O L D   N E W S
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  14.08.2020 Publication
Easing the Monte Carlo sign problem
Quantum Monte Carlo (QMC) methods are the gold standard for studying equilibrium properties of quantum many-body systems. However, in many interesting situations, QMC methods are faced with a sign problem, causing the severe limitation of an exponential increase in the runtime of the QMC algorithm. In this work, we develop a systematic, generally applicable, and practically feasible methodology for easing the sign problem by efficiently computable basis changes and use it to rigorously assess the sign problem. Our framework introduces measures of non-stoquasticity that—as we demonstrate analytically and numerically—at the same time provide a practically relevant and efficiently computable figure of merit for the severity of the sign problem. Complementing this pragmatic mindset, we prove that easing the sign problem in terms of those measures is generally an NP-complete task for nearest-neighbor Hamiltonians and simple basis choices by a reduction to the MAXCUT-problem.
by Dominik Hangleiter, Ingo Roth, Daniel Nagaj and Jens Eisert
Science Advances 6, eabb8341 (2020) | +++ |
QETWORK APVV-14-0878, VEGA MAXAP 2/0173/17
  15.07.2020 Job opening
Postdoc position within project QISS @ IPSAS
Applications are invited for a 18 month postdoc position at the intersection between quantum foundations, quantum information theory, and quantum gravity (previous experience in at least one of these areas is required). The position may start in September 2020 or as soon as possible thereafter.

The post is partly funded by the John Templeton Foundation through the project "The Quantum Information Structure of Spacetime" (www.qiss.fr) and partly by Institute of Physics of Slovak Academy of Sciences. The postdoc is expected to work in close collaboration with M. Ziman, A. Gendiar, M. Sedlák or D.Nagaj. Basic gross salary is 1800 eur / month, however, additional funds, depending on the focus and performance, are expected.

The application consists of CV (including list of publications), research statement (short summary of personal research interests and plans), and three names of potential referees we might ask for recommendation. Please send all this information to Michal Sedlak by email (michal.sedlak@savba.sk) before August 15th 2020, but the sooner the better. Only complete applications are considered.
  04.06.2020 Publication
Finite-m scaling analysis of Berezinskii-Kosterlitz-Thouless phase transitions and entanglement spectrum for the six-state clock model
We investigate the Berezinskii-Kosterlitz-Thouless transitions for the square-lattice six-state clock model with the corner-transfer matrix renormalization group (CTMRG). Scaling analyses for effective correlation length, magnetization, and entanglement entropy with respect to the cutoff dimension m at the fixed point of the CTMRG provide transition temperatures consistent with a variety of recent numerical studies. We also reveal that the fixed-point spectrum of the corner-transfer matrix in the critical intermediate phase of the six-state clock model is characterized by the scaling dimension consistent with the c=1 boundary conformal field theory associated with the effective Z_6 dual sine-Gordon model.
by Hiroshi Ueda, Kouichi Okunishi, Kenji Harada, Roman Krčmár, Andrej Gendiar, Seiji Yunoki, and Tomotoshi Nishino
Phys. Rev. E 101, 062111 (2020) | +++ |
CRIPONT (VEGA 2/0123/19), EXSES (APVV-16-0186)
  19.05.2020 Publication
Energetic footprints of irreversibility in the quantum regime
In classical thermodynamic processes the unavoidable presence of irreversibility, quantified by the entropy production, carries two energetic footprints: the reduction of extractable work from the optimal, reversible case, and the generation of a surplus of heat that is irreversibly dissipated to the environment. Recently it has been shown that in the quantum regime an additional quantum irreversibility occurs that is linked to decoherence into the energy basis. Here we employ quantum trajectories to construct distributions for classical heat and quantum heat exchanges, and show that the heat footprint of quantum irreversibility differs markedly from the classical case. We also quantify how quantum irreversibility reduces the amount of work that can be extracted from a state with coherences. Our results show that decoherence leads to both entropic and energetic footprints which both play an important role in the optimization of controlled quantum operations at low temperature.
by M. H. Mohammady, A. Auffèves and J. Anders
Communications Physics 3, 89 (2020) | +++ |
MoRePro project OPEQ (19MRP0027), COST MP1209
  01.05.2020 Competition
Euroepan Quantum Future Academy 2020
Open physics competition for Slovak university bachelor and master students in quantum physics. The winners will be offered a summer internship and a participation at the final conference in Berlin. The participation is going to be covered from the resources of the QUTE.sk consortium and by the main organizer in Germany. More information available on the dedicated website http://qute.sk/eqfa2020/.
  18.4.2020 Publication
Popescu-Rohrlich box implementation in general probabilistic theory of processes
It is shown that Popescu-Rohrlich nonlocal boxes (beating the Tsirelson bound for Bell inequality) do exist in the existing structures of both quantum and classical theory. In particular, we design an explicit example of measure-and-prepare nonlocal (but no-signaling) channel being the realization of nonlocal and no-signaling Popescu-Rohrlich box within the generalized probabilistic theory of processes. Further we present a post-selection-based spatially non-local implementation and show it does not require truly quantum resources, hence, improving the previously known results. Interpretation and potential (spatially non-local) simulation of this form of process nonlocality and the protocol is discussed.
by Martin Plávala and Mário Ziman
Physics Letters A 384, 126323 (2020) | +++ |
APVV-18-0518 (OPTIQUTE), COST Action CA15220, VEGA 2/0173/17 (MAXAP),
  16.03.2020 Conference
CEQIP 2020
We regret to inform you that due to COVID-19 spread and frozen mobility
we were forced to CANCEL
the workshop. Thank you for your understanding and looking forward to see you at CEQIP 202+.
