The Classical Channel

Quantum Computing

3:49 AM

2

Q: What are some open sources projects on quantum computing?

I am looking to collaborate on open source simulation efforts.

Blue

8:02 AM

@glS @Mithrandir24601 Posted it. Let me know if I made any silly mistake anywhere :P

Quantum Computing

0

Q: Quantum algorithm for linear systems of equations (HHL09): Step 1

I have been trying to get my head around the famous(?) paper Quantum algorithm for linear systems of equations (Harrow, Hassidim & Lloyd, 2009) (more popularly known as the HHL09 algorithm paper) for some time, now. On the very first page, they say: We sketch here the basic idea of our algo...

quantum-algorithms qubit

glS

8:28 AM

@Blue great! but wouldn't it be better to break it up into two separate questions, one about the phase estimation step and the other about the number of qubits needed? You wouldn't need to replicate the background information as you could just put it in one and reference it from the other

Mithrandir24601

@Blue Yeah, I agree with @glS on this - 2 separate questions might be better

Blue

@Mithrandir24601 @glS Okay, on it!

glS

by the way, what's up with all the downvotes in quantumcomputing.stackexchange.com/q/2381/55??

Mithrandir24601

@glS The question's getting a good number of upvotes (and no downvotes), so people seem to be quite happy with the question... Maybe people just want the answers to be more thorough or something? I know if I went and made an answer it would probably be quite long. One thing is that no-one has actually said what a qubit physically is in more detail than 'e.g. spin of an electron', which isn't particularly detailed, I suppose

Quantum Computing

0

Q: Quantum algorithm for linear systems of equations (HHL09): Step 1 - Number of qubits needed

This is a continuation of Quantum algorithm for linear systems of equations (HHL09): Step 1 Questions (contd.): Part 2: I'm not exactly sure how many qubits will be needed for the Step 1 of the HHL09. In Nielsen and Chuang (section 5.2.1, 10th anniversary edition) they say: Thus to succes...

quantum-algorithms qubit

Nelimee

8:42 AM

About the same question as above: I have an answer for most of the question, but I need more precision on something (posted as a comment). Should I post my incomplete answer and then edit with the provided information or should I wait and post a complete answer?

Mithrandir

Wait.

glS

@Mithrandir24601 I mean, I agree that they definitely could have posted better answers, but they probably didn't take the time to do that. Those are not the kind of answers I would expect for this kind of question. Still, I would have just not upvoted the questions rather than downvoted them.

Mithrandir24601

@glS Yeah, same - I haven't upvoted anything but neither have I downvoted anything

Blue

@Nelimee You surely can write an answer for now, and leave behind a note saying that you have things to add.

Editing and improving answers is encouraged

@Nelimee You can ask me if any part of the question is unclear

Nelimee

I didn't get the 6 * log(N) conclusion, especially the factor 6. From where the "6" pops up?

Blue

8:57 AM

@Nelimee That $6$ comes from the formula $t = 3 + \lceil { \log_2(2+\frac{1}{2 (0.1)})\rceil} = 3 + 3 = 6$. It denotes the number of qubits in the "first register" needed to represent each $|\lambda_j\rangle$ or $|\frac{\lambda_j t}{2\pi}\rangle$ to $3$-bits of precision and with $90\%$ accuracy. — Blue 9 mins ago

@Nelimee Did you read that part ^?

Nelimee

I read it. 6 is the number of qubits needed to compute the eigenvalues. Then why multiply it by log(N)? I'm sure it's a silly question, but I can't see why

Blue

@Nelimee To represent each eigenvalue $|\lambda_j\rangle$ or $|\frac{\lambda_j t}{2\pi}\rangle$, you need $6$-qubits. BUT, there are $N$ different eigenvalues to represent! You can't do it with the same set of $6$ qubits. You'd need at least $\lceil{\log_2(N)\rceil}$ times more qubits.

Nelimee

Ok! My question boils down to the comprehension of the quantum phase estimation algorithm. Is the following assertion correct?
In the quantum phase estimation:
1. If the input state is an eigenvector of the considered unitary, then we have the the phase corresponding to its eigenvalue (up to a given probability).
2. If the input state is not an eigenvector, then it is a "superposition" (linear combination) of the eigenvectors of the considered unitary. Then the output is a superposition of the corresponding eigenvalues.

Blue

@Nelimee Yes

I mentioned that part in the question

Nelimee

Then why do we need to encode the log(N) eigenvalues in log(N) different quantum registers?

I go read your edit!

Blue

9:12 AM

@Nelimee How would you represent the superposition of $N$ eigenvalues using just $6$ qubits?

Nelimee

Take a matrix A with eigenvalues 1 and 2 (|01> and |10>). Then 2 qubits is sufficient for the register that represents all the eigenvalues of A: the state (|01> + |10>)/sqrt(2) is a superposition of the eigenvalues of A.

