Agnieszka Skwara – VCQ https://vcq.quantum.at Fri, 19 Feb 2021 16:19:52 +0000 en-US hourly 1 https://wordpress.org/?v=6.6.2 https://vcq.quantum.at/wp-content/uploads/2021/10/cropped-VCQ_SYMBOL-32x32.png Agnieszka Skwara – VCQ https://vcq.quantum.at 32 32 Connecting the Austrian quantum community – literally! https://vcq.quantum.at/2021/02/19/connecting-the-austrian-quantum-community-literally/ Fri, 19 Feb 2021 16:16:14 +0000 https://vcq.quantum.at/?p=4349
AQUnet will establish an Austria-wide glass fiber network for the distribution of quantum information and quantum metrology signals. The existing data backbone infrastructure, operated by ACOnet (Austrian Academic Computer Network), will be upgraded and extended to enable safe and stable transport of quantum signals. Within AQUnet, we will connect a series of locations of the consortium in Vienna and Innsbruck, as well as external users. We will furthermore connect to partnering European initiatives like e.g. REFIMEVE+ (France), PTB-LMU (Germany), ISI-Brno (Czech Republic).
© TU WIen
AQUnet will enable ACOnet (with support by project partners and the scientific community) to extend the existing data infrastructure to quantum signals: where possible, “classical” glass fiber channels will be upgraded used for simultaneous quantum transport, existing dark fibers will be made accessible, new channels established. The notoriously fragile quantum signals will require specific amplification stages (especially along Innsbruck-Vienna). A central node for the star-shaped distribution of an ultrastable optical reference signal (linked to the Austrian primary atomic clock at the federal office for calibration and surveying BEV) will be established.
For further information (in German) visit the website of the TU Wien:
Prof. Thorsten Schumm

Atominstitut
Technische Universität Wien
Stadionallee 2, 1020 Vienna
Austria

E: thorsten.schumm (at)tuwien.ac.at
T: +43-1-58801-141896
]]> Quantum computing: when ignorance is wanted https://vcq.quantum.at/2021/02/10/quantum-computing-when-ignorance-is-wanted/ Wed, 10 Feb 2021 14:39:10 +0000 https://vcq.quantum.at/?p=4259
Quantum technologies for computers open up new concepts of preserving the privacy of input and output data of a computation. Scientists from the University of Vienna, the Singapore University of Technology and Design and the Polytechnic University of Milan have shown that optical quantum systems are not only particularly suitable for some quantum computations, but can also effectively encrypt the associated input and output data. This demonstration of a so-called quantum homomorphic encryption of a quantum computation has now been published in NPJ Quantum Information.
“Artistic image of a homomorphic-encrypted quantum computation using a photonic quantum computer.” Credit: Equinox Graphics, University of Vienna
Quantum computers promise not only to outperform classical machines in certain important tasks, but also to maintain the privacy of data processing. The secure delegation of computations has been an increasingly important issue since the possibility of utilizing cloud computing and cloud networks. Of particular interest is the ability to exploit quantum technology that allows for unconditional security, meaning that no assumptions about the computational power of a potential adversary need to be made.
Different quantum protocols have been proposed, all of which make trade-offs between computational performance, security, and resources. Classical protocols, for example, are either limited to trivial computations or are restricted in their security. In contrast, homomorphic quantum encryption is one of the most promising schemes for secure delegated computation. Here, the client’s data is encrypted in such a way that the server can process it even though he cannot decrypt it. Moreover, opposed to other protocols, the client and server do not need to communicate during the computation which dramatically boosts the protocol’s performance and practicality.
In an international collaboration led by Prof. Philip Walther from the University of Vienna scientists from Austria, Singapore and Italy teamed up to implement a new quantum computation protocol where the client has the option of encrypting his input data so that the computer cannot learn anything about them, yet can still perform the calculation. After the computation, the client can then decrypt the output data again to read out the result of the calculation. For the experimental demonstration, the team used quantum light, which consists of individual photons, to implement this so-called homomorphic quantum encryption in a quantum walk process. Quantum walks are interesting special-purpose examples of quantum computation because they are hard for classical computers, whereas being feasible for single photons.
By combining an integrated photonic platform built at the Polytechnic University of Milan, together with a novel theoretical proposal developed at the Singapore University of Technology and Design, scientist from the University of Vienna demonstrated the security of the encrypted data and investigated the behavior increasing the complexity of the computations.
The team was able to show that the security of the encrypted data improves the larger the dimension of the quantum walk calculation becomes. Furthermore, recent theoretical work indicates that future experiments taking advantage of various photonic degrees of freedom would also contribute to an improvement in data security; one can anticipate further optimizations in the future. “Our results indicate that the level of security improves even further, when increasing the number of photons that carry the data“, says Philip Walther and concludes, “This is exciting and we anticipate further developments of secure quantum computing in the future”.

