arXiv subjects

Šimon Bräuer

Publications and source records attributed to Šimon Bräuer.

7 recordsLinked to original sources

arXiv2026

Symmetry-resolved parent Hamiltonians for entangled bosonic cat resources

Schrödinger cat states, formed from superpositions of well-separated coherent states, are promising resources for bosonic quantum information. We construct parent Hamiltonians for entangled multimode cat states by separating the selection of local coherent-state branches from the correlations and symmetries that determine their global superposition. GHZ-, cluster-, and W-type cat states illustrate three distinct mechanisms for this construction. In the limit of well-separated branches, the resulting oscillator Hamiltonians reduce to familiar qubit parent models based on correlations, stabilizers, and exchange symmetry. The same constraint structure also provides a common basis for coherent preparation, dissipative stabilization, and state certification.

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arXiv2026

Generalization of catability for parity-less cat states

We extend the nullifier-based certification framework of catability from parity-defined coherent cat states to parity-less Kerr cat states. Since Kerr cats cannot be distinguished by parity alone, we replace the parity component of the original nullifier with a displaced-parity operator that captures their characteristic interference structure. The resulting generalized catability remains directly observable and can be evaluated from a finite number of photon-number measurements without full state tomography. Numerical benchmarks show that the method faithfully certifies Kerr-cat features, produces accurate approximations of ideal Kerr cat states, and is more resilient to optical loss than fidelity-based certification. We also identify an exact anti-linear nullifier based on complex conjugation, providing an ideal algebraic description of Kerr cat states. Our results broaden the scope of catability and provide an experimentally practical approach to the characterization of Kerr cat states.

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arXiv2026

Catability as a metric for evaluating superposed coherent states

Superposed coherent states are central to quantum technologies, yet their reliable identification remains a challenge, especially in noisy or resource-constrained settings. We introduce a novel, directly measurable criterion for detecting cat-like features in quantum states, rooted in the concept of nonlinear squeezing. This approach bypasses the need for full state tomography and reveals structure where fidelity fails. The numerical results confirm its robustness under loss and its potential for experimental implementation. The method naturally generalizes to more exotic superpositions, including multiheaded cat states.

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arXiv2026

Witnesses of non-Gaussian features as lower bounds of stellar rank

Quantum non-Gaussian states and operations serve as fundamental resources for universal quantum computation, error correction, and high-precision metrology, extending beyond the Gaussian limits. While the stellar rank provides a rigorous hierarchical measure of non-Gaussianity, it remains challenging to determine experimentally. Conversely, witnesses of non-Gaussian features, based on the expectation values and variances of measurable observables, offer an accessible method for certifying non-Gaussian behavior but lack a direct connection to stellar rank. In this work, we establish a quantitative connection between these witnesses and stellar rank, demonstrating that the former can provide certifiable lower bounds on stellar rank. We introduce normalized expectation value and variance-based quantifiers and show that these witnesses form a consistent hierarchy of thresholds corresponding to stellar rank. Our results bridge the gap between abstract hierarchical measures and experimentally accessible quantifiers, enabling scalable certification of non-Gaussian states.

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arXiv2025

Generalized squeezing as a witness

Quantum systems can be prepared in an infinite continuum of states, but only some of them can be used as resources for quantum technologies. Discerning whether a specific quantum state falls into this class, is often a challenging task. We show that it can be performed by looking at the squeezing of the quantum states - a scenario in which the variance of some observable operator is suppressed below the threshold given by the non-useful states. We discuss the general concept first; then we illustrate the approach by evaluating cubic nonlinear squeezing in the system of collective atomic spins.

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arXiv2022

Nonlinear coherent heat machines and closed-system thermodynamics

All existing heat machines are dissipative open systems. Hence, they cannot operate fully coherently. We propose to replace this conventional thermodynamic paradigm by a completely different one, whereby heat machines are nonlinear coherent closed systems comprised of few field modes. Their thermal-state input is transformed by nonlinear interactions into non-thermal output with controlled quantum fluctuations and the capacity to deliver work in a chosen mode. This new paradigm allows the bridging of quantum coherent and thermodynamic descriptions.

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arXiv2021

Generation of quantum states with nonlinear squeezing by Kerr nonlinearity

Quantum states with nonlinear squeezing are a necessary resource for deterministic implementation of high-order quadrature phase gates that are, in turn, sufficient for advanced quantum information processing. We demonstrate that this class of states can be deterministically prepared by employing a single Kerr gate accompanied by suitable Gaussian processing. The required Kerr coupling depends on the energy of the initial system and can be made arbitrarily small. We also employ numerical simulations to analyze the effects of imperfections and to show to which extent can they be neglected.

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