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arXiv:2511.17250 · npj Quantum Information

Basic cell for a quantum microwave router

Evgeniya Mutsenik, Aidar Sultanov, Leonie Kaczmarek, Matthias Schmelz, Gregor Oelsner, Ronny Stolz, Evgeni Ilichev

quant-phcond-mat.supr-conSubmitted Nov 21, 2025• Announced Nov 24, 2025• 0 citations
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Abstract

We report the first experimental realization of a scalable basic cell for quantum routing, enabling coherent control and exchange of microwave photons between two spatially separated superconducting waveguides coupled via a single transmon qubit. The cell was characterized at 10 mK with an average input signal of approximately 1 photon at approximately 6 GHz, and with the qubit biased to its optimal point to minimize sensitivity to external magnetic fluctuations. By combining steady-state and time-domain measurements, we reconstructed the key parameters of the system, including qubit relaxation and dephasing, waveguide-qubit couplings, and cross-waveguide photon transfer efficiency. The observed performance is consistent with a non-Hermitian Hamiltonian formalism and demonstrates clear limits set by flux bias, temperature, and photon number, in agreement with flux- and temperature-induced dephasing models. Crucially, the cell operates reliably at the single-photon level, and in the high-photon regime we directly observe photon dressing induced by the qubit. These results establish a versatile platform for studying open quantum system phenomena and pave the way for scalable implementations of quantum routing and network nodes.

