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Controle coerente da dinâmica de excitons qubit em cavidades ressonantes

Antonio Freitas Neto

Published Aug 24, 2017• 0 citations
Publisher

Abstract

In this work, we demonstrate theoretically how to use a Gaussian-shaped pulse to control the population inversion in a system composed of a quantum dot treated as a two-level system inserted in resonant nanocavity. For this, we use the Jaynes-Cummings model to describe the system, however, dissipative processes should not be neglected, which were included through the operators of Lindbla. In this sense, we demonstrate how to prepare the system to control population inversion, considering as initial state the superposition of the exciton in the ground state with the cavity in the coherent state. So, we analyze the effects of the dephasing between the cavity and the exciton frequency, as well as, the action of a continuous laser acting exclusively in the cavity generating a protection to the dissipative processes related to the imperfect surface of the cavity. We analyzed the process of exchanging information between two quantum dots coupled through Foster using coherent pulse exclusively at one of the dot and verified the information exchange by means of the inversion of population in the quantum dot unpumped.

DOI
10.14393/ufu.te.2017.170

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In this work, we demonstrate theoretically how to use a Gaussian-shaped pulse to control the population inversion in a system composed of a quantum dot treated as a two-level system inserted in resonant nanocavity. For this, we use the Jaynes-Cummings model to describe the system, however, dissipative processes should not be neglected, which were included through the operators of Lindbla. In this sense, we demonstrate how to prepare the system to control population inversion, considering as initial state the superposition of the exciton in the ground state with the cavity in the coherent state. So, we analyze the effects of the dephasing between the cavity and the exciton frequency, as well as, the action of a continuous laser acting exclusively in the cavity generating a protection to the dissipative processes related to the imperfect surface of the cavity. We analyzed the process of exchanging information between two quantum dots coupled through Foster using coherent pulse exclusively at one of the dot and verified the information exchange by means of the inversion of population in the quantum dot unpumped.
In this paper we demonstrate theoretically how to use an external laser field to control the population inversion of a single quantum dot exciton qubit in a nanocavity. We consider the Jaynes-Cummings model to describe the system, and the incoherent losses were taken into account by using Lindblad operators. We have demonstrated how to prepare the initial state in a superposition of the exciton in the ground state and the cavity in a coherent state. The effects of exciton-cavity detuning, the laser-cavity detunings, the pulse area, and losses over the qubit dynamics are analyzed. We also show how to use a continuous laser pumping in resonance with the cavity mode to sustain a coherent state inside the cavity, providing some protection to the qubit against cavity loss.
We report on coherent control of the resonantly driven exciton state in a single quantum dot in the strong coupling regime. With picosecond laser pulses, the two-level system undergoes Rabi oscillations and a given coherent superposition of states can be addressed. Using a pair of phase-locked pulses, optical manipulation is achieved during the coherence time of the qu-bit. The dephasing processes are also studied and we show that in typical quantum dots, the coherence time T 2 can be of the same order of magnitude as the effective lifetime of the exciton T 1 (a few hundreds of ps) although not reaching the upper theoretical limit of 2T 1 . We conclude that energy relaxation and pure dephasing processes due to virtual scattering with phonons contribute on an equal footing to the loss of coherence.
Introduction Quantum dots are often referred to as artificial atoms, since they trap carriers in discrete energy-levels due to the nanoscale three-dimensional finite potential energy well they provide. As such, dots exhibit a coherent light–matter interaction that is similar to an atom. This is evidenced by observations of atom–optics phenomena such as Rabi oscillations [43, 26], power broadening [27], Autler–Townes doublet [14, 42], Mollow triplet [42, 8], and coherent population trapping [6]. In this chapter, Rabi rotation measurements are used to examine how an exciton transition deviates from an ideal two-level atom due to its interaction with a reservoir of phonons. The neutral exciton transition may be regarded as a two-level system, or qubit, composed of the crystal ground-state ∣0〉 and a single electron-hole pair ∣ X 〉. The state-vector of a qubit can be described as a pseudo spin-half. When an oscillating electro-magnetic field resonantly excites the two-level transition it drives an oscillation in the population inversion known as a Rabi oscillation. This results from the oscillations of the driving field and the dipole of the two-level system being synchronous, such that in its rotating frame, the driving field acts as a static magnetic field that causes the pseudo-spin to rotate. Coherent control of the pseudo-spin can be achieved by applying well-defined driving fields, enabling the preparation, and manipulation of superposition states. Such coherent control concepts have found widespread use in electron spin, and nuclear magnetic resonance spectroscopy.
