arXiv subjects

Shashi Prabhakar

Publications and source records attributed to Shashi Prabhakar.

At least 19 recordsLinked to original sources

arXiv2026

Experimental Demonstration of Free-Space Unidimensional Continuous-Variable Quantum Key Distribution Under High Detector Noise

Continuous-variable quantum key distribution (CV-QKD), which uses quadratures of the electromagnetic field, enables practical quantum communication using standard telecommunication technologies. Unidimensional CV-QKD (UD-CVQKD) simplifies the implementation by restricting modulation to a single quadrature. In this work, we experimentally demonstrate a free-space Gaussian-modulated UD-CVQKD system operating under a high detector electronic-noise regime (1.4 shot-noise units). The system employs polarized coherent states with signal and local oscillator co-propagating in the same spatial mode in orthogonal polarizations, ensuring stable interference. System security is analyzed under both untrusted (UTD) and trusted (TD) detector noise models. While no positive secret key rate is obtained under the UTD model, the TD model enables secure key generation over a finite range of modulation variances, highlighting the critical role of detector trust in high-noise conditions. A maximum secret key rate of 270 kbps is achieved at an optimal modulation variance of 11.57. Furthermore, secure operation requires high-transmittance (low-loss) channels under such noise conditions. This study demonstrates the practical feasibility of free-space UD-CVQKD in realistic high electronic-noise detection constraints and highlights detector electronic noise as a key limiting factor in practical systems.

quant-ph↗

arXiv2025

Strengthening the No-Go Theorem for QRNGs

Quantum random numbers are essential for security against quantum algorithms. Randomness as a beacon is a service being provided for companies and governments to upgrade their security standards from RSA to PQC-QKD or PQC-RSA protocols. Both security mechanisms assume trust in the service provider unless one aims for device-independent protocols. How does an entity ensure that the beacon service has a quantum signature other than relying on faith? Specifically, given a bit-stream, can a user verify a quantum signature in it? Researchers claim this is indecipherable and have stated a no-go theorem for post-processed bit-streams [Physical Review A \textbf{109}, 022243 (2024)]. In this article, we corroborate the results of the no-go theorem while discussing its nuances using two different random number generators and four test methods. These include the NIST statistical test suite and machine learning algorithms that strengthen the theorem. This work is relevant for companies and governments using QRNG, provided to enhance security against quantum threats.

quant-ph↗

arXiv2024

Conservation of angular momentum on a single-photon level

Identifying conservation laws is central to every subfield of physics, as they illuminate the underlying symmetries and fundamental principles. A prime example can be found in quantum optics: The conservation of orbital angular momentum (OAM) during spontaneous parametric down-conversion (SPDC) enables the generation of a photon pair with entangled OAM. In this article, we report on the first study of OAM conservation in SPDC pumped by single photons. Our results present the first implementation of cascaded down-conversion without waveguides, setting the stage for experiments on the direct generation of multi-photon high-dimensional entanglement using all degrees of freedom of light.

quant-ph↗

arXiv2024

Investigating a Device Independence Quantum Random Number Generation

Quantum random number generation (QRNG) is a resource that is a necessity in the field of cryptography. However, its certification has been challenging. In this article, we certify randomness with the aid of quantum entanglement in a device independent setting, where we choose two-photon interference for source characterisation. The CHSH inequality violation and quantum state tomography are used as independent checks on the measurement devices. These measures ensure the unpredictability of quantum random number generation. This work can be easily extended to faster randomness expansion protocols.

quant-ph↗

arXiv2024

Enhancing key rates of QKD protocol by Coincidence Detection

In theory, quantum key distribution (QKD) provides unconditional security; however, its practical implementations are susceptible to exploitable vulnerabilities. This investigation tackles the constraints in practical QKD implementations using weak coherent pulses. We improve on the conventional approach of using decoy pulses by integrating it with the coincidence detection (CD) protocol. Additionally, we introduce an easy-to-implement algorithm to compute asymptotic key rates for the protocol. Furthermore, we have carried out an experimental implementation of the protocol, where we demonstrate that monitoring coincidences in the decoy state protocol leads to enhanced key rates under realistic experimental conditions.

