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Marcin Wieśniak

Publications and source records attributed to Marcin Wieśniak.

At least 19 recordsLinked to original sources
arXiv2026

Micromagnons and long-range entanglement in ferrimagnetic ground states

While significant attention has been devoted to studying entanglement in photonic systems, solid-state spin lattices remain relatively underexplored. Motivated by this gap, we investigate the entanglement structure of one-dimensional ferrimagnetic chains composed of alternating spin-1/2 and spin-3/2 particles. We characterize the ground-state correlations using exact diagonalization and the Density Matrix Renormalization Group method. Although the bipartite entanglement is restricted to nearest neighbors, we reveal the presence of long-range genuine multipartite entanglement between spatially separated spin pairs. These findings advance our understanding of quantum correlations in ferrimagnetic materials. The micromagnon description allows to provide fast approximation of ground states of ferrimagnets and emphasizes presence of multipartite correlations not widely discussed thus far.

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arXiv2025

Long-distance genuine multipartite Entanglement between Magnetic Defects in Spin Chains

We investigate the emergence and properties of long-distance genuine multipartite entanglement, induced via three localized magnetic defects, in a one-dimensional transverse-field XX spin-$1/2$ chain. Using both analytical and numerical techniques, we determine the conditions for the existence of bound states localized at the defects. We find that the reduced density matrix (RDM) of the defects exhibits long-distance genuine multipartite entanglement (GME) across the whole range of the Hamiltonian parameter space, including regions where the two-qubit concurrence is zero. We quantify the entanglement by using numerical lower bounds for the GME concurrence, as well as by analytically deriving the GME concurrence in regions where the RDM is of rank two. Our work provides insights into generating multipartite entanglement in many-body quantum systems via local control techniques.

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arXiv2024

How to be a Copenhagenistic-QBistic Everettist?

This essay reviews a modern understanding of a quantum measurement. Rather than reducing the picture to the observer's experience with quantum system, we try to put it in the context of a broader physical picture. We also attempt to distinguish some basic components and stages of a generic measurement. In this way, we want to understand which aspects of quantum measurements are important to mainstream interpretations of quantum mechanics.

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arXiv2024

Multipartite Entanglement versus Multiparticle Entanglement

Entanglement is defined as presence of quantum correlations beyond those achieved by local action and classical communication. To identify its presence in a generic state, one can, for example, check for existence of a decomposition of separable states. A natural extension is a genuine multipartite entanglement (GME), understood as nonexistenence of a decomposition into biseparable states (later called biseparable decomposition, BD). In this contribution we revisit activation of GME. We discuss few examples of states, which are decomposable into a mixture of biproduct states. However, after merging two copies of these states, we certify nonexistence of BD with witness operators. This seems to challenge our understanding of GME as a separate resource. It turns out that it requires a careful consideration of the physical context. We stress that activation of GME from multiple copies of GME-free states necessarily involves entangling operations.

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arXiv2022

A Variational Approach to the Quantum Separability Problem

We present the variational separability verifier (VSV), which is a novel variational quantum algorithm (VQA) that determines the closest separable state (CSS) of an arbitrary quantum state with respect to the Hilbert-Schmidt distance (HSD). We first assess the performance of the VSV by investigating the convergence of the optimization procedure for Greenberger-Horne-Zeilinger (GHZ) states of up to seven qubits, using both statevector and shot-based simulations. We also numerically determine the (CSS) of maximally entangled multipartite $X$-states ($X$-MEMS), and subsequently use the results of the algorithm to surmise the analytical form of the aforementioned (CSS). Our results indicate that current noisy intermediate-scale quantum (NISQ) devices may be useful in addressing the $NP$-hard full separability problem using the VSV, due to the shallow quantum circuit imposed by employing the destructive SWAP test to evaluate the (HSD). The (VSV) may also possibly lead to the characterization of multipartite quantum states, once the algorithm is adapted and improved to obtain the closest $k$-separable state ($k$-CSS) of a multipartite entangled state.

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arXiv2022

Two-Qutrit entanglement: 56-years old algorithm challenges machine learning

Classifying states as entangled or separable is a highly challenging task, while it is also one of the foundations of quantum information processing theory. This task is higly nontrivial even for relatively simple cases, such as two-qutrit Bell-diagonal states, i.e., mixture of nine mutually orthogonal maximally entangled states. In this article we apply the Gilbert algorithm to revise previously obtained results for this class. In particular we use ``cartography of entanglement'' to argue that most states left in [Hiesmayr, B. C. {\em Scientific Reports} {\bf 11}, 19739 (2021)] as unknown to be entangled or separable are most likely indeed separable, or very weakly entangled. The presented technique can find endless applications in more general cases.

