Joint Seminar on Quantum Information and Technologies
2012/2013 | 2013/2014 | 2014/2015 | 2015/2016 | 2016/2017 | 2017/2018 | 2018/2019 | 2019/2020 | 2020/2021 | 2021/2022 | 2022/2023 | 2023/2024 | 2024/2025 | YouTube channel
until 2023/2024 Quantum Information Seminar | YouTube channel
2018-11-15 (Thursday)
room 1.03, Pasteura 5 at 11:15 
Amit Kumar Pal (Swansea University, UK)Estimating entanglement in large-scale noisy topological codes
Entanglement is considered as resource in quantum information processing tasks. However, computation of the quantity is often challenging, particularly when the system is of large size, or when it is described by a mixed state -- for example, in the presence of noise. In this talk, we discuss how entanglement, as measured by localizable entanglement, in a noisy topological code of large size, such as the Kitaev's surface code and the color code, can be estimated via an experimentally accessible methodology using entanglement witness operators. We also demonstrate how graph states can be employed in the recipe, and discuss how insight about the distance dependence of entanglement can be obtained. The results are particularly relevant in characterizing the stabilizer states in the case of quantum error correction, where the topological codes serve as ideal candidate systems.
2018-11-08 (Thursday)
room 1.03, Pasteura 5 at 11:15
Jędrzej Kaniewski (CFT PAN)Self-testing of quantum devices: recent developments
2018-10-25 (Thursday)
room 1.03, Pasteura 5 at 11:15
Wojciech Gorecki (IFT UW)Quantum error correction in multiparameter metrology
2018-10-18 (Thursday)
room 1.03, Pasteura 5 at 11:15
Mehul Malik (Edinburgh, UK)Experimental multi-photon entanglement beyond qubits
Entanglement is the workhorse of quantum technologies today, ranging from fault-tolerant quantum computation to device-independent quantum communication. The entanglement of more than two quantum particles, commonly known as Greenberger-Horne-Zeilinger (GHZ) entanglement, not only opened the door to the strongest test of local-realism, but also forms a key ingredient of such technologies. Since the discovery of the GHZ theorem, experimental research on multi-particle entanglement has mainly focused on two-dimensional quantum systems with realisations in a diverse range of physical systems including ions, photons, and super-conducting qubits. In all of these systems, a general procedure exists for increasing the number of entangled particles. For example, for photons, a particularly simple experimental scheme uses polarising beam-splitters in combination with post-selection to produce arbitrarily high numbers of photons entangled in a GHZ manner. However, no experiment till date has been able to create a truly high-dimensional and multi-particle entangled state.In this talk, I will discuss the first experimental realisation of a multi-photon entangled state where all photons are genuinely entangled in a high-dimensional manner. Interestingly, our experimental technique was found by using a computer algorithm (MELVIN) but ultimately implemented in the lab by humans. By carefully combining two pairs of high-dimensionally entangled photons in a 27-dimensional multi-port interferometer, we generated a GHZ state consisting of three photons entangled in three dimensions each of their orbital angular momentum. We verified the entanglement through the use of a fidelity-based entanglement witness, and demonstrated three independent violations of the Mermin inequality in three subspaces of our high-dimensional multipartite state. Our results open up a pathway for a further boost to quantum technologies and will enable qualitatively new refutations of local-realistic world views.
2018-10-11 (Thursday)
room 1.03, Pasteura 5 at 11:15
Manuel Gessner (LENS, Florence, Italy)Entanglement and sensitivity in multiparameter quantum metrology
Multiparameter quantum metrology develops strategies to simultaneously estimate several parameters with quantum-enhanced precision and has potential applications in imaging and field sensing. The multiparameter sensitivity is quantified by the covariance matrix of all parameters. We present sensitivity limits for a multimode interferometer as matrix bounds for the covariance matrix. Quantum strategies to improve the precision may consist in entanglement among the parameter-encoding modes or among the particles that enter the interferometer (if their number is fixed). We observe a stepwise enhancement of the achievable precision limit as more modes and particles are entangled. We further discuss the optimal states for the various quantum and classical strategies.
2018-10-04 (Thursday)