[music playing]
- Welcome to the
NASA Ames 2016 Summer Series.
Two questions:
How does the microscopic world
inform the macroscopic world
and vice versa?
What could we learn
and how do they interact?
Can we use quantum mechanics
to advance
real-world applications?
Today's seminar entitled
"Quantum Mechanics at
Macroscopic Scales"
will be given by
Dr. Mark Kasevich.
He is a Professor of Physics
and Applied Physics
at Stanford University.
He received
his Bachelor of Arts
in Physics from Dartmouth,
followed by a Master's
in Physics and Philosophy
from Oxford University
as a Rhodes Scholar
and then a PhD in
Applied Physics from Stanford.
His current research is centered
on the development
of quantum sensors of rotation
and acceleration
based on cold atoms.
He has published
numerous publications
in really relevant journals,
including "Nature" and so on.
And in his spare time,
cofounded AOSense, Incorporated,
a developer and manufacturer
of innovative atom optic sensors
for precision navigation,
time and frequency standards,
and gravity measurements,
and currently serves
as the company's consulting
chief scientist.
Please join me in welcoming
Dr. Mark Kasevich.
[applause]
- Thank you.
Thanks for the introduction.
It's great to be here
this morning.
So I'm gonna tell you about
experimental work
coming out of my laboratory
at Stanford just up the road,
where we're trying to
push the boundaries
of quantum mechanics,
to understand to what scales
we can accept that quantum
mechanics will be true.
And when I say scales,
I'll kind of orient you to the--
to my interest in that
as--as the talk goes on,
but I think it's no secret
that quantum mechanics...
probably one of,
arguably, the most successful
physical theory we have
that had its birth
in the early 20th century,
has left us puzzled.
And many of us who practice it
have both--
by experiment
and pencil and paper work,
find points where it just
doesn't seem to make sense.
And oftentimes that boundary
becomes most puzzling
when you ask, does quantum
mechanics manifest
over the distance scales and
time scales of everyday life?
Meters and seconds
and large numbers of particles.
And so I'm gonna tell you about
experiments today
where we're trying to find
quantum systems
that we can explore
on meter scales
and with large numbers
of particles
and so forth.
Our dream would be to explore,
you know,
quantum mechanics of humans
being in two places at once,
but we're nowhere near that.
But what we have managed to do
is to manipulate atoms
in such a way
that they can exhibit
surprising quantum mechanical
attributes.
So let me introduce you to
the field that I'm in.
I'm an atomic, molecular,
and optical physicist,
and back in the early '90s,
there was some incredible work
that came out of the Konstanz
atomic physics group.
And what they did, Jurgen Mlynek
and his colleagues did,
is they did something
analogous to the Young's
double slit experiment
but they used helium atoms.
And the way
this experiment worked
is they--they took
a beam of helium,
which is at this part
of the diagram,
and they--
they took these atoms
and they fired them
at a narrow slit.
Now, if you're thinking
classically,
the atoms just
go through the slit
and the slit kind of defines
an atomic beam.
If you're thinking
quantum mechanically
and you're a believer in
the de Broglie relationship,
and namely that that atom could
really be considered a wave
whose wavelength
is Planck's constant
divided by its momentum,
then you expect
wavelike behavior.
And by design, Mlynek's group
made this slit narrow enough
so that the helium atom waves
diffracted into this
coherent set of wave fronts
much like you would see
a beam of light diffract
as it propagates through a slit
in a laboratory experiment
and such that it
coherently illuminated
two more slits.
This location and that location.
Well, those two slits
now become sources for
diffracted waves,
and those waves overlap
at a detection screen here.
Now, if they were
water waves or light waves,
you would have
no problem accepting
that those waves would
interfere constructively
and destructively
giving you maxima and minima
in intensity.
But these are atom waves
and how do the atom waves
interfere?
How is that interference
manifested?
Well, it's manifested by
the constructive
and destructive interference
of the atomic wave functions.
And according to the rules
of quantum mechanics,
you add the wave functions
before you square
and then when you square you get
those interference patterns
that you calculate
mathematically.
But what it really means,
again following
the quantum paradigm,
is that those ripples
in the wave function
correspond to ripples
in the probability
of detecting the atom at a given
position along the screen.
And so as they scanned
their detector
along this screen,
they saw the following.
This is
the detector position,
and this is the number of atoms
that they detected
in five minutes.
They saw this beautiful
oscillating pattern,
which is just
the kind of picture
that you associate with the
Young's double slit experiment.
And when they go back and use
the Schrodinger equation
and so forth and calculate the
periodicity of the oscillation,
what you would expect
from the momentum of the atom
and Planck's constant
and the mass of the atom,
turns out it lined up
nearly perfectly
with this data,
demonstrating that
we had reached a position
or they had reached a position
where they could
coherently control
the--the center
of mass wave function
of a beam of helium atoms.
Now, this wasn't a surprise
to anybody.
I mean, if you believed
what was written down
in the early 20th century,
of course,
atoms, under certain
circumstances, will interfere.
That's the law
of quantum mechanics.
Electrons interfere to give us
the hydrogen spectrum
and, you know, Davisson-Germer
showed us diffraction
from surfaces.
So what's the big deal here?
Well, the big deal was there
were some notable physicists
in the, oh,
let's say mid-20th century
who said, that's great,
you know,
quantum mechanics works just
fine for those electrons,
for magnetic spin,
and everything we know about
atomic structure,
but ain't nobody
ever gonna build
an interferometer for atoms.
Why?
Because, well, every time you
build an interferometer,
one of your key
engineering challenges
is to stabilize
the relative paths
of the two interfering beams.
In other words,
if I go back to this slide,
I have to make sure that
the phase acquired by the wave
that goes through
this slit
and hits
the screen there
is stable with respect
to the phase
that goes through
that slit.
And you could do some simple
calculations for atoms
and convince yourself that
there's, you know,
maybe you could build the slits,
but there's no way
you'd get stable phase.
