The h Bar

6:00 PM

but see, I didn't pick a basis (i don't know what I'm doing). i was just like, okay, first qubit is in |0> state, then I apply a NOT gate so I get a |1> state

it's bad.

i guess i don't understand how to do what you're talking about.

DanielSank

For example, with this basis, the matrix representation of a NOT gate on the first qubit is $$ \left[ \begin{array}{cc} 0 & 1 & 0 & 0 \\ 1 & 0 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \end{array} \right] \, .$$

i.e. we turn $|00\rangle \leftrightarrow |01 \rangle$ and leave everything else alone.

@heather That should work too.

Swapnil Das

Hi guys.

heather

@DanielSank and then I do that for all the qubits individually...but there's CNOT gates and stuff so I don't know if that's right. I just look at the state of one and depending on it apply the NOT gate to the other.

and I'm not getting the right answer.

DanielSank

@heather Give me an example of a state on which you want to apply a CNOT.

Swapnil Das

I've a really silly doubt, so please someone care to answer.

Doesn't centripetal force act towards the centre? If so, why's the FBD showing outward direction?

DanielSank

6:05 PM

@SwapnilDas Because the diagram is crap.

Swapnil Das

And the equations above?

DanielSank

@SwapnilDas What about them?

heather

@DanielSank well, the first qubit is in the |1> state and the second qubit is in the |0> state. So I apply a CNOT where the target qubit is the second qubit and the control is the first qubit. So, since the 1st qubit is in the |1> state I said the 1st qubit stays in the |1> state and the second qubit flips to the |1> state.

(one example)

Swapnil Das

@DanielSank Are they correct?

DanielSank

@heather Less words, more math.

Initial state: $$|01\rangle$$

Now you want to apply CNOT with the second qubit as target.

Swapnil Das

6:07 PM

T= mgcos(theta) + mv^2/l

DanielSank

Ok, then you get $$|11\rangle$$

heather

right

DanielSank

Note that the "first" qubit is the rightmost.

Swapnil Das

I was referring this one^^

heather

but I guess I'm not writing it like that.

maybe I'll try to rewrite it like you're describing so it is clearer.

DanielSank

6:08 PM

Well there are a variety of notations.

Some folks write $|\psi\rangle_1 |\phi \rangle_2$ etc.

heather

oh...how do you note the CNOT (with the second qubit as target)?

DanielSank

Where the subscripts are the qubit indices.

@heather Dunno. How about $\text{CNOT}_{1,2}$?

heather

okay...

i'll try.

maybe i'll find the error in my math.

Swapnil Das

Is centripetal force pre existing?

DanielSank

@SwapnilDas I don't know what you mean by "pre existing"

Swapnil Das

6:12 PM

Hmm. Did you have a look at my math?

DanielSank

Your math? You posted a picture of a book.

Swapnil Das

That eqn too.

DanielSank

I don't want to squint at that book photo.

If you'd like to ask a question with a diagram that's easier to read, I'd be happy to help.

Swapnil Das

Lol OK. Thoughts?

Forget about the details of the diagram.

Simple question: if a body is in circular motion, aren't Tension and Centripetal force in the same direction? @DanielSank

DanielSank

@SwapnilDas Yes.

Swapnil Das

6:16 PM

Here, I take the body to be a bob

So, the equation T= mgcos(theta) + mv^2/l is wrong? @DanielSank

DanielSank

dude, use mathjax

Make it easy for people to help.

Swapnil Das

I donno that :P

How's that helpful, actually?

DanielSank

$$T = m g \cos(\theta) + m v^2 / l$$ is a hell of a lot easier to read.

Oh wait a second... do you have chatjax on?

Swapnil Das

I'm on mobile.

Don't think so.

Anyway, could you answer my question? @DanielSank

I'm sure to have centrifugal dreams tonight

DanielSank

Why do you think that equation is wrong?

Draw a free body diagram and balance the forces.

Swapnil Das

6:25 PM

I did. What forces to include?

