arXiv subjects

Magdalena Stobińska

Publications and source records attributed to Magdalena Stobińska.

At least 19 recordsLinked to original sources

arXiv2026

Measurement compatibility in multiparameter quantum interferometry

The Cramér-Rao bound captures completely the performance of single-parameter quantum sensors. On the other hand, its extension to multiple parameters demands more caution. Different aspects need to be captured at once, including, critically, compatibility. In this article we consider compatibility in quantum interferometry for an important class of probe states, measured by double homodyne or photon counters, standard benchmarks for these applications. We include the presence of loss and phase diffusion in the estimation of a phase. Our results illustrate how different weighting of the precision on individual parameters affects their compatibility, adding to the list of considerations for quantum multiparameter estimation.

quant-ph↗

arXiv2025

Experimental neuromorphic computing based on quantum memristor

Machine learning has recently developed novel approaches, mimicking the synapses of the human brain to achieve similarly efficient learning strategies. Such an approach retains the universality of standard methods, while attempting to circumvent their excessive requirements, which hinder their scalability. In this landscape, quantum (or quantum inspired) algorithms may bring enhancement. However, high-performing neural networks invariably display nonlinear behaviours, which poses a challenge to quantum platforms, given the intrinsically linear evolution of closed systems. We propose a strategy to enhance the nonlinearity achievable in this context, without resorting to entangling gates and report the first neuromorphic architecture based on a photonic quantum memristor. In detail, we show how the memristive feedback loop enhances the nonlinearity and hence the performance of the tested algorithms. We benchmark our model on four tasks, a nonlinear function and three time series prediction. In these cases, we highlight the essential role of the quantum memristive element and demonstrate the possibility of using it as a building block in more sophisticated networks.

quant-ph↗

arXiv2024

CHSH Bell Tests For Optical Hybrid Entanglement

Optical hybrid entanglement can be created between two qubits, one encoded in a single photon and another one in coherent states with opposite phases. It opens the path to a variety of quantum technologies, such as heterogeneous quantum networks, merging continuous and discrete variable encoding, and enabling the transport and interconversion of information. However, reliable characterization of the nature of this entanglement is limited so far to full quantum state tomography. Here, we perform a thorough study of Clauser-Horne-Shimony-Holt (CHSH) Bell inequality tests, enabling practical verification of quantum correlations for optical hybrid entanglement. We show that a practical violation of this inequality is possible with simple photon number on/off measurements if detection efficiencies stay above 82%. Another approach, based on photon-number parity measurements, requires 94% efficiency but works well in the limit of higher photon populations. Both tests use no postselection of the measurement outcomes and they are free of the fair-sampling hypothesis. Our proposal paves the way to performing loophole-free tests using feasible experimental tasks such as coherent state interference and photon counting, and to verification of hybrid entanglement in real-world applications.

quant-ph↗

arXiv2024

Ultra-robust topologically protected edge states in quasi-1D systems

In recent years, the study of topologically non-trivial structures in one-dimensional models has been dominated by the Su--Schrieffer--Heeger model due to its simplicity in design and its ability to support edge states with robustness to disorder in couplings, protected by chiral and inversion symmetry. Here, we present a novel study on a zigzag quasi-one-dimensional model, which supports topologically protected edge states without relying on conventional symmetries. Our model utilises next-neighbour couplings to mediate edge states and is simultaneously resilient to dissipation, couplings and on-site energy disorders. In order to understand the topological properties of this model, we introduce a novel way to demonstrate the bulk-boundary correspondence of the edge states and construct a topological invariant that returns quantized values. Our study sheds light on the possibility of constructing topological phases in new ways, even in the absence of conventional symmetries, and opens up new avenues for research in this field. In addition, we demonstrate a possible photonic realization of these models with the help of an orbital-induced synthetic flux.

