arXiv subjects

Saikat Ghosh

Publications and source records attributed to Saikat Ghosh.

30 records · Page 2Linked to original sources

arXiv2016

Nonlinear optical conductivity of a generic two band systems, with application to doped and gapped graphene

We present a general formulation to calculate the dynamic optical conductivity, beyond the linear response regime, of any electronic system whose quasiparticle dispersion is described by a two band model. Our phenomenological model is based on the optical Bloch equations. In the steady state regime it yields an analytic solution for the population inversion and the interband coherence, which are nonlinear in the optical field intensity, including finite doping and temperature effects. We explicitly show that the optical nonlinearities are controlled by a single dimensionless parameter which is directly proportional to the incident field strength and inversely proportional to the optical frequency. This identification leads to a unified way to study the dynamical conductivity and the differential transmission spectrum across a wide range of optical frequencies, and optical field strength. We use our formalism to analytically calculate the nonlinear optical conductivity of doped and gapped graphene, deriving the well known universal ac conductivity of $σ_0={e^2}/4\hbar$ in the linear response regime of low optical intensities (or equivalently high frequencies) and non-linear deviations from it which appear at high laser intensities (or low frequencies) including the impact of finite doping and band-gap opening.

cond-mat.mes-hall↗

arXiv2016

Interplay of classical and quantum dynamics in a thermal ensemble of atoms

In a thermal ensemble of atoms driven by coherent fields, how does evolution of quantum superposition compete with classical dynamics of optical pumping and atomic diffusion? Is it optical pumping that first prepares a thermal ensemble, with coherent superposition developing subsequently or is it the other way round: coherently superposed atoms driven to steady state via optical pumping? Using a stroboscopic probing technique, here we experimentally explore these questions. A 100 ns pulse is used to probe an experimentally simulated, closed three-level, lambda-like configuration in rubidium atoms, driven by strong coherent control and incoherent fields. Temporal evolution of probe transmission shows an initial overshoot with turn-on of control, resulting in a scenario akin to lasing without inversion (LWI). The corresponding rise time is dictated by coherent dynamics, with a distinct experimental signature of half-cycle Rabi flop in a thermal ensemble of atoms. Our results indicate that, in fact, optical pumping drives the atoms to a steady state in a significantly longer time-scale that sustains superposed dark states. Eventual control turn-off leads to a sudden fall in transmission with an ubiquitous signature for identifying closed and open systems. Numerical simulations and toy-model predictions confirm our claims. These studies reveal new insights into a rich and complex dynamics associated with atoms in thermal ensemble, which are otherwise absent in state-prepared, cold atomic ensembles.

physics.atom-ph↗

arXiv2015

Quantum enhanced precision in a collective measurement

We explore the role of $\textit{collective measurements}$ on precision in estimation of a single parameter. Collective measurements are represented by observables which commute with all permutations of the probe particles. We show that with this constraint, quantum bits(qubits) outperform classical bits(non-superposable bits) in optimizing precision. Specifically, we prove that while precision in a collective measurement is loosely bounded by $O\left(\frac{1}{N}\right)$ for $N$ classical bits, using qubits it is tightly bounded by $O\left(\frac{1}{N^2}\right)$. This bound is consistent with quantum metrology protocols with the collective measurement requiring an entangled probe state to saturate. Finally, we construct a canonical measurement protocol that saturates this bound.

quant-ph↗

arXiv2015

A controllable single photon beam-splitter as a node of a quantum network

A model for a controlled single-photon beam-splitter is proposed and analysed. It consists of two crossed optical-cavities with overlapping waists, dynamically coupled to a single flying atom. The system is shown to route a single photon with near-unity efficiency in an effective "weak-coupling" regime. Furthermore, two such nodes, forming a segment of a quantum network, are shown to perform several controlled quantum operations. All one-qubit operations involve a transfer of a photon from one cavity to another in a single node, while two-qubit operations involve transfer from one node to a next one, coupled via an optical fiber. Novel timing protocols for classical optical fields are found to simplify possible experimental realizations along with achievable effective parameter regime. Though our analysis here is restricted to a cavity-QED scenario, basic features of the model can be extended to various other physical systems including gated quantum dots, circuit-QED or opto-mechanical elements.

quant-ph↗

arXiv2014

A model independent approach towards resource count and precision limits in a general measurement

A formulation towards quantifying resource count used in a measurement, that is independent of the model of the measurement dynamics(Quantum/Classical), is considered. For any general measurement with $(M+1)$ discrete outcomes, it is found that there is a unique probability distribution that minimizes the measurement error, with the error scaling as $1/M$. For a measurement with a finite resource$(R)$, this absolute bound implies the resource count to be equal to the possible outcomes i.e. $R=M$. This formulation therefore provides a model independent route towards estimating resource count used in any general measurement scheme.

