Nova (1974–…): Season 46, Episode 2 - Einstein's Quantum Riddle - full transcript

In an effort to disprove quantum mechanics, Albert Einstein proposes one of its strangest features: quantum entanglement.

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High above the Atlantic Ocean,
on one of the Canary Islands,

a team of pioneering scientists
is setting up an experiment

that will span the vast scales
of the cosmos...

..and could confirm our perception
of the universe is an illusion.

I'm checking the signals
from the cosmic

random number generator
and it looks pretty good.

They're connecting two of Europe's
largest telescopes and attempting

to use galaxies billions of light
years away to control intricate

measurements of tiny particles
of light here on Earth.

They're testing the existence
of perhaps the strangest idea

in science - quantum entanglement.



Entanglement is this very powerful
but strange connection that exists

between pairs of particles.

The effect is almost magical.

As if something here can
mysteriously affect something there.

Can particles be connected
as if they're joined together,

even if they're millions
of miles apart?

Entanglement opens up new venues
for communication, for computation

and for understanding the nature
of space and time and the beginning

of the universe.

Einstein rejected the idea, calling
it "spooky action at a distance".

It's so beautiful, but what the heck
is behind it?

So is entanglement real?

Do we live in Einstein's universe
of common sense laws, or a bizarre

quantum reality that allows strange
connections across space and time?



Will the team
prove Einstein right...

Moving.

..or wrong?

Yeah, that's good.

We do have a test.
We are acquiring data right now.

As with so many of the most
important ideas in physics,

the story of quantum entanglement
begins with Albert Einstein.

In 1927, he travelled to Brussels
to attend a meeting

about a new theory that described
the micro world of atoms

and tiny particles -
quantum mechanics.

Quantum mechanics is one of
the most amazing intellectual

achievements in human history.

For the first time, scientists
were able to probe a world

that was, until then,
quite invisible to us.

Looking at the world at the scale
of atoms a million times smaller

than the width of a human hair.

One way to think about the scales
is that if you take an everyday

object like a soccer ball,

and you enlarge that soccer ball

so that actually you can see
the individual atoms, you roughly

have to make it the size
of the Earth.

And then move into that planet.

Then you are in the world
of atoms and particles.

It was the nature of
fundamental particles

that make up the world
we see around us

that Einstein had come to Brussels
to discuss, and it was here

that Einstein entered into
a heated debate that would lead

to the discovery of
quantum entanglement.

A concept that will trouble him
for the rest of his life.

With two PhDs from Harvard,
and now a leading professor at MIT,

David Kaiser has studied
this dispute and has come

to the place where it all began.

This is the original
Solvay Institute building,

beautiful, grand building.

And this is the place,

back in October 1927,

where the fifth
Solvay Conference was held.

This amazing week-long series
of discussions

on really what the world is
made of, and the nature of matter,

and the new quantum theory.

And these steps are the very steps
on which this famous group

photograph was taken.

It is a collection of some
of the most brilliant people

in the world. Here in the front row

we see Albert Einstein and the great
Marie Curie and Max Planck.

In the back row, standing,
the dapper Erwin Schrodinger.

And these sort of brash
20-year-olds, or mid-twenties,

Werner Heisenberg
and Wolfgang Pauli.

I had a huge version of
this photograph up on the wall

as a poster in my college dorm room.

My roommates had their favourite
bands, and I had the 1927

Solvay Conference, which says a lot.

This was one of the greatest
meetings of minds in history.

More than half were or would become
Nobel Prize winners.

Their experiments were showing
that deep inside matter,

tiny particles, like atoms
and their orbiting electrons,

were not solid little spheres.

They seemed fuzzy and undefined.

So this group here,

these were the folks
who had just been plumbing deeper

and deeper and deeper to find
what they hoped would be a bedrock

of what the world is made of.

And to their surprise, they found
things less and less solid

as they dug in.

This world was not tiny little
bricks that got smaller and smaller.

At some point, the bricks
gave way to this mush.

And what looked like solidity,
solidness, in fact became

very confusing and
kind of a whole new way

of thinking about nature.

The theory of quantum mechanics

presented at the meeting
was strange.

It said that a particle like
an electron isn't physically real

until it is observed
and measured by an instrument

that can detect it.

