arXiv subjects

Junaid ur Rehman

Publications and source records attributed to Junaid ur Rehman.

17 recordsLinked to original sources

arXiv2026

QuMIMO Diversity over Discrete-Variable Free-Space Optical Channels

Free-space optical (FSO) links carry quantum states without fiber, but diffraction, pointing error, and atmospheric turbulence couple spatial modes and reduce end-to-end fidelity. Classical multiple-input multiple-output (MIMO) uses spatial channels for multiplexing or diversity. For unknown quantum states, however, the no-cloning theorem prevents classical replication. We therefore formulate an FSO quantum multiple-input multiple-output (QuMIMO) link for an unknown polarization qubit as a single completely positive and trace-preserving (CPTP) map. It combines passive field transfer, bosonic pure loss lifted to Fock space, a receiver map to erasure-augmented polarization qubits, and effective per-port polarization noise. The plane-wave Rytov variance parameterizes turbulence, while an internal-state Gram matrix captures partial photon distinguishability and its effects on path coherence and multiphoton interference. Using Haar-averaged state fidelity, we compare direct transmission, fixed quantum error correction (QEC), approximate quantum cloning, coherent path superposition, and channel-adapted encoder and recovery maps, while distinguishing channel state information (CSI) from endpoint availability. With Full CSI on the two-rail channel, asymmetric cloning and coherent path superposition exceed the fixed single-input single-output (SISO) baseline in average fidelity. Adding rails and their admitted photon-number sectors enlarges the attained general-CPTP gain over the fixed SISO baseline, with the widest margin at moderate turbulence. Fixed stabilizer encoders perform poorly as turbulence makes rail survival unequal and produces errors unlike erasures at known code positions. Thus, QuMIMO realizes channel-adapted spatial diversity without requiring multiple copies of the logical qubit.

quant-ph↗

arXiv2026

Quantum Key Distribution Without Shared Reference Frame Under Unital Noise

We consider a general and practical scenario of quantum key distribution (QKD) over an unknown, stationary, unital qubit channel. Furthermore, due to practical limitations, e.g., relative movement and rotation of communicating parties, a global shared reference frame cannot be established. This scenario can routinely appear in satellite QKD. We propose two methods to overcome the physical qubit noise and the lack of shared reference frame. The first proposed approach involves constructing the Pauli transfer matrix (PTM) description of the channel, which we achieve without requiring a shared reference frame, by absorbing the lack of shared reference frame in the channel definition. This is followed by the identification of singular vectors of PTM as the Bloch vectors for optimal signal states. In the optimized local bases, the resulting correlations are equivalent, up to outcome relabeling, to those of a Pauli channel, allowing us to show the optimality of the BB84 and six-state QKD protocols under these conditions. The second approach, called the sequential basis matching (SBM) involves sequentially identifying the channel-optimized local bases that enable QKD. We show that both of these approaches result in the same effective key exchange rate for QKD.

quant-ph↗

arXiv2025

Trading Quantum Ensembles

We consider an example scenario where we require several copies of a pure quantum state $|ψ\rangle$ for some quantum information processing task. Due to practical limitations, we only have access to $N = 10^3$ depolarized copies of $|ψ\rangle$ such that the fidelity $F$ of each copy with $|ψ\rangle$ is $0.75$. We denote this quantum asset with the ensemble $\mathcal{A}: (10^3, 0.75)_{|ψ\rangle}$. A genie appears and offers to trade $\mathcal{A}$ with either $\mathcal{B}: (10^4, 0.65)_{|ψ\rangle}$ or with $\mathcal{C}: (10^2, 0.90)_{|ψ\rangle}$. Should we accept the trade with either of these two ensembles? In this article, we attempt to answer this question with arbitrary $N$ and $F$. More specifically, we derive resource equivalence curves from quantum resource theory of purity, quantum state distinguishability, quantum state purification, and quantum state tomography. These curves enable ranking of these ensembles according to their operational usefulness for these tasks and allow us to answer the question of trading the aforementioned ensembles.