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arXiv2022

An elegant proof of self-testing for multipartite Bell inequalities

The predictions of quantum theory are incompatible with local-causal explanations. This phenomenon is called Bell non-locality and is witnessed by violation of Bell-inequalities. The maximal violation of certain Bell-inequalities can only be attained in an essentially unique manner. This feature is referred to as self-testing and constitutes the most accurate form of certification of quantum devices. While self-testing in bipartite Bell scenarios has been thoroughly studied, self-testing in the more complex multipartite Bell scenarios remains largely unexplored. This work presents a simple and broadly applicable self-testing argument for N-partite correlation Bell inequalities with two binary outcome observables per party. Our proof technique forms a generalization of the Mayer-Yao formulation and is not restricted to linear Bell-inequalities, unlike the usual sum of squares method. To showcase the versatility of our proof technique, we obtain self-testing statements for N party Mermin-Ardehali-Belinskii-Klyshko (MABK) and Werner-Wolf-Weinfurter-Żukowski-Brukner (WWWŻB) family of linear Bell inequalities, and Uffink's family of N party quadratic Bell-inequalities.

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arXiv2021

Tangible reduction in learning sample complexity with large classical samples and small quantum system

Quantum computation requires large classical datasets to be embedded into quantum states in order to exploit quantum parallelism. However, this embedding requires considerable resources. It would therefore be desirable to avoid it, if possible, for noisy intermediate-scale quantum (NISQ) implementation. Accordingly, we consider a classical-quantum hybrid architecture, which allows large classical input data, with a relatively small-scale quantum system. This hybrid architecture is used to implement an oracle. It is shown that in the presence of noise in the hybrid oracle, the effects of internal noise can cancel each other out and thereby improve the query success rate. It is also shown that such an immunity of the hybrid oracle to noise directly and tangibly reduces the sample complexity in the probably-approximately-correct learning framework. This NISQ-compatible learning advantage is attributed to the oracle's ability to handle large input features.

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arXiv2021

Reviewing pigeonhole effect with transversal modes

We investigate the interference setup claimed to reveal the quantum pigeonhole effect. It is a claim that it is possible that three parties travel across a two-path interferometer, but no pair of them share a path. We demonstrate the inconsistency of the description based on the weak values. We introduce transversal modes and observe the arise of correlations between path and traversal degrees of freedom. Also, the rotational symmetry of the output pattern is studied as a witness of pairwise interaction. It shows no apparent relation to the probability of observing three particles in the same output.

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arXiv2021

Quantum secure learning with classical samples

Studies addressing the question "Can a learner complete the learning securely?" have recently been spurred from the standpoints of fundamental theory and potential applications. In the relevant context of this question, we present a classical-quantum hybrid sampling protocol and define a security condition that allows only legitimate learners to prepare a finite set of samples that guarantees the success of the learning; the security condition excludes intruders. We do this by combining our security concept with the bound of the so-called probably approximately correct (PAC) learning. We show that while the lower bound on the learning samples guarantees PAC learning, an upper bound can be derived to rule out adversarial learners. Such a secure learning condition is appealing, because it is defined only by the size of samples required for the successful learning and is independent of the algorithm employed. Notably, the security stems from the fundamental quantum no-broadcasting principle. No such condition can thus occur in any classical regime, where learning samples can be copied. Owing to the hybrid architecture, our scheme also offers a practical advantage for implementation in noisy intermediate-scale quantum devices.

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arXiv2021

Symmetrized persistency of Bell correlations for Dicke states and GHZ-based mixtures: studying the limits of monogamy

Quantum correlations, in particular those, which enable to violate a Bell inequality \cite{BELL}, open a way to advantage in certain communication tasks. However, the main difficulty in harnessing quantumness is its fragility to, e.g, noise or loss of particles. We study the persistency of Bell correlations of GHZ based mixtures and Dicke states. For the former, we consider quantum communication complexity reduction (QCCR) scheme, and propose new Bell inequalities (BIs), which can be used in that scheme for higher persistency in the limit of large number of particles $N$. In case of Dicke states, we show that persistency can reach $0.482N$, significantly more than reported in previous studies.

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arXiv2020

Hilbert-Schmidt distance and entanglement witnessing

Gilbert proposed an algorithm for bounding the distance between a given point and a convex set. In this article we apply the Gilbert's algorithm to get an upper bound on the Hilbert-Schmidt distance between a given state and the set of separable states. While Hilbert Schmidt Distance does not form a proper entanglement measure, it can nevertheless be useful for witnessing entanglement. We provide here a few methods based on the Gilbert's algorithm that can reliably qualify a given state as strongly entangled or practically separable, while being computationally efficient. The method also outputs successively improved approximations to the Closest Separable State for the given state. We demonstrate the efficacy of the method with examples.

