Entanglement-enabled decoherence-free transmission of two-color photons through a single mode fiber

We demonstrate decoherence-tolerant transmission of a Bell state through a singlemode fiber (SMF) using the photon frequency degree of freedom. To this end, a slightly nondegenerate polarization-entangled singlet, |'F-? = (|HV? - |VH?)/ 2, is localized at the SMF as the depolarization channel subject to random noise due to incessant fiber bending. Two-photon beats and quantum state tomography jointly verify the absence of collective decoherence, showing that |'F-? is the sought-after one-dimensional decoherence-free subspace (DFS) pertaining to SMF. Efficient splitting and combining of photon streams in our DFS help outperform the DFS in time domain. This motivates us to attempt DFS-enabled fault-tolerant fiber transmission of biphoton qubits. Two-photon BB84 protocol is implemented in a polarization-maintaining fiber to which dephasing noise is relevant so that a two-dimensional DFS is appropriate. A low bit error rate 5.4% is achieved by encoding one-qubit information onto the biphoton state in spite of significant polarization fluctuation. Our scalable frequency-based DFS has a natural affinity for wavelength division multiplexing in fiber communication by design and as such is extensible to multi-particle entanglement.