arXiv subjects

Sushil Mujumdar

Publications and source records attributed to Sushil Mujumdar.

At least 19 recordsLinked to original sources

arXiv2026

Super-resolved reconstruction of single-photon emitter locations from $g^{(2)}(0)$ maps

Single-photon sources are vital for emerging quantum technologies. In particular, Nitrogen-vacancy (NV) centers in diamond are promising due to their room-temperature stability, long spin coherence, and compatibility with nanophotonic structures. A key challenge, however, is the reliable identification of isolated NV centers, since conventional confocal microscopy is diffraction-limited and cannot resolve emitter distributions within a focal spot. Besides, the associated intensity scanning is a time-expensive procedure. Here, we introduce a raster-scanned $g^{(2)}(0)$ mapping technique combined with an inversion-based reconstruction algorithm. By directly measuring local photon antibunching across the field of view, we extract the effective emitter number within each focal spot and reconstruct occupancy maps on a sub-focal-spot grid. This enables recovery of the number and spatial distribution of emitters within regions smaller than the confocal focal spot, thereby offering possibilities of going beyond the diffraction limit. Our simulations confirm robust reconstruction of NV-center distributions. The method provides a practical diagnostic tool for locating single-photon sources in an efficient and accurate manner, at much lesser time and effort compared to conventional intensity scanning. It offers valuable feedback for nanophotonic device fabrication, supporting more precise and scalable integration of NV-based quantum photonic technologies.

quant-ph↗

arXiv2026

Efficient imaging of quantum emitters using compressive sensing

Optical imaging of quantum emitters is essential for a wide range of quantum applications. Conventional confocal imaging relies on point-by-point raster scanning, which is inherently time-consuming and photon-inefficient, particularly for sparse emitter distributions and photon-limited samples. Here, we demonstrate a compressive sensing-based imaging approach, where spatially structured wide-field excitation replaces raster scanning, enabling reconstruction of sparse emitters. In our implementation, random binary patterns are used to acquire compressive measurements, from which the spatial fluorescence distribution is reconstructed using a GPSR-BB algorithm. We experimentally demonstrate this approach using nitrogen-vacancy (NV) centers in diamond as a representative platform, with high-fidelity image reconstruction achieved using only approximately $20\%$ of the measurements required for conventional raster scanning. In addition to intensity reconstruction, we extend this framework to reconstruct spatial maps of the second-order correlation function $g^{(2)}(0)$ from compressive measurements. This enables identification of single-photon emitters through antibunching signatures using significantly reduced data.

physics.optics↗

arXiv2025

Generation and certification of pure phase entangled light

Biphoton systems exhibiting entanglement in position-momentum variables, known as spatial entanglement, are among the most intriguing and well-studied phenomena in quantum optics. A notable subset of these are phase entangled states, where entanglement manifests purely through correlations in the spatial phase of the wavefunction. While the generation of such states from biphotons via spontaneous parametric down-conversion has been explored, their physical implications and applications remain under-investigated. In this work, we theoretically and experimentally examine a unique form of phase entanglement known as `pure' phase entanglement. This state exhibits the unusual feature that the position of one photon is correlated with the momentum of the other. Unlike typical spatially entangled states, it shows no direct correlation in position or momentum between the two photons, underscoring that all correlations arise purely from the spatial phase of the wavefunction. We delve deeper into the theory of this state and experimentally construct it from known phase-entangled states. To certify its properties, we propose a setup that performs a "one-particle momentum measurement" and explore the various tunable parameters. We also highlight potential applications of this state in quantum optics and imaging experiments.