Before it's detected,

instead of being a solid particle,

an electron is just a fuzzy wave,

a wave of probability.

These objects,
like electrons and atoms,

when we describe mathematically
their behaviour, the only thing

we can describe is the probability
of being at one place or another.

It's like a wave of all those
different possibilities.

It's not that the electron
is in one place or the other,

we just don't know,

it's that the electron really is
a combination of

every possible place it could be,

until we look at it.

Laws of nature were no longer
definite statements

about what's going to happen next.

They were just statements
about probabilities.

And Einstein felt,
"Well, that's defeat.

"You're giving up on the heart
of what physics has been,"

namely to give a complete
description of reality.

For Einstein, the idea that tiny
particles only pop into physical

existence when they're observed
is akin to magic.

He later asked a friend, "Do you
really believe the moon is not

"there when you are not
looking at it?"

Outside of the formal setting
of the conference...

Bonsoir.

..he challenged the most vocal
supporter of these ideas,

the great Danish physicist
Niels Bohr.

Einstein would show up to breakfast
at the hotel

and Niels Bohr would be there,

and Einstein would present
his latest challenge.

Niels Bohr would sort of mumble
and wonder,

and confer with
his younger colleagues.

They'd head off to the formal
meeting at the Institute,

and somehow, every night, by supper
time, Bohr would have an answer.

One of the observers said that
Einstein was like a jack-in-the-box.

Every day he'd pop up with
a new challenge and Bohr would flip

this way and that, and then
by supper have crushed that one,

and they would start all over again.

To Bohr and his colleagues,

quantum mechanics not only
explained experimental results,

its mathematics were elegant
and beautiful.

And since Einstein hadn't found
flaws in their equations, they left

the Solvay Meeting feeling
more confident than ever

in their ideas.

But Einstein didn't give up.

In 1933, Einstein, a Jewish
scientist and outspoken pacifist,

fled Nazi Germany
and took a position at

the Institute for Advanced Study
near Princeton.

He recruited two physicists
to help him -

Nathan Rosen and Boris Podolsky.

And in 1935, at afternoon tea,
the three men formulated an idea

that would shake the very
foundations of quantum theory.

They discovered
quantum entanglement.

Today, Robert Dijkgraaf is
the director of the institute.

I think scientific progress is
often made in kind of a dialogue.

And I think the informal
moments are crucial.

So at the Institute
for Advanced Study,

we've had tea at three o'clock
from the very beginning.

It was here that a conversation
between Einstein and Rosen sparked

a powerful idea.

Rosen told him he was studying
a system of two hydrogen atoms,

and noticed that there were
some ways in which

these two atoms were coupled,

even if you moved them very
far apart, and then apparently,

Podolsky jumped in and said,
"Well, Professor Einstein,

"this is very important
in your arguments showing

"that quantum theory is incomplete."

So they've got this very
animated discussion.

And what's kind of happened still is
now you have a bunch of scientists

discussing and at some point
someone says,

"Let's write a paper together."
So they did.

Their paper, known today as EPR,
argued that the equations of quantum

mechanics predicted an impossible
connection between particles -

a seemingly magical effect.

It would be like having two
particles, each hidden from view.

Looking at one...

..mysteriously causes the other
to reveal itself too,

with matching properties.

Quantum theory suggested this effect
could happen in the real world.

For example, with particles
of light - photons.

The equations implied that a source
of photons could create pairs

in such a way that when we measure
one, causing it to snap out

of its fuzzy state,
the other mysteriously snaps

out of its own fuzzy state
at the same instant,

with correlated properties.

The 1935 paper that described
this effect has become Einstein's

most referenced work of all.

It has captivated
generations of scientists,

including one of the world's leading
quantum physicists,

Professor Anton Zeilinger.

The Einsten-Podolsky-Rosen Paper
fascinated me

and I had to read it at least five
or six times until I finally

understood what goes on,
and then it didn't let me go again.

Another way to think of the paired
particles is to imagine a game

of chance that's somehow rigged.

Suppose I had a pair
of quantum dice.

I put these two
quantum dice in my little cup.

Throw them.

I look at them.
They show the same number, six.

I put them again in the cup.

Throw them again.

Now they both show three.

I put them in again.

Throw again.

Now they both show one.

The point now being, what I see
here is I see two random processes,

namely each die showing some number,

but these two random processes
do the same.

