arXiv subjects

Marcin Nowakowski

Publications and source records attributed to Marcin Nowakowski.

11 recordsLinked to original sources

arXiv2024

Oracle problems as communication tasks and optimization of quantum algorithms

Quantum query complexity studies the number of queries needed to learn some property of a black box. A closely related question is how well an algorithm can succeed with this learning task using only a fixed number of queries. In this work, we propose measuring an algorithm's performance using the mutual information between the output and the actual value. The task of optimizing this mutual information using a single query, is similar to a basic task of quantum communication, where one attempts to maximize the mutual information of the sender and receiver. We make this analogy precise by splitting the algorithm between two agents, obtaining a communication protocol. The oracle's target property plays the role of a message that Alice encodes into a quantum state, which is subsequently sent over to Bob. The first part of the algorithm performs this encoding, and the second part measures the state and aims to deduce the message from the outcome. Moreover, we formally consider the oracle as a separate subsystem, whose state records the unknown oracle identity. Within this construction, Bob's optimal measurement basis minimizes the quantum correlations between the two subsystems. We also find a lower bound on the mutual information, which is related to quantum coherence. These results extend to multiple-query non-adaptive algorithms. As a result, we describe the optimal non-adaptive algorithm that uses at most a fixed number of queries, for any oracle classification problem. Crucially, this mutual-information perspective carries direct practical utility for algorithmic design, providing the theoretical foundation to optimize iterative subroutines in hybrid quantum--classical schemes. Within the present work, we apply this framework to analyze the stage-by-stage information flow and track partial progress in several standard quantum algorithms.

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arXiv2024

Entanglement of temporal sections as quantum histories and their quantum correlation bounds

In this paper we focus on the underlying quantum structure of temporal correlations and show their peculiar nature which differentiate them from spatial quantum correlations. With a growing interest in representation of quantum states as topological objects, we consider quantum history bundles based on the temporal manifold and show the source of violation of monogamous temporal Bell-like inequalities. We introduce definitions for the mixture of quantum histories and consider their entanglement as sections over the Hilbert vector bundles. As a generalization of temporal Bell-like inequalities, we derive the quantum bound for multi-time Bell-like inequalities.

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arXiv2023

Relativity of spacetime ontology: When correlations in space become correlata in time

Challenging Mermin's perspective that ``correlations have physical reality; that which they correlate does not'' we argue that correlations and correlata are not fundamentally distinct. These are dual concepts depending on the tensor product decomposition defining subsystems. Since the same quantum states may be either entangled or separable, but with respect to alternative tensor product structures, a spatial correlation in one context can become a temporal correlatum in another, and vice versa. In consequence, 2-qubit states invariant under $V\otimes V$ can be either entangled or unentangled, in conflict with the well known uniqueness theorem about the singlet state, a fact with possible implications for the quantum measurement theory.

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arXiv2023

Towards Physics of Internal Observers: Exploring the Roles of External and Internal Observers

In both quantum mechanics and relativity theory, the concept of the observer plays a critical role. However, there is no consensus on the definition of observer in these theories. Following Einstein's thought experiments, one could ask: What would it look like to sit inside a photon or to be a photon? And what type of observer could represent this more global perspective of the photon's interior? To address these questions, we introduce the concepts of internal and external observers with a focus on their relationship in quantum theory and relativity theory. The internal observer, associated with the internal observables super-algebra, glues the external interactions. Drawing inspiration from the advancements in abstract algebraic topology, we propose mathematical representation of the internal observer. We also outline principles for ensuring the consistency of observers in terms of information theory. It becomes evident, through the analysis of the introduced hierarchy of observers, that entanglement is a primitive of space-time causal relationships. While external observers must abide by the relativistic causality linked with the no-signaling principle in quantum mechanics, the internal observer is inherently non-local and may be acausal. However, its consistency is maintained through the formulation of the self-consistency principle. One of the goals of this paper is to construct the representation of the internal observer from the local external algebra of observables, which can be associated with external observers. Additionally, we demonstrate how the concepts of internal and external observers can be applied in the fields of quantum information theory, algebraic quantum field theory, and loop quantum gravity. The concept of internal observer seems to be also fundamental for further development of quantum gravity.

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arXiv2022

Dynamical nonlocality in quantum time via modular operators

We formalize the concept of the modular energy operator within the Page and Wootters timeless framework. As a result, this operator is elevated to the same status as the more studied modular operators of position and momentum. In analogy with dynamical nonlocality in space associated with the modular momentum, we introduce and analyze the nonlocality in time associated with the modular energy operator. Some applications of our formalization are provided through illustrative examples.

