arXiv subjects

G. K. Samanta

Publications and source records attributed to G. K. Samanta.

At least 19 recordsLinked to original sources

arXiv2026

A projection-free approach toward mapping the structured polarization fields

We present a projection-free method for mapping two-dimensional polarization distributions using Hong-Ou -Mandel (HOM) interference. Conventional polarization characterization techniques, such as Stokes polarimetry, rely on sequential intensity measurements under multiple polarization projections, making their accuracy and sensitivity susceptible to the extinction ratio, calibration errors, and stability of the polarization analysis optics. Our approach overcomes these limitations by exploiting the sensitivity of two-photon bunching to polarization indistinguishability at a balanced beam splitter. Using a HOM interferometer driven by a high-brightness spontaneous parametric down-conversion photon-pair source at 810 nm, we introduced a birefringent vortex waveplate with spatially varying polarization rotations in one interferometer arm and measured the resulting coincidence counts with a high signal-to-noise ratio, enabling projection-free characterization of the sample-induced polarization transformations. This configuration maps spatially dependent polarization variations directly onto coincidence counts, providing a projection-free reconstruction of the polarization distribution. We demonstrate high-fidelity ($\sim$95$\%$) reconstruction of the spatial polarization pattern with an angular resolution of approximately $\sim0.4^\circ$. The use an estimator that saturates the Cramer-Rao bound, computed from the Fisher information, can improve resolution further at the cost of longer acquisition. The proposed quantum-optical technique offers a simple, scalable, and high-precision framework for characterizing structured polarization fields in birefringent materials.

quant-ph↗

arXiv2026

Quantum sensing through bosonic-fermionic Bell-state transitions in two-photon interference

Hong-Ou-Mandel (HOM) interference has become a central resource for quantum sensing and metrology owing to its sensitivity to temporal delay and photon indistinguishability. However, existing HOM-based sensing schemes generally rely on inserting a sample into one arm of the interferometer, making the measurement vulnerable to optical loss, alignment instability, and bandwidth-dependent distortion of the interference profile. Here, we demonstrate a symmetry-controlled quantum sensing scheme based on continuous transitions between symmetric (bosonic-like) and antisymmetric (fermionic-like) Bell states in two-photon interference. By imprinting a geometric phase onto the classical pump beam and transferring it to polarization-entangled photons generated via spontaneous parametric down-conversion, we coherently tune the exchange symmetry of the entangled state without altering the temporal or spectral indistinguishability of the photons. The HOM response evolves continuously from bunching to antibunching with a sine square phase dependence, producing a coincidence modulation of approximately 10 * 10^4 counts s^-1 counts/s. In contrast to conventional HOM sensing, the phase-modulation linewidth remains fixed at pi/2, independent of photon bandwidth. Using a birefringent crystal placed directly in the pump beam, we measure thermo-dispersive birefringence with a resolution of the order of 10^{-6} over a broad temperature range. Our results establish exchange symmetry as a controllable resource for robust quantum sensing and symmetry-engineered photonic quantum information processing.

quant-ph↗

arXiv2026

Geometric phase-assisted simple phase compensation enabling quantum key distribution using phase-shifted Bell states

Entanglement-based quantum key distribution (QKD) relies on the distribution of high-fidelity maximally entangled Bell states, typically generated via spontaneous parametric down-conversion (SPDC). In practical systems, unwanted relative phases arise from birefringence, pump-beam contributions, imperfect photon-pair generation, transmission through physical channels, and collection, transforming Bell states into phase-shifted states. This degrades interference visibility, increases the quantum bit error rate (QBER), and limits secure key generation. Conventional compensation techniques, such as birefringent crystals, interferometric stabilization, and spatial light modulators, are often impractical in real-world deployments. Here, we demonstrate a simple and versatile phase-compensation scheme that can be implemented at either the source or the receiver to eliminate arbitrary relative phases in Bell states. We theoretically and experimentally quantify the dependence of QBER in the BBM92 protocol on the relative phase and show that geometric-phase-based control can effectively restore entanglement quality. In a proof-of-concept experiment using a nondegenerate polarization Bell state, we achieve a fidelity exceeding 95% and reduce QBER below the 11% security threshold required for secure QKD. This robust approach enables practical phase control in entangled-photon systems and can be extended to time-bin QKD via time-polarization mapping, offering a promising route toward stable, low-QBER quantum communication.

