Louise Jottrand – VCQ

Vienna Summer School 2020 on Gravitational Quantum Physics

Louise Jottrand — Sat, 01 Aug 2020 06:00:00 +0000

ONLINE
3rd-6th September 2020
REGISTER until 2nd August

WHEN & WHERE

Caslav Brukner
Georgi Dvali
Renate Loll
Eduardo Martin-Martinez
Guglielmo Tino
Robert Wald
Anton Zeilinger
Magdalena Zych

CONFIRMED LECTURERS

· Concepts of General Relativity
· Concepts of Quantum Theory
· Precision tests of Gravity
· Tests of Quantum Theory
· Quantum Field Theory in curved space
· Quantum precision tests of GR (lab scale)
· Quantum precision tests of GR
(astronomical scale)
· Quantum Field Theory and curved spacetime
· Conceptual Challenges
· Quantum tests of Quantum Gravity

TOPICS

ONLINE
3rd-6th September 2020
REGISTER until 2nd August

WHEN & WHERE

Caslav Brukner
Georgi Dvali
Renate Loll
Eduardo Martin-Martinez
Guglielmo Tino
Robert Wald
Anton Zeilinger
Magdalena Zych

CONFIRMED LECTURERS

· Concepts of General Relativity
· Concepts of Quantum Theory
· Precision tests of Gravity
· Tests of Quantum Theory
· Quantum Field Theory in curved space
· Quantum precision tests of GR
· Quantum Field Theory and curved spacetime
· Conceptual Challenges
· Quantum tests of Quantum Gravity

TOPICS

©Bianca Kämpf/Universität Wien

Event Details

The summer school is aimed at young graduate students who want to get familiar with the current status and challenges in exploring the fascinating interface between quantum physics and gravity. The school will provide introductory lectures into the concepts of both general relativity and quantum theory, as well as into the theoretical framework of quantum field theories in and of curved space-time. The school will also provide an introduction to (classical and quantum) tests of gravity and their relevance for fundamental physics.

©Bianca Kämpf/Universität Wien

Event Details

MORE INFORMATION

Summer School Website
Registration Form

Quantum certified – a communication task to test non-classicality

Louise Jottrand — Wed, 20 May 2020 08:56:00 +0000

How can one check whether a system is in a quantum or classical state? In the journal PRL, researchers of the University of Vienna and the IQOQI-Vienna of the Austrian Academy of Sciences propose a new method, the “coherence equivalence”, to certify quantum superpositions without interferometers.

The effect of quantum superposition a.k.a. coherence is the most prominent feature to discriminate between the classical and the quantum domains. In the notorious Schrödinger’s cat paradox, a cat in a closed box is supposedly in a state of superposition between the states of death and life. However, observing such a condition directly is impossible, for when the box is opened and the cat observed the superposition suddenly disappears and the cat is always found in a well-defined state. The same happens in every quantum experiment.

To overcome this issue, physicists rely on the wave-like nature of quantum systems by using interferometers: they split the wave function of a quantum system into two separate paths, introduce a phase difference between the two paths and eventually recombine the system, thus closing a loop. Upon measurements, if an interference pattern appears –the same that arise when overlapping two waves, say when one tosses two stones in a lake– then we are sure that the state was in superposition. This certifies that we are dealing with a quantum system.

In their new theoretical study, Flavio Del Santo and Borivoje Dakić from the University of Vienna and the IQOQI-Vienna of the Austrian Academy of Sciences developed a new tool that allows to tell apart classical and quantum resources namely, the presence of superposition, without closing the loop of a standard interferometer.

Here, they also split the wave function of a quantum system in two separate paths, but they can measure them at separate locations without having to bring the system back together, unlike in a standard interferometry experiment. From the measured quantities, they calculate an expression named “coherence equality”, that is exactly 0 only if the system is classical. Any deviations thereof certify the presence of quantum superposition, thus characterizing the “non-classicality” of a system. “Normally, we do not expect to see the interference fringes by using ‘half of the interferometer’ only – says Borivoje Dakić. – This is exactly what is happening here: we can witness interference ‘at a distance’. I believe this can be turned into a resource for quantum communication.”

