‘Quantum negativity’ can fuel ultra-precise measurements

'Quantum negativity' can fuel ultra-precise measurements

Quantum laser light shines on a chemical molecule that we want to measure. Then light passes our “magic” quantum filter. This filter discards a lot of light, while condensing all the useful information into weak light that eventually reaches the camera’s detector. Credit: Hugo Lepage

Scientists have discovered that a physical property called ‘quantum negativity’ can be used to make more precise measurements of everything from molecular distances to gravitational waves.

Researchers at Cambridge University, Harvard and MIT have shown that quantum particles can carry an unlimited amount of information about the things they have interacted with. The results, reported in the magazine Nature’s CommunicationsIt could allow much more precise measurements and empower new technologies, such as super-precise microscopes and quantum computers.

Metrology is the science of estimations and measurements. If you weighed yourself this morning, you’ve done metrology. In the same way that quantum computing is expected to revolutionize the way complicated calculations are performed, quantum metrology, using the strange behavior of subatomic particles, can revolutionize the way we measure things.

We are used to dealing with odds ranging from 0% (never happens) to 100% (always happens). However, to explain the results of the quantum world, the concept of probability must be expanded to include a quasi-probability call, which may be negative. This quasi-probability allows quantum concepts like Einstein’s ‘creepy action at a distance’ and wave-particle duality to be explained in intuitive mathematical language. For example, the probability that an atom is in a certain position and travels with a specific speed could be a negative number, such as -5%.

An experiment whose explanation requires negative probabilities is said to have ‘quantum negativity’. Scientists have now shown that this quantum negativity can help make more accurate measurements.

All metrology needs probes, which can be simple scales or thermometers. However, in cutting-edge metrology, probes are quantum particles, which can be controlled at the subatomic level. These quantum particles are made to interact with the thing being measured. Then the particles are analyzed by a detection device.

In theory, the higher the number of sounding particles, the more information will be available to the detection device. But in practice, there is a limit to the speed at which detection devices can analyze particles. The same is true in everyday life: putting on sunglasses can filter out excess light and improve vision. But there is a limit to the amount of filtering that can improve our vision: having sunglasses that are too dark is harmful.

“We have adapted standard information theory tools to quasi-probabilities and have shown that filtering quantum particles can condense information from one million particles into one,” said lead author Dr. David Arvidsson-Shukur of the Laboratory. Cavendish from Cambridge and Sarah Woodhead Fellow at Girton College. “That means that detection devices can operate at their ideal input speed while receiving information corresponding to much higher rates. This is prohibited according to normal probability theory, but quantum negativity makes this possible.”

An experimental group at the University of Toronto has already started building technology to use these new theoretical results. Their goal is to create a quantum device that uses single-photon laser light to provide incredibly accurate measurements of optical components. Such measurements are crucial to creating new advanced technologies, such as photonic quantum computers.

“Our discovery opens up exciting new ways to use fundamental quantum phenomena in real-world applications,” said Arvidsson-Shukur.

Quantum metrology can improve measurements of things that include distances, angles, temperatures, and magnetic fields. These more accurate measurements can lead to better and faster technologies, but also to better resources to investigate fundamental physics and improve our understanding of the universe. For example, many technologies rely on the precise alignment of components or the ability to detect small changes in electric or magnetic fields. Greater precision in mirror alignment can allow more precise microscopes or telescopes, and better ways to measure Earth’s magnetic field can lead to better navigation tools.

Quantum metrology is currently used to improve the accuracy of gravitational wave detection at the Nobel Prize-winning LIGO Hanford Observatory. But for most applications, quantum metrology has been too expensive and unattainable with current technology. The recently published results offer a cheaper way of doing quantum metrology.

“Scientists often say that ‘there is no free lunch,’ which means that you cannot earn anything if you are not willing to pay the computational price,” said co-author Aleksander Lasek, Ph.D. Candidate at the Cavendish Laboratory. “However, in quantum metrology, this price can be made arbitrarily low. That is highly contradictory and really surprising!”

Dr. Nicole Yunger Halpern, co-author and ITAMP postdoctoral fellow at Harvard University, said: “Everyday multiplication commutes: six times seven equals seven times six. Quantum theory involves multiplication that does not commute. Lack of commutation allows us to improve metrology using quantum physics.

“Quantum physics improves metrology, computing, cryptography, and more; but rigorously demonstrating that it does is difficult. We showed that quantum physics allows us to extract more information from experiments than we could with just classical physics. The key of the test is a quantum version of the probabilities: mathematical objects that resemble the probabilities but can assume negative and not real values. ”

Final precision limit of multi-parameter quantum magnetometry

More information:
Nature’s Communications (2020). DOI: 10.1038 / s41467-020-17559-w

Provided by the University of Cambridge

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