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  20.05.2019 Publication
Divisibility of qubit channels and dynamical maps
The concept of divisibility of dynamical maps is used to introduce an analogous concept for quantum channels by analyzing the simulability of channels by means of dynamical maps. In particular, this is addressed for Lindblad divisible, completely positive divisible and positive divisible dynamical maps. The corresponding L-divisible, CP-divisible and P-divisible subsets of channels are characterized (exploiting the results by Wolf et al. and visualized for the case of qubit channels. We discuss the general inclusions among divisibility sets and show several equivalences for qubit channels. To this end we study the conditions of L-divisibility for finite dimensional channels, especially the cases with negative eigenvalues, extending and completing the known results. Furthermore we show that transitions between every two of the defined divisibility sets are allowed. We explore particular examples of dynamical maps to compare these concepts. Finally, we show that every divisible but not infinitesimal divisible qubit channel (in positive maps) is entanglement-breaking, and open the question if something similar occurs for higher dimensions.
by David Davalos, Mario Ziman, and Carlos Pineda
Quantum 3, 144 (2019) | +++ |
HiPhoP (Quantera), MAXAP (VEGA 2/0173/17), QETWORK (APVV-14-0878), GACR GA16-22211S
  03.05.2019 Publication
Optimal Probabilistic Storage and Retrieval of Unitary Channels
We address the question of quantum memory storage for quantum dynamics. In particular, we design an optimal protocol for N -> 1 probabilistic storage and retrieval of unitary channels on D-dimensional quantum systems. If we access the unknown unitary gate only N times, the optimal success probability of perfect single-use retrieval is N/(N−1+d2). The derived size of the memory system exponentially improves the known upper bound on the size of the program register needed for probabilistic programmable quantum processors. Our results are closely related to probabilistic perfect alignment of reference frames and probabilistic port-based teleportation.
by Michal Sedlák, Alessandro Bisio, and Mário Ziman
Phys. Rev. Lett. 122, 170502 (2019) | +++ |
HiPhoP (Quantera), MAXAP (VEGA 2/0173/17), QETWORK (APVV-14-0878), GACR GA16-22211S
  03.06.2019 Conference
CEQIP 2019
We are happy to announce that 16th CEQIP workshop will be held in June (3-6) and this year we will explore a completely new venue - kings town Skalica (Vitriol). We expect the registration and the submission will be open in February and closed in mid March. Looking forward to see you there.
  16.04.2019 Publication
Strong electron-electron interactions of a Tomonaga-Luttinger liquid observed in InAs quantum wires
We report strong electron-electron interactions in quantum wires etched from an InAs quantum well, a material generally expected to have strong spin-orbit interactions. We find that the current through the wires as a function of the bias voltage and temperature follows the universal scaling behavior of a Tomonaga-Luttinger liquid. Using a universal scaling formula, we extract the interaction parameter and find strong electron-electron interactions, increasing as the wires become more depleted. We establish theoretically that the spin-orbit interaction cause only minor modifications of the interaction parameter in this regime, indicating that genuinely strong electron-electron interactions are indeed achieved in the device. Our results suggest that etched InAs wires provide a platform with both strong electron-electron interactions and the strong spin-orbit interaction.
by Yosuke Sato, Sadashige Matsuo, Chen-Hsuan Hsu, Peter Stano, Kento Ueda, Yuusuke Takeshige, Hiroshi Kamata, Joon Sue Lee, Borzoyeh Shojaei, Kaushini Wickramasinghe, Javad Shabani, Chris Palmstrøm, Yasuhiro Tokura, Daniel Loss, and Seigo Tarucha
Phys. Rev. B 99, 155304 (2019) | +++ |
  15.04.2019 Publication
Quantum non-demolition measurement of an electron spin qubit
Measurements of quantum systems inevitably involve disturbance in various forms. Within the limits imposed by quantum mechanics, there exists an ideal projective measurement that does not introduce a back action on the measured observable, known as a quantum non-demolition (QND) measurement. Here we demonstrate an all-electrical QND measurement of a single electron spin in a gate-defined quantum dot. We entangle the single spin with a two-electron, singlet–triplet ancilla qubit via the exchange interaction and then read out the ancilla in a single shot. This procedure realizes a disturbance-free projective measurement of the single spin at a rate two orders of magnitude faster than its relaxation. The QND nature of the measurement protocol enables enhancement of the overall measurement fidelity by repeating the protocol. We demonstrate a monotonic increase of the fidelity over 100 repetitions against arbitrary input states. Our analysis based on statistical inference is tolerant to the presence of the relaxation and dephasing. We further exemplify the QND character of the measurement by observing spontaneous flips (quantum jumps) of a single electron spin. Combined with the high-fidelity control of spin qubits these results will allow for various measurement-based quantum state manipulations including quantum error correction protocols.
by Takashi Nakajima, Akito Noiri, Jun Yoneda, Matthieu R. Delbecq, Peter Stano, Tomohiro Otsuka, Kenta Takeda, Shinichi Amaha, Giles Allison, Kento Kawasaki, Arne Ludwig, Andreas D. Wieck, Daniel Loss, and Seigo Tarucha
Nature Nanotechnology, 10.1038/s41565-019-0426-x (2019) | +++ |
  25.03.2019 People
Dr. M. Hamed Mohammedy has joined our research group for 12 months as postdoctoral fellow. His current research interests cover quantum thermodynamics and quantum measurements. Welcome in our group.
  18.03.2019 Lectures
Quantum combs: General theory and applications
Speaker: Michal Sedlák

Series of hands-on lectures on modern mathematical formalism of higher-order quantum structures.

