arXiv subjects

Kasturi Saha

Publications and source records attributed to Kasturi Saha.

At least 19 recordsLinked to original sources

arXiv2026

Quantum Networks Using Color Defects in Diamond: Principles, Progress, and Perspectives

Large-scale quantum networks will enable entirely new applications of quantum information science in fields such as quantum communication, distributed quantum computing, sensing, and metrology. To build nodes of such networks, diamond color defects are one of the promising candidates. Their excellent optical properties, fast spin-qubit control, and long spin coherence times make them well-suited for quantum information processing and quantum memory applications. Additionally, recent advances in the heterogeneous integration of diamond nanophotonic structures with photonic integrated circuits have made these systems more efficient and well-suited for scalable quantum processor architectures. In this comprehensive review, we discuss the optical and spin properties of these systems, recent progress in the building blocks of quantum networks, and demonstrations of metropolitan-scale quantum networks, as well as the challenges associated with these systems at both the fundamental and experimental levels, along with potential solutions.

quant-ph↗

arXiv2026

In-Substrate Imaging of Diamond hBN FET Current via Widefield Quantum Diamond Microscopy

We demonstrate widefield magnetic imaging of current flow in hydrogen terminated diamond field effect transistors (FETs) through in-substrate nitrogen vacancy (NV) centers. Hydrogen termination of the diamond surface induces a two dimensional hole gas (2DHG), while an ensemble of near surface NV centers located $ \sim 1~\mu m$ below the surface enables noninvasive magnetic imaging of current flow with micrometer scale spatial resolution. The FETs were electrically characterized over a range of drain source biases $V_{ds}= 0$ to $-15V$ and gate voltages,$V_{gs}= +3$ to $-9V$ followed by in situ widefield NV magnetometry during device operation. Magnetic field maps and reconstructed current density distributions directly visualize current injection at the source drain contacts and transport beneath the hBN gated channel. Magnetic field maps reveal current density variations in the channel region owing to non-uniformities or defects in the gate dielectric. In addition, we observe a pronounced enhancement of the drain current ($\sim 600-900 \mu A$) and a shift in the apparent threshold voltage during laser illumination, reflecting photo induced changes in channel electrostatics. By correlating gate dependent magnetic images with simultaneous electrical measurements, we directly link spatial current distributions to FET transfer characteristics, providing new insight into buried interface transport and non-uniform gating effects in the transistor channel. As the methodology is compatible with top gated FETs, it can be used to map channel current distributions with micrometer resolution in emerging channel materials, such as 2D materials and wide bandgap channels, and establish widefield NV magnetometry as a powerful platform for probing charge transport in transistors and Van der Waals dielectric heterostructures.

cond-mat.mes-hall↗

arXiv2026

Cavity-Driven Multispectral Gain for High-Sensitivity NV Center Magnetometers

We report a cavity-enabled solid-state magnetometer based on an NV ensemble coupled with a dielectric cavity, achieving 12 pT/$\sqrt{\rm{Hz}}$ sensitivity and a nearly threefold gain from multispectral features. The features originate from cavity-induced splitting of the NV hyperfine levels and leverages robust quantum coherence in the doubly dressed states of the system to achieve high sensitivity. We project simulated near-term sensitivities approaching 100 fT/$\sqrt{\rm{Hz}}$, close to the Johnson-Nyquist limit. Our results establish frequency multiplexing as a new operational paradigm, offering a robust and scalable quantum resource for metrology under ambient conditions.

quant-ph↗

arXiv2025

Observation of Shear Strain in Ion-Implanted Diamond Substrate and Diamond Nanophotonic Structures

