tag:blogger.com,1999:blog-5592542.post4262488419537972783..comments2024-03-18T17:28:44.693-07:00Comments on Rondam Ramblings: A quantum mechanics puzzle, part deuxRonhttp://www.blogger.com/profile/11752242624438232184noreply@blogger.comBlogger17125tag:blogger.com,1999:blog-5592542.post-18786468870735581962018-05-12T16:11:55.566-07:002018-05-12T16:11:55.566-07:00> The profound conclusion is that photons emitt...> The profound conclusion is that <i>photons emitted by a parametric down-converter do not produce interference</i>! [1]<br />><br />> … the photons emitted by a PDC are <i>entangled</i>, and entangled photons do not self-interfere. …<br />><br />> [1] This is not quite true. The strictly correct statement is that entangled photons do not produce first-order interference. They Lukehttps://www.blogger.com/profile/18395549142176242491noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-3375740828630941562018-05-09T23:54:05.010-07:002018-05-09T23:54:05.010-07:00> I'm not sure how to more accurately desc...> I'm not sure how to more accurately describe the properties of the coherent state emitted by the laser in ordinary language, though.<br /><br />Well, there's a lot more to say about laser light than just that. But it's a surprising property (at least it was surprising to me) of laser light that its photons are in superpositions of emission times. The fact that light travels atRonhttps://www.blogger.com/profile/11752242624438232184noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-41460427846578663422018-05-09T23:26:57.766-07:002018-05-09T23:26:57.766-07:00@Ron:
"Singlemode fiber lasers with linewidt...@Ron:<br /><i> "Singlemode fiber lasers with linewidths of a few kHz can have coherence lengths exceeding 100 km."</i><br /><br />Ah, my memory was faulty.<br /><br /><i>for a laser, it actually doesn't make sense to say that a particular photon was emitted by the laser at a particular time or by a particular atom, and this is *essential* to the operation of the laser.</i><br /><br Peter Donishttps://www.blogger.com/profile/09122769947782402203noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-38060254112247074932018-05-09T23:20:19.053-07:002018-05-09T23:20:19.053-07:00@Peter:
Hey, stop steppin' on my lines! ;-)
...@Peter:<br /><br />Hey, stop steppin' on my lines! ;-)<br /><br />Yes, coherence length, blah blah. "Singlemode fiber lasers with linewidths of a few kHz can have coherence lengths exceeding 100 km."<br /><br />But the really interesting thing is [spoiler alert!] that, for a laser, it actually doesn't make sense to say that a particular photon was emitted by the laser at a Ronhttps://www.blogger.com/profile/11752242624438232184noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-22621614637414485862018-05-09T22:43:04.614-07:002018-05-09T22:43:04.614-07:00it's a superposition of the photon being emitt...<i>it's a superposition of the photon being emitted at two different times.</i><br /><br />To clarify a little bit, this sort of superposition is not always possible. Roughly speaking, the requirements for it to work are:<br /><br />(1) The photon state has to be a coherent state (which is true for a laser);<br /><br />(2) The state has to have valid "support" at both times (i.e., Peter Donishttps://www.blogger.com/profile/09122769947782402203noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-51779052055460116352018-05-09T22:18:57.150-07:002018-05-09T22:18:57.150-07:00@Don:
maybe you could contrast it with the "o...@Don:<br /><i>maybe you could contrast it with the "overall state" for this description of Ron's: [shutter/laser only on for short periods]</i><br /><br />In this case (with a caveat, see below), the photons arriving at the detector can't be in a superposition of having been emitted at two different times, because the laser was not on at both of those times. In other words, for Peter Donishttps://www.blogger.com/profile/09122769947782402203noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-69412589058616576492018-05-09T22:12:18.160-07:002018-05-09T22:12:18.160-07:00@Don:
if it takes path s, it should hit the detect...@Don:<br /><i>if it takes path s, it should hit the detector long, long before it can get through path p.<br /><br />Why is there interference? What does the overall state look like?</i><br /><br />In this case, we are not measuring when the photon is emitted from the laser and passed through the filter. So when a photon is detected at some time t at the detector, the quantum state that predicts Peter Donishttps://www.blogger.com/profile/09122769947782402203noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-47187419801257726632018-05-09T21:22:07.145-07:002018-05-09T21:22:07.145-07:00"So the overall state would look something li..."<i>So the overall state would look something like this ... And the first term just says nothing happens inside the apparatus because there is no photon there.</i>"<br /><br />Thanks for the detail, and I understand what you're trying to say ... but I'm still having a hard time incorporating it with the rest of what I think I understand. Can you help me by doing this same Don Geddishttps://www.blogger.com/profile/04214642122689048677noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-88888989817309385572018-05-09T20:56:59.369-07:002018-05-09T20:56:59.369-07:00@me:
