♪ ♪ ANNOUNCER: The following "NOVA" program contains scenes of quantum physics, which is known to cause confusion, anxiety, and even heartbreak.

Please see your physicist if symptoms persist.

NARRATOR: Quantum physics.

It's the science of the very small, but it punches far above its weight.

Quantum physics has not just been important, it's been revolutionary.

NARRATOR: It's the most successful scientific theory of the last 100 years.

Quantum mechanics already permeates everything we do.

NARRATOR: Everything from your computer or cellphone to how we keep time depends on our understanding of the quantum world.

DAVID KAISER: We can say now that we live in a quantum age.

NARRATOR: And it's behind one of the greatest discoveries in the history of science: gravitational waves, tiny ripples in the fabric of space-time itself.

SEAN CARROLL: Gravitational waves give us a whole new way to look at the universe.

NARRATOR: And yet, beyond the mathematics, quantum physics makes a shocking claim: that at its deepest level, reality plays like a game of chance... ELBA ALONSO-MONSALVE: Probabilities are not a measure of what we don't know.

They're just intrinsic to the quantum theory.

NARRATOR: ...with mind-boggling behaviors like superposition and entanglement.

This is weird-- it's strange.

NARRATOR: What quantum physics really means remains deeply mysterious.

But it's created the world we live in today.

Quantum physics actually governs everything around us.

TARA FORTIER: It's not some weird outpost of physics that's far away.

It's completely changed the way we used to live into the way we live now.

NARRATOR: "Decoding the Universe: Quantum."

Right now, on "NOVA."

♪ ♪ NARRATOR: December 12, 1970.

NASA launches a Scout B rocket from a former oil-drilling platform off Kenya's coast.

♪ ♪ Its payload is a small satellite named Uhuru, a Swahili word meaning "freedom."

Uhuru is the first space telescope dedicated to observing X-rays, high-energy light waves invisible to our eyes.

Powerful sources of X-rays constantly bombard Earth, but our atmosphere blocks them.

With this groundbreaking telescope, a new vista for exploration opens.

But buried in the data collected from Uhuru is something ominous.

In 1971, scientists reveal that the constellation Cygnus, the Swan, contains what until then was more of a mythical mathematical beast.

A black hole.

ALONSO-MONSALVE: Black holes are the most mysterious objects in the universe.

Also the most violent.

JANNA LEVIN: Even Einstein didn't think nature would allow such a crazy object.

NARRATOR: Black holes are fearsome monsters, capable of devouring whole planets... ...whole stars... ...and even each other.

A black hole is created when gravitational forces bring together enough mass to put a rip into the fabric of space-time.

KAISER: Some of them are genuinely monstrous.

I mean, millions, billions, maybe even ten billion times the mass of our own sun.

ALONSO-MONSALVE: We don't actually have laws of physics to predict what's going to happen to us when we go in.

Hopefully, none of us will experience it anytime soon.

(laughs) NARRATOR: In the decades since the first sighting, science has learned a lot about these menacing and mysterious objects of destruction.

They aren't that rare.

Supermassive black holes sit at the center of most large galaxies.

We have one in ours.

But it turns out these cosmic behemoths also may have an Achilles' heel.

First predicted by Stephen Hawking in 1974, scientists thought of a black hole as a one-way trip to oblivion.

That past its event horizon, nothing could escape.

But Hawking disagreed.

He theorized something did escape from these mighty giants: radiation.

Ironically, the end result of physics at the tiniest of scales: quantum physics.

♪ ♪ Clifford Johnson is a nonfiction graphic author and also a theoretical physicist.

JOHNSON: One of the key things that was discovered in quantum physics is that empty space itself is not empty.

It's seething with possibility.

Instead of having empty space here, a particle and its antiparticle can appear, dance around a little bit, and then annihilate back into empty space.

Now, imagine that happening near a black hole horizon, which we're told is a one-way door.

What if one of those particles falls in?

And now the partner doesn't have anything to annihilate with.

So it will actually fly off, and a distant observer will see that particle as radiation coming from the black hole.