The authors phrased their analysis in terms of a “communication game” played by two parties, Alice and Bob. A source produces a single information carrier e.g. a particle which is sent to Alice or Bob. A referee stands in each path between the source and the players and can decide whether to block the carrier on its path. This represents the encoding of a binary piece of information, that is whether the carrier goes through or not. The referees then challenge the players to answer a question that requires the knowledge of both the encoded pieces of information. The authors showed that, no matter what strategy the players choose, if the information carrier is a classical object the probability of winning this game – by answering the questions correctly – is always equal to 50%. On the other hand, if the information carrier is allowed to be in a quantum superposition of the two communication channels, the probability deviates from the 50%, hence witnessing a violation of the coherence equality.

“The novelty of this approach, – says Flavio Del Santo – as compared to similar works that aim to detect quantum superposition without closing the loop of an interferometer, is that this result does not rely directly on quantum theory.” In fact, their approach is conceived in the same spirit of Bell’s inequality, which states that any theory that violate its bound cannot uphold the classical feature of locality. In a similar fashion, any theory that violates the coherence equality shows the non-classical behaviour of coherence.

In their new theoretical study, Flavio Del Santo and Borivoje Dakić from the University of Vienna and the IQOQI-Vienna of the Austrian Academy of Sciences developed a new tool that allows to tell apart classical and quantum resources namely, the presence of superposition, without closing the loop of a standard interferometer.

Suggestive representation of a superposition state of a single quantum object (here depicted as a cat to be reminiscent of the notorious Schrödinger's cat gedankenexperiment) delocalized at two different locations where two experimenters, Alice and Bob, are situated. The newly proposed method allows certifying a quantum superposition "at a distance", namely, by Alice's and Bob's local measurements only, without the need of bringing back together the quantum object at a single location. (© Flavio Del Santo, ÖAW)

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F. Del Santo and B. Dakić;

Coherence Equality and Communication in a Quantum Superposition
Phys. Rev. Lett. 124, 190501 (2020)

DOI: 10.1103/PhysRevLett.124.190501

Antibiotic Matter Waves

Louise Jottrand — Tue, 07 Apr 2020 13:32:00 +0000

One of the central tenets of quantum mechanics is the wave-particle duality. It tells us that even massive objects behave like both particles and waves. A number of previous experiments have shown this for electrons, neutrons, atoms and even large molecules. Quantum theory maintains that this is a universal property of matter. However, it had been notoriously difficult to extend this research to complex biomolecular systems. New experiments at the University of Vienna, supported by quantum chemical modelling at Stanford University now demonstrate for the first time the quantum wave nature of a complex antibiotic polypeptide, here gramicidin. The results have been published in Nature Communications.

Quantum interference with building blocks of life
The particle-wave duality is a ubiquitous phenomenon in quantum physics and even though it has been known for nearly a century, it still triggers puzzlement when we see it realized in complex matter: how can an object be delocalized in a wave-like manner? If quantum physics is a universal theory: how complex can an object be to still observe this counterintuitive behaviour? Does it still apply to larger lumps of matter, or even to the building blocks of life, such as for example peptides and proteins?

The research group around Markus Arndt at the University of Vienna is developing sophisticated tools to launch, diffract, interfere and detect complex molecules. However, testing quantum physics with long amino acid chains had remained prohibitive up to now. They had to overcome the challenges related to generate sufficiently intense beams of these biopolymers, to isolate them in high vacuum from any perturbing environment, and to establish coherent tools to probe their quantum nature.

In the new work published in Nature Communications, Armin Shayeghi and colleagues demonstrate for the first time quantum interference of the natural polypeptide gramicidin, an antibiotic made of 15 covalently bound amino acids. A key to this success was the use of ultrafast and intense laser light to desorb the peptides before they could decompose and matter-wave interferometry exploiting diffraction elements based on quantum measurement. These techniques shall pave the way to even more complex biological nanomaterials from proteins to DNA. This research is driven by the fundamental interest in exploring the limits of quantum physics and in establishing novel quantum-enhanced technologies as minimally invasive analytical tools for individual biomolecules isolated in the gas phase.

