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London Underground hosts tests for 'quantum compass' that could replace GPS

theguardian.com

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Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#131

Post by dr_orpheus »

GoldenRacer wrote: post #128
mFixman wrote: post #105

Earlier quoted context omitted.

If 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?

I'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.

Yeah, 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."

Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#133

Post by avianlyric »

robertlagrant wrote: post #122
kitd wrote: post #101

Earlier quoted context omitted.

> It has absolutely no idea where it is from only itself. Neither do GNSS satellites. You can't solve this problem with any method without giving each device a starting reference, and then either continually updating it or enabling some mechanism for multiple devices to vote/agree on reference(T+1). This new tech might allow other improvements, such as the QIN device being a relatively-static "hub", and the user wear…

> Neither do GNSS satellites Is it a similar problem? I hadn't thought about how GPS satellites know where they are. Do they need regular correction?

Yes, satellites are tracked by ground stations, and receive an update every 24 hours with information about their project positions in the future.

GPS satellites transmit this data in the form of an almanac which includes all the high level parameters for estimating the location of every satellite, and ephemeris data which allows you to calculate the precise location of the satellite, when used in tandem with the almanac.

The almanac doesn’t change too often, but ephemeris data is only valid for a few hours. The satellites recalculate the ephemeris data themselves, and normally have a few months of needed data stored on board, just in case they can’t get updates. But the expectation is they’re updated every 24 hours.

Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#134

Post by avianlyric »

playingalong wrote: post #56
contingencies wrote: post #36

Earlier quoted context omitted.

Actually, in a train context, inertial is pretty damn high accuracy because you already have the world's best odometer showing (A) how many times your known size wheel has turned on your known track; and (B) you can test many axels simultaneously to ensure no slippage (I guess you wouldn't test the wheels as they move faster creating a higher frequency sampling requirement for ~no benefit whereas a dedicated axel fea…

Re odometer - don't the wheels of a train slide a little bit? I know the train is big pile of heavy iron, but still. My naive thinking is that when braking or during a rush start some extra distance can be covered. Don't know, just thinking out loud.

Generally trains do everything they can to avoid slipping. They have anti-lock breaks and traction control just like your car, and stuff your car doesn’t have, like “sanders” which pour sand onto the track just in front of the wheels.

Regardless of material, dynamic friction is always lower than static friction. So for maximum acceleration and breaking it’s important to ensure you wheels stay in “rolling” mode of interaction, and don’t slip.

Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#135

Post by ta20240528 »

ngcc_hk wrote: post #120
ta20240528 wrote: post #114

Earlier quoted context omitted.

The USA does belong to the ICJ and has since its inception (the USA not a member of the ICC - different court). Its just that after the invasion of Nicaragua in 1984 it decided to abide by the court "on a case to case basis". Russia also decided to ignore the ICJ in 2022. FWIW I agree that everyone should have nukes, there would be a lot fewer military adventures. But that's not the _current_ USA/UK/FR domestic* and…

Are you sure you meant everyone? Countries fall into disarray all the time. And there are totalitarian countries … It has some use and does prevent someone trying. But there is a downside to it that is too much. But for some cases like Taiwan or Ukraine …

> But for some cases like Taiwan or Ukraine …

Yip, what about Guyana fending off Venzuela? Why can't they play? At some point its immoral to say "this level of security is only for us".

Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#137

Post by romwell »

porphyra wrote: post #9

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…

>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!

Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#138

Post by prewett »

pas wrote: post #124
robertlagrant wrote: post #122

Earlier quoted context omitted.

> Neither do GNSS satellites Is it a similar problem? I hadn't thought about how GPS satellites know where they are. Do they need regular correction?

There's some natural drift, and about yearly boost to keep them in their desired orbits https://space.stackexchange.com/questions/30752/do-operating... As far as I know - while their onboard clock is pretty good - they continuously get corrections from ground stations. (Once per day.) https://gpsbeam.com/ground-stations/ the "navigation message" broadcast by the sats is described here https://gssc.esa.int/navipedia/i…

> their onboard clock is pretty good

They use atomic clocks, so it's accurate to something like 1 part in 10^16, or about 1 second in a billion years! A friend of mine is working on the next generation, which will be even more accurate.

In contrast, the force of gravity experienced by the satellite is known to much less accuracy. In fact, changes on a monthly basis due to rainfall/rivers and tides. The GRACE satellites measure the change, although I couldn't find any information on how accurate their measurements are, except that they can measure the distance between the two satellites to within a micrometer (10^-6), so substantially less precision than the clock!

Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#139

Post by mrguyorama »

dazzlefruit wrote: post #129
perryizgr8 wrote: post #112

Earlier quoted context omitted.

ICBM is likely to deliver a nuke. Which will probably take out a 20 km radius circular area. It is not going to be used to target a base with 100 soldiers.

It is if the goal is to deter without launching a city roast fest. That’s one possible step to e.g. answer a first limited strike without starting a disastrous exchange. And US nukes can be set to low yield.

If NATO ever has any intention of deterrence through detonating a nuke, it will be done in exactly the way it's been specifically planned and openly explained: France has a specific nuclear policy, one of the only ones, that allows them to use nukes before an adversary does. They have a specific nuclear missile fired by plane that will be used as essentially a giant warning shot.

Nobody would fire a warning shot using an ICBM, because if things are hot enough to require a warning shot, that leaves a whole lot of interpretation up to your adversary who is quite likely to respond with their own ICBMs, before finding out yours were loaded with conventional munitions to make a point.

There are however, shorter range ballistic missiles, Russia has a lot of them (the US mostly gave up mid range ballistic missiles in a nuclear treaty) and has even launched a few with concrete "nuclear warhead simulation masses" at Ukraine.

Re: London Underground hosts tests for 'quantum compass' that could replace GPS

#140

Post by Reason077 »

pas wrote: post #124
robertlagrant wrote: post #122

Earlier quoted context omitted.

> Neither do GNSS satellites Is it a similar problem? I hadn't thought about how GPS satellites know where they are. Do they need regular correction?

There's some natural drift, and about yearly boost to keep them in their desired orbits https://space.stackexchange.com/questions/30752/do-operating... As far as I know - while their onboard clock is pretty good - they continuously get corrections from ground stations. (Once per day.) https://gpsbeam.com/ground-stations/ the "navigation message" broadcast by the sats is described here https://gssc.esa.int/navipedia/i…

Amongst other things, the clocks need to be corrected for relativistic effects.

Due to the net effect of both kinetic and gravitational time dilation, clocks onboard a satellite advance more quickly than they would on Earth (when observed from Earth).

https://en.m.wikipedia.org/wiki/Error_analysis_for_the_Globa...

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