O L D   N E W S
: 2024 : 2023 : 2022 : 2021 : 2020 : 2019 : 2018 : 2017 : 2016 : 2015 : 2014 : 2013 : 2012 : 2011 : 2010 : 2009 : 2008 : 2007 :
  23.05.2024 Publication
Link between supercurrent diode and anomalous Josephson effect revealed by gate-controlled interferometry
In Josephson diodes the asymmetry between positive and negative current branch of the current-phase relation leads to a polarity-dependent critical current and Josephson inductance. The supercurrent nonreciprocity can be described as a consequence of the anomalous Josephson effect —a φ 0 -shift of the current-phase relation— in multichannel ballistic junctions with strong spin-orbit interaction. In this work, we simultaneously investigate φ 0 -shift and supercurrent diode efficiency on the same Josephson junction by means of a superconducting quantum interferometer. By electrostatic gating, we reveal a direct link between φ 0 -shift and diode effect. Our findings show that spin-orbit interaction in combination with a Zeeman field plays an important role in determining the magnetochiral anisotropy and the supercurrent diode effect.
by Simon Reinhardt, Tim Ascherl, Andreas Costa, Johanna Berger, Sergei Gronin, Geoffrey C. Gardner, Tyler Lindemann, Michael J. Manfra, Jaroslav Fabian, Denis Kochan, Christoph Strunk, Nicola Paradiso
Nature Communications 15, 4413 (2024) | +++ |
IMPULZ IM-2021-26 (SUPERSPIN), VEGA 2/0183/21 (DESCOM)
  09.05.2024 Publication
Single-shot labeling of quantum observables
We identify and study a particular class of distinguishability problems for quantum observables (positive-operator-valued measures), in which observables with permuted effects are involved, which we call the labeling problem. Consequently, we identify binary observables, which can be labeled perfectly. In this work, we study these problems in the single-shot regime.
by Nidhin Sudarsanan Ragini and Mario Ziman
Phys. Rev. A 109, 052415 (2024) | +++ |
APVV-22-0570 (DEQHOST), VEGA 2/0183/21 (DESCOM)
  01.05.2024 Publication
Emergent Correlated Phases in Rhombohedral Trilayer Graphene Induced by Proximity Spin-Orbit and Exchange Coupling
The impact of proximity-induced spin-orbit and exchange coupling on the correlated phase diagram of rhombohedral trilayer graphene (RTG) is investigated theoretically. By employing ab initio-fitted effective models of RTG encapsulated by transition metal dichalcogenides (spin-orbit proximity effect) and ferromagnetic Cr2Ge2Te6 (exchange proximity effect), we incorporate the Coulomb interactions within the random-phase approximation to explore potential correlated phases at different displacement fields and doping. We find a rich spectrum of spin-valley resolved Stoner and intervalley coherence instabilities induced by the spin-orbit proximity effects, such as the emergence of a spin-valley-coherent phase due to the presence of valley-Zeeman coupling. Similarly, proximity exchange removes the phase degeneracies by biasing the spin direction, enabling a magnetocorrelation effect—strong sensitivity of the correlated phases to the relative magnetization orientations (parallel or antiparallel) of the encapsulating ferromagnetic layers.
by Yaroslav Zhumagulov, Denis Kochan, and Jaroslav Fabian
Phys. Rev. Lett. 132, 186401 (2024) | +++ |
IMPULZ IM-2021-26 (SUPERSPIN), VEGA 2/0183/21 (DESCOM)
  30.04.2024 Colloquium
Since time has the direction - Rudolph Clausius and his legacy today
It is steam engines that marked the dawn of industrialization, yielding both benefits and headache causing challenges. Beyond their practical applications, steam engines profoundly shaped our understanding of the physical world. Among the key figures in this transformation was Rudolph Clausius, whose groundbreaking work during the mid-19th century significantly influenced our comprehension of energy and entropy. Despite his pivotal and broad contributions, Clausius remains somewhat underappreciated in today’s scientific community. His theories, however, serve as the bedrock connecting the microscopic and macroscopic realms, enabling us to control physical and chemical processes across various fields. Clausius was not only a physicist but also an early transdisciplinary scientist, bridging disciplines from physics to chemistry, information science, and economics. In this discussion, we’ll explore Clausius’s central insights, including his concept of the “motive force of heat” (now known as thermodynamics). We’ll delve into his role within the 19th-century scientific community and draw connections to our contemporary world.
Speaker: Prof. Dieter Meschede (University of Bonn, Germany)
Time: May 15th, 2024, 15:00
Place: Lecture hall B, Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynská dolina, Bratislava