cond-mat.mes-hall↗

arXiv2024

Quantum-enhanced joint estimation of phase and phase diffusion

Accurate phase estimation in the presence of unknown phase diffusive noise is a crucial yet challenging task in noisy quantum metrology. This problem is particularly interesting due to the detrimental impact of the associated noise. Here, we investigate the joint estimation of phase and phase diffusion using generalized Holland-Burnett states, known for their experimental accessibility. These states provide performance close to the optimal state in single-parameter phase estimation, even in the presence of photon losses. We adopt a twofold approach by analyzing the joint information extraction through the double homodyne measurement and the joint information availability across all probe states. Through our analysis, we find that the highest sensitivities are obtained by using states created by directing all input photons into one port of a balanced beam splitter. Furthermore, we infer that good levels of sensitivity persist even in the presence of moderate photon losses, illustrating the remarkable resilience of our probe states under lossy conditions.

quant-ph↗

arXiv2023

Entanglement in Resonance Fluorescence

Particle entanglement is a fundamental resource upon which are based many quantum technologies. However, the up-to-now best sources of entangled photons rely on parametric down-conversion processes, which are optimal only at certain frequencies, which rarely match the energies of condensed-matter systems that can benefit from entanglement. In this Article, we show a way to circumvent this issue, and we introduce a new source of entangled photons based on resonance fluorescence delivering photon pairs as a superposition of vacuum and the Bell state $|Φ^-\rangle$. Our proposal relies on the emission from the satellite peaks of a two-level system driven by a strong off-resonant laser, whose intensity controls the frequencies of the entangled photons. Furthermore, the degree of entanglement can be optimized for every pair of frequencies, thus demonstrating a clear advantage over existing technologies. Finally, we illustrate the power of our novel source of entangled single-photon pairs by exciting a system of polaritons and showing that they are left in a maximally entangled steady state.

quant-ph↗

arXiv2023

Rapid training of quantum recurrent neural networks

Time series prediction is essential for human activities in diverse areas. A common approach to this task is to harness Recurrent Neural Networks (RNNs). However, while their predictions are quite accurate, their learning process is complex and, thus, time and energy consuming. Here, we propose to extend the concept of RRNs by including continuous-variable quantum resources in it, and to use a quantum-enhanced RNN to overcome these obstacles. The design of the Continuous-Variable Quantum RNN (CV-QRNN) is rooted in the continuous-variable quantum computing paradigm. By performing extensive numerical simulations, we demonstrate that the quantum network is capable of learning-time dependence of several types of temporal data, and that it converges to the optimal weights in fewer epochs than a classical network. Furthermore, for a small number of trainable parameters, it can achieve lower losses than its classical counterpart. CV-QRNN can be implemented using commercially available quantum-photonic hardware.

quant-ph↗

arXiv2022

Eberhard limit for photon-counting Bell tests and its utility in quantum key distribution

Loophole-free Bell tests are essential if one wishes to perform device-independent quantum key distribution (QKD), since any loophole could be used by a potential adversary to undermine the security of the protocol. Crucial work by Eberhard demonstrated that weakly entangled two-qubit states have a far greater resistance to the detection loophole than maximally entangled states, allowing one to close the loophole with detection efficiency greater than 2/3. Here we demonstrate that this same limit holds for photon-counting CHSH Bell tests which can demonstrate non-locality for higher dimensional multiphoton states such as two-mode squeezed vacuum and generalized Holland-Burnett states. In fact, we show evidence that these tests are in some sense universal, allowing feasible detection loophole-free tests for any multiphoton bipartite state, as long as the two modes are well correlated in photon number. Additionally, by going beyond the typical two-input two-output Bell scenario, we show that there are also photon-counting CGLMP inequalities which can also match the Eberhard limit, paving the way for more exotic loophole-free Bell tests. Finally we show that by exploiting this increased loss tolerance of non maximally entangled states, one can increase the key rates and loss tolerances of QKD protocols based on photon-counting tests.

quant-ph↗

arXiv2022

Proposal for distribution of multi-photon entanglement with optimal rate-distance scaling

We propose a protocol to perform long-range distribution of near-maximally entangled multiphoton states, allowing versatile applications such as quantum key distribution (QKD) and quantum metrology which can provide alternatives to state-of-the-art protocols. Our scheme uses resources available within the current integrated quantum photonic technology: squeezed vacuum states and photon-number-resolving detectors. The distributed entanglement can be certified by Bell tests which have the potential to be loophole free, and may be directly used in well established QKD protocols. Generally, this provides measurement-device-independent (MDI) levels of security, which may be upgraded to fully device-independent (DI) security if the Bell test is loophole-free. In both cases, the protocol is robust to extremely high transmission losses, matching the optimal $O(\sqrtη)$ scaling of key rate with channel transmittance.

