Fig. 3

Rapid, high-fidelity, QND measurement setup incorporating the MPIJIS device. The setup, exhibiting only the main components, includes a two-qubit chip in which only one qubit is measured. The measured qubit is read through a single-port, fast readout resonator and controlled via a separate line, which is capacitively coupled to the qubit. To prevent the qubit from relaxing through the readout resonator, a Purcell filter is added to the resonator port, which connects to the rest of the setup. To enable a reflective measurement of the qubit without utilizing a circulator, a broadband, on-chip directional coupler is incorporated between the Purcell filter and the readout input line. In this configuration, the readout signals input on the directional coupler couple, with about 20 dB of attenuation, to the Purcell filter (and readout resonator), whereas the output readout signals reflecting off the readout resonator are transmitted, with a small insertion loss, to a MPIJIS incorporated into the output chain. Following the MPIJIS, a cryogenic circulator is used to route the readout signal to a quantum-limited Josephson amplifier working in reflection (i.e., JPC) and route the amplified reflected signal off the JPC towards the following amplification stage along the output chain, i.e., the HEMT, which is separated from the JPC circulator by two wideband cryogenic isolators, which further protect the qubit from noise coming down the output chain