arXiv subjects

Ayan Patra

Publications and source records attributed to Ayan Patra.

At least 19 recordsLinked to original sources

arXiv2026

Noise adaptive two-way secure deterministic quantum key distribution

We introduce noise-adaptive quantum key distribution (QKD) protocols, in which the honest parties optimize the encoding (state preparation) and decoding (measurement basis) operations according to the noise models affecting the honest subsystems induced by an eavesdropper. This extends conventional QKD schemes that employ fixed encoding and decoding strategies independent of the noise characteristics of the communication channel. We investigate three representative protocols: entanglement-based secure dense coding (SDC), the entanglement-free Lucamarini and Mancini (LM05), and a two-way prepare-and-measure Bennett Brassard (BB84) protocols. Using entropic uncertainty relations, we derive the corresponding secret key rates for both adaptive and conventional non-adaptive scenarios under collective attacks. For independent but identical noise acting on the forward and backward transmission channels, as well as for correlated and non-Markovian environments, we identify classes of channels for which adaptive schemes yield enhanced secret key rates for the considered protocols. In contrast, we also determine Pauli channels, including depolarizing and bit flip channels, for which adaptive strategies provide no benefit. We further show that these optimal sets are generally non-unique and can differ substantially from the unitaries that maximize dense-coding capacity in the absence of security constraints. Our results establish noise-adaptive encoding and decoding as a powerful framework for improving secure communication over realistic noisy quantum channels.

quant-ph↗

arXiv2026

Beating noise in frequency estimation with squeezing and memory in continuous-variable systems

Quantum metrology promises precision beyond classical limits, yet environmental noise typically degrades the quantum resources required for such enhancement. In this work, we investigate frequency estimation in noisy continuous-variable systems, focusing on two complementary strategies to mitigate decoherence: Hamiltonian engineering and the exploitation of non-Markovian dynamics. By embedding squeezing directly into the system Hamiltonian, we show that the quantum Fisher information (QFI) may acquire a tunable higher-order time dependence, leading to enhanced sensitivity in the short-time regime. Moving beyond the Markovian approximation, we employ the quantum Brownian motion model to demonstrate that structured environments with finite memory can induce information backflow, temporarily restoring and even improving estimation precision relative to the unitary limit. We further assess the achievability of these bounds via Gaussian measurements, identifying regimes where homodyne, heterodyne, and optimized general-dyne measurements saturate the QFI, and noting that stronger squeezing widens the gap, potentially requiring non-Gaussian measurement strategies. Our results establish that jointly tailoring system Hamiltonian and environmental memory offers a viable route toward robust quantum-enhanced frequency estimation in open systems.

quant-ph↗

arXiv2026

Imaginarity-generating power of unitaries: A resource-theoretic approach

Imaginarity, stemming from the complex structure of quantum mechanics, has recently emerged as a fundamental resource, yet its dynamical generation remains largely unexplored. In this work, we introduce the notion of imaginarity-generating power (IGP) of unitary dynamics, which quantifies the ability of unitary operations to produce imaginarity from initially real quantum states. To quantify imaginarity, we employ a measure based on the Hilbert--Schmidt norm, which we show to be monotone under real unital operations. Within the framework of dynamical resource theories, we derive an exact expression for the purity-constrained IGP in arbitrary dimensions and show that, for pure real input states, it depends solely on intrinsic and experimentally accessible properties of the unitary. We further analyze its average behavior over ensembles of states with varying purity under both uniform and Hilbert--Schmidt distributions. We prove that it satisfies the essential properties of a valid resource monotone within the dynamical resource theory of imaginarity. We also characterize the unitaries that maximize the IGP and determine the corresponding bounds. Moreover, for Haar-random unitaries, we show that the IGP concentrates near its maximal value in high dimensions with small fluctuations, indicating that typical high-dimensional quantum dynamics are highly effective at generating imaginarity.

