A Josephson quantum filter (JQF) protects a data qubit (DQ) from the radiative decay into transmission lines in superconducting quantum computing architectures. A transmon, which isa weakly nonlinear harmonic oscillator rather than a pure two-level system, can play a role of a JQF or a DQ. However, in the previous study, a JQF and a DQ were modeled as two-level systems neglecting the effects of higher levels. We theoretically examine the effects of the higher levels of the JQF and the DQ on the control of the DQ. It is shown that the higher levels of the DQ cause the shift of the resonance frequency and the decrease of the maximum population of the first excited state of the DQ in the controls with a continuous wave (cw) field and a pulsed field, while the higher levels of the JQF do not. Moreover, we present optimal parameters of the pulsed field, which maximize the control efficiency.

We investigate theoretically how the ground state of a qubit-resonator system in the deep-strong coupling (DSC) regime is affected by the coupling to an environment. We employ a superpositionof coherent states displaced in the qubit-state-dependent directions as a variational ansatz for the ground state of the qubit-resonator-environment system. We show that the reduced density matrix of the qubit-resonator system strongly depends on types of the resonator-waveguide and resonator-qubit coupling, i.e., capacitive or inductive, because of the broken rotational symmetry of the eigenstates of the DSC system in the resonator phase space. When the resonator couples to the qubit and the environment in different ways (for instance, one is inductive and the other is capacitive), the system is almost unaffected by the resonator-waveguide coupling. In contrast, when the types of two couplings are the same (for instance, both are inductive), by increasing the resonator-waveguide coupling strength, the average number of virtual photons increases and the quantum superposition realized in the qubit-resonator entangled ground state is partially degraded. Since the superposition becomes more fragile when the qubit-resonator coupling strength gets large, there exists an optimal strength of the qubit-resonator coupling to maximize the nonclassicality of the qubit-resonator system.

The rapid development in designs and fabrication techniques of superconducting qubits has helped making coherence times of qubits longer. In the near future, however, the radiativedecay of a qubit into its control line will be a fundamental limitation, imposing a trade-off between fast control and long lifetime of the qubit. In this work, we successfully break this trade-off by strongly coupling another superconducting qubit along the control line. This second qubit, which we call a Josephson quantum filter~(JQF), prevents the qubit from emitting microwave photons and thus suppresses its relaxation, while faithfully transmitting large-amplitude control microwave pulses due to the saturation of the quantum filter, enabling fast qubit control. We observe an improvement of the qubit relaxation time without a reduction of the Rabi frequency. This device could potentially help in the realization of a large-scale superconducting quantum information processor in terms of the heating of the qubit environments and the crosstalk between qubits.

Superconducting circuits offer a scalable platform for the construction of large-scale quantum networks where information can be encoded in multiple temporal modes of propagating microwaves.Characterization of such microwave signals with a method extendable to an arbitrary number of temporal modes with a single detector and demonstration of their phase-robust nature are of great interest. Here we show the on-demand generation and Wigner tomography of a microwave time-bin qubit with superconducting circuit quantum electrodynamics architecture. We perform the tomography with a single heterodyne detector by dynamically changing the measurement quadrature with a phase-sensitive amplifier independently for the two temporal modes. By generating and measuring the qubits with hardware lacking a shared phase reference, we demonstrate conservation of phase information in each time-bin qubit generated.

We report on experimentally measured light shifts of superconducting flux qubits deep-strongly-coupled to an LC oscillator, where the coupling constant is comparable to the qubit’stransition frequency and the oscillator’s resonance frequency. By using two-tone spectroscopy, the energies of the six-lowest levels of the coupled circuits are determined. We find a huge Lamb shift that exceeds 90% of the bare qubit frequencies and inversion of the qubits‘ ground and excited states when there is a finite number of photons in the oscillator. Our experimental results agree with theoretical predictions based on the quantum Rabi model.

Single photon detection is a requisite technique in quantum-optics experiments in both the optical and the microwave domains. However, the energy of microwave quanta are four to fiveorders of magnitude less than their optical counterpart, making the efficient detection of single microwave photons extremely challenging. Here, we demonstrate the detection of a single microwave photon propagating through a waveguide. The detector is implemented with an „impedance-matched“ artificial Λ system comprising the dressed states of a driven superconducting qubit coupled to a microwave resonator. We attain a single-photon detection efficiency of 0.66±0.06 with a reset time of ∼400~ns. This detector can be exploited for various applications in quantum sensing, quantum communication and quantum information processing.

We propose a scheme for continuous detection of itinerant microwave photons in circuit quantum electrodynamics. In the proposed device, a superconducting qubit is coupled dispersivelyto two resonators: one is used to form an impedance-matched Λ system that deterministically captures incoming photons, and the other is used for continuous monitoring of the event. The present scheme enables efficient photon detection: for realistic system parameters, the detection efficiency reaches 0.9 with a bandwidth of about ten megahertz.

By properly driving a qubit-resonator system in the strong dispersive regime, we implement an „impedance-matched“ Λ system in the dressed states, where a resonant singlephoton deterministically induces a Raman transition and excites the qubit. Combining this effect and a fast dispersive readout of the qubit, we realize a detector of itinerant microwave photons. We theoretically analyze the single-photon response of the Λ system and evaluate its performance as a detector. We achieve a high detection efficiency close to unity without relying on precise temporal control of the input pulse shape and under a conservative estimate of the system parameters. The detector can also be reset quickly by applying microwave pulses, which allows a short dead time and a high repetition rate.