arXiv subjects

Jan Chwedeńczuk

Publications and source records attributed to Jan Chwedeńczuk.

At least 19 recordsLinked to original sources

arXiv2026

Scalable quantum resources with short-range interacting spin-$\frac12$ chains

The dynamical generation of quantum resources, such as many-body entanglement, Bell correlations or spin squeezing, can be achieved via one-axis twisting (OAT) dynamics, which require all-to-all couplings. However, current digital and analog quantum simulation platforms natively provide short-range or power-law couplings that decay too quickly for this purpose. We demonstrate that two spin-$\tfrac12$ chain models -- a staggered nearest-neighbor XXX chain and a long-range XXZ chain -- develop an effective OAT nonlinearity when projected onto the symmetric sector. We show that these dynamics generate metrologically useful spin-squeezed states and Greenberger-Horne-Zeilinger coherences that ensure violation of many-body Bell inequalities. We confirm the accuracy of this mapping by comparing it to the exact dynamics and demonstrate that the generated correlations can be read out using a single probe qubit. The resulting dynamics can be simulated with analog and digital quantum simulators.

quant-ph↗

arXiv2026

Operator spreading and recoverability of local quantum Fisher information in a $U(1)$-broken spin chain

While out-of-time-order correlators establish a causal light cone for operator spreading, they do not guarantee that the parameter sensitivity carried by the operator remains locally recoverable. We examine the distinction between operator spreading and metrological recoverability for a parameter encoded in a single site of an XX spin chain subjected to a $U(1)$-breaking transverse field. We evaluate three levels of local metrological accessibility: the bare single-site quantum Fisher information (QFI), the QFI recovered by a variational sweep decoder acting on a finite spatial block, and the exact block QFI. In the integrable limit, the sensitivity propagates as a one-magnon wave packet, and a single-qubit decoder recovers the full block QFI. Breaking magnon-number conservation couples the parameter tangent state to multi-magnon sectors. We analytically demonstrate that the local QFI has no first-order correction in field strength; the leading depletion enters at $\mathcal{O}(h^2)$ through two-magnon scattering. As the field strength increases, the decoded QFI falls below the exact block QFI -- a gap reflecting a generic finite-dimensional compression limitation, as a single output qubit generically cannot capture the full QFI of a block state whose parameter dependence spans more than an effective two-dimensional subspace. The block QFI itself falls below the conserved global value, confirming that the sensitivity has spread beyond the block into non-local correlations. This operational hierarchy provides a precise quantitative distinction between the arrival of operator support and the local accessibility of metrological information.

quant-ph↗

arXiv2026

Topological protection of local quantum Fisher information

In many-body quantum systems, unitary dynamics generically delocalize locally encoded information, causing single-site metrological sensitivity to vanish. We analytically demonstrate that a topological phase can prevent this dispersal. In the open Kitaev chain, a Majorana zero mode fixes the boundary quantum Fisher information (QFI) at a nonzero plateau that persists for times exponentially long in system size. We derive exact analytical expressions for the local QFI and identify the mechanism as the spatial separation of the two Majorana quadratures to opposite ends of the chain. This separation produces a boundary encoding-axis asymmetry that distinguishes topological boundary memory from a generic localized subgap signal. We show numerically that the asymmetry is robust to moderate quenched on-site disorder, while the boundary plateau remains visible under parity-preserving interactions in finite-size real-time simulations. The protocol requires only product-state initialization, Hamiltonian evolution, and single-site readout.

quant-ph↗

arXiv2026

Generation and read-out of many-body Bell correlations with a probe qubit

As demand for quantum technologies increases, so does the need to generate and classify non-classical correlations in complex many-body systems. We introduce a simple and versatile method for creating and certifying entanglement and many-body Bell correlations. This method relies on a single qubit interacting with an $N$-qubit system. We demonstrate that: (i) such pairwise interaction is sufficient to induce many-body quantum correlations, and (ii) the qubit can serve as a probe to extract all information about these correlations. Thus, single-qubit measurements reveal multi-partite entanglement and $N$-body Bell correlations, enabling the rapid and efficient certification of non-classicality in complex systems.

