The increasing need for scaling up quantum computers operating in the microwave domain calls for advanced approaches for control electronics. To this end, integration of componentsat cryogenic temperatures hosting also the quantum devices seems tempting. However, this comes with the limitations of ultra-low power dissipation accompanied by stringent signal-quality requirements to implement quantum-coherent operations. Here, we present a device and a technique to provide coherent continuous-wave microwave emission. We experimentally verify that its operation characteristics accurately follow our introduced theory based on a perturbative treatment of the capacitively shunted Josephson junction as a gain element. From phase noise measurements, we evaluate that the infidelity of typical quantum gate operations owing to this cryogenic source is less than 0.1% up to 10-ms evolution times, which is well below the infidelity caused by dephasing of the state-of-the-art superconducting qubits. Our device provides a coherent tone of 25 pW, corresponding to the total power needed in simultaneous control of thousands of qubits. Thus, together with future cryogenic amplitude and phase modulation techniques, our results may open pathways for scalable cryogenic control systems for quantum processors.

We have fabricated and studied a system of two tunable and coupled nonlinear superconducting resonators. The nonlinearity is introduced by galvanically coupled dc-SQUIDs. We simulatethe system response by means of a circuit model, which includes an additional signal path introduced by the electromagnetic environment. Furthermore, we present two methods allowing us to experimentally determine the nonlinearity. First, we fit the measured frequency and flux dependence of the transmission data to simulations based on the equivalent circuit model. Second, we fit the power dependence of the transmission data to a model that is predicted by the nonlinear equation of motion describing the system. Our results show that we are able to tune the nonlinearity of the resonators by almost two orders of magnitude via an external coil and two on-chip antennas. The studied system represents the basic building block for larger systems, allowing for quantum simulations of bosonic many-body systems with a larger number of lattice sites.

We report on fast tunability of an electromagnetic environment coupled to a superconducting coplanar waveguide resonator. Namely, we utilize a recently-developed quantum-circuit refrigerator(QCR) to experimentally demonstrate a dynamic tunability in the total damping rate of the resonator up to almost two orders of magnitude. Based on the theory it corresponds to a change in the internal damping rate by nearly four orders of magnitude. The control of the QCR is fully electrical, with the shortest implemented operation times in the range of 10 ns. This experiment constitutes a fast active reset of a superconducting quantum circuit. In the future, a similar scheme can potentially be used to initialize superconducting quantum bits.

Superconducting 3D microwave cavities offer state-of-the-art coherence times and a well controlled environment for superconducting qubits. In order to realize at the same time fastreadout and long-lived quantum information storage, one can couple the qubit both to a low-quality readout and a high-quality storage cavity. However, such systems are bulky compared to their less coherent 2D counterparts. A more compact and scalable approach is achieved by making use of the multimode structure of a 3D cavity. In our work, we investigate such a device where a transmon qubit is capacitively coupled to two modes of a single 3D cavity. The external coupling is engineered so that the memory mode has an about 100 times larger quality factor than the readout mode. Using an all-microwave second-order protocol, we realize a lifetime enhancement of the stored state over the qubit lifetime by a factor of 6 with a Z-fidelity of 82%. We also find that this enhancement is not limited by fundamental constraints.

The concept of parity describes the inversion symmetry of a system and is of fundamental relevance in the standard model, quantum information processing, and field theory. In quantumelectrodynamics, parity is conserved and selection rules (SRs) appear when matter is probed with electromagnetic radiation. However, typically large field gradients are required to engineer the parity of the light-matter interaction operator for natural atoms. In this work, we instead irradiate a specifically designed superconducting artificial atom with spatially shaped microwave fields to select the interaction parity in situ. In this way, we observe dipole and quadrupole SRs for single state transitions and induce transparency via longitudinal coupling. Furthermore, we engineer an artificial potassium-like atom with adjustable wave function parity originating from an artificial orbital momentum provided by a resonator. Our work advances light-matter interaction to a new level with promising application perspectives in simulations of chemical compounds, quantum state engineering, and relativistic physics.

Josephson parametric amplifiers (JPA) have become key devices in quantum science and technology with superconducting circuits. In particular, they can be utilized as quantum-limitedamplifiers or as a source of squeezed microwave fields. Here, we report on the detailed measurements of five flux-driven JPAs, three of them exhibiting a hysteretic dependence of the resonant frequency versus the applied magnetic flux. We model the measured characteristics by numerical simulations based on the two-dimensional potential landscape of the dc superconducting quantum interference devices (dc-SQUID), which provide the JPA nonlinearity, for a finite screening parameter βL>0 and demonstrate excellent agreement between the numerical results and the experimental data. Furthermore, we study the nondegenerate response of different JPAs and accurately describe the experimental results with our theory.

We present a systematic analysis of the internal losses of superconducting coplanar waveguide microwave resonators based on niobium thin films on silicon substrates. At millikelvintemperatures and low power, we find that the characteristic saturation power of two-level state (TLS) losses shows a pronounced temperature dependence. Furthermore, TLS losses can also be introduced by Nb/Al interfaces in the center conductor, when the interfaces are not positioned at current nodes of the resonator. In addition, we confirm that TLS losses can be reduced by proper surface treatment. For resonators including Al, quasiparticle losses become relevant above \SI{200}{\milli\kelvin}. Finally, we investigate how losses generated by eddy currents in the conductive material on the backside of the substrate can be minimized by using thick enough substrates or metals with high conductivity on the substrate backside.

A superconducting qubit coupled to an open transmission line represents an implementation of the spin-boson model with a broadband environment. We show that this environment can beengineered by introducing partial reflectors into the transmission line, allowing to shape the spectral function, J({\omega}), of the spin-boson model. The spectral function can be accessed by measuring the resonance fluorescence of the qubit, which provides information on both the engineered environment and the coupling between qubit and transmission line. The spectral function of a transmission line without partial reflectors is found to be Ohmic over a wide frequency range, whereas a peaked spectral density is found for the shaped environment. Our work lays the ground for future quantum simulations of other, more involved, impurity models with superconducting circuits.