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3:49 AM
2
Q: What are some open sources projects on quantum computing?

Enrique SeguraI am looking to collaborate on open source simulation efforts.

 
 
4 hours later…
8:02 AM
@glS @Mithrandir24601 Posted it. Let me know if I made any silly mistake anywhere :P
 
0
Q: Quantum algorithm for linear systems of equations (HHL09): Step 1

BlueI 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...

 
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
 
@Blue Yeah, I agree with @glS on this - 2 separate questions might be better
 
@Mithrandir24601 @glS Okay, on it!
 
glS
by the way, what's up with all the downvotes in quantumcomputing.stackexchange.com/q/2381/55??
 
@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
 
0
Q: Quantum algorithm for linear systems of equations (HHL09): Step 1 - Number of qubits needed

BlueThis 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...

 
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?
 
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.
 
@glS Yeah, same - I haven't upvoted anything but neither have I downvoted anything
 
@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
 
I didn't get the 6 * log(N) conclusion, especially the factor 6. From where the "6" pops up?
 
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 ^?
 
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
 
@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.
 
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.
 
@Nelimee Yes
I mentioned that part in the question
 
Then why do we need to encode the log(N) eigenvalues in log(N) different quantum registers?
I go read your edit!
 
9:12 AM
@Nelimee How would you represent the superposition of $N$ eigenvalues using just $6$ qubits?
 
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
 
@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
 
BTW I have question on the R(lambda^-1) part, but I will make a post for it =)
And yep, I got your point
 
@Nelimee I'm thinking of making one too
 
Thanks for the explanations !
 
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?
 
Yep, my mistake, I was considering what you just said ^^
 
@glS I was a bit confused about that
 
glS
@Blue what's [13]?
 
Now I got the log(N) part.
 
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
 
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
 
@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 $\sum_{j=1}^{\text{whatever}} |u_j\rangle$
 
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!
 
No problem!
 
@glS Okay thanks. I'll think about it!
 
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)
 
@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
 
@Blue For the whole HHL, yes. I'm speaking about QPE when you want improved accuracy
 
9:39 AM
@Nelimee Okay, gotcha
 
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)
 
@Nelimee But those are required for the necessary precision we want, isn't it?
 
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
 
@Nelimee Right
 
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
 
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
 
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 =)
 
@Nelimee I think just $\text{max}(\lceil{\log_2(N)\rceil},6)$ should be good enough. A single register should be sufficient
See you!
 
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?
 
@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 $\sum_{j=1}^{\text{whatever}} |u_j\rangle$
 
Yup!
 
9:54 AM
For this reason I guess ^
@Nelimee Which rotation part?
The R gate?
 
the R(lambda^-1) part
 
@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
 
 
1 hour later…
11:16 AM
1
Q: Generating algebra from commutation

John DoeIn 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...

0
Q: Quantum algorithm for linear systems of equations (HHL09): Step 2 - What is $|\Psi_0\rangle$?

BlueThis 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...

 
11:31 AM
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
 
11:59 AM
@glS It's okay. No hurry !
 
@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 :)
 
12:18 PM
@Nelimee Sure. Take your time :)
 
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?
 
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
 
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 :)
 
@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?
 
@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
 
@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.
 
Honestly, I don't like the question much. It's too broad for any useful answer.
 
@JamesWootton then it's beginning to sound like a discussion on whether or not this question is too broad?
 
2:20 PM
@Blue Ok I edit my posts :)
 
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.
 
Fair. It's not the easiest thing to answer, to my mind, anyway
 
@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.
 
@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.
 
@Blue: I have helped you a tremendous amount on this site. Period.
 
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
 
@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
 
@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.
 
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
 
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
 
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
 
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.
 
That qubit question is so insane. All those answers are correct. Why are they all being downvoted?
2
 
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
 
morning everyone.
 
@heather Late evening here. But well, good morning ;)
 
=P
kinda weird how much answers on that qubit question are being downvoted.
 
Indeed. I think it should be closed before more people spend their time answering such a broad question.
 
no, CW isn't really used anymore
 
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
 
What's CW?
 
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.
 
@Blue: I agree, but I was the only one that voted to close.
 
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?
 
@heather: after CW is clicked, no one gains or loses any points, no matter how many up-votes or down-votes?
 