Negatively charged nitrogen-vacancy (NV) centers and other color centers in diamonds have emerged as promising platforms for quantum communication, quantum information processing, and nanoscale sensing, owing to their long spin coherence times, fast spin control, and efficient photon coupling. Deterministic placement of individual color centers into nanophotonic structures is critical for scalable device integration, and ion implantation is the most viable technique. Nanofabrication processes, including diamond etching, are essential to realize these structures but can introduce crystal strain through lattice damage. In this work, we investigate the impact of ion implantation and nanofabrication-induced strain on the electronic spin levels of NV-centers. We demonstrate that the zero-field continuous-wave optically detected magnetic resonance (CW-ODMR) spectroscopy serves as a sensitive probe of local crystal strain. We report the presence of a shear strain feature in diamond substrates arising from the ion-implantation and nanofabrication processes, as evidenced by the asymmetric splitting black observed in the zero-field CW-ODMR spectrum of NV-centers.

quant-ph↗

arXiv2025

Towards Quantum Limited Spatial Resolution of NV-Diamond Magnetometry

Optically addressable ensembles of solid-state defects, such as nitrogen vacancy (NV) centers, are a leading modality for imaging-based magnetometry, thermometry and strain sensing. However, monitoring the fluorescence of individual defects within a sub-diffraction ensemble remains an outstanding challenge that currently limits access to atomic-scale features and dynamics. For compact clusters of NVs, we formulate imaging-based atomic sensing as a low-dimensional multiparameter estimation task in which one seeks to localize each defect and quantify the field strength in its immediate vicinity. In this work, we employ optical spatial mode demultiplexing (SPADE) to enhance localization and brightness estimation accuracy at sub-diffraction scales. Specifically, we develop a two-stage sensing protocol that augments direct imaging by projecting the incoming optical field onto point spread function (PSF)-adapted, i.e., PAD spatial modes and Yuen-Kennedy-Lax (YKL) spatial modes enabling efficient extraction of emitter positions and brightnesses. The YKL-SPADE measurement employed for brightness estimation is shown to be quantum-optimal in the case of two emitters and establishes a new connection between quantum detection and estimation theories. We numerically evaluate the statistical performance of our protocol for sub-diffraction optically detected magnetic resonance (ODMR) and Rabi sensing experiments. Compared to conventional focal plane intensity measurements, our protocol improves emitter localization accuracy by 6$\times$ and brightness estimation accuracy by 2$\times$ for tightly confined ensembles, residing well below the diffraction limit.

quant-ph↗

arXiv2025

Dictionary-Based Reconstruction of Spatio-Temporal 3D Magnetic Field Images from Quantum Diamond Microscope

Three-dimensional magnetic imaging with high spatio-temporal resolution is critical for probing current paths in various systems, from biosensing to microelectronics. Conventional 2D Fourier-based current source localization methods are ill-posed in multilayer or dynamic systems due to signal overlap and noise. In this work, we demonstrate an innovative nitrogen-vacancy (NV) center-based wide-field magnetic microscopy technique for dynamic three-dimensional imaging and localization of current sources. Using custom-fabricated multilayer micro-coil platform to emulate localized, time-varying currents similar to neuronal activity, we acquire magnetic field maps with micrometre-scale spatial and millisecond-scale temporal resolution using per-pixel lock-in-based detection. Source localization and image reconstruction are achieved using a Least Absolute Shrinkage and Selection Operator (LASSO)-based reconstruction framework that incorporates experimentally measured basis maps as spatial priors. Our method enables robust identification of active current sources across space and time, and significantly advances the accuracy of dynamic 3D current imaging and NV-based magnetometry for complex systems.

physics.ins-det↗

arXiv2024

Optimized Current Density Reconstruction from Widefield Quantum Diamond Magnetic Field Maps

Quantum Diamond Microscopy using Nitrogen-Vacancy (NV) defects in diamond crystals has enabled the magnetic field imaging of a wide variety of nanoscale current profiles. Intimately linked with the imaging process is the problem of reconstructing the current density, which provides critical insight into the structure under study. This manifests as a non-trivial inverse problem of current reconstruction from noisy data, typically conducted via Fourier-based approaches. Learning algorithms and Bayesian methods have been proposed as novel alternatives for inference-based reconstructions. We study the applicability of Fourier-based and Bayesian methods for reconstructing two-dimensional current density maps from magnetic field images obtained from NV imaging. We discuss extensive numerical simulations to elucidate the performance of the reconstruction algorithms in various parameter regimes, and further validate our analysis via performing reconstructions on experimental data. Finally, we examine parameter regimes that favor specific reconstruction algorithms and provide an empirical approach for selecting regularization in Bayesian methods.

