

An accelerometer so good that only quantum mechanics limits it - Reltair
http://arstechnica.com/science/2012/12/an-accelerometer-so-good-that-only-quantum-mechanics-limits-it/

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ISL
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...](http://www.nature.com/nature/journal/v478/n7367/full/nature10461.html)

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>

~~~
ChuckMcM
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.

~~~
ISL
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!

~~~
ChuckMcM
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 ...

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jessriedel
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>

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hkmurakami
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...](http://www.nature.com/nphoton/journal/v6/n11/full/nphoton.2012.245.html#/author-
information)

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aidenn0
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>

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alanctgardner2
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.

~~~
mikeash
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.

~~~
0x0
Yup, the sample code even has a simple filter for this:

[http://developer.apple.com/library/ios/documentation/EventHa...](http://developer.apple.com/library/ios/documentation/EventHandling/Conceptual/EventHandlingiPhoneOS/MotionEvents/MotionEvents.html#//apple_ref/doc/uid/TP40009541-CH4-SW11)

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

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srean
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 ?

~~~
ISL
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.

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Xcelerate
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).

~~~
ISL
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.