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)
 
@heather: have you read my comment on the question?
 
@user1271772 yup, but again, not really used much.
@Blue true, but even for canonical stuff it's not always used anymore.
 
@user1271772 I had voted to close it (around 30 mins ago).
 
@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?
 
@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?
 
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).
 
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
 
downvotes on answers don't necessarily mean it's broad.
but alright, i follow some of your logic.
 
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.
 
well, I can't say I particularly like SO's model - it's rather notorious for being "not nice" regularly to new comers.
 
@heather That's true
 
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.
 
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.
 
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?).
 
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.
 
the smaller the site the tighter knit the community and i think the "nicer" and more welcoming they can be.
 
@heather: Splitting up SO is an excellent idea.
 
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.
 
@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
 
@user1271772 you can retract close votes.
(as long as it is before the question is actually closed; afterwards you must vote to reopen).
 
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.
 
@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)
 
@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" ;)
 
3:47 PM
0
Q: Over-zealous editing by Blue (Part III)

user1271772In 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...

 
@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
 
@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.
 
4:26 PM
0
Q: Over-zealous editing by Blue (Part IV)

user1271772Please 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...

 
This is getting beyond ridiculous now :P
 
@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.)
 
@heather Okay, noted
 
@Blue thanks.
we'd like to get this cleaned up without anything else starting.
 
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.
 
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.
 
@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.
 
@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
 
^
 
@ArtOfCode What brings you here?
Was there a flag?
 
nothing at all
 
4:50 PM
Ah, cool then ;)
 
just a completely and wholly random visit unconnected to any recent event, flag, or message
 
It gets a bit dramatic whenever we have mods from other sites coming in :P
@ArtOfCode Good to know!
 
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.
 
Right - when it's locked, you can't delete it.
 
@user1271772 OK, I've unlocked part IV
 
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.
 
Mithrandir: Have you read my Part III from beginning to end?
 
where is part 2? I can’t seem to find it
 
5:20 PM
@JamesWootton I believe this is part II
While this was, in effect, part I
 
I actually haven't read them, because I was totally disinclined to after reading the accusatory titles.
 
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.
 
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.
 
If you have an issue with what one user is consistently doing, then you can also raise a custom moderator flag.
 
@Mithrandir: How do I raise a custom moderator flag and what is it?
 
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
 
Ok, I think that's quite highly discouraged, and also takes a very long time to get a response.
 
@user1271772 "I think that's quite highly discouraged" --- that's not true (as long as your problem is genuine)
 
We're encouraged to start at lower levels and raise issues on Meta rather than raising the custom moderator flag
 
@user1271772 That is true
 
@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?
 
5:27 PM
@user1271772 It's a biological fact. I don't think we're banned from discussing science ;)
 
tiny steps...
 
@JamesWootton: The abs vs pdf if Part II, the CCNOT -> CNOT is in Part III
 
@user1271772 some things work better on meta, some things work better as custom mod-flags
 
Do you not understand why changing CCNOT -> CNOT is a problem, if I already answered the question "Is it a CCNOT" ?
 
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.
 
5:30 PM
@Vogel612 Huh?
A mod (James Wotton) is clearly asking @user1271772 for clarification. I don't see the issue.
 
@user1271772 noted, thanks for the clarification
 
@Blue FWIW that clarification can be phrased significantly less confrontational, so I preemptively tried to defuse...
 
@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
 
@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?
 
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.
 
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?
 
ugh...
why not just ask me if I have an ambiguous reference?
 
I'll let Vogel clarify :)
 
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" ?
 
@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.
 
^^ that message to be exact
 
5:38 PM
@Vogel612 sorry about that =)
hopefully it's clear which message you are actually pointing to.
 
We have the chat session soon :/
 
@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
 
The conversation appears to be deteriorating. May I suggest taking a breather and continuing later?
 
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.
 
thank you.
let us move on, and the mods can think about the situation and ask you guys if we have any questions.
 
5:44 PM
Bug? O:)
 
probably. I doubt it's going to get much dev-time alotted to it, though
 
Probably some problem in setting the timer
 
@Blue I'll try and hack my way round this for the next session...
 
I don't think it needs dev interference
@Mithrandir24601 Yay!
 
@Mithrandir24601 did you perchance register the event from the community events interface on-site as well?
 