Ho wait

Blue

@Nelimee That's a superposition of eigenvectors

Not eigenvalues

@glS Did this part of the HHL paper make sense to you? ^

I sort of skipped that, and moved on to the $R(\lambda^{-1})$ gate stuff

Don't know if that part is necessary in the algorithm

Nelimee

BTW I have question on the R(lambda^-1) part, but I will make a post for it =)

And yep, I got your point

Blue

@Nelimee I'm thinking of making one too

Nelimee

Thanks for the explanations !

Blue

9:18 AM

It's quite hazy for me

@Nelimee No problem! :)

glS

@Nelimee if I understand, there should be N eigenvalues, but to encode all of them one just needs to have a register fitting a single eigenvalue, as the others are fit there "in superposition". So the dimension of the eigenvalue register is determined only by the precision and probability required. Right?

Nelimee

Yep, my mistake, I was considering what you just said ^^

Blue

@glS I was a bit confused about that

glS

@Blue what's [13]?

Nelimee

Now I got the log(N) part.

Blue

9:21 AM

@glS So do you mean it is possible to represent the superposition of all the eigenvalues (3-bit precision and 90% accuracy) using just $6$ qubits?

"So the dimension of the eigenvalue register is determined only by the precision and probability required"

glS

@Blue I mean, I don't remember exactly where the 6 comes from, but the answer is probably yes. It's similar to how you only need, say, 3 qubits to encode 2^3 possible bit strings

Nelimee

I think he wrote that as a "non-sense that would make me change my mind"

Hummm, I really need to think more about it

Blue

@glS That might be possible, now that I think about it

But still it's confusing until someone manages to write it out clearly

glS

the point is that if you have many vectors $|u_j\rangle$, and each one requires $n$ qubits to be represented, you still only need $n$ qubits to have the state $\sum_{j=1}^{\text{whatever}} |u_j\rangle$

Nelimee

On the other side, you don't choose the phase that will be computed by the QPE (quantum phase estimation). So you can't just apply the algorithm multiple times to different registers to have different phases

glS

9:25 AM

sorry got to go now. I'll try to answer later in the day!

Nelimee

No problem!

Blue

@glS Okay thanks. I'll think about it!

Nelimee

Here the problem comes from the fixed precision versus the non-fixed dimension N. As he has a fixed precision, 3 (or 6) qubits are sufficient to encode the eigenvalues to the given precision, but he might have more than 2^3 (or 2^6) eigenvalues

BTW for me the O(log(1/e)) additional qubits are just ancilla qubits. See arxiv.org/abs/quant-ph/9708016v1 (end of page 11)

Blue

@Nelimee Right. We need at least $\lceil{\log_2(N)\rceil}$ qubits to be able to represent $N$ eigenvalues of the unitary operator $U$.

Considering that the unitary operator is $N\times N$

@Nelimee I guess the necessary number of qubits in the register would be something like $\text{max}(\lceil{\log_2(N)\rceil},6)$.

@Nelimee I was under the impression that only a single ancilla qubit is necessary

Nelimee

@Blue For the whole HHL, yes. I'm speaking about QPE when you want improved accuracy

Blue

9:39 AM

@Nelimee Okay, gotcha

Nelimee

To me those O(log(1/e)) qubits are just ancilla and so should not be counted in the number of necessary qubits (or at least not as he/she did in the question)

Blue

@Nelimee But those are required for the necessary precision we want, isn't it?

Nelimee

They are required *during the computation*. In my link to arxiv they say
"and rounding off the resulting m′-bit string to its most significant m bits"
which means to me that they performed QPE on m' > m qubits but then discarded the additional qubits in order to recover a m-qubit register

And by "precision" he/she probably means "the probability to measure a true eigenvalue". From en.wikipedia.org/wiki/Quantum_phase_estimation_algorithm , "classic" QPE has a 4/pi² probability of success, and this probability can be increased to 1-e with O(log(1/e)) additional qubits

Blue

@Nelimee Right

Nelimee

It's like "classic" QPE is a computation in simple precision for a simple precision result and "improved" QPE (with the additional qubits) is the same computation but using double precision for intermediary results

You will have a more precise result with intermediary results as double, even if the result is in simple precision, but you will pay the price during the computation

Blue

9:48 AM

Agreed, but that precision is usually necessary

I don't how precise the H1N1 simulation they did, is, though

I'm talking about Lloyd et al.'s Hopfield paper

Nelimee

So we still need to figure out this question of "we need log(N) registers to represent all the eigenvalues"...
I'll go eat soon, I will be back in an hour =)

Blue

@Nelimee I think just $\text{max}(\lceil{\log_2(N)\rceil},6)$ should be good enough. A single register should be sufficient

See you!

Nelimee

If we take your representation, then why limiting the precision to 90%? In the case where N is really large, we could compute the phase with an extended precision at no cost

And so we wouldn't need additional registers?

Waow, I'll need to put that on paper to be sure.
@Blue Do we have a way to speak about the rotation part before I post something?

