Researchers break quantum limit in precision of force and position measurements

The technique is unprecedented, but very simple, which will facilitate its practical use in other experiments and laboratories. [Image: Mason et al. - 10.1038 / s41567-019-0533-5]

Precision limit

The precision of the force and position measurements has been raised to a new level thanks to a collaboration of researchers from the University of Copenhagen and the Niels Bohr Institute in Denmark.

The experiment is the first to overcome the so-called "Standard Quantum Limit", or SQL ( Standard Quantum Limit ), which imposes itself as a barrier in the most common and most successful optical techniques for ultra-precise position measurements.

In 2014, a US team detected the smallest force already measured , coming very close to SQL, but did not win. In fact, physicists and engineers have been trying to overcome the Quantum Limit for more than 50 years, using a variety of techniques - but unsuccessfully so far.

David Mason and his colleagues broke the barrier by making a simple modification to the most commonly used approach, which allowed them to cancel the quantum noise in the measurement well enough to push the limit.

The result - and the experiment itself - has important implications for gravitational wave astronomy techniques, atomic force microscopy, various nanotechnology techniques, and the entire field of quantum sensors that have detonated the boundaries of precision in several areas.

Uncertainty and inaccuracy

Quantum actions have quantum consequences. In the context of measurements, this usually means that the very act of measuring a system disturbs you. This effect is called reverse action, or feedback ( backaction ), and is a consequence of the fundamental uncertainties of the systems at the atomic scale, encompassed by the uncertainty principle of Heisenberg .

In many cases, this uncertainty sets a limit to the accuracy with which a measure can be obtained because, in addition to a certain number of figures after the comma, everything is uncertain.

Gravitational wave telescopes, such as LIGO and Virgo, reflect the laser light in a mirror to measure their position in an optical configuration known as an interferometer. The imprecision of this measurement can be improved by increasing the laser power, but eventually the random reflections of the laser photons will disturb the position of the mirror, leading to a less sensitive measurement that will leave astronomical objects weak or distant outside the field of detection.

The Standard Quantum Limit (SQL) is established when an optimal balance between the feedback and the noise responsible for imprecision is achieved. This minimum noise level defines, for example, the best possible accuracy obtained by any interferometer.

A thin silicon nitride membrane (white) is stretched along a silicon (blue) frame. The membrane contains a pattern of holes, with a small island in the center, whose vibrations were measured in the experiment. [Image: Niels Bohr Institute]

Overcoming the Standard Quantum Limit

Mason and his colleagues were able to break the SQL through an ingenious combination of optical and nanomechanical techniques, allowing you to perform the first measurement of an object's position with a precision that exceeds the limit.

Like the LIGO, the new approach uses a laser interferometer to measure a position, in this case the position of a membrane made of ceramic silicon nitride. Although very thin (20 nanometers), the membrane is several millimeters wide and is easily visible to the naked eye.

The trick to going beyond SQL involves doing a double measurement of the light reflected by the membrane. In this configuration, the detector is capable of simultaneously measuring inaccuracy and feedback in a manner that allows those noise sources to mutually cancel each other. In other words, what remains is a "clean" measure.

Using this technique, team measured the position of their membrane with almost 30% better precision than the "allowed" by SQL.

"We are using quantum effects that emerge in the measurement setup itself, so the extra technological effort is really very small. That's good news for possible practical applications," Mason said.


 Continuous force and displacement measurement below the standard quantum limit
David Mason, Junxin Chen, Massimiliano Rossi, Yeghishe Tsaturyan, Albert Schliesser
Nature Physics
DOI: 10.1038 / s41567-019-0533-5

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