quant-ph↗

arXiv2026

Hierarchies of Gaussian multimode entanglement from thermodynamic quantifiers

We develop a thermodynamic characterization of multimode entanglement in pure continuous-variable systems by quantifying the gap between globally and locally extractable work (ergotropy). For arbitrary pure multimode Gaussian states, we prove that the $2$-local ergotropic gap is a faithful entanglement monotone across any bipartition and constitutes a functionally independent upper bound to the Renyi-2 entanglement entropy. We further introduce the $k$-ergotropic score, the minimum $k$-local ergotropic gap, and show that it faithfully quantifies multimode entanglement across $k$ partitions. For pure three-mode Gaussian states, we derive its closed-form relation with the geometric measure for genuine multimode entanglement $(k=2)$, and total Gaussian multimode entanglement $(k=3)$. For systems with more than three modes, the $k$-ergotropic score becomes a functionally independent measure of multimode entanglement to the standard geometric measures. Our results reveal a direct operational hierarchy linking Gaussian multimode entanglement to work extraction under locality constraints, and provide a computable and experimentally accessible thermodynamic framework for characterizing quantum correlations.

quant-ph↗

arXiv2025

Role of quantum state texture in probing resource theories and quantum phase transition

Building on the recently developed quantum state texture resource theory, we exhibit that the difference between maximum and minimum textures is a valid purity monotone in any dimension and provide a lower bound for existing purity measures. We introduce a texture-based resource monotone applicable across general convex resource theories, encompassing quantum coherence, non-stabilizerness, and entanglement. In particular, we propose the notion of non-local texture, which corresponds to the geometric measure of bipartite and multipartite entanglement in pure states. Furthermore, we demonstrate that the texture of the entire ground state or its subsystems can effectively signal quantum phase transitions in the Ising chain under both transverse and longitudinal magnetic fields, offering a powerful tool for characterizing quantum criticality.

quant-ph↗

arXiv2025

Security of deterministic key distribution with higher-dimensional systems

We analyze the security of two-way quantum key distribution using arbitrary finite-dimensional systems, considering both individual and collective eavesdropping attacks, without the effective use of entangled states, by incorporating two mutually unbiased bases and Heisenberg-Weyl operators in higher dimensions. For individual attacks, we consider cloning operations by the eavesdropper and demonstrate a dimensional advantage where secret keys can be generated for greater strengths of interception. To analyze security under collective attacks, we employ a purification scheme and derive the key rate using entropic uncertainty relations. Further, we exhibit how the protocol is more robust against eavesdropping with increasing dimension of the systems used, and compare the performance with that of the entangled two-way secure dense coding protocol when the presence of the eavesdropper is modeled by correlated and uncorrelated noise.

quant-ph↗

arXiv2025

Limits on broadcasting non-stabilizerness through unrestricted operations

In the resource theory of non-stabilizerness, we prove that stabilizer operations cannot replicate or broadcast the "magic" resource of all quantum states in an arbitrary finite dimension. Moreover, we show that even in unrestricted scenarios, there are fundamental limits on cloning the non-stabilizerness content of quantum states. When using an auxiliary system as part of the cloning process, we show that it is impossible to broadcast the non-stabilizerness of qubits that possess a greater degree of non-stabilizerness than the known states on which the transformations are based. We also derive the conditions to broadcast magic perfectly using unrestricted operations. Furthermore, we compare non-stabilizerness broadcasting with traditional state cloning methods, such as state-dependent and -independent cloners, which can achieve perfect broadcasting of states with known magic content. Our findings reveal that state-dependent cloning unitaries designed for specific states have lower average non-stabilizerness-generating power than magic-generating unitaries.

quant-ph↗

arXiv2024

Sequential information theoretic protocols in continuous variable systems

In order to enable the sequential implementation of quantum information theoretic protocols in the continuous variable framework, we propose two schemes for resource reusability, resource-splitting protocol and unsharp homodyne measurements. We demonstrate the advantage offered by the first scheme in implementing sequential attempts at continuous variable teleportation when the protocol fails in the previous round. In the second scheme, unsharp quadrature measurements are employed to implement the detection of entanglement between several pairs of observers. Under specific conditions, our calculations show that it is possible to successfully witness the entanglement of the same two-mode state, sequentially by as many as five observers.

