arXiv subjects

Renu Tripathi

Publications and source records attributed to Renu Tripathi.

12 recordsLinked to original sources

arXiv2025

Millimeter Wave Imaging using Autler-Townes Splitting Induced Fluorescence from Rydberg Atoms in Rubidium Vapor

We demonstrate millimeter wave (mmWave) imaging using Autler-Townes (AT) splitting-induced fluorescence in rubidium vapor. By employing counter-propagating probe and coupling beams, we create a plane of atoms in a dark state that is sensitive to incident mmWave and demonstrate the method with mmWaves resonant with the transition between two neighboring Rydberg states at 91.4 giga hertz. Objects in the mmWave beam path affect the spatial profile of the mmWave field, which, in turn, leads to spatial variation of optical fluorescence at 780 nm. This results in a transduction of the mmWave spatial profile to an optical image that can be acquired by a CCD camera. We demonstrate real-time, diffraction-limited mmWave imaging in transmission geometry, over an effective sensing area of 6.5 cm square with a minimum 600 micro-volt per cm detectable electric field and a rapid 16.4 micro-second response time. In addition, we demonstrate this method with a variety of imaging masks, including 3D-printed vortex phase plates. Our study provides a pathway for high-speed and high-resolution mmWave imaging with potential applications in security, communications, and scientific research.

physics.atom-ph↗

arXiv2025

Dual-Frequency Absorption Spectroscopy in Laser-Cooled Rubidium Atoms: Theoretical Modeling and Experiment

We demonstrate dual-frequency absorption spectroscopy (DFAS) using laser-cooled 87Rb and 85Rb atoms. Doppler-free resonances with high-contrast are produced, which suggest the suitability of using dual-frequency absorption spectroscopy (DFAS) for laser stabilization in a cold-atom-based coherent population trapping (CPT) clock, and for developing a compact, high-performance optical frequency standard using an integrated magneto-optical trap (MOT). We developed a model using density-matrix equations to accurately simulate DFAS in the atomic medium without applying any simplifying approximations. Comprehensive simulations are performed using our multi-level system model to analyze dual-frequency spectra produced in cold atom ensembles under different experimental conditions, including the effect of magnetic field, and two-photon detuning. The simulations accurately yield amplitudes, linewidths, frequency shifts, and lineshapes of DFAS resonances under these experimental conditions. We also demonstrate a simple mechanism for performing CPT spectroscopy by implementing a DFAS laser lock using trapped atoms in the MOT. Additionally, we have extended our model to accurately model the dual-frequency spectrum produced in rubidium cell, which is a medium of practical interest for vapor-cell-based quantum sensing applications.

physics.atom-ph↗

arXiv2025

Spin-squeezed vector atomic magnetometry

Atomic magnetometers based on Zeeman shift measurement have the potential for high sensitivity and long-term stability. Like other atomic sensors including atomic clocks and atom interferometers, the atomic magnetometer could in principle be augmented with spin squeezing for further sensitivity enhancement. However, existing atomic magnetometers are not compatible with spin squeezing because the atoms can hardly be in a pure quantum state during operation. A natural challenge is the arbitrary direction of the magnetic field. In this paper, we propose a cold-atom-based magnetometer with spin squeezing that can measure both the magnitude and the direction of an arbitrary magnetic field. For experimentally accessible parameters, we show that the technique described above could achieve a sensitivity nearly three orders of magnitude higher than that of the best existing magnetometers.

quant-ph↗

arXiv2022

Spin Squeezing Induced Enhancement of Sensitivity of an Atomic Clock using Coherent Population Trapping

The coherent population trapping (CPT) effect is used for making compact atomic clocks. There are two types of CPT clocks: the one in which the Raman beams are applied continuously and the one in which two CPT pulses separated by a dark period are applied (Ramsey scheme). It is obvious that the technique of spin squeezing can only be applied to the Ramsey CPT clock to enhance the sensitivity. However, it is not apparent how to adapt to the CPT clock the protocols for the microwave clock using one-axis-twist squeezing (OATS), since the Ramsey CPT clock is not trivially equivalent to the Ramsey microwave clock. In this paper, we show explicitly how to adapt two protocols using OATS, namely the Schrödinger cat state protocol (SCSP) and the generalization thereof, and the echo squeezing protocol (ESP), to the CPT clock. The ESP magnifies the phase shift by a factor of the square-root of (N/e), while the SCSP magnifies the phase shift by a factor of N/2 , making it able to achieve a higher sensitivity in the presence of excess noise.

