arXiv subjects

Shaurya Aarav

Publications and source records attributed to Shaurya Aarav.

10 recordsLinked to original sources

arXiv2026

Spatial coherence enabled sensorless adaptive optical imaging

Optical aberrations degrade imaging performance in label-free microscopy, where the absence of a guide star often necessitates sensorless adaptive optics (AO). Conventional sensorless AO approaches rely on image-quality metrics whose optimal choice depends on both the specimen and the imaging modality. Here, we present a guide-star-free AO framework based on the spatial coherence properties of spatially incoherent light. The proposed method exploits aberration-induced broadening of the measured spatial correlation distribution as the feedback signal for aberration correction. Experiments in a conventional bright-field imaging system using standard LED illumination demonstrate successful correction of phase aberrations. Furthermore, the approach remains effective even in the presence of spatially structured background noise. These results establish spatial coherence measurements as an effective feedback mechanism for sensorless AO and indicate that the correlation-based feedback principle employed in quantum-assisted AO can likewise be realized using the spatial correlations of incoherent light.

physics.optics↗

arXiv2026

Noise-Resilient Imaging through Coherence Filtering

Noise is a significant challenge in imaging. Conventional intensity-based techniques mitigate noise through various filtering methods, but they often require prior knowledge of noise characteristics and struggle, especially under low-light conditions and with spatially structured noise. Quantum distillation provides enhanced noise rejection; however, its applicability is limited as it requires specialised illumination and substantial modifications to existing imaging setups. In this article, we introduce a coherence-based image distillation approach that separates object from noise by leveraging the difference in their temporal coherence properties. We implement this through our interferometric protocol, which enables imaging based on spatial coherence while simultaneously filtering out noise via temporal coherence. This overcomes the limitations of both intensity-based and quantum distillation methods. We experimentally demonstrate noise resilience by successfully recovering feature-rich objects, such as QR codes and grayscale wheels, obscured by spatially uniform and structured noise 20 times as intense as the object. We further show that our method remains effective for fields with substantial spectral overlap, outperforming spectral filtering in regimes where the latter provides little noise suppression. This approach provides a robust framework for noise-resilient imaging with applications in optical communication, fluorescence microscopy, and biological imaging at both high and low light levels.

physics.optics↗

arXiv2025

Super-resolution with Fourier measurements

Resolving sources beyond the diffraction limit is important in imaging, communications, and metrology. Current image-based methods of super-resolution require phase information (either of the source points or an added filter) and perfect alignment with the centroid of the object. Both inhibit the practical application of these methods, as uniform motion and/or relative jitter destroy their assumptions. Here, we show that measuring intensity in the Fourier plane enables super-resolution without any of the issues of image-based methods. We start with the shift-invariance of the Fourier transform and the observation that the two-point position problem ${x_1,x_2}$ in the near field corresponds to the single-point wavenumber problem $k\ =2\pi/(x_2-x_1)$ in the far field. We consider the full range of mutual coherence and show that for fully coherent sources, the Fourier method saturates the quantum limit, i.e. it gives the best possible measurement. Similar results hold for sub-Rayleigh constellations of $N$ sources, which can act collectively as a spatially averaged metasurface and/or individually as elements of a phased-array antenna. The theory paves the way to merge Fourier optics with super-resolution techniques, enabling experimental devices that are both simpler and more robust than previous designs.

physics.optics↗

arXiv2025

Wavefront correction of high-dimensional two-photon states via coherence-entanglement transfer

Reliable transmission of quantum optical states through real-world environments is key for quantum communication and imaging. Yet, aberrations and scattering in the propagation path can scramble the transmitted signal and hinder its use. A typical strategy is to employ a classical beacon beam to learn and then correct for the wavefront distortions. However, relying on a separate light source increases the overhead in the experimental apparatus. Moreover, the beacon light must closely match the non-classical state in polarization, wavelength, and even temporal bandwidth, which is highly challenging in practice. Here, we introduce a fast and efficient wavefront correction approach where we use the quantum state itself to correct for optical distortion. Via pump shaping, we control the degree of entanglement in the spatially-entangled two-photon state so that it behaves either as a high-dimensional entangled state or as a classical coherent state. The latter case is used to efficiently measure the transmission matrix of the propagation channel and correct its distortions with a spatial light modulator, thereby enabling the transmission of the high-dimensional entangled state with minimal errors. Our approach paves the way for the practical implementation of quantum imaging and communication protocols based on high-dimensional spatially entangled states.

quant-ph↗

arXiv2025

New scattering zones in quantum speckle propagation

Quantum speckles exhibit significantly richer behavior than their classical counterparts due to their higher dimensionality. A simple example is the far-field speckle pattern in 1D light scattering: classical light forms 1D speckles defined by the numerical aperture, whereas biphoton scattering depends in addition on the photon correlation length, forming 2D elliptical speckles. To date, the behavior of quantum speckles for shorter propagation distances has not been considered. We remedy this here by considering the paraxial evolution of two-photon entanglement at arbitrary propagation distances from an isotropic scatterer. We show, theoretically, numerically, and experimentally, that the two length scales of the biphoton introduce a new Fresnel regime between the conventional near and far fields. Further, we show that the quantum near field is characterized by speckles with a square shape that remain constant during propagation. In contrast, the intermediate regime can be engineered to have a constant speckle size along the sum coordinate but a linearly expanding speckle size along the difference coordinate, with a speckle shape that transitions from square to elliptical. The results merge quantum coherence with scattering statistics and suggest new regimes of operation for correlation-based quantum sensing and imaging.