There'd always be some
environmental perturbation
which would shake one wave
with respect to the next.
So that pattern
wouldn't be stable,
those fringes would shake
back and forth,
and as you waited to see
the fringe emerge,
you would never see anything,
and that would look like
just classical behavior.
And so what happened was
as technology since--
in the kind of mid-20th century
to the early '90s,
technology progressed
and we learned,
the field learned,
how to control
the environmental disturbances
on, in this case,
the beam of helium atoms
so that you could have
a stable phase.
And this was--this was
the paradigmatic experiment
that kind of started our field
in the early '90s.
So you could ask, well,
technology has progressed
yet again,
so where are we?
And I could take you through
the entire 25 years,
but I'll take you to a snapshot
of our most recent data.
Where are we today?
How--how--how well
can we control
the trajectories of atoms,
and can we see interference?
And if so, over what types of
distance scales
and time scales?
This distance scale here
is measured in
millionths of a meter,
ten microns.
So here's some data
that came out of my lab
last year.
And what I'm showing you is
two ensembles of rubidium atoms.
I'll tell you how we make
those atoms in--in a moment.
And these rubidium atoms--
this--this peak here has about
10 to the 5 atoms,
and this peak here has about
10 to the 5 atoms.
and they're separated
by a distance
of more than a half a meter.
Basically from here to there,
that's--
those atoms are separated.
So it's not a big deal,
but what is a big deal
is that I claim
each and every atom
that is a member
of this peak
also has a partner
in that peak.
It's actually the same atom
that's been coherently divided
and put in both places
at once.
Now, according to
laws of quantum mechanics,
when I--when I look at a state
like this,
I kill it,
and I collapse the state so that
the atom is either
in the left peak
or the right peak,
which is what happened
when I took this picture.
But before I took that picture,
I claim that I had
this amazing state
where each and every one
of those
10 to the 5 atoms was--
the only way I could
describe physically,
you know, what it was,
was--was an atom that was
in this place and this place
at the same time.
The logical progression
would be putting molecules
in two places at once
or human beings separated
by a meter scale
and so on.
But an atom is,
I want to remind you, is
a pretty complicated object
especially rubidium atom
that has, you know,
a grab bag of nucleons
and a bunch of electrons
all around it.
It's not--it's not
a fundamental particle,
it's a--it's a big collection
of fundamental particles.
So obviously, my job now
is to convince you
that I've had atoms
in two places at once.
This picture alone cannot
convince you of that.
I might as well just put
10 to the 5 on the left
and 10 to the 5 on the right.
How do I convince you that
they're at two places at once?
I have to do
an interference experiment.
I have to bring those clouds
back together,
and I have to overlap them
and I have to see them
interfere as waves.
And so that's the data I'll show
you in the next few slides.
So this is a picture
of the apparatus,
and let me just walk you through
the apparatus
before I show you
some of the data.
So we like to joke in my group
that everything looks better
in CAD.
So here's the CAD rendering
of the apparatus,
which is in the pit
in the basement
of the Varian physics building
at Stanford,
and it's about
a ten-meter-deep pit.
And what you see here is
magnetic shielding
that surrounds
an ultrahigh vacuum system
where we've basically
pumped everything out
in creating an environment that,
if I have a rubidium atom
of the appropriately
conditioned state,
it won't interact with
or be collided with
other gas elements
in the vacuum system.
And what we do is the following.
We--at this location here,
which is at the bottom
of that pit,
we used some modern
atomic physics techniques
to create a cloud
of ultracold,
and I'll explain what
ultracold means in a second,
ultracold rubidium atoms,
about a million of 'em.
What we do is we take rubidium
from a thermal atomic source,
a chunk of metal.
It gets heated up.
You get a stream of atoms
that are moving about
200 meters a second.
You capture them
with laser light.
The laser light is configured
in such a way
it cools them down
to temperatures
of--of millikelvin
to microkelvin,
and what we mean by that is that
if I look at the kinetic energy
or the velocity
of one of those atoms,
I measure it in units
of centimeters per second.
The way--this was something,
a technique
that was discovered
by Steven Chu and others
in the--in the '80s and '90s
for which they received
a Nobel Prize.
After we cool the atoms
that way,
we further cool them in a--
we--we--they're cold enough now
that I can let them sit
in a magnetic field
and that magnetic field
serves as a trap,
and in that trap we can do
a further so-called
evaporative cooling step
and bring them down
to a state of matter
known as
a Bose-Einstein condensate.
This was a technique developed
by Carl Wieman
and Eric Cornell
and Wolfgang Ketterle
in the late '90s, early 2000s,
which also received
a Nobel Prize.
And the miracle of this is
after you've done this
evaporative cooling,
which works just the same way
as when you put your coffee cup
on the counter
and the hot stuff jumps out,
leaving your coffee colder,
here we put the atoms in a trap
and the hot atoms jump out,
leaving colder atoms behind.
We end up with
ensembles of atoms
that have temperatures measured
in nanokelvin,
and we further manipulate them
so their temperature
is just picokelvins,
at which point their velocity
spread is measured in,
you know, units of hundreds
of microns per second.
That is, I have an atom,
I prepare it this way,
I let go of it,
its velocity is defined
to within 100 microns
per second.
Slower than an ant will crawl,
which is kind of amazing
when you consider that
the atoms in this room
are whipping around
at a kilometer per second.
Now, at these low velocities,
per the de Broglie hypothesis
or law,
you expect wavelike behavior,
because the velocity is low,
the wavelength,
the corresponding wavelength
inversely proportional velocity
is long.
And that's the--
getting them cold is key
to observing
these interference effects.
So we've cooled them down here.
That's at the bottom of the pit.
And then we have a process
whereby we launch them
up this ten-meter tower
and they fall back down.
And so that process is--
involves interactions
with beams of light,
and we basically
turn on some lasers
in the appropriate way,
and we give the atoms
a velocity kick
so that they're moving at about
ten meters per second
with respect to the lab frame
and then we let go of them.
