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.
The world is widely regarded as being well and truly into the digital age, also called
the information age.
No longer are economies and industries are 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 a 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, 1’s and 0’s.
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 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 are 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 the weirdness, define the challenge of transmitting and storing
quantum information.
But first, a reminder why we want to muck around with quantum info in the first place.
First there’s the whole quantum computer thing - in those, the ability for a qubit
to hold many simultaneous states can lead to massive speed-ups in certain types of computing.
Partly motivated by the cryptographic-cracking power of the quantum computer, we also want
to 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 classical counterparts.
But these only work if you can actually send entangled quantum states between parties - that
means transmitting qubits over long distances perfectly intact.
So ultimately what is preventing from us 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 en 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 conservation of quantum information, which we’ve 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 to make the copy, you disturb the state in such a way that you will
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 reading in 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 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 intact 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 fibre optic cable, a telephone wire,
the Pony Express, whatever, while the quantum channel needs to carry intact quantum states -
so it's probably fibre 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, say Bill wants to send a message to Ted, and
that message is stored as 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 entangles 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 the measurement.
Let’s look at a more concrete example – although I have to say that this is way oversimplified.
Qubits A and B could be the polarization states of two photons.
They’re entangled so that they have opposite polarization, say one is vertical the other
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 polarization.
Photon B, which was opposite to A, must now be the same polarization as the original photon C.
At this point the original quantum state of photon C, which contains 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 a measurement of his own
qubit B, after which that qubit will be in the state qubit C was at the start.
A minor technical caveat is that you we 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.
OK, sounds easy, right? but there are complications.
It’s pretty much impossible to do all the transmissions, entanglements, and measurements
in perfect 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 a photon.
But storing delicate quantum states for any length of time is hard work – especially
if you don’t want insanely expensive supercooled devices.
Experimentalists have of course come up with a number of ingenious solutions, ranging from
storing entangled photon quantum states in a cloud of caesium atoms, a kind of quantum
atomic disk drive, or the spin-state of a single electron in a nitrogen atom embedded
in diamond crystal.
In the future, if entangled states can be maintained for long periods, it may be possible
for two people to hold a large array 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 all together, 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 matter particles.
So the current state of the art is that entangled quantum states have been transmitted with
photons using fibre 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 matter 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.
We’ve gotten pretty far sending streams of 1’s and 0’s round 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.
I’m guessing we’ll go with “quantum age” - as the quantum internet enables us
to take advantage of the incredible properties of our quantum space time.
In a recent episode we talked about Thorium and the future of Nuclear Energy!
You guys had a lot to say.
austin holbrook says we need to do a better job of breaking the common misconception that
nuclear power plants can explode like Atom bombs.
In fact, they cannot - and I wished we'd a done a better job emphasizing that.
It's really, really hard to even make a critical mass of fissile material explode like an atom
bomb - it's a very precise engineering feat.
And there's absolutely no way that a bunch of fuel rods could do that.
Deri662 points out that proliferation is an especially lower risk from thorium reactors
due to the fact that the U-233 bred by that reactor is very difficult to handle in bomb-making.
That's because it decays to protactinium isotope that itself decays to emit a very high energy
gamma ray, and that gamma radiation can fry bomb electronics and is relatively easily
detected.
This is a good point, it's not quite foolproof because the decay products leading to protactinium
can potentially be separated and removed.
That said, anything that raises the bar on proliferation is good.
I feel that the greatest proliferation threat is easy acquisition of weaponizable material
by small, crackpot outfits that probably couldn't do that separation easily.
Sam Ferguson mentions the pebble bed reactor.
Yeah, that's a good one that we had to cut out the script due to length.
Nuclear fuel - typically uranium - is encased in graphite balls.
The balls act as the moderator so that each pebble emits neutrons with the correct speed
for sustaining fission in surrounding pebbles.
They all lie in a big pile and spent pebbles are cycled out from the bottom.
It's cool because it's passively cooled by gas flow - for example, helium.
A gas coolant means that the coolant can't boil, like it did in Chernobyl.
And in fact the system has much higher temperature tolerance even if the coolant escapes - natural
air circulation should keep it sub-meltdown temperature.
One problem is that it's hard to access and inspect individual pebbles in the middle of
the pile, so cracks in the graphite can go undetected.
Still, it seems a lot safer than a traditional light-water reactor.
TACCOFSX asks why the absorption of neutrons transform U-238 into Pu-239.
Yeah, I breezed over that point.
Absorption of a neutron by itself doesn't change the element type - just the isotope.
So U-238 turns to U-239, still with 92 protons.
But U-239 is unstable and the extra neutron quickly decays into a proton, emitting
an electron and a neutrino, bumping it one up on the periodic table.
This is a common process, and it's how all the so-called trans-uranic elements get created
in fission reactors.
Cory Johnson notes that this discussion about thorium reactors is moot...
Because Fusion reactors are only 20 years away.
For those who don't find that hilarious, let me pedantically explain Cory's joke: fusion
has been projected to be 20 years away for 50 or so years.
KowashiHitori points out that Wind and solar won't build battlemechs... just saying.
Now, this is a very important point.
Funnies aside, you can't run an energy-hungry compact vehicle on solar or batteries for
very long.
Battlemechs, submarines, spaceships, DeLorean time machines - nuclear is probably the easiest
option for the badass vehicles of the future.