Papers by Rupamanjari Ghosh
Journal of Physics B: Atomic, Molecular and Optical Physics, 2015
Optics Letters, 2016
In this letter, we report our experimental results on phase-sensitive amplification (PSA) in non-... more In this letter, we report our experimental results on phase-sensitive amplification (PSA) in non-degenerate signal-idler configuration using ultra-narrow coherent population oscillations in metastable helium at room temperature. We achieved a high PSA gain of nearly 7 with a bandwidth of 200 kHz, by using the system at resonance in a single-pass scheme. Further, the measured minimum gain is close to the ideal value, showing that we have a pure PSA. This is also confirmed from our phase-to-phase transfer curves measurements, illustrating that we have a nearly perfect squeezer, which is interesting for a variety of applications.
Conference on Lasers and Electro-Optics, 2016
We demonstrate both experimentally and theoretically a new form of light storage based on Coheren... more We demonstrate both experimentally and theoretically a new form of light storage based on Coherent Population Oscillations. It is shown to be phase preserving and robust to dephasing effects.
SPIE Proceedings, 2010
ABSTRACT We optimize the simultaneous oscillations at two frequencies in a class-A Vertical Exter... more ABSTRACT We optimize the simultaneous oscillations at two frequencies in a class-A Vertical External Cavity Surface Emitting Laser (VECSEL). We perform this task by measuring the coupling constant between the two perpendicular polarized modes for different values of the transverse spatial separation between the two modes.
Photonics 2010: Tenth International Conference on Fiber Optics and Photonics, 2010
ABSTRACT The nearest-neighbor entanglement of the evolved state of an asymmetric quantum XY spin ... more ABSTRACT The nearest-neighbor entanglement of the evolved state of an asymmetric quantum XY spin chain, in a transverse time-dependent field, exhibits criticalities (dynamical phase transitions) as the initial field parameter is varied at a given fixed time. After a discussion of the dynamical phase transition, we investigate the extent to which the role of an information-theoretic quantum correlation measure, called quantum discord, can be used to understand the entanglement dynamics in this spin model. We show that quantum discord can be associated with the collapse and revival of nearest-neighbor entanglement exhibited in the critical behavior. This behavior of quantum discord leads to the broader question of whether certain classes of measures of nonclassical correlation can help to understand the non-generic features of entanglement in a given system.
Fields generated in a large class of nonlinear optical processes, including those with losses, ha... more Fields generated in a large class of nonlinear optical processes, including those with losses, have a Wigner distribution that is Gaussian centered around the mean value of the field. Various classical and local inequalities that are violated by quantum fields, such as those generated in nonlinear processes, are expressed as inequalities relating to the parameters of the underlying Wigner distribution function.The possibility of distinguishing a quantum system from a classical one in terms of the distribution function parameters is discussed
In the scheme of a quantum nondemolition (QND) measurement, an observable is measured without per... more In the scheme of a quantum nondemolition (QND) measurement, an observable is measured without perturbing its evolution. In the context of studies of decoherence in quantum computing, we examine the `open' quantum system of a two-level atom, or equivalently, a spin-1/2 system, in interaction with quantum reservoirs of either oscillators or spins, under the QND condition of the Hamiltonian of the system commuting with the system-reservoir interaction. For completeness, we also examine the well-known non-QND spin-Bose problem. For all these many-body systems, we use the methods of functional integration to work out the propagators. The propagators for the QND Hamiltonians are shown to be analogous to the squeezing and rotation operators, respectively, for the two kinds of baths considered. Squeezing and rotation being both phase space area-preserving canonical transformations, this brings out an interesting connection between the energy-preserving QND Hamiltonians and the homogeneo...
