arXiv subjects

Hashir Kuniyil

Publications and source records attributed to Hashir Kuniyil.

14 recordsLinked to original sources

arXiv2026

Spectral filtering and crystal length as control parameters for conditional correlations in quantum imaging

We establish spectral filtering as a control parameter for the conditional momentum and position correlations of SPDC biphotons used in quantum imaging. The conditional momentum uncertainty in spontaneous parametric down-conversion (SPDC) is strongly crystal-length and spectral-filter dependent along the walk-off axis; therefore, the effect is observed only in critically phase-matched (CPM) crystals such as $\beta$-barium borate (BBO) crystals, while quasi-phase-matched (QPM) crystals and the non-walk-off axis of BBO remain scaled strictly according to the standard pump waist size ($w_0$) dependent scaling law $1/w_0$-independent of the filter. In position space, the spectral-filter effect is universal and produces a flat-dip-rise (FDR) profile in every crystal class examined. Although this FDR profile was previously demonstrated in BBO only in the nondegenerate regime, our results establish its generality: the FDR dip is also present at exact degeneracy in QPM crystals, an unexpected feature that was previously thought to deliver a resolution advantage in the nondegenerate regime alone. Our treatment applies to all SPDC-based quantum-imaging regimes (including CPM and QPM crystals, walk-off and non-walk-off axes, degenerate and nondegenerate emission, and signal and idler filtering) and offers enhanced quantum-imaging resolution via the design rules presented-most prominently along the walk-off axis of CPM crystals (in far field) and across all transverse axes in the near field.

quant-ph↗

arXiv2026

Phase information beyond entanglement sudden death in coherence-to-entanglement conversion under post-gate noise

An ideal CNOT maps the phase of a coherent qubit onto the coherence between $\ket{00}$ and $\ket{11}$ of a two-qubit state, producing an output that carries both entanglement and estimable phase information. We ask how post-gate noise degrades these two quantities, and find that they are not lost together. For the phase-encoded X states generated by the protocol, the negativity is a thresholded difference of the surviving coherence $z=f\kappa$ and a population penalty $g$, vanishing once $f\kappa\le g$, while the phase quantum Fisher information (QFI) is the smooth ratio $F_\phi=4z^2/(a+b)$, which stays positive for any nonzero coherence. As a result there is an exact region of state space in which the output is separable but still phase-sensitive. We characterize this region, give the residual QFI $F_\phi^\star=4g_\star^2/(1-2g_\star)$ at entanglement death, and show that channels reaching death at the same coordinate share this residual, with global and independent local depolarization forming one such class and $F_\phi^\star=1/6$ at maximal input coherence. Four standard channels appear as trajectories through this common geometry, and asymmetric population transfer adds a third coordinate that changes the entanglement but leaves the QFI unchanged, which marks where the two-coordinate description applies. We identify a measurement that attains the bound and compare with a direct single-qubit probe, which is more precise under matched exposure; the results are therefore reference benchmarks for phase-information retention, not a claim of metrological advantage.

quant-ph↗

arXiv2026

Counterdiabatic Raman Atom Optics for Compact High-Sensitivity Gravimetry

Large-momentum-transfer (LMT) atom interferometry provides a route toward enhanced inertial sensitivity in compact quantum sensors, but its scalability is limited by the accumulation of pulse-transfer errors across long Raman pulse sequences. We investigate theoretically the use of stimulated Raman shortcut-to-adiabatic passage (STIRSAP) for high-fidelity LMT atom optics in a Mach--Zehnder interferometer geometry. The counterdiabatic correction is encoded directly into the Raman pulse envelopes, eliminating the need for auxiliary microwave or radio-frequency control fields. Numerical simulations based on an effective Raman model show that $1~\mu\mathrm{s}$ STIRSAP pulses achieve single-pulse transfer fidelities of $F_\pi = 0.99902$ while maintaining negligible pulse-time overhead even at high momentum order. We analyze the resulting tradeoff between interferometric phase enhancement and compound contrast decay and identify an unconstrained shot-noise optimum near $n\approx270$. The analysis further shows that practical operation at extreme LMT order is constrained by wave-packet separation, vibration noise, Doppler detuning, and accumulated systematic effects rather than by pulse duration itself. These results establish superadiabatic Raman control as a promising approach for scalable high-fidelity atom optics and clarify the physical limitations governing compact high-order atom interferometers.

