Translator: Queenie Lee Reviewer: Rhonda Jacobs
Has anyone here used your quantum devices today?
Anyone? Yes, a few of you.
Here is one of my favorites, my cell phone.
It's literally my entire world here,
right there.
But maybe some of you have used your TV, or a laptop,
or perhaps you have used your iPad, camera.
And maybe some of you are clumsy like me,
and you've dropped one of your fancy devices, and it broke.
And then you had this powerful urge to take it apart and look what's inside.
And then when you did that, you saw there's a bunch of electronics
made up of billions and billions of tiny transistors -
the fundamental building blocks of these devices.
And in order to make those transistors work,
we need to figure out the science of semiconductors.
And to control semiconductors and build a multi-billion dollar industry,
we need to understand structure and interactions at the atomic level.
And that's quantum physics -
the study of the laws that govern microscopic objects
like atoms and electrons;
and even photons, which are particles of light.
You may have heard of another little invention
called the laser.
Quantum device.
So this amazing scientific theory called quantum mechanics
has transformed our lives today.
But my fascination in quantum physics actually started in the stars.
So, this image here may look familiar to you.
It's what we call a solar spectrum.
It's what you see when you pass sunlight through a prism
and it breaks up into all of its constituent colors.
This has been taken by a powerful telescope,
so it's a very high-resolution image.
And what you may notice is a bunch of dark lines.
And that's not dirt or scratches or anything,
that's real.
Those are places where the light is actually missing.
And before quantum mechanics, we didn't really understand
why is it that there are these specific colors that are missing from sunlight.
Well, the reason they are missing is because that light has been absorbed
by gases surrounding the sun, cooler gases.
And the specific colors that are absorbed depends on what that gas is made of.
Is it hydrogen? Is it helium? Calcium?
And in fact, using quantum mechanics,
we can understand the structure of different types of atoms -
hydrogen vs. calcium vs. any other element in the periodic table.
So then, we know which of those particular atoms
will absorb which colors of light.
So each of these missing spots is like a unique bar code
which tells us what the atom is.
So by studying the bar code,
we can figure out what is the gas surrounding the sun.
So for me, this was amazing!
Here is a secret message encoded in sunlight,
about the composition of the sun,
and we can decode it with quantum physics.
But in fact,
that secret message is even bigger,
it's an incredible message about the entire universe,
and the person who decoded it
was a young woman named Cecilia Payne, in 1925.
So Cecilia Payne was a Ph.D. student at Harvard.
Actually back then in the 20s,
you couldn't actually take classes at Harvard if you were a woman,
so she was enrolled in Radcliffe College,
but she did all of her observations and her studies at Harvard Observatory.
And she was studying different spectra just like the one I showed you,
not just for the sun but also for other stars.
And all these different spectra - they look different,
so those bar codes are at different places.
And she was trying to understand what that difference was.
So she used equations from quantum physics and figured out something amazing.
All those differences in the bar code
was because all these different stars had different temperatures.
But in fact, by far the most common element in all those stars
was hydrogen and helium.
There's a hundred billion stars in our galaxy alone,
and there's a hundred billion galaxies out there.
And using these equations from quantum theory,
this young woman was able to figure out something,
a universal property of all these stars.
And that is like figuring out the chemical composition of the universe.
This was actually quite surprising because back in that time,
people thought that the composition of the sun and stars
was very much like the earth.
So in fact, Cecelia Payne didn't even believe her own results,
but very soon after her work, others confirmed this.
And in fact, indeed today we know
that the universe is basically made of hydrogen and helium,
with everything else just in very, very small amounts,
including all of the elements we know here on earth.
So, this was really an incredible thing.
Cecelia Payne, she presented this as part of her Ph.D. thesis.
In fact, it was the first Ph.D. awarded to any student, male or female,
at Harvard Observatory.
And what a way to start.
She chose to stay on at Harvard and continued to do amazing research,
and she taught courses,
but they didn't quite know what to do with this new Ph.D. who is a female.
They weren't really sure what to do,
so they actually could not even give her an official title
for the next decade or so.
And the courses she taught were not even listed on the books.
Finally, in 1938,
they gave her the title of Astronomer,
and then eventually, in 1956, she became a Professor at Harvard,
and very soon afterwards, she became the chair of the astronomy department.
In fact, the first female chair of any department at Harvard.
That's Cecelia Payne-Gaposchkin.
She's not a household name.
She should be! But that's a different talk.
But I was lucky.
I, in fact, did learn about her story and her work
when I was a graduate student myself, starting out.
And it was of course very inspiring to me -
both her science and her experiences as a woman scientist.
Her work brilliantly illustrated the power of this incredible theory
called quantum physics,
but also I learned something else.
I learned that, in fact,
the fundamental laws of nature are accessible to everybody:
male or female, young or old, known or unknown.
And that was a really important message for me,
back then when I was starting out.