  12.03.2020 Publication
Area-law-like systems with entangled states can preserve ergodicity
We study the ground entangled state of the one-dimensional spin-1/2 Ising ferromagnet at its transverse-field critical point. When this problem is expressed in terms of independent fermions, we show that the usual thermostatistical sums emerging within Fermi-Dirac statistics can, for an L-sized subsystem, be indistinctively taken up to L terms or up to lnL terms, providing a neat understanding of the origin of the logarithmic scaling of the entanglement entropy in the system. This is interpreted as a compact occupancy of the phase-space of the L-subsystem, hence standard Boltzmann-Gibbs thermodynamics quantities with an effective system size V ≈ ln(L) are appropriate and are explicitly calculated. The calculations are then to be done in a Hilbert space whose effective dimension is 2ln(L) instead of 2L. In this we can assume ergodicity. Our analysis suggests a scenario where the physical systems are essentially grouped into three classes, in terms of their phase-space occupancy, ergodicity and Lebesgue measure.
by Andre M. C. Souza, Peter Rapčan and Constantino Tsallis
The European Physical Journal Special Topics 229, 759–772 (2020) | +++ |
  10.03.2020 Publication
Coherence of a Driven Electron Spin Qubit Actively Decoupled from Quasistatic Noise
The coherence of electron spin qubits in semiconductor quantum dots suffers mostly from low-frequency noise. During the past decade, efforts have been devoted to mitigate such noise by material engineering, leading to substantial enhancement of the spin dephasing time for an idling qubit. However, the role of the environmental noise during spin manipulation, which determines the control fidelity, is less understood. We demonstrate an electron spin qubit whose coherence in the driven evolution is limited by high-frequency charge noise rather than the quasistatic noise inherent to any semiconductor device. We employ a feedback-control technique to actively suppress the latter, demonstrating a π-flip gate fidelity as high as 99.04±0.23% in a gallium arsenide quantum dot. We show that the driven-evolution coherence is limited by the longitudinal noise at the Rabi frequency, whose spectrum resembles the 1/f noise observed in isotopically purified silicon qubits.
by Takashi Nakajima, Akito Noiri, Kento Kawasaki, Jun Yoneda, Peter Stano, Shinichi Amaha, Tomohiro Otsuka, Kenta Takeda, Matthieu R. Delbecq, Giles Allison, Arne Ludwig, Andreas D. Wieck, Daniel Loss, and Seigo Tarucha
Phys. Rev. X 10, 011060 (2020) | +++ |
  11.02.2019 Job opening
PhD positions available
Interested to join our research team for four years of you life? That is exactly the time the PhD study takes. Currently, we have open several PhD positions in our Institute. We are open for submissions until the positions are filled (latest 05/03/2020) with PhD starting in September 2020. If interested, as the first step, please get in contact with a potential PhD advisor (send them your cv, motivation letter and contact to someone who may write a recommendation letter), discuss the subject and follow his/her instructions. Do not wait until submission deadline and do this as soon as possible, because the number of positions is limited.
  01.02.2020 Job opening
Postdoc position 20.20
We are opening a postdoc position in the area of theoretical quantum information theory. The postdoc is expected to work in close collaboration with core members of RCQI on topics of their common interests (quantum communication, quantum complexity theory, quantum simulations, quantum foundations, etc.). Basic gross salary is 1400 eur / month, however, additional funds, depending on the focus and performance, are expected. The position is offered for one year, but there is a possibility for one-year extension. The position is funded by Institute of Physics of Slovak Academy of Sciences.
       The application consists of CV (including list of publications), research statement (short summary of personal research interests and plans), and three names of potential referees we might ask for recommendation. Please send all this information to Michal Sedlak by email (michal.sedlak@savba.sk) before March 2nd. Only complete applications are considered.
  01.01.2020 Conference
Three kings conference / Trojkáľová konferencia
Annual one day meeting of Czech and Slovak physicists will be held on January 10th in QUTE Auditorium (RCQI, Institute of Physics of Slovak Academy of Sciences) in Bratislava. More information and registration form are available on the meeting website http://quantum.physics.sk/conf/3kk2020.
  19.09.2019 Publication
Influence of applied electric and magnetic fields on a thermally-induced reentrance of a coupled spin-electron model on a decorated square lattice
The combination of an exact and Corner Transfer Matrix Renormalization Group (CTMRG) methods is used to study an influence of external electric and magnetic fields on existence of intriguing reentrant magnetic transitions in a coupled spin-electron model on a decorated square lattice. The two-dimensional (2D) decorated square lattice with localized nodal spins and delocalized electrons is taken into account. It was found that the competition among all involved interactions (the electron hopping, spin-spin and spin-electron interaction, external electric and magnetic fields) in combination with thermal fluctuations can produce new type of reentrant magnetic transitions. Depending on the model parameters the non-zero fields can stabilize or destabilize magnetic reentrance. In addition, an alternative and more effective way, for modulating the magnetic reentrance is found. An origin of intriguing low-temperature round maximum in the specific heat was explained as a consequence of rapid changes in the sublattice magnetizations, which is induced through a competition of all presented interactions.
by Hana Čenčariková, Jozef Strečka, Andrej Gendiar
Physica E 115, 113717 (2020) | +++ |
EXSES APVV-16-0186