An accelerometer so good that only quantum mechanics limits it | Hacker News Reader

An accelerometer so good that only quantum mechanics limits it

arstechnica.com

33 pointsReltair15 comments

The Ars Technica article oversells the shot-noise limit. What you really want to see in this business is a thermally-limited oscillator; the Brownian motion in the spring driving the mass. For a quantum mechanically limited oscillator, check out work like this (which shares an author with the paper linked by Ars Technica): http://www.nature.com/nature/journal/v478/n7367/full/nature1...

A shot noise limit is not an inherent reason for kudos. In particular, this sensor is shot-noise limited at frequencies above a few kHz. In this context, the shot-noise limit may only represent the intrinsic noise of the optical readout, not the intrinsic thermal noise of the oscillator. Their noise figure of 10 ug/rtHz is interesting, but not unprecedented.

The Micro-G FG-5X represents the state of the art at low frequency and can do 15 ug/rtHz at sub-Hz frequencies.

For a more-fair comparison, in a standard MEMS form factor, the Analog Devices ADXL 103 and 203 do 110 ug/rtHz at 100s of Hz and below and cost <$10 each.

It'll be way cool to see what their oscillator will do with improvements. Optical readout has less influence on the detector mass and comparable precision to the best capacitive readout.

Link to the paper on the arXiv: http://arxiv.org/abs/1203.5730

I still think its cool to count cars on the freeway by putting one of these on a pole next to the freeway. Pretty amazingly sensitive.

Awesome. We'd actually have a use in the lab for such a measurement (we see seismic traffic noise from a bridge ~200m away). We've toyed with a magnetometer a bit, but an acceleration-based measurement would be more relevant for our needs.

Did you do any documentation/know of a reference? Thanks!

I read about it on a handout as a blurb of work going on at CalTrans at one of the job recruiting fairs. This is their web site: (http://www.dot.ca.gov/research/researchreports/index.htm) Their search technology sucks though. Still looking ...

FYI, the shot noise limit is one which exists when experimentalists are restricted to "classical" techniques, i.e. they don't use unusual quantum input states or measurements. That's probably a reasonable assumption for anything that could make it into a consumer product, but "quantum-enhanced measurements" can do better. Giovannetti et al.'s article is pretty good for those with the background:

http://www.sciencemag.org/content/306/5700/1330 http://arxiv.org/abs/quant-ph/0412078

The researchers are from Cal Tech and Univ. of Rochester (in case anyone was curious, since the article for some reason fails to mention the institution responsible for the research)

http://www.nature.com/nphoton/journal/v6/n11/full/nphoton.20...

The actual paper[1] states that their prototype has similar performance to the best commercial sensors. However it is much better than any previous optical sensor, and they claim that if they scale the device they can reduce the thermal NEA to 150 ng/rtHz with a 25KHz bandwidth, which would be commercially interesting.

1 http://arxiv.org/abs/1203.5730

Their whole opening about the delay in rotating a phone seems way off base; I thought developers added that to the OS intentionally to avoid over-sensitive sensors triggering rotations too frequently. While the sensitivity of the sensor plays a part, this seems like an entirely software implementation that has little to do with physics.

Furthermore, a lot of recent hardware (starting with the iPhone 4 in Apple-land, not sure about others) includes gyroscopes in order to handle this even better. It doesn't matter how good your accelerometers are, they're going to be subject to noise from the outside when it comes to detecting rotation changes e.g. the motion of the user's hands.

Yup, the sample code even has a simple filter for this:

http://developer.apple.com/library/ios/documentation/EventHa...

The accelerometer + gyroscope combination is refered to as "sensor fusion algorithms" in that document, btw

Given its potential use in inertial navigation system of guided missiles, I would expect the state of the art in realizeable accelerometers to be classified. Can anyone shed any light on how this would compare with what is known about "weapon grade" accelerometers, even if the old ones ?

This technology is not classified secret (hardly anything is), but God help you if you export it from the US without an ITAR and/or Department of Commerce license.

I believe that there's a sensitivity threshold for laser gyros at which the Man gets interested. I've been to a colloquium where the speaker noted that foreign graduate students weren't allowed to work on gyros with sensitivities greater than a specific value. They were careful not to optimize beyond that threshold (sensitivity was ancillary to their science measurement).

I've also seen a civilian science project rename themselves in order to differentiate themselves from missile guidance systems. In general though, I don't think it's much of a problem for science. The sudden classification of a promising technology is a strong signal to other states that the technology is of practical use.

In CS terms, the day that quantum computation goes dark is a day you know that RSA is over.

I would like to link to a post I made about a week ago: http://news.ycombinator.com/item?id=4856642

There was a discussion in the comments about whether enough cumulative integrated acceleration errors would prevent some sort of system like this from replacing GPS. (Granted, we were discussing gyroscopes, but I think this still relates).

10 ug acceleration noise has large consequences in displacement error.

To get an estimate of the size of the effect, recall that for uniform acceleration, x(t) = 1/2 * a * t^2 . So, for 10 ug = 9.8e-5 m/s^2, this works out to ~600 m in an hour or 360 km in a day.

The real effect isn't quite this bad (it's random, not continuous, error), but it illustrates the point. Without an absolute reference for occasional comparison, inertial measurement requires exquisite precision and low-frequency stability in order to be useful for most positioning applications.

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