quant-ph↗

arXiv2026

Recovery-Induced Erasure Attack on QKD Systems

Detector dead time is typically treated as a fixed parameter in quantum key distribution (QKD) security analyses. In practice, however, the effective recovery time of single-photon avalanche photodiodes (SPADs) depends on the incident count rate. In this work, we demonstrate that this count-rate-dependent recovery nonlinearity constitutes a distinct attack primitive. We experimentally characterize the dead time shift of a free-running SPAD under controlled broadband loading and observe a substantial increase in effective recovery time as the detected rate rises into the high photon count regime. We show that recovery-induced availability reduction can be modeled as an adversarial erasure channel and derive a conservative bound on the signal detection probability under loading. Unlike previously studied detector-control or efficiency mismatch attacks, the proposed mechanism does not rely on deterministic blinding or timing discrimination. Instead, count-rate-dependent recovery asymmetry induces basis-dependent suppression of detection probabilities ($p_\perp<p_\parallel$), converting mismatch-induced errors into loss. Particularly, we show in active-basis BBM92 systems, this effect reduces the observed quantum bit error rate (QBER) below the abort threshold while increasing erasure probability. Using experimentally measured detector recovery data, we quantify the parameter regime in which such stealth suppression is achievable. These results establish count-rate-dependent detector recovery as a security-relevant vulnerability and show that countermeasures designed for timing-based efficiency mismatch do not directly address recovery-induced erasure (RIE) attack. Our findings underscore the need to incorporate detector recovery dynamics explicitly into practical QKD security models.

quant-ph↗

arXiv2026

Noise Resilient 1SDIQKD for Practical Quantum Networks

One-sided device-independent quantum key distribution (1SDI-QKD) offers a practical middle ground between fully device-independent protocols and standard QKD, achieving security with detection efficiencies as low as 50.1\% on the untrusted side. However, prior analyses assumed idealized channels, neglecting realistic noise sources. We extend the 1SDI-QKD framework to include amplitude damping, dephasing, and depolarizing noise, quantifying their impact on secure key rates and efficiency requirements. Our results reveal a clear noise hierarchy: dephasing is most tolerable (secure keys achievable at 70\% efficiency with 30\% noise), while amplitude damping and depolarizing noise dramatically elevate requirements to over 90\%. Crucially, we find that security is lost while substantial entanglement remains (concurrence $C \approx 0.7$--$0.8$), demonstrating that steering violation, not merely entanglement, determines 1SDI-QKD security. To mitigate noise effects, we integrate the BBPSSW entanglement purification protocol, showing that 2--4 rounds can restore positive key rates in otherwise insecure regimes. Our resource overhead analysis reveals that effective key rates peak at moderate purification depths; excessive rounds become counterproductive. These findings establish practical boundaries for deploying 1SDI-QKD over metropolitan-scale quantum networks.

quant-ph↗

arXiv2025

Entanglement certification in bulk nonlinear crystals for degenerate and non-degenerate SPDC: spectral filter effects on transverse spatial correlations

Spatial correlations of photon pairs from spontaneous parametric down-conversion (SPDC) underpin quantum imaging and entanglement certification. We present the first systematic study of spectral filter bandwidth effects on transverse spatial correlations in bulk Type-I BBO for degenerate and non-degenerate configurations. In the far field, the degenerate conditional momentum width is pump-limited and filter-invariant, while non-degenerate configurations exhibit monotonic growth in both marginal and conditional momentum widths -- with the walk-off axis $\approx 100$ times more sensitive than the non-walk-off axis. In the near field, we identify a previously unreported flat-dip-rise profile: the conditional position width narrows by $\approx 10\%$ at an optimal bandwidth $\Delta_\mathrm{dip} \approx 1.35\,\Delta\lambda_\mathrm{SPDC}$ before rising due to geometric displacement. When the filter is placed on the idler arm, the dip shifts by the exact factor $(\lambda_i/\lambda_s)^2$. Both results are universal for any non-degenerate SPDC source, requiring only a finite crystal length, $d\theta/d\lambda \neq 0$, and incoherent spectral averaging. The Reid EPR uncertainty product is consistently smaller on the walk-off axis -- a structural advantage of bulk birefringent geometry absent in quasi-phase-matched sources. The optimal filter bandwidth $\Delta_F = \Delta_\mathrm{dip}$ is determined entirely by the intrinsic phase-matching bandwidth of the crystal and is directly readable from the X-entanglement spectral width of the source.