quant-ph↗

arXiv2020

Phase matching in $β$-barium borate crystals for spontaneous parametric down-conversion

Spontaneous parametric down-conversion (SPDC) is the most widely used process for generating entangled photon pairs. In SPDC, a pump photon interacts with a nonlinear optical crystal and splits into two entangled photons called the signal and the idler photons. The SPDC process has been studied extensively in the last few decades for various pump and crystal configurations, and the entangled photon pairs produced by SPDC have been used in numerous experimental studies on quantum entanglement and entanglement-based real-world quantum-information applications. In this tutorial article, we present a thorough study of phase matching in $β$-barium borate (BBO) crystals for spontaneous parametric down-conversion and thereby also investigate the generation of entangled photons in such crystals. First, we present a theoretical derivation of two-photon wavefunction produced by SPDC in the frequency and transverse momentum bases. We then discuss in detail the effects due to various crystal and pump parameters including the length of the crystal, the angle between the optic axis and the pump propagation direction, the pump incidence angle on the crystal surface, the refraction at the crystal surfaces, and the pump propagation direction inside the crystal. These effects are extremely relevant in experimental situations. We then present our numerical and experimental results in order to illustrate how various experimental parameters affect the phase matching and thus the generation of entangled photons. Finally, using the two-photon wavefunction in the transverse wave-vector basis, we show how to derive the two-photon wavefunction in the OAM basis and thereby calculate the two-photon angular Schmidt spectrum. We expect this article to be useful for researchers working in various capacities with entangled photons generated by SPDC in BBO crystals.

physics.optics↗

arXiv2020

Controlling propagation of spatial coherence for enhanced imaging through scattering media

It is known that a spatially partially coherent light field produces better imaging contrast compared to a spatially coherent field and that the contrast increases as the spatial coherence length of the field becomes smaller. The transverse spatial coherence length of most spatially partially coherent fields increases upon propagation. As a result, the field produces progressively decreasing image quality at subsequent transverse planes. By controlling the propagation of spatial coherence, we demonstrate enhanced image quality at different transverse planes along the propagation direction through a scattering medium. Using a source with propagation-invariant spatial coherence function, we report experimental observations of imaging different transverse planes with equal contrast over a significant distance. Furthermore, we generate a spatially partially coherent source that can be tailored to have minimum-possible transverse coherence area at the plane of the object to be imaged, and using this source, we demonstrate imaging spatially separated transverse planes with maximum possible image contrast.

physics.optics↗

arXiv2018

Two-shot measurement of spatial coherence

We propose and demonstrate an interferometric scheme for measuring the two-dimensional two-point cross-spectral density function in a two-shot manner. Our scheme comprises a Michelson interferometer with a converging lens in one of the arms of the interferometer, and the crosss-pectral density function of an input optical field gets encoded in the intensity distribution of the output interferograms. This scheme works for any cross-spectral density function that is real and that depends on the spatial coordinates only through their difference. Using this scheme, we report measurements of several lab-synthesized cross-spectral density functions with very good agreement with theory. Our measurement technique can be very important for applications that are based on utilizing the partial spatial coherence properties of optical fields.

physics.ins-det↗

arXiv2018

Angular Schmidt spectrum of entangled photons: derivation of an exact formula and experimental characterization for non-collinear phase matching

We derive an exact formula for the angular Schmidt spectrum of the orbital angular momentum (OAM)-entangled states produced by parametric down-conversion (PDC). We show that our formula yields true theoretical spectrum without any convergence issue as has been the case with the previously derived formulas. Furthermore, we use this formula to experimentally characterize the angular Schmidt spectrum for non-collinear PDC. We report the measurements of very broad angular Schmidt spectra corresponding to angular Schmidt numbers up to 233, which, to the best of our knowledge, is the highest number reported so far. Our studies can be very important for OAM-based quantum information applications.

physics.optics↗

arXiv2017

Efficient generation of propagation-invariant spatially-stationary partially coherent fields

We propose and demonstrate a novel method for generating propagation-invariant spatially-stationary fields in a controllable manner. Our method relies on producing incoherent mixtures of plane waves using planar primary sources that are spatially completely uncorrelated. The strengths of the individual plane waves in the mixture decide the exact functional form of the generated coherence function. We use LEDs as the primary incoherent sources and experimentally demonstrate the effectiveness of our method by generating several spatially-stationary fields, including a new type, which we refer to as the "region-wise spatially-stationary field." We also experimentally demonstrate the propagation-invariance of these fields, which is an extremely interesting and useful property of such fields. Our work should have important implications for applications that exploit the spatial coherence properties either in a transverse plane or in a propagation-invariant manner, such as correlation holography, wide-field OCT, and imaging through turbulence.

physics.optics↗