quant-ph↗

arXiv2023

LFSR based RNG on low cost FPGA for QKD applications

Linear-feedback shift register (LFSR) based pseudo-random number generator (PRNG) has applications in a plethora of fields. The issue of being linear is generally circumvented by introducing non-linearities as per the required applications, with some being adhoc but fulfilling the purpose while others with a theoretical proof. The goal of this study is to develop a sufficiently ``random" resource for Quantum Key Distribution (QKD) applications with a low computational cost. However, as a byproduct, we have also studied the effect of introducing minimum non-linearity with experimental verification. Starting from the numerical implementation to generate a random sequence, we have implemented a XOR of two LFSR sequences on a low-cost FPGA evaluation board with one of the direct use cases in QKD protocols. Such rigorously tested random numbers could also be used like artificial neural networks or testing of circuits for integrated chips and directly for encryption for wireless technologies.

quant-ph↗

arXiv2023

Use of Non-Maximal entangled state for free space BBM92 quantum key distribution protocol

Satellite-based quantum communication for secure key distribution is becoming a more demanding field of research due to its unbreakable security. Prepare and measure protocols such as BB84 consider the satellite as a trusted device, fraught with danger looking at the current trend for satellite-based optical communication. Therefore, entanglement-based protocols must be preferred since, along with overcoming the distance limitation, one can consider the satellite as an untrusted device too. E91 protocol is a good candidate for satellite-based quantum communication; but the key rate is low as most of the measured qubits are utilized to verify a Bell-CHSH inequality to ensure security against Eve. An entanglement-based protocol requires a maximally entangled state for more secure key distribution. The current work discusses the effect of non-maximality on secure key distribution. It establishes a lower bound on the non-maximality condition below which no secure key can be extracted. BBM92 protocol will be more beneficial for key distribution as we found a linear connection between the extent of violation for Bell-CHSH inequality and the quantum bit error rate for a given setup.

quant-ph↗

arXiv2023

Analysing the Grain size and asymmetry of the particle distribution using auto-correlation technique

Extracting the grain size from the microscopic images is a rigorous task involving much human expertise and manual effort. While calculating the grain size, we will be utilizing a finite number of particles which may lead to an uncertainty in the measurement. To avoid this difficulty, we utilize a simple mathematical tool, the auto-correlation function, to determine the grain size. The random particle distribution and the finite width Gaussian histogram of particle size has motivated us to utilize the auto-correlation function, which has been extensively studied for finding the size of random optical patterns. The finite width of the correlation function provides the grain size, and the difference in correlation length along two mutually independent directions provides information about the asymmetry present in the particle distribution, i.e., the deviation from a spherical shape. The results may find applications in material, pharmaceutical, chemical, and biological studies where extracting the grain size is essential.

physics.data-an↗

arXiv2023

Designing Nonlinear Photonic Crystals for High-Dimensional Quantum State Engineering

We propose a novel, physically-constrained and differentiable approach for the generation of D-dimensional qudit states via spontaneous parametric down-conversion (SPDC) in quantum optics. We circumvent any limitations imposed by the inherently stochastic nature of the physical process and incorporate a set of stochastic dynamical equations governing its evolution under the SPDC Hamiltonian. We demonstrate the effectiveness of our model through the design of structured nonlinear photonic crystals (NLPCs) and shaped pump beams; and show, theoretically and experimentally, how to generate maximally entangled states in the spatial degree of freedom. The learning of NLPC structures offers a promising new avenue for shaping and controlling arbitrary quantum states and enables all-optical coherent control of the generated states. We believe that this approach can readily be extended from bulky crystals to thin Metasurfaces and potentially applied to other quantum systems sharing a similar Hamiltonian structures, such as superfluids and superconductors.

