arXiv subjects

Chandrashekar Radhakrishnan

Publications and source records attributed to Chandrashekar Radhakrishnan.

At least 19 recordsLinked to original sources

arXiv2026

Influence of Noninertial Dynamics on Static Quantum Resource Theories

The effect of noninertial dynamics on static quantum resource theories is investigated. To this end, we first show the equivalence between noninertial effects and a completely positive, trace-preserving (CPTP) map. In this formulation, the Unruh effect is equivalent to a bosonic amplifier channel. The effect of this map on a generic quantum resource is investigated by studying the role of the CPTP map on the three core ingredients of a resource theory, namely, the free states, the free operations and the resource quantifiers. We show several general statements can be made about these three components of a resource theory in the presence of noninertial motion.

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arXiv2025

The Quantum Internet (Technical Version)

Following the emergence of quantum computing, the subsequent quantum revolution will be that of interconnecting individual quantum computers at global level. In the same way that classical computers only realised their full potential with the emergence of the internet, a fully realised quantum internet is the next stage of evolution for quantum computation. This work examines in detail how the quantum internet would evolve in practice, focusing not only on the technology itself but also on the implications it will have economically and politically. We present both original ideas, as well as an extensive review of relevant and related background material. This work begins with a description of classical networks before introducing the key concepts behind quantum networks, such as quantum internet protocols, quantum cryptography, and cloud quantum computing. The work is divided into technical sections (requiring only a basic knowledge of the notation of quantum mechanics), for those interested in mathematical details, as well as non-technical sections for those seeking a more general understanding. We target this work very broadly at quantum and classical computer scientists, classical computer systems, software and network engineers, physicists, economists, artists, musicians, and those just generally curious about the future of quantum technologies and what they might bring to humanity.

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arXiv2024

Environment engineering to protect quantum coherence in tripartite systems under dephasing noise

The practical success of quantum technology hinges on sustaining quantum coherence, which is vulnerable to environmental interactions causing decoherence. We investigate coherence in tripartite quantum systems under the influence of noisy environment. In this study, we explore the dynamics of the relative entropy of coherence for tripartite pure and mixed states in the presence of structured dephasing environments at finite temperatures. Our findings demonstrate that the system's resilience to decoherence is strongly influenced by the bath type and configuration. Specifically, when each qubit interacts with an independent environment, the coherence dynamics differ from those observed in a shared bath setting. In a Markov, memoryless environment, coherence in both pure and mixed states decay faster, whereas coherence is preserved for longer time in the presence of environment memory. This highlights the crucial role of environment memory in enhancing the robustness of tripartite coherence.

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arXiv2024

Pushing Boundaries: Quantum-Enhanced Leader Election and the Limits of Consensus

This work addresses the complexities involved in designing distributed quantum algorithms, highlighting that quantum entanglement does not bypass the Fischer-Lynch-Paterson (FLP) impossibility theorem in asynchronous networks. Although quantum resources such as entanglement offer potential speedups, the inherent constraints of classical communication remain. We develop a leader election algorithm as a proof of concept, demonstrating how entanglement can enhance efficiency while still contending with asynchronous delays. This algorithm serves as a foundation for a broader blueprint for future distributed quantum algorithms, providing insights into both the real performance gains and the limitations that entanglement offers in a distributed setting.

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arXiv2024

R\'enyi relative entropy based monogamy of entanglement in tripartite systems

A comprehensive investigation of the entanglement characteristics is carried out on tripartite spin-1/2 systems, examining prototypical tripartite states, the thermal Heisenberg model, and the transverse field Ising model. The entanglement is computed using the R\'enyi relative entropy. In the traditional R\'enyi relative entropy, the generalization parameter $\alpha$ can take values only in the range $0 \leq \alpha \leq 2$ due to the requirements of joint convexity of the measure. To use the R\'enyi relative entropy over a wider range of $\alpha$, we use the sandwiched form which is jointly convex in the regime $0.5 \leq \alpha \leq \infty$. In prototypical tripartite states, we find that GHZ states are monogamous, but surprisingly so are W states. On the other hand, star states exhibit polygamy, due to the higher level of purity of the bipartite subsystems. For spin models, we study the dependence of entanglement on various parameters such as temperature, spin-spin interaction, and anisotropy, and identify regions where entanglement is the largest. The R\'enyi parameter $\alpha$ scales the amount of entanglement in the system. The entanglement measure based on the traditional and the sandwiched R\'enyi relative entropies obey the Araki-Lieb-Thirring inequality. In the Heisenberg models, namely the XYZ, XXZ, and XY models, the system is always monogamous. However, in the transverse field Ising model, the state is initially polygamous and becomes monogamous with temperature and coupling.