If tension and centripetal force are to be in the same direction, then there's should be a minus in the equation, no?

DanielSank

That equation is for the magnitude of the tension. It has nothing to do with the direction.

Swapnil Das

I meant the plus in between.

$$T + F = mg \cos(\ theta)$$

My first try for mj :P

DanielSank

I don't know what F is.

AccidentalFourierTransform

its a letter

Swapnil Das

Centripetal force.@DanielSank

I want to find the error in my concept, I still can't :(

DanielSank

6:33 PM

I'm not going to do the problem. Sorry.

Busy.

If you have a question about something I'll try to answer.

I'm not interested in trying to figure out what the problem is though.

Unless you want to post a clearer diagram.

heather

well, I rewrote the math @DanielSank

and now it is much easier to read

DanielSank

good

heather

but there is still an error.

first, i think there may be an error in how i'm implementing the toffoli gate using cnot gates

because there is a difference in the results for each.

DanielSank

@heather k

That sounds complicated.

heather

eh...i'm probably just making it complicated (more than it needs to be).

DanielSank

6:36 PM

Why don't you back off and just let the Toffoli be a Toffoli.

Avoid doing all the complex stuff all at once.

heather

i can do that, but the thing is the simulator i'm using doesn't allow the use of a toffoli gate.

but for now, i'll stick with the toffoli gate, i can worry about that later.

the second problem is that the second two qubits are not ending up in the correct state.

only the first qubit (the flipped qubit) is.

I start in the state |001> and end up in the state |111>

where the flipped qubit is the first qubit.

DanielSank

@heather be more specific.

"...not ending up..." when you do what?

heather

in the middle I apply NOT gates CNOT gates and a Toffoli gate. (the bit-flip code)

DanielSank

LESS WORDS, MORE MATH

Use concise, precise notation.

English is crap for describing technical stuff.

heather

|000>

NOT${}_1\to$|001>

CNOT${}_{1,2}\to$|011>

CNOT${}_{1,3}\to$|111>

NOT${}_1\to$|110>

CNOT${}_{1,2}\to$|110>

CNOT${}_{1,3}\to$|110>

DanielSank

6:47 PM

Looks good so far.

heather

Toffoli${}_{2,3,1}\to$|111>

::end::

DanielSank

I forget what a Toffoli is.

Is that a double controlled not?

If so, everything you wrote looks legit.

heather

Toffoli gate

In logic circuits, the Toffoli gate (also CCNOT gate), invented by Tommaso Toffoli, is a universal reversible logic gate, which means that any reversible circuit can be constructed from Toffoli gates. It is also known as the "controlled-controlled-not" gate, which describes its action. It has 3-bit inputs and outputs; if the first two bits are set, it inverts the third bit, otherwise all bits stay the same. == Background == An input-consuming logic gate L is reversible if, for any output y, there is a unique input x such that applying L(x) = y. If a gate L is reversible, there is an inverse gate...

yeah, double controlled not

@DanielSank right, but it should end up with |001>

argh

(circuit shown if you click)

DanielSank

Uh, wut?

hahahaha, @heather you're confused for a funny reason.

You don't need to get the original states of qubits 2 and 3!

The point is the error correct qubit 1!!

The other qubits are helpers to that end alone.

heather

oh!!!

well that explains it

for some reason I thought all the qubits had to come out right

wow, that was stupid of me =P

@DanielSank thanks! =D

DanielSank

6:58 PM

Sure.

@heather, if you could get the original state of all qubits back... physics would be completely broken.

You'd be annihilating the second law of statistical mechanics.

You'd be undoing randomness.

:|

heather

darn it, physics =/

undoing randomness is my favorite.

(though that does make sense, now that I think about it.)

DanielSank

You can do it, but only by moving it elsewhere.

In this case, you're making a trade: you restore the state of qubit 1 at the cost of scrambling qubits 2 and 3.

Any system that appears to have undone randomness is putting it somewhere else.