Blue

@Nelimee That's true. Which is why I used the "max" function :)

So that we will get at least 90% accuracy

30 mins ago, by glS

the point is that if you have many vectors $|u_j\rangle$, and each one requires $n$ qubits to be represented, you still only need $n$ qubits to have the state $\sum_{j=1}^{\text{whatever}} |u_j\rangle$

Nelimee

Yup!

Blue

9:54 AM

For this reason I guess ^

@Nelimee Which rotation part?

The R gate?

Nelimee

the R(lambda^-1) part

Blue

@Nelimee I'll make another new post for that. That's the second step of HHL, which doesn't seem relevant to the current question

You can of course discuss it here, if you wish

Quantum Computing

11:16 AM

1

Q: Generating algebra from commutation

In a paper I am reading it states: For open-loop coherent controllability a quantum system with Hamiltonian $H$ is open-loop controllable by a coherent controller if and only if the algebra $\mathcal{A}$ generated from $\{ H, H_i \}$ by commutation is the full algebra of Hermitian operators for t...

quantum-information quantum-control

0

Q: Quantum algorithm for linear systems of equations (HHL09): Step 2 - What is $|\Psi_0\rangle$?

This is a sequel to Quantum algorithm for linear systems of equations (HHL09): Step 1 - Confusion regarding the usage of phase estimation algorithm and Quantum algorithm for linear systems of equations (HHL09): Step 1 - Number of qubits needed. In the paper: Quantum algorithm for linear system...

quantum-algorithms hamiltonian-simulation

Blue

11:31 AM

@glS In case you get time: quantumcomputing.stackexchange.com/questions/2393/…

That part of the paper looks like complete Greek to me

glS

@Blue I gladly will, but I probably won't make it for today

Blue

11:59 AM

@glS It's okay. No hurry !

Nelimee

@Blue I can answer at nearly all your questions on the third subject, but I'm trying to be sure about the Psi_0 part. I will write my answer when I'll clear on that :)

Blue

12:18 PM

@Nelimee Sure. Take your time :)

Nelimee

12:52 PM

What is the appropriate way of citing? Is just writing the title of the paper (as a link to the arxiv page) sufficient?

Mithrandir24601

1:06 PM

@Nelimee in a post here? Whichever way works, so that's fine - it's not really a citation, just a link, so I wouldn't think too hard about the how and just do whatever is easiest to read

Blue

2:05 PM

@Nelimee Title + author's name(s) + date of publication would be preferable. Very useful when in the future people will be looking for discussions related to a certain paper on our site. Btw I'm reading your answer :)

user1271772

@Mithrandir24601 "One thing is that no-one has actually said what a qubit physically is in more detail than 'e.g. spin of an electron', which isn't particularly detailed, I suppose". That's not true, mine was the first answer and I explained what a qubit physically is, in much more detail than that. I provided a link for what "spin" is because my answer was already getting extremely long, and the question was not "what is spin?", however I did say that an electron can be spin up or spin down

and nothing in between, after explaining what a "quantum" is. The fact that electrons can be spin up or spin down is something we learned in grade 11 (age 15-17) when filling in "orbital box diagrams" for atoms, so that should have been enough, especially after I went through all the detail of what a classical bit is

@glS "I agree that they definitely could have posted better answers, but they probably didn't take the time to do that." what could have been better?

Mithrandir24601

@user1271772 you provided a link, sure, but a link isn't an answer by itself. In any case, I didn't downvote your answer, so I'm not the person/people you have to convince

James Wootton

@Mithrandir24601 " One thing is that no-one has actually said what a qubit physically"
That might be because there is not a single concrete answer yet. If answers need to include details on "What purpose does it serve in quantum computing?" as well as what it is physically, that's a scope that could include any topic in quantum computing hardware or software.

Blue

Honestly, I don't like the question much. It's too broad for any useful answer.

Mithrandir24601

@JamesWootton then it's beginning to sound like a discussion on whether or not this question is too broad?

Nelimee

2:20 PM

@Blue Ok I edit my posts :)

James Wootton

It should be possible to ask what a qubit is without it being too broad. But since it is the starting point for everything, it can be easy to get carried away. Either in the asking or the answering.

I'm not sure exactly what to do about it, except perhaps see what happens.

Mithrandir24601

Fair. It's not the easiest thing to answer, to my mind, anyway

user1271772

@Mithrandir24601 "you provided a link, sure, but a link isn't an answer by itself." Have you even read my answer? The link was for only one tiny part of the answer, and the question was "what is the spin of an electron" it was "what is a qubit". Why would I spend more time explaining what the spin of an electron is when I've already written probably the longest answer out of everyone? Furthermore, I already explained what a classical bit was and that can be spin up

what a classical bit was and that spin of an electron is quantum because it can be spin up or spin down but nothing in between, what more do you want to know about spin @Mithrandir24601?

@Blue "Honestly, I don't like the question much. It's too broad for any useful answer." The question has attracted a lot of negative answers including mine. Maybe it's trolling maybe it's not. Maybe OP and others want the questions to have a negative net score so that more people answer, I don't know, but I think it's getting to the stage where the question should be closed due to attracting too many negative answers. It's discouraging me to spend more time helping people like you here.

