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Nonreciprocal scattering of two superconducting qubits coupled to a waveguide

Andrés Rosario Hamann, Clemens Müller, Markus Jerger, Joshua Combes, Maximilian Zanner, Martin P. Weides, Thomas M. Stace, Arkady Fedorov

Published Jan 1, 2018• 0 citations
OpenAlex

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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.
This article delves into the intricacies of nonreciprocity in a system composed of two superconducting qubits operating in a transmon regime and coupled to a 1-D waveguide. Nonreciprocity, characterized by asymmetric signal transmission, is a phenomenon of increasing interest in quantum technologies, as it enables one-way signal propagation and isolation. Our investigation derives the conditions to obtain nonreciprocity in a qubit-loaded waveguide and discusses how common approximations may impact the observed nonreciprocal effects, highlighting crucial factors that must be considered in designing such systems. By employing realistic system parameters, we unveil significant effects on the system dynamics when higher energy levels are included in the transmon regime. This leads to a reduction of nonreciprocity compared to the ideal scenario, with the transmission ratio falling below the two-level approximation limit of2/32/32/3. Additionally, the statistical properties of the system undergo significant changes, particularly in the coherent functions for transmitted and reflected signals. The emergence of additional population transfer channels and the critical role of quantum correlations between qubits underscore the necessity of careful modeling. Our findings have implications for quantum device engineering, offering insights that can enhance the design of more efficient quantum technology and advanced quantum communication systems.
We study chiral and nonreciprocal single-photon scattering in a chiral-giant-molecule waveguide-QED system. Here, the giant molecule consists of two coupled giant atoms, which interact with two linear waveguides, forming a four-port quantum device. We obtain the exact analytical expressions of the four scattering amplitudes using a real-space method. Under the Markovian limit, we find that the single-photon scattering behavior is determined by the coupling strength between the giant atoms and the waveguides, the coupling strength between the two giant atoms, and the nondipole effect caused by the phase accumulation of photons traveling between the coupling points. It is also found that chiral and nonreciprocal single-photon scattering can be realized by introducing the chiral coupling to break the symmetry in the coupling configuration between the giant molecule and the waveguides. In addition, an ideal chiral emitter-waveguide coupling enables a directional single-photon routing. In the non-Markovian regime, the scattering spectra are characterized by more abundant structures with multiple peaks and dips. In particular, we demonstrate that the non-Markovian retarded effect can induce the nonreciprocal single-photon scattering. Our results have potential applications in the design of optical quantum devices involving giant atoms, which can provide an efficient platform for studying chiral quantum optics.