
A free introduction to quantum computing and quantum mechanics - lisnake
https://quantum.country/
======
Q_is_4_Quantum
Note its not strictly-speaking necessary to learn any linear algebra to
"understand" quantum computing, there is a very simple but universal set of
string-rewriting rules.

I realise for this audience most people wouldn't get much from it, but I find
these rules useful for explaining quantum theory without annoying and fluffy
analogies to high school students. You can find the rules in my book _Q is for
Quantum_ - Part I which covers quantum computing is a free download on the
book website (and the rest will be if I ever get around to producing a
corrected pdf, bleh, but if you read part I and don't want to pay for the rest
let me know and I'll get it to you...)

~~~
gus_massa
Are you sure the easy rewriting rules are not just 2x2 matrix multiplication?

~~~
abdullahkhalids
I looked at the book and it is. His way is perhaps useful for not scaring off
amateurs from learning quantum theories, but ultimately he is tricking them
into learning a part of linear algebra without a foundation upon which more
complex concepts can be built.

~~~
Q_is_4_Quantum
Actually its not quite that simple. The approaches are "equivalent" (in terms
of operational/statistical predictions) but not "isomorphic" (even if we
restrict the regular linear algebra approach to the same gates as I use in the
book). Basically (because of not implementing normalization/unitarity) in
certain situations the "mist" retains a count of the number of "Feynman paths"
that led to the output state. For example, concatenate two PETE boxes
(hadamard gates) and the output is [W,W], concatenate four and its [W,W,W,W].
Operationally identical (you only observe the ball to be certainly white!) and
applying a suitable simplification rule you "drop the extras" of course. But
if someone (unlike me!) wants to think of these states as having ontological
status maybe they would think the distinction is relevant.

A much more interesting example of something similar happens in the Deutch-
Hayden version of Heisenberg picture style quantum theory. There from looking
at the "state of the world" you can determine which total unitary evolution
occurred to get you there. In the Schroedinger picture, if I give you the
inital and final states there are many possible unitaries that get you between
them. Its possible to go further and come up with versions where not only the
total unitary connecting initial and final state is discernable, but also the
sequence of Hamiltonian's that implemented it are too.

These things are mainly of interest to people in foundations of course (since
they like to play the game of blurring the line between our math and what is
"really out there"). I tried to be very careful in the book to maintain the
distinction.

~~~
GuiA
Downloaded part 1 of your book last night and read it, as a fellow computer
scientist and educator, very very solid work! I bought the book!

Are you aware of anyone who made an interactive version (eg in the browser) of
the boxes and balls model? Would be really fun to mess around with it
interactively.

~~~
Q_is_4_Quantum
Thanks for buying! Any money I make eventually works its way back to Malawi
where I grew up, so it not going to be squandered :)

A few weeks ago a high school physics teacher who used the book for some
extension classes sent me some python code he says will do all of the
calculations. I have asked the family friend who maintains the book webpage to
put it on there, and I am sure she will when she gets a chance. However if you
email me - terry @ qisforquantum.org - it would be great to have someone look
at it - I don't know python at all and can't even run it to check it. (I also
have some _atrocious_ octave/matlab code of my own which anyone is also
welcome to.)

------
eightysixfour
One of the creators of this, Andy Matuschak, is the only person I support on
Patreon.

I really believe that we are just now starting to scratch the surface of the
learning opportunities that computers present to us and Quantum Country was an
interesting step forward in that direction. It is worth looking at some of his
previous work with Khan Academy as well, cool stuff.

------
sfg
"Quantum Country is a new kind of book. Its interface integrates powerful
ideas from cognitive science to make memory a choice."

What were the powerful ideas?

~~~
sgdpk
Spaced repetition [1], they send you questions over increasing time periods to
consolidate what you learned in the long term.

[1]
[https://en.wikipedia.org/wiki/Spaced_repetition](https://en.wikipedia.org/wiki/Spaced_repetition)

~~~
da39a3ee
Unfortunately the questions are utterly superficial and laughably different
from the sort of thinking you need to do to understand how a 2-dimensional
complex vector space is involved in quantum physics and quantum computing.

I say that not as a physicist/mathematician looking down at the oiks trying to
underastand this stuff, but as one of the oiks.

~~~
kirrent
Of course they are. By design the flash cards are small atomic pieces of
knowledge that can be answered simply. If the grand overarching concept is a
web of interconnecting ideas, then the flashcards are supposed to help you
maintain that by making sure you don't forget the small ideas the structure is
made of.

It's also by design that some of the questions are, as you say, laughably
simple. It's to catch people who aren't properly engaging with the text as
early as possible.

Nielsen and Matuschak are sensible to these sorts of critiques. Quantum
country is part experiment after all and they've been learning, adjusting, and
experimenting with what works best. If you want a bit of context of where
they've come from and where they want to go, I recommend this:
[https://numinous.productions/ttft/](https://numinous.productions/ttft/)

------
swebs
>At least in popular media accounts, a very common description is that a state
like 0.6∣0⟩ + 0.8∣1⟩ is “simultaneously” in the ∣0⟩ state and the ∣1⟩ state.

>I must confess, I don’t understand what people mean by this.

I think they're just trying to tie it to Schrödinger's cat somehow since
that's the only other topic that seems to be covered in pop science.

------
Mvandenbergh
To me (but then I already know a lot about both of these topics) the format is
actually much more interesting. Although it looks like the material is well
presented.

It does make sense that if we're going to have learning interfaces they have
spaced repetition built in rather than requiring users to do that themselves.

Looking forward to reading through.

------
boromi
Anyone else find the navigation tricky? I wanted to flip through the book to
find out the breadth of topics.