quant-ph↗

arXiv2025

Quantum Mpemba Effect in a Four-Site Bose-Hubbard Model

We investigate relaxation-order inversion, known as the quantum Mpemba effect (QME), in a minimal open many-body system called a one-dimensional four-site Bose--Hubbard chain governed by Lindblad dynamics with local number dephasing. Families of thermal initial states are prepared at a fixed temperature and evolved under a common reference Liouvillian toward the same stationary state. Relaxation is characterized using four complementary diagnostics: trace distance, quantum relative entropy, symmetry-projected entropy imbalance (entanglement asymmetry), and the $\ell_{1}$-norm of coherence in the Fock basis. We find that QME emerges robustly in -the clean interacting regime, where on-site interactions redistribute the overlaps of initial states with slow Liouvillian decay modes, enabling states initially farther from equilibrium to converge faster at late times. In contrast, the noninteracting limit exhibits a monotonic relaxation hierarchy across all metrics. Introducing a linear Stark potential or random on-site disorder suppresses relaxation and eliminates QME signatures by inhibiting transport-assisted mixing and enhancing the dominance of slow modes. Within the explored parameter regime, the Stark field induces significantly stronger retardation than disorder. We further show that symmetry-projected entropy imbalance is particularly sensitive to charge-sector decoherence in reduced subsystems and provides a stringent probe of QME in bosonic platforms. Our results elucidate the essential role of interactions in enabling anomalous relaxation in open lattice systems and connect the suppression of QME under spatial inhomogeneity to localization phenomena in tilted and disordered Bose--Hubbard chains.

cond-mat.quant-gas↗

arXiv2025

Single-photon emission modeling with statistical estimators for the exponential distribution

Single-photon sources are used in numerous quantum technologies, from sensing and imaging to communication, making the accurate modeling of their emissions essential. In this work, we propose a statistical framework for describing single-photon emission processes and implement estimators for the exponential distribution to quantify this phenomenon. Our approach provides a reliable method for estimating the radiative decay time, represented by the inverse rate parameter, which is crucial in quantum optics applications. We explore several statistical estimators, including maximum likelihood estimation, minimum-variance unbiased estimator, and best linear unbiased estimator. To validate our theoretical methods, we test the proposed estimators on experimental data, demonstrating their applicability in real-world settings. We also evaluate the performance of these estimators when dealing with censored data, a frequent limitation in photon emission experiments. The analysis allows us to track the performance of the proposed estimators as the amount of available data decreases, providing insights into their reliability for modeling single-photon emission events under limited resources.

quant-ph↗

arXiv2025

Optimizing Continuous-Wave-Pumped Entanglement-based QKD in Noisy Environments

Quantum key distribution (QKD) has emerged as a promising solution to protect current cryptographic systems against the threat of quantum computers. As QKD transitions from laboratories to real-world applications, its implementation under various environmental conditions has become a pressing challenge. Major obstacles to practical QKD implementation are the loss of photons in the transmission media and the presence of extreme noise, which can severely limit long-range transmission. In this paper, we investigate the impact of extreme noise on QKD system parameters, including timing jitter, rate-dependent timing shifts, changes in effective detector dead time, and rate-dependent detection efficiency. Contrary to manufacturers' specifications, which assume these parameters to be constant, we demonstrate that these parameters exhibit significant variations in extreme noise conditions. We show that changes in these parameters play a key role in determining system performance in noisy environments. To address these nonidealities, we develop a model that adapts to detector-dependent timing distortions and recovery effects. In particular, our model is independent of source parameters and can be implemented using data from the detection unit. Our results show that the model enables reliable characterization and optimization of QKD performance under strong noise.

quant-ph↗

arXiv2024

Towards Scalable Quantum Key Distribution: A Machine Learning-Based Cascade Protocol Approach

Quantum Key Distribution (QKD) is a pivotal technology in the quest for secure communication, harnessing the power of quantum mechanics to ensure robust data protection. However, scaling QKD to meet the demands of high-speed, real-world applications remains a significant challenge. Traditional key rate determination methods, dependent on complex mathematical models, often fall short in efficiency and scalability. In this paper, we propose an approach that involves integrating machine learning (ML) techniques with the Cascade error correction protocol to enhance the scalability and efficiency of QKD systems. Our ML-based approach utilizes an autoencoder framework to predict the Quantum Bit Error Rate (QBER) and final key length with over 99\% accuracy. This method significantly reduces error correction time, maintaining a consistently low computation time even with large input sizes, such as data rates up to 156 Mbps. In contrast, traditional methods exhibit exponentially increasing computation times as input sizes grow, highlighting the superior scalability of our ML-based solution. Through comprehensive simulations, we demonstrate that our method not only accelerates the error correction process but also optimizes resource utilization, making it more cost-effective and practical for real-world deployment. The Cascade protocol's integration further enhances system security by dynamically adjusting error correction based on real-time QBER observations, providing robust protection against potential eavesdropping. Our research establishes a new benchmark for scalable, high-throughput QKD systems, proving that machine learning can significantly advance the field of quantum cryptography. This work continues the evolution towards truly scalable quantum communication.