They still have that ultracold
temperature now,
they're just
moving vertically,
and they fly up this tube
and fly back down that tube.
It's like if you had
a fistful of sand
and you chucked it up.
The sand would go up
and would come back down
and--but and the sand has got
such a low velocity
that it just kind of
doesn't spread out.
These atoms, the size of the
clouds when the come back down
to our detection region
where we have the cameras
to take pictures of them is,
like, hundreds of microns.
Let's see, I've got
one more thing to explain
and that is I'm gonna make
an interferometer for atoms,
so how do I make it?
Well, that's also done with
light and mirrors.
And so we have a beam of--
a bunch of laser beams
that start at this location.
We--we shine them down,
they hit the floor,
they bounce back up,
and due to the way we configure
the interaction,
and I'll explain this
on a subsequent slide,
we precisely manipulate
the atomic wave packets.
We coherently divide them
in two,
we add momentum to them,
we take momentum away,
and we split them up
and recombine them
all with pulses of light.
And this is the mirror here that
that laser beam reflects off of.
That's the only really special
part of this apparatus.
It has to be stable,
pointing in the same direction
with respect to the stars.
Inertially it turns out in order
to see the interference fringes.
And so what we actively do--
we actually do
is we actively tip
the angle of that mirror
so that the beam kind of
when it reflects off of it
always points inertially,
even though the Earth is--
if the Earth is rotating, a
laser beam would rotate with it,
We have to make sure
the laser beam,
as the Earth rotates,
stays pointed straight,
and so there's
a piezo stage there
that makes sure that happens.
how do--this is a little more
experimental detail
on how we coherently divide,
redirect, and recombine atoms
and make an interferometer,
much as you would use
beam splitters and mirrors
and beam splitters to make
an interferometer for light.
And so what we do is,
as I said,
we shine on sequences of pulses
to the atoms.
Here's an example of one of
our pulse sequences.
Some of the data, I'll show you,
we use sequences
of hundreds of pulses.
And each time the pulses
irradiates the atom
in the middle
of the sequence here,
what it does is
it takes one of the--
the--the wave packets,
and I should back up a moment.
This chart here plots
the horizontal axis' time,
the vertical axis'
position.
And what I'm plotting
for you
is the--the center
of the wave packet,
the probability
distribution,
which tells me
where I find the atom.
And the wave packet, of course,
has some spatial distribution.
It's about a millimeter wide--
I'm sorry,
hundreds of microns wide,
but the center
of that wave packet
has a well-defined position
in time--at a given time.
And that's what I'm--I'm--
I'm plotting.
And because I'm building
an interferometer,
a single atom can--is associated
with two wave packets
as it's going through
our apparatus.
So here comes the--the--
the cloud of atoms,
and let me just think of one
of the atoms in that cloud.
At time T equals zero,
it hits this pulse we know
as a pi over 2 pulse,
and what that pulse does is
it exchanges momentum
between the photons
in the laser beam
and the atoms in this
Bose-Einstein condensed
flying cloud in such a way
that it puts the atom in
a coherent super position
of its center of mass state
so that the atom has
two wave packets,
one which has momentum.
We call this
two-photon recoils,
because it absorbed and
stimulated--emitted a photon,
and what that amounts to
is about a centimeter per second
velocity kick.
And the other part
of the wave packet,
which was left behind
in this interaction,
which doesn't see
that velocity kick.
Well, the atomic physics details
are--
you know, I don't want
to go into them here,
but it's just
standard application
of the Schrodinger equation,
which describes the interaction
of a beam of light
with a two-level atom.
And so that's--
that's our beam splitter,
because this part
of the atom there
that has a different velocity,
if I wait long enough,
it drifts apart from the part
that doesn't have
that velocity kick.
Now, we don't have--
we only have so much time
in which we can look at
the atoms,
so we want to give 'em a little
bit more drift velocity
that just two photon recoils,
and so that's where all these
other pulses come in.
Every time
I hit the atoms with--
wave packets
with one of those pulses,
I add,
in the appropriate way,
two photon recoils worth
of momentum.
And so if I'm plotting the
position as a function of time,
and here I've taken off--
I've subtracted out the overall
sag due to gravity.
These atoms are flying up
and they're coming down
and I'm saying, forget about
the gravitational part of it.
Imagine you're flying with the--
the mean of velocity
as you--as you're going up
and coming down.
You just look at the relative
velocity of the wave packets.
These wave packets
drift apart
and then we--
we bang 'em again
so that they come back together,
and at this point, they overlap.
They exit at this point.
This is another
beam splitter pulse.
And we look to see interference
between those two output paths,
which is experimentally observed
by detecting where the atoms are
at this region of space by just
flashing on a beam of light
and taking a picture of 'em.
This is about ten meters.
This splitting here,
that's at 54 centimeters.
And now, what I want to
show you is
after they've been separated
by the 54 centimeters,
when they fall
back down here,
hit the final
beam splitter,
and then fall into
the detection region,
and when I take
a picture,
I actually see
interference fringes.
So this is our key data,
and what I'm showing you
is interference fringes,
which, when I say "fringes,"
what I'm talking about is
probability of finding atoms
in one location
or the other location
at my detection port,
and those two locations are
this location and that location.
And I get excited
when I see that the--the atom's
flopping between one location
and the next location.
The only way that's happening
is--
the only way that can happen,
I claim,
is if I consider the atoms
as waves
and if these waves have gone
through these two paths
separated by a half a meter.
When they come back together,
if I separate them
by just 1.2 centimeters,
I can find relative phases
between those waves
where the addition of the wave
function is constructive
and the atoms are there,
or destructive
and the atoms are there,
and we can--you know, every--
anything in between also
we're capable of detecting,
but I'm just showing you
these two extremes.
For fully constructive
interference,
all the atoms
show up there.
For fully destructive,
all the atoms show up there.
Now, it stands to reason, the
further I separate them apart,
the harder it is for me to get
'em to all come back together.