quant-ph↗

arXiv2025

Tracking phase entanglement during propagation of downconverted photons

High-dimensional entanglement in the form of transverse spatial correlation between a pair of photons generated via spontaneous parametric downconversion is not only a valuable resource in many academic and real-life applications but also provides access to several intriguing quantum phenomena. One such non-intuitive phenomenon is phase entanglement, in which the biphoton state is correlated in the complex phase of its wavefunction. This state, which emerges during the propagation of the biphoton wavefunction, exhibits neither position nor momentum correlation, yet retains full entanglement. In this work, we experimentally explore this state in two distinct ways. The first is by tracking the vanishing spatial photon number correlation over propagation distances lying in $\left[0,\infty\right)$, folded into a finite range using single-lens imaging. These observations show excellent agreement with our theoretical predictions based on the Double Gaussian (DG) approximation of the biphoton state. The second approach involves performing a two-photon interference experiment using a double slit and this state, which reveals the correlated phase front. We show, both theoretically and experimentally, that the observed two-photon interference structure is markedly different from that produced by position-correlated photons, as confirmed by computing the joint probability distribution of photons (JPD) and related metrics. Such interference using phase-entangled light has not been attempted before and opens avenues for advanced experiments and applications in the field of spatial entanglement.

quant-ph↗

arXiv2025

Optimizing the qudit dimensions of position-momentum entangled photons for QKD

We propose an optimization scheme to maximize the secure key rate of a high-dimensional variant of BBM92. We use the position-momentum conjugate bases to encode the higher dimensional qudits, realised in a fully passive optical setup. The setup employs a single lens for the basis measurements and no lossy or slow elements. We optimize the qudit dimension for the protocol by maximizing the number of equiprobable sections (macropixels) of the detected beam while minimizing their overlap error. We show the enhanced key rate by discarding events from the ambiguous border pixels. Our strategy maximizes the overlap between the discarded regions from neighbouring macropixels, thereby globally minimizing the overall loss and error. We calculate the optimal dimension and the secure key rate for certain beam parameters. We experimentally show the feasibility of our scheme. This work paves the way for realistic implementations of high-dimensional device-independent quantum key distribution with enhanced bitrates.

quant-ph↗

arXiv2025

Rapid and efficient wavefront correction for spatially entangled photons using symmetrized optimization

Spatial entanglement is a key resource in quantum technologies, enabling applications in quantum communication, imaging, and computation. However, propagation through complex media distorts spatial correlations, posing a challenge for practical implementations. We introduce a symmetrized genetic algorithm (sGA) for adaptive wavefront correction of spatially entangled photons, leveraging the insight that only the even-parity component of wavefront distortions affects two-photon correlations. By enforcing symmetry constraints, sGA reduces the optimization parameter space by half, leading to faster convergence and improved enhancement within finite number of generations compared to standard genetic algorithms (GA). Additionally, we establish the dependence of enhancement on the signal-to-noise ratio of the feedback signal, which is controlled by detector integration time. This technique enables correction of entanglement degradation, enhancing quantum imaging, secure quantum communication, and quantum sensing in complex environments.

quant-ph↗

arXiv2025

Second-order nonlinear disordered photonic media

The field of complex photonics has garnered significant interest due to its rich physics and myriad applications spanning physics, engineering, biology, and medicine. However, a substantial portion of research focuses primarily on the linear medium. Over the years, optical nonlinearity, particularly the second order denoted as $χ^{(2)}$, has been harnessed for diverse applications such as frequency conversions, three-wave mixing, material characterizations, and bio-imaging. When $χ^{(2)}$-nonlinearity combines with the disorder, a new realm of physics emerges, which in the last 30 years has witnessed substantial progress in fundamental studies and futuristic applications. This review aims to explore fundamental concepts concerning $χ^{(2)}$-nonlinear disordered media, chart the field's evolution, highlight current interests within the research community, and conclude with a future perspective.

physics.optics↗

arXiv2025

Partial-immunity of two-photon correlation against wavefront distortion for spatially entangled photons