Organized and supported by Slovak National Center for Quantum Technologies QUTE.sk and Humboldt Club SK.
  24.04.2024Conference talk
Development of the Slovak National Quantum Communication Infrastructure
In this talk we will present both short-term and long-term vision for our national Slovak QCI infrastructure and put it in the context of the whole EuroQCI endeavour. We will present the latest results building up our quantum testbed with the support of the underlying SANET infrastructure.
Conference talk by Djeylan Aktas at BQIT 2024 workshop held in Bristol (United Kingdom). | +++ |
  21.04.2024 CourseQute
The Quantum World – From Experimental Insight to Technology
A tutorial lectures series on experiments introducing the audience into experimentally controlling quantum objects where single and many atoms will play a central role as simple model systems. This field started about 50 years ago when the non-intuitive world of quantum systems, especially the superposition principle, prompted experimenters to use lasers and light matter interaction to realize ever more illustrations of quantum phenomena. Over time those observations have turned into ever better control which today allows quantum engineers to apply simple quantum systems for tasks in quantum technology ranging from quantum sensing to simulation, communication and computing. The lectures will roughly follow this schedule but may be be adapted to the course of discussions:

• 13/05 09:15 - 10:45 What is Quantum? A General Introduction
• 14/05 09:15 - 10:45 Illustrating Key Quantum Phenomena in Experiments
• 15/05 09:15 - 10:45 Single and Few Atoms: Observing Simple Quantum Systems
• 16/05 09:15 - 10:45 Controlling Quantum Systems
• 17/05 09:15 - 10:45 Visions of Quantum Technology

Speaker: Prof. Dieter Meschede
Prof. Dieter Meschede gained his Ph.D. in 1984 in physics from the Ludwig-Maximilians-Universität München, Since 1994, he is Full Professor of Physics at the University of Bonn, Germany and between 2018-2020 he was appointed as President of the German Physical Society (DPG). His research interests include the field of atomic, molecular and quantum physics. The so-called “conveyor belt of light” – it moves and sorts individual atoms with the aid of laser beams and radio frequency precision – is one of the outstanding research results of his research group. With the help of this “conveyor belt”, atoms could be used as an arithmetic unit for a quantum computer. This work has received great recognition with an Advanced ERC Grant (DQSIM).

Lectures are MSc/PhD friendly, but open for all interested (Elementary Quantum Theory course is assumed) and free of charge. Please register HΞRΞ (no registration deadline) to receive all information.
Place: Auditorium QUTE, Institue of Physics, Bratislava

Organized and supported by Slovak National Center for Quantum Technologies QUTE.sk.
  29.03.2024 Publication
Simple information-processing tasks with unbounded quantum advantage
Communication scenarios between two parties can be implemented by first encoding messages into some states of a physical system which acts as the physical medium of the communication and then decoding the messages by measuring the state of the system. We show that already in the simplest possible scenarios it is possible to detect a definite, unbounded advantage of quantum systems over classical systems. We do this by constructing a family of operationally meaningful communication tasks, each of which, on the one hand, can be implemented by using just a single qubit but which, on the other hand, require an unboundedly larger classical system for classical implementation. Furthermore, we show that even though, with the additional resource of shared randomness, the proposed communication tasks can be implemented by both quantum and classical systems of the same size, the number of coordinated actions needed for the classical implementation also grows unboundedly. In particular, no finite storage can be used to store all the coordinated actions required to implement all possible quantum communication tasks with classical systems. As a consequence, shared randomness cannot be viewed as a free resource.
by Teiko Heinosaari, Oskari Kerppo, Leevi Leppäjärvi, and Martin Plávala
Phys. Rev. A 109, 032627 (2024) | +++ |
SASPRO2 1372/03/01 (SeMIOpT), APVV-22-0570 (DEQHOST), VEGA 2/0183/21 (DESCOM)
  21.03.2024 CourseQute
Quantum photonic device engineering
Series of 5x90 min lectures during 16-18th April covering topics
• Single photon interference and sources
• Photon detectors and Photon counting technology
• Photon statistics: sub-poissonian light and squeezed states
• Multiphoton interference and limits to visibility
• Applications to Quantum key distribution and Metrology
• Linear optics quantum computation and simulation
Lectures are MSc/PhD friendly, but open for all interested (Elementary Quantum Theory course is assumed) and free of charge. Please register HΞRΞ (no registration deadline) to receive all information.

