arXiv subjects

Ramesh Bhandari

Publications and source records attributed to Ramesh Bhandari.

12 recordsLinked to original sources

arXiv2025

A Robust Modular Quantum Processor

We explore the concept of redundancy of critical elements in a quantum computing architecture to circumvent disruption of quantum operations due to a failure of such an element, for example, from a catastrophic cosmic ray event. We illustrate this concept with reference to a recently proposed superconducting modular quantum architecture with a star-like configuration, which has a router at the center that enables superconducting qubit interactions across various modules. Regarding this router as a vital element, we propose a double-star configuration, where a loss of one router is backed by the second one. We also examine the usefulness of this double-star configuration under normal conditions, namely, when the quantum hardware has been rendered safe against cosmic rays due to other mitigating actions like shielding or movement to an underground facility. Simultaneous two qubit-pair interactions like two simultaneous CZ gates and multiqubit gates like CCZS are then easily facilitated.

quant-ph↗

arXiv2023

Updated evaluation of potential ultra-low Q value $β$-decay candidates

"Ultra-low" Q value $β$ decays are referred to as such due to their low decay energies of less than $\sim$1 keV. Such a low energy decay is possible when the parent nucleus decays into an excited state in the daughter, with an energy close to that of the Q value. These decays are of interest as potential new candidates for neutrino mass determination experiments and as a testing ground for studies of atomic interference effects in the nuclear decay process. In this paper, we provide an updated evaluation of atomic mass data and nuclear energy level data to identify potential ultra-low Q value $β$ decay candidates. For many of these candidates, more precise and accurate atomic mass data is needed to determine if the Q value of the potential ultra-low decay branch is energetically allowed and in fact ultra-low. The precise atomic mass measurements can be achieved via Penning trap mass spectrometry.

nucl-ex↗

arXiv2021

Polarization-Induced Interaction as a Perturbation within Orbital Angular Momentum(OAM) Modes of an Optical Fiber

We investigate the role of polarization effects in orbital angular momentum modes of an optical fiber. Specifically, we revisit the removal of the degeneracy of the $HE$ and the $EH$ vector modes that exists within the weakly guiding approximation (WGA); within WGA, these two vector modes coalesce into a single spatial (scalar) orbital angular momentum (OAM) mode, defined by a common topological charge and associated with left or right circular polarization. While the polarization term of the vector wave equation acting as a Hermitian perturbation operator accounts for the splitting of the $HE$ and the $EH$ vector modes, its extension to explain the accompanying field correction, called the polarization-induced field, encounters hurdles; this field is characterized by a topological charge differing by two in magnitude. The problems are critically examined with a possible remedy and its implications.

physics.optics↗

arXiv2021

Nature of the orbital angular momentum (OAM) fields in a multilayered fiber

We provide a theoretical analysis of the nature of the orbital angular momentum (OAM) modal fields in a multilayered fiber, such as the step-index fiber and the ring-core fiber. In a detailed study of the vector field solutions of the step-index fiber (in the exponential basis), we discover that the polarization-induced field component is a modified scalar OAM field (as opposed to a standard OAM scalar field) with a shifted intensity pattern in the weakly guiding approximation (WGA); the familiar intensity donut pattern is reduced or increased in radius depending upon whether it is a case of spin-alignment or anti-alignment with the OAM. Such a shift in the intensity pattern appears to be a general feature of the field of a multilayered fiber as seen from an extension to the ring-core fiber. Additionally, we derive a general expression for the polarization-correction to the scalar propagation constant, which includes, for the first time, the contribution of the polarization-induced field. All the analytic expressions are illustrated and validated numerically with application to a step-index fiber, whose analytic solutions are well-known.

physics.optics↗

arXiv2021

Analytic Expressions for Orbital Angular Momentum Modal Crosstalk in a Slightly Elliptical Fiber

Assuming weakly guiding approximation, we examine orbital angular momentum (OAM) mode mixing on account of ellipticity in a fiber and derive a complete set of analytic expressions for spatial crosstalk, using scalar perturbation theory that incorporates fully the existing degeneracy between a spatial OAM mode and its degenerate partner characterized by a topological charge of opposite sign. These expressions, consequently, include an explicit formula for calculating the 2 pi walk-off length over which an input OAM mode converts into its degenerate partner, and back into itself. We further explore the applicability of the derived expressions in the presence of spin-orbit interaction. The expressions constitute a useful mathematical tool in the analysis and design of fibers for spatially-multiplexed mode transmissions. Their utility is demonstrated with application to a few mode and a multimode step-index fiber.

physics.optics↗

arXiv2019

Orbital Angular Momentum (OAM) Mode Mixing in a Bent Step Index Fiber in Perturbation Theory: Multiple Bends