quant-ph↗

arXiv2024

Magnetic Dipolar Quantum Battery with Spin-Orbit Coupling

We investigate a magnetic dipolar system influenced by the $z$-component of Zeeman splitting, Dzyaloshinsky--Moriya (DM) interaction, and Kaplan--Shekhtman--Entin-Wohlman--Aharony (KSEA) exchange interaction, with emphasis on the role of quantum resources in both closed and open settings. By analyzing the Gibbs thermal state and solving the Lindblad master equation, we study the behavior of quantum coherence, discord, and entanglement under thermal equilibrium and dephasing noise. After exploring these resources, we apply the model to a closed quantum battery (QB). Our results show that while Zeeman splitting degrades quantum resources in noisy and thermal regimes, it enhances QB performance by improving ergotropy, anti-ergotropy, storage capacity, and coherence during cyclic charging. The axial parameter further amplifies performance, leading to coherence saturation and persistent ergotropy growth, in line with the notion of incoherent ergotropy. KSEA interaction and the rhombic term consistently preserve coherence and entanglement under noise, thereby strengthening QB functionality. DM interaction mitigates thermal degradation of resources in the Gibbs state and improves performance, though its effect is limited under Pauli-$X$ dephasing. We reveal diverse behaviors, including increased ergotropy without coherence and the coexistence of coherence with zero extractable work. Finally, we propose Nuclear Magnetic Resonance (NMR) as a feasible platform for experimental implementation.

quant-ph↗

arXiv2021

Noise-Tolerant Object Detection and Ranging Using Quantum Correlations

Imaging, detection and ranging of objects in the presence of significant background noise is a fundamental challenge in optical sensing. Overcoming the limitations imposed in conventional methods, quantum light sources show higher resistance against noise in a time-correlation-based quantum illumination. Here, we introduce the advantage of using not only time correlations but also polarization correlations in photon pairs in the detection of an object that is embedded in a noisy background. In this direction, a time- and polarization-correlated photon pair source using the spontaneous parametric down-conversion process is exploited. We found that the joint measurement of correlated pairs allows distinguishing the signal from the noise photons and that leads to an improved signal-to-noise ratio. Our comparative study revealed that using polarization correlations in addition to time correlations provides improved noise rejection. Furthermore, we show that polarization correlation allows undoing the detector limitation where high background often leads to detector saturation.

quant-ph↗

arXiv2021

Feasibility Study for CubeSat Based Trusted Node Configuration Global QKD Network

Quantum key distribution (QKD) is the most used protocol in the context of quantum cryptography for sharing a private encryption key between two parties. Covid-19 pandemic has raised the ever-increasing need for online communications a lot; this requires enhanced security protocols. QKD has the potential to meet a global scale network's security requirements. Despite considerable progress, all ground-based QKD approaches have distance limitations due to atmospheric or fiber attenuation. A global network scheme can use intersatellite links to establish a trusted node network with constellations. This enables key elements for quantum internet which allows secure exchange of information between quantum computers. The most cost-effective and iterative approach for this goal is to exploit CubeSats. This paper summarizes technical challenges and possible solutions to enable a global QKD network using CubeSats. We discuss practical concerns and alternative paths involved with implementing such systems.

quant-ph↗

arXiv2019

Tailoring the down conversion emission profile via direct imaging with a camera

We present the analysis of emission profile of downconverted photons from a critically phase-matched nonlinear crystal. This is done via direct imaging of down converted photons by a CMOS camera. The effects of nonlinear crystal thickness in collinear and non-collinear geometries on the down converted photon pair rate is directly observed and the experimental results show that the photon pair rate increases linearly with the crystal thickness. However, the rate of the photon pairs collected to a single mode fiber goes quadratically with the crystal length because of the cylindrical asymmetries in the optical path and the exit angle around the pump mode within the nonlinear crystal. The use of cameras for real-time and direct imaging of down conversion emission profile significantly simplifies the phase-matching alignments and the collecting the entangled photons.

quant-ph↗