Atomic Cloud Observations go beyond Heisenberg limits

Researchers used laser light to link caesium atoms and a vibrating membrane. The research, the first of its kind, points to sensors capable of measuring movement with unseen precision beyond Heisenberg limits.

A number of experiments – demonstrate that Heisenberg’s Uncertainty Principle to some degree can be neutralized. This has never been shown before, and the results may spark development of new measuring equipment, new and better sensors.

Professor Eugene Polzik, head of Quantum Optics (QUANTOP) at the Niels Bohr Institute, has been in charge of the research – which has included the construction of a vibrating membrane and an advanced atomic cloud locked up in a minute glass cage.
Light ‘kicks’ object

Heisenberg’s Uncertainty Principle basically says that you cannot simultaneously know the exact position and the exact speed of an object.


If laser light used to measure motion of a vibrating membrane (left) is first transmitted through an atom cloud (center) the measurement sensitivity can be better than standard quantum limits envisioned by Bohr and Heisenberg. Photo: Bastian Leonhardt Strube and Mads Vadsholt

Which has to do with the fact that observations conducted via a microscope operating with laser light inevitably will lead to the object being ‘kicked’. This happens because light is a stream of photons which when reflected off the object give it random ‘kicks’ – and as a result of those kicks the object begins to move in a random way.

This phenomenon is known as Quantum Back Action (QBA) – and these random movements put a limit to the accuracy with which measurements can be carried out at quantum level.

To conduct the experiments at NBI professor Polzik and his team of “young, enthusiastic and very skilled NBI-researchers” used a ‘tailor-made’ membrane as the object observed at quantum level. The membrane was built by Ph.D. Students Christoffer Møller and Yegishe Tsaturyan, whereas Rodrigo Thomas and Georgios Vasikalis – Ph.D. Student and researcher, respectively – were in charge of the atomic aspects. Furthermore Polzik relied on other NBI-employees, assistant professor Mikhail Balabas, who built the minute glass cage for the atoms, researcher Emil Zeuthen and professor Albert Schliesser who – collaborating with German colleagues – were in charge of the substantial number of mathematical calculations needed before the project was ready for publication in Nature.

Nature – Quantum back-action-evading measurement of motion in a negative mass reference frame

Over the last decades scientists have tried to find ways of ‘fooling’ Heisenberg’s Uncertainty Principle. Eugene Polzik and his colleagues came up with the idea of implementing the advanced atomic cloud a few years ago – and the cloud consists of 100 million caesium-atoms locked up in a hermetically closed cage, a glass cell, explains the professor:

“The cell is just 1 centimeter long, 1/3 of a millimeter high and 1/3 of a millimeter wide, and in order to make the atoms work as intended, the inner cell walls have been coated with paraffin. The membrane – whose movements we were following at quantum level – measures 0,5 millimeter, which actually is a considerable size in a quantum perspective”.

The idea behind the glass cell is to deliberately send the laser light used to study the membrane-movements on quantum level through the encapsulated atomic cloud BEFORE (Italics!) the light reaches the membrane, explains Eugene Polzik: “This results in the laser light-photons ‘kicking’ the object – i.e. the membrane – as well as the atomic cloud, and these ‘kicks’ so to speak cancel out. This means that there is no longer any Quantum Back Action – and therefore no limitations as to how accurately measurements can be carried out at quantum level”.


The optomechanical part of the hybrid experiment. The cryostat seen in the middle houses the vibrating membrane whose quantum motion is measured. Photo: Ola J. Joensen

How can this be utilized? More Accurate GPS and improved sensing of gravitational and other waves

“For instance when developing new and much more advanced types of sensors for various analyses of movements than the types we know today from cell phones, GPS and geological surveys”, says professor Eugene Polzik: “Generally speaking sensors operating at quantum level are receiving a lot of attention these days. One example is the Quantum Technologies Flagship, an extensive EU program which also supports this type of research”.

The fact that it is indeed possible to ‘fool’ Heisenberg’s Uncertainty Principle may also prove significant in relation to better understanding gravitational waves – waves in Space moving at the speed of light.

Abstract

Quantum mechanics dictates that a continuous measurement of the position of an object imposes a random quantum back-action (QBA) perturbation on its momentum. This randomness translates with time into position uncertainty, thus leading to the well known uncertainty on the measurement of motion. As a consequence of this randomness, and in accordance with the Heisenberg uncertainty principle, the QBA puts a limitation—the so-called standard quantum limit—on the precision of sensing of position, velocity and acceleration. Here we show that QBA5 on a macroscopic mechanical oscillator can be evaded if the measurement of motion is conducted in the reference frame of an atomic spin oscillator. The collective quantum measurement on this hybrid system of two distant and disparate oscillators is performed with light. The mechanical oscillator is a vibrational ‘drum’ mode of a millimetre-sized dielectric membrane8, and the spin oscillator is an atomic ensemble in a magnetic field9, 10. The spin oriented along the field corresponds to an energetically inverted spin population and realizes a negative-effective-mass oscillator, while the opposite orientation corresponds to an oscillator with positive effective mass. The QBA is suppressed by −1.8 decibels in the negative-mass setting and enhanced by 2.4 decibels in the positive-mass case. This hybrid quantum system paves the way to entanglement generation and distant quantum communication between mechanical and spin systems and to sensing of force, motion and gravity beyond the standard quantum limit.

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