quant-ph↗

arXiv2026

Volume-law protection of metrological advantage

Although entanglement can boost metrological precision beyond the standard quantum limit, the advantage often disappears with particle loss. We demonstrate that scrambling safeguards precision by dispersing information about the encoded parameter into many-body correlations. For Haar-random scrambling unitaries, we derive exact formulas for the average quantum Fisher information (QFI) of the reduced state after tracing out lost particles. The result exhibits a threshold; any remaining subsystem larger than $N/2$ recovers the full QFI, while smaller subsystems contain negligible information. We link this threshold to the scrambling-induced transition from area-law to volume-law entanglement and the associated growth of the Schmidt rank. We outline two realizations -- a brickwork circuit and chaotic XX-chain evolution -- and demonstrate the protection of one-axis-twisted probes against the loss of up to half of the particles.

quant-ph↗

arXiv2025

Many-body quantum resources of graph states

Characterizing the non-classical correlations of a complex many-body system is an important part of quantum technologies. A versatile tool for such a task is one that scales well with the size of the system and which can be both easily computed and measured. In this work we focus on graph states, that are promising platforms for quantum computation, simulation and metrology. We consider four topologies, namely the star graph states with edges, Turán graphs, $r$-ary tree graphs, and square grid cluster states, and provide a method to characterise their quantum content: the many-body Bell correlations, non-separability and entanglement depth for an arbitrary number of qubits. We also relate the strength of these many-body correlations to the usefulness of graph states for quantum sensing. Finally, we characterize many-body entanglement depth in graph states with up to $8$ qubits in $146$ classes non-equivalent under local transformations and graph isomorphisms. The technique presented is simple and does not make any assumptions about the multi-qubit state, so it could find applications wherever precise knowledge of many-body quantum correlations is required.

quant-ph↗

arXiv2025

Transfer of quantum-enhanced information through a many-body system

Forthcoming quantum devices will require high-fidelity information transfer across a many-body system. We formulate the criterion for lossless signal propagation and show that a single qubit can play the role of an antenna, collecting large amounts of information from a complex system. We derive the condition under which the antenna, far from the source and embedded in a many-body interacting medium, can still collect the complete information. A striking feature of this setup is that a single qubit antenna can receive even the full signal amplified by the entanglement of the source. As a consequence, the recovery of this information can be performed with simple single-qubit operations on the antenna (which we fully characterize) rather than with multi-qubit measurements of the source. Finally, we discuss the control of the system parameters necessary for lossless signal propagation. A method discussed here could improve the precision of quantum devices and simplify metrological protocols.

quant-ph↗

arXiv2025

Metrology using atoms in an array of double-well potentials

Quantum effects, such as entanglement, Einstein-Podolsky-Rosen steering, and Bell correlations, can enhance metrological sensitivity beyond the standard quantum limit. These correlations are typically generated through interactions between atoms or molecules, or during the passage of a laser pulse through a birefringent crystal. Here, we consider an alternative method of generating scalable, many-body entangled states, and demonstrate their usability for quantum-enhanced metrology. Our setup is a one-dimensional (1D) array of double-well potentials holding independent and uncorrelated Bose-Einstein condensates. The beam-splitting transformation mixes the signal between adjacent wells and yields a strongly entangled state through a many-body equivalent of the Hong-Ou-Mandel effect. We demonstrate this entanglement can improve the sensitivity of quantum sensors. In our analysis, we account for the effects of atomic fluctuations and identify the optimal measurement that saturates the quantum Cramer-Rao bound.

quant-ph↗

arXiv2025

Engineering interactions by collective coupling of atom pairs to cavity photons for entanglement generation

