@article{Kadlec:24,
title = {Experimental hierarchy of the nonclassicality of single-qubit states via potentials for entanglement, steering, and Bell nonlocality},
author = {Josef Kadlec and Karol Bartkiewicz and Antonín Černoch and Karel Lemr and Adam Miranowicz},
url = {https://opg.optica.org/oe/abstract.cfm?URI=oe-32-2-2333},
doi = {10.1364/OE.506169},
year = {2024},
date = {2024-01-01},
journal = {Opt. Express},
volume = {32},
number = {2},
pages = {2333--2346},
publisher = {Optica Publishing Group},
abstract = {Entanglement potentials are a promising way to quantify the nonclassicality of single-mode states. They are defined by the amount of entanglement (expressed by, e.g., the Wootters concurrence) obtained after mixing the examined single-mode state with a purely classical state; such as the vacuum or a coherent state. We generalize the idea of entanglement potentials to other quantum correlations: the EPR steering and Bell nonlocality, thus enabling us to study mutual hierarchies of these nonclassicality potentials. Instead of the usual vacuum and one-photon superposition states, we experimentally test this concept using specially tailored polarization-encoded single-photon states. One polarization encodes a given nonclassical single-mode state, while the other serves as the vacuum place-holder. This technique proves to be experimentally more convenient in comparison to the vacuum and a one-photon superposition as it does not require the vacuum detection.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Roik2024,
title = {Routing in quantum communication networks using reinforcement machine learning},
author = {Jan Roik and Karol Bartkiewicz and Antonín Černoch and Karel Lemr},
url = {http://dx.doi.org/10.1007/s11128-024-04287-z},
doi = {10.1007/s11128-024-04287-z},
issn = {1573-1332},
year = {2024},
date = {2024-01-01},
journal = {Quantum Information Processing},
volume = {23},
number = {3},
publisher = {Springer Science and Business Media LLC},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{bartkiewicz_synergic_2023,
title = {Synergic quantum generative machine learning},
author = {Karol Bartkiewicz and Patrycja Tulewicz and Jan Roik and Karel Lemr},
url = {https://www.nature.com/articles/s41598-023-40137-1},
doi = {10.1038/s41598-023-40137-1},
issn = {2045-2322},
year = {2023},
date = {2023-08-09},
urldate = {2023-10-18},
journal = {Scientific Reports},
volume = {13},
number = {1},
pages = {12893},
abstract = {We introduce a new approach towards generative quantum machine learning significantly reducing the number of hyperparameters and report on a proof-of-principle experiment demonstrating our approach. Our proposal depends on collaboration between the generators and discriminator, thus, we call it quantum synergic generative learning. We present numerical evidence that the synergic approach, in some cases, compares favorably to recently proposed quantum generative adversarial learning. In addition to the results obtained with quantum simulators, we also present experimental results obtained with an actual programmable quantum computer. We investigate how a quantum computer implementing generative learning algorithm could learn the concept of a maximally-entangled state. After completing the learning process, the network is able both to recognize and to generate an entangled state. Our approach can be treated as one possible preliminary step to understanding how the concept of quantum entanglement can be learned and demonstrated by a quantum computer.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{abo_experimental_2023,
title = {Experimental hierarchy of two-qubit quantum correlations without state tomography},
author = {Shilan Abo and Jan Soubusta and Kateřina Jiráková and Karol Bartkiewicz and Antonín Černoch and Karel Lemr and Adam Miranowicz},
url = {https://www.nature.com/articles/s41598-023-35015-9},
doi = {10.1038/s41598-023-35015-9},
issn = {2045-2322},
year = {2023},
date = {2023-05-26},
urldate = {2023-10-18},
journal = {Scientific Reports},
volume = {13},
number = {1},
pages = {8564},
abstract = {A Werner state, which is the singlet Bell state affected by white noise, is a prototype example of states, which can reveal a hierarchy of quantum entanglement, steering, and Bell nonlocality by controlling the amount of noise. However, experimental demonstrations of this hierarchy in a sufficient and necessary way (i.e., by applying measures or universal witnesses of these quantum correlations) have been mainly based on full quantum state tomography, corresponding to measuring at least 15 real parameters of two-qubit states. Here we report an experimental demonstration of this hierarchy by measuring only six elements of a correlation matrix depending on linear combinations of two-qubit Stokes parameters. We show that our experimental setup can also reveal the hierarchy of these quantum correlations of generalized Werner states, which are any two-qubit pure states affected by white noise.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@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}
}