Research

Spontaneous Emission Rate Modeling of Transmon Qubits

Transmon qubits are one of the leading candidates being pursued to achieve various forms of quantum computation. These qubits consist of superconducting Josephson junctions that are embedded in planar microwave circuitry, such as coplanar waveguides. Generally, the transmon qubit will be coupled to a number of microwave resonators in order to control the qubit, read out its state, and drive qubit-qubit interactions. This requires a careful balance between providing control of the qubit and protecting its fragile quantum state. One of the primary mechanisms that can destroy the qubit's state is by spontaneously emitting its excitation into the many microwave transmission lines around the qubit. Hence, modeling the spontaneous emission rate for practical qubit implementations can provide valuable information about the performance limitations of a device before fabrication.

However, the commonly used theoretical models for analyzing transmon qubits are not conducive to rigorously determining the spontaneous emission rate of practical devices. Further, the detailed numerical electromagnetic analysis of realistic devices can also be challenging due to the strongly multiscale nature of these devices; which have important features sizes that range from sub-micrometer to multiple centimeters in length. We are approaching this problem by developing a more general theoretical model for the interaction of a transmon qubit with electromagnetic fields. With this generalized model, we can show that the spontaneous emission rate of the transmon qubit can be computed using completely classical computational electromagnetics methods. We are now leveraging potential-based time domain integral equation solvers we developed that are well-suited to analyzing multiscale devices to compute the spontaneous emission rate of practical devices.