Quantum computing is getting quite a lot of coverage these days in the media, which I
think is fantastic, but when I read some of the things, there are a few things that are
a bit wrong.
And I'm not really criticising anyone, quantum computing is very hard.
But I think there is some room for some more nuance, so here are my top five clarifications
about quantum computing.
A standard description of quantum computing normally goes something like this.
Quantum computers are made of qubits which can be in a state of 0 and 1 at the same time.
All of the qubits are then entangled so they are all treated as one object that is in many
different states at the same time.
And so a quantum computer is like an infinitely parallel computer.
Now that’s not quite right.
So it’s true that quantum computers are in multiple states at the same time, before
you measure them, but as soon as you measure them, you only get one state back.
Parallel computing deals with breaking down large problems in small chunks which can each
be solved on a processor before the whole thing is recombined.
But the architecture is very different to quantum computers in a parallel computer you’ve
got many different independent processes running at the same time rather than an entangled
state.
And also you can read out any of the states of any of the processors at any time you like.
Number two.
Imagine this situation, you’ve got someone who has developed a quantum algorithm that
needs a hundred qubits in order to run, and you’ve got another person who says I’ve
built this quantum computer and it’s got a hundred qubits in it.
So any normal person would say, okay great you can just take that algorithm and put it
on that machine and get an answer.
But in most cases that’s really not true.
And it’s because they are talking about two different kinds of qubits.
In the algorithms sense what they are talking about is a theoretical ideal qubit which is
commonly referred to as a logical qubit, whereas in a quantum computer sense they are talking
about a real physical device, which could be like a loop of metal, an atom, an electron,
a photon, something like that.
And these physical qubits have noise in them because they are real devices living in the
real world, and noise is a killer for quantum algorithms, it will destroy your quantum state.
So people are all trying to make qubits with as low a noise as possible but the quality
of your qubit matters a huge amount, to how good your quantum computer is.
Now, theoretically there’s a way of getting around noise in qubits using a technique called
quantum error correction which is where you use multiple physical qubits to simulate one
logical qubit.
And how many physical qubits you actually need to simulate a logical qubit depends on
how good quality those physical qubits are.
The higher quality they are the fewer you need, but estimates range from say ten thousand
physical qubits to a hundred physical qubits per logical qubit.
That brings me on to point three, quantum computers and internet encryption.
So a lot of the internet encryption is based on the fact that it’s really hard to factor
large numbers on a classical computer.
And there is this quantum algorithm called Shor’s algorithm that can theoretically
factor these large numbers exponentially faster than the best classical algorithm.
The trouble is for a standard 128 bit encryption you’d need about a thousand qubits, for
Shor’s algorithm to run.
And that means you’d need a million or more physical qubits in order to run it.
And that’s a really massive number of qubits.
Currently we have, the best is 72 qubits that google has in the universal quantum computing
scheme and so it is going to be a very long time until hit a million.
So for the time being, your internet secrets are safe.
Point number four is the fact that, nobody knows for sure that quantum computing will
ever work at scale.
Some people argue that noise is such a significant issue that it will be impossible to get say
a million qubits all working together, without noise coming along and ruining everything.
And it’s a fair point.
Personally I’m optimistic.
I’ve seen that there are.
Human ingenuity can go a very long way.
Some people said that we would never detect gravitational waves on Earth because noise
is a too significant an issue for that, and yet the people at LIGO did and absolute incredible
job over many years and now we can detect gravitational waves on Earth, which is an
amazing achievement.
It’s not exactly equivalent to quantum computing but, you know, the only way we can find out
if we can do it, is by trying to do it.
And like I say, I’m optimistic, but it is always worth bearing mind that it’s not
guaranteed.
And the final point I want to make is about quantum supremacy.
Now quantum supremacy is a bad name because it poorly describes the thing that it’s
trying to describe.
Now, I’ve done a whole video on quantum supremacy so check that out if you want to
find more details about it.
But basically it is the moment in time where are quantum computer can do one thing better
than the best classical supercomputers can do.
Now currently, classical computers can do everything a quantum computer can do, and
more.
But if you think about it, quantum supremacy makes it sound like it’s the time when a
quantum computer can do everything a classical computer can do, and more.
But that’s really not what it is.
It’s a time when a quantum computer can do one tiny little thing better than a classical
computer so it shouldn’t really be called quantum supremacy, it should be called something
more like, a quantum glimmer of hope, or a lot of people are calling it a quantum advantage,
which I think is probably a better name.
Well I hope that cleared up a few things, gave you a bit more subtlety into what quantum
computers can and can’t do
Personally I’m really excited about where quantum computers are going to go in the future.
I think the potential applications of it are massive especially in the realm of quantum
simulation because that’s something that we really struggle to do on classical computers
because, quantum, quantum systems are so hard to simulate.
A quantum computer can do it a lot better, theoretically, and that could be revolutionary
in say like simulating materials, to find out what properties different materials have
with, strength, durability.
Or, exotic things like high temperature superconductors.
Also in chemistry if you could simulate how molecules interact, say, a molecule in a drug,
how that interacts with many different biomolecules in your body, that would be an amazing thing
to do, and something we can’t do right now.
So, I’m very excited to see what happens in the future and, and I’ll keep you updated
here.
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