13th Central European Quantum Information Processing Workshop
16-19 June 2016, Valtice, Czech Republic

Thursday, 16.6.2016
17:30-19:00 EVENING SESSION (chaired by Daniel Reitzner)
17:30-18:10 I Ramon Munoz Tapia The quantum change point   ΞΞΞAbstract: Sudden changes are ubiquitous in nature. The exact time when they happen is usually not known and identifying it is often of crucial importance. We propose a primitive quantum task that encapsulates this situation in its bare-bones. A source assumed to prepare a sequence of identical states starts to prepare a different state after some point. The length of the string of states where the change has occurred is given and the alteration is assumed to happen with equal probability at any point. We find the analytical expression of the optimal success probability of correct identification of the change point for large string lengths, which requires collective measurements on the whole string of states. We also analyse protocols that measure systems individually and provide an online answer at any stage of the measurement process and hence that do no require the use of a quantum memory. We show that these underperform quite substantially the optimal collective procedure.
18:10-18:35 C Daniel Burgarth : Dynamical Decoupling in Infinite Dimensions Ξ slides Ξ
18:35-19:00 C Daniel Nagaj : Quantum proofs can be verified using only single qubit measurements Ξ slides Ξ
19:30 WELCOME DINNER (Valtická rychta)

Friday, 17.6.2016
08:00-08:45 Breakfast
09:00-12:00 MORNING SESSION (chaired by Sergey Filippov)
09:00-09:40 I Miguel Navascues The structure of Matrix Product States   ΞΞΞAbstract: For the past twenty years, Tensor Network States (TNS) have been widely used to model the low energy sector of local Hamiltonians. Their success in doing so has led to the wide-held mantra that TNS of low bond dimension are the `only physical states' of natural condensed matter systems. However, given our experimental limitations to interact with such systems, it is not clear how this conjecture translates into any observable effect. In this talk I will identify particular operational features pertaining to all Matrix Product States (MPS), the class of TNS used to model non-critical one-dimensional spin chains. By exploiting two surprising structural constraints of MPS, we see how to systematically derive `bond dimension witnesses', or k-local operators whose expectation value allows us to lower bound the bond dimension of the underlying quantum state. We extend some of these results to the ansatz of Projected Entangled Pairs States (PEPS). As a bonus, we use our insight on the structure of MPS to: a) derive some limitations on the use of MPS and PEPS for ground state energy computations; b) show how to decrease the complexity and boost the speed of convergence of the semidefinite programming hierarchies described in [Phys. Rev. Lett. 115, 020501 (2015)] for the characterization of finite-dimensional quantum correlations.
09:40-10:05 C Jed Kaniewski : Self-testing of the singlet: analytic bounds from operator inequalities Ξ slides Ξ
10:05-10:30 C Matthias Kleinmann : Device-independent demonstration that a qubit is more than a quantum coin
10:30-11:00 Coffee & Refreshment
11:00-11:40 I Anne Broadbent How to verify a quantum computation   ΞΞΞAbstract: We give a new interactive protocol for the verification of quantum computations in the regime of high computational complexity. Our results are given in the language of quantum interactive proof systems. Specifically, we show that any language in BQP has a quantum interactive proof system with a polynomial-time classical verifier (who can also prepare random single-qubit pure states), and a quantum polynomial-time prover. Here, soundness is unconditional--i.e. it holds even for computationally unbounded provers. Compared to prior work, our technique does not require the encoding of the input or of the computation; instead, we rely on encryption of the input (together with a method to perform computations on encrypted inputs), and show that the random choice between three types of input (defining a computational run, versus two types of test runs) suffice. We also present a new soundness analysis, based on a reduction to an entanglement-based protocol.
11:40-12:05 C Chris Perry : Doubly infinite separation of quantum information and communication
12:05-12:30 C Thomas Bromley : Robustness of asymmetry and coherence of quantum states
12:30-13:30 Lunch
14:00-16:10 AFTERNOON SESSION (chaired by Mario Ziman)
14:00-14:40 I Mark Wilde Trading communication resources in quantum Shannon theory   ΞΞΞAbstract: What are the net rates at which a sender and receiver can generate classical communication, quantum communication, and entanglement by using a quantum channel many times? A priori, this question might seem challenging, but there is a surprisingly simple answer for several channels of interest: Just combine a single protocol with teleportation, super-dense coding, and entanglement distribution. In this talk, I will discuss this theorem in some detail and show how the "triple trade-off" capacity region simplifies for a number of quantum channels of interest. If a particular minimum output entropy conjecture holds, then we have a full characterization of this triple trade-off capacity region for degradable quantum-limited attenuator channels and all quantum-limited amplifier channels. Based on http://arxiv.org/abs/1605.04922, http://arxiv.org/abs/1206.4886, http://arxiv.org/abs/1105.0119, http://arxiv.org/abs/1004.0458, http://arxiv.org/abs/1001.1732, http://arxiv.org/abs/0901.3038, http://arxiv.org/abs/0811.4227 Ξ slides Ξ
14:40-15:05 C Giacomo de Palma : Gaussian optimizers in quantum information
15:05-15:30 C Julio de Vicente : Simple conditions constraining the set of quantum correlations
15:30-16:00 Coffee & Refreshment
16:00-18:30 POSTER SESSION
19:00 DINNER (conference room)
19:00-23:00 CIPHER GAME (18:30 registration)
19:00-19:30 C Tomáš Rybár: The first QCgame puzzle