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We report the first experimental realization of a scalable basic cell for quantum routing, enabling coherent control and exchange of microwave photons between two spatially separated superconducting waveguides coupled via a single transmon qubit. The cell was characterized at 10 mK with an average input signal of approximately 1 photon at approximately 6 GHz, and with the qubit biased to its optimal point to minimize sensitivity to external magnetic fluctuations. By combining steady-state and time-domain measurements, we reconstructed the key parameters of the system, including qubit relaxation and dephasing, waveguide-qubit couplings, and cross-waveguide photon transfer efficiency. The observed performance is consistent with a non-Hermitian Hamiltonian formalism and demonstrates clear limits set by flux bias, temperature, and photon number, in agreement with flux- and temperature-induced dephasing models. Crucially, the cell operates reliably at the single-photon level, and in the high-photon regime we directly observe photon dressing induced by the qubit. These results establish a versatile platform for studying open quantum system phenomena and pave the way for scalable implementations of quantum routing and network nodes.
We have embedded an artificial atom, a superconducting transmon qubit, in an open transmission line and investigated the strong scattering of incident microwave photons (∼6 GHz). When an input coherent state, with an average photon number N≪1 is on resonance with the artificial atom, we observe extinction of up to 99.6% in the forward propagating field. We use two-tone spectroscopy to study scattering from excited states and we observe electromagnetically induced transparency (EIT). We then use EIT to make a single-photon router, where we can control to what output port an incoming signal is delivered. The maximum on-off ratio is around 99% with a rise and fall time on the order of nanoseconds, consistent with theoretical expectations. The router can easily be extended to have multiple output ports and it can be viewed as a rudimentary quantum node, an important step towards building quantum information networks.
The realization of a quantum network composed of quantum nodes which process quantum fields and quantum channels to transfer quantum information has recently been proposed. In recent years, fundamental experiments produced by superconducting circuits suggest that they are promising candidates for realization of a quantum network. Superconducting circuit QED can act as quantum nodes, which can be linked by quantum channels, to transfer flying microwave photons (quantum information) from site to site on chip with high fidelity. Therefore, controlling the propagating microwave photons on chip is an essential step towards realization of a quantum network.In this thesis, we demonstrate a specific quantum node, namely the single photon router. The active element of the router is a single three-level artificial atom, a superconducting transmon type qubit, strongly coupled to a superconducting 1D transmission line. Strong coupling between the artificial atom and propagating microwave photons, revealed in high degree scattering of the resonance waves, has been observed. By exploiting the phenomenon of electromagnetically induced transparency (EIT), we can route a single photon signal from an input port to either of two output ports with an on-off ratio of approximately 90%. The switching time of the router is shown to be a few nanoseconds, consistent with theoretical expectations and the device parameters. Besides the router, we also observed some fundamental phenomenon of single atom, such as strong nonlinearity, anomalous dispersion and the Mollow Triplet.This thesis describes the motivation, theoretical background, design, implementation and the measurement results.
Quantum microwave photonics aims at generating, routing, and manipulating propagating quantum microwave fields in the spirit of optical photonics.To this end, the strong nonlinearities of superconducting quantum circuits can be used to either improve or move beyond the implementation of concepts from the optical domain.In this context, the design of a well-controlled broadband environment for the superconducting quantum circuits is a central task.In this work, we place a superconducting transmon qubit in one arm of an on-chip Mach-Zehnder interferometer composed of two superconducting microwave beam splitters.By measuring its relaxation and dephasing rates we use the qubit as a sensitive spectrometer at the quantum level to probe the broadband electromagnetic environment.For frequencies near the qubit transition frequency, this environment can be well described by an ensemble of harmonic oscillators coupled to the transmon qubit.At low frequencies 0 w  , we find experimental evidence for colored quasi-static Gaussian noise with a high spectral weight, as it is typical for ensembles of two-level fluctuators.Our work paves the way towards possible applications of propagating microwave photons, such as emulating quantum impurity models or a novel architecture for quantum information processing.
Abstract A central challenge for realizing large-scale quantum processors is the design and realization of qubit-qubit connections: we must be able to perform efficient gates between qubits, yet prevent connections from spoiling qubit quality or prohibiting "debugging" the system. In this work, we present a microwave quantum state router that realizes all-to-all couplings among four independent and detachable quantum modules of superconducting qubits. Each module consists of a single transmon, readout mode, and communication mode coupled to the router. The router design centers on a parametrically driven, Josephson-junction based three-wave mixing element which generates photon exchange among the modules' communication modes. We first demonstrate coherent photon exchange among four communication modes, with an average full-iSWAP time of 760 ns and average inter-module gate fidelity of 0.97, limited by our modes' coherence times. We also demonstrate photon transfer and pairwise entanglement between the modules' qubits, and parallel operation of simultaneous iSWAP across the router. The gates demonstrated here can readily be extended to faster and higher-fidelity router operations, as well as scaled to support larger networks of quantum modules.
Abstract We demonstrate an on-demand source of microwave single photons with 71–99% intrinsic quantum efficiency. The source is narrowband (300 kHz) and tuneable over a 600 MHz range around 5.2 GHz. Such a device is an important element in numerous quantum technologies and applications. The device consists of a superconducting transmon qubit coupled to the open end of a transmission line. A π-pulse excites the qubit, which subsequently rapidly emits a single photon into the transmission line. A cancellation pulse then suppresses the reflected π-pulse by 33.5 dB, resulting in 0.005 photons leaking into the photon emission channel. We verify strong antibunching of the emitted photon field and determine its Wigner function. Non-radiative decay and 1/f flux noise both affect the quantum efficiency. We also study the device stability over time and identify uncorrelated discrete jumps of the pure dephasing rate at different qubit frequencies on a time scale of hours, which we attribute to independent two-level system defects in the device dielectrics, dispersively coupled to the qubit. Our single-photon source with only one input port is more compact and scalable compared to standard implementations.
PhD Thesis: Superconducting circuit quantum electrodynamics (QED) has developed into a powerful platform for studying the interaction between matter and different states of light. In this context, superconducting quantum bits (qubits) act as artificial atoms interacting with quantized modes of the electromagnetic field. The field can be trapped in superconducting microwave resonators or propagating in transmission lines. In this thesis, we particularly study circuit QED systems where microwave fields are coupled with superconducting flux and transmon qubits. We optimize the coherence properties of the resonators, by analyzing loss mechanisms at excitation powers of approximately one photon on average. We find that two-level fluctuators associated with oxide layers at substrate and metal surfaces and metal-metal interfaces represent the predominant loss channel. Furthermore, we show how broadband thermal photon fields influence the relaxation and dephasing properties of a superconducting transmon qubit. To this end, we study several second-order loss channels of the transmon qubit and find that the broadband fields introduce a larger decay rate than expected from the Purcell filter defined by the resonator. Additionally, we show that qubit dephasing at the flux-insensitive point as well as low-frequency parameter fluctuations can be enhanced by thermal fields. Finally, we study how artificial atoms react to changes in inherent properties of the light fields. We perform a detailed analysis of the photon statistics of thermal fields using their relation to the qubits coherence properties. We quantitatively recover the expected n2 + nn^{2}\,{+}\,nn2+n-law for the photon number variance and confirm this result by direct correlation measurements. We then show a novel technique for the in-situ conversion of the interaction parity in light-matter interaction. To this end, we couple spatially controlled microwave fields to a flux qubit with two degrees of freedom.