NARRATOR: Without consuming more matter, if it emits radiation, it will gradually shrink in size.

The black hole actually begins to evaporate.

Now, this is a completely stunning revelation.

NARRATOR: Known as Hawking Radiation, its existence is still only a theory.

But perhaps, given enough time-- and it is a very, very, very long time... For many black holes, longer than the current age of the universe.

NARRATOR: ...even a supermassive black hole, like the one at the heart of the Milky Way, may evaporate and disappear, vanquished by the quantum world and the physics of the very small.

♪ ♪ The quantum world is often cast as weird, and it sure can look that way in the movies.

JANET VAN DYNE: You're sending a signal down to the Quantum Realm.

(woman yelps) Cassie!

♪ ♪ (whispers): Where are we?

NARRATOR: But what is quantum physics?

It arose as the solution to a problem.

Science during the 19th century had investigated smaller and smaller amounts of matter and energy.

But by the first two decades of the 20th century, the existing line between the physics of particles and the physics of waves had grown murky, especially when trying to understand the fundamental nature of light.

KAISER: Sometimes it really is important to describe light as a wave, as an extended object that sort of waves in space and travels over time, analogously to an ocean wave in the water.

Other times, as people like Albert Einstein and others began to, to find, they really, really had to describe aspects of light as if it was a collection of particles that traveled almost like miniature billiard balls.

NARRATOR: Ultimately, the answer was a new kind of physics, quantum mechanics, which included an amalgam of ideas about both particles and waves.

Its earliest formulation dates back roughly 100 years.

This 1927 conference in Brussels is where the world's leading physicists met to discuss the newly formed theory.

(people talking in background) NARRATOR: And there was a lot to discuss.

Because quantum mechanics represented a radical departure from the previous paradigm of physics-- what we call today "classical physics."

CARROLL: In classical physics, handed down by Newton, we had determinism.

We had the clockwork universe.

So if you throw a ball-- that's a classical object-- with the same force, the same speed, the same angle, it's always going to go to the same place, right?

In principle, if you knew exactly the state of the whole world all at once, and you knew the laws of physics, you could exactly predict what everything was going to do arbitrarily far in the future and into the past.

NARRATOR: In classical physics, even events that we think of as random aren't, really.

HAKEEM OLUSEYI: There are things that appear random in our everyday lives, like rolling dice.

It looks random, right?

But actually, it's a deterministic set of events which leads to whatever outcome the dice shows.

If I told you exactly how I was going to roll the dice... OLUSEYI: ...you could predict, based on that initial throw, what the final outcome is going to be.

It's a very hard mathematical problem, but it's not intractable.

Quantum mechanically, that's not the case.

NARRATOR: Quantum mechanics tossed out the certainty of the classical clockwork universe for one that only allowed for probabilistic predictions about potential observations.

Probability in quantum physics is different.

Because even if we have the most complete description that the laws of physics will allow us to have, typically, we're unable to predict precisely what we'll see when we observe a quantum system.

CARROLL: Quantum mechanics says we can know everything there is to know about the setup right now.

And still, when we want to make a measurement of it in the future, the best we can do is say, "There's a 50% chance of getting this outcome, 30% chance of that, 20% chance of that."

In the quantum theory, probabilities are not a measure of what we don't know.

They're just intrinsic to the quantum theory.

We cannot get around them.

That is impossible.

NARRATOR: Some physicists, raised on determinism, had trouble accepting this new probabilistic view.

Albert Einstein famously said that he didn't believe God plays dice with the universe.

But there is another related, even stranger aspect to quantum mechanics.

In classical physics, external reality is independent of the observer.

Looking at the moon doesn't change the moon.

And if you look away, the deterministic laws of physics continue to guide the moon on its path.

But in quantum mechanics, things are weirder.

CARROLL: The basic idea of quantum mechanics, the thing that we really struggle with to get our heads around, even as professional physicists, is that unlike any other version of physics, quantum mechanics separates what happens in a system when we're not observing it from what we see when we measure it.

NARRATOR: A few rare exceptions aside, quantum mechanics says that we can't know the position of a particle like an electron when we're not observing it.