quant-ph↗

arXiv2024

Qubit magic-breaking channels

We develop a notion of quantum channels that can make states useless for universal quantum computation by destroying their magic (non-stabilizerness) - we refer to them as magic-breaking channels. We establish the properties of these channels in arbitrary dimensions. We prove the necessary and sufficient criteria for qubit channels to be magic-breaking and present an algorithm for determining the same. Moreover, we provide compact criteria in terms of the parameters for several classes of qubit channels to be magic-breaking under various post-processing operations. Further, we investigate the necessary and sufficient conditions for the tensor product of multiple qubit channels to be magic-breaking. We establish implications of the same for the dynamical resource theory of magic preservability.

quant-ph↗

arXiv2024

Continuous variable dense coding under realistic non-ideal scenarios

We analyze the continuous variable (CV) dense coding protocol between a single sender and a single receiver when affected by noise in the shared and encoded states as well as when the decoding is imperfect. We derive a general formalism for the dense coding capacity (DCC) of generic two-mode Gaussian states. When the constituent modes are affected by quantum-limited amplifiers, pure-loss channels, and environmental interactions together with an inefficient decoding mechanism comprising imperfect double-homodyne detection, we investigate the pattern of DCC of the two-mode squeezed vacuum state (TMSV) by varying the strength of the noise. We further establish that the negative conditional entropy is responsible for providing quantum advantage in CV dense coding and identify a class of pure states capable of furnishing the maximal dense coding capacity equal to that of the TMSV under equal energy. We also demonstrate that, while the TMSV state provides the maximum quantum advantage in the DC protocol, there exists a class of states that is more resilient against noise than the TMSV state in the context of the DCC.

quant-ph↗

arXiv2023

Dimensional advantage in secure information trading via the noisy dense coding protocol

The quantum dense coding (DC) protocol, which has no security feature, deals with the transmission of classical information encoded in a quantum state by using shared entanglement between a single sender and a single receiver. Its appropriate variant has been established as a quantum key distribution (QKD) scheme for shared two-qubit maximally entangled states, with the security proof utilizing the uncertainty relation of complementary observables and the Shor-Preskill entanglement purification scheme. We present the DC-based QKD protocol for higher dimensional systems and report the lower bounds on secret key rate, when the shared state is a two-qudit maximally entangled state, and mixtures of maximally entangled states with different ranks. The analysis also includes the impact of noisy channels on the secure key rates, before and after encoding. In both the noiseless and the noisy scenarios, we demonstrate that the key rate as well as the robustness of the protocol against noise increases with the dimension. Further, we prove that the set of useless states in the DC-based QKD protocol is convex and compact.

quant-ph↗

arXiv2023

Resource Theory of Non-absolute Separability

We develop a resource theory for non-absolutely separable states (non-AS) in which absolutely separable states (AS) that cannot be entangled by any global unitaries are recognised as free states and any convex mixture of global unitary operations can be performed without incurring any costs. We employ two approaches to quantify non-absolute separability (NAS) -- one based on distance measures and the other one through the use of a witness operator. We prove that both the NAS measures obey all the conditions which should be followed by a ``good'' NAS measure. We demonstrate that NAS content is equal and maximal in all pure states for a fixed dimension. We then establish a connection between the distance-based NAS measure and the entanglement quantifier. We illustrate our results with a class of non-AS states, namely Werner states.

quant-ph↗

arXiv2023

Information theoretic resource-breaking channels

We propose the notion of process resource-breaking channels that break the resource for a quantum information processing task. We examine the same using quantum dense coding and teleportation protocols. We prove that the sets DBT (dense coding breaking) and TBT (teleportation breaking) are convex and compact and identify classical-quantum channels as their extreme points. We prove group-covariance to be a sufficient condition for channels to be DBT or TBT when they can destroy the resource of maximally entangled states. We present necessary and sufficient conditions for unital channels to be DBT for a single sender-receiver pair, while for multiple senders, the condition is sufficient. The set of qubit TBT channels is proved equivalent to qubit entanglement-breaking channels provided pre-processing is allowed. We construct witness operators to identify non-TBT(non-DBT) maps.

quant-ph↗

arXiv2023

Perfect transfer of arbitrary continuous variable states across optical waveguide lattices