------
342637
This is a great study resource. A downside is there are no solutions to check
your work, or look up if you get stuck. I’d imagine a lot of people reading
this wouldn’t necessarily be university students with a professor to ask
questions to, so such a resource would be useful.

------
zeepzeep
Are other intros not free?!

------
SiebenHeaven
Beware, there is a sign-in wall on this "free" introduction that comes up
after a couple of topics. Edit: this seems to be the case only on the mobile
version

~~~
Koshkin
I would like to coin the term "A free (as in cheese)..."

~~~
keanebean86
Instead of an open door policy it's a closed but unlocked door policy.

------
da39a3ee
This is a really great resource that introduces some fairly sophisticated
ideas. I'm about to be negative but I genuinely mean that; I'm planning to go
back to it. I was finding it took time to take in the ideas about C^2.

However

> Presented in a new mnemonic medium which makes it almost effortless to
> remember what you read

This is just nonsense and quite distracting, even irritating. Quantum
computing and quantum mechanics is hard and involves sophisticated concepts.
The spaced repetition questions are utterly superficial and represent
unhelpful dumbing down.

Apologies for the rant and slightly silly comparison between quantum physics
and a programming language but the same goes for the Rust book. Rust is a hard
language with sophisticated ideas. Despite the best intentions of the nice
people at Mozilla, no amount of dumbing down changes that. And patronizing
readers does not make the resource more approachable to women and under-
represented minorities, unless you believe that those groups need to be
patronized.

> The set of curly brackets, {}, is a placeholder: think of {} as little crab
> pincers that hold a value in place.

~~~
waynesonfire
> Quantum computing and quantum mechanics is hard and involves sophisticated
> concepts.

It's comments like these that in high school made me avoid calculus. Just
wanted to let you know that you are discouraging. The youngsters reading this
that are still developing their critical thinking skills are being scared away
because of your projections. Frame it different, take responsibility for your
inadequacies.

~~~
da39a3ee
People of the age you're talking about aren't stupid and they're not taken in
by people giving them saccharine messages about how things aren't hard and
anyone can do them.

OK. If you are a student reading this then, well, firstly I'm not at all sure
you're feeling discouraged from learning about the nature of reality and
potentially world-changing computing frontiers because someone on the internet
said it was a hard subject! But if you were, please don't be. Here's why.

Firstly, a lot of incredibly valuable and interesting things do require a bit
of concentration and effort. I'm sure you know this.

But there are reasons to be optimistic! Here are some things to do:

1\. Stay calm! Don't give up.

2\. It's not at all obvious how to read technical stuff to start off with.
Basically, the secret is to read the same paragraph a LOT of times! Everyone
does that. No-one can understand it by reading it through once like a novel.
If you find yourself reading the same paragraph 100 times you're on the right
path because it means that you KNOW you haven't quite grasped all the ideas
yet, and so you are becoming your own teacher in a way; setting your own
standards for yourself.

3\. You've got to be, or become, the sort of person who can concentrate on
reading something challenging, in a quiet room. Talking to others is going to
help a lot also, but there's a private side to this. You can compare it to
training to be on a sports team at a more competitive level. There are going
to be times when you're doing fitness training or whatever, and you're on your
own, and it hurts because you're exhausted. You know this was always part of
the deal.

4\. Don't listen to the people who tell you that it's "easy" and "anyone can
do it". Yes, anyone can do it in in the sense that everyone has the biological
potential to do it (they're human, they have a perfectly good brain). But you
know perfectly well, just by looking around at your classmates that some
people don't have, or at least don't currently have, the attitude that's going
to let them do it. You don't have to be weird; you just need to let one side
of your personality be a studious side, occupying some fraction of your time.

~~~
dang
Would you please read and follow the site guidelines? I appreciate the
positive intention behind your comments, but the way you're going about it is
both against the rules here and guaranteed to make the discussion worse.
That's no coincidence; the rules are the way they are because of what we've
learned about discussion dynamics on the internet.

[https://news.ycombinator.com/newsguidelines.html](https://news.ycombinator.com/newsguidelines.html)

~~~
da39a3ee
Hi dang, sorry, starting off with "What rubbish <username>" was a clear
violation of a very reasonable guideline; I've edited my message to remove
that.

Could you tell me if I violated any of the other guidelines / what else I have
done that is likely to lower the quality of discussion?

Not sure if this is one that I might have violated here, but I don't think I
agree with the guideline saying not to use HN for "ideological battle". That
seems to need a clear definition of what "ideology" is, and even then it seems
to risk ruling out many important contributions to HN, so I cannot promise to
uphold that guideline unless it can be clarified, and possibly not even then.

~~~
dang
The comment you edited is probably ok except for "taken in by people giving
them saccharine messages", but I would say that
[https://news.ycombinator.com/item?id=23563745](https://news.ycombinator.com/item?id=23563745)
breaks the site guidelines about calling names, shallow dismissals, and
generic tangents.

You can't give precise definitions to any of the terms in the guidelines and
it would be a huge mistake to try. All that an attempted formalization would
do is create loopholes to be exploited, and then make moderation harder
because people would excuse themselves with "it's technically not against the
rules". The rules aren't technical. HN is a spirit-of-the-law place, not a
letter-of-the-law place:
[https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...](https://hn.algolia.com/?dateRange=all&page=0&prefix=false&query=by%3Adang%20spirit%20letter&sort=byDate&type=comment).
Here's a version of this point from 6 years ago:
[https://news.ycombinator.com/item?id=7606756](https://news.ycombinator.com/item?id=7606756)

In practice there is an easily observable difference between how people use
the site for curious conversation and how they use it for
political/ideological battle. The former goes with openness, respect, and
exchange of information. The latter goes with being aggressive and wanting to
defeat, not hear, the other side. The former produces comments that are
unpredictable and fresh; the latter produces comments that are repetitive and
nasty.