That's really mind-boggling.

How could two particles act
in unison,

even when they're
separated from each other?

Essential to the EPR argument
is that these particles

can be can be separated
at an arbitrary distance -

one could be here at Princeton, one
could be in the Andromeda galaxy.

And yet, according to quantum
mechanics, a choice to measure

something here is somehow
instantaneously affecting

what can be said about
this other particle.

You can't go from Princeton
to Andromeda instantly.

And yet that, they argued,
is what the equations of quantum

mechanics seem to imply, and that,
they said, so much the worse

for quantum mechanics. The world
simply can't operate that way.

For Einstein, this strange effect
conflicted with the most basic

concept we use
to describe reality - space.

For him, objects, particles,

everything that exists
is located in space.

Space, together with time,
was the key ingredient in his theory

of special relativity,
with his famous equation,

E = MC squared.

Einstein, of course,
was the master of space time.

He thought that if something
happened here, that shouldn't

immediately and instantaneously
change something

that is going on over there.

The principle of locality,
as we currently call it.

For Einstein, it's simply common
sense that if objects are separated

in space, for one to affect
the other, something must

travel between them.

And that travelling takes time.

Quantum particles acting
in unison could be explained

if they were communicating.

One particle instantly
sending a signal to the other,

telling it what properties
it should have.

But that would require
a signal travelling faster

than the speed of light.

Something Einstein's theory
of special relativity

had proven impossible.

And it would mean the particles
were fuzzy and undefined

until the moment they were observed.

Instead, Einstein thought

the particles should be real
all along.

They must carry with them
a hidden layer of deeper physics

that determines their properties
from the start.

Almost the way that magic tricks,
while appearing mysterious,

have a hidden explanation.

But this hidden physics was missing
from quantum mechanics.

So Einstein, Podolsky and Rosen

argued that the theory
was incomplete.

Podolsky was very enthusiastic
about this project.

In fact, he was so enthusiastic
that he ran to the New York Times

and told them the news.

So Einstein read in the newspaper
that he has proven quantum theory

is incomplete and possibly wrong,

and he was very much upset by that.

So he writes a letter to the editor
saying, "This is not the way one

"should communicate in physics."

And he was really upset
with Podolsky.

Apparently he didn't speak
to him any more.

When Niels Bohr heard
of Einstein's paper,

he wrote an obscure response,
arguing that one particle

could instantly influence another.

This seemingly
impossible phenomenon

became known as
quantum entanglement.

But Einstein dismissed it as
"spooky action at a distance".

No-one can think of an experiment
to test whether Einstein

or Bohr was correct.

But that didn't stop physicists
and engineers from making use

of quantum mechanics in the lab.

In the '30s and '40s, the debate

around the EPR Paper
sort of dies down,

but quantum theory
actually takes off.

The mathematics leads to all kinds
of amazing developments.

The equations of quantum mechanics
accurately predict the behaviour

of atoms and tiny particles,
enabling the scientists

of the Manhattan Project
to develop the bomb.

And in the years after the Second
World War, researchers at Bell Labs

in New Jersey used quantum theory
to develop the first lasers.

In our laboratories, men experiment
with a light once undreamed of

in the natural world.

And build small devices that could
control the flow of electricity -

transistors.

It's destined to play a vital role
in your future.

Your electronic future.

Transistors became
the building blocks

of the burgeoning field
of electronics.

Computers, disc drives, the entire
digital revolution, soon followed.

All made possible by the equations
of quantum theory.

Yet Einstein's questions
about entanglement and what it

implied about the incompleteness
of quantum mechanics remained

unanswered until the 1960s...

..when a physicist from
Northern Ireland

made a remarkable breakthrough -
John Bell.

Bell was a very talented
young physics student,

but he quickly grew dissatisfied
with what he considered

almost a kind of dishonesty
among his teachers.

Bell insisted that Einstein's
questions about quantum mechanics

had not been addressed.

He got into shouting matches
with his professors.

"Don't tell us that Bohr
solved all the problems.

"This really deserves
further thought."

Quantum mechanics
has been fantastically successful.

So it is a very intriguing
situation.

At the foundation of all
that impressive success,

there are these great doubts.

It's a very strange thing that ever
since the 1930s, the idea of sitting

and thinking hard about the
foundations of quantum mechanics

has been disreputable among
professional physicists.

