arXiv subjects

Ravi S. Singh

Publications and source records attributed to Ravi S. Singh.

17 recordsLinked to original sources

arXiv2025

Detection of young massive white dwarfs in core-collapsed globular cluster NGC 362

Core-collapsed globular clusters are ideal targets to explore the presence of stellar collision products. Here, we have studied seventeen FUV bright white dwarf members in the globular cluster NGC 362 using data obtained from the Ultra Violet Imaging Telescope (UVIT) mounted on AstroSat and from HST. Multi-wavelength spectral energy distributions (SEDs) are analyzed using UV and optical data sets to characterize and determine the parameters of white dwarfs. Fourteen of the white dwarfs fit single-component SEDs well, while three showed a good fit with a two-component SED model,indicating a binary system comprising a white dwarf and a low-mass main-sequence star. The effective temperature, radius, luminosity, and mass of white dwarfs range between 22000 - 70000 K, 0.008 - 0.028 Rsun, 0.09 - 3.0 Lsun, and 0.30 - 1.13 Msun, respectively. The effective temperature, radius, luminosity, and mass of the companions (low-mass main-sequence stars) are 3500 - 3750 K, 0.150 - 0.234 Rsun,0.003 - 0.01 Lsun, and 0.14 - 0.24 Msun, respectively. The three binary systems (WD-MS), along with the massive WDs may have formed through dynamical processes that occurred during the core collapse of the cluster. This is the first evidence of a massive WD formation in a core-collapsed cluster, which is the missing link in the formation of a fast radio burst (FRB) progenitor in a globular cluster. This study provides evidence that NGC 362 hosts stellar systems that may evolve into exotic stars such as Type Ia supernovae, and/or FRBs in the future. *This is paper VI of the Globular Cluster UVIT Legacy Survey.

astro-ph.SR↗

arXiv2024

Simultaneous Multiparty Quantum Teleportation Supervised by a Controller

Quantum network harbours a technology of multiparty transmission and computation of quantum information. We here design a quantum circuit comprising of Hadamard and controlled-Not gates for preparation of a cluster state of symmetric and antisymmetric Bell pairs involving seventeen qubits which is later on utilized as a quantum channel. This seventeen qubits quantum channel is employed to strategize a simultaneous multiparty (quattro directional) quantum teleportation protocol in which four senders (Alice, Bob, Charlie and David) transmit their arbitrary unknown two-qubit states to respective four receivers (Fancy1, Fancy2, Fancy3 and Fancy4) under the supervision of a controller Elle. After successful accomplishment of the protocol, we assess and compare our scheme with contemporary protocols based on quantum (classical) resource consumption, transmitted qubits, operation complexity and efficiency. We found that intrinsic efficiency of our protocol pegs at 21.65 percent.

quant-ph↗

arXiv2024

Asymmetric Bidirectional Quantum Teleportation: Arbitrary bi-modal Information State

Optical coherent states are experimentally realizable continuous variable quantum states of which preparation by lasers, as well as its manipulation and monitoring by linear optical gadgets are well established. We propose a strategy to send an arbitrary superposition of four-component bimodal entangled coherent states from a sender to a receiver who, simultaneously, tries to transmit an unknown Schrodinger Cat coherent state to sender via employing a cluster consisting of three superposition of two component bimodal entangled coherent states as the quantum channel and utilizing linear optical gadgets. Heralded detection of photons in laboratories of sender and receiver followed by classical communications of even and odd number of photons and local unitary operations, impeccably, accomplishes simultaneous faithful asymmetric bidirectional quantum teleportation with one eighth of probability of success. It is seen that not all detection events implement the protocol and, therefore, one has to locally apply displacement operator, a necessary evil. We analyze near faithful partial asymmetric bidirectional quantum teleportation and associated probability of success therein. We demonstrated that, for an intense coherent optical field, fidelity approach unity.

quant-ph↗

arXiv2024

Unidirectional Cyclic Quantum Teleportation of Arbitrary Schrodinger Cat Coherent-states via Bell Coherent-States

