Quantum Computers Explained – Limits of Human Technology

For most of our history, human
technology consisted of

our brains, fire, and sharp sticks.

While fire and sharp sticks became
power plants and nuclear weapons,

the biggest upgrade has
happened to our brains.

Since the 1960s, the power of our brain
machines has kept growing exponentially,

allowing computers to get smaller
and more powerful at the same time.

But this process is about
to meet its physical limits.

Computer parts are approaching
the size of an atom.

To understand why this is a problem,
we have to clear up some basics.

A computer is made up of very simple
components doing very simple things,

representing data, the means of processing
it, and control mechanisms.

Computer chips contain modules,

which contain logic gates,
which contain transistors.

A transistor is the simplest form
of a data processor in computers,

basically, a switch that
can either block or open

the way for information coming through.

This information is made up of bits,
which can be set to either zero or one.

Combinations of several bits are used to
represent more complex information.

Transistors are combined to create logic
gates, which still do very simple stuff.

For example, an AND gate sends an output
of one if all of its inputs are one

and an output of zero otherwise.

Combinations of logic gates finally
form meaningful modules,

say, for adding two numbers.

Once you can add, you can also multiply,
and once you can multiply,

you can basically do anything.

Since all basic operations are literally
simpler than first-grade math,

you can imagine a computer as
a group of seven-year-olds

answering really basic math questions.

A large enough bunch of them can compute
anything, from astrophysics to Zelda.

However, with parts
getting tinier and tinier,

quantum physics are making things tricky.

In a nutshell, a transistor
is just an electric switch.

Electricity is electrons moving
from one place to another,

so a switch is a passage that can block
electrons from moving in one direction.

Today, a typical scale
for transistors is 14 nm,

which is about 8 times less
than the HIV virus’s diameter

and 500 times smaller
than a red blood cell’s.

As transistors are shrinking
to the size of only a few atoms,

electrons may just transfer themselves to
the other side of a blocked passage

via a process called quantum tunneling.

In the quantum realm, physics
works quite differently from

the predictable ways we’re used to,

and traditional computers
just stop making sense.

We are approaching a real physical
barrier for our technological progress.

To solve this problem,
scientists are trying to

use these unusual quantum
properties to their advantage

by building quantum computers.

In normal computers, bits
are the smallest units of information.

Quantum computers use qubits, which
can also be set to one of two values.

A qubit can be any
two-level quantum system,

such as a spin in a magnetic field
or a single photon.

Zero and one are this
system’s possible states,

like the photon’s horizontal
or vertical polarization.

In the quantum world, the qubit
doesn’t have to be in just one of those;

it can be in any proportions
of both states at once.

This is called superposition.

But as soon as you test its value, say,
by sending the photon through a filter,

it has to decide to be either
vertically or horizontally polarized.

So, as long as it’s unobserved, the qubit
is in a superposition of probabilities

for zero and one, and you can’t
predict which it will be.

But the instant you measure it, it
collapses into one of the definite states.

Superposition is a game-changer.

Four classical bits can be
in one of 2 to the power of 4

different configurations at a time.

That’s 16 possible combinations,
out of which you can use just one.

Four qubits in superposition, however,

can be in all of those
16 combinations at once!

This number grows exponentially
with each extra qubit.

20 of them can already store
a million values in parallel.

A really weird an unintuitive
property qubits can have

is entanglement, a close connection that
makes each of the qubits

react to a change in the other’s
state instantaneously,

no matter how far they are apart.

This means that when measuring
just one entangled qubit,

you can directly deduce properties of
its partners without having to look.

Qubit manipulation
is a mind-bender as well.

A normal logic gate gets
a simple set of inputs

and produces one definite output.

A quantum gate manipulates
an input of superpositions,

rotates probabilities, and produces
another superposition as its output.

So a quantum computer sets up some qubits,
applies quantum gates to entangle them

and manipulate probabilities,
then finally measures the outcome,

collapsing superpositions to an
actual sequence of zeros and ones.

What this means is that you
get the entire lot of calculations

that are possible with your setup
all done at the same time.

Ultimately, you can only
measure one of the results,

and it will only probably
be the one you want,

so you may have to
double-check and try again.

But by cleverly exploiting
superposition and entanglement,

this can be exponentially more efficient

than would ever be possible
on a normal computer.

So, while quantum computers will probably
not replace our home computers,

in some areas they are vastly superior.

One of them is database searching.

To find something in a database,

a normal computer may have
to test every single one of its entries.

Quantum algorithms need only
the square root of that time,

which for large databases
is a huge difference.

The most famous use of quantum
computers is ruining IT security.

Right now, your browsing,
email, and banking data

is being kept secure by an encryption
system in which you give everyone

a public key to encode
messages only you can decode.

The problem is that this
public key can actually be used

to calculate your secret private key.

Luckily, doing the necessary math
on any normal computer

would literally take
years of trial and error.

But a quantum computer
with exponential speedup

could do it in a breeze.

Another really exciting
new use is simulations.

Simulations of the quantum world
are very intense on resources,

and even for bigger structures,
such as molecules,

they often lack accuracy.

So why not simulate quantum physics
with actual quantum physics?

Quantum simulations could provide
new insights on proteins

that might revolutionize medicine.

Right now we don’t know
if quantum computers will be

just a very specialized tool
or a big revolution for humanity.

We have no idea where
the limits of technology are,

and there’s only one way to find out!

This video is supported by
the Australian Academy of Science,

which promotes and
supports excellence in science.

Learn more about this topic
and others like it

at .

It was a blast to work with them,
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