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Shubhrangshu Dasgupta

Publications and source records attributed to Shubhrangshu Dasgupta.

At least 37 records · Page 2Linked to original sources
arXiv2018

Generation of non-classical states of photons from metal-dielectric interface: a novel architecture for quantum information processing

We show that it is possible to generate photons in nonclassical states from a metal-dielectric interface using quantum emitters on the interface. The photons emitted into the surface plasmon mode from the initially excited emitters radiate out in free space in a cone-shaped geometry. When detected at two detectors, these photons exhibit anti-coalescence, a clear signature of nonclassicality. Such a system can also be employed as a building block for a distributed quantum network. We further show that it is indeed feasible to implement our model using available technology.

quant-ph↗
arXiv2018

Effect of 8th Chromophore: A Comprehensive Study of Excitation Energy Transfer in FMO complex

The excitation energy transfer (EET) in Fenna-Matthews-Olson (FMO) complex has been investigated in the presence of realistic inhomogeneous protein environment around different bacteriochlorophyll-a (BChla) sites of a 8-site monomer. We studied the effect of initial coherence and the 8th chromophore on the EET dynamics considering relative orientation of transition dipole moments of all the BChla sites. We choose femtosecond coherent laser pulse to prepare initial coherent superposition state. Our non-Markovian simulation reveals that the initial coherence between excited states of BChla sites accelerate the excitation energy transfer process as compared to initial mixed state. Even more impressively, in the presence of the 8th chromophore, the initial pure state relaxes much faster than in the 7-site monomer. Hence the inclusion of 8th chromophore additionally enhances the EET relaxation.

physics.bio-ph↗
arXiv2018

Influence of pigment-protein coupling on excitation energy transfer in FMO complex

To explain experimentally observed oscillatory dynamics of highly efficient process of excitation energy transfer (EET) in Fenna-Matthews-Olson (FMO) complex, most theoretical models assume the same local protein environment around all the bacteriochlorophyll-a (BChla) sites, contradictory to the structural analysis of FMO complex. Using different values of pigment-protein couplings for different BChla sites, measured in the adiabatic limit of electron transfer, we theoretically investigate the effect of inhomogeneous local protein environment on excitation energy transfer. By employing non-Markovian master equation we demonstrate that the asymmetric system-bath coupling leads to the results consistent with the experimental observations. Quantum dynamical simulation suggests that the correlated fuctuations preserve the oscillation of excitation for long time-scales. Further different BChla sites have asymmetric time-scales of oscillations of excitation due to in-homogeneous pigment-protein couplings also.

physics.bio-ph↗
arXiv2018

Role of Coherence in Excitation Transfer Efficiency to the Reaction Center in Photosynthetic Bacteria Chlorobium Tepidum

We investigate the effect of coherence to the excitation transfer efficiency (ETE) in photosynthetic bacteria Chlorobium Tepidum. We have modeled the monomer of Fenna-Matthews-Olson (FMO) complex as consisting of eight bacteriochlorophyll-a sites, while explicitly consider reaction center core complex (RCC) as an additional site. With the use of realistic bath spectrum and several dominant vibronic modes in the non-Markovian master equation, in an effective 9-site model, we have compared the ETE for an initial pure state and an initial mixed state. We observe that the initial pure state relaxes efficiently to increase the trapping at the RCC. We further illustrate that the coherence play a competitive role to block the back transfer of excitation from RCC pigment to FMO complex and hence to maximize the ETE.

physics.bio-ph↗
arXiv2017

Role of Initial Coherence in Excitation Energy Transfer in Fenna-Matthews-Olson Complex