cond-mat.mes-hall↗

arXiv2024

Real-time Simultaneous Dual Sensing of Temperature and Magnetic Field using NV-based Nano-diamonds

Quantum sensors based on Nitrogen Vacancy (NV) centers in diamond are highly capable of sensing multiple physical quantities. In this study, we use amplitude-modulated lock-in detection of optically detected magnetic resonance of NV nanodiamonds (NVND) to investigate the link between temperature (T) and the zero-field splitting parameter (D) and also the relationship between magnetic field values and the difference of resonance frequencies. We also present NVNDs' capacity to simultaneously sense both thermal and magnetic fields in real time. This dual-sensing approach is beneficial for studying magnetic materials whose magnetization depends on temperature and the applied magnetic field, such as certain ferromagnetic and ferrimagnetic materials. Integrating real-time thermal and magnetic field measurements provides unique opportunities for failure analysis in the integrated circuit (IC) industry and for studying thermodynamic processes in cell physiology. The ability to concurrently monitor temperature and magnetic field variations offers a powerful toolset for advancing precision diagnostics and monitoring in these fields.

quant-ph↗

arXiv2024

Phase-Based Approaches for Rapid Construction of Magnetic Fields in NV Magnetometry

With the second quantum revolution underway, quantum-enhanced sensors are moving from laboratory demonstrations to field deployments, providing enhanced and even new capabilities. Signal processing and operational software is becoming integral parts of these emerging sensing systems to reap the benefits of this progress. This paper looks into widefield Nitrogen Vacancy Center-based magnetometry and focuses on estimating the magnetic field from the Optically Detected Magnetic Resonances (ODMR) signal, a crucial output for various applications. Mapping the shifts of ODMR signals to phase estimation, a computationally efficient approaches are proposed. Involving Fourier Transform and Filtering as pre-processing steps, the suggested approaches involve linear curve fit or complex frequency estimation based on well-known super-resolution technique Estimation of Signal Parameters via Rotational Invariant Techniques (ESPRIT). The existing methods in the quantum sensing literature take different routes based on Lorentzian fitting for determining magnetic field maps. To showcase the functionality and effectiveness of the suggested techniques, relevant results, based on experimental data are provided, which shows a significant reduction in computational time with the proposed method over existing methods

physics.ins-det↗

arXiv2024

Transverse resistance due to electronic inhomogeneities in superconductors

Phase transitions in many-body systems are often associated with the emergence of spatial inhomogeneities. Such features may develop at microscopic lengthscales and are not necessarily evident in measurements of macroscopic quantities. In this work, we address the topic of distribution of current paths in superconducting films. Typical lengthscales associated with superconductivity are in the range of nanometres. Accordingly, measurements of electrical resistance over much larger distances are supposed to be insensitive to details of spatial inhomogeneities of electronic properties. We observe that, contrary to expectations, current paths adopt a highly non-uniform distribution at the onset of the superconducting transition which is manifested in the development of a finite transverse resistance. The anisotropic distribution of current density is unrelated to the structural properties of the superconducting films, and indicates the emergence of electronic inhomogeneities perceivable over macroscopic distances. Our experiments reveal the ubiquitous nature of this phenomenon in conventional superconductors.

cond-mat.supr-con↗

arXiv2024

Observation of Ultra-low AC Susceptibility in Micro-magnets Using Quantum Diamond Microscope