Saturday, 18.6.2016
08:00-08:45 Breakfast
09:00-12:00 MORNING SESSION (chaired by Matej Pivoluska)
09:00-09:40 I Simone Severini Combinatorial Entanglement   ΞΞΞAbstract: We present new combinatorial objects, which we call grid-labelled graphs, and show how these can be used to represent the quantum states arising in a scenario which we refer to as the faulty emitter scenario: we have a machine designed to emit a particular quantum state on demand, but which can make an error and emit a different one. The device is able to produce a list of candidate states which can be used as a kind of debugging information for testing entanglement. By reformulating the Peres-Horodecki and matrix realignment criteria we are able to capture some characteristic features of entanglement: we construct new bound entangled states, and demonstrate the limitations of matrix realignment. We show how the notion of LOCC is related to a generalisation of the graph isomorphism problem. We give a simple proof that asymptotically almost surely, grid-labelled graphs associated to very sparse density matrices are entangled. We develop tools for enumerating grid-labelled graphs that satisfy the Peres-Horodecki criterion up to a fixed number of vertices, and propose various computational problems for these objects, whose complexity remains an open problem. The proposed mathematical framework also suggests new combinatorial and algebraic ways for describing the structure of graphs. The talk is practically arXiv:1605.03564 [quant-ph]
09:40-10:05 C Alexander Müller-Hermes : Relative Entropy Bounds on Quantum, Private and Repeater Capacities
10:05-10:30 C Stefan Baeuml : Bounding multipartite key rate in quantum broadcast networks
10:30-11:00 Coffee & Refreshment
11:00-11:25 C Sergey N. Filippov : Positive tensor products of qubit maps and 2-tensor-stable positive qubit maps
11:25-11:50 C Felix Huber : Characterizing ground and thermal states of few-body Hamiltonians
11:50-12:15 C Thomas Bromley : Accessible quantification of multiparticle entanglement
12:30-13:30 lunch
13:30-18:00 CONFERENCE TRIP (check the map, printed versions will be available) There are several options and you can essentially plan it by yourself. The basic route is about 18 km in total, however, the adventorous option depicted on the map is around 30 km.
19:00 CONFERENCE WINE TASTING (cellars with selection of Top 100 Czech wines, opposite to lecture room)

Sunday, 19.6.2016
08:00-08:45 Breakfast
09:00-12:00 MORNING SESSION (chaired by Frédéric Dupuis)
09:00-09:25 C Yasser Omar : Spatial Search by Quantum Walk is Optimal for Almost all Graphs
09:25-09:50 C Nikolajs Nahimovs : Exceptional configurations of quantum walks with Grover's coin
09:50-10:15 C Mateus Araújo : A purification postulate for quantum mechanics with no causal order
10:15-10:45 Coffee & Refreshment
10:45-11:10 C Jukka Kiukas : Two examples of quantum resource control with single qubit probes
11:10-11:35 C Martin Plávala : Conditions for optimal input states for discrimination of quantum channels Ξ slides Ξ
12:00-13:00 Lunch and LEAVE
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