We demonstrate that perfect state transfer can be achieved in an optical waveguide lattice governed by a Hamiltonian with modulated nearest-neighbor couplings. In particular, we report the condition that the evolution Hamiltonian should satisfy to achieve perfect transfer of any continuous variable input state. The states that can be transmitted need not have any specific properties - they may be pure or mixed, Gaussian or non-Gaussian in character, and comprise an arbitrary number of modes. We illustrate that the proposed protocol is scalable to two- and three-dimensional waveguide geometries. With the help of local phase gates on all the modes, our results can also be applied to realize a SWAP gate between mirror-symmetric modes about the center of the waveguide setup.

quant-ph↗

arXiv2023

Production of genuine multimode entanglement in circular waveguides with long-range interactions

Starting with a product initial state, squeezed (squeezed coherent) state in one of the modes, and vacuum in the rest, we report that a circular waveguide comprising modes coupled with varying coupling strength is capable of producing genuine multimode entanglement (GME), quantified via the generalized geometric measure (GGM). We demonstrate that for a fixed coupling and squeezing strength, the GME content of the resulting state increases as the range of couplings between the waveguides increases, although the GGM collapses and revives with the variation of coupling strength and time. The advantage of long-range coupling can be emphasized by measuring the area under the GGM curve, which clearly illustrates the growing trends of GME with the increasing range of couplings. Moreover, long-range couplings help in generating a higher GGM for a fixed coupling strength. We analytically determine the exact expression of GGM for systems involving an arbitrary number of modes, when all the modes interact with each other equally. The entire analysis is performed in the phase-space formalism. We manifest the constructive effect of disorder in the coupling parameter, which promises a steady production of GME, independent of the coupling strength.

quant-ph↗

arXiv2022

Quantum Dense Coding Network using Multimode Squeezed States of Light

We present a framework of a multimode dense coding network with multiple senders and a single receiver using continuous variable systems. The protocol is scalable to arbitrary numbers of modes with the encoding being displacements while the decoding involves homodyne measurements of the modes after they are combined in a pairwise manner by a sequence of beam splitters, thereby exhibiting its potentiality to implement in laboratories with currently available resources. We compute the closed form expression of the dense coding capacity for the cases of two and three senders that involve sharing of three- and four-mode states respectively. The dense coding capacity is calculated with the constraint of fixed average energy transmission when the modes of the sender are transferred to the receiver after the encoding operation. In both the cases, we demonstrate the quantum advantage of the protocol using paradigmatic classes of three- and four-mode states. The quantum advantage increases with the increase in the amount of energy that is allowed to be transmitted from the senders to the receiver.

quant-ph↗

arXiv2022

Multimode advantage in continuous variable quantum battery

We provide an architecture for a multimode quantum battery (QB) based on the framework of continuous variable (CV) systems. We examine the performance of the battery by using a generic class of multimode initial states whose parameters can be tuned to produce separable as well as entangled states and that can be charged locally as well as globally by Gaussian unitary operations. Analytical calculations show that a separable state is equally advantageous to an entangled one for two- and three-mode batteries when taking the figures of merit as the second moments of the change in energy. In order to produce a stable quantum battery consisting of an arbitrary number of modes, we derive compact analytical forms of the energy fluctuations and prove that for a multimode separable Gaussian initial state, fluctuations decrease as the number of modes increases, thereby obtaining a scaling analysis. Moreover, we demonstrate that local displacement as a charger is better for minimizing the fluctuations in energy than that involving the squeezing unitary operation.

quant-ph↗

arXiv2022

Significance of Fidelity Deviation in Continuous Variable Teleportation

Performance of quantum teleportation is typically measured by the average fidelity, an overlap between the input and output states. Along with the first moment, we introduce the second moment of fidelity in CV teleportation, i.e., the fidelity deviation as the figures of merit to assess the protocol's efficiency. We show that CV states, both Gaussian and non Gaussian, can be better characterized by considering both average fidelity and fidelity deviation, which is not possible with only average fidelity. Moreover, we shed light on the performance of the teleportation protocol in two different input scenarios - one is when input states are sampled from constrained uniform distribution while the other one is Gaussian suppression of the input states which again lead to a different classification of CV states according to their performance. The entire analysis is carried out in noiseless and noisy scenarios with noise being incorporated in the measurement and the shared channels. We also report that one type of noise can make the protocol robust against the other one which leads to a `constructive effect' and identify the noise models which are responsible for decreasing average fidelity and increment in fidelity deviation.

quant-ph↗