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arXiv2024

A magic monotone for faithful detection of non-stabilizerness in mixed states

We introduce a monotone to quantify the amount of non-stabilizerness (or magic for short), in an arbitrary quantum state. The monotone gives a necessary and sufficient criterion for detecting the presence of magic for both pure and mixed states. The monotone is based on determining the boundaries of the stabilizer polytope in the space of Pauli string expectation values. The boundaries can be described by a set of hyperplane inequations, where violation of any one of these gives a necessary and sufficient condition for magic. The monotone is constructed by finding the hyperplane with the maximum violation and is a type of Minkowski functional. We also introduce a witness based on similar methods. The approach is more computationally efficient than existing faithful mixed state monotones such as robustness of magic due to the smaller number and discrete nature of the parameters to be optimized.

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arXiv2024

Dephasing-Induced Distribution of Entanglement in Tripartite Quantum Systems

Preserving multipartite entanglement amidst decoherence poses a pivotal challenge in quantum information processing. However, assessing multipartite entanglement in mixed states amid decoherence presenting a formidable task. Employing reservoir memory offers a means to attenuate the decoherence dynamics impacting multipartite entanglement, thereby slowing its degradation. One of the important measures which can be implemented to quantify entanglement is the relative entropy of entanglement. Although this measure is not monogamous \cite{horodeckirev2009}, it can universally be applied to both pure and mixed states. Based on this fundamental novelty, in this work, therefore, we introduce a quantifier which will investigate how entanglement remain distributed among the qubits of multipartite states when these states are exposed to multipartite dephasing setting. For our study we use various pure and mixed tripartite states subjected to finite temperature in both Markovian and non-Markovian local/common bath. Here, we consider situations where the three qubits interact with a common reservoir as well as a local bosonic reservoir. We also show that the robustness of a quantum system to decoherence depends on the distribution of entanglement and its interaction with various configurations of the bath. When each qubit has its own local environment, the system exhibits different distribution dynamics compared to when all three qubits share a common environment with one exception regarding a mixed state.

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arXiv2024

The role of thermal and squeezed photons in the entanglement dynamics of the double Jaynes-Cummings model

The effects of squeezed photons and thermal photons on the entanglement dynamics of atom-atom, atom-field and field-field subsystems are studied for the double Jaynes-Cummings model. For this purpose, squeezed coherent states and Glauber-Lachs states of radiation are chosen as field states. For the atomic states, we choose one of the Bell state as pure state and a Werner-type state as mixed state. Werner-type state is used to understand the effects of mixedness on entanglement. To measure the entanglement between the two atoms, Wootters' concurrence is used; whereas for the atom-field and field-field subsystems, negativity is chosen. The squeezed photons and thermal photons create, destroy and transfer entanglement within various subsystems. Also, the addition of squeezed photons and thermal photons either lengthens or shortens the duration of entanglement sudden deaths (ESD) associated with atom-atom, atom-field and field-field entanglement dynamics in a complementary way. The effects of Ising-type interaction, detuning and Kerr-nonlinearity on the entanglement dynamics are studied. Each of these interactions removes the ESDs associated with various subsystems. We show that new entanglements are created in this atom-field system by introducing Ising-type interaction between the two atoms. With proper choice of the parameters corresponding to Ising-type interaction, detuning and Kerr-nonliearity, entanglement can be transferred among various subsystems.

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arXiv2024

Enhanced quantum synchronization of a driven qubit under non-Markovian dynamics

Synchronizing a few-level quantum system is of fundamental importance to the understanding of synchronization in the deep quantum regime. We investigate quantum phase synchronization of a two-level system (qubit) driven by a semiclassical laser field, in the presence of a general non-Markovian dissipative environment. The phase preference of the qubit is demonstrated through Husimi Q-function, and the existence of a limit cycle is also shown in our system. Synchronization of the qubit is quantified using the shifted phase distribution. The signature of quantum phase synchronization viz the Arnold tongue is obtained from the maximal value of the shifted phase distribution. Two distinct types of qubit dynamics is considered depending on the reservoir correlation time being very short and a situation when bath correlation time is finite. In the Markov regime of the environment, the phase preference of the qubit goes away in the long time limit, whereas the long-time phase localization persists in the non-Markovian regime. We also plot the maximum of the shifted phase distribution in two ways: (a) by varying the detuning and laser driving strength, and (b) by varying the system-bath coupling and laser driving strength. Various system-environment parameters determine the synchronization regions and the qubit phase synchronization is shown to be enhanced in the non-Markov regime.