Exploiting the cluster of three Bell coherent-states as quantum channel, we presented a scheme wherein quantum-informations encoded in three arbitrary superposed coherent states, i.e., Schrodinger Cat coherent-states are simultaneously transmitted in unidirectional cyclic sequence among three parties by invoking usage of linear optical gadgets such as ideal beam-splitters, phase shifters and photon-number-resolving detectors. Proposed simultaneous unidirectional Cyclic Quantum Teleportation is faithful with one-eighth probability of success. Furthermore, it is seen that not every detection-events provides faithful scheme and, hence, fidelity and probability of success of near-faithful Cyclic Quantum Teleportation is evaluated and numerically interpreted.

quant-ph↗

arXiv2023

UVIT/AstroSat Investigation of a low luminous Blue Straggler Star in NGC 362: Detection of extremely low mass white dwarf as companion

In the present study, we identified an extremely low-mass white dwarf as a companion to a low luminous blue straggler star within the Galactic globular cluster NGC 362. To conduct the analysis, we utilized data obtained from various sources, including AstroSat Ultra Violet Imaging Telescope, UVOT, and the 2.2-m ESO telescope. By examining the spectral energy distribution of the blue straggler star candidate, we successfully identified an extremely low mass white dwarf as its binary companion. We determined the effective temperature, radius, and luminosity for both, the extremely low-mass white dwarf and the blue straggler star candidate. Specifically, the extremely low-mass white dwarf exhibits an effective temperature of 15000 K, a radius of 0.17 Rsun, a luminosity of 1.40 Lsun, and a mass range of 0.19-0.20 Msun. Furthermore, the position of the straggler star within the cluster suggests their formation via the Case A/B mass-transfer mechanism in a low-density environment.

astro-ph.SR↗

arXiv2023

GlobULeS-V. UVIT/AstroSat studies of stellar populations in NGC 362: Detection of Blue Lurkers in a Globular Cluster

We report the discovery of four blue lurkers with low and extremely low-mass white dwarf (ELM WDs) companions in the Galactic globular cluster NGC 362 using AstroSat Ultra Violet Imaging Telescope (UVIT). We analyzed the multi-wavelength spectral energy distribution (SED) of FUV-bright MS stars using data from the UVIT, UVOT, GAIA EDR3, and 2.2m ESO/MPI telescopes. Two each of low-mass WDs and ELM WDs are found as companions for the four blue lurkers by the fitting of two-component SED models. The effective temperatures, radii, luminosities, and masses of two low-mass WDs are (35000, 23000) K, (0.04, 0.05) Rsun , (1.45, 0.22) Lsun , and (0.2, 0.2) Msun, while the two ELM WDs are (14750, 14750) K, (0.09, 0.10) Rsun, (0.34, 0.40) Lsun, and (0.18, 0.18) Msun. The position of blue lurkers within the cluster shows that they originated via the Case A/B mass-transfer mechanism in a low-density environment. This is the first detection of blue lurkers with low-mass WDs and ELM WDs as companions in a globular cluster. The companion cooling age is less than 4 Myr, which suggests that they were just recently formed. These binary systems might have originated due to the cluster recent core collapse.

astro-ph.SR↗

arXiv2023

Intranight Optical Variability of Blazars and Radio-quiet Quasars using the ZTF Survey

We explore the potential of the ongoing Zwicky-Transient-Facility (ZTF) survey for studying Intra-Night Optical Variability (INOV) of active galactic nuclei (AGN), in particular for picking rare events of large INOV amplitudes, whose detection may require extensive temporal coverage. For this, we have used the available high cadence subsets of the ZTF database to build a well-defined large sample of 53 blazars (BLs) and another sample of 132 radio-quiet quasars (RQQs), matched to the blazar sample in the redshift-magnitude plane. High-cadence ZTF monitoring of these two matched samples are available, respectively, for 156 and 418 intranight sessions. Median durations for both sets of sessions are 3.7 hours. The two classes of powerful AGN monitored in these sessions represent opposite extremes of jet activity. The present analysis of their ZTF light curves has revealed some strong INOV events which, although not exceptionally rare for blazars, are indeed so for RQQs, and their possible nature is briefly discussed.

astro-ph.GA↗

arXiv2022

GlobULeS-IV. UVIT/AstroSat detection of extremely low mass white dwarf companions to blue straggler stars in NGC 362