Semiconductor quantum dots have been featured in a number of proposals for quantum computing because of the advantages afforded by confinement on the nanoscale, such as well separated, discrete energy levels and ease of optical manipulation. In addition, they can leverage established semiconductor fabrication techniques and, like the quantum dots used in this work, can be designed to match the telecommunication band, improving the scalability and potential integration of the platform into existing technologies. This thesis work applied optical pulse engineering to manipulate exciton qubits in self-assembled InAs quantum dots. Optimal quantum control theory was used to design pulse shapes that implement high fidelity single-qubit and two-qubit operations, with constraints on the numerical optimization that ensure that the pulses can be accurately implemented using a commercial 4f pulse shaper. In the case of single-qubit operations, the results showed that two uncoupled qubits can be manipulated in parallel using a single phase-shaped laser pulse, provided their optoelectronic properties are sufficiently different. It was demonstrated that targeted differences in inversion and phase between the qubits can be achieved on an ultrafast timescale with high fidelity. In the case of two-qubit operations, the controlled-rotation gate was optimized using amplitude-only and phase-only pulse shaping schemes. The shaped pulses for both schemes were shown to produce higher fidelity operations. The ability to implement robust state inversion on short timescales is particularly useful for technologies such as ultrafast optical switches, single-photon sources, and entangled-photon sources. This work has demonstrated state inversion in a single InAs quantum dot via adiabatic rapid passage using linearly chirped laser pulses. The achieved gate times were an order of magnitude shorter than previous demonstrations. Theoretical predictions of the dependence of the inversion efficiency of the exciton on the sign of the pulse chirp were also verified experimentally, allowing for the identification of phonon-mediated dephasing as the dominant source of decoherence.
The on-demand preparation of higher-order Fock states is of fundamental importance in quantum information sciences.We propose and compare different protocols to generate higher-order Fock states in solid state quantum-dot-cavity systems.The protocols make use of a series of laser pulses to excite the quantum dot exciton and off-resonant pulses to control the detuning between dot and cavity.Our theoretical studies include dot and cavity loss processes as well as the pure-dephasing type coupling to longitudinal acoustic phonons in a numerically complete fashion.By going beyond the two-level approximation for quantum dots, we study the impact of a finite exchange splitting, the impact of a higher energetic exciton state, and an excitation with linearly polarized laser pulses leading to detrimental occupations of the biexciton state.We predict that under realistic conditions, a protocol which keeps the cavity at resonance with the quantum dot until the desired target state is reached is able to deliver fidelities to the Fock state |5 well above 40%.
Abstract A major challenge in generating single photons with a single emitter is to excite the emitter while avoiding laser leakage into the collection path. Ideally, any scheme to suppress this leakage should not result in a loss in the efficiency of the single-photon source. Here, we investigate a scheme in which a single emitter, a semiconductor quantum dot, is embedded in a microcavity. The scheme exploits the splitting of the cavity mode into two orthogonally-polarised modes: one mode is used for excitation, the other for collection. By linking the experiment to theory, we show that the best population inversion is achieved with a laser pulse detuned from the quantum emitter. The Rabi oscillations exhibit an unusual dependence on pulse power. Our theory describes them quantitatively, enabling us to determine the absolute population inversion. By comparing the experimental results with our theoretical model, we determine a population inversion of 98 % − 5 % + 1 % for optimal laser detuning. The Rabi oscillations depend on the sign of the laser-pulse detuning, a phenomenon arising from the non-trivial effect of phonons on the exciton dynamics. The exciton–phonon interaction is included in the theory and gives excellent agreement with all the experimental results.