quant-ph↗

arXiv2025

Entanglement Distribution in Lossy Quantum Networks

Entanglement distribution is essential for unlocking the potential of distributed quantum information processing. We consider an $N$-partite network where entanglement is distributed via a central source over lossy channels, and network participants cooperate to establish entanglement between any two chosen parties under local operations and classical communication (LOCC) constraints. We develop a general mathematical framework to assess the optimal average bipartite entanglement shared in a lossy distribution, and introduce a tractable lower bound by optimizing over a subset of single-parameter LOCC transformations. Our results show that probabilistically extracting Bell pairs from W states is more advantageous than deterministically extracting them from GHZ-like states in lossy networks, with this advantage increasing with network size. We further extend our analysis analytically, proving that W states remain more effective in large-scale networks. These findings offer valuable insights into the practical deployment of near-term networks, revealing a fundamental trade-off between deterministic entanglement distribution protocols and loss-sensitive resources.

quant-ph↗

arXiv2024

Digital Twin for Non-Terrestrial Networks: Vision, Challenges, and Enabling Technologies

This paper investigates the transformative potential of digital twin (DT) technology for non-terrestrial networks (NTNs). NTNs, comprising airborne and space-borne elements, face unique challenges in network control, management, and optimization. DT technology provides a novel framework for designing and managing complex cyber-physical systems with enhanced automation, intelligence, and resilience. By offering a dynamic virtual representation of the NTN ecosystem, DTs enable real-time monitoring, simulation, and data-driven decision-making. This paper explores the integration of DTs into NTNs, identifying technical challenges and highlighting some key enabling technologies. Emphasis is placed on technologies such as the Internet of Things (IoT), machine learning, generative AI, space-based clouds, quantum computing, and others, highlighting their potential to empower DT development for NTNs. To illustrate these concepts, we present a case study demonstrating the implementation of a data-driven DT model for enabling dynamic, service-oriented network slicing within an open radio access network (O-RAN) architecture tailored for NTNs. This work aims to advance the understanding and application of DT technology, contributing to the evolution of network control and management in the dynamic and rapidly changing landscape of non-terrestrial communication systems.

cs.NI↗

arXiv2024

Space-Based Quantum Internet: Entanglement Distribution in Time-Varying LEO Constellations

This paper addresses the complexities of entanglement distribution in LEO satellite networks, particularly those arising from their dynamic topology. Traditional static and dynamic entanglement distribution methods often result in high entanglement drop rates and reduced end-to-end throughput. We introduce a novel framework that leverages the dynamic nature of LEO satellite networks to enhance entanglement distribution efficiency. Employing a space-time graph model to represent the network's temporal evolution, we propose an entanglement distribution strategy based on path utility, incorporating pointing errors, non-ideal link transmittance for intersatellite links, and atmospheric effects for downlinks. Our approach demonstrates superior performance in reducing entanglement drop rates and improving throughput compared to conventional methods. This study advances the field of quantum communication in satellite networks, offering resilient and efficient entanglement distribution strategies that support practical applications such as distributed computing, quantum multipartite cryptography, and distributed quantum sensing. The findings underscore the potential of integrating dynamic satellite networks with quantum technologies to create a reliable and secure quantum internet.

quant-ph↗

arXiv2024

Spatial-Mode Diversity and Multiplexing for Continuous Variables Quantum Communications

We investigate the performance of continuous-variable (CV) quantum communication systems employing diversity schemes to mitigate the effects of realistic channel conditions, including Gaussian lossy channels, fading, and crosstalk. By modeling the transmittivity of the channel as a log-normal distribution, we account for the stochastic nature of fading. We analyze the impact of both post-processing amplification at the receiver and pre-amplification at the transmitter on the fidelity of the communication system. Our findings reveal that diversity schemes provide significant advantages over single-channel transmission in terms of fidelity, particularly in conditions of strong fading and high thermal background noise. We also explore the effect of crosstalk between channels and demonstrate that a noticeable advantage persists in scenarios of strong fading or thermal noise. For CV-QKD, we show that diversity can outperform multiplexing in terms of average secret key rate, revealing a diversity advantage over multiplexing in some regimes.