At best, it can only be described mathematically as a wave, its exact position given in probabilities.

But at the moment that the particle is observed, the probabilistic wave function collapses to one specific location.

To the observer, who never sees this wave-like quality, it is like the particle was a particle all along.

That opens up a whole world of questions, you know?

What happens to the observational outcomes that are not observed?

What picks out which outcome is going to happen?

This is still what we're thinking about today.

NARRATOR: During that mysterious period, when the particle is considered neither here nor there, it is said to be in superposition-- in a sense, a combination of all the possible outcomes.

But what does that really mean?

Is the electron everywhere at the same time?

Is it nowhere at all?

Is it at one particular place and we just don't know?

All of those questions are actually outside of what quantum theory itself actually can answer.

It's not part of the theory at all.

So if you ask me, your guess is as good as mine.

(chuckles): Unfortunately, that's the best I can do.

♪ ♪ NARRATOR: For most people, quantum mechanics remains deeply unintuitive.

And yet it has proven itself again and again by making predictions with uncanny accuracy.

In practical terms, it is the most successful theory science has ever produced, and it has shaped our modern life.

♪ ♪ OLUSEYI: Quantum physics has not just been important, it's been revolutionary.

It's completely changed the way we used to live into the way we live now.

♪ ♪ NARRATOR: Take our sense of time.

(tango music playing) Perhaps there is no better illustration of our intimate relationship with it than music and dance.

♪ ♪ FORTIER: The underlying movement of tango is reliant on the beat, which is reliant on timing, which creates synchronization.

To create a truly smooth dance, it's not enough to just be synchronized on the beat.

It's also the synchronicity between the beats that's important.

That's the real beauty in it, in finding that connection through, stretching out that second.

♪ ♪ NARRATOR: Tara Fortier is a tango professional and a physicist deeply involved in the science of time.

So, we have a number of systems in this lab.

NARRATOR: She works here...

These systems are used to characterize atomic clocks, and also compare atomic clocks.

NARRATOR: ...at the Boulder, Colorado, laboratories of the National Institute of Standards and Technology, or NIST, home to some of the atomic clocks that help set the official time for the country.

Over the centuries, we've tracked time a variety of ways: by the sun's movement, the swing of pendulums, the oscillations of springs, and, in the 20th century, the vibrations of quartz crystals.

But since the 1960s, time has been officially determined using atomic clocks and the quantum characteristics of atoms.

And the idea is that the laws of physics are unchanging, unlike something like the rotation of the Earth.

The rotation of the Earth itself can change because of plate tectonics, because the moon is moving away from the Earth.

Its physics is not truly fundamental.

JUN YE: The reason why we love atomic clock, it's a universally defined time.

No matter who does the experiment, no matter where you do the experiment, you know, in principle, once you've corrected for all the systematic effects, you should produce the same time no matter where.

NARRATOR: The consistency of atomic clocks arises from the very nature of atoms.

Atomic clocks depend crucially on the quantum physics of atoms.

You have a nucleus, around which there are electrons in certain energy levels.

And these energy levels are possible energy states that the electron can have inside the atom.

NARRATOR: Since an electron can only be at certain energy levels and not in between, to get to a higher level, it needs to encounter a very specific helping hand, such as a particular photon.

If it were to absorb an incoming photon, it would have to be of just the right energy to jump from one level to a higher level.

NARRATOR: That special relationship between the electrons of a particular atom and a photon of a specific energy level is a unique signature for that atom.

It's called a "resonant frequency."

JOHNSON: So this characteristic signature of this atom gives us a very specific frequency standard that we can use to build a time-keeping device.

NARRATOR: Atomic clocks work in different ways, but they all use a specific type of atom or molecule as a reference to lock in the frequency of an electromagnetic wave, whose oscillations provide the "ticking" of the clock.

Today, a second is officially defined by counting the oscillations of the primary resonant frequency of a cesium-133 atom.

That's over nine billion oscillations per second.

And you interact with that time reference more than you might think.

