Einstein's Quantum Riddle: The Spooky World of Entanglement

Quantum mechanics challenges our intuitive understanding of the world. Entanglement's implications are revolutionary for technology.

### Quantum Entanglement: Einstein's Quantum Riddle and Its Profound Implications
* 🧠 "A startling phenomenon" known as quantum entanglement challenges the intuitive understanding of the universe at the smallest scales, leading to a **"strange, counterintuitive micro-world of atoms and tiny particles."**
* 🔮 Despite Albert Einstein's skepticism, experiments have shown that "nature's fundamental building blocks" can be "**connected and influence each other instantaneously**," suggesting that **entanglement may be real**.
* The "Canary Islands experiment" used light from quasars, billions of light-years away, to control the measurement of entangled particles, **successfully demonstrating entanglement**.
* 💻 Quantum entanglement has paved the way for revolutionary technologies like "quantum computers," which leverage the **unique properties of entangled qubits for exponential processing power**.
* 🛰 In the field of "quantum communication," **entanglement enables the creation of a "completely secure global communication network"** that is protected by the laws of physics.
* 🌌 The concept of entanglement has profound implications for our understanding of the universe, with some theories, like the "holographic universe," suggesting that **space itself may be "created by entangled quantum particles."**
* 🌟 "Truly understanding quantum mechanics" may require shifting our perspective and **"putting ourselves on the entanglement side"** to grasp the nature of reality beyond our classical intuition.

♪ ♪ NARRATOR:
We live in a world where objects have permanence, and we see cause, then effect. But a startling phenomenon is revealing that this is not how the universe works at the smallest scales of atoms and tiny particles. ♪ ♪ Albert Einstein argued it couldn't possibly be real. DAVID KAISER:
Einstein was like a jack-in-the-box; every day, he'd pop up with a new challenge. NARRATOR:
But after a century of disputes and discoveries... ANTON ZEILINGER:
The experiment is just beautiful. NARRATOR:
...we're using it to create revolutionary new technologies. What we have here is a quantum playground. JIAN-WEI PAN:
We want to push these technologies as far as possible. NARRATOR:
It's perhaps the strangest concept in physics. SHOHINI GHOSE:
We're left with conclusions that make no sense whatsoever.

NARRATOR:
Yet it could be what forms
the very fabric of our cosmos. In the end, we just have
this quantum mechanical world. There is no space anymore. GHOSE:
It's like being
in Alice in Wonderland. Everything is possible. NARRATOR:
Could it be real? It's
Einstein's Quantum Riddle, right now, on NOVA. ♪ ♪ ♪ ♪ NARRATOR:
Is reality an illusion? Could something here
mysteriously affect something there? A century of discoveries
in physics reveals a strange,
counterintuitive micro-world of atoms and tiny particles that challenges
our intuitive understanding of the world we see around us. It's known as quantum mechanics. This strange theory has
enabled us to develop the remarkable technologies
of our digital age. But it makes
a very troubling prediction-- called quantum entanglement. ♪ ♪ GHOSE:
Entanglement is this very powerful
but strange connection

that exists
between pairs of particles. ROBBERT DIJKGRAAF:
Even if they're very far apart, in a way,
they're always coordinated. ♪ ♪ NARRATOR:
Nature's fundamental building
blocks could be connected and influence each other
instantaneously, as if the space between them
doesn't exist. As if two objects can mirror
each other without any apparent connection. ♪ ♪ Einstein called it
spooky action at a distance. He rejected the idea and tried to prove
it couldn't be real. GHOSE:
You could have situations where the cause and the effect
happen at the same time. NARRATOR:
But if entanglement isn't real, cutting-edge technologies
could be in jeopardy. KAISER:
Quantum computers,
quantum encryption-- they depend on entanglement
being a fact in the world. ♪ ♪ NARRATOR:
Underlying it all is a profound question:

Do we live in Einstein's
universe of common-sense laws, or a bizarre quantum reality that allows spooky connections
across space and time? 300 miles off the coast
of West Africa, on one of the Canary Islands, a team of physicists is setting
up a remarkable experiment that will use almost the
entire breadth of the universe to settle the question: Is the seemingly impossible
phenomenon of quantum entanglement an illusion, or is it actually real? Leading the team
is Anton Zeilinger. ZEILINGER:
So we're now going up
the mountain towards
the Roque de los Muchachos. So everything looks perfect
today. NARRATOR:
It's a precarious undertaking. They've got a short window on two of Europe's
largest telescopes. Each one will simultaneously
focus on a different quasar, an extremely distant galaxy emitting huge amounts of light
from its core.