When people tried to do that,
they were kicked out

of physics departments.

And so, for someone like Bell,
he needed to have a day job doing

ordinary particle physics.

But at night, you know, hidden away,

he could do work on the foundations
of quantum mechanics.

Bell became a leading particle
physicist at CERN in Geneva,

but he continued to explore the
debate between Einstein and Bohr.

And in 1964, he published
an astonishing paper.

Bell proved that
Bohr's and Einstein's ideas

made different predictions.

If you could randomly perform one
of two possible measurements

on each particle and check
how often the results lined up,

the answer would reveal whether
we lived in Einstein's world,

the world that followed
common-sense laws,

or Bohr's,

a world that was deeply strange,

and allows
spooky quantum connections.

We now know, with hindsight,

this was one of the most significant
articles in the history of physics,

not just the history of
20th-century physics -

in the history
of the field as a whole.

But Bell's article appears in this,
you know, sort of out of the way

journal, in fact, the journal itself
folds a few years later.

This is not central to
the physics community.

It's sort of dutifully filed
on library shelves,

and then forgotten.

It literally collects dust
on the shelf.

A few years later, completely
by chance, a brilliant experimental

physicist stumbled upon
Bell's article.

I thought, "This is one of the most
amazing papers I had ever read

"in my whole life."

And I kept wondering,
"Well, gee, this is wonderful.

"But where's the experimental
evidence?"

At the University of California,
Berkeley, John Clauser was desperate

to put Bell's idea to the test
in an experiment.

He had a talent for tinkering
in the lab

and building the parts he needed.

I used to rummage around here
and scavenge,

and dumpster dive for old equipment.

He knew where to find hidden storage
rooms like this, which he could raid

to salvage spare parts
for his experiments.

This was a power supply
for diode lasers.

That looks like something I built.

This is wonderful.
This is my book.

I think this may be my old book.

I'll take it!

Deep in the basement
is John's old laboratory.

This is B207, where we did
the original experiment.

He hasn't been back for decades.

I remember distinctly
this old sink.

These computers
were not yet invented.

It was hard work -

we spent long hours, weekends,
evenings, you name it.

We believed, although nobody else
did, that it was actually

a very significant and important
experiment.

Back at home, John has kept parts
and papers from his experiments.

Careful!

Where is this?

So this is my shop,

and laboratory.

Mostly now, I just build sailboat
parts for yacht racing.

Here's a picture
of the experiment I did.

I had more hair in those days!

Here's another picture.
This is Stuart Freedman.

Worked on it with me.

Two photons, entangled photons
would go out

in the opposite directions
through lenses into the polarizers.

They could be, in the simplest
case, we'd put the lenses

in Polaroid sunglasses.

John Clauser and Stuart Freedman
constructed

the world's first Bell Test
experiment.

They focused a laser onto
calcium atoms,

creating pairs of photons

that the equations of quantum theory
suggested should be entangled.

They recorded whether or not
the photons passed through

polarisation filters on
each side, and checked how often

the answers agreed.

After thousands of pairs,

if the results were more correlated
than Einstein's physics predicted,

the photons must be
spookily entangled.

We saw the stronger correlation,
characteristic of quantum mechanics.

We measured it
and that's what we got.

The outcome was exactly what Bohr's
quantum mechanics predicted.

The experiment appeared to show
that the spooky connections

of quantum entanglement
did exist in the natural world.

Could it be that the great
Albert Einstein was wrong?

The first people to react
to this extraordinary result

were not the world's
leading physicists.

Ronald Reagan's definition
of a hippie

was someone who dresses
like Tarzan, has hair like Jane

and smells like Cheeta.

A small group of freethinking
physicists at the heart

of San Francisco's New Age scene
got in touch with John.

They call themselves
the Fundamental Fysics Group.

They spelled physics with an F.

Some members would experiment
with psychedelic drugs.

I mean, they were kind of in the
flow of the kind of hippie scene.

And that group was just mesmerised
by the question of entanglement.

The idea was just to discuss
fringe subjects with an open mind

and I thought, "Well, sure,
that's kind of what I do."

They were doing their best
to link Eastern mysticism

with quantum entanglement.

They sold a lot of
popular textbooks.

There were a lot of followers.

