Abstract We present a non-local quantum system based on a waveguide QED architecture, comprising two spatially separated and largely detuned superconducting transmon qubits. By applying parametric frequency modulation to one of the qubits, we establish a tunable coherent channel between the two far-detuned qubits, thereby forming an Λ -type three-level system. We demonstrate that tuning the modulation amplitude enables the observation of spectral evolution from electromagnetically induced transparency (EIT) to Autler–Townes splitting (ATS). Furthermore, by exploiting the interplay between the non-local waveguide phase and system dissipation, the system achieves significant non-reciprocal microwave transmission and direction-selective photon emission. The scheme operates without external magnetic fields, offering an efficient pathway for realizing on-chip integrated quantum routers and isolators.
Abstract We present a non-local quantum system based on a waveguide QED architecture, comprising two spatially separated and largely detuned superconducting transmon qubits. By applying parametric frequency modulation to one of the qubits, we establish a tunable coherent channel between the two far-detuned qubits, thereby forming an Λ -type three-level system. We demonstrate that tuning the modulation amplitude enables the observation of spectral evolution from electromagnetically induced transparency (EIT) to Autler–Townes splitting (ATS). Furthermore, by exploiting the interplay between the non-local waveguide phase and system dissipation, the system achieves significant non-reciprocal microwave transmission and direction-selective photon emission. The scheme operates without external magnetic fields, offering an efficient pathway for realizing on-chip integrated quantum routers and isolators.
Two transversely coupled and resonant qubits form symmetric and antisymmetric states as their eigenstates. In this paper, we show that parametric modulation of an individual qubit enables direct Rabi swapping between the two states. Its application to set up a $\mathrm{\ensuremath{\Lambda}}$-type system with a pair of strongly coupled superconducting transmon qubits is discussed. The excited state is made of the symmetric state, and the metastable state is the antisymmetric state. The coherence of the metastable state is only limited by the pure dephasing mechanism. Based on this scheme, $\mathrm{\ensuremath{\Lambda}}$-type electromagnetically induced transparency, Autler-Townes splitting, and stimulated Raman adiabatic passage are numerically demonstrated. We highlight the large frequency tunability in such superconducting $\mathrm{\ensuremath{\Lambda}}$-type systems.
We investigate theoretically a type of microwave memory that is composed of a single transmon qubit and a coupling resonator. The synergy of the parametric modulation of the qubit resonance frequency and a microwave directly driving the qubit transition can constitute effective three-level $\mathrm{\ensuremath{\Lambda}}$-type electromagnetically induced transparency. We demonstrate that the dynamical control of such a parametric modulation allows for the possibility of microwave storage and retrieval using a single qubit. Our results pave the way towards the practical implementation of quantum information in superconducting circuits.
Nonreciprocal interaction between two spatially separated subsystems plays a crucial role in signal processing and quantum networks. Here, we propose an efficient scheme to achieve nonreciprocal interaction and entanglement between two qubits by combining coherent and dissipative couplings in a superconducting platform, where two coherently coupled transmon qubits simultaneously interact with a transmission line waveguide. The coherent interaction between the transmon qubits can be achieved via capacitive coupling or via an intermediary cavity mode, while the dissipative interaction is induced by the transmission line via reservoir engineering. With high tunability of superconducting qubits, their positions along the transmission line can be adjusted to tune the dissipative coupling, enabling to tailor reciprocal and nonreciprocal interactions between the qubits. A unidirectional interaction can be achieved when the separation between the two qubits is ( 4 n + 3 ) λ 0 / 4 , where n is an integer and λ 0 is the photon wavelength. This nonreciprocal interaction enables the generation of nonreciprocal entanglement between the two transmon qubits. Furthermore, applying a drive field to one of the qubits can stabilize the system into a nonreciprocal steady-state entangled state. Remarkably, the nonreciprocal interaction in this work does not rely on the presence of nonlinearity or any explicitly nonreciprocal elements, which has more potential applications in designing nonreciprocal quantum devices, processing quantum information, and building quantum networks.
Three-level Lambda systems provide a versatile platform for quantum optical phenomena such as Electromagnetically Induced Transparency (EIT), slow light, and quantum memory. Such Lambda systems have been realized in several quantum hardware platforms including atomic systems, superconducting artificial atoms, and meta-structures. Previous experiments involving superconducting artificial atoms incorporated coupling to additional degrees of freedom, such as resonators or other superconducting atoms. In this work, we performed an EIT experiment in microwave frequency range utilizing a single Fluxonium qubit within a microwave waveguide. The Lambda system is consisted of two plasmon transitions in combination with one metastable state originating from the fluxon transition. In this configuration, the controlling and probing transitions are strongly coupled to the transmission line, safeguarding the transition between 0 and 1 states, and ensuring the Fluxonium qubit is close to the sweet spot. Our observations include the manifestation of EIT, a slowdown of light with a delay time of 217 ns, and photon storage. These results highlight the potential as a phase shifter or quantum memory for quantum communication in superconducting circuits.
We investigate electromagnetically induced transparency (EIT) and Autler-Townes splitting (ATS) in a driven three-level superconducting artificial system which is a dressed-state system resulting from the coupling of a superconducting charge qubit (an artificial atom) and a transmission line resonator. In the frame of the dressed-state approach and steady-state approximation, we study the linear absorption of the dressed artificial system to a weak probe signal in depth. In light of the spectrum-decomposition method and some other restrictions, we obtain the explicit conditions for the dressed-state realization of EIT and ATS and present a corresponding "phase diagram". In contrast to usual bare systems, these conditions given in the dressed system have an extra dependency on the qubit-resonator parameters. And by varying the qubit's Josephson coupling energy we demonstrate a transition from EIT to ATS.
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.