What is a quantum compass?

A recent EU report found that the "average distance of journeys performed in road freight transport in the EU-28 was 131 kilometres in 2017" [1], meaning that this t-shirt or mug you are using has come a long way from the location of its production. The thousands of freight routes daily executed heavily rely on satellite-based navigation, such as the GPS and Galileo systems. Surely this technology is a game-changer and has penetrated most of today’s transportation, agriculture, engineering and more with economic benefits in the range of hundreds of billions of dollars [2]. We know it works. However, there is a caveat: one needs a constant optical contact with a number of satellites; if connection is lost (say, inside a tunnel) the satellites will stop seeing you and hence navigation will fail.

Now imagine that one can estimate their position from the inside; a blind machine that knows at each moment at exactly what position on earth you are! How could this even exist?

Researchers from Imperial College London and MSquared have now announced the remarkable – perhaps revolutionary – accomplishment of a quantum compass [3]. A prototype self-navigating machine that can empower army vehicles, trains, boats and in theory any moving object to trace their exact position without the need of GPS or any other satellite navigation systems.

What it is

First, let us realize that it is not a compass per se, i.e. not a device that will not point you to the magnetic north pole of the earth. It is rather an advanced system that calculates, with astonishing precision, its position using the foremost cutting-edge technology that stems from decades (at least three!) of experimentation in quantum physics and ultracold gases.

Note here that the quantum compass measures the absolute distance traveled from a given initial point. In contrast the GPS gives the position of an object relative to a (set of) satellites.

How it works

How can this even be possible? A simple analogue that I came up with might shed some light on the working principle. Imagine you are holding a cup of coffee. The coffee is at rest and its surface calm. Now start jogging. What happens? The level of your coffee tends to elevate at the direction contrary to the direction of the motion and lowers towards the front. It is the force induced by your acceleration that acts on your coffee and deforms its surface (remember, acceleration times mass equals force). In reality it pulls the liquid backwards, the liquid accelerates, it hits the walls of the cup and reflects back creating a wave that now can interfere with the next incoming wave and so on. Now suppose that I am a tiny observer living inside the cup and sitting on the surface of the water. At the moment that I observe a change in the level of water and, subsequently, a wave propagating I know that someone is moving my house! It turns out that from the shapes of the crests and troughs of the wave, i.e. the ‘interference pattern’, I can, after careful calculations, exactly figure out what the accelerates of the incoming wave was and, with the knowledge of the time elapsed, I find the velocity. Combining the above I can estimate to which direction my cup is moving and also how fast and subsequently what my position is at a given time.

Impressive?

Impressive it is! However no known device can perform this precision measurements in a classical fluid (my coffee); the fluctuations there (random ups and downs of the fluid molecules, together with the friction forces that I cannot theoretically estimate) will curtail the accuracy down to levels far from being any useful.

In a quantum fluid the situation is, luckily, different. Quantum gases are by nature 'clean' and elementary and precision measurements is quantum scientists’ all time favorite!

The quantum

Ultracold quantum gases offer a bypass around the limitations of classical fluids. Simply put, ultracold gases are tiny collections of (most commonly) rubidium or sodium atoms (and nowadays many more species in the various labs around the world) that are really cold. And I mean, tremendously cold. In fact, colder than everything else we have ever created on earth. Even beyond that; at hundreds of billionths of a degree Celsius they are colder than deep space – which makes them colder than anything else in the known universe! The reason we want them so cold is that only in such freezing situations the quantum behavior can be seen; we need to shut everything off, enter the calmest realm of nature and listen to its whisper. Which is quantum! In that regime particles are interchangeably waves and vice versa.

The reason we want them so cold is that only in such freezing situations the quantum behavior can be seen; we need to shut everything off, enter the calmest realm of nature and listen to its whisper. Which is quantum!

Since the mid 90's the “business” of fluids (gases and liquids) of ultracold atoms has boomed. Such systems are nowadays engineered at will and used for their unprecedented accuracy when it comes to measurements. Atomic clocks, that are able to keep time with a breathtaking precision of one second in tens of billions years (!), are based on and made of cold gases (strontium atoms in most cases).

A quantum compass is an accelerometer (think of the device inside your smartphone that knows which way is up and automatically rotates the screen) made of a quantum ultracold gas [4]. The place of the coffee of our classical model now takes a fluid of ultracold atoms. Now recall that this is a quantum thing, so it must behave as a wave too. More precisely, it consists of many waves – one for each particle – that overlap and all together form a giant quantum wave. So, if I shake this wavy substance I expect to see interferences between two (or more) incoming waves. Which in this case are quantum. Generally, the study of interferences in physics accounts for precision measurements and is powerful tool. That powerful that has its own name: interferometry!

The working principle is the same as in the coffee example: one observes the interferences (shape of the outgoing wave) of the quantum fluid and extracts information about the force or acceleration that has caused these interferences [5] and from there the position of the moving object. This principle sounds simple, it takes however powerful lasers and expensive and delicate constructions with precise electronics to make it happen, that usually sit on robust concrete and iron tables inside massive lab rooms. Besides three interferometers (one for each direction of the motion), other devices are necessary too in order to obtain sufficient information: quantum gyroscopes that will tell me if I the object is rotating and how and also an atomic clock that keeps time pretty accurately. The challenge now will be to fit the all the apparatuses inside a portable device and also account for and eliminate potential sources of noise that can tarnish the precision.

Undoubtedly, an industrially challenging endevor with far-reaching implications and elegant quantum physics on its core!

The website https://ultracold.org offers - with my proud involvement (!) - an easy to use, free software with which ultracold quantum systems, such as accelerometers, can be simulated, to an unequaled accuracy on any computer! Give it a shot!

[1] https://ec.europa.eu/eurostat/statistics-explained/index.php/Road_freight_transport_by_journey_characteristics#Average_distance_travelled

[2] Nam Pham, The Economic Benefits of Global Navigation Satellite System and Its Commercial and Non-Commercial Applications, ndp Analytics, (2013)

[3] https://www.imperial.ac.uk/news/188973/quantum-compass-could-allow-navigation-without/

[4] https://www.m2lasers.com/quantum.html

[5] https://www.m2lasers.com/images/M_Squared_Gravimeter_Handout_210x210_V3_Web.pdf