arXiv subjects

Pritam Roy

Publications and source records attributed to Pritam Roy.

12 recordsLinked to original sources

arXiv2026

Beyond Maximal Entanglement: Exact Resources for Multiparty Encrypted Quantum Cloning

Encrypted quantum cloning distributes an unknown $k$-qubit state among $m$ encrypted clones so that no individual clone reveals the input, yet the state can be recovered from any one clone together with a common quantum key. We ask the inverse question: for a fixed encoding architecture, which multipartite pure states can serve as exact resources for this task? For $m\ge2$, we completely characterize the pure resources compatible with a sector-wise two-Pauli encoder, with necessity holding for arbitrary completely positive trace-preserving (CPTP) recovery maps. For even $m$, exact recovery requires maximal entanglement across the signal--noise cut. For odd $m$, less entanglement can suffice, provided that the surviving signal correlations have the structure selected by the encoder. We further show, without fixing the encoder, that exact recovery from every authorized subsystem already implies perfect concealment of each individual signal and requires at least $(m-1)k$ ebits of signal--noise entanglement. For graph states, the resource classification reduces to an exact condition on the kernel of the signal--noise cut matrix, leading to binary certification and constructive Clifford recovery. We identify rank-deficient graph resources that attain the architecture-independent entanglement bound and prove that the entire Dicke family, including $W$ states, is excluded for every sector-wise two-Pauli encoder. Our results show that encrypted recovery depends not only on how much entanglement a resource contains, but also on how its correlations are organized relative to the encoder.

quant-ph↗

arXiv2026

Maximizing Nonclassicality of Massive Objects via Quantum Zeno Effect

For testing quantum mechanics in the macroscopic domain, a major challenge is to devise effective means for enhancing the observable nonclassical signatures despite the ubiquitous presence of environmental decoherence. Toward this goal, we invoke the Quantum Zeno Effect (QZE) for achieving a tunable amplification of an inherently nonclassical quantum disturbance induced by any measurement. Such an enhancement of otherwise small and decoherence-suppressed nonclassicality can arise from the cumulative quantum disturbances generated by repetitive measurements, with the tunability of amplification controlled by the number of measurements. To evidence this, we formulate a testable loophole-free scheme using a massive oscillator, where the system preparation requires trapping and ground-state cooling of a massive object. The required measurements can be realized through a beam-splitter-type interaction between the mechanical oscillator and an optical field, followed by photon detection. Our analysis shows that such amplification, suitably quantified in terms of a testable witness, remains appreciably observable even in the realistic regimes of optomechanical damping, and for sufficiently large masses, thus enabling the demonstration of QZE in the macroscopic domain.

quant-ph↗

arXiv2026

Limits on Broadcasting Genuine Multipartite Entanglement in Quantum Networks

We establish operational limits on the broadcasting of genuine multipartite entanglement (GME) in quantum networks. Using a distributed protocol in which each of N parties locally implements optimal 1 $\to$ 2 cloning via beam-splitter interactions, we derive exact expressions for the broadcast fidelity of Greenberger-Horne-Zeilinger (GHZ), W states, and Cluster states in a limited setting. We show that the fidelity decays exponentially with system size as [c(R)]$^N$, providing a quantitative expression of multipartite entanglement monogamy in the broadcasting setting, and that both state families share a universal normalisation factor arising from independent post-selection probabilities. Most significantly, we prove a no-go result for simultaneous GME certification: for all reflectivities and all system sizes, the two broadcast copies cannot be simultaneously certified as genuinely multipartite entangled within the standard framework of fidelity-based witnesses. We further check this behaviour for three- and four-party cluster states, finding consistent results that support the generality of the no-go beyond the GHZ and W families. This obstruction arises from the redistribution of multipartite coherence, which both reduces the achievable fidelity and increases the corresponding certification threshold. Our results reveal a fundamental trade-off between the broadcastability of multipartite entanglement and its operational certifiability, and delineate intrinsic limits on entanglement distribution in quantum networks.

quant-ph↗

arXiv2026

Security of Device-independent Quantum Key Distribution under Sequential Attack

Device-independent quantum key distribution (DI-QKD) leverages nonlocal correlations to establish cryptographic keys between two honest parties while making minimal assumptions about the underlying systems. The security of DI-QKD is grounded in the validity of quantum theory, with Bell violations ensuring the intrinsic unpredictability of observed statistics, independent of the trustworthiness of the devices. While traditional collective QKD attacks assume that the adversary prepares the shared system, we analyse a scenario where the adversary does not control the source and instead interacts sequentially with the travelling system. In this setting, Eve performs an unsharp measurement that produces effective noise while preserving the observed Bell violation. Although such behaviour is already accounted for in existing DI-QKD security proofs, examining it through an explicit sequential interaction offers a concrete and physically motivated example of how these effective statistics can arise in practice. Our analysis further shows that, within a specific parameter regime, this sequential strategy reproduces some features of an optimal collective attack.

