
Build a toy quantum computer at home - dhruvp
https://www.dhruvonmath.com/2020/07/19/quantum-computers/#
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radioactivist
While this is a nice demonstration of the polarization of light, this is not a
demonstration of quantum mechanics, or quantum computing (though it does have
pedagogical value, if qualified properly).

Polarizers essentially just project the electric field of the wave onto some
axis, zeroing out the perpendicular component. Keeping in mind that light
intensity is the square of the electric field strength, all of this can be
explained through straightforward classical electrodynamics.

An analogous statement would be that interference of light (say through a pair
of slits) is also quantum mechanical in nature. This isn't strictly wrong
(since basically everything is quantum mechanical in nature when you get down
to it) but is a misleading way to present something that can (and was)
understood perfectly well before quantum mechanics came along.

Note: These kind of experiments for a single particle (e.g. photons,
electrons, etc) are a different story and do provide a demonstration of
quantum mechanics (and the combination of wave-like and particle-like
properties intrinsic to it).

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ahelwer
This is incorrect [edit: it isn't] - see the standard "three polarizing
filter" experiment [0] which is impossible to explain with classical mechanics
[edit: it can actually be explained, see coolgod's comment below]. Polarizing
filters don't just zero out the perpendicular component of an electric wave,
they measure each individual photon that passes through and either blocks it
or permits it to pass with some probability proportional to the angle between
the photon's polarization and the filter's polarization.

[0] [https://youtu.be/zcqZHYo7ONs](https://youtu.be/zcqZHYo7ONs)

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coolgod
The three polarizing filter experiment using common sources of light can be
explained perfectly without using quantum mechanics [0]. These effects only
need QM to explain in the single photon regime. The polarized light from the
LED display is definitely not in the single photon regime thus this experiment
does not demonstrate any quantum effects. Without any quantum effects, it is
much more difficult to justify the quantumness of this supposed quantum
computer.

[0]
[http://alienryderflex.com/polarizer/](http://alienryderflex.com/polarizer/)

~~~
ahelwer
I was going to say, this wouldn't explain it because the electric field
strength is projected to the polarization axis (with strength 0.707 at 45
degrees) but to match quantum measurement it should be 0.5 strength (0.707
squared). But as the grandparent comment stresses, light intensity is the
square of electric field strength... so the measurement matches. Makes sense!
I will amend my above comment.

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sasaf5
This is a real-valued computer, not a quantum computer. In the described
algorithm the state is the real-valued angle of the polarizer. One could very
well implement this algorithm using the charge on a capacitor. Also the
algorithm has bugs, it can overshoot the vertical. The author does acknowledge
these shortcomings in the "caveats" section. But with all those caveats, you
are not building a quantum computer at home.

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jerome-jh
The proper term would be "analog computer", but I agree with you. Really this
is not even a computer.

~~~
sasaf5
Indeed, I should have called it "analog computation" instead.

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ThePhysicist
Nice article, but not really a quantum computer or even a system that needs
quantum mechanics to explain it. You could do the same calculation with an
analog system (e.g. a capacitor that you add/remove charges to/from). The
argument from Scott Aaronson about quantum advantage that the author refers to
is really not very relevant, as a single qubit doesn't have any information
encoding advantage over an analog system. A quantum computer simply cannot
produce a speed advantage without relying on entanglement at some point during
the quantum computation. So: no entanglement = no speed advantage.

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dhruvp
Hi! Author here - If you have any feedback on what can improve please let me
know! Thanks for reading and feel free to shoot me a note at
dhruv.parthasarathy@gmail.com if you'd like to see something edited.

~~~
osamagirl69
It is a very nice article, and very well articulated.

However, this article falls into a pet peeve of mine which is that the
behavior exhibited here can also be completely explained classically -- this
is also a standard demo when explaining how polarization works classically. I
feel that it is worth it to at least include a footnote to that effect. The
reason that I bring it up is that I (as someone who first learned classical
optics, but is now learning quantum optics) personally suffered from some deep
rooted misunderstandings about quantum mechanics due to having seen so many of
these simplified demos which do not actually capture the quantum nature of
light.

The way this article is presented it implies that one can also model quantum
phenomenon using maxwells equations -- which is obviously not true. In this
specific case you get the same answer, but as soon as you start looking at the
individual photon statistics your answers will start to diverge. This is where
the actually quantum things like Bells inequality and the Hong–Ou–Mandel
effect come into play. If people had just been up front with their
descriptions 'oh by the way, when you look at the aggregate behavior of
photons they look perfectly classical, it is only when you look at the
statistics do they behave any different' it would have saved me a lot of soul
searching and misguided contempt for the quantum community.

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dhruvp
Hey - this is perfectly reasonable and constructive feedback - thank you! I
see your point that the polarization example can be explained using classical
approaches. I wanted to explain it in terms of individual photons as I wanted
to use this to help provide some visual intuition for qubits. Photon
polarization is a nice, visual way of interpreting qubits and as such lent
itself well to the task.

EDIT: I've gone ahead and added the footnote. Thanks for the suggestion!

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snowwrestler
The 3-polarizer experiment is a very cool way to demonstrate the weirdness of
light.

And the idea of using sequential rotation to keep track of cumulative bias in
coin flips is an interesting concept.

But ultimately I think neither one of those concepts really depends on the
other in this experiment. Checking for light through polarizers is neat, but
keeping track of any other rotating macro-scale object would work just as
well. You can do the same thing by rotating a stick on a piece of graph paper.
If it goes beyond your pre-determined test angle, you declare a bias.

As I understand it, the crazy thing about quantum computing is that you _don
't_ need to go sequentially; you can simultaneously compute every test flip in
one step with qubits. That's why quantum computing could speed up certain
calculations. (Note: please don't ask me to explain how.)

