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V. Ravishankar

Publications and source records attributed to V. Ravishankar.

At least 19 recordsLinked to original sources
arXiv2023

Integrated semi-quantum layered communication

In recent times, secure quantum communication in layered networks has emerged as an important area of study. In this paper, we harness the potential offered by multidimensional states in secure quantum communication with only one quantum participant and all the other classical participants. We propose three protocols for (i) entanglement-based layered semi--quantum key distribution, (ii) layered semi-quantum secret sharing, and, (iii) integrated layered semi-quantum key distribution and secret sharing to share secret information in an arbitrarily layered network. These protocols integrate the features of semi quantum communication in layered networks. All three protocols allow for the simultaneous distribution of secure information in all the layers of a network, thanks to the employment of multidimensional states. We present these protocols for a small network of at most five participants and three layers and show the robustness of the same against various eavesdropping strategies. Finally, we provide a detailed procedure for the generalization of the proposed protocols to distribute keys/secrets in any arbitrarily structured quantum network.

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arXiv2023

Relation between nonclassical features through logical qudits

Scalable modern-time fault-tolerant quantum computation and quantum communication in a network employ a large number of physical qubits. For example, IBM is reported to have made a 127-qubit quantum computer. Unlike classical computation, quantum computation employs different types of logical qubits and qudits in terms of physical multiqubit and multiqudit systems respectively. Given this, of particular interest to us is to enquire on how quantum coherence in logical qubits is a manifestation of underlying quantum correlations in constituent physical multiqubit systems and vice-versa. In a recent work [Asthana, Sooryansh. New J Phys 24.5 (2022): 053026], we have shown that there is reciprocity in nonclassical correlations in physical multiqubit systems and coherence in a single logical qubit system. Subsequently, we have generalised the framework to higher dimensional quantum systems []. The crux of this study is that a single nonclassicality condition derived for quantum coherence in a logical system detects more than one type of nonclassicality in Hilbert spaces of nonidentical dimensions.

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arXiv2023

Task-dependent semi-quantum secure communication in layered networks with OAM states of light

Secure communication in layered networks having differently preferred participants has attracted a lot of research attention. Protocols for key distribution in a layered network have been recently proposed in [M. Pivoluska et al., Phys. Rev. A 97, 032312] by employing asymmetrically entangled multiqudit states. Due to the employment of asymmetrically entangled multiqudit states, the yield of these protocols is very low. To address this issue, in this work, we have proposed semi-quantum secure communication protocols by employing separable states only which give a better yield and a higher key generation rate. As illustrations, we present two representative protocols. The first protocol allows sharing of two keys simultaneously in a network of two layers. The second protocol facilitates direct communication in one layer and key distribution in the other. The separable states, i.e., coherent pulses of orbital angular momentum required in the protocols are easily realizable with current technologies.

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arXiv2023

Boosted quantum and semi-quantum communication protocols

Secure quantum communication protocols based on a prepare-and-measure scheme employ mutually unbiased bases. In these protocols, many runs, in which different participants measure in different bases, simply go wasted. In this paper, we show that it is possible to reduce the number of such runs by a suitable design of the key generation rule. This results in a significant increase in the key generation rate (KGR). We illustrate this advantage by proposing quantum and semi-quantum key distribution protocols by employing effective qubits encoded in higher dimensional quantum systems. None of them demands the preparation of entangled states as resources and a relatively large amount of information can be transferred. For this reason, we believe that our proposals are worth pursuing experimentally.

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arXiv2023

Nonclassical features in higher-dimensional systems through logical qudits

In a recent work [S. Asthana. New Journal of Physics 24.5 (2022): 053026], we have shown the interrelation of different nonclassical correlations in multiqubit systems with quantum coherence in a single logical qubit. In this work, we generalize it to higher-dimensional systems. For this, we take different choices of logical qudits and logical continuous-variable (cv) systems in terms of their constituent physical qudits and physical cv systems. Thereafter, we show reciprocity between conditions for coherence (in logical qudits and logical cv systems) and conditions for nonlocality and entanglement (in their underlying constituent qudits and cv systems). This shows that a single nonclassicality condition detects different types of nonclassicalities in different physical systems. Thereby, it reflects the interrelations of different nonclassical features of states belonging to Hilbert spaces of nonidentical dimensions.

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arXiv2023

Quantum and semi--quantum key distribution in networks

In this paper, we utilize the potential offered by multidimensional separable states (MSS) for secure and simultaneous distributions of keys in a layered network. We present protocols for both quantum and semi-quantum key distribution and discuss their robustness against various eavesdropping strategies. We provide a procedure to identify the requisite resource states to generalise these protocols for arbitrary layered networks. Finally, we study the interrelation between the local dimensionalities of states and achievable key rates in a given layer. These proposals are realisable with current technology, thanks to employment of MSS and many advances in generation, manipulation and measurement of higher-dimensional orbital angular momentum states of light.