Blue

@user1271772 I didn't ask the question so I don't see what you mean by "helping people like you here". No one is forcing you to help anyone or even participate on the site.

user1271772

@Blue: I have helped you a tremendous amount on this site. Period.

Blue

2:35 PM

@user1271772 If you want to believe so, fine.

glS

@user1271772 why should care that much? it's just fake internet points at the end of the day

user1271772

@glS: Well I care about it because I spend time and effort and energy on trying to write the best answer I possibly can.

glS

@user1271772 which is admirable, but you cannot believe that a bunch of people downvoting for whatever reason is a faithful representation of the usefulness of your answer

I mean, it's literally a single downvote that you are talking about

user1271772

@glS: You are right, but it is still discouraging. I didn't spend all that time just to lose 2 points of reputation, you know what I mean?

The problem for this particular question is greater than the single downvote on my answer, there's downvotes all over the place and not all by the same person (I mean how can there be 3 downvotes on one question if they're all from the same person)

glS

@user1271772 sure. But that's how stackexchange works. When you ask/answer you expose yourself to what other people think, and this is the internet so there is really all sorts of people.

user1271772

2:43 PM

In this case it might not be that people think it's a bad answer, but they are just trolling or something. Anyway, I thought I'd write about it here in case it was someone here (which seems likely, based on some discussion I saw in this chat room). The reason I wrote about it here was to let people know it bothers me.

glS

@user1271772 to that I agree to some degree, and indeed I myself mentioned it before. I upvoted some of the questions to make up for some downvotes, but I probably wouldn't have upvoted otherwise. What I believe is the biggest issue with the current answers is that they are not very pedagogical. I mean, I think I know what a qubit is, but still I am confused by some parts of the answers

@user1271772 again, this is the internet. Trolling is a fact of life

user1271772

Thanks for upvoting mine just now, if that was you. As for answers not being pedagocial, I thought mine was extremely pedagogical, what part can be improved?

glS

@user1271772 for example, I find this sentence: "This means for the barcode if the thin line was one "quantum", and the thick line can be two times the size of the thin line, but it cannot be 1.5 times the size of thickness of the thin line." quite confusing

user1271772

That's because the grammar was wrong, I had just edited it right before you wrote that! ;)

glS

anyway, the problem is that it's very hard to answer questions that are so generic, because what a good answer is strongly depends on who is reading it. An example of what I believe is a great answer to a quite generic question is this: stats.stackexchange.com/a/140579/82418

if someone manages to write a comparably well-written answer for this question, the question/answer would likely go to be one of the most upvoted Q/A in the site

user1271772

2:54 PM

Good example, but just because it happened there doesn't mean it will happen here. There seems to be much more of a downvote culture here, especially for this particular question.

vtomole

That qubit question is so insane. All those answers are correct. Why are they all being downvoted?

2

Blue

3:06 PM

Anyhow, today's the chat session. We got to figure out the tag wikis for the incomplete ones.

Probably around 15-20 tags

heather

morning everyone.

Blue

@heather Late evening here. But well, good morning ;)

heather

=P

kinda weird how much answers on that qubit question are being downvoted.

Blue

Indeed. I think it should be closed before more people spend their time answering such a broad question.

heather

no, CW isn't really used anymore

Blue

3:18 PM

In the future, for such canonical Q&A I really think we need to use the CW feature so that the votes don't affect them and they can edit and improve as much as they wish over time

user1271772

What's CW?

heather

it's a little checkbox that can be clicked that basically allows anyone to edit w/o review and means that no reputation changes occur based on voting.

user1271772

@Blue: I agree, but I was the only one that voted to close.

heather

it's not really used anymore except in very specific cases.

i'm not of the opinion the question is too broad though.

@user1271772 if you thought it should be closed then why did you answer?

user1271772

@heather: after CW is clicked, no one gains or loses any points, no matter how many up-votes or down-votes?

Blue

3:20 PM

@heather I know. But such canonical questions and answers do classify as that "rare gem" built jointly by the community as a whole (in words of that blog post)

user1271772

@heather: have you read my comment on the question?

heather

@user1271772 yup, but again, not really used much.

@Blue true, but even for canonical stuff it's not always used anymore.

Blue

@user1271772 I had voted to close it (around 30 mins ago).

heather

@user1271772 one moment.

ah.

the question still stands, though.

why'd you answer if you didn't like the amt of effort or thought it was too broad?