quant-ph↗

arXiv2026

Exclusion reshapes the operational manifestation of preparation contextuality

Replacing the task of retrieval with exclusion changes how preparation contextuality manifests operationally under parity-oblivious constraints, with exclusion showing a quantum advantage where retrieval does not. We introduce the parity-oblivious random exclusion code (POREC) and show that for prime symbol size $m$, classical and preparation-noncontextual encodings provide a tight noncontextual bound. For the first nontrivial case (two digits, three symbols), our derived exact qubit optimum violates this bound, in contrast to parity-oblivious retrieval, which displays no quantum advantage. This characteristic difference is absent without parity constraints. For general prime $m$, qubit strategies achieve a quantum-to-noncontextual gap that grows linearly relative to the random exclusion code (REC) gap, exceeding both parity-oblivious retrieval and standard REC. The exact qubit bound yields a sharp semi-device-independent certification of dimension $d \geq 3$. Our analysis of noise robustness demonstrates POREC to be amenable for experimental implementation on existing prepare-and-measure platforms, establishing parity-oblivious exclusion as a distinct operational probe of preparation contextuality, as well as a practical information processing protocol with wide applications.

quant-ph↗

arXiv2026

Secure One-Sided Device-Independent Quantum Key Distribution Under Collective Attacks with Enhanced Robustness

We study the security of a quantum key distribution (QKD) protocol under the one-sided device-independent (1sDI) setting, which assumes trust in only one party's measurement device. This approach effectively provides a balance between the experimental viability of device-dependent (DD-QKD) and the minimal trust assumptions of device-independent (DI-QKD). An analytical lower bound on the asymptotic key rate is derived to provide security against collective attacks, in which the eavesdropper's information is limited only by the function of observed violation of a linear quantum steering inequality, specifically the three-setting Cavalcanti-Jones-Wiseman-Reid (CJWR) inequality. We provide a closed-form key rate formula by reducing the security analysis to mixtures of Bell-diagonal states by utilizing symmetries of the steering functional. We show that the protocol tolerates higher quantum bit error rates (QBER) than present DI-QKD protocols by benchmarking its performance under depolarizing noise. Furthermore, we explore the impact of detection inefficiencies and show that, in contrast to DI-QKD, which requires near-perfect detection, secure key generation can be achieved even with lower detection efficiency on the untrusted side. These findings highlight the advantages of 1sDI-QKD as a steering-based alternative for secure quantum communication and provide insights relevant for near-future experimental implementations.

quant-ph↗

arXiv2026

Robust certification of quantum instruments through a sequential communication game

We propose a communication game in the sequential measurement scenario, involving a sender and two receivers with restricted communication among the latter parties. In the framework of the prepare-transform-measure scenario, we find a prominent quantum advantage in the receiver's decoding of the message originally encoded by the sender. We show that an optimal trade-off between the success probabilities of the two receivers enables self-testing of the sender's state preparation, the first receiver's instruments, and the measurement device of the second receiver in a semi-device-independent way. Our protocol enables a more robust certification of the unsharp measurement parameter of the first receiver compared to an earlier protocol. We further generalize our game to higher-dimensional systems, revealing greater quantum advantage with an increase in dimensions.

quant-ph↗

arXiv2024

Device-Independent Quantum Secure Direct Communication Under Non-Markovian Quantum Channels

Device-independent quantum secure direct communication (DI-QSDC) is a promising primitive in quantum cryptography aimed towards addressing the problems of device imperfections and key management. However, significant effort is required to tackle practical challenges such as the distance limitation due to the decohering effects of quantum channels. Here, we explore the constructive effect of non-Markovian noise to improve the performance of DI-QSDC. Considering two different environmental dynamics modelled by the amplitude damping and the dephasing channels, we show that for both cases non-Markovianty leads to a considerable improvement over Markovian dynamics in terms of three benchmark performance criteria of the DI-QSDC task. Specifically, we find that non-Markovian noise (i) enhances the protocol security measured by Bell violation, (ii) leads to a lower quantum bit error rate, and (iii) enables larger communication distances by increasing the capacity of secret communication.