quant-ph↗

arXiv2025

Free-space multi-user quantum network with high key rate

Emergent quantum networks are the essential ingredient for securely connecting multiple users worldwide, extensively deployed in both fibre and free-space. An essential element is the multiplexing of entanglement to multiple users, overcoming the peer-to-peer restriction of quantum key distribution (QKD), so far successfully shown in fibre-based architectures. Here, we demonstrate a free-space quantum space division multiplexing architecture using just one entanglement source to realise a fully connected twelve-channel quantum network for seamless QKD connections between six users. The network achieves record coincidence rates exceeding $3 \times 10^{4}$ s$^{-1}$ between any pair of nodes on the network, for sifted key rate of over 400 kbps. Our approach overcomes the active switching hurdle that has hindered the free-space deployment of quantum multiplexing, is fully passive, easily scalable to more nodes and compatible with fibre-based integration, thus opening a new path to scalable and resource-efficient quantum networks that utilise free-space links.

quant-ph↗

arXiv2025

Beam-splitter-free, high-rate quantum key distribution inspired by intrinsic quantum mechanical spatial randomness of entangled photons

Quantum key distribution (QKD) using entangled photon sources (EPS) is a cornerstone of secure communication. Despite rapid advances in QKD, conventional protocols still employ beam splitters (BSs) for passive random basis selection. However, BSs intrinsically suffer from photon loss, imperfect splitting ratios, and polarization dependence, limiting the key rate, increasing the quantum bit error rate (QBER), and constraining scalability, particularly over long distances. By contrast, EPSs based on spontaneous parametric down-conversion (SPDC) intrinsically exhibit quantum randomness in spatial and spectral degrees of freedom, offering a natural replacement for BS-based basis selection. Here, we demonstrate a proof-of-concept QKD scheme that exploits the intrinsic spatial randomness of SPDC without employing beam splitters. The annular SPDC emission ring is divided into four spatial sections, effectively generating two independent EPSs whose photon pairs are distributed to Alice and Bob. Crucially, the measurement basis is not predetermined but is assigned after photon detection by exploiting intrinsic detector timing jitter, thereby concealing the basis information from a potential eavesdropper. This post-detection basis assignment emulates stochastic basis choice while avoiding BS-induced losses and bias. Experimentally, our scheme achieves a 6.4-fold enhancement in sifted key rate, a consistently reduced QBER, and a near-ideal encoding balance between linear and rectilinear bases. Furthermore, the need for four spatial channels can be avoided by employing wavelength demultiplexing to generate two EPSs at distinct wavelength pairs. Harnessing intrinsic spatial/spectral randomness thus enables robust, bias-free, high-rate, and low-QBER QKD, offering a scalable pathway for next-generation quantum networks.

quant-ph↗

arXiv2025

Classical-to-quantum transfer of geometric phase for non-interferometric phase measurement and manipulation of quantum state

The geometric phase, originating from the cyclic evolution of a state, such as polarization on the Poincaré sphere, is typically measured through interferometric approaches that often include unwanted contributions from the dynamic phase. Here, we present a non-interferometric technique based on quantum correlation of pair photons to measure the geometric phase of a classical beam. The transfer of geometric phase of the classical pump beam arising from the cyclic evolution of its polarization state on the Poincaré sphere onto the polarization-entangled pair photons generated via spontaneous parametric down-conversion in a Sagnac interferometer enables easy control over the quantum state. Characterization of the generated quantum states reveals that the geometric phase of the pump beam controls the coincidence counts, entanglement visibility, Bell's parameter, quantum state tomography, and fidelity in close agreement with theoretical predictions. We observe sinusoidal modulation of the Bell's parameter and state fidelity with changes in the geometric phase, resulting in transitions between orthogonal Bell states and Bell-like maximally entangled states. Our results establish the geometric phase of the classical pump as a tunable parameter for quantum state control, offering a compact, passive platform for phase manipulation in quantum photonic systems, enabling geometric phase-based quantum gates, and compensating unwanted phase acquired by the quantum state on propagation.