AC susceptometry, unlike static susceptometry, offers a deeper insight into magnetic materials. By employing AC susceptibility measurements, one can glean into crucial details regarding magnetic dynamics. Nevertheless, traditional AC susceptometers are constrained to measuring changes in magnetic moments within the range of a few nano-joules per tesla. Additionally, their spatial resolution is severely limited, confining their application to bulk samples only. In this study, we introduce the utilization of a Nitrogen Vacancy (NV) center-based quantum diamond microscope for mapping the AC susceptibility of micron-scale ferromagnetic specimens. By employing coherent pulse sequences, we extract both magnitude and the phase of the field from samples within a field of view spanning 70 micro-meters while achieving a resolution of 1 micro-meter. Furthermore, we quantify changes in dipole moment on the order of a femto-joules per tesla induced by excitations at frequencies reaching several hundred kilohertz.

quant-ph↗

arXiv2024

High dynamic-range and portable magnetometer using ensemble nitrogen-vacancy centers in diamond

Nitrogen vacancy (NV) centers in diamonds have been explored for realizing a wide range of sensing applications in the last decade due to their unique quantum properties. Here we realize a compact and portable magnetometer with an ensemble of NV centers which we call the Quantum MagPI (Quantum Magnetometer with Proportional Integral control). Including the sensor head and associated electronics, our sensor assembly can fit inside 10 cm x 10 cm x 7 cm box and control electronics in 30 cm x 25 cm x 5 cm box. We achieve a bandwidth normalized sensitivity of ~ 10 nT/sqrt(Hz). Using closed-loop feedback for locking to the resonance frequency, we extend the linear dynamic range to 200 microT (20x improvement compared to the intrinsic dynamic range) without compromising the sensitivity. We report a detailed performance analysis of the magnetometer through measurements of noise spectra, Allan deviation, and tracking of nT-level magnetic fields in real-time. Additionally, we demonstrate the utility of such a magnetometer by real-time tracking the movement of the elevator car and door opening by measuring the projection of the magnetic field along one of the NV-axes under ambient temperature and humidity.

quant-ph↗

arXiv2023

Ultrafast Green Single Photon Emission from an InGaN Quantum Dot-in-a-GaN Nanowire at Room Temperature

In recent years, there has been a growing demand for room-temperature visible single-photon emission from InGaN nanowire-quantum-dots (NWQDs) due to its potential in developing quantum computing, sensing, and communication technologies. Despite various approaches explored for growing InGaN quantum dots on top of nanowires (NWs), achieving the emission of a single photon at room temperature with sensible efficiency remains a challenge. This challenge is primarily attributed to difficulties in accomplishing the radial confinement limit and the inherent giant built-in potential of the NWQD. In this report, we have employed a novel Plasma Assisted Molecular Beam Epitaxy (PAMBE) growth approach to reduce the diameter of the QD to the excitonic Bohr radius of InGaN, thereby achieving strong lateral confinement. Additionally, we have successfully suppressed the strong built-in potential by reducing the QD diameter. Toward the end of the report, we have demonstrated single-photon emission ($λ$ = 561 nm) at room-temperature from the NWQD and measured the second-order correlation function $g^{2}(0)$ as 0.11, which is notably low compared to other reported findings. Furthermore, the lifetime of carriers in the QD is determined to be 775 ps, inferring a high operational speed of the devices.

cond-mat.mes-hall↗

arXiv2023

Opto-thermoelectric trapping of Fluorescent Nanodiamonds on Plasmonic Nanostructures

Deterministic optical manipulation of fluorescent nanodiamonds (FNDs) in fluids has emerged as an experimental challenge in multimodal biological imaging. Designing and developing nano-optical trapping strategies to serve this purpose is an important task. In this letter, we show how chemically-prepared gold nanoparticles and silver nanowires can facilitate Opto-thermoelectric force to trap individual entities of FNDs using a long working distance lens, low power-density illumination (532 nm laser, 12 $μW/μm^2$). Our trapping configuration combines the thermoplasmonic fields generated by individual plasmonic nanoparticles and the opto-thermoelectric effect facilitated by the surfactant to realise a nano-optical trap down to a single FND 120 nm in diameter. We utilise the same trapping excitation source to capture the spectral signatures of single FNDs and track their position. By tracking the FND, we observe the differences in the dynamics of FND around different plasmonic structures. We envisage that our drop-casting platform can be extrapolated to perform targeted, low-power trapping, manipulation, and multimodal imaging of FNDs inside biological systems such as cells.