So when we separate them
by a half a meter,
we don't have what we call
perfect contrast.
We have somewhat
imperfect contrast where--
but we still see
oscillations
between the majority of the
atoms being at that position
and the majority of the atoms
being at that position.
And for us, this is
a smoking gun signature
that these atoms
had to be interfering
as quantum mechanical particles.
Now, you might say,
shoot, you know,
but quantum mechanics is true,
so what's the big deal
about a half meter
versus a centimeter?
And you know, also,
what have other people done?
So this data here
for massive particles
is--is the world record.
We're about 100 times more
spatially separated
than anybody else has--
has--has done
in any other lab in the world.
But you know, so those are
kind of nice bragging rights
that you use to maybe get
your data into "Nature"
or, you know,
some other journal.
But you know, you--
as scientists, we want to know,
is it--was it worth doing
an experiment like that?
And so that's--I kind of want
to tell you that story
a little bit
on the subsequent slides.
You might also be thinking,
well, you know,
I also know that light
is comprised
of a stream of photons,
and a photon is
a quantum mechanical object,
and when I put a photon
on the beam splitter
and I separate it
and recombine it,
that photon flies over distances
which are much larger
than the 54 centimeters.
And I would say,
if you're thinking that,
that is definitely, you know, a
legitimate and interesting thing
to--to think about, and it's--
it's very interesting,
in my view, to compare,
you know,
the quantum mechanics
of photons,
you know, massless relativistic
particles,
and those of massive particles,
like atoms,
and--and how quantum mechanics
treats them in,
you know, in terms of--
of what the predictions
ought to be.
So what do we learn
when we separate an atom
by a half meter?
Well, there's a lot of effort
all across the world right now
trying to figure out
how macroscopic
we can make a quantum system.
And by macroscopic, how far
apart can we separate it
and recombine it?
How massive can the particle be?
You know, I showed you an atom.
People are actually--in Vienna,
they're interfering molecules,
fairly substantial molecules,
and one of the experiments
they have on their docket
is to interfere a virus,
you know,
so you can make something
that's maybe alive,
separate it and recombine.
They're doing those big stuff
over much smaller
distance scales,
often measured in
hundredths of nanometers.
And also, over what time scale
they're separated.
There's all these different
experiments happening,
and you kind of want to have
some way of comparing them all,
you know, and--you know,
some notion of how macroscopic,
how big, the quantum state
you're making really is.
And so these theorists
in Vienna,
Nimmrichter and Hornberger,
in 2013
did a really interesting set
of theory work
where they created the framework
that allows us experimentalists
to kind of compare
the macroscopicity
of one kind of quantum
experiment with another.
It might be that it's--
it's more interesting
from the point of view
of fundamental test of theory
to interfere a molecule
over 100 nanometers
than an atom
over a half a meter.
And their theory, by the way,
only applies to
massive particles,
not to photons.
And so taken this way,
I don't really have time to go
into the axes on this plot.
My point of showing this is
just to kind of show you
people are
thinking about this.
And our work, the stuff
I just showed you,
is--constrains
quantum mechanics in a way
shown by this green line
and what it does for certain
parameter ranges on that plot,
which kind of tell you about the
separation of the wave packets
and so forth.
We're setting the--
kind of the--the--
the most stringent limits
on macroscopicity
by many orders of magnitudes.
So it turns out,
it does seem like--
it is interesting
to be just exploring the...
The interference patterns of
atoms at these distance scales.
Well, you might be asking,
well, shoot,
but what--what's the point?
How--you know, what would happen
if quantum mechanics was wrong?
You know, how--how might
quantum mechanics break down?
A lot of people
thinking about that.
There's a lot of
kind of crazy theories
that have been proposed
for a long time.
I want to tell you
about one of them
which resurfaced
back just a few months ago
and analyzed our data
in the context of this theory.
When you go--when we--
when we do physics,
when we do engineering, we make
fundamental assumptions
about space and time.
Homogeneous, isotropic,
time progresses uniformly.
but when you look at, you know,
a fine-grain scale,
Planck scale,
10 to the minus 34 meters,
you start to think, well, maybe
space really isn't homogeneous,
maybe it's grainy.
Maybe time isn't continuous.
Maybe it ticks in a funny way
on certain distance scales.
And maybe quantum mechanics
of massively separated objects
might be sensitive to
these kind of perturbations.
It turns out we have to have
extreme control
over the paths of the atoms to
get them to come back together
and interfere constructively.
And if the velocity of one of
those wave packets changes
by nanometers per second,
that's enough to kind of
wipe out the coherence.
And so we make this experiment
by the fact that
we see those fringes
make very good tests
of anything
that's gonna go in there
and shake around the atoms
as they're propagating.
And it may be very--the very
nature of space-time itself
that's doing this.
This Ellis model
imagines a gas of wormholes
that are flying through space,
and this was taken seriously
until the early '90s
when some people
shot this theory down.
And these wormholes,
according to this phenomenology,
collide with the atoms
and give small momentum kicks
and ruin
the interference pattern.
And so there's a bunch of
complicated math these guys did,
and basically, you know,
showed that the data
I just showed you
dramatically constrains
these sort of wormhole theories.
And to put these theories
in context,
there's been a lot of thought
about the quantum mechanics
of black holes and things
recently
and over the past 20 years,
and this was one of
the early types of forays
into quantum and gravity
by people like Lenny Susskind
when they were tying
to figure out,
and they still are trying
to figure out,
the quantum mechanics
of black holes.
I want to change gears
a little bit.
Oftentimes when you build
an interferometer,
you build it
to measure something,
not just to learn
something about,
you know, the structure
of space and time,
and that's kind of
the reason why
we're building these apparatus.
By looking at
the interference fringes,
and assuming now
the interference fringe
is something stable,
we can learn something
about the relative paths
that the two wave packets take.
So what do we learn?
