On-Demand Quantum Storage of Photonic Qubits in an On-Chip Waveguide

Photonic quantum memory is the core element in quantum information processing (QIP). For the scalable and convenient practical applications, great efforts have been devoted to the integrated quantum memory based on various waveguides fabricated in solids. However, on-demand storage of qubits, which is an essential requirement for QIP, is still challenging to be implemented using such integrated quantum memory. Here we report the on-demand storage of time-bin qubits in an on-chip waveguide memory fabricated on the surface of a ${{}^{151}\mathrm{Eu}}^{3+}:{\mathrm{Y}}_2\mathrm{Si}{\mathrm{O}}_5$ crystal, utilizing the Stark-modulated atomic frequency comb protocol. A qubit storage fidelity of $99.3\%\pm 0.2\%$ is obtained with single-photon-level coherent pulses, far beyond the highest fidelity achievable using the classical measure-and-prepare strategy. The developed integrated quantum memory with the on-demand retrieval capability represents an important step toward practical applications of integrated quantum nodes in quantum networks.

Figure 1

(a) Level structure of ${}^{151}{\mathrm{Eu}}^{3+}$ ions at zero magnetic field. (b) Diagram of the experimental setup. The acoustic optic modulators labeled as AOM 1 and AOM 2 are employed to generate the preparation and input beams. The input and preparation beams are combined by a beam splitter (BS) with a reflection-to-transmission ratio of $90:10$. The combined beam is coupled into the waveguide and then collected into a single-mode fiber with a lens group. The mechanical shutter 1 and shutter 2 ensure that the single-photon detector is protected from the strong preparation light. Inset: top view of the on-chip quantum memory under a microscope. Six tracks are fabricated on the sample with a spacing of $23\mu \mathrm{m}$, forming five type IV waveguides. The central one with the minimum insertion loss is employed for the quantum storage. Silver lines provide the electric field for storage time control. FC: Fiber coupler, HWP:half-wave plate.

Figure 2

Spectral-hole burning measurements of the Stark coefficient of the ${{}^7\mathrm{F}}_0$ to ${{}^5\mathrm{D}}_0$ transition of ${}^{151}{\mathrm{Eu}}^{3+}$ ions doped in ${\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal. The prepared absorption peak is split and broadened by the dc electric field. (a) A single class of ions is isolated and the probe laser is swept over a frequency range of 6 MHz. (b) No class cleaning is implemented and the probe laser is swept over a frequency range of 12 MHz.

Figure 3

(a) The storage efficiency as a function of the storage time. The peak width $\gamma$ is fitted as $332\pm 8\mathrm{kHz}$ using Eq. (1). (b) Photon counting histograms of on-demand storage with a weak coherent input of 0.5 photons per pulse. The AFC echo at the first order is completely suppressed after applying the first electric field pulse. After applying the second electric field pulse, the signal can be read out on demand. The histograms in the pink shaded region are enlarged by 5 times.

Figure 4

(a) Sequence for storing, retrieving, and analyzing the time-bin qubits. Inset: illustration of the dynamical phase introduced by the frequency-detuned AFC. (b),(c) The constructive (blue dot) and destructive interference (orange square) fringes for the input state $|e⟩+e^{i\mathrm{\Delta }\alpha }|l⟩$ with a relative phase of (b) $\mathrm{\Delta }\alpha =135°$ and (c) $\mathrm{\Delta }\alpha =90°$. The visibility of $99.6\%\pm 0.2\%$ and $99.1\%\pm 0.3\%$ are obtained, respectively, with an integration window of 100 ns, which is marked by a pair of green dashed lines.