Engineering atom-atom interactions is essential both for controlling novel phases of matter and for efficient preparation of many-body entangled states, which are key resources in quantum communication, computation, and metrology. In this work, we propose a scheme to tailor these interactions by coupling driven atom pairs to optical cavity photons via a molecular state in the dispersive regime, resulting in an effective photon-field-dependent potential. As an illustrative example, by analyzing the quantum Fisher information, we show that such induced interactions can generate robust many-body entanglement in two-mode ultracold bosons in an optical cavity. By tuning the photon-induced interactions through the cavity drive, we identify conditions for preparing highly entangled states on timescales that mitigate decoherence due to photon loss. Our results show that entanglement formation rate scales strongly with both photon and atom number, dramatically reducing the timescale compared to bare atomic interactions. We also identify an optimal measurement for exploiting the metrological potential of the atomic state in an interferometric protocol with significant photon losses, saturating the quantum Cramer-Rao lower bound. Furthermore, we show that despite these losses the atomic state exhibits strong Bell correlations. Our results pave the way for engineering atom-atom interactions to study novel phases of light and matter in hybrid atom-photon systems, as well as for tailoring complex quantum states for new quantum technology protocols and fundamental tests of quantum mechanics.

quant-ph↗

arXiv2025

Inherent quantum resources in stationary spin chains

The standard way to generate many-body quantum correlations is via a dynamical protocol: an initial product state is transformed by interactions that generate non-classical correlations at later times. Here, we show that many-body Bell correlations are inherently present in the eigenstates of a variety of spin-1/2 chains. In particular, we show that the eigenstates and thermal states of the collective Lipkin-Meshkov-Glick model possess many-body Bell correlations. We demonstrate that the Bell correlations can take on quantized values that change discontinuously with variations in the total magnetization. Finally, we show that these many-body Bell correlations persist even in the presence of both diagonal and off-diagonal disorder.

quant-ph↗

arXiv2024

Entanglement classification and \emph{non-k}-separability certification via Greenberger-Horne-Zeilinger-class fidelity

Many-body quantum systems can be characterised using the notions of \emph{k}-separability and entanglement depth. A quantum state is \emph{k}-separable if it can be expressed as a mixture of \emph{k} entangled subsystems, and its entanglement depth is given by the size of the largest entangled subsystem. In this paper we propose a multipartite entanglement measure that satisfies the following criteria: (i) it can be used with both pure and mixed states; (ii) it is encoded in a single element of the density matrix, so it does not require knowledge of the full spectrum of the density matrix; (iii) it can be applied to large systems; and (iv) it can be experimentally verified. The proposed method allows the certification of \emph{non-k}-separability of a given quantum state. We show that the proposed method successfully classifies three-qubit systems into known stochastic local operations and classical communication (SLOCC) classes, namely bipartite, \mbox{W-,} and GHZ-type entanglement. Furthermore, we characterise the \emph{non-k}-separability in known nine SLOCC classes of four-qubit states, absolutely maximally entangled states for five and six qubits and for arbitrary size qubit Dicke states.

quant-ph↗

arXiv2024

Limits to velocity of signal propagation in many-body systems: a quantum-information perspective

The Lieb-Robinson bound (LRB) states that the range and strength of interactions between the constituents of a complex many-body system impose upper limits to how fast the signal can propagate. It manifests in a light cone-like growth of correlation function connecting two distant subsystems. Here we employ the techniques of quantum information to demonstrate that the LRB can be determined from local measurements performed on a single qubit that is connected to a many-body system. This formulation provides an operational recipe for estimating the LRB in complex systems, replacing the measurement of the correlation function with simple single-particle manipulations. We demonstrate the potency of this approach by deriving the upper limit to the speed of signal propagation in the XY spin chain.

quant-ph↗

arXiv2024

Bell correlations of a thermal fully-connected spin chain in a vicinity of a quantum critical point

Bell correlations are among the most exotic phenomena through which quantum mechanics manifests itself. Their presence signals that the system can violate the postulates of local realism, once believed to be the nonnegotiable property of the physical world. The importance of Bell correlations from this fundamental point of view is even straightened by their applications -- ranging from quantum cryptography through quantum metrology to quantum computing. Hence it is of growing interest to characterize the ``Bell content'' of complex, scalable many-body systems. Here we perform the detailed analysis of the character and strength of many-body Bell correlations in interacting multi-qubit systems with particle-exchange symmetry. Such configuration can be mapped onto an effective Schrödinger-like equation, which allows for precise analytical predictions. We show that in the vicinity of the quantum critical point, these correlations quickly become so strong that only a fraction of qubits remains uncorrelated. We also identify the threshold temperature, which, once overpassed, empowers thermal fluctuations that destroy Bell correlations in the system. We hope that the approach presented here, due to its universality, could be useful for the upcoming research on genuinely nonclassical Bell-correlated complex systems.

