Fiber-coupled broadband quantum memory for polarization-encoded photonic qubits

TL;DR

This work addresses the need for fiber-coupled quantum memories with low loss and high bandwidth for polarization-encoded photonic qubits. The authors develop a hybrid loop-and-switch memory that integrates a fast free-space polarization-preserving switch with a fiber storage loop, achieving a pass-through efficiency of $54.1\%$ and an overall storage-efficiency scaling of $\eta_N \approx 0.5^{N+1}$. They demonstrate high-fidelity storage and retrieval of ultrabroadband single-photon polarization qubits over short and long storage times, and show robustness against polarization degradation via a common-path interferometer. The results suggest practical near-term deployment for quantum networking and provide a roadmap for improving losses and extending operation to telecom wavelengths.

Abstract

Various near-term quantum networking applications will benefit from low-loss, fiber-coupled photonic quantum memory devices with high efficiencies. We demonstrate a fiber-coupled loop-and-switch quantum memory platform with a pass-through efficiency of ~54% and an overall storage efficiency that scales as ~0.5^(N+1) where N is the number of storage cycles. We highlight the trade-off between memory lifetime and qubit accessibility in this platform by using two different storage cycle times of ~40 nanoseconds and ~0.5 microseconds, and demonstrate high-fidelity storage and retrieval of ultra-broadband single-photon polarization qubits in both cases.

Figures (4)

• Figure 1: (a) Conceptual overview of the fiber-coupled loop-and-switch (LAS) quantum memory platform. Incident photonic qubits are stored for a total time of $N \Delta \tau$, where $N$ is the user-controlled number of round trips. (b) Overview of our hybrid implementation of a fiber-coupled LAS memory which uses both free-space (green and blue zones) and fiber (pink zone) components. The storage loop is a single-mode fiber terminated by a retroreflector (RR), while the polarization-insensitive switch is comprised of a Pockels cell (PC) and a polarizing beamsplitter (PBS) set in a Sagnac interferometer. The switch and circulator are both free-space units. The key fiber-to-free-space couplers are labeled $C_1$, $C_2$ and $C_3$.
• Figure 2: (a) Experimental schematic: Heralded single photons from a type-I SPDC source are delivered via single-mode fiber (SMF) into the hybrid LAS memory. The circulator (green free-space zone) consists of Faraday rotators in a polarizing beamsplitter (PBS)-based Mach-Zehnder interferometer while the switch (blue free-space zone) consists of a Pockels cell (PC) in a PBS-Sagnac interferometer. A reconfigurable SMF storage line (pink fiber zone) with two different fiber lengths (short: 0.5 m, long: 50 m) enables a demonstration of flexible memory operation on two different timescales. Heralding of the SPDC idler initiates active user-controlled storage and release driven by a pulse generator. The free-space circulator routes released photonic polarization qubits into an SMF output channel for fully fiber-coupled memory operation. (b) Conceptual overview of the $\ket{H}$ (red arrows) and $\ket{V}$ (blue arrows) paths through the 'figure eight' (F8) common-path interferometer, and the bit flip operations ($\hat{X}_C, \hat{X}_S(t), \hat{X}_{DL}$) associated with each of the three zones. (c) Experimental measurements showing memory efficiencies of roughly $\sim\!50\%$ ($\sim\!44\%$) per cycle for the short (long) storage cases using the $\ket{H}$ input state, with solid (dashed) lines representing fits using Eq. \ref{['eq:eff']}. Additional details are included in the main text.
• Figure 3: Example results from output qubit polarization characterization measurements for $N=3$, corresponding to 109.5 ns and 1.58$~\mu$s of total storage time for the short and long cases respectively. (a) and (b): Normalized Malus' law coincidence counting data for input states of $\ket{H}$ (red) and $\ket{D}$ (blue) for short and long storage line cases, respectively. (c) and (d): Reconstructed single photon density matrices for an input state of $\ket{R}$ for the short and long storage cases, respectively. In each pair of plots, the left and right matrices represents the real and imaginary parts of $\ket{R}$ respectively.
• Figure 4: Summary of output state quality vs. memory storage time for both the short and long storage cases. (a) and (b): Best-fit Malus' law visibilities vs. $N$ for $\ket{H}$ (red) and $\ket{D}$ (blue) input states. (c) and (d): Fidelities vs. $N$ for $\ket{H}$ (red), $\ket{D}$ (blue), and $\ket{R}$ (green) input states. The data demonstrates very little polarization state degradation with increasing storage time.