quant-ph↗

arXiv2014

Decay dynamics in a strongly driven atom-molecule coupled system

Within the framework of master equation, we study decay dynamics of an atom-molecule system strongly coupled by two photoassociation lasers. Summing over the infinite number of electromagnetic vacuum modes that are coupled to the laser-dressed atom-molecule system, we obtain an integro-differential master equation for the the system's reduced density matrix. The equation is numerically solved to describe system dynamics in the presence of decay. In particular, we discuss correlated spontaneous emission from a pair of electronically excited diatomic ro-vibrational states due to their laser induced coupling to the ground continuum of atomic scattering states. This allows us to calculate time-dependence of emitted radiation intensity. It exhibits quantum beats due to coherent dynamics. The phase difference between the two driving fields is found to significantly affect the decay dynamics and the beats. Our results demonstrate the possibility to control decay from the molecular excited states and the decoherence between them by changing the relative intensity and the phase between the lasers. We further show that, if the ground-state continuum has a shape resonance at a low energy, then the quantum beats show two distinctive time scales of oscillations in the strong coupling regime. One of the time scales originates from the energy gap between the two excited states while the other time scale corresponds to the collision energy at which free-bound Franck-Condon overlap is resonantly peaked due to the shape resonance.

physics.atom-ph↗

arXiv2013

Information and the foundations of quantum theory

We believe that the hypothesis `it from bit' originates from the assumption that probabilities have a fundamental, irremovable status in quantum theory. We argue against this assumption and highlight four well-known reformulations / modifications of the theory in which probabilities and the measuring apparatus do not play a fundamental role. These are: Bohmian Mechanics, Dynamical Collapse Models, Trace Dynamics, and Quantum Theory without Classical Time. Here the `it' is primary and the `bit' is derived from the `it'.

quant-ph↗

arXiv2008

Heralded single-magnon quantum memory for photon polarization states

We demonstrate a heralded quantum memory based on mapping of a photon polarization state onto a single collective-spin excitation (magnon) shared between two spatially overlapped atomic ensembles. The polarization fidelity is measured by quantum state tomography to be above 90(2)% for any input polarization, which far exceeds the classical limit of 2/3. The process also constitutes a quantum non-destructive probe that detects and regenerates a photon without touching its - potentially undetermined - polarization.

quant-ph↗

arXiv2008

Simplified measurement of the Bell parameter within quantum mechanics

We point out that, if one accepts the validity of quantum mechanics, the Bell parameter for the polarization state of two photons can be measured in a simpler way than by the standard procedure [Clauser, Horne, Shimony, and Holt, Phys. Rev. Lett. 23, 880 (1969)]. The proposed method requires only two measurements with parallel linear-polarizer settings for Alice and Bob at 0 and 45 degrees, and yields a significantly smaller statistical error for a large Bell parameter.

quant-ph↗

arXiv2006

Self-Focusing Dynamics of Coupled Optical Beams

We theoretically and experimentally investigate the mutual collapse dynamics of two spatially separated optical beams in a Kerr medium. Depending on the initial power, beam separation, and the relative phase, we observe repulsion or attraction, which in the latter case reveals a sharp transition to a single collapsing beam. This transition to fusion of the beams is accompanied by an increase in the collapse distance, indicating the effect of the nonlinear coupling on the individual collapse dynamics. Our results shed light on the basic nonlinear interaction between self-focused beams and provide a mechanism to control the collapse dynamics of such beams.

physics.optics↗

arXiv2006

Low-Light-Level Optical Interactions with Rubidium Vapor in a Photonic Bandgap Fiber

We show that a Rubidium vapor can be produced within the core of a photonic band-gap fiber yielding an optical depth in excess of 2000. Our technique for producing the vapor is based on coating the inner walls of the fiber core with an organosilane and using light-induced atomic desorption to release Rb atoms into the core. We develop a model to describe the dynamics of the atomic density, and as an initial demonstration of the potential of this system for supporting ultra-low-level nonlinear optical interactions, we perform electromagnetically-induced transparency with control-field powers in the nanowatt regime, which represents more than a 1000-fold reduction from the power required for bulk, focused geometries.

physics.optics↗

arXiv2004

Coherent resonant interactions and slow light with molecules confined in photonic band-gap fibers

We investigate resonant nonlinear optical interactions and demonstrate induced transparency in acetylene molecules in a hollow-core photonic band-gap fiber at 1.5$μ$m. The induced spectral transmission window is used to demonstrate slow-light effects, and we show that the observed broadening of the spectral features is due to collisions of the molecules with the inner walls of the fiber core. Our results illustrate that such fibers can be used to facilitate strong coherent light-matter interactions even when the optical response of the individual molecules is weak.

physics.optics↗