Jonas Zeuner, Ioannis Pitsios, Si-Hui Tan, Aditya Sharma, Joseph Fitzsimons, Roberto Osellame and Philip Walther;

Experimental Quantum Homomorphic Encryption,

npj Quantum Information 7, 25 (2021);

DOI: 10.1038/s41534-020-00340-8

Philip Walther

Quantum Optics, Quantum Nanophysics and Quantum Information
Faculty of Physics, University of Vienna
Boltzmanngasse 5, 1090 Vienna, Austria

E: philip.walther@univie.ac.at
T: +43-1-4277-72560
M: +43-664-60277-72560
W: https://walther.univie.ac.at/

 

]]> Quantum effects help minimise communication flaws https://vcq.quantum.at/2021/02/01/quantum-effects-help-minimise-communication-flaws/ Mon, 01 Feb 2021 12:06:33 +0000 https://vcq.quantum.at/?p=4243
Particles traveling in a superposition of paths can bypass noise in communication
Artistic illustration of a communication channel between two partners crossing regions of noise. The noise resistance of the channel is enhanced by distributing information over multiple paths in quantum superposition. Credit: © Aloop Visual & Science, University of Vienna
A collaboration between the Universities of Hong-Kong, Grenoble and Vienna, under the lead of Philip Walther, reveals novel techniques to reduce noise in quantum communication. The results, published in the latest issue of Physical Review Research, demonstrate that quantum particles traveling in a superposition of paths enable noise reduction in communications.

Noise hinders performance of modern quantum technologies

Among the most active fields of research in modern physics, both at an academic level and beyond, are quantum computation and communication, which apply quantum phenomena such as superposition and entanglement to perform calculations, or to exchange information. A number of research groups around the world have built quantum devices that are able to perform calculations faster than any classical computer. Yet, there is still a long way to go before these devices can be converted into marketable quantum computers. One reason for this is that both quantum computation and quantum communication are strongly deteriorated by the ease with which a quantum superposition state can be destroyed, or entanglement between two or more quantum particles can be lost. The primary approach to overcome these limitations is the application of so-called quantum error-correcting codes. This, however, requires an amount of resources exceeding that which can be currently achieved in a controlled way. While, in the long run, error correction is likely to become an integral part of future quantum devices, a complementary approach is to mitigate the noise —that is, the cumulative effect of uncorrected errors— without relying on so many additional resources. These are referred to as noise reduction schemes.

Noise mitigation without additional resources through simple quantum schemes

A new approach along this research line was recently proposed to reduce noise in a communication scheme between two parties. Imagine two parties who want to communicate by exchanging a quantum particle, yet the particle has to be sent over some faulty transmission lines (depicted in the artistic illustration). Recently, a team of researchers at Hong-Kong University proposed that an overall reduction in noise could be achieved by directing the particle along a quantum superposition of paths through regions of noise in opposite order. In particular, while classically a particle can only travel along one path, in quantum mechanics it can move along multiple paths at once. If one uses this property to send the particle along two quantum paths, one can, for instance, lead the particle across the noisy regions in opposite order simultaneously. This effect had been demonstrated experimentally by two independent research investigations. These results suggested that, to achieve this noise reduction, it is necessary to place the noisy transmission lines in a quantum superposition of opposite orders. Shortly after this, research groups in Vienna and in Grenoble realised that this effect can also be achieved via simpler configurations, which can even completely eliminate the noise between the two parties.
All of these schemes have now been implemented experimentally and compared with each other by a research team led by Philip Walther at the University of Vienna. In this work, different ways of passing through two noisy regions in quantum superposition are compared for a variety of noise types. The experimental results are also supported with numerical simulations to extend the study to more generic types of noise. Surprisingly, it is found that the simplest schemes for quantum superposition of noisy channels also offer the best reduction of the noise affecting communication. “Error correction in modern quantum technologies is among the most pressing needs of current quantum computation and communication schemes. Our work shows that, at least in the case of quantum communication, already with the technologies currently in use it may be possible to mitigate this issue with no need for additional resources,” says Giulia Rubino, first author of the publication in Physical Review Research. The ease of the demonstrated technique allows immediate use in current long-distance communications, and promises potential further applications in quantum computation and quantum thermodynamics.