♪ ♪ This light will be used to control precise equipment that must be perfectly aligned to make measurements on tiny subatomic particles. (speaking German): MAN (on radio): Okay. NARRATOR: And if that isn't tricky enough, the weather on the mountain is notoriously unpredictable. The team needs perfect conditions for the experiment to work. ♪ ♪ ZEILINGER: In the end it could be running smoothly or there need to be a couple of decisions made, you know, in an excited state in the last instant. ♪ ♪ NARRATOR: With the experiment finally set up, the team takes their positions. ♪ ♪ David Kaiser has worked on this experiment with his colleagues Jason Gallicchio and Andy Friedman for four years. Coordinating it all is Dominik Rauch. The experiment is his thesis project, and it's been years in the making. But as darkness falls, temperatures on the mountain begin to drop.

MAN (speaking German, on radio): (speaking German) Okay, 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. KAISER:
Too icy? Yeah. That's okay. ♪ ♪ ♪ ♪ NARRATOR:
The next day, the team prepares
for another attempt. ♪ ♪ They verify the equipment hasn't
been affected by the weather. But now, the air is thick
with clouds. ZEILINGER:
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. (wind whistling) (phone ringing) NARRATOR:
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. Why are physicists so determined to put this bizarre aspect
of quantum mechanics to the ultimate test? To explore the beginning
of the story, David Kaiser has come
to Brussels, the city that Albert Einstein
traveled to in 1927 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. GHOSE:
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 the planet. Then you are in the world
of atoms and particles. NARRATOR:
It was the nature
of fundamental particles, which 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 would trouble him
for the rest of his life. ♪ ♪ David Kaiser has come
to the place where it all began. ♪ ♪ KAISER:
This is the original
Solvay Institute building, a beautiful grand building. And this is the place, back in October 1927, where the fifth
Solvay Conference was held. This amazing, weeklong series
of discussions on really
what the world was made of, on 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's a collection of the 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 Schrödinger. And these sort of brash
20-year-olds, or mid-20s, Werner Heisenberg,
and Wolfgang Pauli. NARRATOR:
These scientists were the
pioneers of quantum mechanics. KAISER:
I had a huge version
of this photograph up on the wall. It was a poster
in my college dorm room. My roommates had
their favorite bands, and I had
the 1927 Solvay Conference, which says a lot. NARRATOR:
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. KAISER:
So this, this group here, these, 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 was 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. ♪ ♪ NARRATOR:
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's observed-- 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 behavior, the only thing we can describe is the probability of being at one place or another. CARROLL: 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. NARRATOR: Quantum mechanics only tells us the probability of a particle's properties, like location. The 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.

♪ ♪ NARRATOR:
For Einstein, the idea that particles only pop
into existence when they're observed is akin to magic. It's said he asked, Do you really believe
the moon is not there when you are not looking at it? ♪ ♪ Outside of the formal setting
of the conference... KAISER:
Bonsoir. NARRATOR:
...he challenged the most vocal
supporter of these ideas: the great Danish physicist
Niels Bohr. KAISER:
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 suppertime, 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 in the end, by supper,
have crushed that one, and it would start
all over again. NARRATOR:
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
his conviction that quantum mechanics
was flawed. And in his refusal to accept the weird implications
of the theory, he would wind up uncovering
something even weirder. In 1933, with the Nazi Party
in power in Germany, Einstein chose to settle
in America and took a position at the
Institute for Advanced Study in Princeton, New Jersey. He recruited two physicists
to help him.

Nathan Rosen and Boris Podolsky. And in 1935, at afternoon tea, the three men spotted a possible flaw in quantum mechanics that would shake the very foundations of the theory. They noticed that the mathematics of quantum mechanics led to a seemingly impossible situation. Today, Robbert Dijkgraaf is the director of the institute. DIJKGRAAF: Apparently Podolsky would say, Well, Professor Einstein, this is very important in your arguments showing that quantum theory is incomplete. So they got this very animated discussion and what can happen 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. ♪ ♪ NARRATOR: 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 under a cup. 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 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 physicists, including Anton Zeilinger. ZEILINGER:
The Einstein-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. NARRATOR:
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. 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. It's really mind-boggling. ♪ ♪ NARRATOR:
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 could 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 seemed to imply, and that, they said, so much the worse
for quantum mechanics. The world simply can't operate
that way. NARRATOR:
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 its famous equation,
E = MC squared. CARROLL:
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.