quant-ph↗

arXiv2020

Magnetometry using sodium fluorescence with synchronous modulation of two-photon resonant light fields

We report a new technique for generating magnetic resonance with synchronous modulation of two-photon resonant light fields. Magnetic resonances in fluorescence from a sodium cell are measured to demonstrate suitability of this technique for remote magnetometry. A strong magnetic resonance with its dip corresponding to the Larmor frequency is produced in the presence of a transverse magnetic field. An additional resonance at 3\{Omega_L} is observed, which can be used to determine the magnetic field orientation. We have developed a theoretical model based on the density matrix equations to verify our experimental observations. An average magnetic field sensitivity of 41 \mathbf{pT}/\sqrt{\mathbf{Hz}} is measured using light duty cycles ranging from 35% to 10%. We have discussed possible changes that can be made to improve the sensitivity of this scheme further.

physics.atom-ph↗

arXiv2020

Light ellipticity and polarization angle dependence of magnetic resonances in rubidium vapor using amplitude-modulated light: Theoretical and experimental investigations

We report on experimental and theoretical investigations of the polarization dependence of magnetic resonance generated by synchronous optical pumping. Magnetic resonances with narrow linewidth are generated experimentally using a rubidium vapor cell with octade-cyltrichlorosilane (OTS) antirelaxation coating on inner walls. We studied the effect of light ellipticity on the amplitudes and widths of magnetic resonances by matching the light modulation frequency with 2 Ω_L (alignment) and Ω_L(orientation) in a Bell-Bloom interaction geometry, where Ω_L corresponds to the Larmor frequency. Both 2 Ω_L and \OmegaL resonance amplitudes showed a strong dependence on the light ellipticity. In addition, we showed that the duty cycle of light modulation changes the slope of amplitude variations in 2Ω_L and Ω_L resonances with light ellipticity. As a potential application, we showed that the difference between 2Ω_Land Ω_Lresonance amplitudes can be used for in situ measurement of light ellipticity. We also studied the dependence of 2Ω_L and Ω_L resonance amplitudes on the polarization angle of linearly polarized light. These amplitudes oscillate periodically with the polarization angle. We found this oscillatory behavior to be sensitive to the tilt in magnetic field direction from the polarization plane. Such a property could be used to realize a vector magnetometer. A density matrix based theoretical model is developed to simulate the magnetic resonance spectrum for different light polarizations. Our theoretical model accurately reproduces the above mentioned experimental observations.

physics.atom-ph↗

arXiv2019

Magneto-optical resonances in fluorescence from sodium D2 manifold

We report on magneto-optical resonances observed in sodium fluorescence from D2 manifold with an intensity modulated light. Fluorescence resonances are measured in the perpendicular and backward directions to the light propagation, in laboratory experiments using a sodium cell containing neon buffer gas. Properties of these resonances are studied by varying the magnetic field at fixed light modulation frequency, and vice-versa. Modulation with low-duty cycle shows higher-harmonic resonances of the modulation frequency and sub-harmonic resonances of the Larmor frequency. A dark resonance with maximum amplitude for laser wavelength closer to the crossover peak is observed. The origin of this dark resonance observed in Na D2 line is discussed using a theoretical model. Present study is aimed towards improving the understanding of magneto-optical resonances for remote magnetometry applications with mesospheric sodium.

physics.atom-ph↗

arXiv2014

Incorporation of Polar Mellin Transform in a Hybrid Optoelectronic Correlator for Scale & Rotation Invariant Target Recognition

In this paper, we show that our proposed hybrid optoelectronic correlator (HOC), which correlates images using spatial light modulators (SLM), detectors and field programmable gate arrays (FPGAs), is capable of detecting objects in a scale and rotation invariant manner, along with the shift invariance feature, by incorporating polar mellin transform (PMT). We also illustrate a key limitation of the ideas presented in previous papers on performing PMT and present a solution to circumvent this limitation by cutting out a small circle at the center of the Fourier Transform which precedes PMT. Furthermore, we show how to carry out shift, rotation and scale invariant detection of multiple matching objects simultaneously, a process previously thought to be incompatible with PMT based correlators. We present results of numerical simulations to validate the concepts.