Well, the theory for calculating
the relative phase shifts
between atom wave packets
as they propagate through space
and time is well-defined
and it's been articulated since
the beginning of this field,
and it just involves, again,
systematic application
of the Schrodinger equation.
And again, I'm not here to
explain these equations to you,
just to say that
those equations exist
and you can go in
and actually calculate
what the relative phase shifts
for the two paths are.
So you might ask me, well,
what drives the relative phase
of those two paths?
Well, anything that changes
the velocity of the two paths
is gonna lead to a phase shift
because of
the de Broglie relationship.
What changes the velocity?
Well, how about the acceleration
due to gravity?
That's something that, as those
atoms are flying through
the apparatus, their velocity
is changing with time.
Their--their--their phase,
the wavelength is changing
with time.
That turns out to lead to
a huge phase shift
at the output
of the interferometer.
If the interferometer's
rotating,
there's another phase shift
associated with that,
the Coriolis effect.
And there's a whole list
of phase shifts
that you can calculate
along these lines.
Bottom line is these--
these devices
are extremely sensitive
to inertial forces,
rotations and accelerations.
And that's why we--
we use them--
we build sensors with them.
I'll show you a sensor
in just a minute.
This is one of those sensors,
which is to say that--
and I want to maybe go to
this list first.
When I do that calculation,
you say, well, what do--what do
those phase shifts look like?
We--we--we call
a table like this
affectionately
the term list.
And what I'm--
what I'm describing here
is all the different ways
that I can get relative phase
at the output
of the interferometer.
And for the highly engineered
sensors we build,
these term lists
have hundreds of terms,
and I just put in
a simplified one here
to show you some of
the most dramatic shifts.
This "k" is the propagation
vector of the laser beam
that we used
to bat around the atoms.
"g" here is the acceleration
due to gravity,
and "T" is
the time between--
basically the time
of flight
between the first beam splitter
and the exit beam splitter.
And I'll just focus on
this top term
and point out
something amazing
that in that data
I just showed you,
as the atoms are flying through
the interferometer,
about 10 to the 10 radians
of phase evolve
between one path and the other
before the wave functions
overlap.
So if I measure the phase
of the wave function precisely,
I'm basically, and I can count
which fringe I'm on,
I basically have
an awesome accelerometer,
an awesome way
of measuring accelerations,
or if I'm on the Earth,
the gravitational field.
Let me just put
an order of magnitude in there,
10 to the 10 radians,
and typically we can,
on a single experiment,
resolve a phase shift to about
a 10 to the minus 3
to 10 to the minus 2 radians.
It says in a single shot
we are, in principle,
capable of revolve--
resolving accelerations
at the 10 to the minus 12
of little "g" levels.
10 to the minus 12
to 10 to the minus 13
in just a single realization.
How--and that's--that's--I mean,
okay, that's a small number.
How small is it?
Well, the gravitational
acceleration
of two of you guys sitting next
to each other
is 10 to the minus 9
of little "g."
So it means if somebody is
drinking a cup of coffee
next to you,
you know, their mass is changed
by enough
that the gravitational
interaction would be about
10 to the minus 11 of little "g"
or something like that.
That still--and that
would be detectible
with a sensor like this.
So it kind of makes you want
to build these sensors,
maybe not on that
grandiose scale.
And so this company, AOSense,
which is literally
just down the road,
is building smaller versions
of these sensors
for real-world
guidance navigation
and control applications,
and also geodetic applications,
studying of
the Earth's acceleration
due to gravity and so forth.
And in this can here
is a much smaller
atom interferometer
that's measuring the
acceleration due to gravity,
and this is some data
from that sensor.
This is time
in kiloseconds,
and one of these oscillations
is about--is about a day.
And these variations in gravity
are the well-known variations
in acceleration due to gravity
due to the motion
of the Earth and the Moon.
This is about 10 to the minus 7
of the 9.8 meters per second
squared of gravity.
This is to point out that,
yeah, these are
really sensitive sensors
and when you build them on
these grandiose scales,
they become extra sensitive.
So what are we gonna use
that extra sensitivity for?
And this is data that's just
started rolling in
in the past month.
Well, one of the--
I'm in a physics department
so what basic science
might we do with this?
People are interested
in answering at a precise level
the age-old question,
does the brick and the feather,
do they--do they fall
at the same rate?
The Leaning Tower of Pisa
experiment,
the Galileo experiment,
in modern times,
you--it's known as equivalence
principle measurement
and it's--the equivalence
principle is a foundation
for Einstein's theory
of general relativity.
And it's theoretically
interesting,
I'm told by
my theory colleagues,
to probe this principle
at the part in 10 to the 15th
to part in 10 to the 18 level.
Which is to say as the objects
are accelerating together
at basically the acceleration
due to gravity,
there may be some
spurious interaction
due to particles
we know nothing about
that make it so
a rubidium 85 atom
accelerates at a slightly
different rate
than a rubidium 87 atom
because they have different
nuclear composition.
And so that's the experiment
we're doing here.
We have two ensembles
of Bose-Einstein condensed
laser-cooled atoms
that have been launched
and subjected to
a bunch of pulses
that are flying up and down
this tube,
both opening up and closing
interferometers,
and then we measure
simultaneously
the phase shift
from rubidium 85 and 87.
By comparing the phase shifts,
we make a comparative
measurement
of their acceleration
due to gravity,
and then we seek to understand
whether the observed
acceleration due to gravity
is the same for one isotope
or the other.
Now, I don't have results to...
You know, formally discuss,
but I do have some
preliminary data
to show you kind of what this--
these action shots look like now
when we're making a simultaneous
measurement
of the co-acceleration
of--of 87 and 85.
This is a picture
in false color--
as my colleagues joke,
everything looks better
in false color.
So in false color,
rubidium 87 in the upper part
of the camera port,
rubidium 85
in the lower part.
This is one output port
of the 87 interferometer.
This is the other output port
same for 85.
And we introduce a phase--
what we call a phase shear
across the interfering waves
so we can precisely measure
the phase.