~~~
dhruvp
Hey, you're right you could use a stick on a piece of paper etc. Totally fair.
That being said this is in fact a real application where a qubit can model
things a standard bit can't. Professor Aaronson describes it in this paper:
[https://www.scottaaronson.com/papers/qcoin13.pdf](https://www.scottaaronson.com/papers/qcoin13.pdf).
Additionally, it's described in his lecture notes here:
[https://www.scottaaronson.com/qclec/5.pdf](https://www.scottaaronson.com/qclec/5.pdf)

~~~
snowwrestler
Thanks for the links. It doesn't seem like your experiment captures the
interesting part, which is that you don't need more qubits to measure a more
subtle bias.

As I understand the experiment now, it seems like the more subtle the bias in
the coin, the more times you would need to rotate the polarizer to detect the
bias.

If there is something about using the polarizing filters to keep track of
tries that is more efficient than using something like a stick, then I would
emphasize that in your write-up.

~~~
htfu
Yup. As greek to me as the paper is at least it makes very clear what it sets
out to achieve and why (and when) it differs. I suppose it's implicit but I
feel article really ought to explain that in the demonstrated case of heavy
bias, few attempts and fixed, coarse steps there is of course no advantage -
apart from the stick in ground one could also best its resolution off
0b1000000 and ++/\--.

It's a nice explainer on polarization but tries to be more than that and
doesn't achieve it - but with further work (not in form of added caveats but
rather a new approach to tying the two concepts together) I'm sure it could.

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frequentnapper
maybe i'm missing something here, but could we not have just used a stick on
the ground and rotated it accordingly, and still end up with the same result -
if the stick ends up perpendicular to the plane, i.e. you? Why do we need the
light polarization setup?

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dhruvp
Hey - you totally could to mock this exact setup like you're suggesting.
However, in a real quantum computer setup you would take one qubit and apply
either the positive or negative rotation gate to it again and again and rotate
the state of that qubit. The "stick" in the quantum computer would be the
qubit (or photon in this case). So the post is meant to show what would happen
in a real setup. Hope that makes sense.

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keyle
It's more of a quantum demonstration than a computer, no?

~~~
dhruvp
Yes - I used the term computer because it really does allow you to mock this
quantum algorithm:
[https://www.scottaaronson.com/papers/qcoin13.pdf](https://www.scottaaronson.com/papers/qcoin13.pdf).
It's described in Professor Aaronson's lecture notes here:
[https://www.scottaaronson.com/qclec/5.pdf](https://www.scottaaronson.com/qclec/5.pdf)

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srajap06
Analogous statement would be that interference of light (say through a pair of
slits) is also quantum mechanical in nature. This isn't strictly wrong (since
basically everything is quantum mechanical in nature when you get down to it)
but is a misleading way to present something that can (and was) understood
perfectly well before quantum mechanics came along Thatis.

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thdrdt
Is it possible / are there virtual or emulated quantum computers?

I read about the Kyndi model but could not find any implementation.

~~~
jlokier
Yes, it is possible to emulate a quantum computer, and this is done quite
often for research and to try out quantum programs.

The problem is it takes O(2^n) classical computer resources to emulate a
general purpose n-qubit quantum computer. In other words, exponential time or
size.

(We can simulate some larger quantum chemistry systems on a classical
computer, but those aren't general purpose. The simulations are quite
restricted in what they can measure, and there's still a significant practical
size limit.)

So we can only emulate very small general purpose quantum computers or other
quantum systems. For larger quantum computers, in principle those can be
emulated too, except you would need an impossibly fast and large classical
computer to do it. So we can't do so in practice.

This is actually the motivation for building real quantum computers of
significant capacity.

If a real quantum computer can be built with a large number of high quality,
fully coherent qubits, it will be able to do calculations that can't be
emulated on any classical computer we can actually build and run, just because
of the O(2^n) practical limit.

Right now, there are no quantum computers like that. There are some dubious
marketing claims around, and there are also some genuine, but smaller,
devices.

Because we can't even simulate a large quantum computer, we don't know for
certain whether such a device can even be built in the physical world. The
abstracted maths of quantum mechanics, which has proven to be extremely
accurate and correct for everything it's been used on, says it can (subject to
practical engineering details), but the physical world may have a subtle
limitation which we can't detect in smaller systems, that only happens with
larger quantum computers and prevents it from being possible. The maths itself
might even have a subtle reason (such as stability or entropy) why the system
cannot work, but no such reason is known at the moment. We can't "run" the
maths to find out its behaviour on a large system, for the same reason we
can't simulate a large quantum computer without a large quantum computer in
the first place. We can only reason about it in the abstract.

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abhayhegde
I liked the article, but this is not a quantum computer. Please do not take
away the credibility of what a real quantum computer could achieve. This is at
best an algorithm to reveal the angle of polarizer, and also the nature of
light.

Although, appreciate the efforts.

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hetman
Not sure what's up, but this website refuses to scroll on any Chrome based
browser on my phone. Works fine in Firefox though.

Edit: Weird. Now it works. Though there still some initial "resistance" when I
start scrolling...

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imvetri
I like it.

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starpilot
Has anyone cracked SHA with this?

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jmeyer2k
SHA is not crackable with quantum computers. RSA and Diffie-Hellman are though
and nobody has done it yet (or come close).