So, I followed my passion,
and eventually, I earned my own Ph.D. in physics,
not a first at Harvard or anything like that,
but the first woman in my family to do that.
And of course,
I chose to focus my research area in quantum physics.
But instead of studying the largest objects like stars,
I decided that I want to investigate the smallest scales -
the worlds of individual atoms.
So we cannot actually directly see an atom;
it's far too small.
But we can set up experiments
where we can shine a laser beam at a cloud of atoms.
And the atoms will interact with the laser light,
and they will leave a shadow of that interaction in the light,
and we can collect that light and analyze it and extract that shadow,
and from that, we can actually construct a picture to see this atom.
Here's such a picture from data
taken in the lab of my colleague Poul Jessen, at the University of Arizona.
So, as you can see, the picture shows the world of the atom,
which is represented by a sphere.
And the laws that govern this kind of world
is the laws of quantum physics.
And that's actually a very strange set of laws.
For example,
there's something called "the uncertainty principle,"
which says that we cannot know exactly where the atom is located on this surface.
So what you're seeing here is actually an image of likelihood.
Red shows where the atom is most likely to be,
and blue where it's least likely to be.
The other thing that we can do
is we can actually engineer the surface of this sphere,
so actually create a world for the atom.
And, for example, how do we do that?
Well, we use lasers and magnetic fields to control this world.
And one of the things we can do
is we can insert barriers and change the shapes of islands
on the surface of this world.
So this white line shows a barrier that's constructed,
so a barrier is essentially like a wall that you and I could not actually cross.
And then, what we can do
is we can take many such pictures of the atom over time,
and we can put them together and make a movie,
so, the first such movies of this kind ever made.
Here's such a movie.
Watch what happens.
The atom is moving back and forth.
It doesn't seem to be seeing this so-called "barrier,"
and it's not actually climbing over the barrier,
it's passing straight through.
It's an incredible quantum effect called "quantum tunneling."
What you are actually seeing here
is literally an atom walking through a wall.
And that was, of course, an amazing scientific discovery for me,
and I was very excited,
but I did see another one of those secret encoded messages
sent to us by the universe.
The universe is showing us that barriers are meant to be crossed.
And furthermore,
quantum tunneling connects directly back to the stars.
If you look deep in the heart of stars - the core -
it's actually a powerful nuclear fusion reactor.
That's where all of the sunlight and energy is produced,
and some of that comes to us as sunlight from our star.
And, of course,
we know that it's critical for life on earth.
But how does that nuclear reactor work?
Well, thanks to gravity, the hydrogen nuclei in the core combine,
they fuse together to create helium,
and that releases enormous amounts of energy.
But here's the problem:
Hydrogen nuclei, they have positive protons, positive charges.
And as you know, like charges repel each other.
So as you try to bring these hydrogen nuclei close together,
they push against each other.
There is a barrier, an energy barrier,
and the hydrogen atoms cannot cross that barrier.
But yet, they can.
And the reason they cross it is exactly because of quantum tunneling.
They tunnel through the barrier
and are able to complete the fusion process
from which we get light and energy.
So sunlight on earth, and a reason that we have life on earth,
is because quantum particles can walk through walls.
And, now that we understand the physics of quantum tunneling,
we can control it and build amazing devices
like scanning tunneling microscopes, and tunneling diodes.
And these are amazing devices and cutting edge technologies,
but we're just scratching the surface.
Tunneling is just one of many incredible quantum effects
that we are exploring now, to try to build future quantum technologies.
For example,
quantum particles can get connected together
in a powerful connection that we call entanglement.
So if you make a change to one of the quantum particles,
it instantly affects all the other particles,
no matter how far away they are in the universe.
And again,
we find that in order to understand quantum theory
and what it's saying about entanglement,
we have to question our very fundamental ideas
about the nature of reality and space and time itself.
And that's amazing, but there is a nice side effect.
By exploring entanglement, we have found that like tunneling,
it's a resource that we can use to build technologies.
So, it's the fuel that we must have
in order to build future cutting technologies
like quantum computers, or teleportation,
or maybe some technology that we have not even dreamed of as yet.
So quantum physics
has led us on an amazing journey of discovery over the last century.
We have been able to look into the hearts of stars;
it has to lead us
to understand the structure, the composition of the universe;
it has shown us new ways to think about barriers
and connections through space and time.
And, it has shown us
that by studying fundamental scientific laws of nature,
we get really nice side effects,
like cool technologies and awesome devices.
But for me, my fascination remains those secret messages,
those encoded mysteries
out there in the universe.
The universe very generously has left us clues all around us,
and we humans, we get to be nature's detectives.
We are constrained to this planet,
we have limited resources and limited knowledge.
And yet, thanks to physics and developing our theories,
we can expand our understanding to the entire universe.
What will be that next secret message that we will decode,
that will lead to breakthroughs in science as well as society?
I don't know.
And that is the most exciting part.
Thank you.
(Applause)