  Decoherence of quantum states is a major hurdle towards scalable and reliable
quantum computing. Lower decoherence (i.e., higher fidelity) can alleviate the
error correction overhead and obviate the need for energy-intensive noise
reduction techniques e.g., cryogenic cooling. In this paper, we performed a
noise-induced decoherence analysis of single and multi-qubit quantum gates
using physics-based simulations. The analysis indicates that (i) decoherence
depends on the input state and the gate type. Larger number of $|1\rangle$
states worsen the decoherence; (ii) amplitude damping is more detrimental than
phase damping; (iii) shorter depth implementation of a quantum function can
achieve lower decoherence. Simulations indicate 20\% improvement in the
fidelity of a quantum adder when realized using lower depth topology. The
insights developed in this paper can be exploited by the circuit designer to
choose the right gates and logic implementation to optimize the system-level
fidelity.
