Sumanta Das

Decoherence effects in interacting qubits under the influence of various environments

We study competition between the dissipative and coherent effects in the entanglement dynamics of two qubits. The coherent interactions are needed for designing logic gate operations with systems such as ion traps, semiconductor quantum dots and atoms. We show that the interactions lead to a phenomenon of periodic disentanglement and entanglement between the qubits. The disentanglement is primarily caused by environmental perturbations. The qubits are seen to remain disentangled for a finite time before getting entangled again. We find that the phenomenon is generic and occurs for a wide variety of models of the environment. We present analytical results for the time dependence of concurrence for all the models. The periodic disentanglement and entanglement behaviour is seen to be a precursor to the sudden death of entanglement (ESD) and can happen for environments which do not show ESD for non-interacting qubits. Further we also find that this phenomenon can even lead to delayed death of entanglement for correlated environments.

Quantum Interferences in Cooperative Dicke Emission from Spatial Variation of the Laser Phase

We report the generation of a new quantum interference effect in spontaneous emission from a resonantly driven system of two identical two-level atoms due to the spatial variation of the laser phase at the positions of the atoms. This interference affects significantly the spectral features of the emitted radiation and the quantum entanglement in the system. The interference leads to dynamic coupling of the populations and coherences in a basis, determined by the laser phase and represents a kind of vacuum mediated superexchange between the symmetric and antisymmetric states.

Bright and dark periods in the entanglement dynamics of interacting qubits in contact with the environment

Interaction among qubits is the basis to many quantum logic operations. We report how such inter-qubit interactions can lead to new features, in the form of bright and dark periods in the entanglement dynamics of two qubits subject to environmental perturbations. These features are seen to be precursors to the well-known phenomenon of sudden death of entanglement (Yu and Eberly 2004 Phys. Rev. Lett. 93 140404 ) for noninteracting qubits. Further, we find that the generation of bright and dark periods is generic and occurs for wide varieties of environment models. We present explicit results for two popular models.

Protecting bipartite entanglement by quantum interferences

We show that vacuum-induced coherence in three-level atomic systems can lead to preservation of bipartite entanglement when two such atoms are prepared as two initially entangled qubits, each independently interacting with their respective vacuum reservoirs. We explicitly calculate the time evolution of concurrence for two different Bell states and show that a large amount of entanglement can survive in the long time limit. The amount of entanglement left between the two qubits depends strongly on the ratio of the nonorthogonal transitions in each qubit and can be more than 50%. Moreover, we find that as a consequence of vacuum-induced coherence, sudden death of entanglement is prevented for an initial mixed entangled state of the qubits.

Quantum entanglement in coupled lossy waveguides

We investigate the viability of coupled waveguides as basic units of quantum circuits. We study entanglement when the waveguides are fed in by light produced by a down-converter working either in low gain limit or under large gain. We present explicit analytical results for the measure of entanglement in terms of the logarithmic negativity for a variety of input states. We also address the effect of loss on entanglement dynamics of waveguide modes. Our results indicate that the waveguide structures are reasonably robust against the effect of loss and thus quite appropriate for quantum architectures as well as for the study of coherent phenomena like random walks. Our analysis is based on realistic structures used currently.

Photonic controlled-phase gates through Rydberg blockade in optical cavities

We propose a novel scheme for high fidelity photonic controlled phase gates using Rydberg blockade in an ensemble of atoms in an optical cavity. The gate operation is obtained by first storing a photonic pulse in the ensemble and then scattering a second pulse from the cavity, resulting in a phase change depending on whether the first pulse contained a single photon. We show that the combination of Rydberg blockade and optical cavities effectively enhances the optical non-linearity created by the strong Rydberg interaction and thereby reduces the requirements for photonic quantum gates. The resulting gate can be implemented with cavities of moderate finesse which allows for highly efficient processing of quantum information encoded in photons. As a particular example of this, we show how the gate can be employed to increase the communication rate of quantum repeaters based on atomic ensembles.

Quantum-interference-controlled resonance profiles from lasing without inversion to photodetection

In this work we report a quantum interference mediated control of the resonance profiles in a generic three-level system and investigate its effect on key quantum interference (QI) phenomena. Namely in a three-level configuration with doublets in the ground or excited states, we show control over enhancement and suppression of the emission (absorption) profiles. This is achieved by manipulation of the strength of QI and the energy spacing of the doublets. We analyze the application of such QI-induced control of the resonance profile in the framework of two limiting cases of lasing without inversion and photodetection.

Photon-Photon Correlations as a Probe of Vacuum Induced Coherence Effects

We show how the effects of vacuum-induced coherence can be realized by studying photon-photon correlation in the

Quantum interference in timed Dicke basis and its effect on bipartite entanglement

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Collective quantum dot inversion and amplification of photon and phonon waves

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