We report the discovery of extremely low-mass white dwarfs (ELM WDs) as a companion of blue straggler stars (BSSs) in the Galactic globular cluster NGC 362 using images from AstroSats Ultra Violet Imaging Telescope (UVIT). Spectral Energy Distributions (SEDs) for 26 FUV bright member BSSs are created using data from the UVIT, UVOT, Gaia EDR3, and the 2.2 m ESO/MPI telescope. A single SED is fitted to 14 BSSs, whereas double-SED fits revealed ELM WDs as binary companions in 12 of the 26 BSSs studied. The effective temperature, radius, luminosity and mass of the 12 ELM WDs are found to have a range (Teff = 9750-18000 K, R = 0.1-0.4 Rsun, L = 0.4-3.3 Lsun, and M =0.16-0.20 Msun). These suggest that 12 BSSs are post-mass-transfer systems formed through Case A/B mass transfer pathway. To the best of our knowledge, this is the first finding of ELM WDs as companions to BSS in globular clusters. This cluster is known to have a binary BSS sequence and the 12 binary and 14 single BSSs (as classified by the SEDs) follow the mass transfer and collisional sequence of BSS in the colour-magnitude diagram. The cooling ages of 9 of the ELM WDs are found to be younger than 500 Myr. Though the binary BSSs may have formed during the core-collapse (approx 200 Myr) or as part of the dynamical evolution of the cluster, they provide new insights on the dynamics of this cluster.

astro-ph.SR↗

arXiv2021

Optical Flux and Colour Variability of Blazars in the ZTF Survey

We investigate the temporal and colour variability of 897 blazars, comprising 455 BL Lacertae objects (BL Lacs) and 442 Flat Spectrum Radio Quasars (FSRQs), selected from the Roma-BZCAT catalogue, using the multi-band light curves from the Zwicky Transient Facility (ZTF DR6) survey. Assessing the colour variability characteristics over ~2 year timescales, we found that 18.5 per cent (84 out of 455) BL Lacs showed a stronger bluer when brighter (BWB) trend, whereas 9.0 per cent (41 out of 455) showed a redder when brighter (RWB) trend. The majority (70 per cent) of the BL Lacs showing RWB are host galaxy dominated. For the FSRQ subclass, 10.2 per cent (45 out of 442) objects showed a strong BWB trend and 17.6 per cent (78 out of 442) showed a strong RWB trend. Hence we find that BL Lacs more commonly follow a BWB trend than do FSRQs. This can be attributed to the more dominant jet emission in the case of BL Lacs and the contribution of thermal emission from the accretion disc for FSRQs. In analysing the colour behaviour on shorter time windows, we find many blazars evince shorter partial trends of BWB or RWB nature (or occasionally both). Some of such complex colour behaviours observed in the colour-magnitude diagrams of the blazars may result from transitions between the jet-dominated state to the disc-dominated state and vice versa.

astro-ph.GA↗

arXiv2013

On the Polarization of non-Guassian optical quantum field: higher-order optical-polarization

Polarization of light signifies transversal, anisotropic and asymmetrical statistical property of electromagnetic radiation about direction of propagation. Traditionally, optical-polarization is characterized by Stokes theory susceptible to be insufficient in assessing polarization structure of optical quantum fields and, also, does not decipher twin characteristic polarization parameters (ratio of real amplitudes and difference in phases). An alternative way, in spirit of classical description of optical-polarization, is introduced which can be generalized to deal higher-order polarization of quantum light, particularly, prepared in non-Guassian Schrodinger Cat or Cat-like states and entangled bi-modal coherent states. On account of pseudo mono-modal or multi-modal nature of such optical quantum field, higher-order polarization is seen to be highly sensitive to the basis of description.

physics.optics↗

arXiv2013

Degree of Polarization in Quantum Optics through second generalization of Intensity