Experimental approach
Femtosecond short ultraviolet laser pulses knock the fragile molecules off a surface. The particles are swept away in a jet of cold argon atoms. Travelling at velocities of up to 600 m/s the gramicidin molecules have a tiny wavelength of only 350 femtometers, about a ten thousandth of the diameter of the biomolecules themselves. Shayeghi et al. used a very sensitive technique known as time-domain Talbot-Lau interferometry to measure their quantum fringe pattern and find that the molecular coherence is delocalized over more than 20 times the size of the molecules, which can only be explained by quantum mechanics. This conclusion is corroborated by additional high-level quantum chemical calculations, in collaboration with Todd J. Martinez from Stanford University, predicting electronic structure and properties that enter phase-space simulations to model the interference process.

“Our new technique will enable detailed studies of the quantum properties of biomolecules and it paves the way for a new kind of optical spectroscopy of biologically relevant molecules” says Shayeghi.

Peptide source (From: Matter-wave interference of a native polypeptide) Time-domain matter-wave interferometry (From: Matter-wave interference of a native polypeptide)

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A. Shayeghi, P. Rieser, G. Richter, U. Sezer, J. Rodewald, P. Geyer, T. J. Martinez, and M. Arndt;

Matter-wave interference of a native polypeptide,
Nat Commun 11, 1447 (2020)

DOI: 10.1038/s41467-020-15280-2

Website Arndt-Group

VCQ – The next Generation of Quantum Science

Louise Jottrand — Wed, 26 Feb 2020 15:11:00 +0000

Take a look at the new version of the video on the work within VCQ!

Vienna is home to a large and vital quantum community with a diverse research and education programme. It builds on a long tradition in quantum science and joins together creative young minds with fresh ideas for theoretical and experimental scientific ventures. VCQ unites Vienna quantum science from four institutions under one joint umbrella. More than 200 scientists, Ph.D.- and Master students are engaged in world-class research, from fundamental questions to applied quantum technologies. The Ph.D. training within VCQ is spearheaded by the Vienna Doctoral Program on Complex Quantum Systems (CoQuS). The unique environment invites candidates from all over the world to pursue curiosity driven, topical, diverse and unconventional research.

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How Nature Tells us its Formulas

Louise Jottrand — Mon, 03 Feb 2020 09:39:00 +0000

Developing a quantum description for a many-body system is extremely hard. TU Wien (Vienna) and Heidelberg University found a way to obtain quantum theories directly from the experiment.

A multi-particle system. Quantum theories of multi-particle-systems cannot be solved exactly. Approximations are required.
Picture: TU Wien, free to download and use

Many of the biggest questions in physics can be answered with the help of quantum field theories: They are needed to describe the dynamics of many interacting particles, and thus they are just as important in solid state physics as in cosmology. Often, however, it is extremely complicated to develop a quantum field theoretical model for a specific problem – especially if the system in question consists of many interacting particles.

Now a team from the TU Wien and the University of Heidelberg has developed methods with which these models can be directly obtained from experimental measurements. Instead of comparing the experimental results to theoretical model predictions, it is, in a certain sense, possible to measure the theory itself. This should now shed new light on the complicated field of many-body quantum physics.

Quantum Simulators

In recent years, a new method of studying quantum physical systems has gained importance – the so-called “quantum simulators”. “We simply do not have a satisfactory description of some quantum systems, for example high-temperature superconductors. Other systems can just not be observed directly, such as the early universe shortly after the Big Bang. Suppose we still want to learn something about such quantum systems – then we simply choose another system that can be easily controlled in the laboratory and adjust it so that it behaves in a similar way to the system we are actually interested in. For example, we can use experiments on ultracold atoms to learn about systems that we would otherwise not be able to study at all,” explains Jörg Schmiedmayer from the Vienna Center of Quantum Science and Technology (VCQ) at TU Wien. This is possible because there are fundamental similarities between different quantum physical descriptions of different systems.

