Rare-earth ion-doped crystals are of great interest for quantum memories, a central component in future quantum repeaters. To assess the promise of 1 % Tm ³⁺-doped yttrium gallium garnet (Tm:YGG), we report measurements of optical coherence and energy-level lifetimes of its ³H 6 ↔ 3 H ₄ transition at a temperature of around 500 mK and various magnetic fields. Using spectral hole burning (SHB), we find hyperfine ground-level (Zeeman level) lifetimes of several minutes at magnetic fields of less than 1000 G. We also measure coherence time exceeding one millisecond using two-pulse photon echoes. Three-pulse photon echo and SHB measurements reveal that due to spectral diffusion, the effective coherence time reduces to a few µs over a timescale of around two hundred seconds. Finally, temporal and frequency-multiplexed storage of optical pulses using the atomic frequency comb protocol is demonstrated. Our results suggest Tm:YGG to be promising for multiplexed photonic quantum memory for quantum repeaters.

Long optical storage times are an essential requirement to establish high-rate entanglement distribution over large distances using memory-based quantum repeaters. Rare earth ion-doped crystals are arguably well-suited candidates for building such quantum memories. Toward this end, we investigate the 795.32 nm ³H₆ ↔ ³H₄ transition of 1% thulium-doped yttrium gallium garnet crystal (Tm³⁺:Y₃Ga₅O₁₂ : Tm³⁺:YGG). Most essentially, we find that the optical coherence time can reach 1.1 ms, and, using laser pulses, we demonstrate optical storage based on the atomic frequency comb (AFC) protocol up to 100 µs. In addition, we demonstrate multiplexed storage, including feed-forward selection, shifting, and filtering of spectral modes, as well as quantum state storage using members of non-classical photon pairs. Our results show that Tm:YGG can be a potential candidate for creating multiplexed quantum memories with long optical storage times.

In this work, we fabricate a multimode quantum memory out of a thulium-doped crystal and demonstrate storage of laser pulses of up to 100 µsec. A significant step forward for creating quantum memories with long optical storage times.

We characterize the magnetic properties for thulium ion energy levels in the (Tm:YGG) lattice with the goal to improve decoherence and reduce linewidth broadening caused by local host spins and crystal imperfections. More precisely, we measure hyperfine tensors for the lowest level of and excited states using a combination of spectral hole burning, absorption spectroscopy, and optically detected nuclear magnetic resonance. By rotating the sample through a series of angles with an applied external magnetic field, we measure and analyze the orientation dependence of the ion's spin Hamiltonian. Using this spin Hamiltonian, we propose a set of orientations to improve material properties that are important for light-matter interaction and quantum information applications. Our results yield several important external field directions: some to extend optical coherence times, another to improve spin inhomogeneous broadening, and yet another that maximizes mixing of the spin states for specific sets of ions, which allows improving optical pumping and creation of lambda systems in this material.

A proposal for fast-switching broadband frequency-shifting technology making use of frequency conversion in a nonlinear crystal is set forth, whereby the shifting is imparted to the converted photons by creating a bank of frequency-displaced pump modes that can be selected by a photonic switch and directed to the nonlinear crystal. Proof-of-principle results show that the expected frequency-shifting operation can be achieved. Even though the dimensions of the currently employed crystal and significant excess loss in the experimental setup prevented conversion of single-photon-level inputs, thorough experimental and theoretical analysis of the noise contribution allowed for estimation of the system performance in an optimized scenario, where the expected signal-to-noise ratio (SNR) for single-photon conversion and frequency shifting can reach up to 25 dB with proper narrowband filtering and state-of-the-art devices. The proposed frequency-shifting solution figures as a promising candidate for applications in frequency-multiplexed quantum repeater architectures with 25 dB output SNR (with 20% conversion efficiency) and capacity for 16 channels spread around a 100 GHz spectral region.