High-dimensional quantum entanglement in photons offers notable technological advancements over traditional qubit-based systems, including increased information density and enhanced security. However, such high-dimensional states are vulnerable to disruption by complex disordered media, presenting significant challenges in practical applications. Spatially-entangled photons are conventionally generated using a nonlinear crystal via spontaneous parametric down conversion (SPDC). While the effect of disorder on spatially entangled photons in the near field of the crystal is well understood, the impact of disorder in the far field is more complex. In this work, we present a systematic study of the randomization of two-photon correlations caused by arbitrary phase distortions in the far field by breaking it down into odd and even parity components. First, we theoretically show that the two-photon field is only sensitive to the even-parity part of the phase distortion. In follow-up experiments, we employ a deformable mirror to implement random phase distortions, separating the contributions of odd and even parity phases using Zernike polynomials. The experimental results are in agreements with the theoretical predictions. Subsequently, we perform numerical simulations to show that these results extend to stronger degrees of disorder. Our key finding is that, since two-photon correlations are only affected by the even-parity component of phase modulations, the number of independent adaptive optics elements required for optimizing the correlation can be effectively halved, offering a significant practical advantage in managing disorder in quantum systems.

quant-ph↗

arXiv2024

Controlling the degree of entanglement in downconversion by targeted birth zone activation

We explore the consequences of varying the pump beam waist that illuminates a nonlinear crystal, realizing spontaneous parametric down-conversion (SPDC). The coherence is transferred from the marginal one-photon wavefunction to the two-photon wavefunction where it manifests into entanglement in the form of spatial correlation. We interpret this as a consequence of the number of independent emitters, called the biphoton birth zones, targeted by the pump beam on the crystal. The birth zone number $N$ characterises the number of such birth zones that fit along a diameter of the region illuminated by the pump waist. To experimentally observe the duality between the one- and two-photon interference, we employ a double slit and analyse their visibilities $V_m$ and $V_\text{12}$ respectively. We demonstrate the conservation of the quantity $V_m^2+V_\text{12}^2$. Finally, we identify three regimes of entanglement of the down-converted photons based on $N$. We show that changing the pump waist lets us actively control the degree of entanglement letting us access these regimes. We provide implications of each regime, and mention experimental use cases thereof.

quant-ph↗

arXiv2024

Modal complexity as a metric for Anderson localization

We present a thorough study of the complexity of optical localized modes in two-dimensional disordered photonic crystals. Direct experimental measurements of complexity were made using an interferometric setup that allowed for extraction of phases and, hence, complex-valued wavefunctions. The comparison of experimental and theoretical results allows us to propose a metric for Anderson localization based on the average value and statistical distribution of complexity. Being an alternative to other known criteria of localization, the proposed metric exploits the openness of the disordered medium and provides a quantitative characterization of the degree of localization allowing for determining the localization length.

physics.optics↗

arXiv2024

Multifold enhancement of quantum SNR by using an EMCCD as a photon number resolving device

The Electron Multiplying Charge Coupled Devices (EMCCD), owing to their high quantum efficiency and spatial resolution, are widely used to study typical quantum optical phenomena and related applications. Researchers have already developed a procedure that enables one to statistically determine whether a pixel detects a single photon, based on whether its output is higher or lower than the estimated noise level. However, these techniques are feasible at extremely low photon numbers (about 0.15 mean number of photons per pixel per exposure), allowing for at most one photon per pixel. This limitation necessitates a very large number of frames required for any study. In this work, we present a method to estimate the mean rate of photons per pixel per frame for arbitrary exposure time. Subsequently, we make a statistical estimate of the number of photons (greater than or equal to 1) incident on each pixel. This allows us to effectively utilize the EMCCD as a photon number resolving device. This immediately augments the acceptable light levels in the experiments, leading to significant reduction in the required experimentation time. As evidence of our approach, we quantify contrast in quantum correlation exhibited by a pair of spatially entangled photons generated by Spontaneous Parametric Down Conversion process. In comparison to conventional methods, our method realizes an enhancement in the signal to noise ratio by about a factor of 3 for half the data collection time. This SNR can be easily enhanced by minor modifications in experimental parameters such as exposure time etc.

