The h Bar

@heather Phase matching is necessary because the speed of light of the down-converted (IR) photons is different to the original UV. That means that unless conditions are just right, the IR that's produced at one bit of the crystal can interfere destructively with the IR that's produced a bit further down the line.

and that can really hurt your conversion efficiency

BPM = phase matched

QPM = cheating the system into thinking it's phase matched

AccidentalFourierTransform

@EmilioPisanty k

heather

erm...okay, the speed of the light changes?

Emilio Pisanty

3:21 PM

@heather yup

same reason you get rainbows

light is slower inside materials than it is in vacuum

... in a wavelength-dependent way

heather

so when the wavelength changes, the speed changes.

Emilio Pisanty

exactly

heather

but, i assume the speed changes by some constant rate, and if it's all UV on one side of the crystal and IR on the other side, how does the light get out of phase?

Emilio Pisanty

the problem is the propagation inside the crystal

the first half of the crystal will produce some IR

and that will propagate with the UV to the second half

ideally, the UV will produce more IR in the second half, and this will interfere constructively with the existing IR

if, that is, the two waves are still in step when they get to the second half

heather

so the IR doesn't all get produced at the same time...

that sounds really finicky.

Emilio Pisanty

3:25 PM

no, it gets produced in a continuous way throughout the depth of the crystal

and if the IR that gets produced early on drifts out of sync with the UV, it can interfere destructively with the one produced later on

heather

is that why different companies are advertising super thin, small crystals?

Emilio Pisanty

@heather and yes, it can be pretty finicky

@heather probably

heather

huh, okay.

so - is there a way to make it highly probable for the light to stay in sync?

Emilio Pisanty

this is why there is a wavelength dependence

@heather yes. there's a big bag of tricks to make the UV and the IR keep in sync (called phase matching).

the problem is that they're pretty wavelength-specific

heather

so it all depends on which laser i pick.

and which crystal i pick, too, i suppose.

Emilio Pisanty

3:31 PM

(That said, there is also the fact that the SPDC photons will be at double the wavelength you started with. So, if you start with photons in the visible, say, 600nm, then that will produce 1.2um photons, which may be harder to handle. Thus you tend to use UV drivers, so the SPDC produced will be in wavelengths that are easier to handle. 350nm will produce pairs in the visible at 700nm. 400nm will put you at 800nm which is off the edge of the visible.)

@heather so yeah, it depends on both of those choices, which come interlinked

Hi, everybody.

@DanielSank hello =)

hi

@EmilioPisanty Oooooh are we talking about phase matching!?

So fun.

Emilio Pisanty

@DanielSank yup

DanielSank

3:33 PM

We have these special amplifiers called "travelling wave amplifiers" where you have to get that right.

heather

@EmilioPisanty so 350nm lasers are used because it produces light that's visible/easier to work with.

Emilio Pisanty

@heather that's one big pro

DanielSank

They're cool because they're quantum limited, i.e. as noiseless as quantum mechanics will let you make!

Emilio Pisanty

if you're doing this at home and this is your first time aligning optics, I would place a good deal of extra weight on that consideration

heather

@EmilioPisanty I also place a good deal of extra weight on price, and 350nm lasers cost a lot, according to my dad.

Emilio Pisanty

3:34 PM

aligning optics where you're not able to see the beam is no fun

(or so I hear)

DanielSank

@EmilioPisanty yesyesyesyes

heather

@EmilioPisanty yeah, that sounds difficult

Emilio Pisanty

there is also a safety consideration - if you can't see the beam, then you don't have a blink reflex, which makes it easier for stray beams to damage your eyes

though that depends on how much power you put in the beam

heather

does the laser need to be higher power for this operation?

DanielSank

^ Yes

Do not screw around with laser safety.

heather

3:36 PM

because i don't really want to blind myself.

Emilio Pisanty

@heather you probably need a reasonably high power in the UV

which probably means that you're already wearing laser goggles for the duration of the experiment

heather

@EmilioPisanty that sounds like a good idea.

Emilio Pisanty

@heather well, it depends on how much power you have around

(as an extreme case) if you are only putting in a microwatt of power into the system, you can probably do without them

ultimately, you need to check carefully what class is your laser driver, and use laser goggles that are appropriate for that class and wavelength

heather

okay.

hmm, i just found a 355 nm laser that's not too expensive.

DanielSank

link?

