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There are wildly inaccurate descriptions in last week’s Financial Times and Daily Telegraph of the hope that a ‘quantum compass’ will give us more accurate inertial navigation. The funniest quote is the Telegraph’s:

“The whole field hinges upon a new and unusual field of physics called quantum mechanics”

So, this daily paper is only a century behind the news…. similarly amusing is:

“One problem yet to be overcome is that these two possibilities cannot be told apart – a large object to the north of the sensor would give the same reading as acceleration to the south.”

The impossibility, at a fundamental level, of distinguishing between gravity and other forms of acceleration is of course the foundational principle of General Relativity. So Einstein is about to be overthrown?

Not to be outdone the Financial Times gives a worse account, describing the device as:

“a quantum compass that would be able to locate itself based on the subatomic effects of the earth’s magnetic field…. by supercooling trapped ions and reducing the effect of external radiation so they are sensitive only to electromagnetic fluctuations produced by the earth.”

And this is the publication that those in the worlds of finance and politics trust! No wonder the West is going to Hell in a handbasket….

The truth is that that what has been made so far is a bulky but (presumably) accurate linear accelerometer. Accelerometers are useful to a vehicle which is manoeuvring vigorously and not in contact with the Earth’s surface, such as an aircraft or spacecraft. If however you are stationary with respect to the Earth’s surface (or able to move in a straight line in a sedate fashion while an accurate measurement is made, as a submarine can easily do) you do not need accelerometers to find your position. What you need to know is the direction of the centre of the Earth, your own absolute orientation, and the time. In other words a pendulum, a gyrocompass and a clock.

(For good accuracy you also need a map of local gravitational anomalies, because a nearby mountain, including one on the seabed, could be pulling your pendulum slightly to one side – but that also applies to accelerometers.)

The lack of a perfect gyrocompass is the challenge. Existing laser ring gyros have a drift of a few ten-thousandths of a degree per hour: as the centre of the Earth is 6,400 kilometres away, this adds up to about one kilometre a day.

The chip-based source of ultracold atoms which is the new invention is currently being applied to a linear accelerometer based on quantum interference. In principle a similar atomic fountain could be used as the basis for an ultra-accurate gyrocompass, as described several years ago at:

http://physicsworld.com/cws/article/news/2011/oct/07/falling-atoms-measure-the-earths-rotation

But that’s quite some way from what the British group are currently attempting!

This piece has been revised to reflect a more accurate article in Physics World:

http://physicsworld.com/cws/article/news/2014/may/26/quantum-accelerometer-is-being-built-for-navy-submarines

which references the original Nature Nanotechnology paper:

http://www.nature.com/nnano/journal/v8/n5/full/nnano.2013.47.html

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