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  06.05.2022 Publication
Ising ferromagnets and antiferromagnets in an imaginary magnetic field
We study classical Ising spin-1/2 models on a two-dimensional (2D) square lattice with ferromagnetic or antiferromagnetic nearest-neighbor interactions, under the effect of a pure imaginary magnetic field. The complex Boltzmann weights of spin configurations cannot be interpreted as a probability distribution, which prevents application of standard statistical algorithms. In this work, the mapping of the Ising spin models under consideration onto symmetric vertex models leads to real (positive or negative) Boltzmann weights. This enables us to apply accurate numerical methods based on the renormalization of the density matrix, namely, the corner transfer matrix renormalization group and the higher-order tensor renormalization group. For the 2D antiferromagnet, varying the imaginary magnetic field, we calculate with high accuracy the curve of critical points related to the symmetry breaking of magnetizations on the interwoven sublattices. The critical exponent β and the anomaly number C are shown to be constant along the critical line, equal to their values β=1/8 and C=1/2 for the 2D Ising model in a zero magnetic field. The 2D ferromagnets behave in analogy with their 1D counterparts defined on a chain of sites, namely, there exists a transient temperature which splits the temperature range into its high-temperature and low-temperature parts. The free energy and the magnetization are well defined in the high-temperature region. In the low-temperature region, the free energy exhibits singularities at the Yang-Lee zeros of the partition function and the magnetization is also ill-defined: It varies chaotically with the size of the system. The transient temperature is determined as a function of the imaginary magnetic field by using the fact that from the high-temperature side both the first derivative of the free energy with respect to the temperature and the magnetization diverge at this temperature.
by Roman Krčmár, Andrej Gendiar, and Ladislav Šamaj
Phys. Rev. E 105, 054112 (2022) | +++ |
QUASIMODO (VEGA 2/0092/21), SAS-MOST 108-2112-M002-020-MY3, PRESPEED (APVV-20-0150)
  05.05.2022 Publication
Spin relaxation, Josephson effect, and Yu-Shiba-Rusinov states in superconducting bilayer graphene
Bilayer graphene has two nonequivalent sublattices and, therefore, the same adatom impurity can manifest in spectrally distinct ways—sharp versus broad resonances near the charge neutrality—depending on the sublattice it adsorbs at. Employing Green's function analytical methods and the numerical kwant package, we investigate the spectral and transport interplay between the resonances and superconducting coherence induced in bilayer graphene by proximity to an s-wave superconductor. Analyzing doping and temperature dependencies of quasiparticle spin-relaxation rates, energies of Yu-Shiba-Rusinov states, Andreev spectra, and the supercurrent characteristics of Josephson junctions, we find unique superconducting signatures discriminating between resonant and off-resonant regimes. Our findings are in certain aspects going beyond the superconducting bilayer graphene and hold for generic s-wave superconductors functionalized by the resonant magnetic impurities.
by Michael Barth, Jacob Fuchs, and Denis Kochan
Phys. Rev. B 105, 205409 (2022) | +++ |
  02.05.2022 Event
Quantum Rendezvous
National Center for Quantum Technologies QUTE.sk is organising a national strategic meeting focused on development of quantum technologies in Slovakia.
Date: 29/06/2022 – 01/07/2022 http://qute.sk/qrendezvous2022.html
  25.04.2022 Event
Quantum Eastern Europe
Quantum Eastern Europe (QEE) is a 2-day online event that will bring together those stakeholders in Eastern Europe for whom quantum science and technology is relevant. The event will raise awareness and transfer knowledge about quantum science and technology, foster synergies and collaborations between academia, industry, government, and interested individuals, and thereby strengthen the quantum ecosystem in this geographical area.
Date: 5 – 6 May 2022 https://quantumeasterneurope.com/
Our colleagues are participating too. Pavol Neilinger will give a "lightning" talk there and Mario Ziman will be taking part in V4 panel discussion.
  12.03.2022 Job opening
Seeking for quantum PhD students.
Are you a master/engineer student fascinated by quantum physics or future of quantum technologies? Want to run quantum computers, build quantum network, 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 submissions until 10/04/2022, exceptionally also later) with PhD starting in September 2022. If interested, as 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 you to select your quantum destiny.
  07.03.2022 Publication
Experimental implementation of secure anonymous protocols on an eight-user quantum key distribution network
Anonymity in networked communication is vital for many privacy-preserving tasks. Secure key distribution alone is insufficient for high-security communications. Often, knowing who transmits a message to whom and when must also be kept hidden from an adversary. Here, we experimentally demonstrate five information-theoretically secure anonymity protocols on an eight user city-wide quantum network using polarisation entangled photon pairs. At the heart of these protocols is anonymous broadcasting, which is a cryptographic primitive that allows one user to reveal one bit of information while keeping their identity anonymous. For a network of n users, the protocols retain anonymity for the sender, given that no more than n − 2 users are colluding. This is an implementation of genuine multi-user cryptographic protocols beyond standard QKD. Our anonymous protocols enhance the functionality of any fully-connected Quantum Key Distribution network without trusted nodes.
