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Sooryansh Asthana

Publications and source records attributed to Sooryansh Asthana.

At least 19 recordsLinked to original sources
arXiv2026

Quantum Illumination with Symmetry-Constrained Random Unitaries

Quantum illumination provides a quantum advantage in detecting weakly reflecting objects embedded in a noisy environment, even when environmental noise destroys most of the initial entanglement. We investigate this advantage using Haar-random probe states constrained to symmetry-resolved subspaces. Employing tools from quantum channel discrimination and asymptotic hypothesis testing, we derive the discrimination exponents associated with Haar-random probe ensembles and identify the role of symmetry in determining their performance. We show that typical states drawn from fixed-charge sectors achieve the same asymptotic quantum-illumination advantage as maximally entangled probes. In particular, we show that the effective thermal-noise suppression and the corresponding Chernoff exponent are governed by the dimension of the accessible symmetry sector. Our results reveal that the operational resource underlying quantum illumination can be generalized from fine-tuned structure of a specific probe state to the existence of a large symmetry-protected correlation subspace. These findings establish a direct connection between quantum illumination, symmetry-resolved typicality, and quantum channel discrimination, and demonstrate that near-optimal quantum hypothesis testing resources can emerge naturally from generic many-body quantum states constrained by conservation laws.

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arXiv2026

Efficient Many-Body Shadow Metrology via Clifford Lensing

Quantum probes that enable enhanced exploration and characterization of complex systems are central to modern science, spanning applications from biology to astrophysics and chemical design. In large many-body quantum systems, interactions delocalize phase information across many degrees of freedom, dispersing it away from accessible measurements and limiting the scalability of quantum metrology. Here we show that experimentally accessible Clifford operations acting jointly on quantum states and observables can refocus this distributed information. These operations implement what we term {\it Clifford lensing}--transformations that coherently localize phase information onto a reduced set of degrees of freedom, mapping optimal measurements onto observables of reduced Pauli weight. We establish a correspondence between quantum error-correcting codes and interferometric constructions that enforce deterministic phase kickback, and generalize this to circuits that concentrate many-body phase information onto a controllable subset of qubits. We further develop partial shadow tomography protocols for estimating subsystem-supported phases. We experimentally demonstrate these principles in liquid-state nuclear magnetic resonance systems of up to fifteen qubits, achieving optimal sensing with constrained resources. Our results establish a scalable route to coherent control of information flow in interacting quantum systems, enabling many-body quantum sensing and multimode interferometry across complex architectures.

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arXiv2025

Projected Optimal Sensors from Operator Orbits

We unify Ramsey, twist-untwist, and random quantum sensors using operator algebra and account for the Fisher scaling of various sensor designs. We illustrate how the operator orbits associated with state preparation inform the scaling of the sensitivity with the number of subsystems. Using our unified model, we design a novel set of sensors in which a projected ensemble of quantum states exhibits beyond-shot-noise metrological performance. We also show favorable scaling of Fisher information with decoherence models and loss of particles.

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arXiv2023

Indispensability of orbital angular momentum states in secure quantum communication tasks

Quantum key distribution protocols have been designed for layered networks employing multidimensional entangled and separable orbital angular momentum states [Phys. Rev. A 97, 032312 (2018), Int. J. Theor. Phys. 62, 104 (2023)]. This paper seeks an answer to the overarching question -- in the context of secure quantum communication tasks, do orbital angular momentum states act merely as an alternative or do they act as an indispensable resource? We start by showing that the task of quantum key distribution in layered networks can also be accomplished with several copies of lower-dimensional states such as polarization qubits. For this reason, orbital angular momentum states do not offer any intrinsic advantage in layered quantum key distribution. The potential of OAM states unveils itself in the enhancement of key generation rates and integrated quantum communication tasks, which we present in this paper. These tasks can be implemented exclusively with high-dimensional OAM entangled states. In fact, we show that the employment of orbital angular momentum states eliminates the need for entangled state measurements, whose implementation is resource-intensive. We believe that this study opens up a possibility for designing several quantum information processing tasks in which multidimensional OAM states act as an indispensable resource.

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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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arXiv2022

Interrelation of nonclassicality conditions through stabiliser group homomorphism

In this paper, we show that coherence witness for a single qubit itself yields conditions for nonlocality and entanglement inequalities for multiqubit systems. It also yields a condition for quantum discord in two--qubit systems. It is shown by employing homomorphism among the stabiliser group of a single qubit and those of multi--qubit states. Interestingly, globally commuting homomorphic images of single qubit stabilisers do not allow for consistent assignments of outcomes of local observables. As an application, we show that CHSH inequality can be straightforwardly generalised to nonlocality inequalities for multiqubit GHZ states. It also reconfirms the fact that quantumness prevails even in the large $N$--limit, if coherence is sustained. The mapping provides a way to construct many nonlocality inequalities, given a {\it seed} inequality. This study gives us a motivation to gain a better control over multiple degrees of freedom and multi-party systems. It is because in multi-party systems, the same nonclassical feature, {\it viz.}, coherence may appear in many avatars.

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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

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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