CQT Talk by CQT CS Talk by Alexander Belov, University of Latvia
Title: Variations on Quantum Adversary
Date/Time: Wed 29-Apr, 2:00pm
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: The (negative-weighted) quantum adversary bound is a tight characterisation of the quantum query complexity of any partial function. In this talk, we analyse the extent to which this bound can be generalised. Ambainis et al. and Lee et al. generalised this bound to the state generation and state conversion problems, respectively. Using the ideas by Lee et al., we get even further generalisations of the bound.
First, we obtain a version of the bound for general input oracles, which are just arbitrary unitaries. Second, we generalise the bound to the problem of implementing arbitrary unitary transformations. This version of the bound possesses the tight composition property. Finally, we obtain lower bounds on the quantum query complexity of functions and relations with general input oracles.
CQT Talk by Jörg Schmiedmayer, Vienna Center for Quantum Science and Technology (VCQ)
Title: ‘Solving’ a Quantum Many Body Problem by Experiment
Date/Time: Tue 05-May, 2:00pm
Venue: CQT Level 3 Seminar Room S15-03-15
The knowledge of all correlation functions of a system is equivalent to solving the corresponding quantum many-body problem. If one can identify the relevant degrees of freedom, the knowledge of a finite set of correlation functions is in many cases sufficient to determine a sufficiently accurate solution of the corresponding field theory. Complete factorization is equivalent to identifying the relevant degrees of freedom where the Hamiltonian becomes diagonal. I will give examples how one can apply this powerful theoretical concept in experiment.
A detailed study of non-translation invariant correlation functions reveals that the pre-thermalized state a system of two 1-dimensional quantum gas relaxes to after a splitting quench , is described by a generalized Gibbs ensemble . This is verified through phase correlations up to 10th order.
Interference in a pair of tunnel-coupled one-dimensional atomic super-fluids, which realize the quantum Sine-Gordon / massive Thirring models, allows us to study if, and under which conditions the higher correlation functions factorize. This allowed us to characterize the essential features of the model solely from our experimental measurements: detecting the relevant quasi-particles, their interactions and the different topologically distinct vacuum-states the quasi-particles live in. The experiment thus provides a comprehensive insights into the components needed to solve a non-trivial quantum field theory.
Our examples establish a general method to analyse quantum systems through experiments. It thus represents a crucial ingredient towards the implementation and verification of quantum simulators.
Work performed in collaboration with E.Demler (Harvard), Th. Gasenzer und J. Berges (Heidelberg). Supported by the Wittgenstein Prize, the Austrian Science Foundation (FWF): SFB FoQuS: F40-P10 and the EU: ERC-AdG QuantumRelax
 M. Gring et al., Science, 337, 1318 (2012);
 T. Langen et al., Science 348 207-211 (2015).