NARRATOR:
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 traveling 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 traveling 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 theory. So Einstein, Podolsky, and Rosen
argued that quantum mechanics

was incomplete. ♪ ♪ DIJKGRAAF:
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 was really upset
with Podolsky, and apparently, he didn't speak
to him anymore. ♪ ♪ NARRATOR:
When Niels Bohr heard
of Einstein's paper, he wrote an obscure response, arguing that one particle
could somehow mysteriously influence
the other. This seemingly impossible
phenomenon became known
as quantum entanglement. But Einstein dismissed it as
spooky actions at a distance. No one could 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 to do new things. GHOSE:
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. NARRATOR:
Entanglement aside, the equations
of quantum mechanics enabled the scientists
of the Manhattan Project to develop the atomic bomb. And in the years
after the Second World War, researchers at Bell Labs
in New Jersey used quantum theory to develop
one of the first lasers... MAN (in film):
In our laboratories, men experiment with a light once undreamed-of
in the natural world. NARRATOR:
...and build small devices that could control
the flow of electricity-- transistors. MAN 2 (on film):
It's destined to play
a vital role in your future, your electronic future. NARRATOR:
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. KAISER: Bell was a very talented young physics student, but he quickly grew dissatisfied with what he considered almost, almost a kind of dishonesty among his teachers. (talking in background) NARRATOR: Bell insisted that Einstein's questions about quantum mechanics had not been addressed. KAISER: He got into shouting matches with his professors. Don't tell us that Bohr solved all the problems. This really deserves further thought. BELL: Quantum mechanics has been fantastically successful. So it is a very intriguing situation that at the, at the foundation of all that impressive success, there are these great doubts. ♪ ♪ CARROLL: 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.

NARRATOR:
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-- a world that followed
common-sense laws-- or Bohr's-- a world that was deeply strange.

and allowed
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,
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. NARRATOR:
A few years later,
completely by chance, a brilliant
experimental physicist stumbled upon Bell's article. JOHN CLAUSER:
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? NARRATOR:
John worked on Bell's theory

with fellow physicist
Abner Shimony, and at the University
of California, Berkeley, started work on an experiment
to test it. He had a talent for tinkering
in the lab and building the parts
he needed. CLAUSER:
I used to rummage around here and scavenge and dumpster-dive
for old equipment. NARRATOR:
He knew where to find hidden
storage rooms, like this, which he could raid
to salvage spare parts for his experiments. (grunts) CLAUSER:
This was a power supply
for diode lasers. That looks like
something I built. ♪ ♪ Here is a picture
of the experiment I did. (chuckling):
I had more hair in those days. Here's another picture. This is of Stu Freedman, who worked on it with me. NARRATOR:
Piece by piece,
John Clauser and Stuart Freedman constructed the world's first
Bell test experiment. They focused a laser
onto calcium atoms,

causing them to emit
pairs of photons that the equations
of quantum theory suggested should be entangled. They recorded whether or not
the photons passed through filters on each side, and checked how often
the answers agreed. After hundreds of thousands
of measurements, if the pairs were
more correlated than Einstein's physics
predicted, they must be spookily entangled. We saw the stronger correlation characteristic
of quantum mechanics. We measured it,
and that is what we got. ♪ ♪ NARRATOR:
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? Remarkably, the first people to react
to this extraordinary result were not the world's
leading physicists. ♪ ♪ CLAUSER:
Ronald Reagan's definition
of a hippie was someone who dresses
like Tarzan.

has hair like Jane and smells like Cheeta. (laughs) ♪ ♪ NARRATOR: A small group of free-thinking physicists at the heart of San Francisco's New Age scene got in touch with John. KAISER: They called themselves the Fundamental Fysiks Group. They spelled physics with an F. Some members would experiment with psychedelic drugs. I mean, they were, they were kind of in the flow of the kind of hippie scene. And that group was just mesmerized by the question of entanglement. ♪ ♪ CLAUSER: The idea was just to discuss fringe subjects with an open mind. And I thought, Oh, sure! Uh, 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. NARRATOR: 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. It was the great cosmic oneness. NARRATOR:
The group held meetings
at the iconic Esalen Institute. CLAUSER:
It was a marvelous,
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. ♪ ♪ (chuckles):
But it was fun. NARRATOR:
The Fundamental Fysiks 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. ♪ ♪ 40 years later, cutting-edge labs
around the world are now racing
to harness quantum entanglement

to create revolutionary
new technologies... ♪ ♪ ...like quantum computers. ♪ ♪ GHOSE:
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 a 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. ♪ ♪ NARRATOR:
A quantum bit makes use of the
fuzziness of the quantum world. A qubit, as it's known, can be zero or one,
or a combination of both. A particle
or tiny quantum system can be made into a qubit. And today, it's not just
pairs of particles that can be entangled.