Speaker: Prof. John Rarity
Professor of Optical Communication Systems at the University of Bristol. He was one of the early pioneers for experimental quantum information science and performed original quantum cryptography and quantum network communication experiments using single photons and entanglement. He was awarded an ERC Advanced Grant and for his contributions to quantum optics he was elected fellow of the Royal Society in 2015.

    Tuesday 16/04 10:00-11:30, 13:30-15:00
    Wednesday 17/04 10:00-11:30, 13:30-15:00
    Thursday 18/04 10:00-11:30
Place: Auditorium QUTE, Institue of Physics, Bratislava

Organized and supported by Slovak National Center for Quantum Technologies QUTE.sk.
  16.03.2024 Workshop
Mini-workshop: superconductivity, correlations, transport and layered materials
29-30/04/2024, Bratislava, Slovakia
As before, a small workshop targeting current topics from superconductivity, correlated systems, altermagnetism, transport, etc. We asked speakers for more pedagogical talks, accessible to students in master and PhD programs, of course this can not be quarantined :) ... Workshop is free of charge and is open for all people interested to learn new things, meet new people, interact friendly and so on. There is no formal registration, but in order to plan coffee breaks please send "I will come" to email denis.kochan@savba.sk). The collection of speakers is closed and no poster session is organized.

PROGRAM venue: Auditorium QUTE, Institue of Physics, Bratislava
Monday, 29/04/2024
08:30 Registration/Administration duties
09:00 Andreas Costa (Regensburg): Supercurrent Diode Effect and Spin-Orbit Phenomena in Superconducting Junctions
09:40 Aline Ramires (Villigen): Superconductivity-Induced Improper Orders
10:20 Coffee
10:50 Ming Hao Liu (Tainan): Electronic Quantum Transport Simulation for 2D Materials
11:30 Florian Libisch (Wien): Moiré Phonons in Twisted Heterostructures
12:10 Lunch
13:40 Libor Šmejkal (Mainz): Altermagnetism: From Spintronics Anomalies to Unconventional Magnets
14:20 Mathias Scheurer (Stuttgart): Superconducting Diode Effect in Graphene Moiré Systems and Altermagnets
15:00 Marko Milivojevic (Bratislava): Interplay of Altermagnetism and Weak Ferromagnetism in Two-Dimensional RUF4
15:40 Coffee
16:10 Ján Smrek (Wien): Active Polymeric Materials
16:50 Pavol Szabó (Košice): High Spin Polarization in Co 2 FeSn Heusler Nanowires for Spintronics
17:30 Paulo Eduardo Faria Junior (Regensburg): Valley Zeeman Physics in Van der Waals Matter
18:10 End
19:30 Dinner (http://www.sundeck.sk)

Tuesday, 30/04/2024
08:40 Promo project Impulz
09:00 Andor Kormanyos (Budapest): Non-Local Andreev Reflection through Andreev Molecular States
09:40 Jorge Cayao (Uppsala): Majorana Bound States in Superconductor-Semiconductor Hybrids
10:20 Coffee
10:50 Peter Makk 2D antiferromagnets under pressure
11:30 Leonid Golub (Regensburg): Electrical Magnetochiral Current in Tellurium
12:10 Lunch
13:40 Magdalena Marganska (Regensburg): Electronic Interactions and Superconductivity with Fragmented Fermi Surface
14:20 Maksym Serbyn (Wien): Multilayer Graphenes as a Platform for Superconductivity
15:00 Adolfo Otero Fumega (Aalto): Intrinsic Flat Bands and Emergent Phenomena in 2D Materials
15:40 Coffee
16:10 Tomáš Bzdušek (Zürich): Spectra and Topological Numbers in Hyperbolic Lattices
16:50 Roman Martoňák (Bratislava): Kinetic Pathways to the BC8 Structure in Carbon and Silicon
17:30 Juraj Hašík (Amsterdam): Tensor Networks: Applications to Physics and Numerics
18:10 End
18:45 Dinner (http://www.patronskypivovar.sk/)

