June 2017

Mon Tue Wed Thu Fri Sat Sun
Philipp Strasberg

In this talk I will show how to expand the standard thermodynamic framework of a system coupled to a thermal reservoir by considering a stream of independently prepared units repeatedly put into contact with the system. These units can be in any nonequilibrium state and interact with the system with an arbitrary strength and duration. We will see that this stream constitutes an effective resource of nonequilibrium free energy and identify the conditions under which it behaves as a heat, work or information reservoir. We will also briefly discuss that this setup provides a natural framework to analyze information erasure ("Landauer's principle") and feedback controlled systems ("Maxwell's demon"). Furthermore, in the limit of a short system-unit interaction time, I will demonstrate that this setup can be used to provide a thermodynamically sound interpretation to many effective master equations. As an example we will discuss the micromaser, a commonly used experimental setup in quantum optics.

Sudipto Singha Roy


Resonating valence bond states have played a crucial role in the description of exotic

phases in strongly correlated systems, especially in the realm of Mott insulators and the

associated high­Tc superconducting phase transition. In particular, RVB states are

considered to be an important system to study the ground state properties of the doped

quantum spin­1/2 ladder. It is therefore interesting to understand how quantum correlations

are distributed among the constituents of these composite systems. In this regard, we

formulate an analytical recursive method to generate the wave function of doped short­range

resonating valence bond (RVB) states as a tool to efficiently estimate multisite entanglement

as well as other physical quantities in doped quantum spin ladders. Importantly, our results

show that within a specific doping concentration and model parameter regimes, the doped

RVB state essentially characterizes the trends of genuine multiparty entanglement in the

exact ground states of a Hubbard model with large onsite interactions. Moreover, we

consider an isotropic RVB network of spin­1/2 particles with a finite fraction of defects, where

the corresponding wave function of the network is rotationally invariant under the action of

local unitaries. By using quantum information­theoretic concepts like strong subadditivity of

von Neumann entropy and approximate quantum telecloning, we prove analytically that in

the presence of defects, caused by loss of a finite fraction of spins, the RVB network

sustains genuine multisite entanglement, and at the same time may exhibit finite

moderate­range bipartite entanglement, in contrast to the case with no defects.

Masahito Hayashi

Quantum computation offers a novel way of processing information and promises solution of some classically intractable problems. However, if the component of the quantum computor has some errors, it does not output a correct computation outcome. Since the quantum computor is composed of several components, the unexpected correlation causes unexpected error that cannot be corrected by error correction. To resolve this problem, we need to verify the quantum computor. If a problem is in NP, we can verify the correctness of a solution, but a problem that we want to solve with a quantum computer is not necessarily in NP. We need to verify quantum computer. Usually a quantum computer is composed of a combination of so many quantum circuits. It is not easy to predict the outcome of the combination of so many quantum circuits. That is, since we do not know the computation outcome, we cannot verify the computation outcome by itself. To resolve this problem, we employ measurement-based quantum computation (MBQC). MBQC is composed of graph state and local measurements, which are known to us. In particular, the computation resorce is given as the quantum correlation of the graph state, which is in a known form. Hence, we can verify these components by using the method of statistical hypothesis testing. In this talk, we consider the following three settings.

(1) We perfectly trust measurement. So, we need to verify only the graph state.
(2) We trust measurement, but it is noisy. The noise can be converted to noise of graph state. So, we need to verify only the noisy graph state. This protocol works with noisy graph state.
(3) We do not trust measurement as well as graph state. However, it accept only the case when the measurement and the graph state are noiseless.

Setting (3) has weakest assumption, but it works with ideal case. Setting (2) has stronger assumption, but it works with realistic case. This talk is based on joint works with  T. Morimae, K. Fujii, M. Hajdusek, Y. Takeuchi. The detail is available in arXiv:1701.05688,  arXiv:1610.05216, arXiv:1603.02195, Phys. Rev. Lett. 115, 220502 (2015).

Roman Orus

Tensor network states have established themselves as the natural language based on entanglement to describe and numerically simulate quantum many-body systems. In this talk I will first provide a long introduction to what tensor network states are, as well as to some of their related numerical methods. An overall perspective on the field will also be provided. Then, I will review recent progress on this topic conducted in my group. In particular, I will focus on the study of (i) 2d topological order, and (ii) Kagome Heisenberg antiferromagnets on a field, but other relevant developments may also be briefly discussed. Finally, I will discuss future research directions including the role of tensor networks in AdS/CFT, lattice gauge theories, many-body localization, and other recent ideas.

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