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 succeeded
in creating a tiny chip that holds an array
of 72 qubits. The task for researcher
Marissa Giustina and her colleagues is to send signals
to these microscopic qubits to control and entangle them. GIUSTINA:
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. NARRATOR:
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 that 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 get hold of quantum computers? GHOSE:
A quantum computer could crack
our best encryption protocols in minutes, whereas a regular computer, or even a super-computing
network today

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

NARRATOR:
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, Jian-Wei Pan runs a leading
quantum research center. 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 eavesdrop.

Jian-Wei's team has created
a network of optical fibers more than a thousand miles long that can carry
secure information from Beijing to Shanghai. It is used by banks
and data companies. But there's a limit to how far
quantum signals can be sent through optical fibers. To send signals further,

Jian-Wei's team launched the world's first quantum
communication 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 on and allows signals to be sent
and received. The team aims to use
this equipment to create a new, secure
communication system using quantum entanglement. The satellite sends
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. 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. PAN:
So the next step is, we will have ground station,
for example, in Canada.

and also in Africa
and many countries. So, we will use our satellite for the global
quantum communication. We want to push this technology
as far as possible. 

NARRATOR:
These are the first steps in creating a completely
unhackable quantum internet of the future-- made possible
by quantum entanglement. But there's a problem. What if quantum entanglement-- spooky action at a distance-- isn't real after all? It could mean entangled photons
are not the path to complete security. The question goes back to Clauser and Freedman's
Bell test experiment. 

♪ ♪ In the years
after their pioneering work, physicists began to test
possible loopholes in their experiment-- ways in which
the illusion of entanglement might 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 show more agreement 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? KAISER: 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. ♪ ♪ NARRATOR: It is this loophole that the team at the high-altitude observatory

in the Canary Islands is working to tackle. ♪ ♪ 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. The team is 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 about a third of a mile
from each of the detectors. 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. These are among the brightest
objects in the sky, emitting light in powerful jets. Random variations in this light will control
which filters are used to measure the photon pairs. And since the quasars
are so far away--

the light has been traveling
for billions of years to reach Earth-- it makes it incredibly unlikely that anything could be
influencing the random nature of the test. If the experiment is successful, the team will have tackled
the loophole and shown
that quantum entanglement is as spooky
as Bohr always claimed. Dominik and Jason are
at one telescope. Hello, Anton. NARRATOR:
Anton is at the other. (speaking German): ZEILINGER
(speaking German on phone): RAUCH (speaking German): (both speaking German) ♪ ♪ NARRATOR:
With clear skies
finally overhead, the huge telescopes awaken... ♪ ♪ ...poised to collect light
from distant quasars. ♪ ♪ Moving. MAN (speaking German on radio): MAN 2:
All right. Dark count level. MAN (on radio):
Okay, this is good. RAUCH:
So we're doing everything... ...everything at once now.

So the guys for the links are setting the state
of the entangled photon pair. We're trying to acquire
the quasar. We're just centering it and making the field of view
as small as possible, to be sure that we only have
the quasar. Okay. It's guiding now? Yes. Let's wait for
one more image. Okay. Of this one. MAN:
All right. Great, great, great,
great, great. Yeah, that's good. Looks like 90, let's say 91 to be conservative,
of purity. NARRATOR:
With the telescopes
now locked on to two different quasars, the team begins
to take readings. MAN (on radio):
The red counts, 12,000. Blue counts, 7,000. ♪ ♪ We did a full,
the full cosmic Bell test. MAN:
What? Yeah, we're doing
a full cosmic Bell test.

NARRATOR:
It's working. Light from the quasars is
selecting which filters are used to measure
the entangled photons. RAUCH:
It is exciting. It is. Now we do have a test, but it's not clear
what the outcome will be. ♪ ♪ MAN:
Moving. (man talking indistinctly
on radio). MAN:
All right. Everything is exactly the same,
beautiful, perfect, yeah. ♪ ♪ ♪ ♪ NARRATOR:
Two months later,
back in Vienna, the team analyzes
the experimental data. RAUCH:
This might take a second. The numbers look
really great. And it is extremely pleasing
to see that all this worked
so nice. We clearly see
correlations that correspond
to quantum mechanics. NARRATOR:
The results show entanglement. ♪ ♪ 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. ZEILINGER:
The experiment we did
is just fantastic. The big cosmos comes down to control
a small quantum experiment. That, that in itself is a,
is, is beautiful. ♪ ♪ You know, honestly, I still,
I still get chills. I mean... ...when I realize what our team
was able to do, in this intellectual journey
that stretches back to the early years
of the 20th century. There's, 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. ♪ ♪ NARRATOR:
Accepting that entanglement
is a part of the natural world around us

