As the field of superconducting quantum computing advances, networking qubits within a single system becomes essential for building modular processors. Modularity allows the system to circumvent scalability constraints and enable architectures and computational schemes that exploit non-local connectivity to enhance processing capabilities. This work proposes non-local entanglement generation methods based on the theory of chiral quantum waveguide dynamics, which is the quantum-optical framework that describes systems of atoms coupled non-reciprocally to a continuum of modes. We leverage these effects to design a chiral communication module composed of multiple superconducting qubits, capable of both directional single photon routing and the realization of chiral, driven-dissipative entanglement protocols.
As the field of superconducting quantum computing advances, networking qubits within a single system becomes essential for building modular processors. Modularity allows the system to circumvent scalability constraints and enable architectures and computational schemes that exploit non-local connectivity to enhance processing capabilities. This work proposes non-local entanglement generation methods based on the theory of chiral quantum waveguide dynamics, which is the quantum-optical framework that describes systems of atoms coupled non-reciprocally to a continuum of modes. We leverage these effects to design a chiral communication module composed of multiple superconducting qubits, capable of both directional single photon routing and the realization of chiral, driven-dissipative entanglement protocols.
The architectural principle of modularity enables the construction of complex systems from simpler components, each responsible for a particular function. The quantum computer is an intricate system comprising fragile, error-prone parts known as qubits. Entanglement distribution across a network of non-local processing modules facilitates robust and extensible quantum computation. In modular quantum architectures, photons are natural quantum information carriers which propagate through interconnects between processing nodes. In this thesis, we engineer a quantum interconnect between superconducting modules underpinned by the physics of waveguide Quantum Electrodynamics (wQED). First, we realize a multi-qubit module that exploits quantum interference to emit microwave photons into a waveguide with a specified propagation direction. Next, we construct the quantum interconnect by coupling two modules to a common waveguide and demonstrate directional (chiral) photon emission and absorption. Finally, using this chiral quantum interconnect, we generate remote entanglement, establishing a key resource for distributed quantum computation in an all-to-all network architecture.
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.
Abstract Superconducting metamaterials, which are designed and fabricated with structured fundamental circuit elements, have motivated recent developments of exploring unconventional quantum phenomena in circuit quantum electrodynamics (circuit-QEDs). We propose a method to engineer 1D Josephson metamaterial as a chiral waveguide by considering a programmed spatiotemporal modulation on its effective impedance. The modulation currents are in the form of travelling waves which phase velocities are much slower than the propagation speed of microwave photons. Due to the Brillouin-scattering process, non-trivial spectrum regimes where photons can propagate unidirectionally emerge. Considering superconducting qubits coupling with this metamaterial waveguide, we analyze both Markovian and non-Markovian quantum dynamics, and find that superconducting qubits can dissipate photons unidirectionally. Moreover, we show that our proposal can be extended a cascaded quantum network with multiple nodes, where chiral photon transport between remote qubits can be realized. Our work might open the possibilities to exploit SQUID metamaterials for realizing unidirectional photon transport in circuit-QED platforms.
Realizing a fully connected network of quantum processors requires the ability to distribute quantum entanglement. For distant processing nodes, this can be achieved by generating, routing, and capturing spatially entangled itinerant photons. In this work, we demonstrate the deterministic generation of such photons using superconducting transmon qubits that are directly coupled to a waveguide. In particular, we generate two-photon N00N states and show that the state and spatial entanglement of the emitted photons are tunable via the qubit frequencies. Using quadrature amplitude detection, we reconstruct the moments and correlations of the photonic modes and demonstrate state preparation fidelities of 84%. Our results provide a path toward realizing quantum communication and teleportation protocols using itinerant photons generated by quantum interference within a waveguide quantum electrodynamics architecture.
Superconducting quantum circuits are a leading hardware candidate for realizing a large-scale fault-tolerant quantum computer. However, scaling superconducting quantum computers will soon approach the physical limits imposed by their cryogenic operating environments. Consequently, interconnects between multiple processor modules will become increasingly necessary for scaling quantum computers. Optical fiber is a particularly promising candidate for these interconnects due to its low loss and negligible thermal noise at room temperature. However, interfacing microwave-frequency superconducting circuits with optical fibers presents several challenges. In this thesis, we present a modular architecture which addresses many of the current challenges in building this interface. We develop an optomechanical quantum transducer which is capable of generating microwave- and optical-frequency photons in pairs, and demonstrate quantum entanglement between these microwave-optical photon pairs. Further, we develop a superconducting qubit module optimized for efficient absorption of itinerant microwave photons, and demonstrate a high-efficiency photon catch protocol. Finally, we connect the superconducting qubit module to the transducer module via a superconducting microwave cable. We drive the microwave-optical photon pair generation process in the transducer module and employ the photon catch protocol in the qubit module to facilitate state transfer between the qubit and the flying microwave photon from the transducer. We observe correlations between the final qubit state and the detection of an optical photon, demonstrating a crucial step toward establishing quantum entanglement between superconducting qubits and light. We discuss ongoing progress and challenges in realizing hybrid entanglement and in extending this approach to entangle remote superconducting qubits over optical networks.