arXiv subjects

Kanad Sengupta

Publications and source records attributed to Kanad Sengupta.

6 recordsLinked to original sources

arXiv2026

Deterministic Quantum Phase Estimation with Linear Circuit Complexity in a Photonic System

Quantum algorithms solve certain computational problems faster than the best known classical algorithms. Many algorithms, including Shor's for factoring, Grover's for unstructured search, and the HHL for solving linear systems, rely on quantum phase estimation (QPE) as a fundamental subroutine. The QPE protocol proceeds through the initialization of a control register in a uniform superposition, controlled unitary evolution encoding the eigenphase, and a final inverse quantum Fourier transform followed by measurement to extract the phase. Here, we address a special class of unitary operators that frequently appear in quantum Fourier transform-based protocols, cyclic group representations, and periodically evolving quantum systems. We introduce a QPE algorithm that successfully reduces the circuit complexity from $\mathcal{O}(n^2)$ to $\mathcal{O}(n)$ for a special class of unitary operators and implement it on a four-qubit photonic system. The four-qubit system is realised using a photon pair, with two qubits encoded in its polarization degree of freedom and the remaining two in its path modes. In contrast to previous photonic implementations of QPE based on dual-rail encoding and KLM protocol, where controlled operations are inherently probabilistic and thus reduce the overall success probability of phase estimation, our scheme is fully deterministic. Moreover, it is scalable to higher-dimensional unitaries, provided the underlying structure of the unitaries is preserved.

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arXiv2025

Photonic Simulation of Beyond-Quantum Nonlocal Correlations (e.g. Popescu-Rohrlich Box) with Non-Signaling Quantum Resources

Bell nonlocality exemplifies the most profound departure of quantum theory from classical realism. Yet, the extent of nonlocality in quantum theory is intrinsically bounded, falling short of the correlations permitted by the relativistic causality (the no-signaling) principle. A paradigmatic example is the Popescu-Rohrlich correlation: two distant parties sharing arbitrary entanglement cannot achieve this correlation, though it can be simulated with classical communication between them. Here we show how such post-quantum correlations can instead be simulated using intrinsically non-signaling physical resources, and implement the proposed scheme using a quantum circuit on a four-qubit photonic platform. Unlike the conventional approaches, our method exploits dynamical correlations between distinct physical systems, with intrinsic randomness suppressing any signaling capacity. This enables the realization of post-quantum correlations both with and without entanglement. We also analyze how the simulation scheme extends to beyond quantum nonlocal correlations in multipartite systems. Our experimental demonstration using a photonic system establishes a versatile framework for exploring post-quantum correlations in both foundational settings and as a resource for computation and security applications.

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arXiv2025

Deterministic quantum teleportation of a path-encoded state using entangled photons

Quantum teleportation enables a way to transmit an arbitrary qubit state from one place to an other. A standard scheme for teleportation in optical setup involve three photons, an entangled photon pair and a photon carrying quantum state to be teleported. The interaction between the photons in the scheme makes quantum teleportation probabilistic. Here we demonstrate a deterministic teleportation of an arbitrary qubit state using only a polarization entangled photon pair in a linear optical setup. By introducing the path degree of freedom to one of the entangled photon and encoding the arbitrary qubit state into it, we demonstrate 100\% Bell State Measurement (BSM) outcome. This enables a deterministic teleportation of a qubit state in an optical scheme with high fidelity. We report an average teleportation fidelity of 88.00%. The dependency of fidelity on the visibility of single-photon interferometer used for path qubit state shows the possibility of further improving the fidelity of quantum teleportation.

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arXiv2024

Four-qubit photonic system for publicly verifiable quantum random numbers and generation of public and private key

We theoretically propose and experimentally demonstrate the use of a configurable four-qubit photonic system to generate a publicly verifiable quantum random numbers, to perform entanglement verification, and to generate secure public and private key. Quantum circuits, to generate the desired four-qubit states and its experimental realization in the photonic architecture is carried out using photon pairs entangled in polarization and path degree of freedom. By performing measurements on the four-qubit system and accessing partial information of the four-qubit state for public verification, we generate publicly verified and purely secured random bits at the rate of 185 kbps from collective data of 370 kbps. When the system is used for generating public and private keys, an equal number of public and private keys are generated simultaneously. We also record about 97.9% of sampled bits from four-qubit states passing entanglement verification and demonstrate the use of public and private key generated for image encryption-decryption. The theoretical model of noise on the four-qubit state and its effect on the generation rate of verified and secured bits are in perfect agreement with the experimental results. This demonstrates the practical use of the small-scale multi-qubit photonic system for quantum-safe applications by providing the option for real-time verification of the security feature of the quantum system.

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arXiv2024

Experimental realization of universal quantum gates and six-qubit entangled state using photonic quantum walk

For quantum computation using photons, performing deterministic quantum gate operations is a challenge due to the probabilistic nature of the photon-photon interaction. Encoding qubits in multiple degrees-of-freedom of photons and controlling operations between them is one of the promising ways to navigate the probabilistic behavior. Using single-photon discrete-time quantum walk in combination with polarization and path degrees-of-freedom, we experimentally demonstrate the realization of a universal set of quantum gates with high fidelity at room temperature. The deterministic realization of quantum gates through photonic quantum walk are characterized via quantum state tomography. For a three-qubit system using a single photon, the first qubit is encoded using polarization information, and the other two qubits are encoded using path information, closely resembling a Galton-board setup. To generate a six-qubit Greenberger-Horne-Zeilinger state, entangled photon pairs are used to entangle the two three-qubit modules on which gate operations are performed. We also provide insights into the mapping of photonic quantum walk operations to quantum circuits and propose methods to resourcefully scale. This demonstration marks a significant progress towards using quantum walks for quantum computing and provides a framework for using fewer photons in combination with different degrees-of-freedom of photon to scale the number of qubits.

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arXiv2024

Quantum illumination using polarization-entangled photon pairs for enhanced object detection

Entangled light sources for illuminating objects offer advantages over conventional illumination methods by enhancing the detection sensitivity of reflecting objects. The core of the quantum advantage lies in effectively exploiting quantum correlations to isolate noise and detect objects with low reflectivity. This work experimentally demonstrates the benefits of using polarization-entangled photon pairs for quantum illumination and shows that the quantum correlation measure, using CHSH value and normalized CHSH value, is robust against losses, noise, and depolarization. We report the detection of objects with reflectivity ($\eta$) as low as 0.05 and an object submerged in noise with a signal-to-noise ratio of 0.003 using quantum correlation and residual quantum correlation measures, surpassing previous results. Additionally, we demonstrate that the normalized CHSH value aids in estimating the reflectivity of the detected object. Furthermore, we analyze the robustness of the correlation measure under photon attenuation in atmospheric conditions to show the practical feasibility of real-time applications.

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