arXiv subjects

Kunal K. Das

Publications and source records attributed to Kunal K. Das.

At least 37 records · Page 2Linked to original sources

arXiv2015

Spin squeezing by tensor twisting and Lipkin-Meshkov-Glick dynamics in a toroidal Bose-Einstein condensate with spatially modulated nonlinearity

We propose a scheme for spin-squeezing in the orbital motion of a Bose-Einstein condensate (BEC) in a toroidal trap. A circular lattice couples two counter-rotating modes and squeezing is generated by the nonlinear interaction spatially modulated at half the lattice period. By varying the amplitude and phase of the modulation, various cases of the twisting tensor can be directly realized, leading to different squeezing regimes. These include one-axis twisting and the two-axis counter-twisting which are often discussed as the most important paradigms for spin squeezing. Our scheme naturally realizes the Lipkin-Meshkov-Glick model with the freedom to vary all its parameters simultaneously.

cond-mat.quant-gas↗

arXiv2013

Boosting the Rotational Sensitivity of Matter-wave Interferometry with Nonlinearity

We propose a mechanism to use nonlinearity arising from inter-particle interactions to significantly enhance rotation sensitivity of matter-wave interferometers. The method relies on modifying Sagnac interferometers by introducing a weak circular lattice potential that couples modes with opposite orbital angular momenta (OAM). The primary observable comprises of the modal population distributions measured at particular times. This provides an alternate mechanism for rotation sensing that requires substantially smaller ring size, even in the linear non-interacting regime. Nonlinearity can improve the sensitivity, as well as operation timescales, by several orders of magnitude.

cond-mat.quant-gas↗

arXiv2012

An atomtronics transistor for quantum gates

We present a mechanism for quantum gates where the qubits are encoded in the population distribution of two component ultracold atoms trapped in a species-selective triple-well potential. The gate operation is a specific application of a new design for an atomtronics transistor where inter-species interaction is used to control transport, and can be realized with either individual atoms or aggregates like Bose-Einstein condensates (BEC). We demonstrate the operational principle with a static external potential, and show feasible implementation with a smooth dynamical potential.

cond-mat.quant-gas↗

arXiv2012

Scattering by an oscillating barrier: quantum, classical, and semiclassical comparison

We present a detailed study of scattering by an amplitude-modulated potential barrier using three distinct physical frameworks: quantum, classical, and semiclassical. Classical physics gives bounds on the energy and momentum of the scattered particle, while also providing the foundation for semiclassical theory. We use the semiclassical approach to selectively add quantum-mechanical effects such as interference and diffraction. We find good agreement between the quantum and semiclassical momentum distributions. Our methods and results can be used to understand quantum and classical aspects of transport mechanisms involving time-varying potentials, such as quantum pumping.

cond-mat.quant-gas↗

arXiv2011

Transparent, Non-local, Species-selective Transport in an Optical Superlattice Containing Two Interacting Atom Species

In an optical superlattice of triple wells, containing two mutually interacting atom species in adjacent wells, we show that one species can be transported through the positions of the other species, yet avoiding significant overlap and direct interaction. The transfer protocol is optimized to be robust against missing atoms of either species in any lattice site, as well as against lattice fluctuations. The degree and the duration of the inter-species overlap during passage can be tuned, making possible controlled large-scale interaction-induced change of internal states.

cond-mat.quant-gas↗

arXiv2009

Quantum Pumping with Ultracold Atoms on Microchips: Fermions versus Bosons

We present a design for simulating quantum pumping of electrons in a mesoscopic circuit with ultra-cold atoms in a micro-magnetic chip trap. We calculate theoretical results for quantum pumping of both bosons and fermions, identifying differences and common features, including geometric behavior and resonance transmission. We analyze the feasibility of experiments with bosonic $^{87}$Rb and fermionic $^{40}$K atoms with an emphasis on reliable atomic current measurements.

cond-mat.quant-gas↗

arXiv2008

Conditions for Vanishing Central-well Population in Triple-well Adiabatic Transport