This is such a common point of confusion, that you can get a gold badge for figuring it out.

;-)

heather

wow =)

DanielSank

Yeah, noise is often misunderstood.

That's probably part of why I like it.

JamalS

Has anyone encountered a sort of Taylor expansion, using the Lie derivative? I came across it in a paper on cosmic strings.

AccidentalFourierTransform

7:17 PM

that shit cray

JamalS

It's described as a 'Taylor expansion off the worldsheet', defined as, $Q = Q \rvert_0 + \xi^i \mathcal L_i Q \rvert_0 + \frac12 \xi^i \xi^j \mathcal L_i \mathcal L_j Q \rvert_0 + \dots$

AccidentalFourierTransform

I thought that $Q=CV$ =P

> "This chapter discusses how a natural extension of the concept of directional derivatives in $\mathbb R^n$ can be defined for functions on Lie groups.1 This “Lie derivative” is closely related to the differential geometric properties of the group. Functions on a Lie group can be expanded in a Taylor series using Lie derivatives.2 Explicit expressions for these Lie derivatives..."

@JamalS ^ see this. Just google "Taylor expansion, using the Lie derivative" and check the first result

JamalS

@AccidentalFourierTransform Hmm, this is not in the context of a Lie group though. It's purely geometric, in the Gauss-Codazzi formalism

AccidentalFourierTransform

argg

JamalS

I'll just email the author

She's replied to me before

AccidentalFourierTransform

7:25 PM

oh cool

JamalS

Because the string is being treated as a submanifold, and in the paper they are expanding using the normal vectors, I guess the geometric interpretation is that you're sort of going away from the manifold, as the normal space is orthogonal to the tangent space

I guess that's probably what is meant by, 'off the worldsheet.'

dmckee

Anyone have an opinion of good resources for how to build on-line courses?

7

AccidentalFourierTransform

@dmckee no idea

but, out of curiosity, what are you up to?

dmckee

@AccidentalFourierTransform My department head has offered summer work teaching the first on-line run of one of our bread and butter services classes.

I'm worrying about two things: first, is there enough time to be ready by summer and second how in the heck do you build a good on-line class in the first place?

And I need to be able to give her an answer in a week or two, so I don't have a lot of time to find these things out.

Summer pay is a good thing in my current fiscal situation, but for complicated reasons teaching in person is difficult this year, so...

AccidentalFourierTransform

I see

well, I have no idea and I don't even know who to ask :-/

but good luck with that

dmckee

7:43 PM

Thanks. I know its a long shot for any particular denizen on the h bar, but there are a bunch of folks who drop it, so maybe I get lucky.

JamalS

8:15 PM

@dmckee Can you contact anyone in charge of the MIT OCW website?

I once also built an online class myself...

dmckee

@JamalS The MIT opencourseware for the related subject is just a package of notes and videos of a normal lecture. That kind of misses the point and most students won't engage properly. I'd like to build something that takes advantage of the medium. At least in the easy ways.

JamalS

@dmckee What about contacting the edX then?

I mean, if it's how to do it technologically, then that's another question. But as for what you should implement, there are so many great examples online, you should have no problem.

dmckee

My problem is that I don't know where the great examples are.

I've never spent any time on this task at all, and the only online course I have taken was in mathematical cryptography. For fun.

heather

9:12 PM

@dmckee, well, my first thought is Khan Academy plus extra stuff - videos/tutorials, short checks, submitting homework, and then occasional tests. The results of all of these would of course be submitted to the teacher. Maybe chatrooms for discussion with teachers or other students (one "official" chat plus students can create their own for study groups).

and then simulations maybe that they can mess with

to replace experiments (plus a video of you doing the experiment or whatever)

dmckee

@heather Simulations never replace experiments. Simulation do what the programmer told them to. Experiments do what nature tells them to. You can supplement experiments and demos with simulation but they simply don't replace the real thing.

heather

@dmckee, sorry...would the simulations at least be useful to some extent?

