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Papers by Ankita Niranjan

Research paper thumbnail of Landau-Zener transitions and adiabatic impulse approximation in an array of two Rydberg atoms with time-dependent detuning

Physical Review A

We study the Landau-Zener (LZ) dynamics in a setup of two Rydberg atoms with time-dependent detun... more We study the Landau-Zener (LZ) dynamics in a setup of two Rydberg atoms with time-dependent detuning, both linear and periodic, using both the exact numerical calculations as well as the method of adiabatic impulse approximation (AIA). By varying the Rydberg-Rydberg interaction strengths, the system can emulate different three-level LZ models, for instance, bow-tie and triangular LZ models. The LZ dynamics exhibits nontrivial dependence on the initial state, the quench rate, and the interaction strengths. For large interaction strengths, the dynamics is well captured by AIA. In the end, we analyze the periodically driven case, and AIA reveals a rich phase structure involved in the dynamics. The latter may find applications in quantum state preparation, quantum phase gates, and atom interferometry.

Research paper thumbnail of Dynamics and quantum correlations in two independently driven Rydberg atoms with distinct laser fields

Journal of Physics B: Atomic, Molecular and Optical Physics

We study the population dynamics in a two-atom setup in which each atom is driven independently b... more We study the population dynamics in a two-atom setup in which each atom is driven independently by different light fields, but coupling the same Rydberg state. In particular, we look at how an offset in the Rabi frequencies between two atoms influences the dynamics. We find novel features such as amplifying the Rabi frequency of one atom, together with strong Rydberg-Rydberg interactions freezes the dynamics in the second atom. We characterize the Rydberg-biased freezing phenomenon in detail, with effective Hamiltonians obtained for various limits of the system parameters. In the absence of Rabi-offset, the doubly excited state population exhibits a Lorentzian profile as a function of interaction, whereas for very small offsets it shows splitting and thus peaks. Using an effective Hamiltonian as well as the perturbation theory for weak interactions, we show that the peak arises from a competition between Rabi-offset and Rydberg-Rydberg interactions when both are sufficiently small, together with the Rydberg blockade at large interactions. The effective Hamiltonians provide us with analytical results which are in an excellent agreement with full numerical solutions. Also, we analyze the growth and the dynamics of quantum correlations such as entanglement entropy and quantum discord for the coherent dynamics. We extend our studies to the dissipative case in which the spontaneous emission from the Rydberg state is taken into account and in particular, we look at the purity and quantum discord of the steady states. To conclude, our studies reveal that the local manipulation of an atom using Rabi-offset can be an ideal tool to control the quantum correlations and in general, quantum states of the composite two-qubit systems.

Research paper thumbnail of Comparison of Mo/Si and NbC/Si lamellar multilayer gratings near Si absorption edge

ABSTRACT Multilayer gratings are important optical elements for obtaining a high efficiency high ... more ABSTRACT Multilayer gratings are important optical elements for obtaining a high efficiency high resolution performance in the soft x-ray region. Though the Mo/Si multilayer poses a poor thermal stability, it is a preferred combination for multilayer coating near the Si L edge region (λ=124Å). As a substitute to Mo/Si, a structure comprised of compound material NbC was proposed recently which exhibits superior thermal stability with identical reflectivity performance. In the present study, an analytical solution for grating efficiency calculation presented in Ref 2 by Kozhevnikov et al. is used to calculate theoretical performance of lamellar multilayer grating (LMG) comprised of Mo/Si and NbC/Si structure. Simulation results suggest that the performance of lamellar grating comprised of NbC/Si multilayer is identical with that obtained using Mo/Si multilayer grating.

Research paper thumbnail of Vortex dynamics of rotating Bose-Einstein condensate of microcavity polaritons

The European Physical Journal B, 2015

In this work we perform a numerical study of a rotating, harmonically trapped, Bose-Einstein cond... more In this work we perform a numerical study of a rotating, harmonically trapped, Bose-Einstein condensate of microcavity polaritons. An efficient numerical method (toolbox) to solve the complex Gross-Pitaevskii equation is developed. Using this method, we investigate how the behavior of the number of vortices formed inside the condensate changes as the various system parameters are varied. In contrast to the atomic condensates, we show, there exists an (experimentally realizable) range of parameter values in which all the vortices can be made to vanish even when there is a high rotation. We further explore how this region can be tuned through other free parameters and also discuss how this study can help to realize the synthetic magnetic field for polaritons and hence paving the way for the realization of the quantum Hall physics and many other exotic phenomena.