quant-ph↗

arXiv2021

Modelling lattices of organic polaritons

Microcavity-polaritons in two-dimensional lattice geometries have been used to study a wide range of interesting physics. Meanwhile, organic materials have shown great promise on the road towards polaritonic devices, as the strong binding energy of their Frenkel excitons permits room temperature condensation and lasing. Whilst there are theoretical treatments of the condensation processes in planar organic microcavities, models of lattice geometries are lacking. Here, we introduce a model for describing the dynamics of lattices of zero-dimensional organic microcavities, where the dominant condensation mechanism involves the emission of a vibrational phonon. The vibronic resonance provides a tool for targeted condensation in a particular eigenmode of the system, which we highlight by examining a dimer and a dimerised chain. For the dimer, we observe a double resonance in the condensation efficiency that arises from tuning the condensate-reservoir detuning in to resonance with either the symmetric or antisymmetric mode. This selective condensation was also exploited in the dimerised chain, to condense the system into the topological edge states under homogeneous pumping of all cavities. We also showed an interesting signature of nonreciprocal transport when pumping a single cavity in the chain, where the direction of propagation depends on the sublattice being pumped.

cond-mat.mes-hall↗

arXiv2020

Extreme renormalisations of dimer eigenmodes by strong light-matter coupling

We explore by theoretical means an extreme renormalisation of the eigenmodes of a dimer of dipolar meta-atoms due to strong light-matter interactions. Firstly, by tuning the height of an enclosing photonic cavity, we can lower the energy level of the symmetric `bright' mode underneath that of the anti-symmetric `dark' mode. This is possible due to the polaritonic nature of the symmetric mode, that shares simultaneously its excitation with the cavity and the dimer. For a heterogeneous dimer, we show that the polariton modes can be smoothly tuned from symmetric to anti-symmetric, resulting in a variable mode localisation from extended throughout the cavity to concentrated around the vicinity of the dimer. In addition, we reveal a critical point where one of the meta-atoms becomes `shrouded', with no response to a driving electric field, and thus the field re-radiated by the dimer is only that of the other meta-atom. We provide an exact analytical description of the system from first principles, as well as full-wave electromagnetic simulations that show a strong quantitative agreement with the analytical model. Our description is relevant for any physical dimer where dipolar interactions are the dominant mechanism.

cond-mat.mes-hall↗

arXiv2019

Anderson localisation in steady states of microcavity polaritons

We present an experimental signature of the Anderson localisation of microcavity polaritons, and provide a systematic study of the dependence on disorder strength. We reveal a controllable degree of localisation, as characterised by the inverse-participation ratio, by tuning the positional disorder of arrays of interacting mesas. This constitutes the realisation of disorder-induced localisation in a driven-dissipative system. In addition to being an ideal candidate for investigating localisation in this regime, microcavity polaritons hold promise for low-power, ultra-small devices and their localisation could be used as a resource in quantum memory and quantum information processing.

cond-mat.quant-gas↗

arXiv2019

Correlation evolution in dilute Bose-Einstein condensates after quantum quenches

The universal forms of quantum density and phase correlations after an interaction quench are found for dilute 1d, 2d, and 3d condensates. A Bogoliubov approach in a local density aproximation is used. We obtain compact expressions for the most visible effects. Our results show how loss of phase coherence and antibunching are built up after the quench by quantum fluctuations. We demonstrate further that the density correlations can be observed even with imaging resolution much worse than healing length. This indicates that the direct measurement of counterpropagating atom pairs in situ in a continuum system is realistic. The conditions in contemporary 1d experiments are especially favorable for the correlation wave observations.

cond-mat.quant-gas↗

arXiv2016

Quantum steering inequality with tolerance for measurement-setting-errors: experimentally feasible signature of unbounded violation