quant-ph↗

arXiv2024

Diversity and Multiplexing in Quantum MIMO Channels

Characterization and exploitation of multiple channels between the transmitter and the receiver in multiple-input multiple-output (MIMO) communications brought a paradigm shift in classical communication systems. The techniques developed around MIMO communication systems not only brought unprecedented advancements in the communication rates but also substantially improved the reliability of communication, measured by low error rates. Here, we explore the same ideas in the paradigm of quantum MIMO communication. Specifically, we utilize approximate quantum cloning to transmit multiple copies of the same quantum state over a MIMO channel that incorporates crosstalk, losses, and depolarizing noise. With this strategy, we find an achievable tradeoff between the average fidelity and communication rate over this MIMO setup.

quant-ph↗

arXiv2024

Physical Layer Aspects of Quantum Communications: A Survey

Quantum communication systems support unique applications in the form of distributed quantum computing, distributed quantum sensing, and several cryptographic protocols. The main enabler in these communication systems is an efficient infrastructure that is capable to transport unknown quantum states with high rate and fidelity. This feat requires a new approach to communication system design which efficiently exploits the available physical layer resources, while respecting the limitations and principles of quantum information. Despite the fundamental differences between the classic and quantum worlds, there exist universal communication concepts that may proven beneficial in quantum communication systems as well. In this survey, the distinctive aspects of physical layer quantum communications are highlighted in a attempt to draw commonalities and divergences between classic and quantum communications. More specifically, we begin by overviewing the quantum channels and use cases over diverse optical propagation media, shedding light on the concepts of crosstalk and interference. Subsequently, we survey quantum sources, detectors, channels and modulation techniques. More importantly, we discuss and analyze spatial multiplexing techniques, such as coherent control, multiplexing, diversity and MIMO. Finally, we identify synergies between the two communication technologies and grand open challenges that can be pivotal in the development of next-generation quantum communication systems.

quant-ph↗

arXiv2023

Characterizing and Utilizing the Interplay between Quantum Technologies and Non-Terrestrial Networks

Quantum technologies are increasingly recognized as groundbreaking advancements set to redefine the landscape of computing, communications, and sensing by leveraging quantum phenomena, like entanglement and teleportation. Quantum technologies offer an interesting set of advantages such as unconditional security, large communications capacity, unparalleled computational speed, and ultra-precise sensing capabilities. However, their global deployment faces challenges related to communication ranges and geographical boundaries. Non-terrestrial networks (NTNs) have emerged as a potential remedy for these challenges through providing free-space quantum links to circumvent the exponential losses inherent in fiber optics. This paper delves into the dynamic interplay between quantum technologies and NTNs to unveil their synergistic potential. Specifically, we investigate their integration challenges and the potential solutions to foster a symbiotic convergence of quantum and NTN functionalities while identifying avenues for enhanced interoperability. This paper not only offers useful insights into the mutual advantages but also presents future research directions, aiming to inspire additional studies and advance this interdisciplinary collaboration.

eess.SY↗

arXiv2021

Matrix-Completion Quantum State Tomography

The deployment of intermediate- and large-scale quantum devices necessitates the development of efficient full state tomographical techniques for quantum benchmarks. Here, we introduce a matrix filling-based method for tomography of pure quantum states, called the matrix-completion quantum state tomography. This method requires only 2n + 1 local Pauli operators and minimal post-processing for n-qubit states. Numerical results show that our method is highly efficient on superconducting real quantum devices and achieves better fidelity estimates of multiqubit quantum states as compared to contemporary pure state tomography methods. These desirable features of the matrix-completion quantum state tomography protocol make it suitable for the benchmarking of intermediate- and large-scale quantum devices dealing mainly with pure quantum states.

quant-ph↗

arXiv2021

Error-mitigated photonic variational quantum eigensolver using a single-photon ququart

We report the experimental resource-efficient implementation of the variational quantum eigensolver (VQE) using four-dimensional photonic quantum states of single-photons. The four-dimensional quantum states are implemented by utilizing polarization and path degrees of freedom of a single-photon. Our photonic VQE is equipped with the quantum error mitigation (QEM) protocol that efficiently reduces the effects of Pauli noise in the quantum processing unit. We apply our photonic VQE to estimate the ground state energy of He--H$^{+}$ cation. The simulation and experimental results demonstrate that our resource-efficient photonic VQE can accurately estimate the bond dissociation curve, even in the presence of large noise in the quantum processing unit.

