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We demonstrate decoherence-tolerant transmission of a Bell state through a single-mode fiber (SMF) using the photon frequency degree of freedom. To this end, a slightly non-degenerate polarization-entangled singlet, | Ψ - ⟩=(|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 | Ψ - ⟩ 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.

Ghost imaging (GI) allows retrieval of the images of an uninterrogated object nonlocally based on cross correlation of photons. Central to GI is the integration of sparse detection events, i.e., bucket detection, even in the time domain. Here we report temporal single-pixel imaging of a non-integrating class as a viable GI variant that obviates the need for constant vigil. Mere division of the otherwise distorted waveforms by the known impulse response function of the detector makes the corrected waveforms readily available. This tempts us to use slow and as such less costly commercially available optoelectronic devices like light-emitting diodes and solar cells for imaging purposes on a one-time readout basis.