  High-fidelity quantum gate operations are essential for achieving scalable
quantum circuits. In spin qubit quantum computing systems, metallic gates and
antennas which are necessary for qubit operation, initialization, and readout,
also cause detriments by enhancing fluctuations of electromagnetic fields.
Therefore evanescent wave Johnson noise (EWJN) caused by thermal and vacuum
fluctuations becomes an important unmitigated noise, which induces the decay of
spin qubits and limits the quantum gate operation fidelity. Here, we first
develop a quantum electrodynamics theory of EWJN. Then we propose a numerical
technique based on volume integral equations to quantify EWJN strength in the
vicinity of nanofabricated metallic gates with arbitrary geometry. We study the
limits to two spin-qubit gate fidelity from EWJN-induced relaxation processes
in two experimentally relevant quantum computing platforms: (a) silicon quantum
dot system and (b) NV centers in diamond. Finally, we introduce the Lindbladian
engineering method to optimize the control pulse sequence design and show its
enhanced performance over Hamiltonian engineering in mitigating the influence
of thermal and vacuum fluctuations. Our work leverages advances in
computational electromagnetics, fluctuational electrodynamics and open quantum
systems to suppress the effects of thermal and vacuum fluctuations and reach
the limits of two-spin-qubit gate fidelity.