For example, through the Global Positioning System: GPS.

♪ ♪ GPS: Turn left.

FORTIER: I think that GPS is actually kind of crazy, when you think about it.

How did we do anything before GPS?

NARRATOR: The U.S.-based GPS system uses over 30 dedicated orbiting satellites, each with multiple atomic clocks.

When you use the GPS on your cell phone, its receiver checks the signals from four or more satellites.

The signal contains information about the satellite's position and the time it sent the signal.

That time stamp is critical.

Your phone uses it to calculate how long it took to receive the signal, and from that, knows the distance to the satellite.

With that information from multiple satellites, it is possible to triangulate the phone's position within a few yards.

But the whole system depends on knowing the time.

FORTIER: In the end, I find it amazing, how strongly we're committed and tied to atomic clocks and how much we take it for granted.

Even though I build atomic clocks, but when I'm driving, being guided by this GPS service, you don't really become aware of how much atomic clock technology has permeated everywhere in modern life.

♪ ♪ Have you had a chance to look at more systematically varying the V-Z?

NARRATOR: Jun Ye is a physicist with joint appointments: with NIST, the University of Colorado- Boulder, and their joint institute, JILA.

What if you locked exactly on top of each other and see whether that peak disappears completely?

NARRATOR: He works on the new generation of atomic clocks, known as optical atomic clocks.

While cesium clocks use microwaves, optical clocks use lasers, which run at higher frequencies.

That also means using a different atom.

Instead of cesium, Jun's work mostly uses strontium atoms, along with a laser carefully tuned to one of strontium's resonant frequencies.

It puts one of the strontium electrons into superposition, so it is both excited and unexcited at the same time, creating what Jun calls a quantum pendulum.

This pendulum is swinging at a speed of nearly one million billion cycles per second.

It's going back and forth, back and forth.

And this superposition creates this quantum pendulum.

NARRATOR: And when it comes to accuracy, more swings or higher frequency equals more precision.

If you think of swings as marks on a ruler, the more marks you have, the more exactly you can measure.

So, compared to a cesium clock, Jun's strontium clock is around 100,000 times more precise.

And that much sensitivity makes all the more apparent some of the stranger aspects of time, including one first predicted by Einstein: gravitational time dilation.

In the movie "Interstellar," part of the crew of a spaceship descends in a shuttle to a planet orbiting a supermassive black hole.

When the shuttle returns, those on the mission feel they've only been gone for three hours, but not the crew member who remained in orbit.

Hello, Rom.

I've waited years.

CASE: 23 years, four months, eight days.

NARRATOR: The difference in time is another effect of the black hole's warping of the fabric of space-time.

The warping not only means gravity gets stronger closer to the black hole, but time gets slower, too.

And you don't need a black hole to be able to measure it.

Even on Earth, gravity varies, and so does time, based on the distance from the planet's center.

So a person at the top of the Empire State Building experiences weaker gravity and time going faster than a person at street level, where gravity is stronger.

But all that happens imperceptibly.

Our wristwatches just aren't accurate enough to show the difference.

But Jun's optical clocks are so accurate that even a small difference in elevation between two clocks will reveal a discrepancy in the passage of time.

When the clock changes elevation by a few hundred microns, basically size of a human hair, you will start to be able to see that time is actually running differently.

♪ ♪ NARRATOR: With that much accuracy, a clock transforms into something more than a timepiece.

It becomes a new window into the nature of the universe.

♪ ♪ YE: Making a clock is much more than just a piece to keep time.

It is a sensor to explore fundamental physics, to expand our curiosity, to build new technologies that can connect to quantum computing, quantum information processing, and communication.

♪ ♪ NARRATOR: Central to making Jun's precision atomic clocks work are ultra-stable lasers, which themselves are also a quantum technology.

They date back to the 1960s.

GOLDFINGER: You are looking at an industrial laser, which emits an extraordinary light not to be found in nature.

I will show you.

(laser cracks) NARRATOR: This scene from 1964's "Goldfinger" is said to be one of the first popular depictions of this new, cutting-edge tech.

