Photonic qubit encoding interconversion for heterogeneous quantum networking

Abstract

Quantum information processing, communication, and sensing networks are being developed with various qubit platforms that use different encoding schemes. Connecting quantum network nodes to distribute entanglement requires matching photon qubit basis encoding. In this work, we implement an interconversion protocol which converts photon qubit encoding from the polarization basis to the time-bin basis, transmits the photons through a transport fiber with large fluctuations in polarization, and converts back to polarization encoding for ease of measurement. This interconversion scheme faithfully transmits a polarization Bell state across the transport fiber by converting sources of infidelity to changes in transmission rate. These results illustrate a practical approach for interfacing distinct qubit platforms to enable modular and flexible operation in heterogeneous quantum networks.

Figures (6)

• Figure 1: Representation of a heterogeneous quantum network and photonic qubit interconversion protocol. (a) Four example qubit platforms supporting varied photonic qubit basis encodings can interfere with each other via photon encoding interconversion. Additionally, encoding interconversion can be used to convert polarization encoding to time-bin encoding, suitable for long haul photon transmission. Time-bin qubits are converted back to polarization basis before detection for ease of measurement, as shown with photons from the ion trap node. (b) Idler photons from a polarization Bell state produced from a PIC source are sent through interconversion modules connected via a transport fiber. Signal photons are routed directly to a two qubit QST module for analysis.
• Figure 2: Diagram for polarization Bell state generation using PIC. Amplified pump light is coupled to the chip and distributed via cascaded $1 \times 2$ splitters. Spectrally entangled biphotons generated by SFWM from two spiral waveguides, having the state $\ket{H_sH_i}$, are combined using an FPBS. The relative phase between the spirals is stabilized to generate the Bell state $\ket{\phi^+}$ (Eq. \ref{['eq:bell_state_spiral']}). To increase Bell state fidelity, we balance attenuation between both arms of the interferometer by controlling the amount of light that gets coupled to the output of the FPBS using manual polarization controllers (MPC).
• Figure 3: Schematic for polarization to time-bin conversion and photon coincidence results. (a) An FPBS separates $\ket{H}$ and $\ket{V}$ polarized photons. $\ket{V}$ polarized photons are sent through a 10.2m delay line to add extra path length relative to $\ket{H}$ photons, thereby creating time-bins separated by 50. The two paths are recombined using an FBS, with MPCs in both arms to make the polarizations of both the time-bins the same. Active control to the FPS stabilizes the interferometric phase. A VOA equalizes attenuation across both arms, minimizing polarization dependent loss. (b) Early ($\ket{e}$) and late ($\ket{l}$) time-bins separated by 50 observed by taking coincidences of signal and idler H (early) and V (late) photons.
• Figure 4: Schematic for time-bin to polarization conversion and photon coincidence results. (a) The FPBS allows a fixed polarization through to the interconversion module. An FBS separates photons into two paths having a relative path length of 10.2m, which are combined using an FPBS to convert back to polarization basis, with MPCs in both arms ensuring maximum transmission through the final FPBS. (b) Plot showing the three arrival times of the time-bin encoded idler photons that pass through a transport fiber and then the time-bin to polarization interconversion module. Using an FBS distributes both $\ket{e}$ and $\ket{l}$ photons to the two arms of the AMZI. By post-selecting photons traversing a combination of short and long paths (indicated by the middle peak), we complete the time-bin to polarization conversion and retrieve the polarization qubit.
• Figure 5: Fidelity and transmission rate variation for time-bin and polarization encoded photons. (a) Coincidence counts (normalized) of signal and idler photons as polarization in the transport fiber is perturbed by rotating the HWP in the MPP by 1° every 5. Idler photons passing through the interconversion modules (connected via transport fiber) show huge changes in the transmission rate while the fidelity remains fairly constant (0.94 average fidelity), whereas idler photons passing through only the transport fiber show reduction in fidelity as the polarization changes while the coincidence rate remains fairly constant. (b) Each fidelity measurement is plotted against total coincidences, and a linear fit gives a slope of $8.7\times 10^{-6}$, signifying minimal dependence of fidelity on photon transmission rate.