Analytical expressions are derived for coherent tunneling via adiabatic passage (CTAP) in a triple well system with negligible central-well population at all times during the transfer. It is shown that a manipulation of the depths of the extreme-wells, correlated with the time variation of the \emph{non-adjacent} barriers is essential for maintaining vanishing population of the central well. The validity of our conditions are demonstrated with a numerical solution of the time-dependent Schrödinger equation. The transfer process is interpreted in terms of a current through the central well.

cond-mat.mes-hall↗

arXiv2008

What is Quantum in Quantum Pumping: The Role of Phase and Asymmetries

We show that quantum pumping does not always require a quantum description or a quantum phase. Quantum pumping is shown to encompass different types of processes, some of which intrinsically rely on phase while others do not. We also show that many pumping processes have a hidden antisymmetric component that contributes significantly to the instantaneous current at the terminals without causing net charge transfer in a period. We have also computed the exact pumped current for some cases over a full range of time variation from adiabatic to non-adiabatic.

cond-mat.mes-hall↗

arXiv2007

A Quantum Singlet Pump

We provide provide a detailed study of biasless coherent transport of singlet electron pairs in one-dimensional (1D) channels induced by electron-electron interactions that are time-varying in certain spatially localized regions of the channel. When the time variation is cyclic, the mechanism is analogous to the adiabatic quantum pumping of charge and spin previously studied. However, the presence of interactions that vary only in localized regions of space requires an intrinsically two-body description which is irreducible to the 1D single particle scattering matrix elements that are sufficient to describe quantum pumping of charge and spin. Here we derive a generalized theory for the pumping of such interacting pairs starting from first principles. We show that the standard description of charge pumping is contained within our more broadly applicable expressions. We then apply our general results to a concrete lattice model and obtain an exact analytical expression for the pumped singlet current. We further demonstrate that such a model can be implemented with a chain of currently available quantum dots with certain minor modifications that we suggest; we present a detailed numerical feasibility analysis of the characteristics of such experimentally realizable quantum dots, showing that the requirements for a measurable pumped singlet current are within experimental range.

cond-mat.mes-hall↗

arXiv2007

Adiabatic quantum pumping in an Aharonov-Bohm loop and in a Si-like nanowire: Role of interference in real space and in momentum space

We study the consequences of interference effects on the current generated by adiabatic quantum pumping in two distinct one-dimensional (1D) lattice model. The first model contains an Aharonov-Bohm (AB) loop within a tight-binding chain of lattice sites. The static AB phase is shown to strongly affect interference between the two arms of the loop, serving as an on-off switch and regulator for the pumped current. The second model simulates pumping in semiconductors with indirect band-gaps, by utilizing a tight-binding chain with next-nearest-neighbor coupling. The model exhibits signatures of interference between degenerate conduction band states with different Fermi wavevectors.

cond-mat.mes-hall↗

arXiv2007

Macroscopic consequences of calcium signaling in microdomains: A first passage time approach

Calcium (Ca) plays an important role in regulating various cellular processes. In a variety of cell types, Ca signaling occurs within microdomains where channels deliver localized pulses of Ca which activate a nearby collection of Ca-sensitive receptors. The small number of channels involved ensures that the signaling process is stochastic. The aggregate response of several thousand of these microdomains yields a whole-cell response which dictates the cell behavior. Here, we study analytically the statistical properties of a population of these microdomains in response to a trigger signal. We apply these results to understand the relationship between Ca influx and Ca release in cardiac cells. In this context, we use a first passage time approach to show analytically how Ca release in the whole cell depends on the single channel kinetics of Ca channels and the properties of microdomains. Using these results, we explain the underlying mechanism for the graded relationship between Ca influx and Ca release in cardiac cells.