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arXiv2020

Distance between Bound Entangled States from Unextendible Product Bases and Separable States

We discuss the use of the Gilbert algorithm to tailor entanglement witnesses for unextendibleproduct basis bound entangled states (UPB BE states). The method relies on the fact that an optimalentanglement witness is given by a plane perpendicular to a line between the reference state, entanglementof which is to be witnessed, and its closest separable state (CSS). The Gilbert algorithm finds anapproximation of CSS. In this article, we investigate if this approximation can be good enough toyield a valid entanglement witness. We compare witnesses found with Gilbert algorithm and those givenby Bandyopadhyay-Ghosh-Roychowdhury (BGR) construction. This comparison allows us to learnabout the amount of entanglement and we find a relationship between it and a feature of the constructionof UPB BE states, namely the size of their central tile. We show that in most studied cases, witnessesfound with the Gilbert algorithm in this work are more optimal than ones obtained by Bandyopadhyay,Ghosh, and Roychowdhury. This result implies the increased tolerance to experimental imperfections ina realization of the state.

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arXiv2019

General mapping of multi-qu$d$it entanglement conditions to non-separability indicators for quantum optical fields

We show that any multi-qudit entanglement witness leads to a non-separability indicator for quantum optical fields, which involves intensity correlations. We get, e.g., necessary and sufficient conditions for intensity or intensity-rate correlations to reveal polarization entanglement. We also derive separability conditions for experiments involving multiport interferometers, now feasible with integrated optics. We show advantages of using intensity rates rather than intensities, e.g., a mapping of Bell inequalities to ones for optical fields. The results have implication for studies of non-classicality of "macroscopic" systems of undefined or uncontrollable number of "particles".

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arXiv2018

Spectra in nested Mach-Zehnder interferometer experiments

By the means of the standard quantum mechanics formalism I present an explicit derivation of the structure of power spectra in Danan {\em et al.} and Zhou {\em et al.} experiments with nested dynamically changing Mach-Zehnder interferometers. The analysis confirms that we observe prominent, first-order peaks on frequencies related to some of the elements of the interferometer, but not on others. However, as I shall demonstrate, there are also other, weaker effects related to all relevant elements of the setup. In case of the Danan {\em et al.} setup, there are even peaks at all frequencies of element oscillations. When confronted in an experiment, these observations shall challenge the interpretation of the experiments based on anomalous trajectories of light.

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arXiv2018

Entanglement indicators for quantum optical fields: three-mode multiport beamsplitters EPR interference experiments

We generalize a new approach to entanglement conditions for light of undefined photons numbers given in [Phys. Rev. A {\bf 95}, 042113 (2017)] for polarization correlations to a broader family of interferometric phenomena. Integrated optics allows one to perform experiments based upon multiport beamsplitters. To observe entanglement effects one can use multi-mode parametric down-conversion emissions. When the structure of the Hamiltonian governing the emissions has (infinitely) many equivalent Schmidt decompositions into modes (beams), one can have perfect EPR-like correlations of numbers of photons emitted into "conjugate modes" which can be monitored at spatially separated detection stations. We provide entanglement conditions for experiments involving three modes on each side, and three-input-three-output multiport beamsplitters, and show their violations by bright squeezed vacuum states. We show that a condition expressed in terms of averages of observed rates is a much better entanglement indicator than a related one for the usual intensity variables. Thus the rates seem to emerge as a powerful concept in quantum optics, especially for fields of undefined intensities.

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arXiv2018

Entanglement conditions involving intensity correlations of optical fields: the case of multi-port interferometry

Normalized quantum Stokes operators introduced in [Phys. Rev. A {\bf 95}, 042113 (2017)] enable one to better observe non-classical correlations of entangled states of optical fields with undefined photon numbers. For a given run of an experiment the new quantum Stokes operators are defined by the differences of the measured intensities (or photon numbers) at the exits of a polarizer divided by their sum. It is this ratio that is to be averaged, and not the numerator and the denominator separately, as it is in the conventional approach. The new approach allows to construct more robust entanglement indicators against photon-loss noise, which can detect entangled optical states in situations in which witnesses using standard Stokes operators fail. Here we show an extension of this approach beyond phenomena linked with polarization. We discuss EPR-like experiments involving correlations produced by optical beams in a multi-mode bright squeezed vacuum state. EPR-inspired entanglement conditions for all prime numbers of modes are presented. The conditions are much more resistant to noise due to photon loss than similar ones which employ standard Glauber-like intensity, correlations.

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