That interference, which is what's being...@me:<br /><i> That interference, which is what's being detected, is only produced for photons that get through the filter, and is produced by the interferometer. And the two possibilities for the photon in the interferometer (i.e., the pieces of its wave function in the two arms) are not decohered and are not macroscopically</i><br /><br />Agh, this got garbled, should have ended with "Peter Donishttps://www.blogger.com/profile/09122769947782402203noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-77007604376638237112018-05-09T20:52:53.506-07:002018-05-09T20:52:53.506-07:00How would you describe the "overall pure stat...<i>How would you describe the "overall pure state of the system including all branches"? What do you think that encompasses?</i><br /><br />It depends on how much you want to include. If you want to include the filter itself, then the overall pure state would have to include the two states of the filter: "absorbed photon" and "did not absorb photon". And, as above, Peter Donishttps://www.blogger.com/profile/09122769947782402203noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-30538846751887479822018-05-09T20:41:42.294-07:002018-05-09T20:41:42.294-07:00@Don:
Assuming we agree on Ron's post
I agree...@Don:<br /><i>Assuming we agree on Ron's post</i><br /><br />I agree with Ron's analysis of all the cases he presents, yes.<br /><br /><i>The (MWI) "actual truth" is that individual photons both go through the filter, and also get blocked. Simultaneously, for a single photon. (Hence, interference, even for single photons.)</i><br /><br />No, these possibilities can't Peter Donishttps://www.blogger.com/profile/09122769947782402203noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-17214962969114670562018-05-09T20:12:21.328-07:002018-05-09T20:12:21.328-07:00"there is no interference effect at all, not ..."<i>there is no interference effect at all, not even if you look at all of the "worlds" taken together</i>"<br /><br />I may well still be confused, but I don't yet understand your claim.<br /><br />Assuming we agree on Ron's post, he starts the setup with this description: "<i>The photons are blocked by the filter at random, so there is no way to tell when a Don Geddishttps://www.blogger.com/profile/04214642122689048677noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-27865379959232951202018-05-09T19:40:31.016-07:002018-05-09T19:40:31.016-07:00@Don:
if you were ever going to be able to determi...@Don:<br /><i>if you were ever going to be able to determine which-way, then a previous event would have already entangled (aka "split") you (the observer himself) into the component of the photon that is already on only one side of the interference (and thus that slice of you will no longer be able to observe the full interference effect).</i><br /><br />But this isn't actually Peter Donishttps://www.blogger.com/profile/09122769947782402203noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-25835463646733324092018-05-09T19:29:56.021-07:002018-05-09T19:29:56.021-07:00@Ron:
It's entanglement, not measurement per s...@Ron:<br /><i>It's entanglement, not measurement per se, that destroys interference.</i><br /><br />I think if you had said "the principle that entanglement destroys interference" instead of "the principle that which-way information destroys interference", it would have made your point with much less risk of the kind of misinterpretation that Don is describing.<br />Peter Donishttps://www.blogger.com/profile/09122769947782402203noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-65868086015467261902018-05-08T23:43:52.735-07:002018-05-08T23:43:52.735-07:00@Don:
> I fear that many laypeople may interp...@Don:<br /><br />> I fear that many laypeople may interpret that sentence as something preciously close to magic, or something about consciousness<br /><br />Hm, I thought I made it clear that this was not the case when I wrote "this setup will not produce interference even if we don't actually measure the idler photon". It's entanglement, not measurement per se, that Ronhttps://www.blogger.com/profile/11752242624438232184noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-21081316260192853022018-05-08T21:05:30.130-07:002018-05-08T21:05:30.130-07:00P.S. I do feel a little vindicated, that on your o...P.S. I do feel a little vindicated, that on your original post I guessed an answer of theory #2, which was: "<i>There is interference after T2, but it goes away if the laser is dim enough</i>". On this post, you say: "<i>Then at time Tb the photons arrive from the long path. All the photons are identical, so we no longer know which path they took. So interference.</i>" and Don Geddishttps://www.blogger.com/profile/04214642122689048677noreply@blogger.comtag:blogger.com,1999:blog-5592542.post-27598340144563086222018-05-08T20:58:20.395-07:002018-05-08T20:58:20.395-07:00Not disagreeing with any of your conclusions, but ...Not disagreeing with any of your conclusions, but ... it makes me uncomfortable to use language like "<i>the principle that which-way information destroys interference</i>". That may be true, but there is a question of causation vs. correlation here. I fear that many laypeople may interpret that sentence as something preciously close to magic, or something about consciousness ("Don Geddishttps://www.blogger.com/profile/04214642122689048677noreply@blogger.com