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The way a new type of state called a hybrid state, which contains more than one degree of freedom, is used in many practical applications of quantum communication tasks with lesser amount of resources. Similarly, our aim is here to perform multi-quantum communication tasks in a protocol to approach quantum information in multipurpose and multi-directional. We propose a hybrid multi-directional six-party scheme of implementing quantum teleportation and joint remote state preparation under the supervision of a controller via a multi-qubit entangled state as a quantum channel with 100% success probability. Moreover, we analytically derive the average fidelities of this hybrid scheme under the amplitude-damping and the phase-damping noise., Comment: 3 figures, 16 pages

Quantum key distribution (QKD) is a technique to establish a secret key between two parties through a quantum channel. Several QKD protocols have been proposed and implemented over optical fibers or free space links. The main challenge of operating QKD protocols over a free space link is atmospheric losses. In this paper, we have studied and compared the performance of single and entangled photon based QKD protocols by evaluating the quantum bit error rate and secure key rate for terrestrial free-space quantum communication by considering different free-space losses, such as geometrical losses, atmospheric losses as well as device imperfections., Comment: 9 Figures, 2 Table, 17 Pages

In this study, we have theoretically presented a prepare-and-measure-based SARG04 protocol over free space. It has shown that the highest secret key rate is possible even under free-space losses with a maximum tolerance of noise., Comment: 4 pages, 1 figure

Recently, Ri-Gui Zhou et al. [Int. J. Theor. Phys. 59, 166-172 (2020)] proposed a scheme for bidirectional quantum teleportation of two-two and two-three qubit states by utilizing a six-qubit entangled state as a quantum channel. It is observed that the intrinsic efficiency calculated by Zhou et al. is not correct. In this improved scheme, we show that bidirectional teleportation of two-two and two-three qubit quantum states can be done by using optimized quantum resource and less consumption of classical resource. We also calculate the intrinsic efficiency of our improved scheme which is much better than the Zhou's scheme. Additionally, we also discuss about the security of the protocol., Comment: 6 pages

Bidirectional quantum teleportation scheme is a two-way quantum communication process, in which two parties simultaneously receive each other information. Recently, in paper [1] (Zhou et al., IEEE Access, 7, 44269 (2019)), a six-qubit cluster state has been used to teleport three-qubit entangled state (Alice) and arbitrary single-qubit unknown quantum state (Bob) bidirectionally. An experimental point of view, the preparation and the maintenance of cluster type entangled state is very difficult and costly. So, in this paper we have designed an optimal scheme for bidirectional quantum teleportation scheme and shown that proposed scheme requires optimized amount of quantum resources., Comment: 1 figure, 2 table

In the present study, we have proposed a scheme for two-way quantum communication in which the two legitimate participants mutually exchange their quantum information to each other by using a four-qubit cluster state as the quantum channel. Recently, by utilizing a four-qubit cluster state as the quantum channel, Kazemikhah et al. [Int. J. Theor. Phys., 60 (2021) 378] tried to design a scheme for the mutual exchange of quantum information between two legitimate participants. However, in the present study, it has been shown that in their scheme the transmission of quantum information cannot be realized because the two participants are not entangled to each other due to a trivial conceptual mistake made by Kazemikhah et al. in the description of the quantum channel. Here, we have shown that two legitimate participants can teleport quantum information states to each other by using a four-qubit cluster state as the quantum channel, provided they co-operate with each other and perform non-local controlled phase gate operation. If both participants do not co-operate with each other, then no one can reconstruct the information sent to them, and therefore the exchange of information is possible only when both participants are honest to each other., Comment: 8 pages, 2 tables

We proposed two entanglement concentration protocols (ECPs) to obtain maximally entangled Greenberger-Horne-Zeilinger (GHZ)-type entangled coherent state (ECS) from the corresponding partially entangled GHZ-type ECSs. We obtained the first ECP using a partially entangled GHZ-type ECS assisted with a superposition of single-mode coherent state, however the second ECP is designed using two copies of partially entangled GHZ-type ECSs. The success probabilities have also been calculated and discussed for both the ECPs. We have further compared the success probabilities of our first ECP for 3-mode GHZ-type ECS with an ECP of 3-mode W-type ECS and found that our ECP is more efficient (maximal success probabilities) for larger value (\beta=0.7) of state parameter. For the physical realization, two optical circuits (for two ECPs) using linear optical elements, viz 50:50 beam splitter, phase shifter, and photon detectors are provided, which support the future experimental implementation possible with the present technology., Comment: 9 pages, 5 figures

