Our work has included computer simulations, time-resolved spectroscopic measurements of quantum emitters and the nanofabrication of graphene. In the computer simulations we demonstrate a novel method for entanglement generation by means of plasmon-mediated interactions between quantum emitters. In the experimental part of the project, we have carried out spectroscopic measurements to select the most suitable kind of quantum emitter for the hybrid quantum system, following the requirements of narrow inhomogeneous broadening. Also, using magneto-optical Kerr spectroscopy, in combination with numerical Stoner-Wohlfarth model, we have design an experimental method in which the coupling of quantum emitters to plasmonic cavities can be tested by assessing the populations of Zeeman sublevels. We have not been able to reach the visible or near infrarred sepectrum in the nanofabricated plasmonic nanocavities. For this reason, we have theoretically investigated a four-level scheme, in which the probe laser, in the range of optical communication wavelengths, can couple a long-lived state of the emitter to an excited state, while this is coupled to another excited state by the plasmon.
In the theoretical part of the project, we have developed a software that numerically simulates the dynamics of the hybrid quantum system and its interaction with light. For that task, we have used computer techniques of quantum optics, such as the Monte Carlo method of quantum trajectories, in combination with numerical techniques for the calculation of the near field of graphene nanostructures. Our investigations have covered the strong-coupling regime as well as the weak coupling regime:
A. Strong-coupling regime: In this situation we can assume that cavity-mediated interactions induce collective energy shifts in some states of the quantum emitters. In our protocol the population transfer between two qubit states of the emitters is conditional on the cavity-mediated interactions. Our method uses four energy levels in such a way that the wavelength of the graphene plasmons can be different from those of the lasers.
B. Weak-coupling regime: This situation is more realistic than the strong-coupling regime. Here, we cannot assume energy shifts, but we can assume cooperative spontaneous emission into the plasmonic modes. We have calculated the generation of quantum non-local correlations between the emitters as a result of the cooperative spontaneous emission. We notice that only the ions at distances from graphene below 5nm are the subject of strong cooperative emission. This may be a drawback for absorption spectroscopy.
In the experimental part of the project, time-resolved spectroscopic measurements of a variety of quantum emitters have provided us with rich information to select the most suitable quantum emitters to couple to graphene plasmons. We have measured colloidal quantum dots, covalent organic frameworks, laser dyes and Erbium-doped yttria thin films. Erbium ions in yttria thin films show the narrowest inhomogeneous linewidth at 1550 nm. These are the quantum emitters of preference.
The interaction between quantum emitters and graphene plasmons is effective only at very short distances below 5nm. For such thin films, magneto-optical spectroscopy can be much more precise than absorption spectroscopy if the sample has unbalanced populations in the Zeeman sublevels. In our coupling schemes the qubit states can be two Zeeman sublevels that fulfil the requirements for Kerr spectroscopy. Parallel to the research activities with quantum emitters and graphene, I carried out various measurements of magnetic thin films. For the analysis of the measured data, I developed a software to fit the measured magnetic remanence to a Stoner-Wohlfarth model in order to obtain the anisotropic magnetic constants.