DanielSank

10:00 PM

@dmckee Excuse me, um, you appear to have turned into some sort of duck.

Just thought you'd like to know.

@dmckee Depends. What's the course?

I'm coming from zero experience here, but I think some online courses focus too much on their online-ness and forget what makes a course good in the first place. If it were me in the teacher's chair I'd release reading material and homework, just like a normal course. That leaves only lectures and office hours. Lectures can be done with any variety of video-conferencing software. Same with office hours. You can use something like Google calendar to schedule office hours with particular students.

If you want something that integrates all of the above, I have no idea ;)

Loong

@DanielSank Sula nebouxii

DanielSank

@Loong Eh?

oh

Blue-footed booby

The blue-footed booby (Sula nebouxii) is a marine bird native to subtropical and tropical regions of the Pacific Ocean. It is one of six species of the genus Sula – known as boobies. It is easily recognizable by its distinctive bright blue feet, which is a sexually selected trait. Males display their feet in an elaborate mating ritual by lifting them up and down while strutting before the female. The female is slightly larger than the male and can measure up to 90 cm (35 in) long with a wingspan of up to 1.5 m (5 ft). The natural breeding habitats of the blue-footed booby are the tropical a...

G. Bergeron

10:33 PM

@DanielSank I have been questioning why you said that for a time, thinking of why you might say this sequence of operations would not be reversible (really, almost answering to the contrary) and trying to understand your reference to noise until I read the words bit-flip....

@DanielSank This happened to me a couple days ago

dmckee

@DanielSank The course is first semester college physics. Meaning mechanics through waves with a algebra/trig math basis.

It our second highest volume course (right after the gen ed survey), but we haven't taught it on-line up til now.

@DanielSank The thing that doesn't work with most students is recording 30+ minute lectures and supplying the in-class material as a lump.

Sure the really motivated ones do fine with that (I took a Coursera on mathematical cryptography that was run that way, and slotting into it very nicely, but by then I had a Ph.D.).

As I understand it, the thing that makes it native to on-line is cutting it into smaller pieces and lumping the presentation, practice exercises and graded work into self-contained units of a size the students will digest.

But how big is that? And how do I chain them? And what degree of bondage and discipline should I try to enforce over the schedule (there must be some or they'll get too far behind, but too much breaks one of the advantages of on-line)?

G. Bergeron

@DanielSank But also, I followed the link and the fluctuation-dissipation theorem is a nice one, but is only valid for systems in equilibrium. I would say the exact reason for the phenomenon in the question is that dissipation implies an interaction with many ignored degrees of freedom which can then influence the degrees of freedom of interest. In essence the content of the theorem but as a more general principle.

dmckee

I have a forum available, and a kind of limited video chat with a shared whiteboard, but when I've administered courses from other folks' templates the students have barely used the forum and shown not the slightest interest in chatting.

G. Bergeron

@dmckee Aren't the grades motivations enough?

dmckee

@G.Bergeron ::chuckles::

The very first thing they tell you when you get to be a professor is "Remember, your students aren't younger versions of you."

And then you try to teach the first term as if they were anyway.

G. Bergeron

10:45 PM

@dmckee Getting a textbook thrown at me with a deadline in the form of an exam is certainly enough for me

dmckee

@G.Bergeron Me, too.

G. Bergeron

@dmckee I wanted grants...

dmckee

But more than half your majors are different and as are nearly everybody in the service courses.

After the disaster you make of the first term you start coming to terms with what is needed to get the typical student to do an adequate amount of learning in the course of one term.

G. Bergeron

@dmckee Ok, maybe I'm wrong on that point, but how come knowing that passing the class sits between them and what they want not be enough? I thought hand-holding was not necessary at university level anymore.

dmckee

It's probably worse for me. I'm at the newest and smallest "university" my state runs.

@G.Bergeron I don't do a lot of hand-holding and I get some negative comments on reviews for that.

