this post was submitted on 19 Jun 2024

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‘It’s the perfect place’: London Underground hosts tests for ‘quantum compass’ that could replace GPS (www.theguardian.com)

submitted 5 months ago by [email protected] to c/[email protected]

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[–] [email protected] 41 points 5 months ago

Phones have had accelerometer/gyros for a while now. Problem with pinpointing one's location is how to get a starting fix and how to deal with drift and loss of signal.

The way devices have dealt with it is to periodically confirm and baseline with a satellite fix.

If this method does away with all that, it could remove the reliance on overhead signals and those trying to jam them in hostile zones.

Pretty cool. Lots of potential.

[–] [email protected] 9 points 5 months ago (1 children)

Can anyone explain how lasers are used to cool? I would prefer ELI18 but i probably need this ELI5

[–] [email protected] 18 points 5 months ago (2 children)

If you use the right color of light, then the doppler effect means that the atoms will only absorb (and be pushed by) light that they are headed towards. That means that the light will always act as a brake for the atoms and never an accelerator, so the fluid will cool. If you do this from all directions, the fluid will start to stay still in one place and get very close to absolute zero. Idk, I just read the Wikipedia article, but that is my best attempt at an ELI18

[–] [email protected] 6 points 5 months ago

You got it pretty much on point. Shooting a laser at atoms is like shooting a machine gun at an indestructible target. If it moves towards you, you can slow it down. But preventing it from accelerating when the target is stationary is where quantum mechanics comes in. That is your explanation: The laser light only acts as a force when the light is resonant with the atom and the Doppler effect means that the resonance condition changes depending on the speed of the atoms.

[–] [email protected] 3 points 5 months ago

I love you just a little bit!

[–] [email protected] 30 points 5 months ago (3 children)

Even a perfect sensor will accumulate errors in the nav solution over time because there's no such thing as a perfect gravity model. No free-running INS will ever replace GPS long term. This shit is so frustrating to see in the press.

[–] [email protected] 2 points 5 months ago

Yeah, more accurate dead reckoning is always useful, but you'll still need some sort of of ground-based or satellite based navigation system if you're using this for any system that requires any reasonable amount of accuracy.

[–] [email protected] 27 points 5 months ago (1 children)

It will definitely require corrections from GPS or other systems, but if made sufficiently accurate; it could be months or even years before the accumulated errors necessitate a correction.

What seems more concerning to me is a system like this would require 100% up time between outside corrections.

A gps receiver can acquire its position from a completely powered off state. Inertial guidence though, needs to be told its current position; then it can keep track of where it goes from there. If there's any hiccup with power, you've completely lost your location fix and can't reacquire it alone.

Put the two together though, and the inertial guidence can accurately fill in the gaps between gps service while also getting regular updates/corrections when you do have that signal available.

[–] [email protected] 7 points 5 months ago

I don't think you'll ever see an INS going months without needing a correction. Imperfect gravitational compensation applies directly to the specific force measurements and those errors are then accumulated twice.

[–] [email protected] 8 points 5 months ago (1 children)

Especially since, to calculate current location, it needs an input of initial location (i.e. it needs GPS coordinates to begin with so it can track direction and velocity relative to that initial position). You can’t replace something you depend upon.

[–] [email protected] 11 points 5 months ago (2 children)

the initial location doesn't need to be GPS, just a known anchor location. Which is trivial to implement in the case of trains, since stations don't move that drastically.

[–] [email protected] 5 points 5 months ago (1 children)

"Fixed" ground points move a surprising amount. The local ground can shift, and of course whole continents are constantly drifting.

[–] [email protected] 5 points 5 months ago (1 children)

surely these are things that should be considered, but they move in relation to what? And is this surprising amount of any significance for tens or hundreds of miles of rail?

[–] [email protected] 5 points 5 months ago

In relation to all other points of interest, which are themselves all moving.

It's not really relevant for rail, no, but not because of inaccuracy and drift, but because the trains are on fixed paths already. Inertial navigation and dead reckoning are accurate enough to get from station to station, and each station can have local markers, even something as simple as a reflector at the end of the platform.

But they're not developing it just for rail. It would be incredibly valuable for submarines and mining, for example.

[–] [email protected] 3 points 5 months ago (1 children)

But wouldn't you scramble the precision with that? Stations can be quite big and anchoring to the station location means you already start with an offset to your location.

Depending on the accuracy over time, they could pinpoint a location while the user is sleeping and than use that as an anchor for the day.

But everything about that is speculative; let's see where this goes first.

[–] [email protected] 7 points 5 months ago* (last edited 5 months ago) (1 children)

you're thinking anywhere on the platform, I'm suggesting a known place near a station by which the train passes and its location - at that moment - is known.

All the system needs is a ground-truth location after a certain amount of time. GPS is just a cheap and convenient way to do it almost anywhere, but this location correction doesn't need to be satellite-based at all.

[–] [email protected] 1 points 5 months ago

Yeah maybe that could work. I definitely agree that there's ways to get good anchor points. Maybe through cross-check with wireless networks even.

[–] [email protected] 80 points 5 months ago (1 children)

The article describes the device working in ways that violate relativity, but the actual technical description is a lot cooler.

It's not a quantum compass, really. It's a quantum accelerometer and gyroscope. The hope is that its accuracy will lend itself to long-term inertial guidance, which normally needs regular GPS updates to correct errors which accumulate over time.

[–] [email protected] 34 points 5 months ago (2 children)

a lot cooler

ice what you did there

long-term inertial guidance

this is exactly what I got from the article: a more accurate inertial navigation. What part violates relativity?

[–] [email protected] 35 points 5 months ago* (last edited 5 months ago) (2 children)

It is being used to develop a quantum compass – an instrument that will exploit the behaviour of subatomic matter in order to develop devices that can accurately pinpoint their locations no matter where they are placed,

[...]

The aim of the Imperial College project [...] is to create a device that is not only accurate in fixing its position, but also does not rely on receiving external signals.

These statements imply the device can know exactly where it is in space just by measuring some purely internal quantum effect, which conflicts with the principles of Lorentz invariance and relativity.

Both are constructed around the same idea that there's nothing special in the laws of physics that changes with where you are or how fast you're going. That observation is what led the conclusion that the speed of light is the same in every reference frame, and to Einstein developing the theory of relativity.

In reality, the device needs an external signal to learn its initial position. And it's unlikely to be perfectly accurate so it may still need periodic updates, just hopefully a lot less frequently.

The London Underground is actually kind of a dumb use-case because it's fixed infrastructure. You can just have something like RFID tags around the track that the train reads as it goes by. And there's going to be sensors in the track that report trains' presence to a central control room. It's just a good setting to test the device.

What it's really potentially quite useful for is nuclear submarines since they can stay underwater pretty much as long as their food supplies last, and knowing their position without using sonar or being able to receive GPS signals is quite important for navigation and obstacle avoidance. But the author was probably told to downplay potential military applications.

[–] [email protected] 25 points 5 months ago

The London Underground is actually kind of a dumb use-case because it’s fixed infrastructure.

On the other hand it's a perfect test bed, because there's sufficient changes of direction and speed, and the fixed infrastructure lets you measure drift. Plus it being underground helps simulate GPS signal being weak or unavailable.

[–] [email protected] 10 points 5 months ago

It would certainly make jamming of guided weapons quite difficult; missiles, drones, UAVs, etc.

[–] [email protected] 9 points 5 months ago* (last edited 5 months ago)

You can’t use it to determine both your location and velocity at the same time. (/s)

[–] [email protected] 17 points 5 months ago

"replacing GPS" is a stretch, but it's some sci-fi tech to use when GPS can't be used