From U Copenhagen Niels Bohr Institute: “Smart atomic cloud solves Heisenberg’s observation problem”

University of Copenhagen

Niels Bohr Institute bloc

Niels Bohr Institute

13 July 2017
Eugene Polzik
polzik@nbi.dk
+45 2338 2045

Quantum physics: Scientists at the Niels Bohr Institute, University of Copenhagen have been instrumental in developing a ‘hands-on’ answer to a challenge intricately linked to a very fundamental principle in physics: Heisenberg’s Uncertainty Principle. The NBI-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.

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From the left: Phd student Rodrigo Thomas, Professor Eugene Polzik and PhD student Christoffer Møller in front of the experiment demonstrating quantum measurement of motion. Photo: Ola J. Joensen.

Our lives are packed with sensors gathering all sorts of information – and some of the sensors are integrated in our cell phones which e.g. enables us to measure the distances we cover when we go for a walk – and thereby also calculate how many calories we have burned thanks to the exercise. And this to most people seems rather straight forward.

When measuring atom structures or light emissions at the quantum level by means of advanced microscopes or other forms of special equipment, things do, however, get a little more complicated due to a problem which during the 1920’s had the full attention of Niels Bohr as well as Werner Heisenberg. And this problem – this has to do with the fact that in-accuracies inevitably taint certain measurements conducted at quantum level – is described in Heisenberg’s Uncertainty Principle.

In a scientific report published in this week’s issue of Nature, NBI-researchers – based on 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 as well as 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 small glass cage.

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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.

Light ‘kicks’ object

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

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.

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The atomic part of the hybrid experiment. The atoms are contained in a micro-cell inside the magnetic shield seen in the middle. Photo: Ola J. Joensen.

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 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”.

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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?

“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 the 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 the fabric of space-time itself of light.

In September of 2015 the American LIGO-experiment was able to publish the first direct registrations and measurements of gravitational waves stemming from a collision between two very large black holes.

However, the equipment used by LIGO is influenced by Quantum Back Action, and the new research from NBI may prove capable of eliminating that problem, says Eugene Polzik.

See the full article here .

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Niels Bohr Institute Campus

The Niels Bohr Institute (Danish: Niels Bohr Institutet) is a research institute of the University of Copenhagen. The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics.

The Institute was founded in 1921, as the Institute for Theoretical Physics of the University of Copenhagen, by the Danish theoretical physicist Niels Bohr, who had been on the staff of the University of Copenhagen since 1914, and who had been lobbying for its creation since his appointment as professor in 1916. On the 80th anniversary of Niels Bohr’s birth – October 7, 1965 – the Institute officially became The Niels Bohr Institute.[1] Much of its original funding came from the charitable foundation of the Carlsberg brewery, and later from the Rockefeller Foundation.[2]

During the 1920s, and 1930s, the Institute was the center of the developing disciplines of atomic physics and quantum physics. Physicists from across Europe (and sometimes further abroad) often visited the Institute to confer with Bohr on new theories and discoveries. The Copenhagen interpretation of quantum mechanics is named after work done at the Institute during this time.

On January 1, 1993 the institute was fused with the Astronomic Observatory, the Ørsted Laboratory and the Geophysical Institute. The new resulting institute retained the name Niels Bohr Institute.

The University of Copenhagen (UCPH) (Danish: Københavns Universitet) is the oldest university and research institution in Denmark. Founded in 1479 as a studium generale, it is the second oldest institution for higher education in Scandinavia after Uppsala University (1477). The university has 23,473 undergraduate students, 17,398 postgraduate students, 2,968 doctoral students and over 9,000 employees. The university has four campuses located in and around Copenhagen, with the headquarters located in central Copenhagen. Most courses are taught in Danish; however, many courses are also offered in English and a few in German. The university has several thousands of foreign students, about half of whom come from Nordic countries.

The university is a member of the International Alliance of Research Universities (IARU), along with University of Cambridge, Yale University, The Australian National University, and UC Berkeley, amongst others. The 2016 Academic Ranking of World Universities ranks the University of Copenhagen as the best university in Scandinavia and 30th in the world, the 2016-2017 Times Higher Education World University Rankings as 120th in the world, and the 2016-2017 QS World University Rankings as 68th in the world. The university has had 9 alumni become Nobel laureates and has produced one Turing Award recipient

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