Emilio Pisanty

3:40 PM

but bottom line is you still need someone that's trained in laser safety to be around and check that you're not going to blind yourself

heather

@DanielSank qphotonics.com/…

DanielSank

Oh, diodes. Yeah those should be cheap.

What are you doing, anyway?

Besides trying to blind yourself?

@SevenSidedDie what brings you here?

heather

@DanielSank basically, take two entangled photons, phase shift one, and then show that they're still coherent.

DanielSank

o_O

heather

well, if you consider that i was originally going to use gamma photons produced by sodium-22, it's already gotten a lot more feasible

SevenSidedDie

3:42 PM

@DanielSank Good morning/afternoon/evening! It's in my Favourites list, so I get parachuted in whenever I reconnect.

heather

i have to head off for a bit. thanks @EmilioPisanty for putting up with my questions =)

Emilio Pisanty

@heather no worries

Sid

@SevenSidedDie GamEn!

heather

(and @DanielSank of course)

Emilio Pisanty

@heather for when you come back, this looks useful

(which you've probably seen, but still)

DanielSank

3:46 PM

I'm puzzled.

What entangled state are you wishing to create?

If you entangle the polarizations then phase shifting one of the photons shouldn't do anything particularly interesting.

Emilio Pisanty

@DanielSank I'm not sure there's a definite goal at this point, but I think she's going for some form of energy/time entangled state

Just got an email peddling this

academia.edu is impressively sleazy

DanielSank

Image too small. Cannot read text in green button.

Emilio Pisanty

@DanielSank image embiggens on click, but essentially says

DanielSank

@EmilioPisanty Does not. Tried.

Emilio Pisanty

"we found 25 papers that mention your name but we won't tell you which ones unless you pay us"

@DanielSank small green button on left just says "Get Started"

DanielSank

3:50 PM

Yeah, typical.

Emilio Pisanty

@DanielSank I dunno, I'm pretty sure academia.edu are on the edge of the envelope with these kinds of shenanigans

particularly as far as academic papers go

DanielSank

It's hard to be worse than journals.

1) Pay us moneyz to publish your stuff.
2) Review submissions for us for free. We no pay u.
3) Pay us to read published papers.

This is pretty good:

Recuerdo

►

Emilio Pisanty

@DanielSank well, yes, there's that

@heather another relevant link: xqp.physik.uni-muenchen.de/publications/files/theses_bachelor/…

but you might want to get your laser goggles on before you read it

@DanielSank no can see =|

DanielSank

@EmilioPisanty Why not?

Emilio Pisanty

@DanielSank "this video is not available :/"

DanielSank

3:54 PM

gardangit

Emilio Pisanty

@DanielSank proxy's up

DanielSank

Not sure what that means.

Emilio Pisanty

i.e. got it to work

why would anyone do this?

DanielSank

^ incompetence?

Emilio Pisanty

@DanielSank presumably

DanielSank

3:56 PM

Do you like the music? It's not your standard fare.

dmckee

@heather Er ... 350 nm is near UV. But it will show on bleached paper so it is relatively easy to track the beam through an apparatus. //used a 321 nm nitrogen laser for a while

John Doe

Hi all

Emilio Pisanty

@DanielSank I have many standard fares

DanielSank

@JohnDoe Hi.

dmckee

Make sure you have and wear the appropriate safety googles. You don't want cataracts in your teens.

John Doe

3:58 PM

@DanielSank Are you familiar with the stochastic Schrödinger equation?

AccidentalFourierTransform

TIL search engines block UV radiation

DanielSank

I am familiar with stochastic things and with the Schrodinger equation.

I am not familiar with the stochastic Schrodinger equation.

John Doe

@DanielSank Oh okay cool .

@DanielSank Did you see my comment on your post about rotated frames?

DanielSank

@JohnDoe Yeah but I was on my phone and the mathjax didn't render. Link?

Emilio Pisanty

@DanielSank presumably

6

A: What is the "interaction picture" or "rotating frame" in quantum mechanics?

Basic idea: the rotating frame "unwinds" part of the evolution of the quantum state so that the remaining part has a simpler time dependence. The interaction picture is a special case of the rotating frame. Consier a Hamiltonian with a "simple" time independent part $H_0$, and a time dependent p...

John Doe

4:04 PM

That's the one.

@DanielSank Is the rotated frame reformulation of the Hamiltonian used mostly or only when considering a time dependent perturbation?

