Realization of Reliable Solid-State Quantum Memory for Photonic Polarization Qubit

Faithfully storing an unknown quantum light state is essential to advanced quantum communication and distributed quantum computation applications. The required quantum memory must have high fidelity to improve the performance of a quantum network. Here we report the reversible transfer of photonic polarization states into collective atomic excitation in a compact solid-state device. The quantum memory is based on an atomic frequency comb (AFC) in rare-earth ion-doped crystals. We obtain up to 0.999 process fidelity for the storage and retrieval process of single-photon-level coherent pulse. This reliable quantum memory is a crucial step toward quantum networks based on solid-state devices.

Figure 1

(a) Illustration of the sample used as the memory hardware for arbitrary polarizations. The arrows represent the $c$ axis of the crystals. Details about the sample are given in the Supplemental Information [26]. (b) The near 880 nm input photons are absorbed at the ${}^{4}I_{9/2}\to {}^{4}F_{3/2}$ transition of ${\mathrm{Nd}}^{3+}$. This transition is strongly $H$ polarized. After a programmable time, the photon will be collectively emitted in a desirable spatial mode. (c) The preparation and storage sequence used in the experiment.

Figure 2

The experimental setup for quantum storage of a photonic polarization-qubit in solids. The upper acousto-optic modulators (AOM) produces pump light for the AFC preparation. The lower AOM produces a weak probe light for the storage sequence. The light’s polarizations are initialized by the polarization beam splitters (PBS). The probe light’s polarizations are controlled by the half-wave plate (${\mathrm{HWP}}_1$) and quarter-wave plate (${\mathrm{QWP}}_1$). The samples are placed in a cryostat at a temperature of 1.5 K. Beyond the sample, a phase plate ($\theta$) and ${\mathrm{HWP}}_4$ corrects any polarization rotation caused by the sample. The probe light’s polarizations are then analyzed with ${\mathrm{QWP}}_2$, ${\mathrm{HWP}}_5$, and a Wollaston prism. The two MCs are used to protect the single-photon detector (SPD) from classical pump pulses.

Figure 3

(a) With an AFC prepared with a periodicity of 5 MHz, the $H+V$ polarized single-photon pulses are collectively reemitted after a 200 ns storage time in the sample. (b) The real part numbers in the process matrix $\chi$ as obtained from a quantum process tomography. All of the imaginary numbers are close to zero, with the largest amplitude being 0.017.

Figure 4

The storage efficiency as a function of storage time. The storage sequence is repeated 400 times at a frequency of 400 kHz.