quant-ph↗

arXiv2023

Photon diffusion in space and time in a second-order nonlinear disordered medium

We report experimental and theoretical investigations on photon diffusion in a second-order nonlinear disordered medium under conditions of strong nonlinearity. Experimentally, photons at the fundamental wavelength ($λ=1064$ nm) are launched into the structure in the form of a cylindrical pellet, and the second-harmonic ($λ=532$ nm) photons are temporally analyzed in transmission. For comparison, separate experiments are carried out with incident green light at $λ=532$ nm. We observe that the second harmonic light peaks earlier compared to the incident green photons. Next, the sideways spatial scattering of the fundamental as well as second-harmonic photons is recorded. The spatial diffusion profiles of second-harmonic photons are seen to peak deeper inside the medium in comparison to both the fundamental and incident green photons. In order to give more physical insights into the experimental results, a theoretical model is derived from first principles. It is based on the coupling of transport equations. Solved numerically using a Monte Carlo algorithm and experimentally estimated transport parameters at both wavelengths, it gives excellent semi-quantitative agreement with the experiments for both fundamental and second-harmonic light.

physics.optics↗

arXiv2023

Quantum echo route towards exceptional points in Anderson localized lasers

Exceptional points, that are spectral degeneracies in the parameter space of non-Hermitian systems, have evoked a massive interest in the optical domain owing to their striking consequences on optical behavior of commonly known systems. Through careful engineering of gain and loss, exceptional points have been demonstrated in a variety of photonic systems ranging from optical fibers to chaotic cavities, exhibiting extra-ordinary phenomena and augmented functionalities. However, in the domain of disordered systems, there are still no realizations of exceptional points even though mode-coupling and non-Hermitian behavior is amply demonstrated. The obvious challenge lies in the probabilistic nature of disorder, which is a difficult candidate for parametric control. Here, we exploit the probabilistic nature of Anderson localizing systems by implementing thousands of disorder configurations. We demonstrate statistical occurrences of lasing over exceptional points. Our route towards exceptional points begins with detection of quantum echoes, which are temporal signatures of coupling between modes. Quantum echoes unambiguously set apart two coupled modes from a pair of two isolated modes that are spectrally close perchance. Simultaneous temporal, spectral and spatial investigations provide corroborative evidence of the convergence of eigenvalues and eigenvectors in the approach to the exceptional points. Ultimately, the vanishing of the echo and coalescence of spectral peaks and spatial intensity distributions, accompanied by the square-Lorentzian lineshape of lasing peaks, identify the exceptional point, at which the lasing intensity is seen to be significantly higher.

physics.optics↗

arXiv2022

Speckle decorrelation in fundamental and second-harmonic light scattered from nonlinear disorder

Speckle patterns generated in a disordered medium carry a lot of information despite the apparent complete randomness in the intensity pattern. When the medium possesses $χ^{(2)}$ nonlinearity, the speckle is sensitive to the phase of the incident fundamental light, as well as the light generated within. Here, we examine the speckle decorrelation in the fundamental and second-harmonic transmitted light as a function of varying power in the fundamental beam. At low incident powers, the speckle patterns produced by successive pulses exhibit strong correlations, that decrease with increasing power. The average correlation in the second-harmonic speckle decays faster than in the fundamental speckle. Next, we construct a theoretical model, backed up by numerical computations, to obtain deeper physical insights on the faster decorrelations in the second-harmonic light. Whilst providing excellent qualitative agreement with the experiments, the model sheds important light on the contribution of two effects in the correlations, namely, the generation of second-harmonic light, and the propagation thereof.

physics.optics↗

arXiv2022

Anomalous transport regime in non-Hermitian, Anderson-localizing hybrid systems

In a disordered environment, the probability of transmission of a wave reduces with increasing disorder, the ultimate limit of which is the near-zero transmission due to Anderson localization. Under localizing conditions, transport is arrested because the wave is trapped in the bulk of the sample with decaying-exponential coupling to the boundaries. Any further increase in disorder does not modify the overall transport properties. Here, we report the experimental demonstration of a hitherto-unrealized anomalous transport of hybrid particles under localizing disorder in a non-Hermitian setting. We create hybrid polariton-photon states in a one-dimensional copper sample with a comb-shaped periodic microstructure designed for microwave frequencies. Metallic dissipation realizes the necessary non-Hermiticity. Disorder is introduced by deliberate alterations of the periodic microstructure. Direct measurement of wave-functions and phases was achieved by a near-field probe. At a particular disorder, We observe the onset of Anderson localization of the hybrid states endorsed by exponential tails of the wavefunction. However, at stronger disorder and under conditions that support localization, an unexpected enhancement in the transmission was facilitated by an emergent mini-band. The transmission was traced to the hopping of the hybrid particle over multiple co-existing localized resonances that exchange energy due to the non-orthogonality. These emergent states are manifested in all configurations under strong disorder, suggesting the formation of a novel transport regime. This is verified by measuring the averaged conductance which endorses an anomalous transport regime in the hybrid, non-Hermitian environment under strong disorder. These experimental observations open up new unexplored avenues in the ambit of disorder under non-Hermitian conditions.