G. Rubino, L. A. Rozema, D. Ebler, H. Kristjánsson, S. Salek, P. Allard Guérin, A. A. Abbott, C. Branciard, C. Brukner, G. Chiribella, P. Walther; High-Experimental quantum communication enhancement by superposing paths, Physical Review Research 3, 013093 (2021).

DOI: 10.1103/PhysRevResearch.3.013093

Dr. Giulia Ribino

Faculty of Physics, University of Vienna
Vienna Center for Quantum Science and Technology (VCQ)
Boltzmanngasse 5, 1090 Vienna, Austria

T: +43 (0)1 4277 72563
E: giulia.rubino@univie.ac.at
W: http://walther.quantum.at

]]> Optimal information about the invisible https://vcq.quantum.at/2021/01/25/optimal-information-about-the-invisible/ Mon, 25 Jan 2021 11:38:53 +0000 https://vcq.quantum.at/?p=4173
How do you measure objects that you can’t see under normal circumstances? Utrecht University and TU Wien (Vienna) open up new possibilities with special light waves.
When light gets deflected by a disordered structure it becomes difficult to estimate where the target is located. In this new study a procedure is presented that allows one to reach the optimal estimation precision in such challenging scenarios.
Instead of using straight, ordinary laser beams to estimate the position of a hidden object inside a disordered environment (see top panels), the optimal procedure works by imprinting a pattern on the incoming laser beam that yields the maximum information output on the object and allows one to estimate its position precisely (see bottom panels).

Laser beams can be used to precisely measure an object’s position or velocity. Normally, however, a clear, unobstructed view of this object is required – and this prerequisite is not always satisfied. In biomedicine, for example, structures are examined, which are embedded in an irregular, complicated environment. There, the laser beam is deflected, scattered and refracted, often making it impossible to obtain useful data from the measurement.

However, Utrecht University (Netherlands) and TU Wien (Vienna, Austria) have now been able to show that meaningful results can be obtained even in such complicated environments. Indeed, there is a way to specifically modify the laser beam so that it delivers exactly the desired information in the complex, disordered environment – and not just approximately, but in a physically optimal way: Nature does not allow for more precision with coherent laser light. The new technology can be used in very different fields of application, even with different types of waves, and has now been presented in the scientific journal “Nature Physics”.

The vacuum and the bathroom window

“You always want to achieve the best possible measurement accuracy – that’s a central element of all natural sciences,” says Stefan Rotter from TU Wien. “Let’s think, for example, of the huge LIGO facility, which is being used to detect gravitational waves: There, you send laser beams onto a mirror, and changes in the distance between the laser and the mirror are measured with extreme precision.” This only works so well because the laser beam is sent through an ultra-high vacuum. Any disturbance, no matter how small, is to be avoided.

But what can you do when you are dealing with disturbances that cannot be removed? “Let’s imagine a panel of glass that is not perfectly transparent, but rough and unpolished like a bathroom window” says Allard Mosk from Utrecht University. “Light can pass through, but not in a straight line. The light waves are altered and scattered, so we can’t accurately see an object on the other side of the window with the naked eye.” The situation is quite similar when you want to examine tiny objects inside biological tissue: the disordered environment disturbs the light beam. The simple, regular straight laser beam then becomes a complicated wave pattern that is deflected in all directions.

The optimal wave

However, if you know exactly what the disturbing environment is doing to the light beam, you can reverse the situation: Then it is possible to create a complicated wave pattern instead of the simple, straight laser beam, which gets transformed into exactly the desired shape due to the disturbances and hits right where it can deliver the best result. “To achieve this, you don’t even need to know exactly what the disturbances are,” Dorian Bouchet, the first author of the study explains. “It’s enough to first send a set of trial waves through the system to study how they are changed by the system.”

The scientists involved in this work jointly developed a mathematical procedure that can then be used to calculate the optimal wave from this test data: “You can show that for various measurements there are certain waves that deliver a maximum of information as, e.g., on the spatial coordinates at which a certain object is located.”

Take for example an object that is hidden behind a turbid pane of glass: there is an optimal light wave that can be used to obtain the maximum amount of information about whether the object has moved a little to the right or a little to the left. This wave looks complicated and disordered, but is then modified by the turbid pane in such a way that it arrives at the object in exactly the desired way and returns the greatest possible amount of information to the experimental measuring apparatus.

