- When we finally have a quantum internet, you'll be able to simultaneously like and dislike this video.

But we don't yet, so I hope you like it.

(upbeat music) The world is widely regarded as being well and truly into the Digital Age, also called the Information Age.

No longer are our economies and industries solely characterized by the physical goods they produce.

And, in fact, some of the largest companies in the world produce no physical goods at all.

Digital information is a commodity in its own right.

As discussed in our previous episode, this worldwide digital economy is fundamentally reliant on certain cryptographic processes.

Currently, these processes work in the realm of classical cryptography, but one day soon, this may not be enough.

And so quantum cryptographic methods and algorithms are being developed.

However, it's one thing to design a protocol, it's something else entirely to build a system to support it.

To understand what needs to be done, we need to get to the foundations of quantum mechanics.

We need to talk about quantum information theory.

First, plain old non quantum information theory, the study of the creation, storage, and transmission of information, typically in the form of classical bits, ones and zeros.

Claude Shannon started it all with his 1948 paper A Mathematical Theory of Communication, which quantified the rate of digital information that can be transferred without error given the amount of noise in a communication channel.

Information theory has since blossomed into a full science, ultimately connecting the concept of information and certain fundamentals of physics, such as entropy and also quantum theory.

Quantum information theory parallels classical information theory, but instead of using classical bits, it deals in bits of quantum information, qubits.

Qubits enjoy all of the weirdness of quantum mechanics.

They can be in a superposition of many states at once, defined only when they're measured.

Two qubits can be entangled with each other so both of their states are determined when one is measured.

They can even be teleported.

Qubits are also subject to some fundamental restrictions, which I'll get to.

Those restrictions, on top of all of that weirdness, define the challenge of transmitting and storing quantum information.

But first, a reminder of why we want to muck around with quantum info in the first place.

First, there's the whole quantum computer theme.

Now, in those, the ability for a qubit to hold many simultaneous states can lead to massive speed ups in certain types of computing.

And partly motivated by the cryptographic cracking power of the quantum computer, we also wanna think about a quantum internet.

In fact, we already did.

In our episode on quantum key distribution, we talked about two schemes for sharing cryptographic keys that should be far more secure than their classical counterparts.

But these only work if you can actually send entangled quantum states between parties.

That means transmitting qubits over long distances perfectly in tact.

So, ultimately, what is preventing us from just setting up these networks and getting on with it?

We can already send photons of light very long distances using lasers or fiber optics, and those photons are pretty quantum.

The problem is that to transmit quantum information, we have to pay attention to individual photons, quanta of light.

To transfer classical information using light, each bit is encoded with many photons and many can be lost or altered on route without compromising the signal.

If too many photons are lost, you can just run the channel through a repeater, which reads the signal and boosts it with extra photons.

It's much harder to transmit single photons in a way that perfectly maintains their quantum state.

And it's fundamentally impossible to boost that signal by duplicating those photons.

This impossibility is referred to as the no-cloning theorem.

It simply states that you cannot take a quantum state and copy it perfectly and end up with two copies of the same state existing at the same time.

This is connected to the law of conversation of quantum information, which we have also talked about before.

It comes from the fact that every quantum state in the universe must be perfectly traceable, single quantum state to single quantum state, both forwards and backwards in time.

That prohibits a quantum state vanishing but also splitting in two or being copied.

The no-cloning theorem means that as soon as you try to read a qubit, which you have to do at some point in order to make a copy, you disturb the state in such a way that you'll never end up with two exact copies of the same quantum state.

Plus, even if you could copy it, you wouldn't really be able to transmit an entangled quantum state because the act of ridding the state to copy it would destroy the entanglement through a phenomenon called decoherence.

While it's impossible to copy a qubit, it is possible to overwrite one.

And it can be overwritten with exactly the same state but in a completely different location.

In other words, qubits can be teleported.

This doesn't allow faster than light communication nor a teleportation of actual matter, because a classical sub-light speed channel is still needed to extract the information.

But quantum information does allow us to massively extend the range over which we can send an in-tact qubit, no copying or boosting needed.

Think of it this way, two people, let's say Bill and Ted, are connected by a classical channel and a quantum channel.

The classical channel can be anything, a fiber optic cable, a telephone wire, the Pony Express, whatever.

