@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{Perina2022quantum,
title = {Quantum Liouvillian exceptional and diabolical points for bosonic fields with quadratic Ħamiltonians: Ŧhe Ħeisenberg-Langevin equation approach},
author = {Jan Perina Jr and Adam Miranowicz and Grzegorz Chimczak and Anna Kowalewska-Kudłaszyk},
url = {https://doi.org/10.22331/q-2022-12-22-883},
doi = {10.22331/q-2022-12-22-883},
issn = {2521-327X},
year = {2022},
date = {2022-12-10},
journal = {Quantum},
volume = {6},
pages = {883},
publisher = {Verein zur Förderung des Open Access Publizierens in den Quantenwissenschaften},
abstract = {Equivalent approaches to determine eigenfrequencies of the Liouvillians of open quantum systems are discussed using the solution of the Heisenberg-Langevin equations and the corresponding equations for operator moments. A simple damped two-level atom is analyzed to demonstrate the equivalence of both approaches. The suggested method is used to reveal the structure as well as eigenfrequencies of the dynamics matrices of the corresponding equations of motion and their degeneracies for interacting bosonic modes described by general quadratic Hamiltonians. Quantum Liouvillian exceptional and diabolical points and their degeneracies are explicitly discussed for the case of two modes. Quantum hybrid diabolical exceptional points (inherited, genuine, and induced) and hidden exceptional points, which are not recognized directly in amplitude spectra, are observed. The presented approach via the Heisenberg-Langevin equations paves the general way to a detailed analysis of quantum exceptional and diabolical points in infinitely dimensional open quantum systems.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{shilan2022,
title = {Hybrid photon–phonon blockade},
author = {Shilan Abo and Grzegorz Chimczak and Anna Kowalewska-Kudłaszyk and Jan Peřina Jr and Ravindra W. Chhajlany and Adam Miranowicz },
url = {https://www.nature.com/articles/s41598-022-21267-4},
doi = {https://doi.org/10.1038/s41598-022-21267-4},
issn = {2045-2322},
year = {2022},
date = {2022-10-21},
journal = {Scientific Reports},
volume = {12},
pages = {17655},
abstract = {We describe a novel type of blockade in a hybrid mode generated by linear coupling of photonic and
phononic modes. We refer to this effect as hybrid photon–phonon blockade and show how it can
be generated and detected in a driven nonlinear optomechanical superconducting system. Thus,
we study boson-number correlations in the photon, phonon, and hybrid modes in linearly coupled
microwave and mechanical resonators with a superconducting qubit inserted in one of them. We find
such system parameters for which we observe eight types of different combinations of either blockade
or tunnelling effects (defined via the sub- and super-Poissonian statistics, respectively) for photons,
phonons, and hybrid bosons. In particular, we find that the hybrid photon–phonon blockade can be
generated by mixing the photonic and phononic modes which do not exhibit blockade.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}