But no matter which quantum system is studied, scientists always come across a fundamental problem: “If there are too many particles involved, the formulas of quantum theory quickly become so complicated that they cannot be solved, not even with the best supercomputers in the world,” explains Sebastian Erne. “That’s a pity, because systems consisting of many particles are particularly interesting. In everyday life, it is always the case that many particles play a role at the same time.”

Getting Rid of the Details

In general, it is not possible to solve the exact quantum theory for a many-particle-system, in which every single particle is considered. One has to find a simplified quantum description that contains all the essential properties, but no longer relies on details about the individual particles. “This is similar to describing a gas,” explains Jörg Schmiedmayer. “We’re not interested in every single atom, but in more general variables such as pressure and temperature.”

But how do you arrive at such theories for many-body systems? Deriving them purely mathematically from the laws of nature that apply to individual particles is extremely complicated. But as it now turns out, this is not necessary. “We have found a method of reading the quantum field theoretical description directly from the experiment,” says Schmiedmayer. “In a certain sense, nature provides the formulas, with which it must be described, all by itself.”

We know that every quantum theory has to obey certain formal rules – we talk for example about correlations, propagators, vertices, Feynman diagrams – the basic building blocks of every quantum physical model. The research team of TU Wien and the University of Heidelberg has found a way to make these individual basic building blocks experimentally accessible. The experimental measurements result in an empirically obtained quantum theory for a many-body system, without having to work with paper and pencil.

“For years, we have suspected that this is theoretically possible, but not everyone believed us that it actually works,” says Jörg Schmiedmayer. “Now we have shown that we were right – by looking at a special case where the theory can also be found and (in certain limits) solved mathematically. Our measurement results provide exactly the same theory building blocks.”

Ultracold Atomic Clouds

The experiment was done with clouds of thousands of ultracold atoms that are trapped in a magnetic trap on an atomic chip. “From the quantum wave patterns of these atomic clouds, we can determine the correlation functions from which the basic building blocks of the appropriate theory can be derived,” explains Schmiedmayer.

The results have now been published in the journal “Physical Review X”. The team hopes that this will significantly simplify the study of quantum many-particle systems. Perhaps it will shed some light on some of the big questions in physics.

The project was funded by the Austrian Science Fund (FWF) as part of Austria’s participation in the SFB1225: ISOQUANT.

A multi-particle system. Quantum theories of multi-particle-systems cannot be solved exactly. Approximations are required. Picture: TU Wien, free to download and use Sketch of the quantum simulator used by Zache et al., which measures the phase correlations between two one-dimensional superfluids separated by a barrier. From the experiments, the teams determine the “irreducible vertices” (red) that fully define a quantum field theory for the so-called sine-Gordon model. ©APS/Alan Stonebraker Atom chip: The atom chip (in gold) at TU Wien, at which the experiment is performed. Photo: TU Wien, free to download and use.

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T. V. Zache, T. Schweigler, S. Erne, J. Schmiedmayer, and J. Berges;

Extracting the Field Theory Description of a Quantum Many-Body System from Experimental Data
Phys. Rev. X 10, 011020 – Published 29 January 2020

DOI: 10.1103/PhysRevX.10.011020

A Remote Control for Everything Small

Louise Jottrand — Mon, 18 Nov 2019 15:18:47 +0000

Atoms, molecules or even living cells can be manipulated with light beams. At TU Wien a method was developed to revolutionize such “optical tweezers”.

Taylored light wave - Intensity distribution of an electric wave field that applies a well-defined torque onto the quadratic target. (free to download and reprint © TU Wien)

They are reminiscent of the “tractor beam” in Star Trek: special light beams can be used to manipulate molecules or small biological particles. Even viruses or cells can be captured or moved. However, these optical tweezers only work with objects in empty space or in transparent liquids. Any disturbing environment would deflect the light waves and destroy the effect. This is a problem, in particular with biological samples because they are usually embedded in a very complex environment.