This is like what happens
if you misalign
an optical max into
interferometer,
you see fringes.
And basically,
by comparing the phase of--
of these--this is
a lot of atoms,
and this is a few atoms.
By comparing the phase
of those peaks
to the phases
of those peaks,
we can measure
their relative phase
and then get a handle on
the equivalence principle.
The data I'm showing you here
right now,
we're--is good
to about 12 digits,
and we think that when we get
this apparatus all tuned up,
we're gonna have 14 digits
of sensitivity or more.
And at that level,
our theoretical colleagues
start getting interested
in these results.
Thanks.
Let's see, so what's--
You know, where do really
interesting things happen?
Well, people think
the really interesting stuff
is gonna be at 16 digits.
The world record for equivalence
principle right now is in
the University of Washington
gravity group,
where they measure
equivalence principle
using torsion pendulum
to 13 digits,
and they're pushing into
the 14-digit level.
And so we're hoping to at least
have a measurement
that's competitive
with what they're doing
and complementary
in the sense that we're using,
you know, this completely
quantum mechanical method
for making the measurement.
What else does equivalence
principle measurement tell you?
Well, you know,
what's the impact
of EP being true or false?
And so here, there's a lot of
recent theoretical work
motivated by the fact--
by cosmology
of, basically,
there's dark matter out there
and we don't know what it is
and certain flavors
of dark matter
lead to equivalence principle
violating interactions
that we might see in our lab.
And so this is a chart from one
of my colleagues,
Peter Graham, at Stanford,
that shows the possible impact
of the measurements
we're doing right now
on some of his favorite
equivalence principle
violating theories.
And again, I'm not gonna explain
the axes,
just to say that
this yellow region
is what we know
to be excluded right now,
and the kind of data
we're taking is--is--
is gonna further
push the bounds
on the possible validity
or lack of validity
of these obscure theories
about dark matter.
I want to change gears to
some more wholesome physics.
So...
[laughs]
You probably all have seen,
in one form or another,
this data, which is the--
the unbelievable data
that was taken by
the LIGO Collaboration when--
for their first observation
of gravitational radiation.
What they did was
they built two conventional
optical interferometers at two
regions in the United States.
And the idea is that if...
Black holes merge,
spin and merge,
at way far away,
they give rise to perturbations,
ripples in space-time,
which propagate
at the speed of light
through space-time to us here
on planet Earth
and these space-time ripples
basically have
the--the effect of stretching
and contracting space
as a function of time
in such a way
that if you build one of these
precision interferometers,
it can be observed.
And so just to take you--
if you haven't seen this data,
take you through
some of their signals.
This is the output of one
of the interferometers
and this is the output of
the other interferometer,
you know, that one there
and that one there.
And when you see those fringes,
when you see that shaking,
that means that something came
and changed the path length
of one of the interferometer
arms with respect to the other,
that something being
a gravitational wave,
and in such a way
that you can observe
the cataclysmic event,
which in this case
was way far away,
two black holes coalescing.
And so I just explained
that I'm building
interferometers,
and the interferometers
I'm building
I think are gonna be
very sensitive.
And so you might ask, you--
could we see
gravitational radiation
in a way that's similar to LIGO
but now using atom interference?
And if we could, what--
why would you want to build
an instrument like that?
And so I'll spend
just a few minutes
kind of describing
our thinking on that.
We haven't built anything that's
nearly as sensitive
as this LIGO instrument,
but we think that a space-based
instrument holds promise
for observing certain types
of gravitational wave sources.
And here my philosophy
is the following:
this was the breakthrough
discovery
that taught us that, you know,
observing--the physics community
and the astronomy community and
the astrophysicist community,
that observing
gravitational waves
is really interesting.
And, you know, you go back
in time, you say,
well, we know that, you know,
using telescopes
to look at the cosmos
is also interesting.
And right now, when you do
optical astronomy,
there are lots of different
telescopes that you build
depending on what you want
to look at.
So as gravitational wave
astronomy progresses as a field,
I think it stands to reason that
there are gonna be
a diversity of telescopes that
you're gonna bring to the table
for making interesting--
scientifically interesting
observation.
So here's a--here's a plot
generated by Sesana
and colleagues
that basically traces
the evolution
of the--the black hole merger
that I just showed you
through time.
And what's happening
in the LIGO signal
is that the LIGO antenna
only captures
a transient signal for the--
the very final moments
of the merger.
When those black holes
are spinning really quick
and then merge,
that's when you see blips
in the interferometer.
Well, in this...
Situation,
those black holes
are spinning slowly
before they hit this
cataclysmic event here
for a long period
of time.
And it's interesting to go
and look at these black holes
in this region
of parameter space
before they finally hit
their merger situation.
How might you build an antenna
that is capable of detecting
these low frequency
perturbations?
Well, the world has been
thinking a lot about that,
and that's where I think
our atom interferometer detector
may have something to say.
When I say the world's been
thinking a lot about that,
there's a planned
ESA mission,
and NASA may--
is looking like it will
participate in that,
that is designed
to detect
very low frequency
gravitational waves.
This is frequency
and this is the--
this axis here is the amplitude
of the gravitational wave.
And you notice that
this antenna, this telescope,
can see
the very low frequencies.
LIGO can see
the very high frequencies,
but maybe you want to see
these intermediate frequencies.
So this is the antenna we
propose to build with atoms.
And I'm showing you kind of
a theory view
of something that we published
in this paper
a number of years ago.
This is the space-time diagram
for the positions
of wave packets
at one region in space
and another region of space.
This is--
this region of space now
I want to be separated
by about a gigameter,
10 to the 9 meters,
such that if
a gravitational wave
comes rolling through this--
this intervening space,
that will lead to
a relative phase shift
of this interferometer
with respect to
that interferometer,
that I can observe
very much like
the differential phase shift
that we're observing right now
in our equivalence
principle work.
The only difference is
these interferometers now
are separated by
a long distance
and I have to correlate the
phases across that baseline.