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arXiv2023

Entanglement preservation in tripartite quantum systems under dephasing dynamics

Protecting entanglement from decoherence is a critical aspect of quantum information processsing. For many-body quantum systems evolving under decoherence, estimating multipartite entanglement is challenging. This challenge can be met up by considering distance based measure such as relative entropy of entanglement which decisively measures entanglement in both pure as well as mixed states. In this work, we investigate the tripartite entanglement dynamics of pure and mixed states in the presence of a structured dephasing environment at finite temperature. We show that the robustness of the quantum system to decoherence is dependent on the distribution of entanglement and its relation to different configurations of the bath. If the bath is structured individually such that each qubit has its own environment, the system has different dynamics compared to when the bath is common to all the three qubits. From the results we conjecture that there is a connection between the distribution of entanglement among the qubits and the distribution of bath degrees of freedom, and the interplay of these two distributions determines the decay rate of the entanglement dynamics. The sustainability of tripartite entanglement is shown to be enhanced significantly in presence of reservoir memory.

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arXiv2022

Exploring quantum properties of bipartite mixed states under coherent and incoherent basis

Quantum coherence and quantum entanglement are two different manifestations of the superposition principle. In this article we show that the right choice of basis to be used to estimate coherence is the separable basis. The quantum coherence estimated using the Bell basis does not represent the coherence in the system, since there is a coherence in the system due to the choice of the basis states. We first compute the entanglement and quantum coherence in the two qubit mixed states prepared using the Bell states and one of the states from the computational basis. The quantum coherence is estimated using the l1-norm of coherence, the entanglement is measured using the concurrence and the mixedness is measured using the linear entropy. Then we estimate these quantities in the Bell basis and establish that coherence should be measured only in separable basis, whereas entanglement and mixedness can be measured in any basis. We then calculate the teleportation fidelity of these mixed states and find the regions where the states have a fidelity greater than the classical teleportation fidelity. We also examine the violation of the Bell-CHSH inequality to verify the quantum nonlocal correlations in the system. The estimation of the above mentioned quantum correlations, teleportation fidelity and the verification of Bell-CHSH inequality is also done for bipartite states obtained from the tripartite systems by the tracing out of one of their qubits. We find that for some of these states teleportation is possible even when the Bell-CHSH inequality is not violated, signifying that nonlocality is not a necessary condition for quantum teleportation.

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arXiv2022

Quantum synchronization due to information backflow

The phase synchronization of a single qubit in a dissipative bath in the absence of driving field is demonstrated. Using the Husimi $Q$-function we show that the phase preference is present in the long time limit only during non-Markovian evolution with a finite detuning. This happens due to the information backflow signifying that non-Markovianity is a resource for quantum synchronization. To quantify synchronization we use the shifted phase distribution as well as its maximal value. From the maximal value of the shifted phase distribution we observe the signatures of quantum synchronization {\it viz} the Arnold tongue. In our case the region ofsynchronization is outside the tongue region and the region inside the tongue is the desynchronized region. This is in contrast to the results in the literature, where the synchronization is within the tongue region.

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arXiv2022

Accessible and inaccessible quantum coherence in relativistic quantum systems

The quantum coherence of a multipartite system is investigated when some of the parties are moving with uniform acceleration and the analysis is carried out using the single mode approximation. Due to acceleration the quantum coherence is divided into two parts as accessible and inaccessible coherence and the entire analysis has been carried out in the single-mode approximation. First we investigate tripartite systems, considering both GHZ and W-states. We find that the quantum coherence of these states does not vanish in the limit of infinite acceleration, rather asymptoting to a non-zero value. These results hold for both single- and two-qubit acceleration. In the GHZ and W-states the coherence is distributed as correlations between the qubits and is known as global coherence. But quantum coherence can also exist due to the superposition within a qubit, the local coherence. To study the properties of local coherence we investigate separable state. The GHZ state, W-state and separable states contain only one type of coherence. Next we consider the $W \bar{W}$ and star states in which both local and global coherences coexist. We find that under uniform acceleration both local and global coherence show similar qualitative behaviour. Finally we derive analytic expressions for the quantum coherence of N-partite GHZ and W-states for n < N accelerating qubits. We find that the quantum coherence of a multipartite GHZ state falls exponentially with the number of accelerated qubits, whereas for multipartite W-states the quantum coherence decreases only polynomially. We conclude that W-states are more robust to Unruh decoherence and discuss some potential applications in satellite-based quantum communication and black hole physics.