has profound implications. It means we 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. It's almost like being
in Alice in Wonderland, right? Where everything is possible. ♪ ♪ NARRATOR:
It was first seen as an unwelcome
but unavoidable consequence of quantum mechanics. Now, after nearly 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 described, researchers are now using it in their search for 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 have 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. NARRATOR:
What if space itself
is actually created by the tiny quantum world? Just like temperature,
warm and cold, consists simply of the movement
of atoms inside an object, perhaps space as we know it emerges from networks
of entangled quantum particles. It's a mind-blowing idea. DIJKGRAAF:
What we are learning these days is that we might have to give up
that 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. NARRATOR:
In a radical theory-- known
as the holographic universe-- space and time are created
by entangled quantum particles on a sphere
that's 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. NARRATOR:
It may be impossible
to intuitively understand this wild mathematical idea, but it suggests that 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 anymore. And so in some sense,
the paradoxes of entanglement... The EPR paradox
disappears into thin air. ♪ ♪ CARROLL:
Truly understanding
quantum mechanics will only happen
when we put ourselves on the entanglement side, and we stop privileging
the world that we see, and start thinking
about the world as it actually is. ♪ ♪ KAISER:
Science has made
enormous progress for centuries by sort of breaking complicated
systems down into parts. When we come to a phenomenon
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. ♪ ♪ ZEILINGER:
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. To order this program on DVD, visit ShopPBS
or call 1-800-PLAY-PBS. Episodes of NOVA are available
with Passport. NOVA is also available
on Amazon Prime Video.

Einstein's Quantum Riddle | Full Documentary | NOVA | PBS

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Quantum entanglement is a strange phenomenon where particles can instantly influence each other, challenging our understanding of space and time.

The experiment on the Canary Islands aims to determine if quantum entanglement is real, using distant quasars to make measurements on subatomic particles.

The challenging setup and unpredictable weather on the mountain create obstacles for the team, putting their time-sensitive experiment at risk.

Physicists are determined to test quantum entanglement, which allows particles to instantaneously influence each other's state.

The concept of quantum mechanics, which describes the behavior of particles at the atomic and subatomic levels, was born out of heated debates among brilliant scientists in Brussels in 1927.

The theory's strange principles, including the idea that particles are not physically real until they are observed, troubled Einstein and led to ongoing debates with Niels Bohr.

Einstein's Challenge to Quantum Mechanics

Einstein and his colleagues were confident in their ideas about quantum mechanics, despite Einstein's refusal to accept its implications.

In 1935, they published the EPR paper, which pointed out a seemingly impossible connection between particles, challenging the foundations of the theory.

The EPR paper's argument about the unison behavior of separated particles conflicted with Einstein's concept of space and the principle of locality, leading to his challenge of quantum mechanics.

Podolsky's enthusiasm for a project led to a rift between him and Einstein.

Niels Bohr's response to Einstein's paper introduced the concept of quantum entanglement.

John Bell's groundbreaking paper and the experiment by John Clauser and Abner Shimony provided the evidence that quantum entanglement was real, challenging Einstein's worldview.

The Discovery of Quantum Entanglement and its Modern Applications

Quantum entanglement was confirmed through an experiment showing stronger correlation than predicted by Einstein's physics.

A group of free-thinking physicists, the Fundamental Fysiks Group, was ahead of their time in recognizing the significance of entanglement.

Today, quantum entanglement is being harnessed for revolutionary technologies, such as quantum computers, which utilize entangled qubits for greater processing power.

Quantum computers can solve complex problems, like finding the shortest route between multiple cities, in a fraction of the time it would take a regular computer.

They also pose a threat to traditional encryption but offer a secure communication system protected by the laws of physics.

Researchers in China are using quantum entanglement to create a global communication network, using optical fibers and the world's first quantum communication satellite.

Testing Quantum Entanglement on a Cosmic Scale

A team in the Canary Islands is conducting a giant version of Clauser and Freedman's Bell test, using entangled particles and light from distant quasars.

The experiment aims to settle the Einstein-Bohr debate and demonstrate the spooky nature of quantum entanglement.

The results show strong evidence of entanglement, closing the loophole and strongly supporting the validity of quantum mechanics.

The Profound Implications of Entanglement in Modern Physics

Entanglement is now at the heart of modern physics, challenging common sense notions about the universe.

Researchers are exploring entanglement in the search for a unified theory of the universe, aiming to reconcile quantum mechanics and general relativity.

A radical theory suggests that space and time may be created by entangled quantum particles, potentially forming the true fabric of the universe.