Quantum steering is a relatively simple test for quantumness of correlations, proving that the values of quantum-mechanical measurement outcomes come into being only in the act of measurement. By exploiting quantum correlations Alice can influence -- steer -- Bob's physical system in a way inaccessible in classical world, leading to violation of some inequalities. Demonstrating this and similar quantum effects for systems of increasing size, approaching even the classical limit, is a long-standing challenging problem. Here we provide experimentally feasible signature of unbounded violation of a steering inequality. We derive its universal form where tolerance for measurement-setting-errors is explicitly build-in by means of the Deutsch-Maassen-Uffink uncertainty relation. Then, generalizing the mutual unbiasedness, we apply the inequality to the multi-singlet and multi-particle bipartite Bell-state. However, the method is general and opens the possibility of employing multi-particle bipartite steering for randomness certification and development of quantum technologies, e.g.\ random access codes.

quant-ph↗

arXiv2016

Numerical modeling of exciton-polariton Bose--Einstein condensate in a microcavity

A novel, optimized numerical method of modeling of an exciton-polariton superfluid in a semiconductor microcavity was proposed. Exciton-polaritons are spin-carrying quasiparticles formed from photons strongly coupled to excitons. They possess unique properties, interesting from the point of view of fundamental research as well as numerous potential applications. However, their numerical modeling is challenging due to the structure of nonlinear differential equations describing their evolution. In this paper, we propose to solve the equations with a modified Runge--Kutta method of 4th order, further optimized for efficient computations. The algorithms were implemented in form of C++ programs fitted for parallel environments and utilizing vector instructions. The programs form the EPCGP suite which have been used for theoretical investigation of exciton-polaritons.

cond-mat.quant-gas↗

arXiv2016

Exciton-polariton X-waves in a microcavity

We investigate the possibility of creating X-waves, or localized wave packets, in resonantly excited exciton-polariton superfluids. We demonstrate the existence of X-wave traveling solutions in the coupled exciton-photon system past the inflection point, where the effective mass of lower polaritons is negative in the direction perpendicular to the wavevector of the pumping beam. Contrary to the case of bright solitons, X-waves do not require nonlinearity for sustaining their shape. Nevertheless, we show that nonlinearity is important for their dynamics, as it allows for their spontaneous formation from an initial Gaussian wave packet. Unique properties of exciton-polaritons may lead to applications of their X-waves in long-distance signal propagation inside novel integrated optoelectronic circuits based on excitons.

cond-mat.quant-gas↗

arXiv2015

Quantum leap: how to complete a quantum walk in a single step

Quantum walks provide simple models of various fundamental processes. It is pivotal to know when the dynamics underlying a walk lead to quantum advantages just by examining its statistics. A walk with many indistinguishable particles and measurements of non-classical multi-particle correlations is likely to reveal the quantum nature. The number of elements $O(n)$ in a setup realizing walks grows with their length or spread $n$. We introduce the concept of a quantum leap, a process which can be achieved with fewer or complementary resources and which in a single step simulates another long process. The process and its leap are described by the same Hamiltonian but, the latter parametrizes the evolution with a tunable parameter of a setup. In the case of walks, a leap immediately gives a probability distribution which results only after many steps. This may be appealing for simulation of processes which are lengthy or require dynamical control. We discuss a leap based on the multi-particle Hong--Ou--Mandel interference, an inherently quantum phenomenon. It reproduces a quantum walk enabling perfect state transfer through spin chains. It requires a beam splitter, two detectors and $n$ particles to mimic a walk on a chain of size $O(n)$, for time fixed by beam-splitter's reflectivity. Our results apply to a broad class of systems where the HOM-like effects can be observed, and may constitute a new approach to simulation of complex Hamiltonians with passive interferometers.

quant-ph↗

arXiv2015

Photon distribution at the output of a beam splitter for imbalanced input states

In the Hong--Ou--Mandel interferometric scheme, two identical photons that illuminate a balanced beam splitter always leave through the same exit port. Similar effects have been predicted and (partially) experimentally confirmed for multi-photon Fock-number states. In the limit of large photon numbers, the output distribution follows a $(1-x^2)^{-1/2}$ law, where $x$ is the normalized imbalance in the output photon numbers at the two output ports. We derive an analytical formula that is also valid for imbalanced input photon numbers with a large total number of photons, and focus on the extent to which the hypothesis of perfect balanced input can be relaxed, discussing the robustness and universal features of the output distribution.

quant-ph↗