quant-ph↗

arXiv2023

Generation of scalable many-body Bell correlations in spin chains with short-range two-body interactions

Dynamical generation of strong and scalable quantum resources, like many-body entanglement and Bell correlations, in spin-$1/2$ chains, is possible with all-to-all interactions, either for constant interaction strength realizing one-axis twisting protocol or for power-law decaying potentials. We show, however, that such quantum resources can also be dynamically generated with a finite range of interactions. We identify a necessary critical range and indicate a critical time when scalable quantum correlations appear. Finally, we show that the certification of generated states is accessible in the modern quantum simulator platforms.

quant-ph↗

arXiv2022

One-axis twisting as a method of generating many-body Bell correlations

We demonstrate that the one-axis twisting (OAT), a versatile method of creating non-classical states of bosonic qubits, is a powerful source of many-body Bell correlations. We develop a fully analytical and universal treatment of the process, which allows us to identify the critical time at which the Bell correlations emerge, and predict the depth of Bell correlations at all subsequent times. Our findings are illustrated with a highly non-trivial example of the OAT dynamics generated using the Bose-Hubbard model.

quant-ph↗

arXiv2020

Characterizing quantum correlations in spin chains

The growth in the demand for precisely crafted many-body systems of spin-$1/2$ particles/qubits is due to their top-notch versatility in application-oriented quantum-enhanced protocols and the fundamental tests of quantum theory. Here we address the question: how quantum is a chain of spins? We demonstrate that a single element of the density matrix carries the answer. Properly analyzed it brings information about the extent of the many-body entanglement and the non-locality. This method can be used to tailor and witness highly non-classical effects in many-body systems with possible applications to quantum computing, ultra-precise metrology or large-scale tests of quantum mechanics. As a proof of principle, we investigate the extend of non-locality and entanglement in ground states and thermal states of experimentally accessible spin chains.

quant-ph↗

arXiv2018

Multipartite-Entanglement Dynamics in Regular-to-Ergodic Transition: a Quantum-Fisher-Information approach

The characterization of entanglement is a central problem for the study of quantum many-body dynamics. Here, we propose the quantum Fisher information as a useful tool for the study of multipartite-entanglement dynamics in many-body systems. We illustrate this by considering the regular-to-ergodic transition in the Dicke model---a fully-connected spin model showing quantum thermalization above a critical interaction strength. We show that the QFI has a rich dynamical behavior which drastically changes across the transition. In particular, the asymptotic value of the QFI, as well as its characteristic timescales, witness the transition both through their dependence on the interaction strength and through the scaling with the system size. Since the QFI also sets the ultimate bound for the precision of parameter estimation, it provides a metrological perspective on the characterization of entanglement dynamics in many-body systems. Here we show that quantum ergodic dynamics allows for a much faster production of metrologically useful states.

cond-mat.stat-mech↗

arXiv2018

Decoherence-assisted detection of entanglement of two qubit states

We show that the decoherence, which in the long run destroys quantum features of a system, can be used to reveal the entanglement in a two-qubit system. To this end, we consider a criterion that formally resembles the Clauser-Horne-Shimony-Holt (CHSH) inequality. In our case the local observables are set by the coupling of each qubit to the environmental noise, controlled with the dynamical decoupling method. We demonstrate that the constructed inequality is an entanglement criterion---it can only be violated by non-separable initial two-qubit states, provided that the local noises are correlated. We also show that for a given initial state, this entanglement criterion can be repurposed as a method of discriminating between Gaussian and non-Gaussian noise generated by the environment of the qubits. The latter application is important for ongoing research on using qubits to characterize the dynamics of environment that perturbs them and causes their decoherence

quant-ph↗