18.3.2019 (Monday) 10:00 - 12:00
20.3.2019 (Wednesday) 9:00 - 11:00
22.3.2019 (Friday) 10:00 - 12:00
25.3.2019 (Monday) 10:00 - 12:00
27.3.2019 (Wednesday) 10:00 - 12:00
29.3.2019 (Friday) 10:00 - 12:00

Place: seminar room of Research Center for Quantum Information, Bratislava
  22.02.2019 Publication
Orbital effects of a strong in-plane magnetic field on a gate-defined quantum dot
We theoretically investigate the orbital effects of an in-plane magnetic field on the spectrum of a quantum dot embedded in a two-dimensional electron gas (2DEG). We derive an effective two-dimensional Hamiltonian where these effects enter in proportion to the flux penetrating the 2DEG. We quantify the latter in detail for harmonic, triangular, and square potential of the heterostructure. We show how the orbital effects allow one to extract a wealth of information, for example, on the heterostructure interface, the quantum dot size and orientation, and the spin-orbit fields. We illustrate the formalism by extracting this information from recent measured data [L. C. Camenzind et al., arXiv:1804.00162; Nat. Commun. 9, 3454 (2018)].
by Peter Stano, Chen-Hsuan Hsu, Leon C. Camenzind, Liuqi Yu, Dominik Zumbühl, and Daniel Loss
Phys. Rev. B 99, 085308 (2019) | +++ |
  11.02.2019 Job opening
Postdoc position
We are opening a one-year 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 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 Mario Ziman by email (ziman@savba.sk) before February 28th. We are aiming to start as early as possible.
       Selected candidates will be interviewed (online) and/or invited to give a seminar at our research center. The research performance and experience of the candidate will be the only selection criteria.
  01.02.2019 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 31/05/2019) with PhD starting in September 2019. If interested, as the first step, please get in contact with a potential PhD advisor (send them your cv and motivation 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.
  30.01.2019 Popular
Čo nedokážu ani počítače budúcnosti? / What we won't be able to do on the computers we don't have yet?
Our colleague Daniel Nagaj is giving a SAVinci popular-science talk [in Slovak] on quantum computers. It is part of a series of science-coffee events organized by Slovak Academy of Sciences. ... read more ...
Place: Westend Gate Lobby, Dúbravská cesta, Bratislava - Patrónka
Time: Wednesday 30/01/2019, 17:30
  23.01.2019 Publication
Grover search under localized dephasing
Decoherence in quantum searches, and in the Grover search, in particular, has already been extensively studied, leading very quickly to the loss of the quadratic speedup over the classical case, when searching for some target (marked) element within a set of size N. The noise models used were, however, almost always global. In this paper, we study Grover search under the influence of localized partially dephasing noise of rate p. We find that, in the case when the size k of the affected subspace is much smaller than N and the target is unaffected by the noise, namely when kp≪√N, the quadratic speedup is retained. Once these restrictions are not met, the quadratic speedup is lost. If the target is affected by the noise, the noise rate needs to scale as 1/√N to keep the speedup. We also observe an intermediate region, where if k∼Nμ and the target is unaffected, the speedup seems to obey Nμ, which for μ>0.5 is worse than the quantum, but better than the classical case. We also put obtained results for quantum searches into perspective of quantum walks and searches on graphs.
by Daniel Reitzner and Mark Hillery
Phys. Rev. A 99, 012339 (2019) | +++ |
SASPRO 0055/01/01, MAXAP (VEGA 2/0173/17), QETWORK (APVV-14-0878) GACR GA16-22211S
  22.01.2019 Publication
Quantum information processing and composite quantum fields
Some beautiful identities involving hook contents of Young diagrams have been found in the field of quantum information processing, along with a combinatorial proof. We here give a representation theoretic proof of these identities and a number of generalizations. Our proof is based on trace identities for elements belonging to a class of permutation centralizer algebras. These algebras have been found to underlie the combinatorics of composite gauge invariant operators in quantum field theory, with applications in the AdS/CFT correspondence. Based on these algebras, we discuss some analogies between quantum information processing tasks and the combinatorics of composite quantum fields and argue that this can be fruitful interface between quantum information and quantum field theory, with implications for AdS/CFT.
by Sanjaye Ramgoolam and Michal Sedlák
J. High Energ. Phys. 2019, 170 (2019) | +++ |
HIPHOP (QuantERA project 731473), QETWORK (APVV-14-0878), MAXAP (VEGA 2/0173/17)