Submarines may turn to quantum positioning for guidance
Electronics Weekly, 15 May 2014
In 2016 a British submarine will slip its moorings and set sail under the guidance of the quantum world. The navigation system it will be testing should record the vessel’s position with 1000 times more accuracy than anything before.
If successful, the system, known as quantum positioning, could be miniaturised for use in aircraft, trains, cars and even cellphones. This would provide a backup navigation tool in cities’ concrete canyons, or in autonomous vehicles, where a loss of GPS signal can be dangerous.
GPS doesn’t work underwater, so submarines navigate using accelerometers to register every twist and turn of a vessel after it submerges and loses its last positioning fix. But this isn’t very accurate.
“Today, if a submarine goes a day without a GPS fix we’ll have a navigation drift of the order of a kilometre when it surfaces,” says Neil Stansfield at the UK Defence Science and Technology Laboratory (DSTL) at Porton Down. “A quantum accelerometer will reduce that to just 1 metre.”
To create the supersensitive quantum accelerometers, Stansfield’s team was inspired by the Nobel-prizewinning discovery that lasers can trap and cool a cloud of atoms placed in a vacuum to a fraction of a degree above absolute zero. Once chilled, the atoms achieve a quantum state that is easily perturbed by an outside force – and another laser beam can then be used to track them. This looks out for any changes caused by a perturbation, which are then used to calculate the size of the outside force.
The DSTL team wants this set-up to be usable in the real-world setting of a submarine, where the size of the force would correspond to the movements as the sub swings around in the sea.
Their prototype quantum accelerometer, which resembles a 1-metre-long shoe box, will be trialled on land in September 2015, the team will say at a conference at the UK National Physical Laboratory in Teddington this week. It will initially operate along just one axis, before two more sets of lasers and trapped atoms are added to accommodate motion in all three dimensions. Each will cool 1 million atoms of rubidium. “Once we have understood the first generations, we’ll start to miniaturise it for other applications,” says Stansfield.
It’s not a done deal yet, though, because the accelerometer can’t distinguish between tiny gravitational effects and accelerations caused by a vessel’s movement. “If the submarine passes an underwater mountain whose gravity attracts it to the west, that feels exactly like an acceleration to the east,” says Edward Hinds at the Centre for Cold Matter at Imperial College London, who is developing the accelerometer for the DSTL. “This means that very good gravity maps will be required to navigate correctly.”
The DSTL isn’t alone in pursuing quantum navigation: teams in the US, China and Australia are chasing the same prize.
“Super-accurate navigation makes sleeping easier for the captain of a submarine,” says John Powis, head of the NATO Submarine Rescue Service in Faslane, UK, and a former navigator on Royal Navy submarines. It will also make it easier to go on patrol undetected, as submarines will no longer have to expose a mast to GPS, he says.
But Powis thinks this technology may have the greatest impact in future generations of weapons – once it has shrunk down in size. “The submarine does not need to know its position in metres and centimetres,” he says. “But a projectile like a missile or shell might.”
The DSTL team believes the technology has applications beyond warfare, though.”Ten to 20 years ago this would have needed a huge cryogenic cooler, but laser-cooled atom clouds are changing all that,” says team leader Stephen Till. He says future generations of the technology are likely to make their way into everything from cars to our smartphones. “We’re convinced the size and power will come down for broad use.”