But scientists at TU Wien (Vienna) have now shown how virtue can be made of necessity: A special calculation method was developed to determine the perfect wave form to manipulate small particles in the presence of a disordered environment. This makes it possible to hold, move or rotate individual particles inside a sample – even if they cannot be touched directly. The tailor-made light beam becomes a universal remote control for everything small. Microwave experiments have already demonstrated that the method works. The new optical tweezer technology has now been presented in the journal “Nature Photonics”.

Optical tweezers in disordered environments

“Using laser beams to manipulate matter is nothing unusual anymore,” explains Prof. Stefan Rotter from the Institute for Theoretical Physics at TU Wien. In 1997, the Nobel Prize in Physics was awarded for laser beams that cool atoms by slowing them down. In 2018, another Physics Nobel Prize recognized the development of optical tweezers.

But light waves are sensitive: in a disordered, irregular environment, they can be deflected in a highly complicated way and scattered in all directions. A simple, plane light wave then becomes a complex, disordered wave pattern. This completely changes the way light interacts with a specific particle.

“However, this scattering effect can be compensated,” says Michael Horodynski, first author of the paper. “We can calculate how the wave has to be shaped initially so that the irregularities of the disordered environment transform it exactly into the shape we want it to be. In this case, the light wave looks rather disordered and chaotic at first, but the disordered environment turns it into something ordered. Countless small disturbances, which would normally render the experiment impossible, are used to generate exactly the desired wave form, which then acts on a specific particle.

Calculating the optimal wave

To achieve this, the particle and its disordered environment are first illuminated with various waves and the way in which the waves are reflected is measured. This measurement is carried out twice in quick succession. “Let’s assume that in the short time between the two measurements, the disordered environment remains the same, while the particle we want to manipulate changes slightly,” says Stefan Rotter. “Let’s think of a cell that moves, or simply sinks downwards a little bit. Then the light wave we send in is reflected a little bit differently in the two measurements.” This tiny difference is crucial: With the new calculation method developed at TU Wien, it is possible to calculate the wave that has to be used to amplify or attenuate this particle movement.

“If the particle slowly sinks downwards, we can calculate a wave that prevents this sinking or lets the particle sink even faster,” says Stefan Rotter. “If the particle rotates a little bit, we know which wave transmits the maximum angular momentum – we can then rotate the particle with a specially shaped light wave without ever touching it.”

Successful experiments with microwaves

Kevin Pichler, also part of the research team at TU Wien, was able to put the calculation method into practice in the lab of project partners at the University of Nice (France): he used randomly arranged Teflon objects, which he irradiated with microwaves – and in this way he actually succeeded in generating exactly those waveforms which, due to the disorder of the system, produced the desired effect.

“The microwave experiment shows that our method works,” reports Stefan Rotter. “But the real goal is to apply it not with microwaves but with visible light. This could open up completely new fields of applications for optical tweezers and, especially in biological research, would make it possible to control small particles in a way that was previously considered completely impossible.”

Taylored light wave Intensity distribution of an electric wave field that applies a well-defined torque onto the quadratic target. (free to download and reprint © TU Wien) Andre Branstötter, Michael Horodynski, Kevin Pichler, Stefan Rotter, Matthias Kühmayer (left to right) (free to download and reprint © TU Wien) Aluminium waveguide with a quadratic target in the middle (the cover plate is removed and not shown). The white teflon elements to the left and right of the target mimic a disordered medium. (free to download and reprint © TU Wien)

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M. Horodynski, M. Kühmayer, A. Brandstötter, K. Pichler, Y. V. Fyodorov, U. Kuhl & S. Rotter;

Optimal wave fields for micromanipulation in complex scattering environments
Nature Photonics (2019)
https://www.nature.com/articles/s41566-019-0550-z

DOI: 10.1038/s41566-019-0550-z

Free version: https://arxiv.org/abs/1907.09956
Website Rotter-Group