Research paper thumbnail of Landau-Zener transitions and adiabatic impulse approximation in an array of two Rydberg atoms with time-dependent detuning

Physical Review A

We study the Landau-Zener (LZ) dynamics in a setup of two Rydberg atoms with time-dependent detun... more We study the Landau-Zener (LZ) dynamics in a setup of two Rydberg atoms with time-dependent detuning, both linear and periodic, using both the exact numerical calculations as well as the method of adiabatic impulse approximation (AIA). By varying the Rydberg-Rydberg interaction strengths, the system can emulate different three-level LZ models, for instance, bow-tie and triangular LZ models. The LZ dynamics exhibits nontrivial dependence on the initial state, the quench rate, and the interaction strengths. For large interaction strengths, the dynamics is well captured by AIA. In the end, we analyze the periodically driven case, and AIA reveals a rich phase structure involved in the dynamics. The latter may find applications in quantum state preparation, quantum phase gates, and atom interferometry.

Research paper thumbnail of Dynamics and quantum correlations in two independently driven Rydberg atoms with distinct laser fields

Journal of Physics B: Atomic, Molecular and Optical Physics

We study the population dynamics in a two-atom setup in which each atom is driven independently b... more We study the population dynamics in a two-atom setup in which each atom is driven independently by different light fields, but coupling the same Rydberg state. In particular, we look at how an offset in the Rabi frequencies between two atoms influences the dynamics. We find novel features such as amplifying the Rabi frequency of one atom, together with strong Rydberg-Rydberg interactions freezes the dynamics in the second atom. We characterize the Rydberg-biased freezing phenomenon in detail, with effective Hamiltonians obtained for various limits of the system parameters. In the absence of Rabi-offset, the doubly excited state population exhibits a Lorentzian profile as a function of interaction, whereas for very small offsets it shows splitting and thus peaks. Using an effective Hamiltonian as well as the perturbation theory for weak interactions, we show that the peak arises from a competition between Rabi-offset and Rydberg-Rydberg interactions when both are sufficiently small, together with the Rydberg blockade at large interactions. The effective Hamiltonians provide us with analytical results which are in an excellent agreement with full numerical solutions. Also, we analyze the growth and the dynamics of quantum correlations such as entanglement entropy and quantum discord for the coherent dynamics. We extend our studies to the dissipative case in which the spontaneous emission from the Rydberg state is taken into account and in particular, we look at the purity and quantum discord of the steady states. To conclude, our studies reveal that the local manipulation of an atom using Rabi-offset can be an ideal tool to control the quantum correlations and in general, quantum states of the composite two-qubit systems.

Research paper thumbnail of Comparison of Mo/Si and NbC/Si lamellar multilayer gratings near Si absorption edge

ABSTRACT Multilayer gratings are important optical elements for obtaining a high efficiency high ... more ABSTRACT Multilayer gratings are important optical elements for obtaining a high efficiency high resolution performance in the soft x-ray region. Though the Mo/Si multilayer poses a poor thermal stability, it is a preferred combination for multilayer coating near the Si L edge region (λ=124Å). As a substitute to Mo/Si, a structure comprised of compound material NbC was proposed recently which exhibits superior thermal stability with identical reflectivity performance. In the present study, an analytical solution for grating efficiency calculation presented in Ref 2 by Kozhevnikov et al. is used to calculate theoretical performance of lamellar multilayer grating (LMG) comprised of Mo/Si and NbC/Si structure. Simulation results suggest that the performance of lamellar grating comprised of NbC/Si multilayer is identical with that obtained using Mo/Si multilayer grating.

Research paper thumbnail of Vortex dynamics of rotating Bose-Einstein condensate of microcavity polaritons

The European Physical Journal B, 2015

In this work we perform a numerical study of a rotating, harmonically trapped, Bose-Einstein cond... more In this work we perform a numerical study of a rotating, harmonically trapped, Bose-Einstein condensate of microcavity polaritons. An efficient numerical method (toolbox) to solve the complex Gross-Pitaevskii equation is developed. Using this method, we investigate how the behavior of the number of vortices formed inside the condensate changes as the various system parameters are varied. In contrast to the atomic condensates, we show, there exists an (experimentally realizable) range of parameter values in which all the vortices can be made to vanish even when there is a high rotation. We further explore how this region can be tuned through other free parameters and also discuss how this study can help to realize the synthetic magnetic field for polaritons and hence paving the way for the realization of the quantum Hall physics and many other exotic phenomena.