Their books became bestsellers,
like The Tao of Physics,

which highlighted that Eastern
philosophy and quantum entanglement

both described a deep connectedness
of things in the universe.

The Great Cosmic Oneness.

The group held meetings
at the iconic Esalen Institute.

It was a marvellous, beautiful place
where they would discuss

all of these ideas.

It was right on the Pacific coast
with the overflow from the hot tubs

cascading down the cliffs
into the Pacific Ocean.

To my knowledge, no useful
connections to Eastern mysticism

were ever discovered by the group.

But it was fun!

The Fundamental Fysics Group
may not have uncovered the secrets

of cosmic oneness, but in seeing
entanglement as central to physics,

they were decades ahead
of their time.

In the years after Clauser
and Freedman's pioneering work,

physicists began to test possible
loopholes in their experiment.

Ways in which the illusion
of entanglement may be created

so the effect might not be
so spooky after all.

One loophole
is especially hard to rule out.

In modern Bell Test experiments,
devices at each side test

whether the photons can pass
through one of two filters

that are randomly chosen,

effectively asking one of
two questions

and checking how often
the answers agree.

After thousands of photons,

if the results line up

more than Einstein's physics
predicts, the particles

must be spookily entangled.

But what if something had

mysteriously influenced
the equipment

so that the choices of the filters
were not truly random?

Is there any common cause, deep
in the past before you even turn

on your device, that could
have nudged the questions

to be asked and the types of
particles to be emitted?

Maybe some strange particle, maybe
some force that had not been taken

into account, so that what looks
like entanglement might indeed

be an accident, an illusion.

Maybe the world still acts
like Einstein thought.

It's this loophole that the team
of Austrian and American physicists

is working to tackle at the high
altitude observatory on the island

of La Palma in the Canaries.

With quantum mechanics

now more established than ever,
they're determined to put

entanglement to the ultimate test,

and finally settle
the Einstein-Bohr debate

beyond all reasonable doubt.

Professor Anton Zeilinger
is leading the team.

So we are now going up the mountain
towards the Roque de los Muchachos.

Everything looks perfect today.

They're creating a giant
version of Clauser and

Freedman's Bell Test,
with the entire universe

as their lab bench.

In this cosmic Bell Test, the source
of the entangled particles is almost

exactly 500 metres from each
of the two telescopes.

The team must send perfectly timed
pairs of photons through the air

to each side.

At the same time,

the telescopes will collect
light from two extremely far off,

extremely bright galaxies
called quasars.

Random variations in the colour
of the quasar light will determine

which filters the photons
must pass through.

And since the light from the quasars
has been travelling

for billions of years
to reach Earth,

it makes it incredibly unlikely
that anything

could be influencing the random
nature of the test.

At this experiment,
we use three locations.

One is behind me here,

where you see this container
with the blue frame.

In that container we have the source
for the entangled pairs of photons.

One of the two photons is sent
to one telescope,

the other one to the other.

The first telescope is here.

The Telescopio Nazionale Galileo.

It's an Italian telescope.

You can see the small container
on the bridge there

where the photon arrives.

And the telescope then will look up
on the sky, pick up the light

from a quasar, which is very, very
far away, and take the signal

of the quasar to steer the kind
of measurement done on the photon

which arrives.

Over there is Herschel - William
Herschel telescope, the big one.

The photon arrives there,

and again, that telescope picks up

the light from
a different quasar,

and that steers the measurement
on the photon here.

The preparation right now
is that at the source,

I would claim that we have some
of the best people in the world

working on these quantum sources.

By firing a laser through
a specially made crystal,

the team creates a pair of photons

that quantum theory suggests
should be entangled.

We have some entanglement,
just about the best we've seen here.

We still have to tune the state
a bit to get to what we usually

achieve in the lab.

Then the next step also
being done in daylight now

is to try to couple the two
positions to each other

with lasers, to make sure
that they really see each other,

that everything works.

OK.

Eins, zwei, drei, vier, funf...

OK.

And also in parallel,
they are making big lists

of possible quasars.

This is our favourite pair
of quasars.

One of the quasars emitted its light
something like 12 billion years ago.

The William Herschel Telescope
is going to look at a quasar that's

coming up from the horizon

throughout our two-hour
observation period,

getting higher and higher
in the sky.