by Zixin Huang, Siddarth Koduru Joshi, Djeylan Aktas, Cosmo Lupo, Armanda O. Quintavalle, Natarajan Venkatachalam, Sören Wengerowsky, Martin Lončarić, Sebastian Philipp Neumann, Bo Liu, Željko Samec, Laurent Kling, Mario Stipčević, Rupert Ursin & John G. Rarity
npj Quantum Information 8, 25 (2022) | +++ |
  18.02.2022 Publication
J1-J2 fractal studied by multirecursion tensor-network method
We generalize a tensor-network algorithm to study the thermodynamic properties of self-similar spin lattices constructed on a square-lattice frame with two types of couplings, J1 and J2 , chosen to transform a regular square lattice (J1 = J2 ) onto a fractal lattice if decreasing J2 to zero (the fractal fully reconstructs when J2 = 0). We modified the higher-order tensor renormalization group (HOTRG) algorithm for this purpose. Single-site measurements are performed by means of so-called impurity tensors. So far, only a single local tensor and uniform extension-contraction relations have been considered in HOTRG. We introduce 10 independent local tensors, each being extended and contracted by 15 different recursion relations. We applied the Ising model to the J1 − J2 planar fractal whose Hausdorff dimension at J2 = 0 is d(H ) = ln 12/ ln 4 ≈ 1.792. The generalized tensor-network algorithm is applicable to a wide range of fractal patterns and is suitable for models without translational invariance.
by Jozef Genzor, Andrej Gendiar, Ying-Jer Kao
Phys. Rev. E 105, 024124 (2022) | +++ |
JSR SAS-MOST 108-2112-M-002-020-MY3, APVV-18-0518 (OPTIQUTE), APVV-16-0186 (EXSES), VEGA 2/0123/19 (CRIPONT), JTF ID# 61466 (QISS)
  12.02.2022 Job opening
Postdoc opening
We are seeking for postdocs in the area of quantum communication technologies. We are interested for candidates of experimental and theoretical background in the area of quantum communication engineering, or experimental quantum optics, or quantum information theory. The positions are offered for one year (extensions for couple of more years are possible, but details depend on the particular financial source) with a basic gross salary from 1800 eur/month and flexible expected start (earliest from June 2022). The application consists of CV (including list of publications), research statement (short summary of personal research interests and plans), and three names of potential referee we might ask for recommendation letters. Please send all this information to Mario Ziman by email (ziman@savba.sk) before 31/03/2022. Only complete applications will be considered.
  03.02.2022 Publication
Energy Scale Deformation on Regular Polyhedra
A variant of energy scale deformation is considered for the S = 1/2 antiferromagnetic Heisenberg model on polyhedra. The deformation is induced by the perturbations to the uniform Hamiltonian, whose coefficients are determined by the bond coordinates. On the tetrahedral, octahedral, and cubic clusters, the perturbative terms do not affect the ground state of the uniform Hamiltonian when they are sufficiently small. On the other hand, for the icosahedral and dodecahedral clusters, it is numerically confirmed that the ground state of the uniform Hamiltonian is almost insensitive to the perturbations unless they lead to a discontinuous change in the ground state. The obtained results suggest the existence of a generalization of sine-square deformation in higher dimensions.
by Takuya Eguchi, Satoshi Oga, Hosho Katsura, Andrej Gendiar, and Tomotoshi Nishino,
J. Phys. Soc. Jpn. 91, 034001 (2022) [6 Pages] | +++ |
  19.01.2022 Publication
Creation of quantum coherence with general measurement processes
Quantum measurement not only can destroy coherence but also can create it. Here, we estimate the maximum amount of coherence, one can create under a complete non-selective measurement process. For our analysis, we consider projective as well as POVM measurements. Based on our observations, we characterize the measurement processes into two categories, namely, the measurements with the ability to induce coherence and the ones without this ability. Our findings also suggest that the more POVM elements present in a measurement that acts on the quantum system, the less will be its coherence creating ability. We also introduce the notion of raw coherence in the POVMs that helps to create quantum coherence. Finally, we find a trade-off relation between the coherence creation, entanglement generation between system and apparatus, and the mixedness of the system in a general measurement setup.
by Sanuja D. Mohanty, Gautam Sharma, Sk Sazim, Biswajit Pradhan, Arun K. Pati
Quantum Information Processing 21, 48 (2022) | +++ |
Schwarz, OPTIQUTE (APVV-18-0518), HOQIP (VEGA 2/0161/19)
  04.12.2021 Publication
Measurement uncertainty and dense coding in a two-qubit system: Combined effects of bosonic reservoir and dipole–dipole interaction
The uncertainty principle imposes a limitation on the measurement accuracy of two incompatible observables and has potential applications in quantum information science. We explore the bipartite entropic uncertainty relation and dense coding for two qubits coupled via dipole–dipole interaction and subject to a bosonic reservoir. It is shown that there exists a trade-off between the uncertainty bound and the dense coding capacity which results in their opposite dynamical behaviors. Moreover, the behaviors of the measurement uncertainty and the dense coding capacity rely crucially on the initial system–reservoir correlation, the strength of the dipole–dipole interaction, and the degree of non-Markovianity, and one can reduce the measurement uncertainty and enhance the dense coding capacity by tuning these parameters to appropriate values.
by Saeed Haddadi, Ming-Liang Hu, Youssef Khedi, Hazhir Dolatkhah, Mohammad Reza Pourkarimi, Mohammed Daoud
Results in Physics 32, 105041 (2022) | +++ |
APVV-18-0518 (OPTIQUTE) VEGA 2/0161/19 (HOQIT)