Protecting Quantum Modes in Optical Fibers

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 an alternative 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 fiber, we experimentally demonstrate that using appropriate nonorthogonal modes, the polarization-preserving fiber does not fully scramble these modes over the full Poincare 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 allows for more efficient measurements of quantum excitations in nonorthogonal modes.

Automated summary

Polarization-preserving fibers cannot preserve two nonorthogonal states required for quantum communication. We propose an alternative scheme using helically twisted photonic crystal fibers to enable polarization encoding in the continuous-variable regime. Experimental results show that appropriate nonorthogonal modes in the polarization-preserving fiber do not fully scramble over the full Poincare sphere, but remain on a great circle, forming a one-dimensional protected subspace that can be parametrized by a single variable. This allows for more efficient measurements of quantum excitations in nonorthogonal modes.