This thesis is focused on the design and analysis of quantum communication protocols. Several schemes for quantum communication have been introduced in the recent past. For example, quantum teleportation, dense coding, quantum key distribution, quantum secure direct communication, etc., have been rigorously studied in the last 2-3 decades. Specifically, a specific attention of the present thesis is to study the quantum teleportation schemes with entangled orthogonal and nonorthogonal states and their experimental realization, but not limited to it. We have also studied some aspects of quantum cryptography., Comment: Ph.D. Thesis. Optimal quantum communication schemes with proof of principle experiments on IBM quantum experience and optical designs of quantum cryptographic schemes. Based on arXiv:1610.02514, arXiv:1705.00670, arXiv:1704.05294, arXiv:1804.01776

Several quantum cryptographic schemes have been proposed and realized experimentally in the past. However, even with an advancement in quantum technology and escalated interest in the designing of direct secure quantum communication schemes there are not many experimental implementations of these cryptographic schemes. In this paper, we have provided a set of optical circuits for such quantum cryptographic schemes, which have not yet been realized experimentally by modifying some of our theoretically proposed secure communication schemes. Specifically, we have proposed optical designs for the implementation of two single photon and one entangled state based controlled quantum dialogue schemes and subsequently reduced our optical designs to yield simpler designs for realizing other secure quantum communication tasks, i.e., controlled deterministic secure quantum communication, quantum dialogue, quantum secure direct communication, quantum key agreement, and quantum key distribution. We have further proposed an optical design for an entanglement swapping based deterministic secure quantum communication and its controlled counterpart., Comment: Explicit optical designs for the implementation of secure direct quantum communication are provided

We proposed two linear optics based entanglement concentration protocols (ECPs) to obtain maximally entangled 4-mode Cluster-type entangled coherent state (ECS) from less (partially) entangled Cluster-type ECS. The first ECP is designed using a superposition of single-mode coherent state with two unknown parameters, whereas the second ECP is obtained using a superposition of single-mode coherent state and a superposition of two-mode coherent state with four unknown parameters. The success probabilities have been calculated for both the ECPs. Necessary quantum circuits enabling future experimental realizations of the proposed ECPs are provided using linear optical elements. Further, the benefit of the proposed schemes is established in the context of long distance quantum communication where photon loss is an obstruction., Comment: 11 pages, 3 figures

Recently, various quantum computing and communication tasks have been implemented using IBM's superconductivity-based quantum computers which are available on the cloud. Here, we show that the circuits used in most of those works were not optimized and the use of the optimized circuits can considerably improve the possibility of observing unique features of quantum mechanics. Specifically, a systematic procedure is used here to obtain optimized circuits (circuits having reduced gate count and number of levels) for a large number of Clifford+T circuits which have already been implemented in the IBM quantum computers. Optimized circuits implementable in IBM quantum computers are also obtained for a set of reversible benchmark circuits. With a clear example, it is shown that the reduction in circuit costs enhances the fidelity of the output state (with respect to the theoretically expected state in the absence of noise) as lesser number of gates and levels introduce lesser amount of errors during evolution of the state. Further, considering Mermin inequality as an example, it's shown that the violation of classical limit is enhanced when we use an optimized circuit. Thus, the approach adopted here can be used to identify relatively weaker signature of quantumness and also to establish quantum supremacy in a stronger manner., Comment: Circuits implemented in IBM quantum processors have been optimized using a systematic procedure and considering Mermin inequality as an example, it's shown that the violation of classical limit is enhanced when an optimized circuit is used