cond-mat.dis-nn↗

arXiv2019

Optical Thouless conductance and level-spacing statistics in two-dimensional Anderson localizing systems

We experimentally investigate spectral statistics in Anderson localization in two-dimensional amorphous disordered media. Intensity distributions captured over an ultrabroad wavelength range of $\sim 600$~nm and averaged over numerous configurations provided the Ioffe-Regel parameter to be $\sim2.5$ over the investigated wavelength range. The spectra of the disordered structures provided access to several quasimodes, whose widths and separations allowed to directly estimate the optical Thouless conductance $g_{Th}$, consistently observed to be below unity. The probability distribution of $g_{Th}$ was measured to be a log-normal. Despite being in the Anderson localization regime, the spacings of energy levels of the system was seen to follow a near Wigner-Dyson function. Theoretical calculations based on the tight-binding model, modified to include coupling to a bath, yielded results that were in excellent agreement with experiments. From the model, the level-spacing behavior was attributed to the degree of localization obtained in the optical disordered system.

physics.optics↗

arXiv2019

Discrepant transport characteristics under Anderson localization at the two limits of disorder

Anderson localization is a striking phenomenon wherein transport of light is arrested due to the formation of disorder-induced resonances. Hitherto, Anderson localization has been demonstrated separately in two limits of disorder, namely, amorphous disorder and nearly-periodic disorder. However, transport properties in the two limits are yet unstudied, particularly in a statistically consistent manner. Here, we experimentally measure light transport across two-dimensional open mesoscopic structures, wherein the disorder systematically ranges from nearly-periodic to amorphous. We measure the generalized conductance, which quantifies the transport probability in the sample. Although localization was identified in both the limits, statistical measurements revealed a discrepant behavior in the generalized conductance fluctuations in the two disorder regimes. Under amorphous disorder, the generalized conductance remains below unity for any configuration of the disorder, attesting to the arrested nature of transport. Contrarily, at near-periodic disorder, the distribution of generalized conductance is heavy-tailed towards large conductance values, indicating that the overall transport is delocalized. Theoretical results from a model based on the tight-binding approximation, augmented to include open boundaries, are in excellent agreement with experiments, and also endorse the results over much larger ensembles. These results quantify the differences in the two disorder regimes, and advance the studies of disordered systems into actual consequences of Anderson localization in light transport.

physics.optics↗

arXiv2018

Direct experimental determination of critical disorder in one-dimensional weakly disordered photonic crystals

We report experimental measurement of critical disorder in weakly disordered, one-dimensional photonic crystals. We measure the configurationally-averaged transmission at various degrees of weak disorder. We extract the density of states (DoS) after fitting the transmission with theoretical profiles, and identify the Lifshitz tail realized by weak disorder. We observe the vanishing of Van Hove singularities and the flattening of the DoS with increasing disorder in our system. Systematic variation of disorder strength allows us to study the behavior of Lifshitz exponent with the degree of disorder. This provides a direct handle to the critical disorder in the one-dimensional crystal, at which the transport behavior of the system is known to change. The contradictory behavior at very weak disorder in the DoS variation at the bandedge and the midgap are seen to resolve into synchronous behavior beyond the critical disorder. The experimentally measured transmission is shown to carry a clear signature of the critical disorder, which is in very good agreement with the theoretically expected disorder.

physics.optics↗