Design and modeling of a cryogenic system for circuit QED experiments

Brian Vlastakis, Gerhard Kirchmair, Hanhee Paik +5 more

Gerhard Kirchmair, Brian Vlastakis, Hanhee Paik +5 more

Jinsu Son, Gahyun Choi, Taewan Noh +9 more

A transmon qubit in the 3-dimensional microwave cavity is a versatile system for various circuit QED experiments. We have performed numerical calculations of the vacuum Rabi coupling g in 3D transmon circuit QED system, and compared the values with the experimental measurements. We used COMSOL Multiphysics RF package with an appropriate model to efficiently calculate the coupling strength. The calculation agrees with the measurement within a few percents in simple cases. Depending on the position of the qubit in the cavity, we calculated the coupling strengths to different eigenmodes. We compared our result for the fundamental mode (TE101) and a higher mode (TE201) with the experiment by changing the qubit position, as shown in Fig. 2. This simulation and characterization enable efficient design methods for future 3D circuit QED experiments. We confirm that we can design and predict the qubit coupling to the cavity modes with enough accuracy in 3D circuit QED system.

Grzegorz H. Kasprowicz

Michael Peterer

Superconducting qubits are a promising technology for building a scalable quantum computer. An important architecture employed in the field is called Circuit Quantum Electrodynamics (circuit QED), where such qubits are combined with high quality microwave cavities to study the interaction between artificial atoms and single microwave photons. The ultra-strong coupling achieved in these systems allows for control and readout of the quantum state of qubits to perform quantum information processing. The work on circuit QED performed in this thesis consisted of realizing an experimental setup for qubit experiments in a new laboratory, investigating the coherence and decay of higher energy levels of superconducting transmon qubits and finally demonstrating a novel coaxial form of circuit QED. Designing and building a 3D circuit QED setup involved the following main accomplishments: producing high quality 3D cavities; designing and installing the cryogenic microwave setup as well as the room temperature amplification and data acquisition circuitry; successfully developing a recipe for the fabrication of Josephson junctions; controlling and measuring superconducting 3D transmon qubits at 10mK. Several qubits were fully characterised and have shown coherence times of several microseconds and relaxation times up to 25μs. Superconducting qubits in fact possess higher energy levels that can provide significant computational advantages in quantum information applications. In experiments performed at MIT, preparation and control of the five lowest states of a transmon qubit was demonstrated, followed by an investigation of the phase coherence and decay dynamics of these higher energy levels. The decay was found to proceed mainly sequentially with relaxation times in excess of 20μs for all transitions. A direct measurement of the charge dispersion of these levels was performed to explore their characteristics of dephasing. This experiment was also reproduced on a 3D transmon fabricated and measured in Oxford, where due to a higher effective qubit temperature a multi-level decay model including thermal excitations was developed to explain the observed relaxation dynamics. Finally, a coaxial transmon, which we name the coaxmon, is presented and measured with a coaxial LC readout resonator and input/output coupling ports placed inline along the third dimension. This novel coaxial circuit QED architecture holds great promise for developing a scalable planar grid of qubits to build a quantum computer.

Yu‐qiang Liu, Yi‐jia Yang, Chang‐shui Yu

Precisely controlling heat transfer in a quantum mechanical system is particularly significant for designing quantum thermodynamical devices. With the technology of experiment advances, circuit quantum electrodynamics (circuit QED) has become a promising system due to controllable light-matter interactions as well as flexible coupling strengths. In this paper, we design a thermal diode in terms of the two-photon Rabi model of the circuit QED system. We find that the thermal diode can not only be realized in the resonant coupling but also achieve better performance, especially for the detuned qubit-photon ultrastrong coupling. We also study the photonic detection rates and their nonreciprocity, which indicate similar behaviors with the nonreciprocal heat transport. This provides the potential to understand thermal diode behavior from the quantum optical perspective and could shed new insight into the relevant research on thermodynamical devices.