Measurement setup and device description.
(a) Simplified diagram of the measurement setup at Millikelvin temperatures. The device under test (DUT) is a microwave cavity which properties are tested either by transmission or reflection measurements from its ports. To perform reflection measurements, a microwave tone (RF1) is applied to the output port of the cavity via a circulator (C). The reflected signal is amplified by a high-electron-mobility transistor (HEMT) amplifier at 4 K and a chain of room temperature (RT) amplifiers. The sample sits at 20 mK and is isolated from the higher temperature stages by an additional circulator in series. The amplified signal is down-converted to an intermediate frequency of 25 MHz in an IQ mixer, driven by a dedicated LO, and is digitized by an analog-to-digital converter (ADC) for data analysis. To perform transmission measurement a microwave signal (RF2) is applied to the input port of the cavity. For spectroscopy of the qubits, an additional excitation microwave tone (RF3) is applied together with the measurement signal. The magnetic field inside the cavity is controlled by three coils attached to the exterior of the cavity and wired to individual current sources (DC) at room temperature. (b) 3D model of the cavity with a chip containing three flux tunable transmon qubits. Most of the cavity body is made of Al (gray) with a Cu insert (brown). The Cu insert allows magnetic field from the three mounted coils to penetrate the interior of the cavity for individual control of all qubits frequencies. In addition, the Cu insert provides a beneficial thermal link to the chip. (c) Magnified image of the three transmon qubits on the chip. Each qubit consists of two Al capacitor plates connected via a line interrupted by a micron size DC SQUID (not visible at this scale). (d) Photograph of the cavity parts. To reduce losses due to possible gaps between Cu and Al parts, ∼2 μm of Al were evaporated on the surfaces of the Cu insert which get in contact with Al.