A three-level atom in a Λ configuration trapped in an optical cavity forms a basic unit in a numb... more A three-level atom in a Λ configuration trapped in an optical cavity forms a basic unit in a number of proposed protocols for quantum information processing. Through control with an appropriate laser, this system allows for efficient storage of cavity photons into long-lived atomic excitations, and their retrieval with high fidelity. This process presumes an adiabatic transfer through the `dark state', a coherent superposition of the two lower levels of the Λ system, by a slow variation of the intensity of the control laser. We study the full quantum mechanics of this transfer process with a view to examine the nonadiabatic effects, as the control laser is varied in time. The nonadiabatic effects arise due to inevitable excitations of the system to states involving the highest level of the Λ configuration, which is radiative. We formulate the problem in terms of the instantaneous eigenstates and solve it numerically, allowing for spontaneous decays from the excited level. We pre...
This review is intended for readers who want to have a quick understanding on the theoretical und... more This review is intended for readers who want to have a quick understanding on the theoretical underpinnings of coherent states and squeezed states which are conventionally generated from the prototype harmonic oscillator but not always restricting to it. Noting that the treatments of building up such states have a long history, we collected the important ingredients and reproduced them from a fresh perspective but refrained from delving into detailed derivation of each topic. By no means we claim a comprehensive presentation of the subject but have only tried to re-capture some of the essential results and pointed out their inter-connectivity.
We analyze the phase-locking phenomena in a mutually delay-coupled diode laser system. The system... more We analyze the phase-locking phenomena in a mutually delay-coupled diode laser system. The system stability, within different dynamical regimes, is investigated as a function of the coupled-cavity delay time and the optical coupling strength. Different regimes of phase-correlated dynamics are discussed. The occurrence of amplitude quenching is shown experimentally as well as numerically. The visibility measure is used which unveils a signature of global phase locking stability within the window between amplitude-death islands.
A three-level atom in a $\Lambda$ configuration trapped in an optical cavity forms a basic unit i... more A three-level atom in a $\Lambda$ configuration trapped in an optical cavity forms a basic unit in a number of proposed protocols for quantum information processing. Through control with an appropriate laser, this system allows for efficient storage of cavity photons into long-lived atomic excitations, and their retrieval with high fidelity. This process presumes an adiabatic transfer through the `dark state', a coherent superposition of the two lower levels of the $\Lambda$ system, by a slow variation of the intensity of the control laser. We study the full quantum mechanics of this transfer process with a view to examine the nonadiabatic effects, as the control laser is varied in time. The nonadiabatic effects arise due to inevitable excitations of the system to states involving the highest level of the $\Lambda$ configuration, which is radiative. We formulate the problem in terms of the instantaneous eigenstates and solve it numerically, allowing for spontaneous decays from t...
Physical Review A, 2009
We present a realistic theoretical treatment of a three-level Λ system in a hot atomic vapor inte... more We present a realistic theoretical treatment of a three-level Λ system in a hot atomic vapor interacting with a coupling and a probe field of arbitrary strengths, leading to electromagneticallyinduced transparency and slow light under the two-photon resonance condition. We take into account all the relevant decoherence processes including collisions. Velocity-changing collisions (VCCs) are modeled in the strong collision limit effectively, which helps in achieving optical pumping by the coupling beam across the entire Doppler profile. The steady-state expressions for the atomic density-matrix elements are numerically evaluated to yield the experimentally measured response characteristics. The predictions, taking into account a dynamic rate of influx of atoms in the two lower levels of the Λ, are in excellent agreement with the reported experimental results for 4 He*. The role played by the VCC parameter is seen to be distinct from that by the transit time or Raman coherence decay rate.
physica status solidi (a), 2008
ABSTRACT 1 Introduction The electrical Hall effect, which has been observed by E. H. Hall in the ... more ABSTRACT 1 Introduction The electrical Hall effect, which has been observed by E. H. Hall in the late 70&#39;s of the 19th century, has become the standard technique for the electri-cal characterization of free charge carrier properties in semiconductors [1]. This technique, however, requires electrical contacts and known current paths, which ex-cludes application to complex heterostructures and nano-structured materials. For example, the true free charge car-rier properties can be obscured by space charge regions caused by the contact potentials of the electrical contacts. Furthermore, it is often difficult to determine the exact car-rier path within the sample such that the free charge carrier concentrations and mobilities determined for a complex layered sample cannot be assigned separately to the indi-vidual sample constituents.
