arXiv:2305.11220
Protecting quantum modes in optical fibres
Muhammad Abdullah Butt, Paul Roth, George K. Wong, M. H. Frosz, Luis Lorenzo Sánchez-Soto, Elena A. Anashkina, Alexey V. Andrianov, P. Banzer, Philip St.J. Russell, Gerd Leuchs
Abstract
Polarization-preserving fibers maintain the two polarization states of an orthogonal basis. Quantum communication, however, requires sending at least two nonorthogonal states and these cannot both be preserved. We present a new scheme that allows for using polarization encoding in a fiber not only in the discrete, but also in the continuous-variable regime. For the example of a helically twisted photonic-crystal fibre, we experimentally demonstrate that using appropriate nonorthogonal modes, the polarization-preserving fiber does not fully scramble these modes over the full Poincaré sphere, but that the output polarization will stay on a great circle; that is, within a one-dimensional protected subspace, which can be parametrized by a single variable. This will allow for more efficient measurements of quantum excitations in nonorthogonal modes.