I think you've made your point, Goldfinger.

Thank you for the demonstration.

♪ ♪ NARRATOR: Today, lasers are everywhere.

There are medical lasers to correct vision, lasers at the checkout counter, lasers for cutting, communicating, entertaining cats, and, of course, for light shows.

(crowd cheering) NARRATOR: Which encourage us all to trip the light fantastic.

(band playing) (cheering) (plays note) (note stops) NARRATOR: Which may be why... ♪ ♪ ...experimental physicist Rana Adhikari is laser-focused on lasers.

When I talk about how, how beautiful a laser is as a instrument, I don't want to gush about it too much.

Like, I'm in love with lasers, I don't know.

I feel like a weirdo fanatic or something like that, but...

They're just, there's something about them.

NARRATOR: To understand what makes laser light so special, it makes sense to look at an ordinary light bulb-- the old-fashioned kind, with a tungsten filament.

It produces light through thermal radiation-- an electric current passing through the filament heats it up.

Its tungsten atoms become excited and vibrate at different speeds, which causes them to emit photons in all directions, across a variety of wavelengths.

Compared to a laser, this is chaos.

ADHIKARI: The way you should think about a light bulb is something like, they're just a mob of people, all singing at different pitch, so it's like a rock concert audience.

CROWD (singing): ♪ We will, we will rock you ♪ But a laser, a laser is more like if you go to Juilliard or Berklee School of Music and you go to a concert.

(singing on one pitch) ADHIKARI: It's like a choir of people who have got perfect pitch, but it's a choir of something like a million trillion people singing at the same time, the same tone.

NARRATOR: That's because laser light is generated in an entirely different way, a fact hidden in its name: stimulated emission.

Let's say we have, inside an atom, an electron that's at some excited state, some higher energy level, and now a photon of just the right frequency passes by the atom.

It triggers the atom to do something interesting.

The electron loses energy and goes to a lower energy, and emits a photon of precisely the same frequency as the one that came in.

It's going in the same direction and it has the same phase.

So what we have there is a quantum mechanical amplification process.

NARRATOR: If we place a group of those same excited atoms inside a chamber with mirrors at both ends, the emitted photons will bounce back and forth, continuing to stimulate the emission of more photons, which in turn stimulate even more photons.

One of the mirrors is only partially reflective.

It allows some of the light to escape.

Now, that light's very special.

It's composed of photons that are all the same frequency-- so, the same color-- and they're all the same phase, and all going in the same direction.

So you have this intense pure beam of light, and that's the laser.

♪ ♪ NARRATOR: Lasers have proven to be an extremely versatile tool, including for measuring distance.

Rana's work with stable high-frequency lasers takes that to an extreme.

When you use them, you're in a whole different realm of measurement than anything else that has to do with rulers and any of that other stuff.

Anybody who is, like, a real pro knows that the only thing that you ever measure is frequency.

If you're measuring anything else, you're kind of an amateur.

NARRATOR: Thanks to the fixed speed of light, the beam of a high-frequency laser has an incredibly short wavelength, perfect for measuring extremely small changes in distance.

Since 1996, Rana has been part of a project that uses laser light to measure something incredibly, unimaginably small-- and weird: tiny fluctuations in the fabric of space and time itself.

Space and time ripple.

They're not fixed things, and so, the distance between my two hands is not always going to be this if I hold them steady.

NARRATOR: The idea, like so many, goes back to Einstein.

In the early 20th century, his work led to the merging of space and time into one concept: space-time.

And he theorized that gravity was the warping of that space-time fabric by the mass of objects.

But that carried a startling implication, that the acceleration of objects with mass would create ripples in space-time that spread at the speed of light: gravitational waves.

TIFFANY NICHOLS: Gravitational waves were first predicted by Einstein, and he didn't believe it at first.

So he went back and forth through, I believe, the mid-'30s.

But his first prediction was they were too minute to ever be detected.

♪ ♪ NARRATOR: By the 1980s, that sentiment had changed, and LIGO-- the Laser Interferometer Gravitational Wave Observatory-- was founded as a joint Caltech and M.I.T.

project.

