When an atomic/molecular system controlled by an external field (to imprint information on it, such as preparing a qubit), is brought near a plasmonic nanostructure, the control problem changes. On one hand, the field experienced by the atom is modified due to the plasmon effects of the nanostructure. On the other hand, due to atom-nanostructure interactions the quantum state spontaneous emission is increased. Purification of states is fundamental to the implementation of quantum technologies. My approach for state purification is based on the interplay of control fields and spontaneous emission from a two-level quantum system in presence of a gold nanoparticle (GNP). For a specific location of the system around the GNP, I can find a control field that increases the purity of the system more than without the GNP. Unlike previous efforts of state purification that involve cooling to the ground state, here the excited state population remains significant.
When an atomic/molecular system controlled by an external field (to imprint information on it, such as preparing a qubit), is brought near a plasmonic nanostructure, the control problem changes. On one hand, the field experienced by the atom is modified due to the plasmon effects of the nanostructure. On the other hand, due to atom-nanostructure interactions the quantum state spontaneous emission is increased. Purification of states is fundamental to the implementation of quantum technologies. My approach for state purification is based on the interplay of control fields and spontaneous emission from a two-level quantum system in presence of a gold nanoparticle (GNP). For a specific location of the system around the GNP, I can find a control field that increases the purity of the system more than without the GNP. Unlike previous efforts of state purification that involve cooling to the ground state, here the excited state population remains significant.
The ability to prepare desired states without allowing time for the atomic system to spontaneously decay to the ground state is limited by the fact that Hamiltonian controls cannot affect the purity of the state in question. In this paper, we discuss how a continuous off-resonant electromagnetic wave can be used to prepare a desired final quantum state. With the aid of plasmonic surface-enhancement, by placing a two-level atomic system near a gold nanoparticle, preparation is accomplished in a reduced period of time and with lower electric field intensity while the system purity is improved when compared to an isolated system.
In the field of quantum information and quantum computing, entanglement plays an essential role. Entanglement preservation is an important issue as realistic quantum systems are affected by decoherence and entanglement losses due to the interaction with their environment. For example, in spite of an exponential decay of a single qubit, the entanglement between two qubits may completely disappear at a finite time; a phenomenon known as “entanglement sudden death” [1]. Recently, interest has been given in cases that qubits can be strongly coupled to plasmonic nanostructures, like, for example, an one-dimensional plasmonic waveguide [2] or a two-dimensional lattice of metal-coated dielectric nanoparticles [3]. In such systems the strong interaction with the surface plasmons leads to significant entanglement between the two qubits. Here, we consider the interaction of two initially entangled qubits interacting individually with a two-dimensional lattice of metal-coated dielectric nanoparticles. We consider two cases for the qubits, a pair of regular two-level systems and a pair of V-type systems where one transition is the qubit and the other level acts as an umbrella level [4]. We consider the entanglement dynamics for different initial conditions of the qubits. The specific plasmonic nanostructure leads to strongly modified spontaneous emission rates of individual quantum systems (strong suppression in certain cases) and, in addition, to strongly anisotropic Purcell effect for orthogonal dipoles, that in turn can be used for simulating quantum interference in spontaneous emission [5]. We use these effects for significantly prolonging entanglement dynamics near the plasmonic nanostructure in both cases, in comparison to the cases that the qubits are in free space. 1. T. Yu and J. H. Eberly, Phys. Rev. Lett. 93, 140404 (2004). 2. A. Gonzalez-Tudela, D. Martin-Cano, E. Moreno, L. Martin-Moreno, C. Tejedor, and F. J. Garcia-Vidal, Phys. Rev. Lett. 106, 020501 (2011). 3. N. Iliopoulos, A. F. Terzis, V. Yannopapas, and E. Paspalakis, Ann. Phys. 365, 38 (2016). 4. S. Das and G. S. Agarwal, Phys. Rev. A 81, 052341 (2010). 5. V. Yannopapas, E. Paspalakis, and N. V. Vitanov, Phys. Rev. Lett. 103, 063602 (2009).
