arXiv subjects

Umakant D. Rapol

Publications and source records attributed to Umakant D. Rapol.

At least 19 recordsLinked to original sources

arXiv2025

Evaporative cooling by pulse width modulation (PWM) of optical dipole traps

We introduce a method for cooling atoms in an optical dipole trap using pulse-width modulation (PWM) technique, without reducing the laser power of the dipole trap. The PWM technique involves digital modulation of the trap at a fixed frequency. The effective time-averaged dipole potential is lowered by adjusting the duty cycle of the modulation, thereby implementing evaporative cooling. We show that, this technique effectively reduces temperature and enhances phase space density. A comparison with the standard method of evaporative cooling has also been made. Apart from the atom loss due to reduction of the effective trapping potential, we observe an additional loss channel originating from the lack of trapping potential during the trap off time. This atom loss is observed at different modulation frequencies which are an order of magnitude higher compared to trapping frequency of dipole trap. The PWM technique provides an alternative to traditional evaporative cooling in scenarios where it is preferred that the laser power of the trap should be constant.

physics.atom-ph↗

arXiv2024

Asymmetric dynamical localization and precision measurement of BEC micromotion

We employ a Bose Einstein Condensate (BEC) based atom-optic kicked rotor to generate an asymmetrically localized momentum distribution that depends upon initial velocity of the BEC. Asymmetric features are shown to arise from the early-time dynamics induced by the broken parity symmetry and, asymptotically freeze as the dynamical localization stabilizes. The asymmetry in the momentum distribution critically depends upon the initial launch velocity and is sensitive to very small initial velocities ('micromotion') of the BEC. In this work, we also perform a precise measurement of the 'micromotion'. By utilizing the technique of measuring the early-time asymmetry of momentum distribution, we report measurement of micromotion down to (230 \pm 17 , μ\text{m/s}).

quant-ph↗

arXiv2023

Comparative Analysis of Phase Noise for different configurations of Bragg lattice for an Atomic Gravimeter with Bose-Einstein Condensate

We perform a comparative study of the phase noise induced in the lasers used for Bragg diffraction in a Bose-Einstein condensate-based quantum gravimeter where the Bragg beams are generated using two different configurations. In one of the configurations, the Bragg beams that form the moving optical lattice are generated using two different acousto-optic modulators. In the second configuration, the Bragg beams are generated using a single acousto-optic modulator carrying two phase-locked frequencies. The second configuration shows a suppression of phase noise by a factor of 4.7 times in the frequency band upto 10 $kHz$, the primary source of noise, which is the background acoustic noise picked up by optical components and the optical table. We report a sensitivity of 99.7 $μGal/\sqrt Hz$ for an interferometric time of 10 $ms$.

quant-ph↗

arXiv2023

Electromagnetically Induced Transparency (EIT) aided cooling of strontium atoms

The presence of ultra-narrow inter-combination spectroscopic lines in alkaline earth elements places them as promising candidates for optical atomic clocks, quantum computation, and for probing fundmental physics. Doppler cooling of these atoms is typically achieved through two subsequent stages: the initial cooling is on the 1s0-1p1 transition followed by cooling using the narrow-line 1s0-3p1 transition. However, due to significantly lower linewidth of the second stage cooling transition, efficient transfer of atoms into the second stage becomes technically challenging. The velocity distribution of the atoms after the first stage of cooling is too broad for atoms to be captured efficiently in the second stage cooling. As a result, the capture efficiency of atoms into the second stage Magneto-Optical Trap is low, even if the linewidth of the second stage cooling laser is artificially broadened.

physics.atom-ph↗

arXiv2022

Control of dynamical localization in atom-optics kicked rotor

Atom-optics kicked rotor represents an experimentally realizable version of the paradigmatic quantum kicked rotor system. After a short initial diffusive phase the cloud settles down to a stationary state due to the onset of dynamical localization. In this work we realise an enhancement of localization by modification of the kick sequence. We experimentally implement the modification to this system in which the sign of the kick sequence is flipped by allowing for a free evolution of the wavepackets for half the Talbot time after every $M$ kicks. Depending on the value of $M$, this modified system displays a combination of enhanced diffusion followed by asymptotic localization. This is explained as resulting from two competing processes -- localization induced by standard kicked rotor type kicks, and diffusion induced by half Talbot time evolution. The evolving states display a localized but non-exponential wave function profiles. This provides another route to quantum control in kicked rotor class of systems. The numerical simulations agree well with the experimental results.

