Dr. Joseph Cotter takes some unusual pieces of luggage on his travels on the London underground. It includes a stainless steel vacuum chamber, several billion atoms of rubidium and a series of lasers used to cool its equipment to a temperature just above absolute zero.
While not the average kit you’d expect to find hauled in carriages on the District Line, it’s the equipment Cotter – who works at Imperial College London’s Center for Cold Matter – uses on his underground journeys.
While the luggage may be bizarre, it has an ambitious purpose. It is being used to develop a quantum compass – a tool that will exploit the behavior of subatomic matter to develop devices that can accurately determine their locations regardless of where they are placed, paving the way for the creation of a new generation underground and underwater sensors.
The ideal place to test this is the London Underground, Cotter and his team discovered. “We are developing very precise new sensors using quantum mechanics, and they show great promise in the laboratory,” he told the Observer last week. “However, they are less accurate in real-world environments. So we take our gear to the London Underground. It’s the perfect place to smooth out the rough edges and make our gear work in real life.”
The idea of a quantum compass is to bypass or complement current methods of determining the locations of planes, cars and other objects. It usually relies on Global Navigation Satellite Systems (GNSS) – such as GPS – which has become decisive in the transport of goods and services by road, sea and air. Using external signals, these systems can pinpoint the positions of vehicles.
But GNSS devices are vulnerable to bad weather and interference, and don’t work underwater or underground, and their signals are often blocked by tall buildings and other obstructions. The aim of the Imperial College project – which was backed by UK Research and Innovation’s Technology Mission Fund and the UK National Quantum Technology Programme – is to create a device that is not only accurate in determining its position, but also does not rely on receiving external signals.
“Then you don’t have to worry about signals being lost or blocked by high-rise apartments,” said Dr Aisha Kaushik, another member of the Imperial Center for Cold Matter team. “You will have more confidence in knowing where you or your vehicle is at a specific time.”
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At the heart of the quantum compass – which could be ready for widespread use in a few years – is a device known as an accelerometer that can measure how an object’s velocity changes over time. This information, combined with the starting point of that object, allows its future positions to be calculated. Cell phones and laptops have accelerometers, but these versions cannot maintain their accuracy over long periods of time.
However, quantum mechanics offers scientists a way to provide new precision and accuracy by measuring properties of supercool atoms. At extremely low temperatures, atoms behave in a “quantum” manner. They act like matter and like waves. “When atoms are ultra-cold, we can use quantum mechanics to describe how they move, and this allows us to make precise measurements that tell us how our device changes its position,” Cotter said.
In the devices – which were carried on board London Underground rail test trains and not on shuttle services – rubidium is placed in the vacuum chamber that lies at the heart of the machine. Powerful lasers are then used to cool these atoms to a fraction of a degree above absolute zero (−273.15C). In these conditions, the wave properties of the rubidium atoms are affected by the acceleration of the vehicle carrying the equipment, and these small changes can be accurately measured.
The system has been found to work well in a stable lab, but it needs to be tested in more extreme conditions if it is to be transformed into a transportable, self-contained device that can be used in remote or complex locations, Cotter added.
Tube train tunnels are ideal for this task, and London Underground will benefit from new quantum sensors that will remove the need for the hundreds of kilometers of cables currently installed to track the location of the 540 trains zipping under the capital at peak times.
