Systematic Dimensionality Reduction for Quantum Walks: Optimal Spatial Search and Transport on Non-Regular Graphs

Scientific Reports
Leonardo NovoYasser Omar

Abstract

Continuous time quantum walks provide an important framework for designing new algorithms and modelling quantum transport and state transfer problems. Often, the graph representing the structure of a problem contains certain symmetries that confine the dynamics to a smaller subspace of the full Hilbert space. In this work, we use invariant subspace methods, that can be computed systematically using the Lanczos algorithm, to obtain the reduced set of states that encompass the dynamics of the problem at hand without the specific knowledge of underlying symmetries. First, we apply this method to obtain new instances of graphs where the spatial quantum search algorithm is optimal: complete graphs with broken links and complete bipartite graphs, in particular, the star graph. These examples show that regularity and high-connectivity are not needed to achieve optimal spatial search. We also show that this method considerably simplifies the calculation of quantum transport efficiencies. Furthermore, we observe improved efficiencies by removing a few links from highly symmetric graphs. Finally, we show that this reduction method also allows us to obtain an upper bound for the fidelity of a single qubit transfer on an XY spin network.

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Citations

Dec 30, 2017·Physical Review Letters·Shantanav ChakrabortyYasser Omar
Mar 26, 2016·Physical Review Letters·Shantanav ChakrabortyYasser Omar
Jan 14, 2021·Entropy·Luca RazzoliPaolo Bordone
Jul 3, 2021·Physical Review Letters·Dylan LewisSougato Bose
Jan 8, 2020··Abdelmajid El MoutaouakkilFatima El Barakaz

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