We introduce a three-qubit waveguide-QED system to achieve a cavity-QED strong-coupling regime for a probe qubit embedded in atomlike mirrors, which was realized in recent experiments. We then extend this system into the deep non-Markovian regime and demonstrate the profound role that retardation plays on the dressed resonances, allowing one to significantly improve the polariton lifetimes (by several orders of magnitude), tune the resonances, as well as realize Fano-like resonances and simultaneous coupling to multiple cavity modes. Exact Green's function solutions are presented for the classical spectral resonances, and the full quantum dynamics are simulated using matrix product states, where we demonstrate additional control of the cavity-QED system using chiral probe qubits. We show example qubit dynamics with both one and two quantum excitations.
We introduce a three-qubit waveguide-QED system to achieve a cavity-QED strong-coupling regime for a probe qubit embedded in atomlike mirrors, which was realized in recent experiments. We then extend this system into the deep non-Markovian regime and demonstrate the profound role that retardation plays on the dressed resonances, allowing one to significantly improve the polariton lifetimes (by several orders of magnitude), tune the resonances, as well as realize Fano-like resonances and simultaneous coupling to multiple cavity modes. Exact Green's function solutions are presented for the classical spectral resonances, and the full quantum dynamics are simulated using matrix product states, where we demonstrate additional control of the cavity-QED system using chiral probe qubits. We show example qubit dynamics with both one and two quantum excitations.
We study a three-qubit waveguide system in the presence of optical pumping, when the side qubits act as atomlike mirrors, manifesting in a strong light-matter coupling regime. The qubits are modeled as two-level systems, where we account for important saturation effects and quantum nonlinearities. Optically pumping this system into a nonlinear regime is shown to lead to a rich manifold of dressed states that can be seen in the emitted spectrum, and we show two different theoretical solutions using a medium-dependent master-equation model in the Markovian limit, as well as using matrix product states without invoking any Markov approximations. We demonstrate how a rich nonlinear spectrum is obtained by varying the relative decay rates of the mirror qubits as well as their spatial separation and show the limitations of using a Markovian master equation. Our model allows one to directly model giant-atom phenomena, while including important retardation effects and multiphoton nonlinearities. We also show how the excited three-qubit system, in a strong-coupling regime, deviates significantly from a Jaynes-Cummings model when entering the nonlinear regime.
Experiments utilizing quantum optics have progressed rapidly in last few decades, particularly in the context of quantum computation, simulation, and communication. Early work in this field focused on implementations of cavity quantum electrodynamics (QED), where atoms, either natural or artificial, are strongly coupled to the confined photonic modes of cavities. However, in recent years, achieving strong coupling between atoms and itinerant photons has also gained significant interest for its applications in quantum networking. To this end, many atomic platforms are attempting to realize the waveguide QED architecture: atoms that interact with the continua of propagating photonic modes within a waveguide. In this thesis, we realize the strong-coupling regime of a waveguide QED architecture by coupling superconducting artificial atoms, typically operated as qubits, to one-dimensional transmission lines. We first demonstrate that superconducting qubits in a waveguide QED system can be used as high-quality quantum emitters. We then leverage the quantum interference between the simultaneous emission from multiple qubits in order to generate non-classical, spatially entangled, and directional itinerant microwave photons. These types of photons are particularly useful for remote entanglement and quantum communication protocols. Finally, we demonstrate that superconducting qubits can be engineered to enter novel regimes of light-matter interactions that are difficult, or even impossible, to achieve in other atomic platforms. In particular, we realize the giant-atom regime of waveguide QED, where the atom can no longer be treated as a point-like object. We use our giant atoms to implement tunable atom-waveguide couplings, as well as decoherence-free waveguide-mediated interactions between multiple atoms.
Experiments based on cavity quantum electrodynamics (QED) are widely used to study the interaction of a light field with a discrete frequency spectrum and emitters. More recently, the field of waveguide QED has attracted interest due to the strong interaction between propagating photons and emitters that can be obtained in nanophotonic waveguides, where a continuum of frequency modes is allowed. Both cavity and waveguide QED share the common goal of harnessing and deepening the understanding of light-matter coupling. However, they often rely on very different experimental setups and theoretical descriptions. Here, we experimentally investigate the transition from cavity to waveguide QED with an ensemble of cold atoms that is coupled to a fiber-ring resonator, which contains a nanofiber section. By varying the length of the resonator from a few meters to several tens of meters, we tailor the spectral density of modes of the resonator while remaining in the strong coupling regime. When increasing the resonator length, we observe a continuous transition from the paradigmatic Rabi oscillations of cavity QED to non-Markovian dynamics reminiscent of waveguide QED.
In quantum communication and distributed quantum computing, one-dimensional (1D) waveguides provide directional transfer of quantum information. A single-mode waveguide has a density-of-states singularity at the lower cut-off frequency, which resembles sharp resonances of a cavity but with non-Markovian dynamics. Thus we put forward schemes of coupling atomic transitions and a waveguide continuum edge. We first present a scheme of spin-photon quantum interface operating in the non-Markovian regime for a $\ensuremath{\Lambda}$-type three-level system coupled to a waveguide. Then we show that strong coupling between atomic transitions and a waveguide continuum edge can lead to vacuum Rabi oscillations and bound polariton states.
Manipulating the nature of photons emission is one of the basic tasks in quantum optics and photonics. The ever growing list of quantum applications requires a robust means of controlling the strongly coupled coherent interaction of photons and matter. Here, we investigate three-photon transmission spectra in a strongly coupled cavity polariton system and show that the correlation functions and transmitted photon stream can be optically manipulated. The dynamics of single photons and photon pairs at the polariton resonances can be changed by light from a single external coupling laser. At the "dark-state polariton," three-photon transmission is a perfectly coherent field in contrast to the strong photon-bunching behavior of a typical cavity quantum electrodynamics system. When the detuned probe light is tuned to the "bright polariton," the light exhibits a dramatic photon antibunching effect. Remarkably, the Fano-resonant asymmetric three-photon transmission caused by the interference between the dressed states leads to a new quantum feature that is strongly nonclassical (the third-order correlation function g((3))(0, 0) ≪ 1) and has a wide and tunable bandwidth. The dependence of the intrinsic third-order correlation and time symmetry of the photon stream on the controlled parameters is also examined. Strongly nonclassical, all-optically controllable multi-photon dynamics are very important for future quantum devices and metrology.
We introduce a picture to describe and interpret waveguide-QED problems in the non-Markovian regime of long photonic retardation times resulting in delayed coherent feedback. The framework is based on an intuitive spatial decomposition of the waveguide into blocks. Among these, the block directly coupled to the atoms embodies an effective lossy multimode cavity leaking into the rest of the waveguide, in turn embodying an effective white-noise bath. The dynamics can be approximated by retaining only a finite number of cavity modes, which grows with the time delay. This description captures the atomic as well as the field’s dynamics, even with many excitations, in both emission and scattering processes. As an application, we show that the recently identified non-Markovian steady states can be understood by retaining very few or even only one cavity mode.