Emilio Pisanty

@DanielSank I would like to confirm your reasoning for why "$R(t) := U_0(t,t_0)^{\dagger}$ has the particularly useful property that the first term in (*) cancles the $H_0(t)$ part of the second term." Is this working correct: $$i \hbar \dot{R} (t) R(t)^{\dagger} + R(t)H(t)R(t)^{\dagger} = i \hbar \partial_t U_0^{\dagger}U_0 + U_0^{\dagger}[H_0 + V(t)]U_0 = i \hbar \partial_t U_0^{\dagger}U_0 + U_{0}^{\dagger}H_0 U_0 + U_0^{\dagger}V(t)U_0 = i \hbar \partial_tU_0^{\dagger}U_0 + U_0^{\dagger}(i \hbar \partial_t U_0) = i \hbar[\partial_t(U_0^{\dagger}U_0)] = i \hbar[\partial_t 1] = 0$$ Thanks. — John Doe 2 hours ago

no, it's not correct

John Doe

Dammit

Emilio Pisanty

you drop $U_0^\dagger V(t) U_0$ for no reason

AccidentalFourierTransform

^

the rest is fine

Emilio Pisanty

but otherwise it's OK

AccidentalFourierTransform

4:06 PM

more comments please

John Doe

Oh okay cool thanks.

AccidentalFourierTransform

I want that formula away from the starboard, pls

Emilio Pisanty

you

want

what

?

more

AccidentalFourierTransform

almost there

Emilio Pisanty

space?

something?

or

DanielSank

4:07 PM

@JohnDoe so are we ok?

John Doe

@DanielSank Yeah thanks just wanted to confirm my understanding of that part.

DanielSank

ok, maybe delete that comment then.

Emilio Pisanty

also @JohnDoe try and avoid comments that muck up the formatting that badly ;-)

John Doe

@EmilioPisanty You joking?

Slereah

uridimma.hopto.org/misc/Regge

Collisions work a bit better now

Though it's a bit of a kludge

Emilio Pisanty

4:10 PM

@JohnDoe no, but no stress

Slereah

I'm not quite sure when two objects actually merge

I think I need to rewrite everything properly

Though that's not easy since Javascript is awful

The code's a bit of a kludge

I think what I need to do is to do a real collision system

Plot out trajectories in advance and check for collisions

djsmiley2k

4:30 PM

It's time for me to be really dumb

So this uncrackable encryption thing that china is claiming

Whats to stop a middleman reading the values, as from what I understand you can't test if the other party has already measured the value or not,

-- theregister.co.uk/2017/07/19/…

this is their claims...

And I'm just a small boy....

0celoñe7

@ACuriousMind what's the best BW ending?

Slereah

there's already quantum resistant cryptography

It's called lattice encryption

0celoñe7

@djsmiley2k 😼

ACuriousMind

@0celoñe7 I don't know what different endings there are, I only played it once so far

djsmiley2k

lol -- I thought of it like this. The cat is dead or alive until someone (anyone) checks. So the middleman spying on the whole situation checks. The cat is now dead. The middle man 'closes the box'. The end point recieve the box, the cat is dead.

Prior to them opening the box at the end point, do they 'know' it's been opened previously?

Suddenly I've had the realisation of what the thing in china is doing.

The cat is a number, it's constantly changing, however if the middleman looks, the number becomes static... and will no longer match what the sender is setting it to.

0celoñe7

4:41 PM

@ACuriousMind I imagine going to see the ancient vampire will end very badly.

@djsmiley2k l-that cat does not render well on windows

its supposed to be smirking

djsmiley2k

4:59 PM

lol

Secret

5:15 PM

RE: Regg: Wedge shaped orbits anormally no longer produced

Today's PhD comics

phdcomics.com/comics/archive.php?comicid=1963

Now thinking of ways to turn a professor into a grad student

0celoñe7

5:41 PM

@ACuriousMind this is a loooooooooong game

How is this even possible

djsmiley2k

Ah well I think I answered my own question in the end

Secret

Scenario: consider the following reference frame looking object A:

So before we computed A, we have the following simple kinetics going on: Two slabs moving past each other inopposite direction with some relative velocity v (and assume nonrelativistic)

Now, the computation of A makes distance (not just displacements) time dependent. The result is that you have a reference frame like object such that the slabs never moved, but their distance changes due to a time varying metric

ACuriousMind

@0celoñe7 Yeah, the sheer volume of hand-crafted content in that game puts most AAA publishers to shame.