quant-ph↗

arXiv2025

Device-independent, megabit-rate quantum random number generator with beam-splitter-free architecture and live Bell test certification

Device-independent quantum random number generators (DI-QRNGs) are crucial for information processing, ensuring certified quantumness and genuine randomness. However, existing implementations often face low bit rates due to quantumness testing challenges. Here, we present a high-bit-rate DI-QRNG with live quantumness certification through the Bell test. Using spontaneous parametric down-conversion in a polarization Sagnac interferometer, we generate entangled pair-photons at diametrically opposite points on an annular ring with strong spatial and temporal correlations. Dividing the ring into six diametrically opposite sections, we create three entangled photon sources that exhibit bias-free quantum mechanical randomness from a single resource. By utilizing the coincidence counts of pair-photons from two sources, we generate raw bits, while the third source simultaneously measures the Bell's parameter without any loss of QRNG bits. We have generated 90 million raw bits in 46.4 seconds with the Bell parameter (S $>$ 2), with a minimum entropy extraction ratio exceeding 97$\%$. Post-processed using a Toeplitz matrix, the DI-QRNG achieves a bit rate of 1.8 Mbps, passing all NIST 800-22 and TestU01 tests. In the absence of Bell's parameter for a non-maximally entangled state, $g^{(2)}(0)$ can be the metric for quantumness measure. Scalable and beam-splitter-free, this megabit-rate DI-QRNG is ideal for practical applications.

quant-ph↗

arXiv2025

Dead-zone-free single-beam atomic magnetometer based on free-induction-decay of Rb atoms

Free-induction-decay (FID) magnetometers have evolved as simple magnetic sensors for sensitive detection of unknown magnetic fields. However, these magnetometers suffer from a fundamental problem known as a "dead zone," making them insensitive to certain magnetic field directions. Here, we demonstrate a simple experimental scheme for the dead-zone-free operation of a FID atomic magnetometer. Using a single laser beam containing equal strength of linear- and circular-polarization components and amplitude-modulation at a low-duty cycle, we have synchronously pumped the rubidium-87 atoms with both first- and second-order frequency harmonics. Such a pumping scheme has enabled us to observe the free Larmor precession of atomic spins at a frequency of $Ω_L$ (orientation) and/or 2$Ω_L$ (alignment) in a single FID signal, depending on the direction of the external magnetic field. We observed that the amplitude of the FID signal does not go to zero for any magnetic field direction, proving the absence of dead zones in the magnetometer. The magnetometer has a sensitivity in the range of 3.2 - 8.4 pT/$\sqrt{Hz}$ in all directions. Our experimental scheme can be crucial in developing miniaturized atomic magnetometers for various practical applications, including geomagnetic applications.

physics.atom-ph↗

arXiv2024

Fast measurement of group index variation with ultimate precision using Hong-Ou-Mandel interferometry

Hong-Ou-Mandel (HOM) interferometry has emerged as a valuable tool for quantum sensing applications, particularly in measuring physical parameters that influence the relative optical delay between pair photons. Unlike classical techniques, HOM-based quantum sensors offer higher resolution due to their intrinsic dispersion cancellation property. Despite this advantage, achieving precise measurements of optical delay crucial for practical applications often involves time-consuming integration and post-processing with traditional statistical methods. To address this challenge, our recent work focused on optimizing optical delay measurements in a time-efficient manner. By carefully selecting the length of a 1 mm periodically-poled KTP (PPKTP) crystal for pair photon generation, we achieved a remarkable group index measurement precision of $\sim 6.75\times 10^{-6}$ per centimeter of sample length, surpassing the previous maximum precision by over 400$\%$. These current measurements maintain fast detection and high photon counts, which are essential for practical quantum sensing applications. The HOM-based method, while limiting the measurement range, can be extended by compensating for photon delay using an optical delay stage. As a proof-of-principle, we measured the group index variation of PPKTP over a temperature range up to 200$^{\circ}$C with a precision in the range of one part per million ($\sim$10$^{-6}$). This advancement not only contributes to quantum sensing but also holds promising implications for high-precision and long-range measurements in quantum optical coherence tomography.

