Bolometric techniques for circuit quantum electrodynamics

Superconducting quantum bits, or qubits, provide a promising platform for realizing large-scale quantum computers. As the number of qubits on a chip keeps increasing, so does the amount of required electronics and instruments needed for qubit control and readout. Eventually, it will be impossible to fit all the control lines into a cryostat. Therefore, there is an on-going effort to move the conventional room-temperature components into the lower-temperature stages. So-called Josephson junction is one of the fundamental building blocks used to realize these components in superconducting circuits. In this thesis, we study a particular type of a Josephson junction, a superconductor—normal-metal—superconductor junction. We experimentally measure its admittance at typical frequencies used in superconducting circuits. Furthermore, we show theoretically and experimentally that Josephson junctions can be used to realize one of the components aiding in scaling up quantum computers, a magnetic-flux-tunable phase shifter. The main topic of this thesis is bolometry. It is one of the oldest forms of thermal radiation detection invented over a century ago. Yet, it remains as a tool-of-choice in many applications ranging from consumer electronics to particle physics and astronomy. State-of-the-art bolometers have noise equivalent power (NEP) of 300 zW√Hz and can detect single 1.6-THz photons, which corresponds to the energy of 1.1 zJ. However, superconducting quantum computers utilize light at microwave frequencies in their operation. Thermal detectors have yet to reach this frontier due to orders of magnitude lower energy of the microwave photons than far-infrared photons. In this thesis, we bring the performance of bolometers deep into the microwave regime. First, we study a gold-palladium-nanowire-based bolometer. We are able to decrease the NEP record down to 50 zW√Hz. By introducing a Josephson parametric amplifier to the measurement circuitry, we can further improve the NEP to 20 zW√Hz. Together with the low thermal time constant below 100 µs, this result indicates an energy resolution of 0.32 zJ. Second, we investigate graphene as a substitute for gold-palladium. Graphene is a promising candidate for bolometry due to its two-dimensional nature. We find NEP on par with the gold-palladium bolometer. Importantly, the thermal time constant with graphene is about two orders of magnitude shorter, which implies an energy resolution of 20 yJ, corresponding to the energy of a single 30-GHz photon.