Orgs: Denis Kochan, František Herman
supported by IMPULZ project IM-2021-26 SuperSpin and SASPRO2 project No. 945478 AuDS
  06.03.2024 Publication
Bloch sphere analog of qudits using Heisenberg-Weyl Operators
We study an analogous Bloch sphere representation of higher-level quantum systems using the Heisenberg-Weyl operator basis. We introduce a parametrization method that will allow us to identify a real-valued Bloch vector for an arbitrary density operator. Before going into arbitrary d-level (d>=3) quantum systems (qudits), we start our analysis with three-level ones (qutrits). It is well known that we need at least eight real parameters in the Bloch vector to describe arbitrary three-level quantum systems (qutrits). However, using our method we can divide these parameters into four weight, and four angular parameters, and find that the weight parameters are inducing a unit sphere in fourdimension. And, the four angular parameters determine whether a Bloch vector is physical. Therefore, unlike its qubit counterpart, the qutrit Bloch sphere does not exhibit a solid structure. Importantly, this construction allows us to define different properties of qutrits in terms of Bloch vector components. We also examine the two and three-dimensional sections of the sphere, which reveal a non-convex yet closed structure for physical qutrit states. Further, we apply our representation to derive mutually unbiased bases (MUBs), characterize unital maps for qutrits, and assess ensembles using the Hilbert-Schmidt and Bures metrics. Moreover, we extend this construction to qudits, showcasing its potential applicability beyond the qutrit scenario.
by Gautam Sharma, Sibasish Ghosh and Sk Sazim
Physica Scripta 99, 045105 (2024) | +++ |
APVV-22-0570 (DEQHOST), VEGA 2/0183/21 (DESCOM)
26.02.2024 Workshop
CEQIP 2024
Dear Bob,
I did not see you for couple of months. How are you doing? Nevermind you forgot gifts for Christmas. I know you did research. Quantum research, of course. Any new algorithm? Not? Some cryptographic improvement? ... No? Some interesting calculation and new insight? Think about it. Remember CEQIP? Also not? Do not worry. No one remembers everything.
      See you in June.
                            Your Sk.Alica
  21.02.2024 Publication
Advantages of quantum communication revealed by the reexamination of hyperbit theory limitations
Pawłowski and Winter’s hyperbit theory, proposed in 2012, presented itself as an alternative to quantum theory, suggesting novel ways of redefining entanglement and classical communication paradigms. This research undertakes a meticulous reevaluation of hyperbit theory, uncovering significant operational constraints that question its equivalence with quantum mechanics. Crucially, the supposition that hyperbit theory and quantum theory are equivalent relies on the receiver having unattainable additional knowledge about the sender’s laboratory, indicating that the work by Pawłowski and Winter is incorrect. This study accentuates the constraints of hyperbits in information processing and sheds light on the superiority of quantum communication, thereby advancing the investigation at the intersection of classical and quantum communication.
by Giovanni Scala, Seyed Arash Ghoreishi, and Marcin Pawłowski
Phys. Rev. A 109, 022230 (2024) | +++ |
VEGA 2/0183/21 (DESCOM)
  12.02.2024 Publication
Gravitational quantum switch on a superposition of spherical shells
In the absence of a complete theory of quantum gravity, phenomenological models built upon minimal assumptions have been explored for the analysis of possible quantum effects in gravitational systems. Implications of a superposition of geometries have been considered in such models, including the occurrence of processes with indefinite order. In a gravitational quantum switch, in particular, the order of operations applied by two agents on a target system is entangled with the state of the geometry. We consider a model describing the superposition of geometries produced by distinct arrangements of spherical mass shells, and show that a protocol for the implementation of a gravitational quantum switch can be formulated in such a system. The geometries in superposition are identical in an exterior region outside a given radius, and differ within such a radius. The exterior region provides a classical frame from which the superposition of geometries in the interior region can be probed. One of the agents crosses the interior region and becomes entangled with the geometry, which is explored as a resource for the implementation of the quantum switch. Novel features of the protocol include the superposition of nonisometric geometries, the existence of a region with a definite geometry, and the fact that the agent that experiences the superposition of geometries is in free fall, preventing information on the global geometry to be obtained from within its laboratory.