Quantum process tomography of each directly implementable quantum gate used in the IBM quantum processors is performed to compute gate error in order to check viability of complex quantum operations in the superconductivity-based quantum computers introduced by IBM and to compare the quality of these gates with the corresponding gates implemented using other technologies. Quantum process tomography (QPT) of C-NOT gates have been performed for three configurations available in IBM QX4 processor. For all the other allowed gates QPT have been performed for every allowed position (i.e., by placing the gates in different qubit lines) for IBM QX4 architecture, and thus, gate fidelities are obtained for both single-qubit and 2-qubit gates. Gate fidelities are observed to be lower than the corresponding values obtained in the other technologies, like NMR. Further, gate fidelities for all the single-qubit gates are obtained for IBM QX2 architecture by placing the gates in the third qubit line ($q[2]$). It's observed that the IBM QX4 architecture yields better gate fidelity compared to IBM QX2 in all cases except the case of $\operatorname{Y}$ gate as far as the gate fidelity corresponding to the third qubit line is concerned. In general, the analysis performed here leads to a conclusion that a considerable technological improvement would be inevitable to achieve the desired scalability required for the realization of complex quantum operations., Comment: Quantum Process tomography has been done for all the gates used in IBM QX2 and IBM QX4

Recently, Zhao et al., (Int. J. Theor. Phys. 57, 516-522 (2018)) have proposed a scheme for quantum teleportation of an eight-qubit quantum state using a six qubit cluster state. In this comment, it's shown that the quantum resource (multi-partite entangled state used as the quantum channel) used by Zhao et al., is excessively high and the task can be performed using any two Bell states as the task can be reduced to the teleportation of an arbitrary two qubit state. Further, a trivial conceptual mistake made by Zhao et al., in the description of the quantum channel has been pointed out. It's also mentioned that recently a trend of proposing teleportation schemes with excessively high quantum resources has been observed and the essence of this comment is applicable to all such proposals., Comment: It is shown that teleportation of multi qubit-states can be done using Bell states

A scheme for distributed quantum measurement that allows nondestructive or indirect Bell measurement was proposed by Gupta et al., (Int. J. Quant. Infor. \textbf{5} (2007) 627) and subsequently realized experimentally using an NMR-based three-qubit quantum computer by Samal et al., (J. Phys. B, \textbf{43} (2010) 095508). In the present work, a similar experiment is performed using the five-qubit super-conductivity-based quantum computer, which has been recently placed in cloud by IBM Corporation. The experiment confirmed that the Bell state can be constructed and measured in a nondestructive manner with a reasonably high fidelity. A comparison of the outcomes of this study and the results obtained earlier in the NMR-based experiment has also been performed. The study indicates that to make a scalable SQUID-based computer, errors by the gates (in the present technology) have to be reduced considerably., Comment: 7 figures,13 pages including 1 appendix

An explicit scheme (quantum circuit) is designed for the teleportation of an $n$-qubit quantum state. It is established that the proposed scheme requires an optimal amount of quantum resources, whereas larger amount of quantum resources has been used in a large number of recently reported teleportation schemes for the quantum states which can be viewed as special cases of the general $n$-qubit state considered here. A trade off between our knowledge about the quantum state to be teleported and the amount of quantum resources required for the same is observed. A proof of principle experimental realization of the proposed scheme (for a 2-qubit state) is also performed using 5-qubit superconductivity-based IBM quantum computer. Experimental results show that the state has been teleported with high fidelity. Relevance of the proposed teleportation scheme has also been discussed in the context of controlled, bidirectional, and bidirectional-controlled state teleportation., Comment: 11 pages 4 figures

The effect of non-orthogonality of an entangled non-orthogonal state based quantum channel is investigated in detail in the context of the teleportation of a qubit. Specifically, average fidelity, minimum fidelity and minimum assured fidelity (MASFI) are obtained for teleportation of a single qubit state using all the Bell type entangled non-orthogonal states known as quasi Bell states. Using Horodecki criterion, it is shown that the teleportation scheme obtained by replacing the quantum channel (Bell state) of the usual teleportation scheme by a quasi Bell state is optimal. Further, the performance of various quasi Bell states as teleportation channel is compared in an ideal situation (i.e., in the absence of noise) and under different noise models (e.g., amplitude and phase damping channels). It is observed that the best choice of the quasi Bell state depends on the amount non-orthogonality, both in noisy and noiseless case. A specific quasi Bell state, which was found to be maximally entangled in the ideal conditions, is shown to be less efficient as a teleportation channel compared to other quasi Bell states in particular cases when subjected to noisy channels. It has also been observed that usually the value of average fidelity falls with an increase in the number of qubits exposed to noisy channels (viz., Alice's, Bob's and to be teleported qubits), but the converse may be observed in some particular cases., Comment: 14 pages, 4 figures