Below you will find a list of seminars organised by ICTQT. For comprehensive list of quantum events in other institutions please see the KCIK website.
Speaker: Andrew Nemec (Duke University)
Abstract
Hybrid codes simultaneously encode both quantum and classical information together, giving an advantage over coding schemes where the quantum and classical information are transmitted separately. We construct the first known families of hybrid codes that are guaranteed to provide an advantage over quantum codes, as well as also giving a construction of hybrid codes from subsystem codes that allow for different minimum distances for the encoded quantum and classical information . We also show how hybrid codes can be applied to the problem of faulty syndrome measurements and lead to the construction of new quantum data-syndrome codes.Speaker: Andrew Nemec (Duke University, USA)
Abstract
Hybrid codes simultaneously encode both quantum and classical information together, giving an advantage over coding schemes where the quantum and classical information are transmitted separately. We construct the first known families of hybrid codes that are guaranteed to provide an advantage over quantum codes, as well as also giving a construction of hybrid codes from subsystem codes that allow for different minimum distances for the encoded quantum and classical information . We also show how hybrid codes can be applied to the problem of faulty syndrome measurements and lead to the construction of new quantum data-syndrome codes.Speaker: Seungbeom Chin (ICTQT / Sungkyunkwan University)
Abstract
We propose a graph method to systematically search for schemes that obtains genuine entanglement in arbitrary N-partite linear quantum networks (LQNs). While the indistinguishability of quantum particles is widely used as a resource for the generation of entanglement, it is challenging to devise a suitable LQN that carries a specific entangled state. Our research presents a mapping process of arbitrary LQNs to graphs, which provides an organized strategy for designing LQNs to generate multipartite entanglement with and without postselection. This talk is based on Quantum 5 (2021), 611 and arXiv:2211.04042.Speaker: Carlos L. Benavides-Riveros (MPI for the Physics of Complex Systems, Dresden / University of Trento)
Abstract
Determining the properties of the excitations in quantum many-body systems is a fundamental problem across almost all sciences. For instance, quantum excited states underpin new states of matter, support biological processes such as vision, or determine optoelectronic properties of photovoltaic devices. Yet, while ground-state properties can be determined by rather accurate computational methods, there remains a need for theoretical and computational developments to target excited states efficiently. Inspired by the duplication of the Hilbert space used to study black-hole entanglement and the electronic pairing of conventional superconductivity, we have recently developed a new variational scheme to compute the full spectrum of a quantum many-body Hamiltonian, rather than only its ground or the lowest-excited states. An important feature of our proposed scheme is that these spectra can be computed in a one-shot calculation. The scheme thus provides a novel variational platform for excited-state physics. In the talk, I will show an explicit calculation for a Fermi-Hubbard Hamiltonian, based on a unitary coupled-cluster ansatz. Since our approach is suitable for efficient implementation on quantum computers, we believe this “variational quantum diagonalizer” has the potential to enable unprecedented calculations of excited-state processes of quantum many-body systems. The talk is based on C. L. Benavides-Riveros et al., Phys. Rev. Lett. 129, 066401 (2022)Speaker: Máté Farkas (ICFO, Barcelona)
Abstract
Mutually unbiased bases (MUBs) correspond to measurements in quantum theory that are complementary: if a measurement in a basis yields a definite outcome on a given quantum state, then a measurement in a basis unbiased to the first one yields a uniformly random outcome on the same state. Simple examples of MUBs are photon polarisation measurements in the horizontal and vertical directions, or spin measurements in the z and x directions of a spin-1/2 particle. Their complementary property makes MUBs highly useful in various quantum information processing tasks, such as quantum state tomography, communication tasks, Bell inequalities, and quantum cryptography.In this talk—after an introduction to MUBs and their use in quantum information—I will introduce a generalisation of MUBs termed mutually unbiased measurements (MUMs). MUMs retain the complementary property of MUBs in a “device-independent” manner: in order to define MUMs, one does not need to refer to the Hilbert space dimension (the number of degrees of freedom, which is not an observable property), only to the outcome number of the measurements (an operational property). I will discuss the mathematical characterisation and constructions of MUMs, and the fundamental similarities and differences between MUBs and MUMs. Then, I will introduce a family of Bell inequalities tailored to MUMs, and show how to use these inequalities for device-independent quantum cryptography, as well as how to use these Bell inequalities to tackle a long-standing open problem on the number of MUBs in a given Hilbert space dimension.