arXiv subjects

Jan Dziewior

Publications and source records attributed to Jan Dziewior.

13 recordsLinked to original sources

arXiv2025

Interactions of pre- and postselected quantum particles

An approach for analysis of effective interaction between pre- and postselected quantum particles is developed. It is argued that the cases of complete pre- and postselection of particles are more profound than the cases of partial pre- and postselection, since the former goes beyond modification of the average of interactions on an ensemble of experiments. Recently discussed paradoxical phenomena such as the pigeonhole paradox and the modification of the interaction from repulsion to attraction are analyzed within the introduced formalism, and a few new surprising examples are presented.

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arXiv2024

Valid and efficient entanglement verification with finite copies of a quantum state

Detecting entanglement in multipartite quantum states is an inherently probabilistic process, typically with a few measured samples. The level of confidence in entanglement detection quantifies the scheme's validity via the probability that the signal comes from a separable state, offering a meaningful figure of merit for big datasets. Yet, with limited samples, avoiding experimental data misinterpretations requires considering not only the probabilities concerning separable states but also the probability that the signal came from an entangled state, i.e. the detection scheme's efficiency. We demonstrate this explicitly and apply a general method to optimize both the validity and the efficiency in small data sets providing examples using at most 20 state copies. The method is based on an analytical model of finite statistics effects on correlation functions which takes into account both a Frequentist as well as a Bayesian approach and is applicable to arbitrary entanglement witnesses.

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arXiv2023

Comment on "Photons can tell 'contradictory' answer about where they have been''

Yuan and Feng [Eur. Phys. J. Plus 138:70, 2023] recently proposed a modification of the nested Mach-Zehnder interferometer experiment performed by Danan et al. [Phys. Rev. Lett. 111:240402, 2013] and argued that photons give "contradictory" answers about where they have been, when traces are locally imprinted on them in different ways. They concluded that their results are comprehensible from what they call the "three-path interference viewpoint", but difficult to explain from the "discontinuous trajectory" viewpoint advocated by Danan et al. We argue that the weak trace approach (the basis of the "discontinuous trajectory" viewpoint) provides a consistent explanation of the Yuan-Feng experiment. The contradictory messages of the photons are just another example of photons lying about where they have been when the experimental method of Danan et al. is applied in an inappropriate setup.

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arXiv2023

Analysing quantum systems with randomised measurements

Measurements with randomly chosen settings determine many important properties of quantum states without the need for a shared reference frame or calibration. They naturally emerge in the context of quantum communication and quantum computing when dealing with noisy environments, and allow the estimation of properties of complex quantum systems in an easy and efficient manner. In this review, we present the advancements made in utilising randomised measurements in various scenarios of quantum information science. We describe how to detect and characterise different forms of entanglement, including genuine multipartite entanglement and bound entanglement. Bell inequalities are discussed to be typically violated even with randomised measurements, especially for a growing number of particles and settings. Furthermore, we also present an overview on the estimation of non-linear functions of quantum states and shadow tomography from randomised measurements. Throughout the review, we complement the description of theoretical ideas by explaining key experiments.

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arXiv2023

Photons are lying about where they have been, again

Bhati and Arvind [Phys. Lett. A, 127955 (2022)] recently argued that in a specially designed experiment the timing of photon detection events demonstrates photon presence at a location at which they are not present according to the weak value approach. The alleged contradiction is resolved by a subtle interference effect resulting in anomalous sensitivity of the signal imprinted on the postselected photons for the interaction at this location, similarly to the case of a nested Mach-Zehnder interferometer with a Dove prism [Quant. Stud.: Mat. Found. 2, 255 (2015)]. We perform an in depth analysis of the characterization of the presence of a pre- and postselected particle at a particular location based on information imprinted on the particle itself. The theoretical results are tested by a computer simulation of the proposed experiment.

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arXiv2022

Tracing Information Flow from Open Quantum Systems

Open quantum systems are highly relevant, both for practical applications as well as for fundamental questions about the nature of information and its transfer, encompassing for example decoherence and memory effects. Quantum mechanics introduces additional complexity to the transfer of information, e.g., storage of information in non-classical correlations. Yet, some of these aspects tend to be neglected by the usual framework of open system research. In this work we use photons in a waveguide array to implement a quantum simulation of the coupling of a qubit with a low-dimensional discrete environment. Using the trace distance between quantum states as a measure of information, we analyze different types of information transfer. Extending the usual perspective which is focused on the system alone, we also investigate the presence of information in the environment.