by Natália S. Móller, Bruna Sahdo, Nelson Yokomizo
Quantum 8, 1248 (2024) | +++ |
Schwarz stipend, APVV-22-0570 (DeQHOST), VEGA 2/0183/21 (DESCOM), VEGA 2/0156/22 (QuaSiModo), JTF-61466 (QISS)
  12.02.2024 Publication
Incompatibility of quantum instruments
Quantum instruments describe outcome probability as well as state change induced by measurement of a quantum system. Incompatibility of two instruments, i. e. the impossibility to realize them simultaneously on a given quantum system, generalizes incompatibility of channels and incompatibility of positive operator-valued measures (POVMs). We derive implications of instrument compatibility for the induced POVMs and channels. We also study relation of instrument compatibility to the concept of non-disturbance. Finally, we prove equivalence between instrument compatibility and postprocessing of certain instruments, which we term complementary instruments. We illustrate our findings on examples of various classes of instruments.
by Leevi Leppäjärvi, Michal Sedlák
Quantum 8, 1246 (2024) | +++ |
SASPRO-2-945478, APVV-22-0570 (DeQHOST), VEGA 2/0183/21 (DESCOM)
  04.02.2024 Job opening
Looking for PhD in quantum technologies and foundations ΞΞΞ
Are you a master/engineer student fascinated by quantum physics or future of quantum technologies? Want to understand quantum foundations, run quantum computers, build quantum systems, use quantum networks, or encrypt quantum messages? Interested to join our research team for four years of you life? That is the time it takes to do the research and become expert (with PhD title) in quantum simulations, or optical quantum communication networks, or quantum security, or foundations of quantum phenomena. All of these fields are waiting for your contribution. Currently, we have open several PhD positions at our Institute. We are open for your email ideally before 29/02/2024. As for the first step please get in contact with a potential PhD advisor (send him your cv, motivation letter and contacts to potential references), discuss the subject and follow his/her instructions. Do not wait until the submission deadline and do this as soon as possible. If you are uncertain who to contact, just choose any of us. We are all happy to help. \ΞΞΞ
  01.02.2024 Job opening
IPSAS fellowship call - seeking for a colleague
Within the call for IPSAS fellowship we are looking for motivated postdoc, or a colleague willing to spent his/her sabbatical time with us. If interested please send your CV before February 29th (the sooner the better) directly to one of us. Choose the one who matches the best with your experience and interests. Making the right choice is part of the preselection process. If undecided email to ziman@savba.sk. Final interviews will happen in the second week of March.
  29.01.2024 Publication
Bose-Einstein condensates and the thin-shell limit in anisotropic bubble traps
Within the many different models, that appeared with the use of cold atoms to create BECs, the bubble trap shaped potential has been of great interest. However, the relationship between the physical parameters and the resulting manifold geometry remains yet to be fully understood for the anisotropic bubble trap physics in the thin-shell limit. In this paper, we work towards this goal by showing how the parameters of the system must be manipulated in order to allow for a non-collapsing thin-shell limit. In such a limit, a dimensional compactification takes place, thus leading to an effective 2D Hamiltonian which relates to up-to-date bubble trap experiments. At last, the resulting Hamiltonian is perturbatively solved for both the ground-state wave function and the excitation frequencies in the leading order of deviations from a spherical bubble trap.
by Elias J. P. Biral, Natália S. Móller, Axel Pelster, F. Ednilson A. dos Santos
New J. Phys. 26, 013035 (2024) | +++ |
Schwarz stipend, APVV-18-0518 (OPTIQUTE), VEGA 2/0156/22 (QUASIMODO), JTF-61466 (QISS)