We theoretically show that the initial coherence plays a crucial role in enhancing the speed of excitation energy transfer (EET) in Fenna-Matthews-Olson (FMO) complex. We choose a simplistic eight-level model considering all the bacateriochlorophyll-a sites in a monomer of FMO complex. We make a comparative numerical study of the EET, in terms of non-Markovian evolution of an initial coherent superposition state and a mixed state. A femto-second coherent laser pulse is suitably chosen to create the initial coherent superposition state. Such an initial state relaxes much faster than a mixed state thereby speeding up the EET. In this analysis, we have taken into account the relative orientation of the transition dipole moments of the bacateriochlorophyll-a sites and their relative excitation energies. Our results reveal that for 2D electronic spectroscopy experiments, the existing two-pathway model of energy transfer in FMO complex may not be suitable in our understanding of EET.

physics.bio-ph↗
arXiv2017

Coherence and its Role in Excitation Energy Transfer in Fenna-Mathews-Olson Complex

We show that the coherence between different bacteriochlorophyll-a (BChla) sites in the Fenna-Mathews-Olson complex is an essential ingredient for excitation energy transfer between various sites. The coherence delocalizes the excitation energy, which results in the redistribution of excitation among all the BChla sites in the steady state. We further show that the system remains partially coherent at the steady state. In our numerical simulation of the non-Markovian density matrix equation, we consider both the inhomogeneity of the protein environment and the effect of active vibronic modes.

physics.chem-ph↗
arXiv2017

Population trapping in the excited states using vacuum-induced coherence and adiabatic process

We theoretically investigate how population can be trapped in the excited doublet in presence of vacuum-induced coherence (VIC). We employ delayed pulses to transfer population from a metastable state to the excited states. Subsequently, spontaneous emission from the excited doublet builds coherence between them. This coherence can be probed by using chirping, which leads to the decoupling of the excited doublet from the ground state thereby ensuring population transfer via delayed pulses. Our results indicate that the existence of VIC leads to the generation of a mixed state in the excited state manifold, where trapping of the population occurs even in the presence of large decay. This trapping may be realized in molecular systems and can be understood as a sensitive probe of VIC. We present suitable numerical analysis to support our results.

quant-ph↗
arXiv2016

Light deflection by light: Effect of incidence angle and inhomogeneity

We study the angular deflection of the circular polarized components of a linearly polarized probe field in a weakly birefringent atomic system in tripod configuration. A spatially inhomogeneous control field incident obliquely onto an atomic vapor cell facilitates a large angular divergence between circular components. We show that the angular resolution can be dynamically controlled by optimally choosing the angle of incidence and the transverse profile of the control beam. For instance, by employing a Laguerre-Gaussian profile of the control field, one can impart a large angular divergence to the circular components close to the entry face of the atomic vapor cell. We further demonstrate how such a medium causes the focusing and refocusing of the probe field, thereby acting as a lens with multiple foci. The absorption in the medium remains negligible at resonance due to electromagnetically induced transparency (EIT).

quant-ph↗
arXiv2016

Optical switching and bistability in a four-level atomic systems

We explore the coherent control of nonlinear absorption of intense laser fields in four-level atomic systems. For instance, in a four-level ladder system, a coupling field creates electromagnetically induced transparency (EIT) with Aulter-Townes doublet for the probe field while the control field is absent. A large absorption peak appears at resonance as the control field is switched on. We show how such a large absorption leads to optical switching. Further, this large absorption gets diminished and a transparency window appears due to the saturation effects as the strength of the probe field is increased. We further demonstrate that the threshold of the optical bistability can be modified by suitable choices of the coupling and the control fields. In a four-level Y-type configuration, the effect of the control field on saturable absorption (SA) and reverse saturable absorption (RSA) is highlighted in the context of nonlinear absorption of the probe field. We achieve RSA and SA in a simple atomic system just by applying a control field.

quant-ph↗
arXiv2016

Probing vacuum-induced coherence via magneto-optical rotation in molecular systems

We investigate theoretically the effects of vacuum-induced coherence (VIC) on magneto-optical rotation (MOR). We carry out a model study to show that VIC in the presence of a control laser and a magnetic field can lead to large enhancement in the rotation of the plane of polarization of a linearly polarized weak laser with vanishing circular dichroism. This effect can be realized in cold molecular gases and may be used as a sensitive probe for VIC. Such a large MOR angle can also be used to detect weak magnetic field with large measurement sensitivity.

