Describing systems of superconducting atoms coupled to a continuum of photonic modes at multiple separated locations in a waveguide, waveguide quantum electrodynamics (QED) with giant atoms has emerged as a promising platform for realizing quantum interconnect. Such systems have been reported to exhibit rich phenomena that differ from those of natural atoms. Going beyond the widely used Born-Markov and Wigner-Weisskopf approximations, we investigate the non-Markovian dynamics of one and two giant atoms interacting with a waveguide formed by an array of coupled resonators. We discover that the diverse dynamical behaviors of the giant atoms are intrinsically determined by the energy spectrum of the composite system consisting of the giant atoms and the photonic modes in the waveguide. As long as one and more bound states are present in the energy spectrum, their excited-state probabilities, respectively, tend to stable finite values and lossless Rabi-like oscillations with frequencies proportional to the differences of the bound-state eigenenergies. Our result provides an insightful guideline for suppressing the decoherence of giant atoms and facilitates the development of quantum interconnect devices using giant-atom waveguide QED.
Describing systems of superconducting atoms coupled to a continuum of photonic modes at multiple separated locations in a waveguide, waveguide quantum electrodynamics (QED) with giant atoms has emerged as a promising platform for realizing quantum interconnect. Such systems have been reported to exhibit rich phenomena that differ from those of natural atoms. Going beyond the widely used Born-Markov and Wigner-Weisskopf approximations, we investigate the non-Markovian dynamics of one and two giant atoms interacting with a waveguide formed by an array of coupled resonators. We discover that the diverse dynamical behaviors of the giant atoms are intrinsically determined by the energy spectrum of the composite system consisting of the giant atoms and the photonic modes in the waveguide. As long as one and more bound states are present in the energy spectrum, their excited-state probabilities, respectively, tend to stable finite values and lossless Rabi-like oscillations with frequencies proportional to the differences of the bound-state eigenenergies. Our result provides an insightful guideline for suppressing the decoherence of giant atoms and facilitates the development of quantum interconnect devices using giant-atom waveguide QED.
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.
We study the bound states in the continuum (BIC) in a system of two identical two-level giant atoms coupled to a one-dimensional waveguide. By deriving general dark-state conditions, we clarify how coupling configurations and atomic parameters influence decay suppression. Through analysis of the long-time dynamical behaviors of atoms and bound photons, we carry out a detailed classification of bound states and explore the connections between these dynamical behaviors and the system's intrinsic light-matter interactions. The system supports static bound states with persistent atomic excitations, and oscillating bound states with periodic atom-photon or atom-atom excitation exchange. Under certain conditions, oscillating bound states can contain more harmonic components owing to the emergence of additional quasi-dark modes, rendering them promising platforms for high-capacity quantum information processing. These findings advance the understanding of BIC in waveguide quantum electrodynamics with multiple giant atoms and reveal their prospective applications in quantum technologies.
We study the generation of quantum entanglement between two giant atoms coupled to a common one-dimensional waveguide. Here each giant atom interacts with the waveguide at two separate coupling points. Within the Wigner-Weisskopf framework for single coupling points, we obtain the time-delayed quantum master equations governing the evolution of the two giant atoms for three different coupling configurations: separated, braided, and nested couplings. For each coupling configuration, we consider both the Markovian and non-Markovian entanglement dynamics of the giant atoms, which are initially in two different separable states: single- and double-excitation states. Our results show that the generated entanglement depends on the phase shift, time delay, atomic initial state, and the coupling configuration. For the single-excitation initial state, there exists the steady-state entanglement for each coupling in both the Markovian and non-Markovian regimes due to the appearance of the dark state. For the double-excitation initial state, we observe entanglement sudden birth via adjusting the phase shift in both regimes. In particular, the maximally achievable entanglement for the nested coupling is about one order of magnitude larger than those of separate and braided couplings. We also find that the maximal entanglement for these three coupling configurations can be enhanced in the case of small time delays. This work can be utilized for the generation and control of entanglement in quantum networks based on giant-atom waveguide-QED systems, which have wide potential applications in quantum information processing.
Abstract We investigate phase-controlled bound states in a one-dimensional photonic waveguide coupled to an artificial giant atom at two distant sites. Specifically, we identify the bound state out of the continuum (BOC) and the bound state in the continuum (BIC) and derive the exact existence condition for the BOC. Furthermore, we analytically determine the BIC’s frequency and photonic distribution profile. Remarkably, our analysis reveals quantum beats in both atomic and photonic dynamics, arising from coherent oscillations between the BIC and BOC. These results establish a novel approach for manipulating waveguide quantum electrodynamics via engineered bound states, with potential applications in quantum information processing.
We study the properties of bound states in waveguide-QED systems consisting of multiple giant atoms coupled to a coupled-resonator waveguide. Based on the general analytical expressions for these states and the corresponding energy spectra, we analyze in detail the threshold conditions for the appearance of bound states and the photon-mediated interactions between dressed atoms for different configurations. In addition, when multiple giant atoms are coupled to the waveguide, different types of interacting atomic chain can be obtained by manipulating the coupling configurations. Accordingly, the energy spectra of the bound states form metaband structures in the photonic band gaps. This makes the system a useful platform for quantum simulation and quantum information processing.
Giant atoms, quantum emitters that couple to light at multiple discrete points, are emerging as a new paradigm in quantum optics due to their many promising properties, such as decoherence-free interaction. While most previous work has considered giant atoms coupled to open continuous waveguides or a single giant atom coupled to a structured bath, here we study the interaction between two giant atoms mediated by a structured waveguide, e.g., a photonic crystal waveguide. This environment is characterized by a finite energy band and a band gap, which affect atomic dynamics beyond the Markovian regime. Here we show that, inside the band, decoherence-free interaction is possible for different atom-cavity detunings, but is degraded from the continuous-waveguide case by time delay and other non-Markovian effects. Outside the band, where atoms interact through the overlap of bound states, we find that giant atoms can interact more strongly and over longer distances than small atoms for some parameters, for instance, when restricting the maximum coupling strength achievable per coupling point. The results presented here may find applications in quantum simulation and quantum gate implementation.