Laser experiments in Utrecht

The fact that the method actually works was confirmed experimentally at Utrecht University: Laser beams were directed through a disordered medium in the form of a turbid plate. The scattering behaviour of the medium was thereby characterised, then the optimal waves were calculated in order to analyse an object beyond the plate – and this succeeded, with a precision in the nano-meter range.

Then the team carried out further measurements to test the limits of their novel method: The number of photons in the laser beam was significantly reduced to see whether one then still gets a meaningful result. In this way, they were able to show that the method not only works, but is even optimal in a physical sense: “We see that the precision of our method is only limited by the so-called quantum noise,” explains Allard Mosk. “This noise results from the fact that light consists of photons – nothing can be done about that. But within the limits of what quantum physics allows us to do for a coherent laser beam, we can actually calculate the optimal waves to measure different things. Not only the position, but also the movement or the direction of rotation of objects.”

These results were obtained in the context of a program for nanometer-scale imaging of semiconductor structures, in which universities collaborate with industry. Indeed, possible areas of application for this new technology include microbiology but also the production of computer chips, where extremely precise measurements are indispensable.

D. Bouchet, S. Rotter, A.P. Mosk;

Maximum information states for coherent scattering measurements

Nature Physics (2021), DOI: 10.1038/s41567-020-01137-4

Prof. Stefan Rotter
Institute for Theoretical Physics
TU Wien

Wiedner Hauptstraße 8–10,  1040 Vienna, Austria

Phone: +43 1 58801 13618
Mail: stefan.rotter(at)tuwien.ac.at

]]> 50-year-old riddle resolved! https://vcq.quantum.at/2020/11/23/50-year-old-riddle-resolved/ Mon, 23 Nov 2020 13:33:35 +0000 https://vcq.quantum.at/?p=3650

Physicists at TU Wien clarify a long-lasting theoretical debate about the vacuum states of cavity QED.
© TU Wien

Quantum electrodynamics (QED) is the fundamental theory of electromagnetism, which also describes how atoms, molecules and solids interact with the quantized radiation field.  The success of this theory and our current very precise understanding of such light-matter interaction processes is rooted in a key feature of the electromagnetic force: it is weak at the quantum level. This property allows us to model and interpret the coupling of atoms and fields in terms of well-defined electromagnetic excitations, the photons, which are emitted and absorbed by matter in discrete units. As a consequence, the vacuum of QED, which is the state with the lowest energy in this theory, is very simple: All the atoms are deexcited and there are no photons.

For a long time it has been speculated that this general picture could break down when atoms or other particles are coupled to a single electromagnetic mode that is confined to a very small region in space (“cavity QED”). This confinement drastically increases the atom-field coupling strength compared to that of free space. In the early 1970s it has first been predicted that under such conditions a new, so-called ‘superradiant’ vacuum state with a large number of photons can emerge. This counterintuitive and very surprising result immediately sparked a controversial scientific debate and in many follow-up studies alternating claims about the existence and the non-existence of this new vacuum phase have been made. But until today no consent about this topic has been reached.

In a recent publication in SciPost Physics, researchers from the TU Wien and the University of Innsbruck present a first exact numerical study of the vacuum phases in cavity QED. These calculations, carried out on Austria’s most powerful supercomputer, the Vienna Scientific Cluster, definitely confirm the existence the superradiant phase. However, the study also shows that this state is not as spectacular as initially thought and can be simply reinterpreted as a modified ferroelectric phase. To the surprise of the authors, at even higher coupling strengths several completely new vacuum configurations appeared, which were unknown before and, intuitively, shouldn’t even exist.  Therefore, while closing one debate, these numerical findings show that our current understanding of light-matter interactions in this ultrastrong coupling regime must be completely rethought. Beyond a purely scientifc interest, these insights can be important for various future applications, ranging from vacuum-assisted chemical reactions to quantum technologies based on ultrastrongly coupled superconducting circuits.

M. Schuler, D. De Bernardis, A. M. Läuchli, & P. Rabl;

The Vacua of Dipolar Cavity Quantum Electrodynamics
SciPost Phys. 9, 066 (2020).

https://scipost.org/SciPostPhys.9.5.066

DOI: 10.21468/SciPostPhys.9.5.066

]]> 2 ERC Synergy Grants for UNIVIE’s Physicists! https://vcq.quantum.at/2020/11/12/two-erc-synergy-grants-for-university-of-viennas-physicists/ Thu, 12 Nov 2020 08:40:02 +0000 https://vcq.quantum.at/?p=3582

ERC Synergy Grants for Thomas Pichler and Markus Aspelmeyer.