0celoñe7

5:58 PM

@ACuriousMind do swords break if they get to 0?

there's no way to repair in this damn quest it seems

ACuriousMind

6:15 PM

@0celoñe7 Actually...no idea, I never let that happen

Secret

https://chat.stackexchange.com/transcript/message/38880932#38880932
hmm, ok, it is nothing very new in terms of the computation, but it does give a useful perspective of effectively freezing all motion by having the distances between the objects changes rather than the object moving about (as we suppose them to be fixed on a certain grid which is a reference frame we select)

0celoñe7

@ACuriousMind I don't really have a choice. Haven't seen a smith in 3 hours.

Secret

So using the simple case of two slabs passing by each other at constant v << c, then viewing the metric, the stabs are stationary inside the box we have chosen, but when we try to measure the distance between the two slabs (imagine an interface with a <---->arrow drawn and then a number pops on top of it), then the distance changes (the number itself varies as we step through the time of the simulation but things stay put in the box)

Hmm, I think this interpretation suggest I might be in some kind of comoving frame of the two objects since all notions of motion disappears and I only get time varying distances

I wonder if such notion of comoving frame can be found for any dynamics where the momentum of n objects are complicated functions of time...

ACuriousMind

6:32 PM

@0celoñe7 You don't have any tools with you?

0celoñe7

@ACuriousMind I used almost all of them

There was some other quest line where I didn't have access to smiths for a long time

0celoñe7

6:51 PM

@ACuriousMind ayyy wtf is this fight

Detlaff has an instakill through Quen

AccidentalFourierTransform

7:24 PM

this edit doesn't make sense as an edit, but perhaps it could be good to post it as a new answer?

as CW?

what do you guys think?

> I’ll have you know I graduated top of my class in the Navy Seals, and I’ve been involved in numerous secret raids on Al-Quaeda, and I have over 300 confirmed kills. I am trained in gorilla warfare and I’m the top sniper in the entire US armed forces. You are nothing to me but just another target. I will wipe you the fuck out with precision the likes of which has never been seen before on this Earth, mark my fucking words....

heather

8:05 PM

hello

Sid

Hey!

0celoñe7

@ACuriousMind Ok, done

Too many side quests left

that game is too much

David Z

8:22 PM

@AccidentalFourierTransform The person who suggested the edit would have to post it as a new answer, as presumably it only applies to them

heather

> Measurements of physical properties such as position, momentum, spin, and polarization, performed on entangled particles are found to be appropriately correlated. - wikipedia

would this include phase?

(i assume so, but just making sure.)

Slereah

Define "phase"

If you mean phase of EM fields, yes

the phase and amplitude of EM fields are correlated

heather

8:39 PM

i mean the phase of a wave, yes (specifically of a photon).

AccidentalFourierTransform

@DavidZ yeah but its a pity - the edit contains useful (?) information

oh well

whatever

0

A: How does breathing affect the reading on weighing machine?

Assume you inhale 5 litres of air. That's only 6 grams. So it doesn't affect much (air density 1.2g/l)

^ mod delete?

djsmiley2k

8:53 PM

wait, I'm wrong

I do still have a question

Is it possible, to know that a photon is entangled to another, without deciding it's state?

0celoñe7

9:05 PM

@ACuriousMind people unironically defined Detlaff

wtf

0celoñe7

9:27 PM

@ACuriousMind Definitely burnt out for a while. I'll go back and finish the side quests later. One of the best games I've ever played

Pretty amazing, really

ACuriousMind

@0celoñe7 I'm not sure what that means.

@0celoñe7 Toldja ;)

0celoñe7

@ACuriousMind I got the ending where Regis killed Dettlaff and the sisters reuinited.

People on reddit and other places say the best ending is where you let Dettlaff live

David Z

@AccidentalFourierTransform it was already flagged

ACuriousMind

@0celoñe7 ::shrug:: as I already said, I don't think there are "best" endings in this game, but I really didn't see much reason for Geralt to let him live. The sisters...didn't reunite in my playthrough, though...

0celoñe7

@ACuriousMind Did you romance Syanna?

ACuriousMind

9:39 PM

Nope

0celoñe7

@ACuriousMind My Geralt is very promiscuous ;)

AccidentalFourierTransform

@DavidZ I know, I stumbled upon it on the LQ list

I usually flag for mod attention anyway

bc reviewers usually "Looks Ok" that kind of posts

the answer is correct, but it is an explicit answer to a HW question

actually, that post has two "Looks OK" atm

physics.stackexchange.com/review/low-quality-posts/182965

David Z

@AccidentalFourierTransform I'm really not so sure about that... you may be right, but it's far from obvious

AccidentalFourierTransform

¯\_(ツ)_/¯

it wont get deleted by reviewers

if you mods think it should be deleted, you'll have to delete it yourselves

no big deal anyway

David Z

Yeah, we've been aware of that one.