q-bio.SC↗

arXiv2006

Adiabatic quantum pumping of a desired ratio of spin current to charge current

We present a prescription for generating pure spin current or spin selective current, based on adiabatic quantum pumping in a tight-binding model of a one dimensional conductor. A formula for the instantaneous pumped current is derived without introducing the scattering matrix. Our calculations indicate that some pumping cycles produce the maximum value 2 of pumped spin while others reverse the direction of current as a result of small alterations of the pumping cycle. We find pumping cycles which produce essentially any ratio of spin current to charge current.

cond-mat.mes-hall↗

arXiv2005

Radial Dependence of the Carrier Mobility in Semiconductor Nanowires

The mobility of charge carriers in a semiconductor nanowire is explored as a function of increasing radius, assuming low temperatures where impurity scattering dominates. The competition between increased cross-section and the concurrent increase in available scattering channels causes strongly non-monotonic dependence of the mobility on the radius. The inter-band scattering causes sharp declines in the mobility at the wire radii at which each new channel becomes available. At intermediate radii with the number of channels unchanged the mobility is seen to maintain an exponential growth even with multiple channels. We also compare the effects of changing the radial scaling of the impurity distribution. We use transverse carrier wavefunctions that are consistent with boundary conditions and demonstrate that the $δ$-function approximate transverse profile leads to errors in the case of remote impurities.

cond-mat.mtrl-sci↗

arXiv2005

Controlled Flow of Spin-Entangled Electrons via Adiabatic Quantum Pumping

We propose a method to dynamically generate and control the flow of spin-entangled electrons, each belonging to a spin-singlet, by means of adiabatic quantum pumping. The pumping cycle functions by periodic time variation of localized two-body interactions. We develop a generalized approach to adiabatic quantum pumping as traditional methods based on scattering matrix in one dimension cannot be applied here. We specifically compute the flow of spin-entangled electrons within a Hubbard-like model of quantum dots, and discuss possible implementations and identify parameters that can be used to control the singlet flow.

quant-ph↗

arXiv2004

A Langevin analysis of fundamental noise limits in Coherent Anti-Stokes Raman Spectroscopy

We use a Langevin approach to analyze the quantum noise in Coherent Anti-Stokes Raman Spectroscopy (CARS) in several experimental scenarios: with continuous wave input fields acting simultaneously and with fast sequential pulsed lasers where one field scatters off the coherence generated by other fields; and for interactions within a cavity and in free space. In all the cases, the signal as well as the quantum noise due to spontaneous decay and decoherence in the medium are shown to be described by the same general expression. Our theory in particular shows that for short interaction times, the medium noise is not important and the efficiency is limited only by the intrinsic quantum nature of the photon. We obtain fully analytic results \emph{without} making an adiabatic approximation, the fluctuations of the medium and the fields are self solved consistently.

cond-mat.stat-mech↗

arXiv2004

Thermodynamic and noise considerations for the detection of microscopic particles in a gas by Photoacoustic Raman spectroscopy

We develop a simple thermodynamic model to describe the heat transfer mechanisms and generation of acoustic waves in photoacoustic Raman spectroscopy by small particulate suspensions in a gas. Using Langevin methods to describe the thermal noise we study the signal and noise properties, and from the noise equivalent power we determine the minimum number density of the suspended particles that can be detected. We find that for some relevant cases, as few as 100 particles per cubic meter can be detected.

quant-ph↗

arXiv2002

Bose-Fermi Mixtures in One Dimension

We analyze the phase stability and the response of a mixture of bosons and spin-polarized fermions in one dimension (1D). Unlike in 3D, phase separation happens for low fermion densities. The dynamics of the mixture at low energy is independent of the spin-statistics of the components, and zero-sound-like modes exist that are essentially undamped.

cond-mat.soft↗

arXiv2002

Interference of a thermal Tonks gas on a ring

A nonzero temperature generalization of the Fermi-Bose mapping theorem is used to study the exact quantum statistical dynamics of a one-dimensional gas of impenetrable bosons on a ring. We investigate the interference produced when an initially trapped gas localized on one side of the ring is released, split via an optical-dipole grating, and recombined on the other side of the ring. Nonzero temperature is shown not to be a limitation to obtaining high visibility fringes.

cond-mat.soft↗