The physicist Thomas Pichler and his international team will receive an ERC Synergy Grant of 14 million Euros to build a new electron nano-spectrometer at the University of Vienna, which combines microscopy with spectroscopy as a new key technology for materials analysis. With their 13 million Euros ERC Synergy Grant, the Viennese quantum physicist Markus Aspelmeyer and the team around Oriol Romero-Isart from Innsbruck want to explore the limits of the quantum world by positioning a solid-state object containing billions of atoms at two locations simultaneously for the first time.
© Universität Wien

“ERC grants enable leading-edge research and are an important indicator of the international competitiveness of a university. Thomas Pichler and Markus Aspelmeyer, members of the Faculty of Physics, have both been awarded a prestigious, highly remunerated ERC Synergy Grant for their outstanding research projects,” says Rector Heinz W. Engl.

MORE-TEM - a revolutionary research tool for materials science and technology

Nanomaterials are at the heart of innovation in basic science and technology, for instance to find energy solutions in the context of the European Green Deal. For their application, it is necessary to see the atomic structure of the nanomaterial as well as to determine its local properties. However, this is not feasible with any existing technology.

The ERC Synergy Grant MORE-TEM, led by Thomas Pichler of the University of Vienna, will now close this gap. For this purpose, a fundamentally new and worldwide unique electron nano-spectrometer will be realized in Vienna with the contribution of several international partners. The set-up can be imagined as a much smaller, cheaper, large-scale research facility downsized to a “table-top” version. This represents a breakthrough for the characterization of all nanomaterials, combining the advantages of electron microscopy with high-resolution spectroscopy in a unique nano-spectrometer.

“I am extremely pleased that the generous support of the ERC and the University of Vienna will now allow us to build this revolutionary scientific instrument with the possibilities of a large-scale research facility in Austria. I am convinced that this is a globally unique lighthouse project that will establish Europe as a world leader in the analysis of modern materials for science and technology for years to come”, explains Thomas Pichler.

The possible applications range from fundamental studies on the origin of quantum phase transitions such as superconductivity, the construction of nanoscale matter with atomistic control, the nanometer-resolved performance of nanoscale devices in-situ and in operando, to direct technological applications such as the improvement of battery electrodes of the widely used lithium-ion batteries. New measurement methods will also enable the direct structural analysis of individual biomolecules in liquid state, which is expected to lead to a breakthrough in biomedical studies.

MORE-TEM brings together complementary expertise from world-leading researchers and companies: coordinator Thomas Pichler from the University of Vienna is a renowned expert in electron spectroscopy and optical spectroscopy. The renowned scientist Kazu Suenaga from Japan is an expert in electron microscopy. Francesco Mauri from the University La Sapienza in Rome contributes to the project with his competence in theoretical ab-initio spectroscopy, and Max Haider from CEOS GmbH in Heidelberg, a pioneer in aberration correction recognized by the Wolf and Kavli prizes, with his unsurpassed expertise in electron optics.

MORE-TEM represents a real game changer in the analysis and optimization of nanoscale materials and devices. The fundamentally new and globally unique infrastructure thus provides an innovative approach pushing the application of nanotechnology in physics, materials science, (bio)chemistry and engineering to the next level.

Explore the limits of the quantum world

The superposition principle states that a single object can behave as if it were in several places at once. This behaviour is confirmed in milestone experiments: at the scale of elementary particles, atoms and even molecules containing thousands of atoms. But is this also true for the macroscopic domain, say for solid state objects visible to the naked eye? In answering this question, the Austrian-Swiss research team around Markus Aspelmeyer (University of Vienna), Oriol Romero-Isart (University of Innsbruck) with Lukas Novotny und Romain Quidant (ETH Zürich) now wants to take a big step forward.

Over the last years, the four researchers have developed new experimental and theoretical techniques to levitate and control such nanoparticles in the quantum regime in individual ERC projects. “We combine our know-how in fundamental science, nanotechnology and engineering to create a radical new approach to this question”, says experimental physicist Markus Aspelmeyer from the University of Vienna, who is also Scientific Director at the IQOQI Vienna (Austrian Academy of Science). In a joint effort, they will study the nanoparticles in a high vacuum, its center-of-mass motion cooled down to near absolute zero and levitating in a combination of optical, electric, and magnetic fields.

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