quant-ph↗
arXiv2015

Estimation of temporal separation of slow light pulses in atomic vapors by weak measurement

We show how two circular polarization components of a linearly polarized pulse, propagating through a coherently driven dilute atomic vapor, can be well resolved in time domain by weak measurement. Slower group velocity of one of the components due to electromagnetically induced transparency leads to a differential group delay between the two components. For low number density, this delay may not be large enough to temporally resolve the two components. We show how this can be enhanced in terms of mean time of arrival of the output pulse through a post-selected polarizer. We demonstrate the idea with all the analytical and numerical results, with a specific example of alkali atoms.

quant-ph↗
arXiv2014

Sharply tunable group velocity in alkali vapors using a single low-power control field

We show how a single linearly polarized control field can produce a sharply tunable group velocity of a weak probe field at resonance in a four-level atomic configuration of alkali vapors. The dispersion can be switched from normal to anomalous along with vanishing absorption, just by changing intensity of the resonant control field. In addition, by allowing different intensities of the different polarization components of the control field, the anomalous dispersion can be switched back to the normal. This thereby creates a "valley of anomaly" in group index variation and offers two sets of control field intensities, for which the system behaves like a vacuum. The explicit analytical expressions for the probe coherence are provided along with all physical explanations. We demonstrate our results in $J = 1/2 \leftrightarrow J = 1/2$ transition for D_1 lines in alkali atoms, in which one can obtain a group index as large as $3.2\times10^{8}$ and as negative as $-1.5\times10^{5}$ using a control field with power as low as 0.017 mW/cm$^2$ and 9.56 mW/cm$^2$ .

quant-ph↗
arXiv2006

Decoherence-induced geometric phase in a multilevel atomic system

We consider the STIRAP process in a three-level atom. Viewed as a closed system, no geometric phase is acquired. But in the presence of spontaneous emission and/or collisional relaxation we show numerically that a non-vanishing, purely real, geometric phase is acquired during STIRAP, whose magnitude grows with the decay rates. Rather than viewing this decoherence-induced geometric phase as a nuisance, it can be considered an example of "beneficial decoherence": the environment provides a mechanism for the generation of geometric phases which would otherwise require an extra experimental control knob.

quant-ph↗
arXiv2006

Two-bit Deutsch-Jozsa algorithm using an atomic ensemble

The first optical proposal for the realization of the two-bit version of the Deutsch-Jozsa algorithm [D. Deutsch and R. Jozsa, Proc. R. Soc. London A {\bf 493}, 553 (1992)] is presented. The proposal uses Stark shifts in an ensemble of atoms and degenerate sources of photons. The photons interact dispersively with an atomic ensemble, leading to an effective Hamiltonian in atom-field basis, which is useful for performing the required two-qubit operations. Combining these with a set of one-qubit operations, the algorithm can be implemented. A discussion of the experimental feasibility of the proposal is given.

quant-ph↗
arXiv2005

Quantum interferences from cross talk in $J=1/2\leftrightarrow J=1/2$ transitions

We consider the possibility of a control field opening up multiple pathways and thereby leading to new interference and coherence effects. We illustrate the idea by considering the $J=1/2\leftrightarrow J=1/2$ transition. As a result of the additional pathways, we show the possibilities of nonzero refractive index without absorption and gain without inversion. We explain these results in terms of the coherence produced by the opening of an extra pathway.

quant-ph↗
arXiv2003

Magneto-optical rotation of spectrally impure fields and its nonlinear dependence on optical density

We calculate magneto-optical rptation of spectrally impure fileds in an optically thick cold atmic medium. We show that the spectral impurity leads to non-linear dependence of the rotation angle on optical density. Using our calculations, we provide a quanttative analysis of the recent experimental results of G. Labeyrie et al. [Phys. Rev. A 64, 033402 (2001)] using cold Rb$^{85}$ atoms.

physics.optics↗