5:47 PM
@Vogel612 How did you know? :P Let me guess, there's no workaround?
 
the workaround is to not do that
It should be sufficient to just register the event from the chatroom schedule
 
@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...)
 
It seems to be fixed now @Mithrandir24601
Did you do something?
 
"the workaround is to not do that" ~ best answer!
 
@Blue I may have done :P
 
5:52 PM
I think you can cheat with the "Display Date Override"
but I have yet to try.
 
@Vogel612 Interesting. I'll try this one for the next session and we can see if it works then
 
0
Q: Implementing a CCCNOT gate

chusterA 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...

 
Your feed has a pretty boring name...
 
@Vogel612 Suggest something! ;)
 
maybe "Measure" from the Measure operation?
 
6:00 PM
@Vogel612 Oh yeah, never got round to/totally forgot about asking about that...
 
@Vogel612 boring......:P
 
better than the sitename :)
 
We need something "crazy" and "funny" :D
 
CR's default feed is called "Captain Obvious" so don't go stealing that one.
 
We could call the two feeds 'Alice' and 'Bob'
 
6:01 PM
that's infosec for you ...
 
@Mithrandir24601 That's a nice one
 
"Superposition" and "Collapse"?
 
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
 
Oooh, 'Merlin' and 'Arthur', after the QMA complexity class
 
No one would understand that one XD
 
6:04 PM
QMA for meta and BQP for main?
 
Merlin and Arthur are from the classical MA class
 
I think Alice and Bob is nice
 
@user1271772 what about Guinevere?
;)
@Blue i agree.
 
glS
@Mithrandir24601 I vote for that! Otherwise Alice and Bob is also great
 
@Blue The more obscure, the better, right?
 
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
 
Alice and Bob is the default infosec example, usually
 
@Mithrandir24601 Not necessarily, lol
@heather Okay, so now let's vote for which site's messenger Alice should be :P
 
Alice, Bob and Eve. I'm hearing Merlin and Arthur the first time and I'm studying a theory-focused cs course
 
Normally who sends the message in quantum protocols? Alice to Bob or Bob to Alice?
 
6:06 PM
@Blue Alice, I think?
 
Alice to Bob
 
yeah, i'm pretty sure Alice sends to Bob.
 
But yeah, it's true that I heard about Alice and Bob way before doing quantum info or computing
 
@glS Alice and Bob are used in pretty much every cryptography example ever, I think.
 
@heather unless it's about CA-Root-Certificates
 
6:07 PM
Is this about renaming the Classical channel?
 
glS
mh, you are probably right. Still relevant for us!
 
@JamesWootton nah. about renaming the feeds
 
We could do something like the qubit and the _ - i.e., the messages in the channel.
 
I find the current name to be ok, and doesn't require modification in my opinion
 
@heather Or name them after different protocols?
 
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
 
Call one of them Cow and the other ...?
 
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!
 
@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 :)
 
@Mithrandir24601 That sounds sweet, indeed :P
 
@Blue How about calling one Alice and the other Merlin or Arthur?
 
6:13 PM
Alice and Merlin
 
@Mithrandir24601 Eh, there's shouldn't be any Alice without Bob XD
 
If it is going to be Merlin or Arthur, it has to be Merlin
 
Not morally acceptable to me :P
 
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
 
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 :))
 
6:17 PM
i think i'm becoming in favor of Merlin and Arthur.
 
@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?
 
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
 
@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?

BlueTL;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
 
6:43 PM
hello!
 
@meowzz Sup? We're sort of sitting out this chat session :)
@meowzz Please consider introducing yourself if you don't mind!
It seems you're new here
 
7:14 PM
indeed! not sure where to start..
i ended up here after a series of less than stellar experiencs elsewhere. so far i really like it!
 
@Mithrandir24601 somehow i'm reminded of the fact that one silly name for nonlocal correlations is "passion-at-a-distance"
 
i didnt really expect it & have been in a bit of a daze the last week
 
@meowzz Good to know :)
You're always welcome to give any kind of feedback about how we can improve the user experience for newcomers
 
7:29 PM
so far i am really impressed by the community here & cant think of anything!
 
Hehe :P
 
@Semiclassical haha! This is a great name in this context though!
 
@Mithrandir24601 yup
 
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