Seminarium ISQI -  Prof. Jan Perina Jr., Dr Javid Naikoo and Kishore Thapliyal

We investigate the hierarchy of quantum correlations in a quadratic bosonic parity-time-symmetric system (PTSS) featuring distinct dissipation and amplification channels [1,2,3]. The hierarchy includes global nonclassicality, entanglement, asymmetric quantum steering, and Bell nonlocality. We elucidate the interplay between the system {physical} nonlinearity—which serves as a source of quantumness—and the specific dynamics of bosonic PTSSs, which are qualitatively influenced by their damping and amplification characteristics. Using a set of quantifiers—including local and global nonclassicality depths, negativity, steering parameters, and the Bell parameter—we demonstrate that the standard PTSS typically exhibits weaker quantumness than its counterparts affected solely by damping or solely by amplification. Both the maximum values attained by these quantifiers and the speed and duration of their generation are generally lower in the standard PTSS. A comparative analysis of three two-mode PTSSs—standard, passive, and active—with identical eigenvectors and real parts of eigenfrequencies, but differing in their damping and amplification strengths, reveals the crucial role of quantum fluctuations associated with gain and loss. Among them, the passive PTSS yields the most strongly nonclassical states. Nevertheless, under suitable conditions, the standard PTSS can also generate highly nonclassical states. The supremacy of the passive PTSS is further supported by its fundamental advantages in practical realizations.

[1] J. Peřina Jr., K. Bartkiewicz, G. Chimczak, A. Kowalewska‑Kudłaszyk, A. Miranowicz, J. K. Kalaga, W. Leonski: Quantumness and its hierarchies in PT-symmetric down-conversion models, Phys. Rev. A, to appear; http://arxiv.org/abs/2510.06171.
[2] J. Peřina Jr., A. Miranowicz, J. K. Kalaga, W. Leoński: Unavoidability of nonclassicality loss in PT-symmetric systems, Phys. Rev. A 108, 033512 (2023).
[3] J. Peřina, Jr., A. Lukš, J. K. Kalaga, W. Leoński, A. Miranowicz: Nonclassical light at exceptional points of a quantum PT-symmetric two-mode system, Phys. Rev. A 100, 053820 (2019).

Singularities such as exceptional and hybrid exceptional-diabolic (HED) points naturally arise in the study of open quantum systems, where the interplay of loss and gain gives rise to rich and unconventional dynamical behaviour. In this talk, I will discuss how these singularities can be harnessed to achieve enhanced sensitivity in quantum sensing. Drawing on methods from quantum estimation theory and perturbative analysis of singular matrices, we examine how the sensitivity of a quantum system is governed by the structure of its response near these points. In particular, we show that operating at HED points can yield a twofold improvement in estimation precision compared to standard exceptional points. The analysis also identifies heterodyne detection as an optimal measurement strategy that achieves the fundamental quantum limit of precision. Overall, the results highlight how the unique features of open-system dynamics can be leveraged to develop ultrasensitive metrological protocols and advance next-generation quantum technologies.

[1] Javid Naikoo, Ravindra W. Chhajlany, Janek Kołodynski, Phys. Rev. Lett.  131, 220801 (2023)
[2] Javid Naikoo, Ravindra W. Chhajlany, Adam Miranowicz, New J. Phys. 27 064505 (2025)

We present a comprehensive study of quantum exceptional points, quantum diabolical points, and quantum hybrid points arising in multimode bosonic systems described by quadratic Hamiltonians. This study, based on a two-part investigation [1-2], focuses on the classification, physical realization, and dynamical implications of these spectral singularities within the framework of open quantum systems. We demonstrate how these singularities manifest directly in the Heisenberg picture dynamics of bosonic operators. We begin with multi-mode bosonic systems composed of up to five coupled modes described by non-Hermitian Hamiltonians under generalized PT-symmetric conditions. These models are physically well-grounded, and their predictions are widely accepted within the field. Thereafter, we address more unconventional configurations, where PT-symmetric behavior is not globally defined but appears only within restricted regions of the Liouville space. We refer to this as nonconventional PT symmetry, and explore its implications for both spectral structure and physical interpretation. We distinguish between inherited and genuine spectral singularities: the former being artifacts of system decomposition, and the latter intrinsic to the reduced system’s non-Hermitian dynamics. Finally, we analyze systems with unidirectional coupling, a strongly non-Hermitian interaction that can be realized, for example, via counter-propagating bosonic modes. While such systems enable the engineering of higher-order inherited degeneracies through concatenation, we find that the preservation of quantum consistency (e.g., operator commutation relations and physicality) is limited to short-time dynamics. Our results offer a unified framework for understanding and controlling complex singular behaviors in non-Hermitian bosonic systems and open pathways toward their application in quantum sensing, mode control, and non-Hermitian quantum engineering.

Semianria odbywają sie w sali 16 przy dziekanacie i są organiowane w trybie hybrydowym - link do spotkania w MS Teams znajduje sie na stronie: https://isik.amu.edu.pl/seminars/