| dc.description.abstract | The prospect of incorporating strong light-matter interaction into the greater cause of optical information processing has become one of the ultimate goals of quantum optics. Currently, one of the most useful resources for the design of quantum photonic devices is waveguide-integrated op-tics [1]. The most lucrative feature of waveguide-based interfaces is the remarkable controllability over photonic transport properties, a feature that can be conveniently harnessed for the design of purely optical devices. However, this is not the sole aspect that makes these systems so special. Quantum emitters interfacing with a waveguide offers the perfect platform to study the effects of vacuum-mediated long-range couplings between distant emitters. The tunability of the nature and strength of waveguide-induced couplings is providing useful insights into the wider scope and applicability of these models. This thesis attempts to bring these two different approaches to the theoretical investigation of waveguide-integrated architectures under a common theme, highlighting some of the intriguing outcomes of these treatments deduced in the recent past.
The first half of the thesis is dedicated to the theoretical study of single-photon transport in a waveguide interfacing with a periodic string of atomic-scale dipoles, like qubits or two-level atoms. Radiation fields in a one-dimensional structured environment can strongly modify the spontaneous emission from these dipoles. The ensemble of dipoles can exhibit a variety of interference phenomena, which lends greater control to these setups on photon transport. For identical atoms, these interference effects are systematically studied and connections to the lattice periodicity are established. Other notable features include flat-banded reflection profiles owing to the periodicity and cooperative Dicke-type superradiance. Certain non-Markovian signatures are also studied numerically, as they would be relevant in very-strong-coupling regimes. Going beyond the assumption of identical atoms, we next solve photon transport across an array of non-identical dipoles differing in their transition frequencies. By tailoring our analysis in a setting where the periodicity is an integer or half-integer multiple of the resonant wavelength, we work out the exact condition to generate transparency by modulating the individual transition frequencies. We demonstrate the specific applicability of this scheme to an even-sized atomic chain. Since there is no control field used, this mechanism is clearly distinct from the standard paradigm of EIT. We also establish analogies with the linear excitation regime of cavity QED, where an identical manipulation of the atomic frequencies can render the system transparent to a weak input field. However, photon transport in a waveguide has one major distinction from cavity-QED setups. Waveguide-integrated devices display diode-like characteristics owing to the non-reciprocity of photon transport. This additional perk in a waveguide setup is a direct consequence of phase couplings between quantum emitters mediated by the waveguide, which widens the space of possibilities, offering clear advantages over cavities in various applications. All our theoretical results are deduced in the single-photon regime of waveguide QED, which requires a dedicated quantum mechanical treatment. With the advent of high-quality single-photon quantum sources, whether in the optical or the microwave regime, as well as the possibility of producing enhanced light-matter couplings in superconducting circuitry and quantum-dot-based structures, the laboratory verification of our analytical observations, should be within reach.
The second half of this thesis addresses some acute ramifications of a reservoir-mediated coupling between physical systems. Even in the absence of a driving field, the vacuum modes in a waveguide can introduce coupling and coherence between any two otherwise non-interacting systems exchanging energy with the waveguide. The coupling is commonly referred to as dissipative coupling as it originates from leakage into a common bath and requires an open-system description. The mean-field dynamics of dissipatively coupled systems can be modeled in terms of an effective non-Hermitian Hamiltonian, which can be tailored to exhibit anti-Parity-Time (PT) symmetry. This symmetry characterizes a Hamiltonian that flips signs upon the joint action of parity and time-reversal operators and has attracted enormous attention in the last decade. By tap-ping into the remarkable potential of anti-PT symmetry, we propose two intriguing applications. First, we demonstrate how this can serve as a sensor for weak anharmonicities. Nonlinearities are of fundamental interest in optics, leading to a myriad of important physical effects, such as multistability/switching, generation of squeezed and entangled states, electromagnetically induced transparency, and so on. A fine-grained estimation of nonlinearities is, therefore, a prerequisite to the primed control of these effects. We present explicit results in the context of cavity magnonics, illustrating the efficient detection of magnonic anharmonicity (∼ 0.01 − 10 nHz) even at a very weak drive power of 1 µW. Since the theoretical analysis guiding our results is absolutely general, the sensing protocol can be applied to a broad class of systems. That said, the prime reason for choosing a magnon-based model is the continued escalation of interest in hybrid magnon-photon interfaces. New interest has also shown up in utilizing these interfaces for the reversible conversion between microwave and optical photons. This pursuit is still in its infancy and achieving appre-ciable conversion efficiency is still a far cry. We took a significant step in this direction when we proposed the idea of a dissipatively coupled, anti-PT symmetric cavity-magnon interface support-ing large enhancements in the theoretical efficiency. The enhancement is made possible on account of a dark mode in the system. While the conversion in our model is reversible, the efficiencies in the microwave-to-optical and the optical-to-microwave conversions are found to be unequal. This asymmetry is unique to dissipatively coupled systems and emerges both from the indirect nature of the coupling as also the spatial separation between the cavity and magnetic sample. In light of the recent experimental realizations of dissipative magnon-photon couplings, both the applications proposed above are likely to secure practical implementation in the foreseeable future. | |
