwaves
on for is what determines how far that magnet  rotates.
So I can d- can leave that, uh, those microwaves
on to rotate from zero to one, so flip completely
the magnet, or I can actually go anywhere
in between.
And if I stop somewhere in between, that's
a superposition.
Okay.
Right.
So now just like in a regular computer, we
have a bit, actually a qubit now, and we know
how to flip it between zero and one.
So there's one more ingredient, if you'll
remember, that we need to do computation.
It's just like in a regular computer.
We need the bit, we need to flip it and we
need to do gates.
All right.
So how do we do gates?
To do gates, we're going to use that same
little magnet property of the electron but
I have to tell you a little bit more about
how that little magnet actually effects how
the ion moves in space.
So it turns out that if I have my ion and
it's in the state zero and the magnet is pointing
down, and I apply some magnetic field around
my ion and this magnetic field looks basically
like a hill.
It goes from low magnetic field over there
on that side of the board to high magnetic
field on this side here.
Okay.
It turns out that if my ion is in this state
where the bar magnet's pointing down, it wants
to go to the area of low magnetic field.
It likes to go down the hill.
Okay.
However, if I flip the magnet,
the electron's magnet, the spin
it wants to go the opposite way.
It actually wants to go up the hill.
You can kind of think of this as if you flip
that electron spin, it's kind of like you
flipped the world for it and it wants to go
up instead of down.
Okay.
Right.
So how's, how's this useful?
Well let's say that now I, instead of making
a hill with magnetic field,
I make a bowl of magnetic field.
So I have low field in the middle and I have
high field on the edges,
and I put two ions in that field.
Okay.
Now they are both pointing down.
south is on the top.
So what do they want to do?
They want to go to the point of low field.
They both want to go to the point of low field.
So what's going to happen in this scenario
is they're going to move that way.
Okay.
Now what happens if I flip the bowl?
Well now low field is that way so now they
want to go the opposite way.
Okay So what about if I flip it again and
I just keep flipping it?
Well now I'll excite a motion that's like
this, okay.
And if you want to see what this actually
looks like with four ions in a trap,
it looks like that.
Okay.
Now I have to give you a caveat here that
this isn't exactly the motion I'm describing.
It's a very technical caveat but I have to
give it to you anyway.
But it looks very similar to that.
Okay.
Right.
So based on what the qubit state of my ions
was, they moved a certain way in the trap.
I just want to you to keep that in mind, okay.
Now let me show you what would happen if I
flip the state of one of my qubits.
Well, the one that's flipped now likes to
go to high field instead of low field, right?
So actually that one is going to go that way,
and that one with south on the top is going
to go that way too.
to the point of low field.
So they're now both going to move like this.
And when I flip the field they're going to
go that way.
And so the type of motion I'm going to excite
instead of this one,
which is what we saw before, is this one.
And let's look at what that looks like with
ions, it looks like that, okay.
So this is actually very powerful because
depending on what the qubit state of my ions was
I have created a different motion in the trap.
Okay, so we're going to use that to do gates
but I have one more piece of information that
I need to give you to complete the cycle and
explain to you how we're going to do this gate.
And to do that we actually need to go back
to classical physics a little bit
and we're now going to think of the ions as just masses on strings just masses on springs.
Okay.
And I'm going to tell you that this system
is very analogous
to the two ions in that bowl, okay.
And you can think of these two masses as the
two ions.
Now, I'm going to show you some videos of
somebody exciting both this mode
and this mode on that spring.
So the top one he's exciting this mode,
the bottom one he's exciting this mode, okay.
And there are two things, if I slow the videos
down, that I want you to note about these videos.
The first one is that to get the ions moving,
well the masses  in this case moving,
all the person had to do was push it once and release.
And then there was some kind of natural oscillation
going on, okay.
The second thing is that that oscillation
is interesting first of all because it happens by itself.
Once you push it once, it just happens, okay.
And it happens at a sort of natural rate,
at a sort of natural speed, you know.
And this is just like if you go to a park
and you, um, go onto a swing
and you ask your friend to push you.
If they push you, that string, that swing
also has a natural frequency.
And if they push you at the wrong rate, they
don't push you like alongside at the
same rate as this, the swing wants to move, you're
actually not going to move very well
and the swing is eventually going to stop.
But if you move in phase with the way the
swing naturally wants to go, you can get that
swing to move really far, right.
So it's the same thing here.
it's really, really hard to see this in these
videos but it turns out that the natural speed
at which this one happens is faster than the
one at which this one happens.
So this one goes a little bit faster than
this one.
Okay.
Because there is a difference in rate at which
this happens and at which this happens,
by choosing how fast I flip that field that makes
the ions move,
I can choose which one of these motions I excite.
Make sense?
I just make the, the rate at which I flip
up to down equal to the rate that, for instance,
the ions like to naturally move like this.
Okay.
And that's exactly what I'm going to do to
do the gate it's called a motional gate,
funnily enough, because it's all about motion.
And if I put in two qubits, one pointing up and the other one pointing down
you will remember that in that configuration
they wanted to go this way, okay.
But I'm not going to flip the field at the
rate that is correct to drive this one.
I'm going to flip it at the rate that is correct
to drive this one.
So what happens if my states, my input states
are one and zero or zero and one is nothing.
I will see exactly the same output with no
motion.
However, if I then flip one of the bits and
they are now both either zero, zero or one, one
you will remember that if they were the
same they move like this, right.
And I am now changing my magnetic field at
the that this likes to move.
So now what I see is motion.
And this I can tell you is a gate.
Why?
Because depending on what the input state
of my qubits was,
I get a different thing happening, a different output state.
So this is the final ingredient you needed
to be able to do any operation you would like
to do in a quantum computer.
You have your gate, you have the ability to
flip your qubit and that's it.
Okay.
Now, just to finish I wanted to tell you that
ions, trapped atoms, are actually a very exciting
candidate for making quantum computers that
are very large scale, because they can actually
stay in those quantum superpositions for a
very, very long time comparatively.
The second thing that's cool about them is
that you can do these type of operation,
this type of gate with extremely high precision,
which means that you can do
many of these gates without making many mistakes.
And this is essential if you're going to do
very complicated, long computations because
if you're going to get something that makes
sense out of your calculation,
you need to not make mistakes very often, right.
And the last thing I want to tell you is that
quantum computing is really exciting.
Not just because of what you might have heard
like, you know, you might have heard that
there are certain problems, very specific
problems, that quantum computers can solve
much faster than a classical computer.
Like for instance factoring very large prime numbers.
But for me, what really excited me about quantum
computers when I first saw this field,
first learned about it, was that nature in its most
basic building blocks is quantum.
So if you want to exactly simulate any process
in nature, you need a quantum simulator, right.
For instance, if you would like to design
some new medicines you need to understand
how a protein folds and this is very complicated.
But yet can be done with quantum simulation.
You can even simulate things that you could
never imagine would be possible
to do experiments in, in the lab.
Like for instance, a black hole.
You can simulate that with a quantum simulator.
So, that's why I find this field extremely exciting
and I want to thank some collaborators here.
And to thank you very much for coming and
listening to the talk.
[Music; applause]