Part of Rana's work at Caltech has been to continuously improve the essential art of LIGO: laser interferometry.

This is the, where it all begins.

I'm going to show you the whole laser interferometer in here that's a prototype of the LIGO system.

NARRATOR: The basic design is easy to understand.

The LIGO interferometer has two arms at right angles to each other.

A very stable infrared laser feeds into a beam splitter, which directs half the beam down each arm.

ADHIKARI: Half of the light goes one way and half goes the other way.

And then you have mirrors at the ends, and they reflect the light back.

NARRATOR: The phase of one arm of the laser is the reverse of the other.

If all is normal, when recombined, they will cancel each other out, resulting in no signal.

But if a gravitational wave passes through, distorting space-time, the length of each arm will change, shifting the phase of the two beams.

For a brief moment, the equipment will register a signal.

Instead of having exact cancellation and destructive interference, you have a little bit of light leaking out.

And that little bit of light that leaks out is what we detect.

NARRATOR: But there is a key difference between Rana's working testbed and the real deal: size.

This is one of two LIGO installations in the United States.

While the arms of the Caltech instrument are about 44 yards long, the ones here cover about two-and-a-half miles each.

Costing hundreds of millions of dollars, LIGO was a huge gamble on an unproven idea... ...that paid off.

♪ ♪ In 2015, a signal was detected.

And it was a doozy.

LEVIN: The first event that LIGO detected was the most powerful event human beings had recorded since the Big Bang itself.

More power came out of that collision of those two black holes than was emanated by all the stars in the universe combined.

All of that power came out in the ringing of the drum of space time.

NARRATOR: Since the original event, LIGO has confirmed the detection of more than 80 others.

It is hard to overstate the significance of the discovery.

♪ ♪ LIGO is massive.

Albert Einstein predicted that gravitational waves should exist, and now we measure them.

This is the most direct observation of black holes that we've ever had.

This is a complete revolution in science.

NARRATOR: And it's all possible because of that quantum technology that has become completely embedded in our lives: the laser.

The more stable your laser is, the more things in the universe you can measure.

And there's no limit to it.

So every year, when we get lasers better and better, we'll be able to see further out into the universe and see tinier things in the microscopic nature of reality, matter, space and time-- anything like that.

You just have to keep working on this one tool and make it better and better.

♪ ♪ NARRATOR: Arguably, the most important change in quantum physics in recent decades is a deeper understanding of a special kind of shared state called quantum entanglement.

Imagine a machine that spits out pairs of coins, which, on the surface, look like ordinary coins.

If you flip one, it comes up heads or tails about 50% of the time.

Nothing strange there.

But using a pair of coins fresh out of the machine, you flip one, it comes up heads.

And then the other, it also comes up heads.

That could just be luck.

(machine chirping, crowd cheering) NARRATOR: So then you do the same thing with another fresh pair.

This time, the first coin is tails, and so is the second-- agreement again.

So you flip another pair, and then another, and another, and another.

Pair after pair, the two coins always agree on the first flip.

What's going on?

Maybe the first flipped coin, once it comes up heads or tails, is somehow telling the other coin how to behave.

To make sure that can't happen, you separate the coins by flying one to the moon and flip them at the same time, so no message could possibly travel between them.

Still, they come up in agreement.

♪ ♪ It all sounds too strange to be true, but particles really can behave like those coins.

In quantum physics, it's called "entanglement."

KAISER: Entanglement is really just a stubborn, stubborn, exciting and/or frustrating fact that takes a long time to try to get our heads around.

Entanglement is certainly the most interesting and the most confusing aspect of quantum.

It's one of these things we don't see, you know, naively in the world around us, but it is taking place deep in the materials that exist around us every day.

NARRATOR: And while you probably won't come across a coin entangler anytime soon, in the lab, scientists routinely generate pairs of entangled particles that share a quantum state so fully, they can be thought of as one quantum object.

You simply can't differentiate between them.

It's just one pure state.

It's as though you have a single entity that's spatially separated without a physical connection.