quant-ph↗

arXiv2020

Atom interferometry using temporal Talbot effect on a Bose-Einstein condensate

We experimentally investigate a uniform pulse sequence in which atom interference is realized using the temporal matter-wave Talbot effect in an atom-optic kicked rotor system. Multi-path interference is obtained in asymmetric configuration with momentum differences up to $\pm$14 $\hbar k$, by virtue of Talbot resonance. We experimentally confirm the theoretical limit placed on the performance of this interferometer by the finite momentum distribution of the initial ensemble consisting of a Bose-Einstein condensate (BEC). This limitation on sensitivity, occurring due to the degradation of resonant dynamics is also important in the realization of a one-dimensional continuous-time quantum walk in the implementation of quantum search algorithms.

physics.atom-ph↗

arXiv2019

Study of loss dynamics of strontium in a magneto-optical trap

Collisions with background atoms are known to induce a significant shift in the frequency of state-of-the-art optical atomic clocks and contribute to state decoherence in cold atom experiments. The effects of these collisions can be quantified by measuring their cross sections. We experimentally measured the collision cross section between $^{88}$Sr$-$N$_{2}$ in a Magneto-Optical Trap (MOT). The measurement was carried out by monitoring the atom number loss rate as a function of background pressure of N$_{2}$ and the cross section thus obtained was 8.1(4)$\times 10^{-18}$ m$^{2}$. The measured collision cross section has been utilized for the determination of C$_{6}$ coefficient of the ground state (${^1S}_0$) of $^{88}$Sr atom, which can be useful to estimate the relative frequency shift in the clock transition. We also estimate the loss rate induced by the combined effect of the decay of atoms in the long-lived ${^3P}_0$ state and temperature-induced atomic losses from the capture volume of the MOT. We find that the contribution due to the latter is dominant in comparison to the other atomic loss channels and must be included in the studies that rely on the total loss rate measurement.

physics.atom-ph↗

arXiv2018

Design, Fabrication and Characterization of nanoplasmonic lattice for trapping of ultracold atoms

Ultracold atom-traps on a chip enhances the practical application of atom traps in quantum information processing, sensing, and metrology. Plasmon mediated near-field optical potentials are promising for trapping atoms. The combination of plasmonic nanostructures and ultracold atoms has the potential to create a two dimensional array of neutral atoms with lattice spacing smaller than that of lattices created from interfering light fields -- the optical lattices. We report the design, fabrication and characterization of a nano-scale array of near-field optical traps for neutral atoms using plasmonic nanostructures. The building block of the array is a metallic nano-disc fabricated on the surface of an ITO-coated glass substrate. We numerically simulate the electromagnetic field-distribution using Finite Difference Time Domain method around the nanodisc, and calculate the intensity, optical potential and the dipole force for $^{87}$Rb atoms. The optical near-field generated from the fabricated nanostructures is experimentally characterized by using Near-field Scanning Optical Microscopy. We find that the optical potential and dipole force has all the desired characteristics to trap cold atoms when a blue-detuned light-field is used to excite the nanostructures. This trap can be used for effective trapping and manipulation of isolated atoms and also for creating a lattice of neutral atoms having sub-optical wavelength lattice spacing. Near-field measurements are affected by the influence of tip on the sub-wavelength structure. We present a deconvolution method to extract the actual near-field profile from the measured data.

physics.optics↗

arXiv2018

A simple atomic beam oven with a metal thermal break

We report the design and construction of a simple, easy to machine high-temperature oven for generating an atomic beam in laser cooling experiments. This design eliminates the problem of thermal isolation of the oven region from the rest of the vacuum system without using a glass or ceramic thermal break. This design simplifies the construction and operation of high-temperature ovens for elements having low vapor pressure. We demonstrate the functionality of such a source for Strontium (Sr) atoms. We generate a high flux of Sr atoms for use in laser cooling and trapping experiments. The optimization of the design of the metal thermal break is done using a finite element analysis.