It turns out this is
a pretty good way
of detecting
a gravitational wave.
If you're building instruments,
what we propose--are
essentially proposing doing is
there's this LISA instrument,
which is very good
for the ultimate
low frequencies.
We're proposing replacing
a macroscopic proof mass,
which sits inside the satellite
and which has recently been
verified
by the LISA Pathfinder
collaboration,
spectacularly verified,
with a cloud of atoms.
And due to the atomic physics
processing,
it turns out we can
build this antenna
with just two satellites.
The LISA configuration requires
three satellites,
and from a system engineering
perspective,
we think this may be favorable.
And if you want to think
more deeply about this,
it turns out the atoms
are serving as
precision proof masses
and position references
and the lasers that we use
to interrogate the atoms
are providing
a really excellent ruler
by which we're--we're
measuring the time evolution
of the two distances
as the gravitational wave
comes through.
And if you say,
well, what--
how good
is this telescope?
The telescope is--I characterize
it by its frequency
and its strain response, how big
of a wave it can measure.
This is what the LISA antenna
can do,
and this is what we think the
antennas that we are envisioning
can do.
And if you want to take that
a step further,
there's some new ideas
we have out there
where the LISA strain curve
is sitting up here,
and what we want to do
with atoms
is a couple of orders of
magnitude better.
So this is not--this is probably
the generation of telescopes--
you know, two generations away.
LISA has to be built first,
but eventually, I hope that
we'll be starting to build
these gravitational
wave detectors
with these atom sensors.
More practical applications?
Well, I'm measuring--
when I build a gravitational
wave detector,
I'm measuring perturbations
in space-time
due to gravity
very precisely.
In that case, due to
a gravitational wave.
If I have a satellite
in low-Earth orbit,
I can do something more mundane.
I can look at the perturbations
in the relative positions
between two clouds of atoms
this time separated by meters,
not 10 to the 9 meters.
In the same way, by building
two interferometers
and subtracting,
and it turns out,
that makes a very good
differential acceleration
sensor,
which I can use to characterize
the Earth's gravitational field
and perturbations in
this gravitational field.
And this is considered
interesting
because the perturbations
in the gravitational field,
as observed from orbit,
tell you a lot about
the mass distribution on Earth.
And we're interested in
the mass distribution
because some of that mass
is water,
and as we know,
due to climate change,
that water's moving around.
And so ice is melting
in one place,
it's going someplace else,
and how can we figure out
where it's going?
There's a guy at Goddard,
and there's some other people
at JPL looking at this.
This is--
this data here is from--
This analysis is from
Scott Luthke's group
at Goddard of where
they're actually looking at
the gravity gradient signals
of--of water table.
And this is one of the maps
they generate,
and, you know, they kind of show
you the gravity contrast
from--from water
as it's moving around.
And so we're, with NASA Goddard,
are building an instrument which
would be a prototype
for one of these--
thanks--
one of these space-based
gravity gradiometers.
So I have--
I have five minutes left,
and I want to,
just in this last five minutes,
change gears a little bit.
I started the talk
by telling you about a--
you know, a story about
macroscopicity
and quantum mechanics
in terms of distance separation.
I want to finish the talk by
telling you
a different type of
macroscopicity.
That is, how can we make states
and manipulate states of--
that are quantum mechanical
but contain large ensembles
of atoms?
And the state I'm gonna
tell you about
is one where we have
collections of atoms,
thousands of atoms
all glued together
in a fundamentally
quantum mechanical way
that--so-called
entangled states--
that are doing something
very useful for us
in the context
of interferometry.
And so my intro to
this last subject is
let's talk about noise
for a little bit.
When--when we go
and build our interferometer
and look at
an interference fringe,
we don't measure
a perfect phase.
We always get a little noise
on top of that fringe,
and I illustrate that
schematically here.
If I'm building
an interferometer,
I don't care
what interferometer it is,
I scan the phase,
I'm detecting some number of
particles in an exit port,
and there's always some noise
at a particular phase.
Here, I've frozen this--
this cartoon
at this particular
phase value
and plotted a distribution
of the number of particles
that I detect,
photons or atoms,
at an output port,
and it fluctuates.
It fundamentally
has to fluctuate
because what I'm doing is
I'm collapsing wave packets
when those two interfering
photons or atoms
come back together,
and it's a fundamentally
statistical process.
Now, that collapse happens often
in a way
where the statistics between
one particle and the next
are completely uncorrelated,
and it gives us what's called
shot noise.
And the--the--
the well-known theory
which predicts the amount of
noise you expect to see
for the number of particles
you have
coursing through
your interferometer.
And to make a long story short,
using kind of laser
and atomic physics techniques,
we are now manipulating
that noise
in--in a fundamentally
quantum mechanical way
to reduce the noise
at the output port
of the interferometer.
And just to show you that
in data,
this is the output port
of an interferometer
which was tuned to operate
exactly at the mid-phase point
that I showed you
on the previous slide.
And if I use
just regular atoms,
not fancily entangled
or correlated with some tricks
I don't have time
to tell you about,
if I measure the fluctuations,
I get a distribution
shown here in blue.
And those fluctuations
for virtually every
interferometric sensor
that's built today,
those shot-noise fluctuations
fundamentally limit the
sensitivity of that instrument.
Now, using
quantum entanglement tricks,
we now have manipulated this--
this ensemble of atoms
so that those fluctuations
are ten times narrower.
And it stands to reason,
if you're associating with
the mean position of that
distribution of phase
and you care about
measuring the phase,
having a narrower distribution
really helps you
in your precision measurement.
And we demonstrate that
in this data
by applying a tiny phase shift
to this interferometer
that shifts this distribution
from an output of
that number of atoms
to this number of atoms.
This is the number of atoms
at the output port,
and this is the probability of
observing that number of atoms.
This entanglement
is really helping us.