quant-ph↗

arXiv2024

Beamsplitter-free, high bit-rate, quantum random number generator based on temporal and spatial correlations of heralded single-photons

The spontaneous parametric down-conversion (SPDC), an inherently random quantum process, produces a non-deterministic photon-pair with strong temporal and spatial correlations owing to both energy and momentum conservation. Therefore, the SPDC-based photon pairs are used for quantum random number generation (QRNG). Typically, temporal correlation in association with an ideal unbiased beam splitter is used for QRNG without fully exploring the spatial correction. As a result, SPDC-based QRNG has a low bit rate. On the other hand, due to the spatial correlation, the photon pairs in non-collinear phase-matched geometry are generated randomly in diametrically opposite points over an annular ring spatial distribution. Therefore, exploring the temporal correlation between photon pairs from different sections of the annual ring can lead to multi-bit QRNG at a high rate, avoiding the need for a beam splitter. As a proof-of-concept, we report on high-bit-rate QRNG by using spatial correlation of photon-pairs by sectioning the SPDC ring of a non-collinear, degenerate, high-brightness source and temporal correlation between the diametrically opposite sections. Dividing the annular ring of the high-brightness photon-pair source based on a 20 mm long, type-0 phase-matched, periodically-poled KTP crystal into four sections, recording the timestamp of the coincidences (widow of 1 ns) between photons from diametrically opposite sections and assigning bits (0 and 1), we extracted 90 million raw bits over 27.7 s at a pump power of 17 mW. We determined the extraction ratio using the minimum entropy evaluation of more than 95% in our case. Using Toeplitz matrix-based post-processing, we achieved a QRNG with a bit-rate of 3 Mbps, passing all NIST 800-22 and TestU01 test suites. The generic scheme shows the possibility of further enhancement of the bit rate through more sectioning of the SPDC ring.

quant-ph↗

arXiv2024

Talbot effect-based sensor measuring grating period change in subwavelength range

Talbot length, the distance between two consecutive self-image planes along the propagation axis for a periodic diffraction object (grating) illuminated by a plane wave, depends on the period of the object and the wavelength of illumination. This property makes the Talbot effect a straightforward technique for measuring the period of a periodic object (grating) by accurately determining the Talbot length for a given illumination wavelength. However, since the Talbot length scale is proportional to the square of the grating period, traditional Talbot techniques face challenges when dealing with smaller grating periods and minor changes in the grating period. Recently, we demonstrated a Fourier transform technique-based Talbot imaging method that allows for controlled Talbot lengths of a periodic object with a constant period and illumination wavelength. Using this method, we successfully measured periods as small as a few micrometers and detected sub-micrometer changes in the periodic object. Furthermore, by measuring the Talbot length of gratings with varying periods imaged through the combination of a thick lens of short focal length and a thin lens of long focal length and large aperture, we determined the effective focal length of the thick lens in close agreement with the theoretical effective focal length of a thick lens in the presence of spherical aberration. These findings establish the Talbot effect as an effective and simple technique for various sensing applications in optics and photonics through the measurement of any physical parameter influencing the Talbot length of a periodic object.