quant-ph↗

arXiv2022

Endless Fun in high dimensions -- A Quantum Card Game

Quantum technologies, i.e., technologies benefiting from the features of quantum physics such as objective randomness, superposition, and entanglement, have enabled an entirely different way of distributing and processing information. The enormous progress over the last decades has also led to an urgent need for young professionals and new educational programs. Here, we present a strategic card game in which the building blocks of a quantum computer can be experienced. While playing, participants start with the lowest quantum state, play cards to "program" a quantum computer, and aim to achieve the highest possible quantum state. Thereby they experience quantum features such as superposition, interference, and entanglement. By also including high-dimensional quantum states, i.e., systems that can take more than two possible values, and by developing different multi-player modes, the game can help the players to understand complex quantum state operations and can also be used as an introduction to quantum computational tasks for students. As such, it can also be used in a classroom environment to increase the conceptual understanding, interest, and motivation of a student. Therefore, the presented game contributes to the ongoing efforts on gamifying quantum physics education with a particular focus on the counter-intuitive features which quantum computing is based on.

quant-ph↗

arXiv2022

Anisotropic Spatial Entanglement

The photon-pairs generated through spontaneous parametric down-conversion (SPDC) possess strong correlations in their transverse position and momentum degrees of freedom. For such photon pairs, the transverse position correlation length depends on the crystal thickness and the pump wavelength, whereas the transverse momentum correlation length depends on the beam waist and spatial coherence length of the pump. By controlling these parameters, it is possible to engineer the spatial entanglement. Here, we utilize the circular asymmetry of the pump by using an elliptical-Gaussian beam to change the degree of entanglement in transverse directions, which we call anisotropic entanglement. We show the interrelation between the degree of anisotropic entanglement and the asymmetry in the beam width. In addition, we also show that for a highly asymmetric pump beam, the entanglement along the y-direction completely vanishes, whereas the entanglement in the x-direction remains intact.

quant-ph↗

arXiv2022

Intensity correlations in perturbed optical vortices: Diagnosis of the topological charge

We propose and experimentally verify a novel scheme for diagnosing the order of a perturbed optical vortex using its 2-D spatial autocorrelation function. The order of a vortex was found to be equal to the number of dark rings or zero points present in the correlation function. We have provided a compact analytical expression for the correlation function in the form of Laguerre polynomials. Further, we have utilized the divergence of the first zero point of Laguerre polynomials upon propagation to obtain information about higher-order spatial modes, and compared it with our experimental results. It has been shown that the accuracy of obtained information can further be enhanced by increasing the collection area of scattered vortex beams. These results find applications in free-space optical and satellite communication as the proposed technique is alignment-free, and information can be obtained even with a small portion of the scattered beam.

physics.optics↗

arXiv2022

BBM92 quantum key distribution over a free space dusty channel of 200 meters

Free space quantum communication assumes importance as it is a precursor for satellite-based quantum communication needed for secure key distribution over longer distances. Prepare and measure protocols like BB84 consider the satellite as a trusted device, which is fraught with security threat looking at the current trend for satellite-based optical communication. Therefore, entanglement-based protocols must be preferred, so that one can consider the satellite as an untrusted device too. The current work reports the implementation of BBM92 protocol, an entanglement-based QKD protocol over 200 m distance using an indigenous facility developed at Physical Research Laboratory (PRL), Ahmedabad, India. Our results show the effect of atmospheric aerosols on sift key rate, and eventually, secure key rate. Such experiments are important to validate the models to account for the atmospheric effects on the key rates achieved through satellite-based QKD.

quant-ph↗

arXiv2021

Size invariant twisted optical modes for efficient generation of higher dimensional quantum states

Optical vortex beams are profiled as helical wavefronts with a phase singularity carrying an orbital angular momentum (OAM) associated with their spatial distribution. The transverse intensity distribution of a conventional optical vortex has a strong dependence on the carried topological charge. However, perfect optical vortex (POV) beams have their transverse intensity distribution independent of their charge. Such `size-invariant' POV beams have found exciting applications in optical manipulation, imaging and communication. In this article, we investigate the use of POV modes in the efficient generation of high dimensional quantum states of light. We generate heralded single photons carrying OAM using spontaneous parametric down-conversion (SPDC) of POV beams. We show that the heralding efficiency of the SPDC single photons generated with POV pump is greater than that with normal optical vortex beams. The dimensionality of the two-photon OAM states is increased with POV modes in the pump and projective measurements using Bessel-Gaussian vortex modes that give POV, instead of the Laguerre-Gaussian modes.

