UK team aims for commercial quantum-based gravity sensor

The aim is to build a portable technology demonstrator by April 2021, that is twice as sensitive as the industry standard (falling mass) technique and, importantly because the aims is for this to lead to a commercial product, 10x faster in operation. “Now it takes a few days to measure across a football field, we want to get that down to less than a day,” Cliff Weatherup, strategic technology manager of Teledyne e2v told Electronics Weekly – the company will integrate the components in the final instrument.

The technique still senses the movement of a mass, but in this case it is a cloud of ultra-cold atoms, likely to be rubidium, propelled upward in a vacuum, then falling back under gravity. (See How it works below)

Its quantum nature links any measurement to the characteristics of the atoms used, which as far as anyone can tell are completely invariant, which means, in theory, the gravity measurement could be absolute, with no need for calibration.

Teledyne e2v is part of the consortium, called Gravity Pioneer, which is led by engineering services company RSK and also includes: Fraunhofer UK, Altran, Geomatrix Earth Science, Magnetic Shields, UniKLasers, Silicon Microgravity, Optocap, QinetiQ, the University of Birmingham and the University of Southampton.

The project has received £6m from UK Research and Innovation, after the consortium submitted a bid in response to the £20m Quantum Technologies Pioneer Fund, part of the Industrial Strategy Challenge Fund.

Speaking on receiving the research funding, Dr Richard Murray, business development manager at Teledyne e2v said, “The project was proposed by a UK consortium of the best scientific and engineering companies the UK has to offer, from component manufacturers to instrument developers and end-users.

A factor in the bid’s success, according to Murray’s firm, is the inclusion of the whole potential supply chain as well as end-users in the consortium – RSK is an end-user, whose clients include BP, Network Rail, HS2 and Airbus.

“The UK is world leading in quantum technology and the project brings together the best the UK has to offer in this field. Once we can demonstrate the advanced performance of quantum cold-atom sensors, the economic and societal benefits of this new quantum industry in the UK will be significant,” said Murray.

How it works

The instrument involves a vertical vacuum chamber, clusters of rubidium atoms and lasers – no cryogenics are needed.

Lasers isolate a cluster of atoms, cool them close to absolute zero, and then launch them upwards.

Left un-molested, gravity would cause slow this cluster to a stop and then accelerating back down to the starting point – much as a tennis ball thrown straight up would do. The vertical height of the cluster flight is around 300mm.

However, during this flight, the cluster is hit three times by lasers, which both measure it and alter it.

By the way, at no point are the atoms in the cluster required to be entangled with each other – the scheme would work with one atom.

What it does require, is for each atom to act as a wave rather than a particle (Louis de Broglie, 1920s) – in this case, a wave with mass that gravity can act on. It also requires that each atom can be put into a superposition of two states (Schrödinger’s cat), and that these two superimposed waveforms from the same atom can interfere with each other.

The first laser strike is applied as the cluster rises in its trajectory and puts each atom into a superposition.

In one possible state, the phase of the atom’s waveform remains untouched. In the other, its phase is slightly altered.

Back in the particle world, this means that the altered part of the superimposed atom gets a little more upward velocity and therefore travels higher.

The second strike occurs near the top of the flight, removing any phase/velocity difference, and the third occurs when the cloud gets back to the point of the first strike.

Any height difference in the trajectories is revealed from interference data extracted during the three measurements – interference due to phase-shifts in the waveform nature of the atoms – and it is possible to extract from this the value of the acceleration (gravity) applied to the atoms in the cloud.

If one atom was used, building up a measurement would take minutes, which is why a cloud is used to provide a higher interference signal-to-noise ratio.

This is an absolute measurement of gravity, and would be extraordinarily valuable as it is, if its extreme sensitivity did not mean that that the gravity signature is swamped by signals due to physical vibration within the measurement system – the point that minutes of averaging would be required to get a reading.

Differential measurement comes to the rescue

To cancel-out signals due to system vibration, exactly the same measurement is performed further up the vacuum tube, at the same time, using the same lasers, on a second cloud of atoms.

By extracting the difference signal in the laser signal, noise due to vibration is inherently cancelled out of the raw data.

The absolute value of the gravity measurement is also cancelled out, and the output of the instrument becomes the difference in gravity due to the slightly different altitudes at which the two clouds were measured – it is no longer a gravimeter, but a gravity gradiometer.

And changes in gravity gradient are exactly what surveyors are looking for when searching for underground voids and masses.

For a fuller explanation, try this University of Birmingham link (click on Atom Interferometry)

It helps to know the the ‘space’ axis in the diagram there is the vertical, and the diagram is as though you were traveling with the atom as the three laser pulses change its phase.

With thanks to University of Birmingham cold atom scientist Dr Michael Holynski, who patiently explained the principle of operation until I had some idea of what is going on.