NARRATOR: Entangled particles remain connected even when they're separated by hundreds of miles-- and likely far more.

KAISER: So does that mean it can go between here and Andromeda?

Probably-- the equations give us no reason to think it wouldn't.

NARRATOR: Entanglement sounds bizarre.

Einstein derided the idea as "spooky action at a distance."

But since the 1970s, experiment after experiment has confirmed entanglement is a real quantum phenomenon.

Now, of course, many, many decades later, we know that entanglement is undeniably a part of the world.

It's how the world works at the quantum mechanical level.

We better get used to that, and now see, what can we do with it?

Because it's powerful, let's try to use it.

It's become this new tool.

Being able to create and control it might be arguably thought of as one of the biggest scientific and engineering developments of the 21st century.

NARRATOR: And that's happening on several fronts.

Entanglement has been put to work in quantum cryptography and quantum communication, in atomic clocks, and in continuing improvements to LIGO, but perhaps with the greatest fanfare in quantum computing.

And that starts with this: the qubit.

The qubit gets its name from its cousin in classical computing, the binary bit.

♪ ♪ Like its name suggests, a binary bit can only be set to zero or one.

But from such humble beginnings, much has flowed-- more or less all the computing that makes up the modern world.

All the calculations, all emails.

Whether you're talking to your friend or whether you are a NASA scientist doing some rocket calculation, all of that can boil down to just zeros and ones switching inside your computer, which is kind of amazing, that it's that universal.

NARRATOR: Despite its many successes, the binary bit is the equivalent of a light switch-- on or off.

The qubit is far more subtle.

ALONSO-MONSALVE: The special thing about a qubit is that it operates by the laws of quantum mechanics.

It doesn't have to be just in the zero state or just in the one state.

It can be in a superposition of both.

NARRATOR: That superposition creates a mathematical space often represented by a sphere.

SOPHIE HERMANS: Where a classical bit can only sit at the South Pole or the North Pole, a quantum bit can be anywhere on the surface of the sphere.

It opens up a whole new array of possibilities of mathematical operations.

NARRATOR: But a single qubit will only take you so far in computing.

LANES: One qubit by itself is not a computer, or it would be the world's smallest, most useless computer.

But when you combine them, it can provide enough computation and calculations that you can get something on the other end.

NARRATOR: Using several qubits together opens up the power of entanglement and unleashes mind-boggling levels of complexity.

PRESKILL: If I wanted to give a complete description of what's happening with just a few hundred qubits, very highly entangled with one another, I would have to write down more bits than the number of atoms in the visible universe.

And it's that extravagance of the quantum language that we wish to exploit in a quantum computer.

NARRATOR: Beyond the work being done at universities, there are about 100 companies developing qubits and quantum computing hardware.

Major players include Google, Microsoft, Amazon, and IBM.

Its hardware development effort is centered here, at the Thomas J. Watson Research Center in Yorktown Heights outside New York City.

Okay, let me introduce you to our IBM Quantum System Two.

Actually, inside here is three quantum processors, and the team is working on how you investigate algorithms that use multiple different processors.

NARRATOR: IBM's qubits employ small loops of superconducting metal.

Since superconductors require cold temperatures to operate, the center section of the computer is a refrigeration unit.

In fact, the cooling unit of a quantum computer can look so cool, it's often confused for the star of the show.

♪ ♪ LANES: So this is a dilution refrigerator.

A lot of people think this entire cool shiny machine here is a quantum computer, but that's actually not the case.

This is not a quantum computer.

This is a quantum computer, this tiny little chip down here.

(laughing): This is a freezer, basically.

But you can't deny that it is amazing-looking.

All of these fancy shiny parts are just plumbing parts and cables designed to keep the quantum computer insanely cold.

I mean, it's, like, minus-400-something degrees Fahrenheit.

Like, there's absolute zero.

We are .015 above that.

It has to be so insanely cold because we use superconductors to make our qubits.

And then furthermore, we want to remove any type of noise or thermal excitations, which can disturb the qubits and make them behave in ways that we don't like.