Blue

@heather Well, just because it isn't generally used doesn't mean we can't use it. Imo it's fine if we frame a policy regarding such canonical questions, for our community. (As long as the community mods and devs are fine with it we shouldn't have a problem)

Maybe ask on Meta SE?

heather

3:22 PM

@Blue of course we can use it, i just personally am of the opinion it's not really necessary (though i'd be interested in seeing what the other mods think about it).

user1271772

Heather: First of all it was asked by Mithrandir, who might not have a high reputation here but is a mod on other SE's, and the question also got positive feedback from Blue and Mithrandir2 (our mod) in the comments.... I didn't think it was "too broad" at the time. It was AFTER i wrote the answer and was shocked to see a negative vote come so quick, and then the next day saw THREE negative votes on James Wootton's answer, and TWO negative votes on some other people's answers, that I started to

think it was too broad

heather

downvotes on answers don't necessarily mean it's broad.

but alright, i follow some of your logic.

user1271772

In SO, it's common for questions that "attract many negative answers" to be closed.

Also common to close questions that lack basic research

When I first answered it I didn't think it was too broad, I actually thought I could give a "perfect" answer, but it turns out I was wrong.

heather

well, I can't say I particularly like SO's model - it's rather notorious for being "not nice" regularly to new comers.

Blue

@heather That's true

user1271772

3:35 PM

I agree that SO is not very nice to new comers

SO gets 7.2k quesitons per day so perhaps they need to have a different model.

heather

i've always kinda thought (though it'll never be done) that if they split SO up into a python site, a C site, a java site, etc, that would help.

Blue

I had mentioned once before I would like mods to not interfere and close questions unless in absolutely obvious cases. But this one is sort of an "edge case". We don't have any agreed-upon-by-community policy for this one. I think this deserves its own meta post (i.e. How to deal with canonical (broad) questions?).

user1271772

Well in this case of "what is a qubit" if the answers were more positively received, I wouldn't find it to be such a problem, but people seem to be "wasting" their time on trying to answer it.

heather

the smaller the site the tighter knit the community and i think the "nicer" and more welcoming they can be.

user1271772

@heather: Splitting up SO is an excellent idea.

heather

3:38 PM

it'll never be done, of course.

@Blue i think with the way that question is i would not feel comfortable mod-hammering it. if y'all want it closed, y'all are gonna have to close it. taking off my mod cap though, i think it's fine. it's asking for a crash course, not a textbook.

user1271772

@Blue: I also don't want to interfere too much, and still hope I can retract my "close vote" for the "qubit" question, but only if the answers get appreciated a bit more

heather

@user1271772 you can retract close votes.

(as long as it is before the question is actually closed; afterwards you must vote to reopen).

user1271772

At the moment I'm worried that keeping it open might attract more people to fall victim to the downvoters, and if they are newcomers they will be discouraged from ever answering something else again. However if something changes I will re-tract the close vote.

Blue

@heather It's not always about what the OP wants. The matter is also that it's not possible to be rigorous in a "crash course". I had spent a long time yesterday thinking about how to frame an answer for it, using only the fundamental postulates of QM, but then gave up (partly because I don't have sufficient knowledge to answer it completely rigorously, and partly because it would take me way too long to write an answer)

heather

@Blue the answer doesn't need to be rigorous.

"I'd prefer an explanation that is easily understood by laypeople" usually means the exact opposite of "rigor" ;)

Quantum Computing Meta

3:47 PM

0

Q: Over-zealous editing by Blue (Part III)

In my first complaint I was asked to give examples which would reveal who was doing it and I said: "I thought about doing that, but if I do that, then everyone will see who he/she is, which might help to get the problem solved faster, but at the expense of someone's dignity. Do you reall...

discussion feature-request

Blue

@heather Sure, but I prefer to not write non-rigorous answers (as far as possible). Also, it's not always true that rigorous answers can't be clear enough for newbies (they can be...just that you need to spend a lot of time on it)

glS

@user1271772 I really don't see what's the big deal in a bunch of downvotes. People put effort in answering, others possibly didn't appreciate the result. It's not nice when it happens sure, but tough, that's life. When a great answer will pop up it will be heavily upvoted, I have no doubt about it

Ah Heather pointed out, the question being broad/ot is not related to the votes, especially in the short term. In physics.se it's not rare to see highly upvoted questions getting closed as off topic, for example

user1271772

@gIS: For the most part, I agree with you, but I don't know if any answer was going to get a lot of upvotes on the "qubit" question, based on the way things were going.

Maybe now something will, after so many people have made note of the problem, but the way things were going it didn't look promising

glS

4:14 PM

@user1271772 things don't have to happen now. Questions on SE sometimes get good answers after years of standing there, and that is fine.

but I have to say, I have the feeling that the some of the answers are downvoting each other. I understand the temptation, as no one wants to be the most downvoted one, but still it's not ideal.

Quantum Computing Meta

4:26 PM

0

Q: Over-zealous editing by Blue (Part IV)

Please read Part III first. Here's another example: My answer quoted the question, but then part of the question that was quoted got changed, which affects my answer. In this case the change to the question wasn't absolutely obligatory, and at the very least there was no discussion about it befor...

discussion feature-request

Blue

This is getting beyond ridiculous now :P

heather

@Blue @user1271772 the mods are figuring out how to deal with this whole situation. Please refrain from commenting about it, or flagging about it, or angering each other, or making new posts, or anything related to this, until we figure out what to do. Thanks.