quant-ph↗

arXiv2021

Adaptive quantum state tomography with iterative particle filtering

Several Bayesian estimation based heuristics have been developed to perform quantum state tomography (QST). Their ability to quantify uncertainties using region estimators and include a priori knowledge of the experimentalists makes this family of methods an attractive choice for QST. However, specialized techniques for pure states do not work well for mixed states and vice versa. In this paper, we present an adaptive particle filter (PF) based QST protocol which improves the scaling of fidelity compared to nonadaptive Bayesian schemes for arbitrary multi-qubit states. This is due to the protocol's unabating perseverance to find the states's diagonal bases and more systematic handling of enduring problems in popular PF methods relating to the subjectivity of informative priors and the invalidity of particles produced by resamplers. Numerical examples and implementation on IBM quantum devices demonstrate improved performance for arbitrary quantum states and the application readiness of our proposed scheme.

quant-ph↗

arXiv2021

Entanglement-Free Parameter Estimation of Generalized Pauli Channels

We propose a parameter estimation protocol for generalized Pauli channels acting on $d$-dimensional Hilbert space. The salient features of the proposed method include product probe states and measurements, the number of measurement configurations linear in $d$, minimal post-processing, and the scaling of the mean square error comparable to that of the entanglement-based parameter estimation scheme for generalized Pauli channels. We also show that while measuring generalized Pauli operators the errors caused by the Pauli noise can be modeled as measurement errors. This makes it possible to utilize the measurement error mitigation framework to mitigate the errors caused by the generalized Pauli channels. We use this result to mitigate noise on the probe states and recover the scaling of the noiseless probes, except with a noise strength-dependent constant factor. This method of modeling Pauli channel as measurement noise can also be of independent interest in other NISQ tasks, e.g., state tomography problems, variational quantum algorithms, and other channel estimation problems where Pauli measurements have the central role.

quant-ph↗

arXiv2021

Self-Guided Quantum State Learning for Mixed States

We provide an adaptive learning algorithm for tomography of general quantum states. Our proposal is based on the simultaneous perturbation stochastic approximation algorithm and is applicable on mixed qudit states. The salient features of our algorithm are efficient ($O \left( d^3 \right)$) post-processing in the dimension $d$ of the state, robustness against measurement and channel noise, and improved infidelity performance as compared to the contemporary adaptive state learning algorithms. A higher resilience against measurement noise makes our algorithm suitable for noisy intermediate-scale quantum applications.

quant-ph↗

arXiv2020

Quantum Anonymity for Quantum Networks

We present the first quantum anonymous notification (QAN) protocol that introduces anonymity and paves the way for anonymous secure quantum communication in quantum networks. QAN protocol has applications ranging from multiparty quantum computation to quantum internet. We utilize the QAN protocol to propose an anonymous quantum private comparison protocol in an $n$-node quantum network. This protocol can compare private information of any $2 \leq k \leq n$ parties with the help of the remaining $n-k$ parties and a semi-honest third party. These protocols feature a traceless property, i.e., encoding operations cannot be traced back to their originating sources. Security analysis shows that this protocol is robust against external adversaries and malicious participants.

quant-ph↗

arXiv2020

Holevo Capacity of Discrete Weyl Channels

Holevo capacity is the maximum rate at which a quantum channel can reliably transmit classical information without entanglement. However, calculating the Holevo capacity of arbitrary quantum channels is a nontrivial and computationally expensive task since it requires the numerical optimization over all possible input quantum states. In this paper, we consider discrete Weyl channels (DWCs) and exploit their symmetry properties to model DWC as a classical symmetric channel. We characterize lower and upper bounds on the Holevo capacity of DWCs using simple computational formulae. Then, we provide a sufficient and necessary condition where the upper and lower bounds coincide. The framework in this paper enables us to characterize the exact Holevo capacity for most of the known special cases of DWCs.

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