quant-ph↗

arXiv2021

High-accuracy longitudinal position measurement using self-accelerating light

Radially self-accelerating light exhibits an intensity pattern that describes a spiraling trajectory around the optical axis as the beam propagates. In this article, we show in simulation and experiment how such beams can be used to perform a high-accuracy distance measurement with respect to a reference using simple off-axis intensity detection. We demonstrate that generating beams whose intensity pattern simultaneously spirals with fast and slow rotation components enables a distance measurement with high accuracy over a broad range, using the high and low rotation frequency, respectively. In our experiment, we achieve an accuracy of around 2~$μ$m over a longitudinal range of more than 2~mm using a single beam and only two quadrant detectors. As our method relies on single-beam interference and only requires a static generation and simple intensity measurements, it is intrinsically stable and might find applications in high-speed measurements of longitudinal position.

physics.optics↗

arXiv2020

Spatial structuring of light for undergraduate laboratory

In recent times, spatial light modulators have become a common tool in optics laboratories as well as industrial environment to shape the spatial structure of a beam. Although these devices are often easy to use, they usually come at a high cost such that they are far from being implemented in a lot of undergraduate laboratories. However, over the last years, the progress in developing more cost-effective projectors has led to affordable spatial light modulators in the form of so-called Digital Micromirror Devices (DMD). This reduction in price, as well as their simple usage, make such devices increasingly suitable for undergraduate laboratories to demonstrate optical effects and the shaping of light fields. Here, we show one of the most cost-effective ways to make a DMD available, namely turning a projector evaluation module into a computer-controlled spatial light modulator. We explain the underlying functioning and how this low-cost spatial light modulator can be used in undergraduate laboratories. We further characterize the efficiency of the device for the most commonly used laser wavelengths and demonstrate various exemplary optics experiments suitable for undergraduate laboratories ranging from single and multi-slit diffraction, to optical Fourier transformations. Lastly, we show that using amplitude holography, the device can be used to generate transverse spatial modes, e.g. Laguerre-Gaussian beam, which are one of the most commonly used spatially structured beams.

physics.ed-ph↗

arXiv2020

Two-photon Quantum Interference and Entanglement at 2 μm

Quantum-enhanced optical systems operating within the 2- to 2.5-$μ$m spectral region have the potential to revolutionize emerging applications in communications, sensing, and metrology. However, to date, sources of entangled photons have been realized mainly in the near-infrared 700- to 1550-nm spectral window. Here, using custom-designed lithium niobate crystals for spontaneous parametric down-conversion and tailored superconducting nanowire single-photon detectors, we demonstrate two-photon interference and polarization-entangled photon pairs at 2090 nm. These results open the 2- to 2.5-$μ$m mid-infrared window for the development of optical quantum technologies such as quantum key distribution in next-generation mid-infrared fiber communication systems and future Earth-to-satellite communications.

quant-ph↗

arXiv2019

Selective tuning of Hilbert spaces in states encoded with spatial modes of light

Spatial modes of light directly give the most easily accessible degree of freedom that span an infinite dimensional Hilbert space. The higher dimensional spatial mode entanglement realized using spontaneous parametric down conversion (SPDC) process is generally restricted to the subspace defined by a single spatial mode in pump. Access to other modal subspaces can be realized by pumping beams carrying several easily tunable transverse modes. As a proof-to-principle experiment, we generate twin-photon states in a nonlinear SPDC process with pump as a superposition of first order Laguerre-Gaussian (or Hermite-Gaussian) modes. We show that the generated states can be easily tuned between different subspaces by controlling the respective modal content in the pump superposition.

quant-ph↗