(You can continue to do other stuff, just don't do anything related to this. Please.)

Blue

@heather Okay, noted

heather

@Blue thanks.

we'd like to get this cleaned up without anything else starting.

Mithrandir

Well.

I thought I'd just ask a simple, beginner question about a term I'd seen thrown around. Didn't expect everything to blow up.

Blue

4:41 PM

@Mithrandir It's not your fault. We don't yet have any agreed-upon policies to deal with such questions, that's all. Anyhow, as I had mentioned yesterday, I'd like to see a more focused question, from you.

heather

@Mithrandir definitely not your fault, especially this more recent issue. As for the too broad thing - nothing's blown up yet there =P

I think your question is fine; we'll see what the community thinks in the review queues.

Mithrandir24601

@Mithrandir I'm glad the question is asked. It might be too broad, might not be (it's not up to me to decide). It'll maybe cause a discussion in meta about this, which could be a good thing

heather

^

Blue

@ArtOfCode What brings you here?

Was there a flag?

ArtOfCode

nothing at all

Blue

4:50 PM

Ah, cool then ;)

ArtOfCode

just a completely and wholly random visit unconnected to any recent event, flag, or message

Blue

It gets a bit dramatic whenever we have mods from other sites coming in :P

@ArtOfCode Good to know!

user1271772

5:11 PM

I notice now after Mithrandir's rollback that I made a mistake with "over-zealous editing by Blue Part IV". I am trying to delete the question but can't figure out how. Is the delete button gone because it's locked? Sorry Blue about the 4th one, but parts I, II, and III are still very valid in my opinion.

Mithrandir

Right - when it's locked, you can't delete it.

Mithrandir24601

@user1271772 OK, I've unlocked part IV

Mithrandir

Also, @user1271772, a bit of advice from my travels around the network: Don't word the question like "This user did a bad thing"; instead, ask "was the action taken in this case the correct thing to do?". When phrased the latter way, it's a.) not calling out a specific user in public, which is very heavily discouraged b.) more likely to result in a productive conversation than something that comes across as an accusation.

Accusations don't go over very well, but a polite discussion is the right way to go about it.

user1271772

Mithrandir: Have you read my Part III from beginning to end?

James Wootton

where is part 2? I can’t seem to find it

Mithrandir24601

5:20 PM

@JamesWootton I believe this is part II

While this was, in effect, part I

Mithrandir

I actually haven't read them, because I was totally disinclined to after reading the accusatory titles.

Blue

A tad unrelated: There is an old saying along the lines of "If you want someone to agree with you, first try to be sympathetic with their viewpoint and then gradually try to change them with reason". It's a well know psychological trick. I think D Carnegie's popular book covers it well.

As a matter of fact, humans don't respond well to accusations and immediately become defensive.

user1271772

Blue: What do you mean it's a "tad unrelated", you are clearly doing what Heather told you NOT to do.

Mithrandir: I think you should read my Part III from beginning to end, it's the least you can do after calling me out the way you've done.

Mithrandir

If you have an issue with what one user is consistently doing, then you can also raise a custom moderator flag.

user1271772

@Mithrandir: How do I raise a custom moderator flag and what is it?

Blue

5:24 PM

@user1271772 If you click the "flag" option on any post there's something called "flag for moderator attention"

Where you can describe your problem

user1271772

Ok, I think that's quite highly discouraged, and also takes a very long time to get a response.

Blue

@user1271772 "I think that's quite highly discouraged" --- that's not true (as long as your problem is genuine)

user1271772

We're encouraged to start at lower levels and raise issues on Meta rather than raising the custom moderator flag

Blue

@user1271772 That is true

James Wootton

@user1271772 I read part III but couldn’t see the issue. Blue’s edit was to make an abs link, which you seem to agree is better. Do you disagree that an abs link is better, or do you disagree that Blue created an abs link?

Blue

5:27 PM

@user1271772 It's a biological fact. I don't think we're banned from discussing science ;)

Vogel612

tiny steps...

user1271772

@JamesWootton: The abs vs pdf if Part II, the CCNOT -> CNOT is in Part III

Vogel612

@user1271772 some things work better on meta, some things work better as custom mod-flags

user1271772

Do you not understand why changing CCNOT -> CNOT is a problem, if I already answered the question "Is it a CCNOT" ?

Vogel612

didn't heather put it clear enough?

56 mins ago, by heather

@Blue @user1271772 the mods are figuring out how to deal with this whole situation. Please refrain from commenting about it, or flagging about it, or angering each other, or making new posts, or anything related to this, until we figure out what to do. Thanks.

Blue

5:30 PM

@Vogel612 Huh?

A mod (James Wotton) is clearly asking @user1271772 for clarification. I don't see the issue.

James Wootton

@user1271772 noted, thanks for the clarification

Vogel612

@Blue FWIW that clarification can be phrased significantly less confrontational, so I preemptively tried to defuse...