quant-ph↗

arXiv2023

Stationary state of harmonic chains driven by boundary resetting

We study the nonequilibrium steady state (NESS) of an ordered harmonic chain of $N$ oscillators connected to two walls which undergo diffusive motion with stochastic resetting. The intermittent resettings of the walls effectively emulate two nonequilibrium reservoirs that exert temporally correlated forces on the boundary oscillators. These reservoirs are characterized by the diffusion constant and resetting rates of the walls. We find that, for any finite $N$, the velocity distribution remains non-Gaussian, as evidenced by a non-zero bulk kurtosis that decays $\sim N^{-1}$. We calculate the spatio-temporal correlation of the velocity of the oscillators $\langle v_l(t) v_{l'}(t') \rangle$ both analytically as well as using numerical simulation. The signature of the boundary resetting is present at the bulk in terms of the two-time velocity correlation of a single oscillator and the equal-time spatial velocity correlation. For the resetting driven chain, the two-time velocity correlation decay as $t^{-\frac{1}{2}}$ at the large time, and there exists a non-zero equal-time spatial velocity correlation $\langle v_l(t) v_{l'}(t') \rangle$ when $l \neq l'$. A non-zero average energy current will flow through the system when the boundary walls reset to their initial position at different rates. This average energy current can be computed exactly in the thermodynamic limit. Numerically we show that the distribution of the instantaneous energy current at the boundary is independent of the system size. However, the distribution of the instantaneous energy current in the bulk approaches a stationary distribution in the thermodynamic limit.

cond-mat.stat-mech↗

arXiv2011

Magnifying Lens Abstraction for Stochastic Games with Discounted and Long-run Average Objectives

Turn-based stochastic games and its important subclass Markov decision processes (MDPs) provide models for systems with both probabilistic and nondeterministic behaviors. We consider turn-based stochastic games with two classical quantitative objectives: discounted-sum and long-run average objectives. The game models and the quantitative objectives are widely used in probabilistic verification, planning, optimal inventory control, network protocol and performance analysis. Games and MDPs that model realistic systems often have very large state spaces, and probabilistic abstraction techniques are necessary to handle the state-space explosion. The commonly used full-abstraction techniques do not yield space-savings for systems that have many states with similar value, but does not necessarily have similar transition structure. A semi-abstraction technique, namely Magnifying-lens abstractions (MLA), that clusters states based on value only, disregarding differences in their transition relation was proposed for qualitative objectives (reachability and safety objectives). In this paper we extend the MLA technique to solve stochastic games with discounted-sum and long-run average objectives. We present the MLA technique based abstraction-refinement algorithm for stochastic games and MDPs with discounted-sum objectives. For long-run average objectives, our solution works for all MDPs and a sub-class of stochastic games where every state has the same value.

cs.GT↗

arXiv2010

Safety-Guarantee Controller Synthesis for Cyber-Physical Systems

The verification and validation of cyber-physical systems is known to be a difficult problem due to the different modeling abstractions used for control components and for software components. A recent trend to address this difficulty is to reduce the need for verification by adopting correct-by-design methodologies. According to the correct-by-design paradigm, one seeks to automatically synthesize a controller that can be refined into code and that enforces temporal specifications on the cyber-physical system. In this paper we consider an instance of this problem where the specifications are given by a fragment of Linear Temporal Logic (LTL) and the physical environment is described by a smooth differential equation. The contribution of this paper is to show that synthesis for cyber-physical systems is viable by considering a fragment of LTL that is expressive enough to describe interesting properties but simple enough to avoid Safra's construction. We report on two examples illustrating a preliminary implementation of these techniques on the tool PESSOALTL.

cs.LO↗

arXiv2010

Interface Building for Software by Modular Three-Valued Abstraction Refinement

Verification of software systems is a very hard problem due to the large size of program state-space. The traditional techniques (like model checking) do not scale; since they include the whole state-space by inlining the library function codes. Current research avoids these problem by creating a lightweight representation of the library in form of an "interface graph" (call sequence graph). In this paper we introduce a new algorithm to compute a safe, permissive interface graph for C-type functions. In this modular analysis, each function transition is summarized following three-valued abstraction semantics. There are two kinds of abstraction used here. The global abstraction contains predicates over global variables only; however the local abstraction inside each function may also contain the local variables. The abstract summary needs refinement to guarantee safety and permissiveness. We have implemented the algorithms in TICC tool and compared this algorithm with some related interface generation algorithms. We also discuss the application of interface as an offline test-suite. We create an interface from the model program (specification) and the interface will act as a test-suite for the new implementation-under-test (IUT).

cs.LO↗