physics.atom-ph↗

arXiv2018

Diffraction of a CW atom laser in the Raman-Nath regime

Atom interferometry is the most successful technique for precision metrology. However, current interferometers using ultracold atoms allows one to probe the interference pattern only momentarily and has finite duty cycle, resulting in an aliasing effect and a low-bandwidth measurement -- also known as Dick effect. Interferometry with a continuous-wave atom laser shows promise in overcoming these limitations due a continuous monitoring of the interference pattern. In this work, we demonstrate a key step towards such an interferometry by demonstrating a diffraction of an `atom laser' in the Raman-Nath regime. We outcouple a continuous beam of coherent atoms from a reservoir of $^{87}$Rb Bose-Einstein condensate (BEC) upto 400 ms. The `atom laser' interacts with a grating formed by a standing wave of a far detuned laser light. The atom laser diffracts into several orders going up to 9$^{th}$ order or up to momenta of $\pm 18\ \hbar k$. We have characterized the diffraction of atom laser for different conditions and the results match with numerical simulations. Such atom laser will allow for construction of an atom-interferometer to probe physics phenomenon continuously up to a time of the order of few hundred millisecond.

physics.atom-ph↗

arXiv2017

Dark solitons generation and their instability dynamics in two dimensional condensates

We analyze numerically the formation and the subsequent dynamics of two-dimensional matter wave dark solitons in a Thomas-Fermi rubidium condensate using various techniques. An initially imprinted sharp phase gradient leads to the dynamical formation of a stationary soliton as well as very shallow grey solitons, whereas a smooth gradient only creates grey solitons. The depth and hence, the velocity of the soliton is provided by the spatial width of the phase gradient, and it also strongly influences the snake-instability dynamics of the two dimensional solitons. The vortex dipoles stemming from the unstable soliton exhibit rich dynamics. Notably, the annihilation of a vortex dipole via a transient dark lump or a vortexonium state, the exchange of vortices between either a pair of vortex dipoles or a vortex dipole and a single vortex, and so on. For sufficiently large width of the initial phase gradient, the solitons may decay directly into vortexoniums instead of vortex pairs, and also the decay rate is augmented. Later, we discuss alternative techniques to generate dark solitons, which involve a Gaussian potential barrier and time-dependent interactions, both linear and periodic. The properties of the solitons can be controlled by tuning the amplitude or the width of the potential barrier. In the linear case, the number of solitons and their depths are determined by the quench time of the interactions. For the periodic modulation, a transient soliton lattice emerges with its periodicity depending on the modulation frequency, through a wave number selection governed by the local Bogoliubov spectrum. Interestingly, for sufficiently low barrier potential, both Faraday pattern and soliton lattice coexist. The snake instability dynamics of the soliton lattice is characteristically modified if the Faraday pattern is present.

cond-mat.quant-gas↗

arXiv2017

Non-exponential decoherence and subdiffusion in atom-optics kicked rotor

Quantum systems lose coherence upon interaction with the environment and tend towards classical states. Quantum coherence is known to exponentially decay in time so that macroscopic quantum superpositions are generally unsustainable. In this work, slower than exponential decay of coherences is experimentally realized in an atom-optics kicked rotor system subjected to non-stationary Lévy noise in the applied kick sequence. The slower coherence decay manifests in the form of quantum subdiffusion that can be controlled through the Lévy exponent. The experimental results are in good agreement with the analytical estimates and numerical simulations for the mean energy growth and momentum profiles of atom-optics kicked rotor.

quant-ph↗

arXiv2017

A compact atomic beam based system for Doppler-free laser spectroscopy of Strontium atoms

We report the construction of a simple, light weight and compact atomic beam spectroscopy cell for Strontium atoms. The cell is built using glass blowing technique and includes a simple Titanium Sublimation Pump for active pumping of the residual and background gases to maintain ultra-high vacuum. Commercially available and electrically heated dispenser source is used to generate the beam of Sr atoms. We perform spectroscopy on the $5s^2\ ^1S_0\longrightarrow 5s\ 5p\ ^1P_1$ transition to obtain a well resolved Doppler free spectroscopic signal for frequency stabilization of the laser source. This design can be easily extended to other alkali and alkaline earth metals.