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arXiv2021

Cooperation and dependencies in multipartite systems

We propose an information-theoretic quantifier for the advantage gained from cooperation that captures the degree of dependency between subsystems of a global system. The quantifier is distinct from measures of multipartite correlations despite sharing many properties with them. It is directly computable for classical as well as quantum systems and reduces to comparing the respective conditional mutual information between any two subsystems. Exemplarily we show the benefits of using the new quantifier for symmetric quantum secret sharing. We also prove an inequality characterizing the lack of monotonicity of conditional mutual information under local operations and provide intuitive understanding for it. This underlines the distinction between the multipartite dependence measure introduced here and multipartite correlations.

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arXiv2020

Multipartite Entanglement Analysis From Random Correlations

Quantum entanglement is usually revealed via a well aligned, carefully chosen set of measurements. Yet, under a number of experimental conditions, for example in communication within multiparty quantum networks, noise along the channels or fluctuating orientations of reference frames may ruin the quality of the distributed states. Here we show that even for strong fluctuations one can still gain detailed information about the state and its entanglement using random measurements. Correlations between all or subsets of the measurement outcomes and especially their distributions provide information about the entanglement structure of a state and also enable to witness genuine multipartite entanglement. As long as the rate of entanglement generation is sufficiently high, this method overcomes any type and strength of localized unitary noise.

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arXiv2019

Universality of local weak interactions and its application for interferometric alignment

The modification of the effect of interactions of a particle as a function of its pre- and postselected states is analyzed theoretically and experimentally. The universality property of this modification in the case of local interactions of a spatially pre- and postselected particle has been found. It allowed to define an operational approach for characterization of the presence of a quantum particle in a particular place: the way it modifies the effect of local interactions. The experiment demonstrating this universality property provides an efficient interferometric alignment method, in which the beam on a single detector throughout one phase scan yields all misalignment parameters.

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arXiv2019

Measurements of nonlocal variables and demonstration of the failure of the product rule for a pre- and postselected pair of photons

We report the first implementation of the von Neumann instantaneous measurements of nonlocal variables which becomes possible due to technological achievements in creating hyperentangled photons. Tests of reliability and of the nondemolition property of the measurements have been performed with high precision showing the suitability of the scheme as a basic ingredient of numerous quantum information protocols. The method allows to demonstrate for the first time with strong measurements a special feature of pre- and postselected quantum systems: the failure of the product rule. It has been verified experimentally that for a particular pre- and postselected pair of particles a single measurement on particle $A$ yields with certainty $σ_x^A=-1$, a single measurement on particle $B$ yields with certainty $σ_y^B=-1$, and a single nonlocal measurement on particles $A$ and $B$ yields with certainty $σ_x^A σ_y^B=-1$.

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arXiv2018

Measurement-Disturbance Tradeoff Outperforming Optimal Cloning

One of the characteristic features of quantum mechanics is that every measurement that extracts information about a general quantum system necessarily causes an unavoidable disturbance to the state of this system. A plethora of different approaches has been developed to characterize and optimize this tradeoff. Here, we apply the framework of quantum instruments to investigate the optimal tradeoff and to derive a class of procedures that is optimal with respect to most meaningful measures. We focus our analysis on binary measurements on qubits as commonly used in communication and computation protocols and demonstrate theoretically and in an experiment that the optimal universal asymmetric quantum cloner, albeit ideal for cloning, is not an optimal procedure for measurements and can be outperformed with high significance.

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arXiv2017

An Improved Experiment to Determine the `Past of a Particle' in the Nested Mach-Zehnder Interferometer

We argue that the modification proposed by Li et al. [Chin. Phys. Lett. 32, 050303 (2015)] to the experiment of Danan et al. [Phys. Rev. Lett. 111, 240402 (2013)] does not test the past of the photon as characterised by local weak traces. Instead of answering the questions: (i) Were the photons in A? (ii) Were the photons in B? (iii) Were the photons in C? the proposed experiment measures a degenerate operator answering the questions: (i) Were the photons in A? (ii) Were the photons in B and C together? A negative answer to the last question does not tell us if photons were present in B or C. A simple variation of the modified experiment does provide good evidence for the past of the photon in agreement with the results Danan et al. obtained.

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arXiv2016

Weak value beyond conditional expectation value of the pointer readings

It is argued that a weak value of an observable is a robust property of a single pre- and post-selected quantum system rather than a statistical property. During an infinitesimal time a system with a given weak value affects other systems as if it were in an eigenstate with eigenvalue equal to the weak value. This differs significantly from the action of a system pre-selected only and possessing a numerically equal expectation value. The weak value has a physical meaning beyond a conditional average of a pointer in the weak measurement procedure. The difference between the weak value and the expectation value has been demonstrated on the example of photon polarization. In addition, the weak values for systems pre- and post-selected in mixed states are considered.

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