physics.optics↗

arXiv2023

Polarization coverage and self-healing characteristics of Poincaré-Bessel beam

As a vector version of scalar Bessel beams, Poincaré-Bessel beams (PBBs) have attracted a great deal of attention due to the presence of polarization singularities and their nondiffraction and self-healing properties. Previous studies of PBBs have been restricted primarily to understanding the disinclination patterns in the spatially variable polarization, and many of the properties of PBBs remain unexplored. Here, we present a theoretical and experimental study of the polarization characteristics of PBBs, investigating a variety of their features. Using a mode transformation of a full Poincaré (FP) beam in a rectangular basis, ideally carrying 100$\%$ polarization coverage of polarization states represented on the surface of the Poincaré sphere, we observe the PBB as the superposition of an infinite number of FP beams, as each ring of PBB has polarization coverage >75$\%$. We also observe the resilience of a PBB's degree of polarization to perturbation. The polarization-ellipse orientation map of PBBs shows the presence of infinite series of C-point singularity pairs. The number of such series pairs is decided by the number of C-point singularity pairs of the FP beam. The dynamics of C-point singularity pairs in the self-healing process show a non-trivial creation of new singularities and recombination of existing singularities. Such dynamics provide insight into ``Hilbert Hotel'' style evolution of singularities in light beams. The present study can be useful for imaging in the presence of depolarizing surroundings, studying turbulent atmospheric channels, and exploring the rich mathematical concepts of transfinite numbers.

physics.optics↗

arXiv2023

Near-video frame rate quantum sensing using Hong-Ou-Mandel interferometry

Hong-Ou-Mandel (HOM) interference, the bunching of two indistinguishable photons on a balanced beam-splitter, has emerged as a promising tool for quantum sensing. There is a need for wide spectral-bandwidth photon pairs (for high-resolution sensing) with high brightness (for fast sensing). Here we show the generation of photon-pairs with flexible spectral-bandwidth even using single-frequency, continuous-wave diode laser enabling high-precision, real-time sensing. Using 1-mm-long periodically-poled KTP crystal, we produced degenerate, photon-pairs with spectral-bandwidth of 163.42$\pm$1.68 nm resulting in a HOM-dip width of 4.01$\pm$0.04 $μ$m to measure a displacement of 60 nm, and sufficiently high brightness to enable the measurement of vibrations with amplitude of $205\pm0.75$ nm and frequency of 8 Hz. Fisher-information and maximum likelihood estimation enables optical delay measurements as small as 4.97 nm with precision (Cramér-Rao bound) and accuracy of 0.89 and 0.54 nm, respectively, therefore showing HOM sensing capability for real-time, precision-augmented, in-field quantum sensing applications.

physics.optics↗

arXiv2023

Simple experimental realization of optical Hilbert Hotel using scalar and vector fractional vortex beams

Historically, infinity was long considered a vague concept - boundless, endless, larger than the largest - without any quantifiable mathematical foundation. This view changed in the 1800s through the pioneering work of Georg Cantor showing that infinite sets follow their own seemingly paradoxical mathematical rules. In 1924, David Hilbert highlighted the strangeness of infinity through a thought experiment now referred to as the Hilbert Hotel paradox, or simply Hilbert's Hotel. The paradox describes an "fully" occupied imaginary hotel having infinite number of single-occupancy rooms, the manager can always find a room for new guest by simply shifting current guests to the next highest room, leaving first room vacant. The investigation of wavefield singularities has uncovered the existence of a direct optical analogy to Hilbert's thought experiment. Since then, efforts have been made to investigate the properties of Hilbert's Hotel by controlling the dynamics of phase singularities in``fractional'' order optical vortex beams. Here, we have taken such proposals to the next level and experimentally demonstrated Hilbert's Hotel using both phase and polarization singularities of optical fields. Using a multi-ramped spiral-phase-plate and a supercontinuum source, we generated and controlled fractional order vortex beams for the practical implementation of Hilbert's Hotel in scalar and vector vortex beams. Using a multi-ramped spiral-phase-plate, we show the possibility for complicated transitions of the generalized Hilbert's Hotel. The generic experimental scheme illustrates the usefulness of structured beams in visualizing unusual mathematical concepts and also for fractional vector beams driven fundamental and applied research.

physics.optics↗

arXiv2022

Single-pass, nonlinear frequency conversion of full Poincaré beams

Full Poincaré (FP) beams, a special class of fully correlated beams generated through the coaxial superposition of a Laguerre-Gauss and fundamental Gaussian modes of orthogonal polarizations, contain all possible polarization states on the surface of the Poincaré sphere in a single beam. While the presence of all unconventional polarization states makes the FP beams useful for various applications, the dependence of the refractive index on the polarization restricts the efficient generation of FP beams across the electromagnetic spectrum through nonlinear frequency processes. To avoid such difficulty, we use two contiguous BIBO crystals with orthogonal optic axes and generate an ultrafast FP beam at 405 nm with average power as high as 18.3 mW at a single-pass conversion efficiency of 2.19%. Using Stokes parameters and Stokes phases, we observed the doubling of C-points and L-lines singularities and the orbital angular momentum in the SHG process. We have also devised a new technique to estimate the polarization coverage of the pump and SHG FP beams and observed the variation in polarization coverage with the intensity weightage of the constituting beams. Interestingly, the SHG beam has the highest polarization coverage for the FP beam of equal intensity weightage of the superposed beams. We validated our experimental results in close agreement with the theoretical results.

