Probing Decoherence in Plasmonic Waveguides in the Quantum Regime

We experimentally investigate the decoherence of single surface-plasmon polaritons in metal stripe waveguides. We use a Mach-Zehnder configuration previously considered for measuring decoherence in atomic, electronic, and photonic systems. By placing waveguides of different lengths in one arm, we are able to measure the amplitude damping time T-1 = 1.90 +/- 0.01 x 10(-14) s, pure phase damping time T-2* = 11.19 +/- 4.89 x 10(-14) s and total phase damping time T-2 = 2.83 +/- 0.32 x 10(-14) s. We find that decoherence is mainly due to amplitude damping, and thus loss arising from inelastic electron and photon scattering plays the most important role in the decoherence of plasmonic waveguides in the quantum regime. However, pure phase damping is not completely negligible. The results will be useful in the design of plasmonic waveguide systems for carrying out phase-sensitive quantum applications, such as quantum sensing. The probing techniques developed may also be applied to other plasmonic nano-structures, such as those used as nanoantennas, as unit cells in metamaterials, and as nanotraps for cold atoms.