Now, this entanglement,
this--this correlation
of creating these
correlated states of atoms,
which again we do with
lights and mirrors,
is doing something amazing
to a cloud of otherwise
10 to the 5 atoms,
which are noninteracting.
What it's doing is they're
making them fundamentally linked
in a quantum mechanical way so
that when I make a measurement,
I--if I--if I measure
an atom here,
somehow that affects
the wave function
of an atom over there
in--in--in such a way
that when I measure them all,
I can't--
they're correlated in such a way
that I can't have fluctuations
bigger than
what we observe there.
I've been using this word
"entanglement" a lot.
This is--I'll skip through this.
We do this with a cavity.
That's what the apparatus
looks like.
We use this word
entanglement a lot.
What do I mean by that?
Well, I mean when I try
and write the wave function
for all those atoms,
and I--if you've had that
quantum mechanics course,
you write--one particle you
write its wave function down,
and then in the Hilbert space
you write the wave function
of the next atom down
and the next atom down.
If they're uncorrelated
straightforward,
you write the wave function
for all those
10 to the 5 atoms down.
Once you do this, we call it
a squeezing operation.
We're squeezing out the noise.
You have trouble writing down
that wave function in such a way
that you can separate out the
contributions from each atom
and factorize each atom
independently.
In fact, for this data, this
analysis shown in this plot,
says that the wave functions
for these atoms
are in--are correlated
in such a way
that I can't think of the atoms
in smaller groups than 1,000
independently.
They're--they're--they're
correlated in clusters
of at least 1,000 atoms.
This is like a world record
for quantumness
of ensembles of atoms.
And what's--what's interesting,
if you've read about quantum
computers and things like that,
it's this very
entanglement property
which is thought to be--
gonna be useful
for speeding up computation.
Well, for building sensors,
it's also useful.
And if by reliably
entangling the particles,
I can dramatically reduce
the sensor noise,
making a better sensor
and actually making
these fancy ideas
of quantum mechanics useful.
And so this work is
demonstrating macroscopicity
in a different way.
I now have 1,000-atom
quantum systems
truly behaving as quantum.
At that, I have to thank
the people in my research group
and my theory collaborations--
collaborators,
and you guys
for your attention.
[applause]
- Thank you very much.
Thank you.
So we have time
for a few questions.
If you have a question,
please raise your hand,
wait for the microphone,
and ask one question only.
Thank you.
This, up front here.
- Thank you.
Thank you for
interesting talk.
You mentioned this
very sensitive accelerometer
based on atomic interferometry,
but they all required
this cryogenic,
very low temperature
sort of operation, right?
So you cannot make them
very small.
They should be heavy.
They should be bulky.
So how can you put something
that big
to satellites or...
- Yeah, so when we say
ultracold,
the only cryogen
I'm using is laser light.
So I need
a single-frequency laser
which is, you know,
smaller than my--
the semiconductor laser
flash
is smaller than my thumbnail,
and it's the interaction
of the laser light
with the atom cloud
that reduces their temperature.
And so the smallest sensors now
are approaching the size
of my fist.
I showed you that--
that gravimeter,
which is about the size of
a two-liter Coke bottle.
And as we, you know,
kind of start engineering these
and use--replace
kind of bulk optics
with integrated photonics,
which we're now manufacturing
with some partners
in the valley,
and state-of-the-art
semiconductor laser sources,
the size of these sensors
is being crushed.
They're really becoming small.
In fact, there are some--
AOSense and my interest aren't
in making them tiny, tiny, tiny,
like competing with
MEMS sensors.
We want them to be big enough
to be high performance.
But we have gyroscopes now that
are about--about this big
that have performance figures
of merit
that are really
pretty extraordinary.
So it's all about, you know,
engineering with optics,
and the--
that's--that's the main
technical challenge.
There's a little bit
of new engineering knowledge
that we have to gain,
figuring out how to make
reliable sources of atoms
and all the rest.
Much of that draws on some work
from the atomic clock community
in the--in the '50s.
Another way of answering
your question is,
you're comfortable with going
out and buying an optical--
I'm sorry, an atomic clock from,
used to be Hewlett-Packard
and now it's Microsemi,
that's rack mounted.
That, you know,
and has a beam of atoms in it
and it's interrogated,
and kind of what we're doing
is very similar.
But we do not need
liquid helium.
[laughs]
- If you can entangle
1,000 atoms,
any chance of using this
in quantum computing
in the near future?
- For what?
- Quantum computing.
Quantum computer.
- Oh, that's
an interesting question.
So what are the links between--
the field I just discussed
is receiving a great deal
of interest right now,
it's called quantum metrology,
and that's using entanglement to
improve performance of sensors,
but we all sort of also want
to build a quantum computer.
And so what's the link between
this kind of entanglement
and the entanglement you need
for quantum computation?
Well, I view it as a baby step
towards a quantum computer.
So a quantum computer,
you have to go in there
and reliably entangle
determine--
almost deterministically,
I should say,
the way functions of--
of each and every atom
in a way that you--
you sort of understand.
Here I have this grab bag
of atoms
that are sort of magically
being entangled
but in a way that I have even
trouble writing down
what the wave function is.
So there's a big step between
what I've just demonstrated
to you and--demonstrated
and the type of entanglement
you need for a quantum computer.
Nonetheless, there are--
if you follow this field,
there's a lot of discussion
about what makes a good qubit
for a quantum computer,
how you're gonna error-correct
that qubit and so forth.
And some of these
cavity atom ideas
are interesting players
in that discussion,
but it gets pretty technical
and, you know,
you have to talk about the gate
fidelities and things like that,
and it boils down to a bunch of
technical details
that maybe aren't, you know,
they're surprising
on first blush.
So part of my program is
to--to--to figure out
what role this type of
entanglement might have
in quantum computation,
and I say that.
I'm much more optimistic
about other systems than that
for quantum computation.
Thank you.
- Okay, so please join me
in thanking Dr. Kasevich
for an excellent talk.
Thank you.
- Thank you.
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