physics.atom-ph↗

arXiv2005

Doppler-free spectroscopy in driven three-level systems

We demonstrate two techniques for studying the features of three-level systems driven by two lasers (called control and probe), when the transitions are Doppler broadened as in room-temperature vapor. For $Λ$-type systems, the probe laser is split to produce a counter-propagating pump beam that saturates the transition for the zero-velocity atoms. Probe transmission then shows Doppler-free peaks, which can even have sub-natural linewidth. For V-type systems, the transmission of the control beam is detected as the probe laser is scanned. The signal shows Doppler-free peaks when the probe laser is resonant with transitions for the zero-velocity group. Both techniques greatly simplify the study of three-level systems since theoretical predictions can be directly compared without complications from Doppler broadening and the presence of multiple hyperfine levels in the spectrum.

physics.optics↗

arXiv2003

Precise measurement of hyperfine structure in the $5P_{3/2}$ state of $^{85}$Rb

We demonstrate a technique to measure hyperfine structure using a frequency-stabilized diode laser and an acousto-optic modulator locked to the frequency difference between two hyperfine peaks. We use this technique to measure hyperfine intervals in the $5P_{3/2}$ state of $^{85}$Rb and obtain a precision of 20 kHz. We extract values for the magnetic-dipole coupling constant $A=25.038(5)$ MHz and the electric-quadrupole coupling constant $B=26.011(22)$ MHz. These values are a significant improvement over previous results.

physics.atom-ph↗

arXiv2003

Precise measurements of UV atomic lines: Hyperfine structure and isotope shifts in the 398.8 nm line of Yb

We demonstrate a technique for frequency measurements of UV transitions with sub-MHz precision. The frequency is measured using a ring-cavity resonator whose length is calibrated against a reference laser locked to the $D_2$ line of $^{87}$Rb. We have used this to measure the 398.8 nm ${^1S}_0 \leftrightarrow {^1P}_1$ line of atomic Yb. We report isotope shifts of all the seven stable isotopes, including the rarest isotope $^{168}$Yb. We have been able to resolve the overlapping $^{173}$Yb($F = 3/2$) and $^{172}$Yb transitions for the first time. We also obtain high-precision measurements of excited-state hyperfine structure in the odd isotopes, $^{171}$Yb and $^{173}$Yb. The measurements resolve several discrepancies among earlier measurements.

physics.atom-ph↗

arXiv2002

Laser cooling and trapping of Yb from a thermal source

We have successfully loaded a magneto-optic trap for Yb atoms from a thermal source without the use of a Zeeman slower. The source is placed close to the trapping region so that it provides a large flux of atoms that can be cooled and captured. The atoms are cooled on the ${^1S_0} \leftrightarrow {^1P_1}$ transition at 398.8 nm. We have loaded all seven stable isotopes of Yb into the trap. For the most abundant isotope ($^{174}$Yb), we load more than $10^7$ atoms into the trap within 1 s. For the rarest isotope ($^{168}$Yb) with a natural abundance of only 0.13%, we still load about $4 \times 10^5$ atoms into the trap. We find that the trap population is maximized near a detuning of $-1.5Γ$ and field gradient of 75 G/cm.

physics.atom-ph↗

arXiv2002

Precise measurement of hyperfine intervals using avoided crossing of dressed states

We demonstrate a technique for precisely measuring hyperfine intervals in alkali atoms. The atoms form a three-level $Λ$ system in the presence of a strong control laser and a weak probe laser. The dressed states created by the control laser show significant linewidth reduction. We have developed a technique for Doppler-free spectroscopy that enables the separation between the dressed states to be measured with high accuracy even in room-temperature atoms. The states go through an avoided crossing as the detuning of the control laser is changed from positive to negative. By studying the separation as a function of detuning, the center of the level-crossing diagram is determined with high precision, which yields the hyperfine interval. Using room-temperature Rb vapor, we obtain a precision of 44 kHz. This is a significant improvement over the current precision of ~ 1 MHz.

physics.atom-ph↗