Well, the quantum channel needs to carry in-tact quantum states, so it's probably fiber optics.

A pair of entangled particles are created and Bill and Ted receive one each via the quantum channel.

Bill has qubit A and Ted has qubit B.

Now, let's say Bill wants to send a message to Ted, and that message is stored in the state of a third qubit, qubit C, that would be only one bit of information, but you could always use more qubits.

To send that message, Bill performs a particular type of measurement on his qubits called a Bell measurement, not a Bill measurement.

Performing this measurement simultaneously on A and C entangled these qubits, but breaks the entanglement between A and B.

However, qubit B then has to be in whatever state C was in prior to this measurement.

Let's look at a more concrete example, though I have to say this is way oversimplified.

Qubits A and B could be the polarization states of two photons.

They're entangled so they have opposite polarization.

Say one is vertical and one is horizontal.

Measure one and you immediately know the other.

Now, Bill takes photon A and entangles it with photon C using a Bell measurement, so that now A and C have opposite polarizations.

Photon B, which was opposite to A, must now have the same polarization as the original photon C. At this point, the original quantum state of photon C, which contained the message, has been almost completely teleported to photon B.

The reason it's an incomplete explanation is that there is more to the quantum state of C than simply the aspect of polarization that is fixed by the entanglement.

The remaining information of the quantum state is actually obtained by observing the outcome of the process that generated the entanglement itself.

This measurement outcome is encoded in two classical bits which Bill sends to Ted along the classical channel.

Using the information in these two bits, Ted can then calibrate the measurement of his own qubit B, after which that qubit will be in the state that qubit C was at the start.

Now, a minor technical caveat is that if you are only using photonic qubits, then it's not so easy to perform a Bell measurement that will give all of the information we need for this final step.

But all of this is definitely possible with matter qubits.

Combined with a quantum key distribution protocol, this can give a mechanism for secure communication.

It can also be used to transmit quantum information over longer distances than we could normally send entangled particles, just position repeaters along the quantum channel between Bill and Ted.

Bill performs the above trick with the nearest repeater, that repeater communicates with the next repeater, and so on, until we reach Ted, who should still get a copy of the original qubit C. In principle, this can be done without the quantum channel ever becoming un-quantum, which means it stays secure.

Okay, sounds easy, right?

Well, there are complications.

It's pretty much impossible to do all the transmissions, entanglements, and measurements in prefect synchrony.

Quantum states have to somehow be stored, by Bill, by Ted, and by the repeaters in between.

This typically means transferring a quantum state between a photon and a matter particle, say an electron whose up or down spin direction can be entangled with the polarization state of the photon.

But storing delicate quantum states for any length of time is hard work, especially if you don't want insanely expensive supercool devices.

Experimentalists have, of course, come up with a number of ingenious solutions, ranging from storing entangled photon quantum states in a cloud of cesium atoms, a kind of quantum atomic disc drive, or in the spin state of a single electron in a nitrogen atom embedded in a diamond crystal.

In the future, if entangled states `can be maintained for long periods, it may be possible for two people to hold a larger ray of mutually entangled qubits, which they could use to communicate by exchanging classical Bell measurement data.

This could also be done between many individuals in a centralized node, a sort of quantum switchboard.

There are also proposals for removing the need for physical storage altogether with repeaters that are entirely photonic.

These are great because they're much, much faster than repeaters that have to transfer quantum states between photons and meta particles.

So the current state of the art is that entangled quantum state have been transmitted with photons using fiber optics and lasers, some researchers have even succeeded in bouncing entangled photons off a satellite.

These photons can then transfer their entangled states into a variety of meta-storage systems which may eventually serve as repeaters to extend the range and connect a network of these quantum channels.

Reliability and speed are not where we need them to be, but the progress is fast.

We currently live in the Information Age, but it's a Classical Information Age.

Now, we've gotten pretty fast sending streams of ones and zeros around the world, but if we could build truly quantum networks, we'll also be able to build the next generation of cryptographic protocols, distributed quantum computers, as well as achieve new levels of atomic clock synchronization and extreme precision in our interferometric telescopes.

The Quantum Information Age is around the corner, but I'm guessing we'll just go with Quantum Age as the quantum internet enables us to take advantage of the incredible properties of our quantum space time.