I think the case for deleting it may not be strong enough to override a review, but we'll see

AccidentalFourierTransform

9:45 PM

sure

Neutral Response

►

0celoñe7

9:57 PM

@ACuriousMind in the definition of projective limit, what are the $\pi_{ij}$ supposed to be, exactly?

For direct limits they are the gluing functions

Is the projective limit supposed to be "zooming in"?

ACuriousMind

10:13 PM

@0celoñe7 They are "projections", although they're not required to be surjective

The "standard" example is getting the p-adic integers from the projective limit of $\mathbb{Z}/p\mathbb{Z} \leftarrow \mathbb{Z}/p^2\mathbb{Z} \leftarrow \mathbb{Z}/p^3\mathbb{Z} \leftarrow \dots$

0celoñe7

Only algebraists know what p-adic integers are

ACuriousMind

An element of that projective limit is an infinite tupel of numbers $a_i$, where each $a_i$ is in $\mathbb{Z}/p^i\mathbb{Z}$ and to be thought of as the remainder modulo $p^i$ of the full p-adic integer.

0celoñe7

That's not even an exaggeration, after looking it up I see no reason to know what they are

Do you have a more relevant example?

@ACuriousMind Yes, that's clear-ish.

But in that case the "projections" are surjective, no?

ACuriousMind

Yes, they are

The only non-algebraic example I know off the top of my head is a bunch of subsets of a set (or topological space) ordered by reverse inclusion

In that case all the "projectors" are injective, and the result is their intersection

0celoñe7

Yeah, that's also clear.

What's what I meant by "zooming in."

ACuriousMind

10:18 PM

I think it really depends on what the maps are doing

The p-adic case "feels" very different to me than the intersection of sets, although the construction is "the same"

0celoñe7

@ACuriousMind projective limits only seem to be addressed in the exercises in this book

I wish they gave more examples...the only example I can find is the dyadic integers.

ACuriousMind

Well, a more general example is the I-adic completion of a ring with respect to a maximal ideal I ;)

0celoñe7

@ACuriousMind What does that have to do with operator theory?

ACuriousMind

I really can't remember any non-algebraic occurence of the inverse limit, though.

@0celoñe7 I don't have the slightest idea

0celoñe7

@ACuriousMind The direct limit is central in functional analysis...

I wonder why the projective limit is abstent

Emilio Pisanty

10:29 PM

seriously?

0celoñe7

@EmilioPisanty pic for sub-10k scrubs?

Emilio Pisanty

> I can't get the image to appear right-side up so if you can save it and work on the questions.

0celoñe7

It appears to be upside down.

@EmilioPisanty Strange.

Emilio Pisanty

at least s/he deleted it after about two minutes

but still

user129412

I'm looking for a paper, but I can't find it anymore. Perhaps someone here knows it. I think it is from 1992, and it is about measurements of III-V semiconductors covered with gold. They measure either the fermi level pinning or the charge neutrality level or the Schottkey barrier, something like that. It has a figure in which a bunch of III-Vs are shown, from which you can for example see that InAs has an accumulation layer (or that the fermi level is pinned in teh conduction band)

I would think that I should be able to find it with the above, but so far no luck

Mithrandir24601

10:40 PM

That took all of a few seconds...

0celoñe7

@Mithrandir24601 wow

idk how to do that

Mithrandir24601

@0celoñe7 It was so ridiculously hard ;P

Emilio Pisanty

11:35 PM

Please do not vandalize your posts. By posting on the Stack Exchange network, you've granted a non-revocable right for SE to distribute that content (under the CC BY-SA 3.0 license). By SE policy, any vandalism will be reverted. If you would like to disassociate this post from your account, see What is the proper route for a disassociation request? — NobodyNada 21 mins ago

harsh

0celoñe7

@EmilioPisanty from profile "I'm a homeschooled Christian teenager from the Portland, Oregon area."

There's something special about people who identify themselves with a religion

Who would actually introduce themselves like that

@JohnRennie I made a cucumber and tomato salad, put a load of raw garlic in there. Fire!

user129412

@0celoñe7 Not too uncommon for an American to put a lot of weight on their Christian values. It is a large part of their identity. Not of all Americans of course, but for a certain subsection.