G. Bergeron

10:49 PM

@dmckee Wow, that is dedication! I really see this as not the responsibility of the university

dmckee

But I do structure the course in bite-sized pieces with some explicit foreshadowing of the fact that we're going to need this in $N$ weeks, so get used to it now.

G. Bergeron

The way I see it, it's up to the students to know they're getting behind to much!

vzn

@heather what simulator are you using?

Bernardo Meurer

@dmckee You don't seem to be on ratemyprof

dmckee

@G.Bergeron The current structure of the economy and the easy availability of loans is driving many students into college. Many that wouldn't have come in previous generations, and many who should come yet because they aren't adult enough.

G. Bergeron

10:50 PM

@dmckee That's more then enough. You shouldn't even have to do this. Just say be ready for everything at the exam! :)

dmckee

And my place gets the tough end of that.

At least the math department shields us from the really bad cases.

There are students who take remedial pre-algebra three or more times before they either pass of give up.

G. Bergeron

@dmckee But I know, a colleague who is teaching at something that would correspond to first year undergrads was expecting things to be like I said and told me he was proven otherwise

dmckee

@BernardoMeurer As far as I can tell students aren't using it much any more. I had a couple of lackluster comments when I did some adjunct work at other schools.

G. Bergeron

@dmckee And what would be the problem of them failing? University is not meant to be easy.

dmckee

@G.Bergeron SO the key question is who was he teaching.

Bernardo Meurer

10:53 PM

It seems like the grades are based on how easy it is to get an A in part

DanielSank

@G.Bergeron yes.

G. Bergeron

@dmckee Children of rich parents...

Bernardo Meurer

also, "caring" seems like a major factor in your grading

dmckee

@G.Bergeron People who can't pass that course on two tries probably shouldn't be here in the first place, but there are pressures that drive them to it even when it is not a natural decision for them.

G. Bergeron

@dmckee I know, but still, they fail and then what?

dmckee

10:55 PM

@G.Bergeron Who breaks into three groups: those who have developed self-discipline, those who are disciplined by their parents, and those who put more energy and effort into avoiding doing the course work than would be required to do it.

I taught some of those at one of my adjunct gigs.

It's easier, because the class mostly tries, but the wailing and gnashing of teeth when not doing the work is reflected in the grades is a sight to behold.

G. Bergeron

@dmckee Yeah well he was not expecting ''hand-holding'' in any shape or form.

vzn

@dmckee google turns up these apparently halfway decent refs nea.org/assets/docs/onlineteachguide.pdf uwex.edu/disted/conference/Resource_library/proceedings/… elearnmag.acm.org/featured.cfm?aid=1609990

dmckee

@vzn Google turns up a lot of things. I wanted the opinion of someone I know, because then I can judge how their experience might relate to my own.

G. Bergeron

@dmckee Anyway, good luck! I can't help you much seeing I never did that and I would assume it is not my responsibility to motivate students.

dmckee

People have hugely different styles and philosophies of teaching so you have to know who is giving advice as well as what they advise.

vzn

11:01 PM

@dmckee it seems almost nobody else here teaches. maybe some ppl take online classes (but not in college "for-credit" setting)...

rob

@dmckee Do you have other people doing online classes at your institution, or would you be a local pioneer?

dmckee

One existing professor in my department has done this recently (he did the the gen ed survey) and I have an appointment to sit and chat with him, but he's playing single parent while his wife does some consulting work about five hundred miles away, so scheduling is tough.

JamalS

"You've earned the tumbleweed badge (asked a question with zero score, no answers, no comments, and low views for a week)"

So glad I achieved that - always wanted to have that badge...

dmckee

And the distance learning center has been more helpful to me in the past with questions about what the software does and how to run it than with question about how to make it work for creating learning.

@JamalS It is kind of a distinction. I have it somewhere, but I can't recall on which site(s).

JamalS

I can't believe this didn't get a single vote: physics.stackexchange.com/questions/301551/…

I found it quite interesting. (Well, I guess it goes without saying since I asked it.)

heather

11:56 PM

@vzn ibm quantum experience

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