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arXiv2022

Combating errors in propagation of orbital angular momentum modes of light in turbulent media

There is a wealth of simulation, experimental and analytical studies on propagation of orbital angular momentum (OAM) modes through atmospheric and oceanic turbulence.} Using the data of these studies and generalising the framework proposed in [Bala et al., [arXiv:2208.04555] for error-immune information transfer, we accomplish two tasks. First, we identify invariants for propagation of OAM modes through atmospheric and oceanic turbulence, in which error-immune information can be encoded. A closer look at the data reveals two universal features: (i)coherence lasts for a much longer distance in turbulence than entanglement, and, (ii) the crosstalk among different OAM modes depends very weakly on the initial OAM mode index in the weak turbulence regime. Keeping these in mind, we next develop a method for combating errors in what we call an idealised crosstalk channel. In an idealised crosstalk channel, the crossover probabilities are independent of the initial mode index (IMI). We lay down a procedure that allows to retrieve full information in a state by identifying invariant quantities. Finally, we construct quantum error correction and rejection codes for idealised crosstalk channels, without any need for multiparty entanglement.

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arXiv2022

Combating quantum errors: an integrated approach

Near-term quantum communication protocols suffer inevitably from channel noises, whose alleviation has been mostly attempted with resources such as multiparty entanglement or sophisticated experimental techniques. Generation of multiparty higher dimensional entanglement is not easy. This calls for exploring realistic solutions which are implementable with current devices. Motivated particularly by the difficulty in generation of multiparty entangled states, in this paper, we have investigated error-free information transfer with minimal requirements. For this, we have proposed a new information encoding scheme for communication purposes. The encoding scheme is based on the fact that most noisy channels leave some quantities invariant. Armed with this fact, we encode information in these invariants. These invariants are functions of expectation values of operators. This information passes through the noisy channel unchanged. Pertinently, this approach is not in conflict with other existing error correction schemes. In fact, we have shown how standard quantum error-correcting codes emerge if suitable restrictions are imposed on the choices of logical basis states. As applications, for illustration, we propose a quantum key distribution protocol and an error-immune information transfer protocol.

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arXiv2021

State transfer with separable optical beams and variational quantum algorithms with classical light

Classical electromagnetic fields and quantum mechanics -- both obey the principle of superposition alike. This opens up many avenues for simulation of a large variety of phenomena and algorithms, which have hitherto been considered quantum mechanical. In this paper, we propose two such applications. In the first, we introduce a new class of beams, called equivalent optical beams, in parallel with equivalent states introduced in [Bharath & Ravishankar, https://doi.org/10.1103/PhysRevA.89.062110]. These beams have the same information content for all practical purposes. Employing them, we show how to transfer information from one degree of freedom of classical light to another, without any need for classically entangled beams. Next, we show that quantum machine learning can be performed with OAM beams through the implementation of a quantum classifier circuit. We provide explicit protocols and explore the possibility of their experimental realization.

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arXiv2021

Error-immune quantum communication

Environmental effects on the transmission of a state result, in general, in a change in the information carried by it. To mitigate this, many techniques such as quantum error--correcting codes, decoherence--free--subspaces [Rev Mod Phys, 88(4):041001, 2016] are employed. The basic idea underlying them is to protect/recover the state. These techniques require multi-party entanglement, whose generation is a difficult task. Further, retrieval of information would require complete tomography, which inevitably requires a large number of copies. Taking this into account, in this work, a formalism has been laid down which does not require recovery of a state. The formalism employs scaling laws to obtain quantities that remain invariant under a noisy evolution of a state. The information encoded in these invariant quantities can be transmitted in an error-immune manner. Since multiparty entanglement and error detection/correction will not be required, the proposed scheme would be cost-effective and may be reliably employed for error-free information transfer. Employing the formalism, we have obtained invariant quantities for various noisy channels of a qubit and a quNit.