In this paper, we address the impact of multiple fiber bends on orbital angular mode (OAM) mode propagation in a fiber. In particular, we extend the OAM mode-mixing due to a single-fiber bend studied earlier in detail for the step-index fiber, to the case of a succession of bends. The bends may have relative orientation with respect to each other. Analytic expressions leading to crosstalk are given. The OAM mode intensity at the output of a pair of bends is especially studied with emphasis on the intensity pattern changes that arise from the changes in the relative orientation of the bends.

physics.optics↗

arXiv2019

Orbital Angular Momentum (OAM) Mode Mixing in a Bent Step Index Fiber in Perturbation Theory

Within the framework of perturbation theory, we explore in detail the mixing of orbital angular momentum(OAM) modes due to a fiber bend in a step-index multimode fiber. Using scalar wave equation, we develop a complete set of analytic expressions for mode-mixing, including those for the $2π$ walk-off length, which is the distance traveled within the bent fiber before an OAM mode transforms into its negative topological charge counterpart, and back into itself. The derived results provide insight into the nature of the bend effects, clearly revealing the mathematical dependence on the bend radius and the topological charge. We numerically simulate the theoretical results with applications to a few-mode fiber and a multimode fiber, and calculate bend-induced modal crosstalk with implications for mode-multiplexed systems. The presented perturbation technique is general enough to be applicable to other perturbations like ellipticity and easily extendable to other fibers with step-index-like profile as in the ring fiber.

physics.optics↗

arXiv2016

On the general constraints in single qubit quantum process tomography

We briefly review single-qubit quantum process tomography for trace-preserving and nontrace-preserving processes, and derive explicit forms of the general constraints for fitting experimental data. These new forms provide additional insight into the structure of the process matrix. We illustrate their utility with several examples, including a discussion of qubit leakage error models and the intuition which can be gained from their process matrices.

quant-ph↗

arXiv2015

On single qubit quantum process tomography for trace-preserving and nontrace-preserving maps

We review single-qubit quantum process tomography for trace-preserving and nontrace-preserving processes, and derive explicit forms of the general constraints for fitting experimental data. These forms provide additional insight into the structure of the process matrix as well as reveal a tighter bound on the trace of a nontrace-preserving process than has been previously stated. We also describe, for completeness, how to incorporate measured imperfect input states.

quant-ph↗

arXiv2014

Alternate Forms of the T-Matrix in Quantum State Tomography

In this paper, we focus on alternate forms of the T-matrix used in the Maximum Likelihood Estimate (MLE) procedure for fitting the experimental data collected in quantum state tomography experiments. In particular, we analyze the single quantum state tomography case, deriving in the process three new valid alternate forms for achieving optimality. These alternative forms then serve as a consistency check, thus enhancing the robustness of the MLE fitting process. One form, in particular, serves as a useful compliment to the standard form normally employed. We subsequently provide a generalization of these forms to the case of multiqubit state tomography.

quant-ph↗

arXiv2014

Quantum Error Correcting Codes and the Security Proof of the BB84 Protocol

We describe the popular BB84 protocol and critically examine its security proof as presented by Shor and Preskill. The proof requires the use of quantum error correcting codes called the Calderbank-Shor-Steanne (CSS) quantum codes. These quantum codes are constructed in the quantum domain from two suitable classical linear codes, one used to correct for bit-flip errors and the other for phase-flip errors. Consequently, as a prelude to the security proof, the report reviews the essential properties of linear codes, especially the concept of cosets, before building the quantum codes that are utilized in the proof. The proof considers a security entanglement-based protocol, which is subsequently reduced to a "Prepare and Measure" protocol similar in structure to the BB84 protocol, thus establishing the security of the BB84 protocol. The proof, however, is not without assumptions, which are also enumerated. The treatment throughout is pedagogical, and this report, therefore, serves a useful tutorial for researchers, practitioners, and students, new to the field of quantum information science, in particular, quantum cryptography, as it develops the proof in a systematic manner, starting from the properties of linear codes, and then advancing to the quantum error correcting codes, which are critical to the understanding of the security proof.

quant-ph↗

arXiv2014

On Single Qubit Quantum State Tomography

In this paper, we derive analytic expressions for the starting (initial) values of the parameters of the T-matrix that is frequently employed in the construction of a theoretical density matrix in a Maximum Likelihood Estimate (MLE) procedure; this optimization procedure is used to fit the experimental data pertaining to single qubit state tomography. Appropriate starting values are critical in achieving a global minimum in the fitting process. We also indicate an analytic way of making the experimentally determined density matrix physical without resorting to the MLE process, if optimality in quantum state tomography can be disregarded, since the ultimate goal is optimality in quantum process tomography.

quant-ph↗