Fig. 1. Characterization of the conjoined-tube fiber (CTF) and experimental setup for polarization quantum state transmission. (a) SEM image of the CTF. (b) Cut-back measured loss spectrum. The minimum loss and quantum transmission wavelength are marked by the purple circle and blue dashed line, respectively. (c) Simulated phase birefringence and PDL based on cross-section micrographs. (d) Experimental setup consisting of a heralded single-photon source (upper left), polarization entangled photon-pair source (lower left), and fiber-testing bench. BBO1/BBO2, β-barium-borate crystals for frequency doubling and generation of entangled photon pairs, respectively; KDP, potassium dihydrogen phosphate crystal for generation of single photons; HWP, half-wave plate; QWP, quarter-wave plate; PBS, polarization beam splitter with an extinction ratio of over 30 dB; polarizer, linear polarizer with an extinction ratio of over 40 dB.

Fig. 2. Experimental results of single-photon polarization state transmission. (a) Fidelities and purities of the six transmitted polarization states. The average fidelity and purity are ∼0.980 and ∼0.998, respectively. Error bars represent one standard deviation. (b) Process matrix of the 36.4 m CTF link, suggesting a fidelity of 0.983 with respect to an identity process matrix.

Fig. 3. Experimental results of single-photon polarization state transmission. (a) Polarization correlations between the local and transmitted photons. The visibilities in the H/V and D/A bases are measured to be 97.1% and 97.8%, respectively. (b) Density matrix of the distributed entangled state. The fidelity with respect to the initial state is 0.977.

Fig. 4. Characterization of chromatic dispersion (CD), group index, and single-photon latency. (a) Measured CDs and group indices of a 20 cm long CTF and a 5 cm long SMF using a free-space Mach–Zehnder interferometer. δ=0.466±0.003. (b) Relative time delay of single photons transmitting through the CTF and SMF with length of 36.4±0.1 m. The timing jitter induced uncertainty is estimated to be 0.6 ns.

Fig. 5. Simulated MF overlapping coefficients with (a) glass and (b) interface for our CTF. The fundamental core modes with the two polarizations are calculated.

Fig. 6. Simulated (a) effective indices and (b) confinement losses of different orders of modes.

Fig. 7. Comparison of experimentally measured and numerically calculated losses of our CTF.

Fig. 8. (a) Measured fidelities and purities of the six single-photon polarization states after transmission through a 36.4 m SMF. Error bars represent one standard deviation. (b) Process matrix (χmn) of the SMF.

Fig. 9. (a) Measured fidelities and purities of the six single-photon polarization states after a 65.5 m CTF using weak coherent source. Error bars represent one standard deviation. (b) Process matrix (χmn) of the CTF.