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arXiv2020

Consistency of quantum computation and the equivalence principle

The equivalence principle, being one of the building blocks of general relativity, seems to be also crucial for analysis of quantum effects in gravity. In this paper we consider the question if the equivalence principle has to hold for consistency of performing quantum computation in gravitational field. We propose an analysis with a looped evolution consisting of steps both in the gravitational field and in the accelerated reference frame. We show that without the equivalence principle the looped quantum evolution cannot be unitary and looses its consistency. For this reasoning the equivalence principle is formulated in terms of the gauge transformations and is analyzed for particles acquiring an appropriate phases associated with the actions over the looped path. In consequence, to keep consistency of quantum operations in gravitational field, it is required to keep some quantum variant of the equivalence principle. This proves importance of the quantized versions of this fundamental gravitational principle for quantum information processing.

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arXiv2020

Are temporal quantum correlations generally non-monogamous?

In this paper we focus on the underlying quantum structure of temporal correlations and show their peculiar nature which differentiate them from spatial quantum correlations. We show rigorously that a particular entangled history, which can be associated with a quantum propagator, is monogamous to conserve its consistency throughout time. Yet evolving systems violate monogamous Bell-like multi-time inequalities. This dichotomy, being a novel feature of temporal correlations, has its roots in the measurement process itself which is discussed by means of the bundles of entangled histories. We introduce and discuss a concept of a probabilistic mixture of quantum processes by means of which we clarify why the spatial-like Bell-type monogamous inequalities are further violated. We prove that Tsirelson bound on temporal Bell-like inequalities can be derived from the entangled histories approach and as a generalization, we derive the quantum bound for multi-time Bell-like inequalities. It is also pointed out that what mimics violation of monogamy of temporal entanglement is actually just a kind of polyamory in time but monogamy of entanglement for a particular evolution still holds.

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arXiv2018

Entangled Histories vs. the Two-State-Vector Formalism - Towards a Better Understanding of Quantum Temporal Correlations

The Two-State-Vector formalism and the Entangled Histories formalism are attempts to better understand quantum correlations in time. The main objective of this paper is to show that, with appropriately defined scalar products, both formalisms can be made isomorphic. We show that they treat operators and states on equal footing leading to the same statistics for all measurements. In particular, we discuss the topic of quantum correlations in time and show how they can be generated and analyzed in a consistent way in these formalisms. Furthermore, we elaborate on a novel behavior of quantum histories of evolving multipartite systems which do not exhibit global non-local correlations in time but nevertheless can lead to entangled reduced histories characterizing evolution of an arbitrarily chosen sub-system.

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arXiv2018

Comment on "Measurements without probabilities in the final state proposal"

The final state proposal [G.T. Horowitz and J.M. Maldacena, J. High Energy Phys. 2004(2), 8 (2004)] is an attempt to relax the apparent tension between string theory and semiclassical arguments regarding the unitarity of black hole evaporation. The authors of [R. Bousso and D. Stanford, Phys. Rev. D 89, 044038 (2014)] analyze thought experiments where an infalling observer first verifies the entanglement between early and late Hawking modes and then verifies the interior purification of the same Hawking particle. They claim that "probabilities for outcomes of these measurements are not defined" and therefore suggest that "the final state proposal does not offer a consistent alternative to the firewall hypothesis." We show, in contrast, that one may define all the relevant probabilities based on the so-called ABL rule [Y. Aharonov, P.G. Bergmann, and J.L. Lebowitz, Phys. Rev. 134, 1410 (1964)], which is better suited for this task than the decoherence functional. We thus assert that the analysis of Bousso and Stanford cannot yet rule out the final state proposal.

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arXiv2017

Quantum Entanglement in Time

In this paper we present a concept of quantum entanglement in time in a context of entangled consistent histories. These considerations are supported by presentation of necessary tools closely related to those acting on a space of spatial multipartite quantum states. We show that in similarity to monogamy of quantum entanglement in space, quantum entanglement in time is also endowed with this property for a particular history. Basing on these observations, we discuss further bounding of temporal correlations and derive analytically the Tsirelson bound implied by entangled histories for the Leggett-Garg inequalities.

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arXiv2016

Monogamy of quantum entanglement in time

In this paper we state a fundamental question about the structure of correlations in time and analyze temporal monogamy relations. We show that the nature of temporal correlations is inherently different from the spatial ones but in similarity to quantum spatial correlations, we expose a phenomenon of monogamy of quantum entanglement in time. We perform this task applying the entangled histories framework as a modifcation of the consistent histories approach. These considerations are supported by introduction of necessary tools specific for the tensor algebra used for representation of spatial correlations. We show that Tsirelson bound on temporal Bell-like inequalities can be derived from the entangled histories approach. Finally, we point out that in a context of the tensor algebra used for linking states in different times further studies on mathematical structure of the state representing evolving systems are needed.

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