Controlling the dynamics of open quantum systems; i.e. quantum systems that decohere because of interactions with the environment, is an active area of research with many applications in quantum optics and quantum computation. My thesis expands the scope of this inquiry by seeking to control open systems in proximity to an additional system. The latter could be a classical system such as metal nanoparticles, or a quantum system such as a cluster of similar atoms. By modelling the interactions between the systems, we are able to expand the accessible state space of the quantum system in question.For a single, three-level quantum system, I examine isolated systems that have only normal spontaneous emission. I then show that intensity-intensity correlation spectra, which depend directly on the density matrix of the system, can be used detect whether transitions share a common energy level. This detection is possible due to the presence of quantum interference effects between two transitions if they are connected. This effect allows one to asses energy level structure diagrams in complex atoms/molecules. By placing an open quantum system near a nanoparticle dimer, I show that the spontaneous emission rate of the system can be changed ``on demand" by changing the polarization of an incident, driving field. In a three-level, Λ system, this allows a qubit to both retain high qubit fidelity when it is operating, and to be rapidly initialized to a pure state once it is rendered unusable by decoherence. This type of behaviour is not possible in a single open quantum system; therefore adding a classical system nearby extends the overall control space of the quantum system. An open quantum system near identical neighbours in a dense ensemble is another example of how the accessible state space can be expanded. I show that a dense ensemble of atoms rapidly becomes disordered with states that are not directly excited by an incident field becoming significantly populated. This effect motivates the need for using multi-directional basis sets in theoretical analysis of dense quantum systems. My results demonstrate the shortcomings of short-pulse techniques used in many recent studies. Based on my numerical studies, I hypothesize that the dense ensemble can be modelled by an effective single quantum system that has a decoherence rate that changes over time. My effective single particle model provides a way in which computational time can be reduced, and also a model in which the underlying physical processes involved in the system's evolution are much easier to understand. I then use this model to provide an elegant theoretical explanation for an unusual experimental result called ``transverse optical magnetism''. My effective single particle model's predictions match very well with experimental data.
We study the dynamics of two initially entangled qubits, where each one interacts locally and independently of the other, with a plasmonic nanostructure. By considering two different cases for the qubits, two identical two-level systems and two identical V-type quantum systems, where one two-level transition plays the role of the qubit while the third level acts as an ``umbrella level'', we study the corresponding entanglement dynamics for several pure and mixed initial states. As the plasmonic nanostructure we take a two-dimensional lattice of metal-coated dielectric nanoparticles. The presence of this nanostructure leads to highly suppressed spontaneous emission rates of the individual quantum systems, as well as to highly anisotropic spontaneous decay rates for orthogonal dipole matrix elements due to the anisotropic Purcell effect, leading to quantum interference in spontaneous emission. Both of the effects can be used for significantly prolonging the time evolution of entanglement for several system parameters.
A five-level atomic system is proposed in vicinity of a two-dimensional (2D) plasmonic nanostructure with application in atom-photon entanglement. The behavior of the atom-photon entanglement is discussed with and without a control laser field. The amount of atom-photon entanglement is controlled by the quantum interference created by the plasmonic nanostructure. Thus, the degree of atom-photon entanglement is affected by the atomic distance from the plasmonic nanostructure. In the presence of a control field, maximum entanglement between the atom and its spontaneous emission field is observed.
This review is devoted to the study of effects of quantum optics in nanostructures. The mechanisms by which the rates of radiative and nonradiative decay are modified are considered in the model of a two-level quantum emitter (QE) near a plasmonic nanoparticle (NP). The distributions of the intensity and polarization of the near field around an NP are analyzed, which substantially depend on the polarization of the external field and parameters of plasmon resonances of the NP. The effects of quantum optics in the system NP + QE plus external laser field are analyzed-modification of the resonance fluorescence spectrum of a QE in the near field, bunching/antibunching phenomena, quantum statistics of photons in the spectrum, formation of squeezed states of light, and quantum entangled states in these systems.