physics.optics↗

arXiv2021

Imaging inspired characterization of single photons carrying orbital angular momentum

We report on an imaging-inspired measurement of orbital angular momentum (OAM) using only a simple tilted lens and an Intensified Charged Coupled Device (ICCD) camera, allowing us to monitor the propagation of OAM structured photons over distance, crucial for free-space quantum communication networks. We demonstrate measurement of OAM orders as high as 14 in a heralded single-photon source (HSPS) and show, for the first time, the imaged self-interference of photons carrying OAM in a modified Mach-Zehnder Interferometer (MZI). The described methods reveal both the charge and order of a photons OAM, and provide a proof of concept for the interference of a single OAM photon with itself. Using these tools, we are able to study the propagation characteristics of OAM photons over distance, important for estimating transport in free-space quantum links. By translating these classical tools into the quantum domain, we offer a robust and direct approach for the complete characterization of a twisted single-photon source, an important building block of a quantum network.

quant-ph↗

arXiv2019

Controlled generation of array beams of higher order orbital angular momentum and study of their frequency doubling characteristics

We report on a simple and compact experimental scheme to generate high power, ultrafast, higher order vortex array beams. Simply by using a dielectric microlens array (MLA) and a plano-convex lens we have generated array beams carrying the spatial property of the input beam. Considering the MLA as a 2D sinusoidal phase grating, we have numerically calculated the intensity pattern of the array beams in close agreement with the experimental results. Using Gaussian embedded vortex beams of order as high as l= 6, we have generated vortex array beams with individual vortices of order as high as l= 6. We have also theoretically derived the parameters controlling the intensity pattern, size and the pitch of the array and verified experimentally. The single-pass frequency-doubling of the vortex array at 1064 nm in a 1.2 mm long BiBO crystal produced green vortex array of order, l_sh= 12, twice the order of the pump beam. Using lenses of different focal lengths, we have observed the vortex array of all orders to follow the focusing dependent conversion similar to the Gaussian beam. The maximum power of the green vortex array is measured to be 138 mW at a single-pass efficiency as high as ~3.65%. This generic experimental scheme can be used to generate array beams of desired spatial intensity profile across wide wavelength range by simply changing the spatial profile of the input beam.

physics.optics↗

arXiv2019

Single-pass, second harmonic generation of ultrafast, higher order vector vortex beams at blue

We report a novel experimental scheme for single-pass second harmonic generation (SHG) of vector vortex beam in the blue. Using an ultrafast Ti:Sapphire laser of pulse width ~17 fs and a set of spiral phase plates in polarization based Mach-Zehnder interferometer (MZI) we have generated vector vortex beams of order as high as lp = 12 at an average power of 860 mW. Given the space-variant polarization of the vector vortex beam, and the dependence of nonlinear frequency conversion processes on the polarization of the interacting beams, using two contiguous bismuth borate crystals with optic axis orthogonal to each other, we have frequency-doubled the near-IR vector vortex beam into visible vector vortex beams with order as high as lsh=24. The maximum output power of the vector vortex beam of order, lsh =2 is measured be as high as 20.5 mW at a single-pass SHG efficiency of 2.4 %. Controlling the temporal delay in the MZI, we have preserved the vector vortex nature of the beams at both pump and frequency-doubled beams at ultrafast timescales. The measurement on mode purity confirms the generation of high quality vector vortex beams at pump and SHG wavelengths. The generic experimental scheme can be used to generate vector vortex beams across the electromagnetic spectrum.

physics.optics↗