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I remember doing laser cooling of rubidium atoms in physics class. It's really cool how it works: * Rubidium will absorb photons of a certain wavelength and re
by porphyra 2y ago
I remember doing laser cooling of rubidium atoms in physics class. It's really cool how it works:
* Rubidium will absorb photons of a certain wavelength and re-emit it in a random direction. Since photons have momentum, it will get some net momentum change from absorbing photons all coming from a particular direction, but the re-emissions are in random directions so they have net zero momentum.
* Shine laser light at it from all directions but at a wavelength slightly longer than the wavelength at which it will absorb. Now, if the rubidium atom is moving, the light hitting it head on will be Doppler shifted into the wavelength that it absorbs, slowing it down, whereas the other light wouldn't affect it. So, no matter how it is moving, it will slow down.
Rubidium is a good material since it has an S shell on the outside as though it were a big hydrogen atom, but it is so massive that it is a lot nicer to work with for this application.
- tanseydavid 2y ago>> It's really cool how it works Pun intended? Please say yes.
- contravariant 2y agoMaybe I'm missing something but hitting a gas just above absolute zero with lasers from every direction doesn't sound as something that could be miniaturised.
- gridspy 2y agoMost technology has a limit like this (for instance CPU transistor size in the past). However once this is a proven technology, money will be invested in solving these issues.
- porphyra 2y agoTheoretically you might be able to do it with some VCSELs, not sure though...
- nine_k 2y agoMaybe not down to a pea size, but possibly down to an orange size. Lasers are tiny, and "every direction" does not involve many of them, it involves some fiber optics, I suppose. The bulk of the device would be thermal insulation, and a volume for liquid helium + some piping to let it evaporate, and to replenish it.
- deleted 2y ago[deleted]
- horstbort 2y agoYou don't need any helium or nitrogen here, cooling happens only by laser cooling and evaporative cooling from magnetic or optical traps. The atoms are perfectly insulated in an ultra high vacuum. Electronics still take the bulk of the volume here, as does the laser system. While the lasers themselves are tiny indeed, the light needs to be manipulated before reaching the atoms. And yes, it involves quite a lot of fiber optics :-).
- greenavocado 2y ago[flagged]
- ziddoap 2y agoYou don't need to copy/paste the exact same comment three times in one topic. We read it the first time.
- greenavocado 2y agoFixed
- Waterluvian 2y agoThe “nuke Moscow” case doesn’t require GPS. You’ll get plenty sufficient precision from inertial systems.
- roywiggins 2y agoPrecision is, counterintuitively, still a big deal in the logic of nuclear weapons strategy. Nuking Moscow is easy, nuking hardened targets can be a lot harder. > Before the invention of this new fuzing mechanism, even the most accurate ballistic missile warheads might not detonate close enough to targets hardened against nuclear attack to destroy them. But the new super-fuze is designed to destroy fixed targets by detonating above and around a target in a much more effective way. Warheads that would otherwise overfly a target and land too far away will now, because of the new fuzing system, detonate above the target... > As a consequence, the US submarine force today is much more capable than it was previously against hardened targets such as Russian ICBM silos. A decade ago, only about 20 percent of US submarine warheads had hard-target kill capability; today they all do. https://thebulletin.org/2017/03/how-us-nuclear-force-modernization-is-undermining-strategic-stability-the-burst-height-compensating-super-fuze/amp/ https://thebulletin.org/2017/03/how-us-nuclear-force-moderni...
- Dylan16807 2y agoBut importantly, making sure a nuke can still hit Moscow despite geolocation countermeasures has lots of worrying consequences for normal people that are in normal cities. If nukes are hitting random countryside then a lot less people die. But when it's the difference between hitting a hardened target or being off by 200 meters, that doesn't really affect normal people.
- BurningFrog 2y agoFor many purposes, "every direction" is only three directions in our 3D space.
- tonyarkles 2y agoIt's pretty amazing what's slowly but surely becoming possible. This company, for example, has successfully made chip-scale "fiber" ring gyros with incredible performance that would have been very large heavy units 5-10 years ago: https://www.anellophotonics.com/products/x3 https://www.anellophotonics.com/products/x3
- dekhn 2y agoThere's a whole bunch of laser cooling science/applications: https://en.wikipedia.org/wiki/Laser_cooling https://en.wikipedia.org/wiki/Laser_cooling
- 4gotunameagain 2y agoDoes it mean that using this mechanism you can only achieve cooling for a narrow temperature range ? As I assume you cannot shine laser light at a spread spectrum to capture all the possible ΔT
- o11c 2y agoI imagine at higher temperatures, the lasers that hit at an angle would have the correct doppler shift.
- shadowgovt 2y agoThank you for this explanation. I've often wondered how laser cooling works and never found a clean explanation.
- bl0rg 2y agoGreat explanation :)
- mFixman 2y agoIf I understood this correctly, would this allow you to make an extremely good estimation of direction of velocity? If you want to use dead reckoning to estimate position, how would you know the abs of the velocity?
- deleted 2y ago[deleted]
- GoldenRacer 2y agoI'm way out of my depth here but I believe this would function as a very sensitive accelerometer. The rubidium would be still relative to the lasers so when you accelerated, it would have inertia and take some time to catch up with the rest of the device. You'd then integrate the acceleration to find the velocity and integrate that to find position.
- dr_orpheus 2y agoYeah, it is an extremely sensitive accelerometer which it calls out about halfway through the article. "At the heart of the quantum compass – which could be ready for widespread use in a few years – is a device known as an accelerometer that can measure how an object’s velocity changes over time."
- pmx 2y agoThank you for this, it's the first question that entered my head when I read the article.
- romwell 2y ago>Now, if the rubidium atom is moving, the light hitting it head on will be Doppler shifted into the wavelength that it absorbs, slowing it down, whereas the other light wouldn't affect it Spooky. It's like having a Maxwell's demon for photons sitting on top of every atom!