Blue

@Vogel612 Which part?

4 mins ago, by user1271772

Do you not understand why changing CCNOT -> CNOT is a problem, if I already answered the question "Is it a CCNOT" ?

Ah, OK

user1271772

@JamesWootton: As for Part II: The problem wasn't just abs <-> pdf, but the whole formatting of my reference was changed un-necessarily. Maybe I like Harvard citation while you like PRL citation while Blue likes Chicago-style or someone else likes Humanities style (Wootton et al. 2017). I don't think such edits are a valuable use of Blue's time since they are not mandatory grammatical edits. But this is not just about helping teach Blue how to more effectively use their time, it is very

@Blue : When and where did Vogel612 say it was that comment that could be phrased less controversial?

Blue

3 mins ago, by Vogel612

@Blue FWIW that clarification can be phrased significantly less confrontational, so I preemptively tried to defuse...

"that clarification"

5 mins ago, by Blue

A mod (James Wotton) is clearly asking @user1271772 for clarification. I don't see the issue.

user1271772

5:36 PM

Why do you have to make it look like vogel was talking about my comment?

What if Vogel was talking about yours?

Vogel612

ugh...

why not just ask me if I have an ambiguous reference?

Blue

I'll let Vogel clarify :)

Vogel612

it's not like I've vanished

what I was originally talking about being "confrontational" was the rhethoric question that posed as clarification for James

9 mins ago, by user1271772

Do you not understand why changing CCNOT -> CNOT is a problem, if I already answered the question "Is it a CCNOT" ?

heather

@user1271772 stop. you may talk to mods to clarify about the situation, but please don't talk to @Blue about this because it seems to just degenerate into arguing.

Vogel612

^^ that message to be exact

heather

5:38 PM

@Vogel612 sorry about that =)

hopefully it's clear which message you are actually pointing to.

Blue

We have the chat session soon :/

Vogel612

@Blue thank you for trying your best to defuse this, but I think it's easier for all participants if we all disengage for a bit

Mithrandir

The conversation appears to be deteriorating. May I suggest taking a breather and continuing later?

user1271772

Thank you all for clarifying. Heather I will not talk to Blue about this situation anymore for the time being, although I don't quite appreciate the way you told me to stop. Vogel, it was not meant to be a rhetorical question but a genuine one where I was going to further clarify if he didn't understand the problem I complained about in my meta post.

@Mithrandir: I think we are okay.

heather

thank you.

let us move on, and the mods can think about the situation and ask you guys if we have any questions.

Blue

5:44 PM

Bug? O:)

Vogel612

probably. I doubt it's going to get much dev-time alotted to it, though

Blue

Probably some problem in setting the timer

Mithrandir24601

@Blue I'll try and hack my way round this for the next session...

Blue

I don't think it needs dev interference

@Mithrandir24601 Yay!

Vogel612

@Mithrandir24601 did you perchance register the event from the community events interface on-site as well?

Mithrandir24601

5:47 PM

@Vogel612 How did you know? :P Let me guess, there's no workaround?

Vogel612

the workaround is to not do that

It should be sufficient to just register the event from the chatroom schedule

Mithrandir24601

@Vogel612 It only creates the message on the board about 30 minutes before the session is due to start though

Another workaround would be to get a non-mod to create the event, but then we ran into issues with this before (although hopefully they wouldn't happen again...)

Blue

It seems to be fixed now @Mithrandir24601

Did you do something?

user1271772

"the workaround is to not do that" ~ best answer!

Mithrandir24601

@Blue I may have done :P

Vogel612

5:52 PM

I think you can cheat with the "Display Date Override"

but I have yet to try.

Mithrandir24601

@Vogel612 Interesting. I'll try this one for the next session and we can see if it works then

Quantum Computing

0

Q: Implementing a CCCNOT gate

A CCCNOT gate is a four-bit reversible gate that flips its fourth bit if and only if the first three bits are all in the state 1. How would I implement a CCCNOT gate using Toffoli gates? Assume that bits in the workspace start with a particular value, either 0 or 1, provided you return them to th...

quantum-computer quantum-gate

Vogel612

Your feed has a pretty boring name...

Blue

@Vogel612 Suggest something! ;)

Vogel612

maybe "Measure" from the Measure operation?

Mithrandir24601

6:00 PM

@Vogel612 Oh yeah, never got round to/totally forgot about asking about that...

Blue

@Vogel612 boring......:P

Vogel612

better than the sitename :)

Blue

We need something "crazy" and "funny" :D

Vogel612

CR's default feed is called "Captain Obvious" so don't go stealing that one.

Mithrandir24601

We could call the two feeds 'Alice' and 'Bob'

Vogel612

6:01 PM

that's infosec for you ...

Blue

@Mithrandir24601 That's a nice one

Vogel612

"Superposition" and "Collapse"?