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arXiv2021

Quantum communication with $SU(2)$ invariant separable $2\times N$ level systems

Information is encoded in a qubit in the form of its Bloch vector. In this paper, we propose protocols for remote transfers of information in a known and an unknown qubit to qudits using $SU(2)$- invariant $\frac{1}{2}\otimes S$ discordant states as a channel. These states have been identified as separable equivalents of the two-qubit entangled Werner states in [Bharath & Ravishankar, Phys. Rev. A 89, 062110]. Due to $SU(2) \times SU(2)$ invariance of these states, the remote qudit can be changed by performing appropriate measurements on the qubit. We also propose a protocol for transferring information of an unknown qudit to a remote qudit using a $\frac{1}{2}\otimes S$- state as a channel. Finally, we propose a protocol for swapping of quantum discord from $\frac{1}{2}\otimes S$- systems to $S\otimes S$- systems. All the protocols proposed in this paper involve separable states as quantum channels. Employing these protocols, we believe that quantum information processing can be performed using highly mixed separable higher dimensional states.

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arXiv2021

Contextuality based quantum conferencing

Nonlocality inequalities for multi-party systems act as contextuality inequalities for single qudit systems of suitable dimensions. In this paper, we propose the procedure for adaptation of nonlocality-based quantum conferencing protocols (QCPs) to contextuality-based QCPs. Unlike the nonlocality-based protocols, the proposed protocols do not involve nonlocal states. These protocols can also be implemented in a class of noisy channels without any compromise in the key generation rate. As an illustration of the procedure, we present a QCP based on Mermin's contextuality inequality. As a significant improvement, we propose a QCP based on CHSH contextuality inequality involving only four-dimensional states irrespective of the number of parties sharing the key. The key generation rate of the latter is twice that of the former. Although these QCPs allow for an eavesdropping attack which has no analog in nonlocality-based QCPs, a way out of this attack is demonstrated. Finally, we show the feasibility of experimental implementation of these protocols with orbital angular momentum (OAM) states.

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arXiv2021

Weak measurements, non-classicality and negative probability

This paper establishes a direct, robust and intimate connection between (i) non classicality tests for various quantum features, e.g., non-Boolean logic, quantum coherence, nonlocality, quantum entanglement, quantum discord; (ii) negative probability, and (iii) anomalous weak values. It has been shown [Adhikary et al. Eur. Phys. J. D, 74(68):68, 2020] that nonexistence of a classical joint probability scheme gives rise to sufficiency conditions for nonlocality, a nonclassical feature not restricted to quantum mechanics. The conditions for nonclassical features of quantum mechanics are obtained by employing pseudo probabilities, which are expectation values of the parent pseudo projections. The crux of the paper is that the pseudo-probabilities, which can take negative values, can be directly measured as anomalous weak values. We expect that this opens up new avenues for testing nonclassicality via weak measurements, and also gives deeper insight into negative pseudo probabilities which become measurable. A quantum game, based on violation of classical probability rule is also proposed that can be played by employing weak measurements.

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arXiv2020

Non-locality and entanglement in multi-qubit systems from a unified framework

Non-classical probability is the underlying feature of quantum mechanics. The emergence of Bell-CHSH non-locality for bipartite systems and linear entanglement inequalities for two-qubit systems has been shown in Adhikary et al. 2020 [Eur. Phys. J. D 74, 68 (2020)], purely as violations of classical probability rules. In this paper, we improve upon that work by showing that violation of any nonlocality inequality implies violation of classical probability rules, manifested through negative probabilities, without recourse to any underlying theory. Moving on to entanglement, we employ parent pseudoprojections to show how any number of linear and nonlinear entanglement witnesses for multiqubit systems can be obtained as violations of classical probability rules. They include the ones that have been derived earlier by employing different methods. It provides a perspective complementary to the current understanding in terms of the algebraic approaches.

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arXiv2019

A generalised framework for non-classicality of states II: Emergence of non locality and entanglement

A unified formalism was developed in [S. Adhikary et. al., arXiv:1710.04371 [quant-ph]], for describing non-classicality of states by introducing pseudo projection operators in which both quantum logic and quantum probability are naturally embedded. In this paper we show, as the first practical application, how non-locality and entanglement emerge as two such important manifestations. It provides a perspective complementary to (i) the understanding of them that we have currently (in terms of LHV models) and (ii) to the algebraic approaches employed. The work also makes it possible to obtain, in a systematic manner, an infinite number of conditions for non-classicality, for future applications.

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arXiv2019

A generalised framework for non-classicality of states

Non-classical probability (along with its underlying logic) is a defining feature of quantum mechanics. A formulation that incorporates them, inherently and directly, would promise a unified description of seemingly different prescriptions of non-classicality of states that have been proposed so far. This paper sets up such a formalism. It is based on elementary considerations, free of ad-hoc definitions, and is completely operational. It permits a systematic construction of non-classicality conditions on states and also to quantify the non-classicality, at the same time. This quantification, as shown for the example of two level systems, can serve as a measure of coherence and can be furthermore, harnessed to obtain a measure for pure state entanglement for coupled two level systems.

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