Quantum distributed metrology - reducing measurement noise now in reach

Scientists have devised a new way of accurately measuring multiple quantities at once which could be used across geophysics and even to help global clocks tell the same time.

Physicists at the Quantum Science and Technology Hub at the University of Portsmouth led by Dr Vincenzo Tamma, in collaboration with the University of Bari, Italy and the Naval Postgraduate School in California, US, have just published as a Rapid Communication in Physical Review Research a new, scalable measurement scheme which marks a step forward in distributed quantum metrology.

The team devised the scheme to try to address one of physics’ greatest problems – how to ensure a measurement is pinpoint accurate.

Dr Tamma said: “With our method we are able to infer with high accuracy not only one quantity but any linear combination of distributed physical quantities of interest in an optical network, beating the shot-noise limit and reaching in fact the Heisenberg limit. This technology could potentially find applications in global clock synchronisation, estimation of magnetic field gradients, and geophysics.”

The team say the key is the use of squeezed light – a beam of photons in a particular quantum state which can be easily generated in a laboratory – which exploits the Heisenberg uncertainty principle by exponentially reducing the uncertainty in one of the variables of the photon field. The squeezed beam of light is then distributed across various locations by an optical network of beam splitters. After it has been coupled to the parameters of interest, the beams are recombined with an inverse optical network.

Their work was inspired by a historic difficulty in accurate measurements.

This technology could potentially find applications in global clock synchronisation, estimation of magnetic field gradients, and geophysics.

Dr Vincenzo Tamma, Reader in Physics

Dario Gatto, Dr Tamma’s PhD student working on this project, said: “Whenever we perform a measurement of any kind, the outcome is inevitably affected by uncertainty. In our daily life, this is mostly due to macroscopic errors in the measurement procedure of the observer.

“Think, for example, of measuring the length of a very wide wall. You might use a tape measure. If the tape isn’t placed perfectly perpendicular along the whole wall, by even the smallest angle, the measurement will be incorrect.

“To avoid this, you might pull the tape straight, but any stretching of the tape would also result in a measurement error.

“In the end, no matter how carefully you perform a measurement, it is virtually impossible to get exactly the same result twice.”

In physics, such inaccuracies are expected so scientists rely on a series of measurements. The more measurements we perform, the more confident we can be about the value of the measured quantity.

Dr Tamma said: “However, due to the microscopic nature of our world, which is governed by the strange laws of quantum mechanics, the practical skills of the observer, the errors in the instrumentation, and the number of measurements performed are not the only factors – every measurement is fundamentally uncertain, and there is simply no way to measure any physical quantity more precisely than a certain threshold dictated by the infamous Heisenberg uncertainty principle.

“In a nutshell, this principle dictates that if the precision in the measurement of a certain variable (say, the position of a particle) is reduced, there is another variable, called the conjugated variable (in this case, velocity), such that the corresponding precision is increased.”

Hourglass on stony ground

The problem of measuring multiple parameters has only recently started to be explored - Photo by Aron Visuals on Unsplash

It is precisely this notion that offers an exciting possibility for developing new technologies based on quantum physics.

“The Heisenberg limit to uncertainty is considerably lower than its counterpart in classical physics, called the shot-noise limit. This suggests that by exploiting the quantum nature of the microscopic world, we can perform more and more precise measurements, beating the shot-noise limit and reaching the ultimate precision allowed by quantum mechanics.”

This paradigm has, he added, blossomed into a full-fledged field of study known as quantum metrology. The most promising platform for this is the exploitation of photons (quanta of light) and their coupling with the unknown parameter to be measured.

While a variety of proposals for the measurement of a single physical quantity has been put forward, the problem of measuring multiple parameters has only recently started to be explored. In particular, one is often interested in combining the outcomes of measurements of parameters distributed across several spatially separated locations.

“This technology would allow us to perform super-sensitive distributed measurements for a wide range of potential applications where a high degree of precision is needed, such as imaging and sensing, for medical and biological applications, gravitational wave detection, and more,” Dr Tamma said.

The researchers at the Quantum Science and Technology Hub at the University of Portsmouth will keep pushing in this direction, developing new quantum technologies to tackle the challenges of our society.

REFERENCE PUBLICATION: Dario Gatto, Paolo Facchi, Frank A. Narducci, and Vincenzo Tamma, "Distributed quantum metrology with a single squeezed-vacuum source" Phys. Rev. Research 1, 032024(R) (2019). DOI: 10.1103/PhysRevResearch.1.032024

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