Blue

But there's bound to be gender discrimination depending upon whom you choose to be the main site feed and whom you choose for meta site feed :P

Mithrandir24601

Oooh, 'Merlin' and 'Arthur', after the QMA complexity class

Blue

No one would understand that one XD

Vogel612

6:04 PM

QMA for meta and BQP for main?

user1271772

Merlin and Arthur are from the classical MA class

Blue

I think Alice and Bob is nice

heather

@user1271772 what about Guinevere?

;)

@Blue i agree.

glS

@Mithrandir24601 I vote for that! Otherwise Alice and Bob is also great

Mithrandir24601

@Blue The more obscure, the better, right?

user1271772

6:04 PM

Am I user 1271772?

Oh I am, hahaha didn't notice that was my number

glS

though Alice and Bob is more specific to QC, while Merlin and Arthur is more cstheory

Vogel612

Alice and Bob is the default infosec example, usually

Blue

@Mithrandir24601 Not necessarily, lol

@heather Okay, so now let's vote for which site's messenger Alice should be :P

Vogel612

Alice, Bob and Eve. I'm hearing Merlin and Arthur the first time and I'm studying a theory-focused cs course

Blue

Normally who sends the message in quantum protocols? Alice to Bob or Bob to Alice?

Mithrandir24601

6:06 PM

@Blue Alice, I think?

user1271772

Alice to Bob

heather

yeah, i'm pretty sure Alice sends to Bob.

Mithrandir24601

But yeah, it's true that I heard about Alice and Bob way before doing quantum info or computing

heather

@glS Alice and Bob are used in pretty much every cryptography example ever, I think.

Vogel612

@heather unless it's about CA-Root-Certificates

James Wootton

6:07 PM

Is this about renaming the Classical channel?

glS

mh, you are probably right. Still relevant for us!

Vogel612

@JamesWootton nah. about renaming the feeds

heather

We could do something like the qubit and the _ - i.e., the messages in the channel.

user1271772

I find the current name to be ok, and doesn't require modification in my opinion

Mithrandir24601

@heather Or name them after different protocols?

Blue

6:09 PM

"Although Alice and Bob were invented with no reference to their personality, authors soon began adding colorful descriptions. In 1983, Blum invented a backstory about a troubled relationship between Alice and Bob, writing, "Alice and Bob, recently divorced, mutually distrustful, still do business together. They live on opposite coasts, communicate mainly by telephone, and use their computers to transact business over the telephone."

Lol

Mithrandir24601

Call one of them Cow and the other ...?

Blue

Alice and Bob

Alice and Bob are fictional characters commonly used as placeholder names in cryptology, as well as science and engineering literature. The Alice and Bob characters were invented by Ron Rivest, Adi Shamir, and Leonard Adleman in their 1978 paper "A method for obtaining digital signatures and public-key cryptosystems." Subsequently, they have become common archetypes in many scientific and engineering fields, such as quantum cryptography, game theory and physics. As the use of Alice and Bob became more popular, additional characters were added, each with a particular meaning. == Overview == Alice...

@heather So let's just use alphabetical order: Alice for main site and Bob for meta, to avoid controversies...lol

@F.Siciliano Holla!

Mithrandir24601

@Blue One of my friends came up with an amazing story about them once, saying that they met in a future world, only had to travel some distance apart in the galaxy. That was sweet :)

Blue

@Mithrandir24601 That sounds sweet, indeed :P

Mithrandir24601

@Blue How about calling one Alice and the other Merlin or Arthur?

user1271772

6:13 PM

Alice and Merlin

Blue

@Mithrandir24601 Eh, there's shouldn't be any Alice without Bob XD

James Wootton

If it is going to be Merlin or Arthur, it has to be Merlin

Blue

Not morally acceptable to me :P

James Wootton

The existence of Bob implies the existence of Alice, but Alice can exist independently

in theory, that is. I don' think I've ever observed an isolated Alice

Blue

Don't ruin the amazing love story @Mithrandir24601's friend came up with...lol. (BTW it's sort of fun to quarrel about these petty things after all the heavy discussion before this :))

heather

6:17 PM

i think i'm becoming in favor of Merlin and Arthur.

Mithrandir24601

@Blue Ah, but that's long gone now. Quantum information can only travel at the speed of light, so Bob may be left forever wondering if Alice agreed to marry him :'(

Although it was possibly Alice that asked Bob, I've forgotten

@heather Could post a meta question and see what people think?

Vogel612

somewhat unrelated: I was reading through the wikipedia article on QMA and was surprised to read that it's not clear whether P \subsetneq PSPACE, only that NL \subsetneq PSPACE

Blue

@Mithrandir24601 coughs...wormholes...coughs :P

@Vogel612 That is true

6

A: Is there any general statement about what kinds of problems can be solved more efficiently using a quantum computer?

TL;DR: No, we do not have any precise "general" statement about exactly which type of problems quantum computers can solve, in complexity theory terms. However, we do have a rough idea. According to Wikipedia's sub-article on Relation to to computational complexity theory The class of proble...

@meowzz Heyo