Realization of a globe-spanning quantum network is a current worldwide goal, where near and long term implementations will benefit from connectivity between platforms optimized for specific tasks. Towards this goal, a quantum network architecture is herewith proposed whereby quantum processing devices based on NV- colour centers act as quantum routers (QR) and, between which, long-distance entanglement distribution is enabled by spectrally-multiplexed quantum repeaters based on absorptive quantum memories in rare-earth ion-doped crystals and imperfect entangled photon-pair sources. The inclusion of a quantum buffer structure between repeaters and routers is shown to, albeit the increased complexity, improve the achievable entanglement distribution rates in the network. Although the expected rate and fidelity results are presented for a simple linear network (point-to-point), complex topologies are compatible with the proposed architecture through the inclusion of an extra layer of temporal multiplexing in the QR's operation. Figures of merit are extracted based on parameters found in the literature for near-term scenarios and attest the availability of the proposed buffered-router-assisted frequency-multiplexed automated repeater chain network.

We argue that long optical storage times are required to establish entanglement at high rates over large distances using memory-based quantum repeaters. Triggered by this conclusion, we investigate the 795.325 nm3 H6↔H34 transition of Tm:Y3Ga5O12 (Tm:YGG). Most importantly, we find that the optical coherence time can reach 1.1 ms, and, using laser pulses, we demonstrate optical storage based on the atomic frequency comb protocol during up to 100 μs as well as a memory decay time Tm of 13.1 μs. Possibilities of how to narrow the gap between the measured value of Tm and its maximum of 275 μs are discussed. In addition, we demonstrate multiplexed storage, including with feed-forward selection, shifting and filtering of spectral modes, as well as quantum state storage using members of nonclassical photon pairs. Our results show the potential of Tm:YGG for creating multiplexed quantum memories with long optical storage times, and open the path to repeater-based quantum networks with high entanglement distribution rates.

Entangling quantum systems with different characteristics through the exchange of photons is a prerequisite for building future quantum networks. Proving the presence of entanglement between quantum memories for light working at different wavelengths furthers this goal. Here, we report on a series of experiments with a thulium-doped crystal, serving as a quantum memory for 794-nm photons, an erbium-doped fiber, serving as a quantum memory for telecommunication-wavelength photons at 1535 nm, and a source of photon pairs created via spontaneous parametric down-conversion. Characterizing the photons after re-emission from the two memories, we find nonclassical correlations with a cross-correlation coefficient of g12(2)=53±8; entanglement preserving storage with input-output fidelity of FIO≈93±2%; and nonlocality featuring a violation of the Clauser-Horne-Shimony-Holt Bell inequality with S=2.6±0.2. Our proof-of-principle experiment shows that entanglement persists while propagating through different solid-state quantum memories operating at different wavelengths.

Long-lived sub-levels of the electronic ground-state manifold of rare-earth ions in crystals can be used as atomic population reservoirs for photon echo-based quantum memories. We measure the dynamics of the Zeeman sublevels of erbium ions that are doped into a lithium niobate waveguide, finding population lifetimes at cryogenic temperatures down to 0.7 K as long as seconds. Then, using these levels, we prepare and characterize atomic frequency combs (AFCs), which can serve as a memory for quantum light at 1532 nm wavelength. The results allow predicting a 0.1% memory efficiency, limited mainly by unwanted background absorption that we believe to be caused by excitation-induced erbium spin flips and frequency shifting due to two-level systems or non-equilibrium phonons. Hence, while it should be possible to create an AFC-based quantum memory in Er^{3 +}:Ti^{4 +}:LiNbO₃, improved crystal growth together with optimized AFC preparation will be required to make it suitable for applications in quantum communication.

Large-scale fiber-based quantum networks will likely employ telecommunication-wavelength photons of around 1550 nm wavelength to exchange quantum information between remote nodes, and quantum memories, ideally operating at the same wavelength, that allow the transmission distances to be increased, as key elements of a quantum repeater. However, the development of a suitable memory remains an ongoing challenge. Here, we demonstrate the storage and reemission of single heralded 1532-nm-wavelength photons using a crystal waveguide. The photons are emitted from a photon-pair source based on spontaneous parametric down-conversion and the memory is based on an atomic frequency comb of 6 GHz bandwidth, prepared through persistent spectral-hole burning of the inhomogeneously broadened absorption line of a cryogenically cooled erbium-doped lithium niobate waveguide. Despite currently limited storage time and efficiency, this demonstration represents an important step toward quantum networks that operate in the telecommunication band and the development of integrated (on-chip) quantum technology using industry-standard crystals.