Classical definition of degree of polarization is expressed in quantum domain by replacing intensities through quantum mechanical average values of relevant number operators and is viewed as first generalization of Intensity. This definition assigns inaccurately the unpolarized status to some typical optical fields such as amplitude coherent phase randomized and hidden polarized, which are not truly unpolarized light. The apparent paradoxical trait is circumvented by proposing a new definition of degree of polarization in Quantum Optics through second generalization of Intensity. The correspondence of new degree of polarization to usual degree of polarization in Quantum Optics is established. It is seen that the two definitions disagree significantly for intense optical fields but coincides for weak light (thermal light) or for optical fields in which occupancy of photons in orthogonal mode is very feeble. Our proposed definition of degree of polarization, similar to other proposals in literature, reveals an interesting feature that states of polarization of optical quantum fields depend upon the average photons (intensity) present therein.

quant-ph↗

arXiv2012

Quasi-Probability Distribution Functions for Optical-Polarization

Cahill-Glauber C(s)-correspondence is employed to construct Quasi-Probability Distribution Functions (QPDFs) for optical-polarization in phase space following equivalent description of polarization in Classical Optics. The proposed scheme provides pragmatic insights as compared to obscure SU (2) quasi-distributions on Poincare sphere. QPDF (Wigner function) of bi-modal quadrature coherent states is evaluated and numerically investigated to demonstrate the application.

quant-ph↗

arXiv2012

Higher (2nd)-order polarization-Wigner function for `even' entangled bi-modal coherent states

Higher (2nd)-order Wigner distribution function in quantum phase space for entangled bi-modal coherent states, a representative of higher (2nd)-order optical-polarization, is introduced by generalizing kernel (transiting) operator in Cahill-Glauber C(s)-correspondence rule. The nature is analyzed which reveals the occurrence of oscillating three peaks: 'two' for individual bi-modes and third for interference between modes. Also, the graphics of 2nd-order polarization-Wigner distribution function, incisively, demonstrates that it is of non-Gaussian nature attaining non-negative values in quantum phase space.

quant-ph↗

arXiv2011

Generation of Hidden Optical-Polarization: Squeezing and Non-Classicality

A monochromatic double-mode coherent light endowed with orthogonally polarized photons propagating collinearly is studied in Degenerate Parametric Amplification. Generation of Hidden Optical- Polarized States is shown by non-zero values of Index of Hidden Optical-Polarization. Squeezing in HOPS is demonstrated by recognizing a Squeezing function. The Non-Classical feature of HOPS is observed by 'degree of Hidden Optical-Polarization' which attains non-classical value 'greater than unity'. The dynamical nature of Generation, Squeezing and Non-Classicality are numerically presented.

quant-ph↗

arXiv2011

Higher-order optical-polarization

Polarization properties of optical field in Schrödinger Cat states or Even- or Odd-Coherent states are investigated by generalizing usual concept of optical-polarization to render the concept of Higher-order optical-polarization. It is observed that Higher-order optical-polarization is a basis-dependent property of optical field.

physics.optics↗

arXiv2010

Generation and Analysis of Hidden Optical-Polarization States

A hybrid Optical-Device (Phase-Conjugating Mirror Michelson Interferometer), made up of Phase-conjugate mirror along with ordinary mirror and Polarization Beam-splitter, is formally designed and investigated for the generation of an unusual Optical-Polarization States. This weird yet 'essentially single-mode' optical-polarization state has non-random 'ratio of amplitudes' and non-random 'sum of phases' in orthogonal bases-modes contrary to usual 'truly single-mode' optical-polarization states of which non-random 'ratio of amplitudes' and non-random 'difference of phases' serves as optical-polarization characteristic parameters. Since it is seen not to be characterized by Stokes parameters, one may, therefore, assign the name 'Hidden Optical-Polarization States (HOPS)'. HOPS are characterized by a set of parameters, namely, Hidden Optical-Polarization parameters. Formal experimental schemes are presented to experimentally measure these parameters and, thus, offering characterization of HOPS.

quant-ph↗

arXiv2010

Observation of Squeezing in Hidden Optical-Polarization States

The dynamic feature of monochromatic bi-modal chaotic optical field, enriched with orthogonally-polarized basis-modes propagating collinearly and undergoing Degenerate Parametric Amplification, is investigated to demonstrate Squeezing in Hidden Optical-Polarization states. The Variance (Noise) of Hidden Optical-Polarization Parameters showing squeezing therein is numerically studied by identifying a Squeezing - Function. This squeezing in HOPS is seen to depend critically on interaction time and an Onset-time responsible for the same is demarcated.

quant-ph↗