Galileo Telescope is going to look
at this other quasar that's

sweeping out on the sky.

In the end, it could be
running smoothly,

or there will need to be

a couple of decisions made,
you know, in an excited state,

in the last instance.

With the experiment finally set up,
the team take their positions.

Professor David Kaiser has worked

on this experiment
with his colleague

Jason Gallicchio for four years.

Coordinating it all
is Dominik Rauch.

The experiment is his
thesis project.

He's also been preparing for years.

But as darkness falls, temperatures
on the mountain begin to drop.

OK.

OK. There's bad news.

They have been told to leave
the William Herschel

because the road will be
so dangerous, too dangerous.

So they have to go down now.

Yeah.

The team has been instructed
to leave the telescopes.

The extreme weather could
quickly make their route

off the mountain too dangerous.

After all the preparation,

it's devastating news.

Yes. There seems to be a serious
ice problem on the road.

OK.

We'll be called back if things
work out, I guess. Happens. Yeah.

What can you do?

The next day, the team begins
to prepare for another attempt.

As the sun begins to set,

and the experiment's
start time approaches,

Johannes Handsteiner checks
the equipment hasn't been affected

by the weather.

What are you doing, Johnny?

I'm currently checking the signals

from the cosmic random number
generator,

checking if we have
a high enough signal,

and it looks pretty good.

But now, the air is thick
with clouds.

Here's the humidity
at the various telescopes,

and you see the humidity
is 100%.

So as long as this lasts...

..we can't do much.

The teams at both telescopes wait.

But the clouds don't clear.

All the preparation
has come to nothing.

Time on these huge telescopes
is precious and theirs has run out.

This ambitious test of
quantum entanglement must wait.

Anton Zeilinger was first captivated
by the concept of entanglement

as a young researcher in the 1980s.

I did not go to a single hour at the
University of Quantum Mechanics.

I learned it all from books.

And when I read the books,

it was clear that people evaded
the question, what does it mean?

They pretended everything is clear,
but it was not so clear.

The mathematics is fantastic.

It's so beautiful that one can only
regret that not everybody

can appreciate it.

But what the heck is behind it?

Anton began to experiment
with entanglement in the lab.

As new technologies developed,

he was able to perform
remarkable quantum trickery.

What happens if, instead of
two particles, you have three?

Nobody knew.

And it was my goal from then on
to realise that in the lab,

and he took me ten years
until I had that.

What at that time we could do
was create pairs

of entangled particles.

So you have a pair and another pair.

And then, what you do
is you kind of mix

these two photons.

Mix these two photons,
and take one out of the system,

and then you are left with
one, two, three, one, two, three.

And if you do it right,
then this this photon doesn't know

whether it belongs to this partner
or this partner.

And that way, all three
become entangled.

Today, cutting-edge labs
around the world are racing

to harness Anton's
multi-particle entanglement

to create revolutionary
new technologies.

Like quantum computers.

In our everyday computers,
the fundamental unit of computing

is a bit - a binary digit,
zero or one.

And inside the computer,
there's all these transistors

which are turning on and off
currents.

On is one, off is zero.

And these combinations lead
to universal computing.

With a quantum computer,

you start with the fundamental unit
that's not a bit, but a quantum bit,

which is not really a zero or a one,
but it can be fluid.

A quantum bit makes use
of the fuzziness

of the quantum world.

A qubit, as it's known,

can be a zero or one,
or a combination of both.

A particle or tiny quantum system
can be made into a qubit.

And groups of qubits can be linked
with entanglement

to create a quantum computer.

The more qubits, the greater
the processing power.

At Google's Quantum Computing
Laboratory in Santa Barbara,

the team has recently created
a single chip that holds 72 qubits,

more than any other
quantum chip produced to date.

The task for researcher
Marissa Giustina and her colleagues

is to send signals
to these microscopic qubits

to control and entangle them.

Mounted on the underside
of this plate, we have the quantum

processing chip itself.

In essence, a quantum playground,

you could say. Each qubit
is a quantum object

that we should be able
to control at will.

Thinking about it as the faster
version of that PC over there

would be a great slight to this.

It can be much more than that.

For instance, if we want
to understand how some chemical

reaction works, rather than using
a mathematical model, we can use

these qubits to model the chemistry.