physics.optics↗

arXiv2013

Hybrid Optoelectronic Correlator Architecture for Shift Invariant Target Recognition

In this paper, we present theoretical details and the underlying architecture of a hybrid optoelectronic correlator that correlates images using Spatial Light Modulators (SLM), detector arrays and Field Programmable Gate Array (FPGA). The proposed architecture bypasses the need for nonlinear materials such as photorefractive polymer films by using detectors instead, and the phase information is yet conserved by the interference of plane waves with the images. However, the output of such a Hybrid Opto-electronic Correlator (HOC) has four terms: two convolution signals and two cross-correlation signals. By implementing a phase stabilization and scanning circuit, the convolution terms can be eliminated, so that the behavior of an HOC becomes essentially identical to that of a conventional holographic correlator (CHC). To achieve the ultimate speed of such a correlator, we also propose an opto-electronic chip which would perform all the electrical processes in a parallel manner. The HOC architecture along with the phase stabilization technique would thus be as good as a CHC, capable of high speed image recognition in a translation invariant manner.

physics.optics↗

arXiv2010

A Close Companion Search around L Dwarfs using Aperture Masking Interferometry and Palomar Laser Guide Star Adaptive Optics

We present a close companion search around sixteen known early-L dwarfs using aperture masking interferometry with Palomar laser guide star adaptive optics. The use of aperture masking allows the detection of close binaries, corresponding to projected physical separations of 0.6-10.0 AU for the targets of our survey. This survey achieved median contrast limits of Delta_K ~ 2.3 for separations between 1.2 - 4 lambda/D, and Delta_K ~ 1.4 at (2/3)lambda/D. We present four candidate binaries detected with moderate to high confidence (90-98%). Two have projected physical separations less than 1.5 AU. This may indicate that tight-separation binaries contribute more significantly to the binary fraction than currently assumed, consistent with spectroscopic and photometric overluminosity studies. Ten targets of this survey have previously been observed with the Hubble Space Telescope as part of companion searches. We use the increased resolution of aperture masking to search for close or dim companions that would be obscured by full aperture imaging, finding two candidate binaries. This survey is the first application of aperture masking with laser guide star adaptive optics at Palomar. Several new techniques for the analysis of aperture masking data in the low signal to noise regime are explored.

astro-ph.SR↗

arXiv2005

Controllable Anomalous Dispersion and Group Index Nulling via Bi-Frequency Raman Gain in Rb Vapor for application to Ultraprecision Rotation Sensing

We have recently proposed [9], the use of fast-light media to obtain ultrahigh precision rotation sensing capabilities. The scheme relies on producing a critically anomalous dispersion, in a suitable dispersive medium, which is introduced in the arms of a Sagnac interferometer. We present here an experimental investigation of the anomalous dispersion properties of bi-frequency Raman gain in rubidium vapor, with the goal of using this medium for producing the critically anomalous dispersion condition. A heterodyne phase measurement technique is used to measure accurately the index variation associated with the dispersion. The slope of the negative linear dispersion (or group index) is experimentally varied by more than two orders of magnitude while changing the frequency separation between pump fields, responsible for producing gain. Using this result, we have identified the experimental parameters for achieving a null value of the group index, corresponding to the critically anomalous dispersion condition necessary for enhanced rotational sensitivity.

quant-ph↗

arXiv2005

Experimental Constraints of Using Slow-Light in Sodium Vapor for Light-Drag Enhanced Relative Rotation Sensing

We report on experimental observation of electromagnetically induced transparency and slow-light (vg ~ c/607) in atomic sodium vapor, as a potential medium for a recently proposed experiment on slow-light enhanced relative rotation sensing [11]. We have performed an interferometric measurement of the index variation associated with a two-photon resonance to estimate the dispersion characteristics of the medium that is relevant to the slow-light based rotation sensing scheme. We also show that the presence of counter-propagating pump beams in an optical Sagnac loop produces a backward optical phase conjugation beam that can generate spurious signals, which may complicate the measurement of small rotations in the slow-light enhanced gyroscope. We identify techniques for overcoming this constraint.

quant-ph↗