EPL (Europhysics Letters), 2008
PACS 42.50.Gy-Effects of atomic coherence on propagation, absorption, and amplification of light;... more PACS 42.50.Gy-Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption PACS 42.25.Bs-Wave propagation, transmission and absorption PACS 42.50.Nn-Quantum optical phenomena in absorbing, amplifying, dispersive and conducting media Abstract-Electromagnetically induced transparency (EIT) is observed in gaseous 4 He at room temperature. Ultra-narrow (less than 10 kHz) EIT windows are obtained for the first time for purely electronic spins in the presence of Doppler broadening. The positive role of collisions is emphasized through measurements of the power dependence of the EIT resonance. The measurement of slow light opens up possible ways to applications.
We study a generic open quantum system where the coupling between the system and its environment ... more We study a generic open quantum system where the coupling between the system and its environment is of an energy-preserving quantum nondemolition (QND) type. We obtain the general master equation for the evolution of such a system under the influence of a squeezed thermal bath of harmonic oscillators. From the master equation it can be seen explicitly that the process involves decoherence or dephasing without any dissipation of energy. We work out the decoherence-causing term in the high and zero temperature limits and check that they match with known results for the case of a thermal bath. The decay of the coherence is quantified as well by the dynamics of the linear entropy of the system under various environmental conditions. We make a comparison of the quantum statistical properties between QND and dissipative types of evolution using a system of two-level atom and a harmonic oscillator.
arXiv: Quantum Physics, 2013
Electromagnetically induced transparency (EIT) in metastable helium at room temperature is experi... more Electromagnetically induced transparency (EIT) in metastable helium at room temperature is experimentally shown to exhibit remarkable light storage capabilities for intermediate values of the detuning between the coupling and probe beams and the center of the atomic Doppler profiles. For an optical depth lower than 3, an 11% efficiency after a $3\ \mu\mathrm{s}$ storage time has been achieved for a detuning measuring twice the Doppler linewidth. The associated storage lifetime has been measured to be equal to $11\ \mu\mathrm{s}$. Moreover, an additional phase shift is shown to be imposed to the retrieved pulse of light when the EIT protocol is performed at non-zero optical detunings. The origin and possible applications of this detuning are discussed.
arXiv: Quantum Physics, 2006
In the scheme of a quantum nondemolition (QND) measurement, an observable is measured without per... more In the scheme of a quantum nondemolition (QND) measurement, an observable is measured without perturbing its evolution. In the context of studies of decoherence in quantum computing, we examine the `open' quantum system of a two-level atom, or equivalently, a spin-1/2 system, in interaction with quantum reservoirs of either oscillators or spins, under the QND condition of the Hamiltonian of the system commuting with the system-reservoir interaction. The propagators for these QND Hamiltonians are shown to be connected to the squeezing and rotation operators for the two baths, respectively. Squeezing and rotation being both phase space area-preserving canonical transformations, this brings out an interesting analogy between the energy-preserving QND Hamiltonians and the homogeneous linear canonical transformations.
In a recent article, Ford, Lewis and O'Connell (PRA 64, 032101 (2001)) discuss a thought expe... more In a recent article, Ford, Lewis and O'Connell (PRA 64, 032101 (2001)) discuss a thought experiment in which a Brownian particle is subjected to a double-slit measurement. Analyzing the decay of the emerging interference pattern, they derive a decoherence rate that is much faster than previous results and even persists in the limit of vanishing dissipation. This result is based on the definition of a certain attenuation factor, which they analyze for short times. In this note, we point out that this attenuation factor captures the physics of decoherence only for times larger than a certain time t_mix, which is the time it takes until the two emerging wave packets begin to overlap. Therefore, the strategy of Ford et al of extracting the decoherence time from the regime t < t_mix is in our opinion not meaningful. If one analyzes the attenuation factor for t > t_mix, one recovers familiar behaviour for the decoherence time; in particular, no decoherence is seen in the absence...
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Papers by Rupamanjari Ghosh