By using entangled qubits,

quantum computers could tackle
real world problems

that traditional computers
simply can't cope with.

For example -

a salesman has to travel
to several cities,

and wants to find
the shortest route.

Sounds easy.

But with just 30 cities,
there are so many possible routes,

it would take an ordinary computer,
even a powerful one,

hundreds of years to try each one
and find the shortest.

But with a handful of entangled
qubits, a quantum computer

could resolve the optimal path

in a fraction of the number
of steps.

There's another reason teams
like Marissa's are racing to create

a powerful quantum computer.

Cracking secret codes.

In today's world, everything
from online shopping to covert

military communications
is protected from hackers

using secure digital codes,
a process called encryption.

But what if hackers could make use
of quantum computers?

A quantum computer could crack

our best encryption protocols
in minutes,

whereas a regular computer or even
a supercomputing network today

couldn't do it,
you know, given months of time.

But while quantum entanglement
may be a threat to traditional

encryption, it also offers an even
more secure alternative.

A communication system that
the very laws of physics protect

from secret hacking.

Researchers in China
are leading the way.

Here in Shanghai, at the University
of Science and Technology,

Professor Jian-Wei Pan runs
a leading quantum research centre.

His teams are working to harness
the properties of the quantum world.

They can send secret messages

using a stream of photons
in a system

that instantly detects any attempt
to steal the information.

Including digital phone calls

that, in theory, are impossible
to hack into.

So what have we got in here?

So here is a system for
the secure telephone, yeah?

This is a fibre connection
between two systems,

to send secure information.

PHONE RINGS

Hello?

You can hear, it's very clear.

Jian-Wei's team has created
a network of optical fibres

more than 1,000 miles long,
that can carry secure information

from Beijing to Shanghai.

It's used by banks
and data companies.

Here is all the city which are
connect to the network.

So here is Shanghai,
here is China Bank,

and the here is our control centre.

So people are really using
that secure information transfer.

But there is a limit to how far
quantum signals can be sent

through optical fibres.

To send signals further,

Jian-Wei's team launched
the world's first

quantum communications satellite.

Above Earth's atmosphere,
there are fewer obstacles,

and quantum particles
can travel much further.

Each night, teams on the ground
prepare to track the satellite

across the sky.

Laser guidance equipment locks
onto the satellite to allow signals

to be sent and received.

The team aims to use this equipment
to create a new secure

communications system using
quantum entanglement.

The satellite will send
entangled photons to two users.

An eavesdropper could intercept
one of the entangled photons,

measure it and send on
a replacement photon.

But it wouldn't be
an entangled photon.

Its properties wouldn't match.

And it would be clear
an eavesdropper was on the line.

In theory, this technique
could be used to create a totally

secure global communication network.

So the next step is we will
have ground station, for example

in Canada, and also in Africa,
and in many country,

so we will use our satellite

for the global quantum
communication.

We want to push this technology
as far as possible.

These are the first steps
of a completely unhackable

quantum internet of the future,

made possible by
quantum entanglement.

But there's a problem.

What if the loophole

in our experimental proof
of entanglement exists,

and spooky action at a distance
isn't real after all?

It could mean entangled
photons are not the path

to complete security.

So by using light released
from quasars, the team

in the Canary Islands is hoping
to close this loophole and prove

that entanglement is as spooky
as Bohr always claimed.

Dominik and Jason
are at one telescope.

Anton is at the other.

OK, ciao.

With clear skies finally overhead,
the huge telescopes awaken.

Poised to collect light
from distant galaxies.

Moving.

Dark count level.

We're doing everything, we're doing
everything at once now.

The guys are setting the state

of the entangled photon pair.

We'll try to acquire the quasar.

We're just centring it and making
the feeder view as small as possible

to be sure that we only have
the quasar.

OK. It's guiding now. Yes.

One more image. OK.

All right. Good, good, good. Yes.

OK. Yeah, that's good.

Looks like... Let's say 91,
to be conservative, of purity.

With the telescopes now locked
onto two different quasars,

the team begins to take readings
from their equipment.

We did the full...
the full cosmic Bell Test.

What?

Yeah, we're doing
the full cosmic Bell Test.

It's working. Light from quasars
is selecting which filters are used

to measure the entangled photons.

It is exciting. It is...

Now we do have a test, but it's not
clear what the outcome will be.

