In the context of a discrete analog of the classical Heisenberg model, we investigate the critical behavior of the icosahedron model, where the interaction energy is defined as the inner product of neighboring vector spins of unit length pointing to the vertices of the icosahedron. The effective correlation length and magnetization of the model are calculated by means of the corner-transfer-matrix renormalization group (CTMRG) method. A scaling analysis with respect to the cutoff dimension m in CTMRG reveals a second-order phase transition characterized by the exponents ν=1.62±0.02 and β=0.12±0.01. We also extract the central charge from the classical analog of entanglement entropy as c=1.90±0.02, which cannot be explained by the minimal series of conformal field theory.

Šikovní vedci prišli s otázkou: Mohli by na seba dvaja ľudia nahlas rozprávať bez toho, aby im hocikto iný mohol rozumieť? Spolu sa nad tým zamyslíme a prídeme na to, že sa to nedá pomocou nového vymysleného jazyka, pretože aj ten by sa počúvajúci postupne naučili. Jednou z odpovedí je je kryptografia s verejnými a tajnými kľúčmi, založená na tom, že niektoré úlohy je veľmi ťažké vypočítať. Kvantové počítače (ktoré ešte nemáme) ale tieto úlohy budú schopné riešiť celkom rýchlo. Znamená to koniec šifrovania? Nie. Nakoniec uvidíme, ako sa sen o bezpečne utajených správach stáva skutočnosťou v tajomnom svete malinkých, rýchlo letiacich častíc - kvantovo previazaných fotónov. Budeme sa hrať so svetlom a ukážeme si, ako sa pomocou nich dajú posielať naozaj tajné správy.

V piatok 29. septembra 2017 sa uskutočnil kvantovo zašifrovaný videohovor medzi prezidentmi čínskej a rakúskej akadémie vied. Kvantové šifrovanie tak dosiahlo nový rekord vo vzdialenosti – z rádovo stoviek kilometrov sme sa týmto experimentom skokom posunuli na 7400 km. Na komunikáciu sa použil čínsky satelit Micius, ktorý sprostredkoval kvantovú výmenu šifrovacieho kľúča. Odhliadnuc od unikátnosti použitých technológií a významného vedeckého prínosu, tento experiment ukazuje, že kvantovo zabezpečená komunikácia je naozaj otázka relatívne blízkej budúcnosti. Ako to funguje a na čo všetko máme byť pripravení? ...

Spája sa v ňom odborník na kvantovú fyziku a kvantové informácie, ale aj ohromný nadšenec pre tento odbor. Je doslova vedecký optimista: myslí si, že kvantové senzory zlepšia medicínu, kvantové počítače nájdu nové materiály a lieky a kvantový internet nám naozaj umožní bezpečnú komunikáciu a online anonymitu. Docent Mário Ziman, vedúci Centra pre výskum kvantovej informácie Fyzikálneho ústavu SAV.
Ste fyzik. Ako ste sa k tomu dostali? ....

Although quantum physics was founded more than ninety years ago this very useful and successful theory still seems unintuitive and puzzling to everyone. Devices relying on quantum effects already found their way into our everyday lives (Lasers, LED lights, Magnetic Resonance Imaging, …), yet they have not changed much our thinking. However, the thoughts of modern physics like R. Feynman, D. Deutsch and Peter Shor may revolutionize the way we think about information and computation. The aim of this talk is to describe basic ideas behind advantages of quantum computers and to explain how they ruin common cryptographic schemes, but gave us unconditionally secure quantum cryptography.

Understanding the dynamics of open quantum systems is important and challenging in basic physics and applications for quantum devices and quantum computing. Semiconductor quantum dots offer a good platform to explore the physics of open quantum systems because we can tune parameters including the coupling to the environment or leads. Here, we apply the fast single-shot measurement techniques from spin qubit experiments to explore the spin and charge dynamics due to tunnel coupling to a lead in a quantum dot-lead hybrid system. We experimentally observe both spin and charge time evolution via first- and second-order tunneling processes, and reveal the dynamics of the spin-flip through the intermediate state. These results enable and stimulate the exploration of spin dynamics in dot-lead hybrid systems, and may offer useful resources for spin manipulation and simulation of open quantum systems.

The divisibility of dynamical maps is visualized by trajectories in the parameter space and analyzed within the framework of collision models. We introduce ultimate completely positive (CP) divisible processes, which lose CP divisibility under infinitesimal perturbations, and characterize Pauli dynamical semigroups exhibiting such a property. We construct collision models with factorized environment particles, which realize additivity and multiplicativity of generators of CP divisible maps. A mixture of dynamical maps is obtained with the help of correlated environment. The mixture of ultimate CP divisible processes is shown to result in a class of eternal CP indivisible evolutions. We explicitly find collision models leading to weakly and essentially non-Markovian Pauli dynamical maps.

We show that it is possible to use a quantum walk to find a path from one marked vertex to another. In the specific case of M stars connected in a chain, one can find the path from the first star to the last one in O(M√N) steps, where N is the number of spokes of each star. First we provide an analytical result showing that by starting in a phase-modulated highly superposed initial state we can find the path in O(M√N logM) steps. Next, we improve this efficiency by showing that the recovery of the path can also be performed by a series of successive searches when we start at the last known position and search for the next connection in O(√N) steps leading to the overall efficiency of O(M√N). For this result we use the analytical solution that can be obtained for a ring of stars of double the length of the chain.

We investigate the influence of nuclear spins on the resistance of helical edge states of two-dimensional topological insulators (2DTIs). Via the hyperfine interaction, nuclear spins allow electron backscattering, otherwise forbidden by time-reversal symmetry. We identify two backscattering mechanisms, depending on whether the nuclear spins are ordered or not. Their temperature dependence is distinct but both give resistance, which increases with the edge length, decreasing temperature, and increasing strength of the electron-electron interaction. Overall, we find that the nuclear spins will typically shut down the conductance of the 2DTI edges at zero temperature.

In the exact quantum query model a successful algorithm must always output the correct function value. We investigate the function that is true if exactly k or l of the n input bits given by an oracle are 1. We find an optimal algorithm (for some cases), and a nontrivial general lower and upper bound on the minimum number of queries to the black box.