♪ ♪ NARRATOR: Since 2016, IBM has made its quantum computers accessible to the public over the internet.

Anyone can come up with a quantum algorithm, akin to a classical computer program, and submit it to be run.

GAMBETTA: Since we first put it on the cloud, people have run over three trillion jobs on the quantum computers.

NARRATOR: Running an algorithm on a quantum computer involves setting the initial state of the qubits, and then manipulating them in a series of steps.

To do that, on its systems, IBM uses microwave pulses.

GAMBETTA: These microwave pulses essentially either flip the qubit, create it in a superposition, or measure it.

NARRATOR: After all the manipulation, the qubits are read, collapsing their quantum state into either a zero or one.

But there's a catch.

LANES: On a quantum computer, the chip can spontaneously decay from the excited state, or the one state, into the zero state when we don't want it to.

And this can occur, you know, about every millisecond or so.

These errors are basically inherent to the quantum nature of the device.

NARRATOR: Correcting these errors is one of the built-in challenges of quantum computing.

The current generation of quantum computers are not yet able to do it themselves.

So there's one final step.

GAMBETTA: The information then comes back out.

Then it gets sent over to a computer where we do things like error mitigation, post-process the results, correct for any extra noise, and then we send it back through the cloud.

♪ ♪ NARRATOR: It is easy to imagine that quantum computing is the next phase of classical computing.

That soon, you'll see a box that says "New Qubitium chip inside!"

The most common question people always ask me, which is, like, "When will I be able to play 'Minecraft,' when will I be able to play 'Doom' on my quantum computer?"

Quantum computers are not good for everything.

In the future, there won't be quantum PowerPoint, there won't be quantum Word.

We don't need to do that, because we have classical computers and Xboxes that are perfectly suitable for those types of applications.

NARRATOR: Quantum computers function very differently and are aimed at very different tasks.

Experts see a role for quantum computers in areas like simulation of quantum behaviors in chemistry and materials, or optimization of complex systems ranging from energy distribution to database searches.

In any case, the future of quantum computing is far from written.

GHOSE: So because things are really speeding up all over the world, I think we're going to very quickly see a demonstration of a task that's been done with a quantum computer that just is well, well outside the capability of current computers.

And that'll probably happen within the next five to ten years, I would say.

The future of computing is going to have classical accelerators, it's going to have A.I.

accelerators, and it's going to have quantum computing accelerators all working together.

And for me, that's one of the most exciting things, is, how do we actually take advantage of all these different accelerators?

CARROLL: I think that a well-functioning quantum computer will be able to do certain things much, much faster.

But number one, we don't know for sure.

And number two, it might turn out, the pessimistic view of this, that those problems are kind of limited, that they're very, very specialized.

But that's all exciting, fun work in progress.

That's what makes it interesting.

♪ ♪ NARRATOR: The roots of quantum physics go back 100 years.

But only in recent decades have we started to gain control over the quantum realm.

And that has already transformed the way we live.

There has been astonishing change in the kinds of quantum systems we can build and manipulate.

Quantum mechanics itself already permeates everything we do.

They're part of how we manipulate the world.

They're part of every transistor and every computer.

LANES: It's about how things interact on a fundamental level, but it turns out we need to know how things interact on a fundamental level to do big things, as well.

KAISER: There are still deep mysteries to puzzle with.

That part hasn't gone away.

What's increased, in a way that I still find remarkable, is that these same curious, mind-boggling quantum features are now built into how people navigate the world every single day.

NARRATOR: But what about the future?

What will quantum technology offer in the coming decades?

Just like we can tell our kids, "Oh, yeah," you know, "I was born before the internet, I was born before smartphones," 50 years from now, people are going to be telling stories about technologies that are normal that today we can't even fathom.

I believe 50 years from now, people growing up won't think twice about entanglement, superposition...

I think that will be commonplace.

One of the things I love about quantum mechanics is that it seems non-intuitive to us.

It tells us that there's something beyond just what we think we understand.

We can't always rely on our intuition.

We have to rely on our understanding to make progress.

And quantum mechanics just shows us that so clearly.

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