@article{Chimczak2023,
title = {The effect of thermal photons on exceptional points in coupled resonators.},
author = {Grzegorz Chimczak and Anna Kowalewska‑Kudłaszyk and Ewelina Lange and Karol Bartkiewicz and Jan Peřina Jr.},
url = {https://www.nature.com/articles/s41598-023-32864-2},
doi = {https://doi.org/10.1038/s41598-023-32864-2},
year = {2023},
date = {2023-04-11},
journal = {Scientific Reports},
volume = {13},
pages = {5859},
abstract = {We analyse two quantum systems with hidden parity-time ( PT ) symmetry: one is an optical device,
whereas another is a superconducting microwave-frequency device. To investigate their symmetry, we
introduce a damping frame (DF), in which loss and gain terms for a given Hamiltonian are balanced.
We show that the non-Hermitian Hamiltonians of both systems can be tuned to reach an exceptional
point (EP), i.e., the point in parameter space at which a transition from broken to unbroken hidden
PT symmetry takes place. We calculate a degeneracy of a Liouvillian superoperator, which is called
the Liouvillian exceptional point (LEP), and show that, in the optical domain, LEP is equivalent to
EP obtained from the non-Hermitian Hamiltonian (HEP). We also report breaking the equivalence
between LEP and HEP by a non-zero number of thermal photons for the microwave-frequency system.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ROIK2022128270b,
title = {Entanglement quantification from collective measurements processed by machine learning},
author = {Jan Roik and Karol Bartkiewicz and Antonín Černoch and Karel Lemr},
url = {https://www.sciencedirect.com/science/article/pii/S0375960122003528},
doi = {https://doi.org/10.1016/j.physleta.2022.128270},
issn = {0375-9601},
year = {2022},
date = {2022-09-15},
journal = {Physics Letters A},
volume = {446},
pages = {128270},
abstract = {This paper investigates how to reduce the number of measurement configurations needed for sufficiently precise entanglement quantification. Instead of analytical formulae, we employ artificial neural networks to predict the amount of entanglement in a quantum state based on results of collective measurements (simultaneous measurements on multiple instances of the investigated state). We consider collective measurement limited to two copies of the investigated state. This approach allows us to explore the precision of entanglement quantification as a function of measurement configurations in a relevant scenario for practical quantum communications. For the purpose of our research, we consider general two-qubit states and their negativity as entanglement quantifier. We outline the benefits of this approach in future quantum communication networks.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Jirakova2021b,
title = {Experimental hierarchy and optimal robustness of quantum correlations of two-qubit states with controllable white noise},
author = {Kate{ř}ina Jirákov á and Antonín Č and Karel Lemr and Karol Bartkiewicz and Adam Miranowicz},
url = {https://doi.org/10.1103/physreva.104.062436},
doi = {10.1103/physreva.104.062436},
year = {2021},
date = {2021-12-21},
journal = {Physical Review A},
volume = {104},
number = {6},
pages = {062436},
publisher = {American Physical Society (APS)},
abstract = {We demonstrate a hierarchy of various classes of quantum correlations on experimentally prepared two-qubit Werner-like states with controllable white noise. Werner states, which are white-noise-affected Bell states, are prototypal examples for studying such a hierarchy as a function of the amount of white noise. We experimentally generate Werner states and their generalizations, i.e., partially entangled pure states affected by white noise. These states enable us to study the hierarchy of the following classes of correlations: separability, entanglement, steering in three- and two-measurement scenarios, and Bell nonlocality. We show that the generalized Werner states (GWSs) reveal fundamentally different aspects of the hierarchy compared to the Werner states. In particular, we find five different parameter regimes of the GWSs, including those steerable in a two-measurement scenario but not violating Bell inequalities. This regime cannot be observed for the usual Werner states. Moreover, we find threshold curves separating different regimes of the quantum correlations and find the optimal states which allow for the largest amount of white noise which does not destroy their specific quantum correlations (e.g., unsteerable entanglement). Thus, we could identify the optimal Bell-nondiagonal GWSs which are, for this specific meaning, more robust against the white noise compared to the Bell-diagonal GWSs (i.e., Werner states).},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Bartkiewicz2021b,
title = {Accuracy of Entanglement Detection via Artificial Neural Networks and Human-Designed Entanglement Witnesses},
author = {Jan Roik and Karol Bartkiewicz and Antonín Černoch and Karel Lemr},
url = {https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.15.054006},
doi = {https://doi.org/10.1103/PhysRevApplied.15.054006},
year = {2021},
date = {2021-05-04},
journal = {Phys. Rev. Applied},
volume = {15},
pages = {054006},
abstract = {The detection of entangled states is essential in both fundamental and applied quantum physics. However, this task proves to be challenging, especially for general quantum states. One can execute full state tomography but this method is time demanding, especially in complex systems. Other approaches use entanglement witnesses: these methods tend to be less demanding but lack reliability. Here, we demonstrate that artificial neural networks (ANNs) provide a balance between the two approaches. In this paper, we make a comparison of ANN performance with witness-based methods for random general two-qubit quantum states without any prior information on the states. Furthermore, we apply our approach to a real experimental data set.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}