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arXiv2022

Dynamical Crossover from Markovian to Non-Markovian dynamics in the strong coupling regime

The transient dynamics of quantum coherence of Gaussian states are investigated. The state is coupled to an external environment which can be described by a Fano-Anderson type Hamiltonian. Solving the quantum Langevin equation, we obtain the Greens functions which are used to compute the time evolved first and second moments of the quadrature operators. From the quadrature operator moments, we construct the covariance matrix which is used to measure the coherence in the system. The coherence is measured using the relative entropy of coherence measure. We consider three different classes of spectral densities in our analysis viz, the Ohmic, the sub-Ohmic, and the super-Ohmic densities. In our work, we study the dynamics of the coherent state, squeezed state, and displaced squeezed state. For all these states we observe that when the coupling with the system and the environment is weak, the coherence monotonically decreases and eventually vanishes in a long time. Thus all the states exhibit Markovian evolution in the weak coupling limit. In the strong coupling limit, the dynamics for the initial period is Markovian and after a certain period, it becomes non-Markovian where we observe an environmental backaction on the system. Thus in the strong coupling limit, we observe a dynamical crossover from Markovian nature to non-Markovian behavior. This crossover is very abrupt under some environmental conditions and for some parameters of the quantum state. Using a quantum master equation approach we verify the crossover from the dynamics of the dissipation and fluctuation parameters and the results endorse those obtained from coherence dynamics.

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arXiv2021

Prediction of mass of $η_{c}$ (2S) using variational method

The suitability of using non-relativistic quantum mechanics to investigate heavy quark mesons is illustrated through a study of the charmonium meson. We consider a limiting form of the QCD potential which is a simple combination of the linear and Coulomb potential. The experimentally determined masses of $J/ψ(1S)$ and $χ_{c1}(1P)$ are reproduced for $m_{c} \approx 1.1 GeV$. For $ψ(2S)$ we have three different sets of variational parameters and to choose the appropriate one we use the leptonic decay width of $ψ(2S)$ and $J/ψ(1S)$. Finally we use a spin-spin interaction to investigate the hyperfine splitting of Charmonium and use it to calculate the mass of $η_{c}(2S)$. Our theoretical results agree with the experimentally measured values of $η_{c}(2S)$ and thereby verifies the usefulness of non-relativistic quantum mechanics in the study of heavy quark meson.

hep-ph↗

arXiv2021

Thermal coherence of the Heisenberg model with Dzyaloshinsky-Moriya interactions in an inhomogenous external field

The quantum coherence of the two-site XYZ model with Dzyaloshinsky-Moriya (DM) interactions in an external inhomogenous magnetic field is studied. The DM interaction, the magnetic field and the measurement basis can be along different directions, and we examine the quantum coherence at finite temperature. With respect to the spin-spin interaction parameter, we find that the quantum coherence decreases when the direction of measurement basis is the same as that of the spin-spin interaction. When the spin-lattice interaction is varied, the coherence always increases irrespective of the relation between its direction and the measurement basis. Similar analysis of quantum coherence based on the variation of the external inhomogenous magnetic field is also carried out, where we find that the coherence decreases when the direction of the measurement basis is the same as that of the external field.

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arXiv2020

Experimental study of quantum coherence decomposition and trade-off relations in a tripartite system

Quantum coherence is the most fundamental of all quantum quantifiers, underlying other well-known quantities such as entanglement, quantum discord, and Bell correlations. It can be distributed in a multipartite system in various ways -- for example, in a bipartite system it can exist within subsystems (local coherence) or collectively between the subsystems (global coherence) and exhibits a trade-off relation. In quantum systems with more than two subsystems, there are more trade-off relations, due to the various decomposition ways of the coherence. In this paper, we experimentally verify these coherence trade-off relations in adiabatically evolved quantum systems using a spin system by changing the state from a product state to a tripartite entangled state. We study the full set of coherence trade-off relations between the original state, the bipartite product state, the tripartite product state, and the decohered product state. We also experimentally verify the monogamy inequality and show that both the quantum systems are polygamous except for the initial product state. We find that despite the different types of states involved, the properties of the state in terms of coherence and monogamy are equivalent. This illustrates the utility of using coherence as a characterization tool for quantum states.

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arXiv2020

A multipartite generalization of quantum discord

A generalization of quantum discord to multipartite systems is proposed. A key feature of our formulation is its consistency with the conventional definition of discord in bipartite systems. It is by construction zero only for systems with classically correlated subsystems and is a non-negative quantity, giving a measure of the total non-classical correlations in the multipartite system with respect to a fixed measurement ordering. For the tripartite case, we show that the discord can be decomposed into contributions resulting from changes induced by non-classical correlation breaking measurements in the conditional mutual information and tripartite mutual information. The former gives a measure of the bipartite non-classical correlations and is a non-negative quantity, while the latter is related to the monogamy of the non-classical correlations.

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