physics.optics↗

arXiv2023

Decoupling Nuclear Spins via Interaction-Induced Freezing in Nitrogen Vacancy Centers in Diamond

Nitrogen-Vacancy (NV) centers in diamonds provide a room-temperature platform for various emerging quantum technologies, e.g. the long nuclear spin coherence times as potential quantum memory registers. We demonstrate a freezing protocol for an NV center to isolate its intrinsic nuclear spin from a noisy electromagnetic environment. Any initial state of the nuclear spin can be frozen when the hyperfine-coupled electron and nuclear spins are simultaneously driven with unequal Rabi frequencies. Through numerical simulations, we show that our protocol can effectively shield the nuclear spin from strong drive or noise fields. We also observe a clear suppression of quantum correlations in the frozen nuclear spin regime by measuring the quantum discord of the electron-nuclear spin system. These features can be instrumental in extending the storage times of NV nuclear-spin based quantum memories in hybrid quantum systems.

quant-ph↗

arXiv2022

Mapping AC Susceptibility with Quantum Diamond Microscope

We present a novel technique for determining the microscale AC susceptibility of magnetic materials. We use magnetic field sensing properties of nitrogen-vacancy (\ce{NV-}) centers in diamond to gather quantitative data about the magnetic state of the magnetic material under investigation. In order to achieve the requisite speed in imaging, a lock-in camera is used to perform pixel-by-pixel lock-in detection of \ce{NV-} photo-luminescence. In addition, a secondary sensor is employed to isolate the effect of the excitation field from fields arising from magnetic structures on \ce{NV-} centers. We demonstrate our experimental technique by measuring the AC susceptibility of soft permalloy micro-magnets at excitation frequencies of up to \SI{20}{\hertz} with a spatial resolution of \SI{1.2}{\micro \meter} and a field of view of \SI{100}{\um}. Our work paves the way for microscopic measurement of AC susceptibilities of magnetic materials relevant to physical, biological, and material sciences.

physics.app-ph↗

arXiv2022

Sub-second Temporal Magnetic Field Microscopy Using Quantum Defects in Diamond

Wide field-of-view magnetic field microscopy has been realised by probing shifts in optically detected magnetic resonance (ODMR) spectrum of Nitrogen Vacancy (NV) defect centers in diamond. However, these widefield diamond NV magnetometers require few to several minutes of acquisition to get a single magnetic field image, rendering the technique temporally static in it's current form. This limitation prevents application of diamond NV magnetometers to novel imaging of dynamically varying microscale magnetic field processes. Here, we show that the magnetic field imaging frame rate can be significantly enhanced by performing lock-in detection of NV photo-luminescence (PL), simultaneously over multiple pixels of a lock-in camera. A detailed protocol for synchronization of frequency modulated PL of NV centers with fast camera frame demodulation, at few kilohertz frequencies, has been experimentally demonstrated. This experimental technique allows magnetic field imaging of sub-second varying microscale currents in planar microcoils with imaging frame rates in the range of 50 to 200 frames per second (fps). Our work demonstrates that widefield per-pixel lock-in detection of frequency modulated NV ODMR enables dynamic magnetic field microscopy.

cond-mat.mes-hall↗

arXiv2021

Detecting Initial Correlations via Correlated Spectroscopy in Hybrid Quantum Systems

Generic mesoscopic quantum systems that interact with their environment tend to display appreciable correlations with environment that often play an important role in the physical properties of the system. However, the experimental methods needed to characterize such systems either ignore the role of initial correlations or scale unfavourably with system dimensions. Here, we present a technique that is agnostic to system-environment correlations and can be potentially implemented experimentally. Under a specific set of constraints, we demonstrate the ability to detect and measure specific correlations. We apply the technique on a hybrid quantum system of Nitrogen Vacancy Centers (NV) coupled to an optical cavity with initial correlations. We extract the interaction strength and effective number of interacting NVs from the initial correlations using our technique.

quant-ph↗