Everything's exactly the same.
Beautiful, perfect.

Two months later, back in Vienna,

the team analyses
the experimental data.

This might take a second.

The numbers look really great,

and it is extremely pleasing to see
that all this works out nice.

We clearly see correlations that
correspond to quantum mechanics.

The results show entanglement.

The data shows that the pairs
of photons correlated

more than Einstein's physics
predicts.

And since the light from the quasars
controlling the test

was nearly eight billion years old,

it's extremely unlikely
that anything

could have affected
its random nature.

This remaining loophole
seems to be closed.

It seems Einstein was wrong.

The experiment we did
is just fantastic.

The big cosmos comes down

to control a small
quantum experiment.

That in itself is beautiful.

I think to do these experiments
is a question of honesty.

You have to do them -
as long as there are loopholes,

you have to close them,
as much as possible.

This, I consider an obligation.

You know, honestly, I still...
I still get chills.

I mean, when I realise
what our team was able to do

in this intellectual journey
that stretches back to the early

years of the 20th century.

There's hardly any room left
for a kind of alternative,

Einstein-like explanation.

We haven't ruled it out,

but we've shoved it into such
a tiny corner of the cosmos,

as to make it even more implausible
for anything

other than entanglement
to explain our results.

Accepting that entanglement is part
of the natural world around us

has profound implications.

It means you must accept
that an action in one place

can have an instant effect anywhere
in the universe, as if there's no

space between them.

Or that particles only
take on physical properties

when we observe them.

Or we must accept both.

We're left with conclusions

about the universe that make
no sense whatsoever.

Science is stepping outside of all
of our boundaries of common sense,

and yet finding ways
to explain the universe.

It is almost like being
in Alice in Wonderland,

where everything is possible.

It was first seen as an unwelcome
but unavoidable consequence

of quantum mechanics.

Now, after a century of disputes
and discoveries,

spooky action at a distance

is finally at the heart of
modern physics.

At the Institute for Advanced Study,
where the concept of entanglement

was first discovered, researchers
are using it in powerful new ideas

that could lead to a single unified
theory of the universe.

The Holy Grail of physics.

Einstein's theories of special
and general relativity perfectly

describe space, time and gravity

at the largest scales
of the universe,

while quantum mechanics perfectly
describes the tiniest scales.

Yet these two theories have never
been brought together.

So far, we've not yet had a single
complete theory that is both quantum

mechanical, and reproduces
the prediction of Einstein's

wonderful theory of
general relativity.

Maybe the secret is entanglement.

What we are learning these days

is that in some sense,
quantum theory

is really the dominant force,

and that we might have to give up

what Einstein holds sacred,

namely space and time.

So he was always thinking, "Well,
we have little pieces of space

"and time, and out of this,
we build the whole universe."

In a radical theory,
known as the holographic universe,

space and time are created
by entangled quantum particles

on a sphere that is
infinitely far away.

What's happening in space in some
sense, all described in terms

of a screen outside here.

The ultimate description
of reality resides on this screen.

Think of it as kind of
quantum bits living on that screen,

and this, like a movie projector,
creates an illusion

of the three-dimensional reality
that I'm now experiencing.

This wild idea may sound
far-fetched, but it reveals

that mathematically, the space
and time we experience every day

could be created by
the quantum world.

Entanglement could be what forms
the true fabric of the universe.

The most puzzling element
of entanglement, that, you know,

somehow two points in space
can communicate,

becomes less of a problem, because
space itself has disappeared.

In the end, we just have
this quantum mechanical world.

There is no space any more.

And so in some sense, the paradoxes
of entanglement,

the EPR paradox,
disappears into thin air.

Truly understanding
quantum mechanics will only happen

when we put ourselves
on the entanglement side.

When we stop privileging the world
that we see, and start

thinking about the world
as it actually is.

Science has made enormous progress
for centuries by sort of breaking

complicated systems down into parts.

When we come to phenomena

like quantum entanglement,
that scheme breaks.

When it comes to the bedrock
of quantum mechanics,

the whole is more than the sum
of its parts.

The basic motivation is just
to learn how nature works,

what's really going on.

Einstein said it very nicely.

He's not interested in this detailed
question or that detailed question.

He just wanted to know,

"What were God's thoughts
when he created the world?"