Reliable quantum communication over hundreds of kilometers is a daunting yet necessary requirement for a quantum internet. To overcome photon loss, the deployment of quantum repeater stations between distant network nodes is necessary. A plethora of different quantum hardware is being developed for this purpose, each platform with its own opportunities and challenges. Here, we propose to combine two promising hardware platforms in a hybrid quantum repeater architecture to lower the cost and boost the performance of long-distance quantum communication. We outline how ensemble-based quantum memories combined with single-spin photon transducers, which can transfer quantum information between a photon and a single spin, can facilitate massive multiplexing, efficient photon generation, and quantum logic for amplifying communication rates. As a specific example, we describe how a single Rubidium (Rb) atom coupled to nanophotonic resonators can function as a high-rate, telecom-visible entangled photon source with the visible photon being compatible with storage in a Thulium-doped crystal memory (Tm-memory) and the telecom photon being compatible with low-loss fiber propagation. We experimentally verify that the Tm and Rb transitions resonate with each other. Our analysis shows that by employing up to nine repeater stations, each equipped with two Tm-memories capable of holding up to 625 storage modes, along with four single Rb atoms, one can reach a quantum communication rate of about 10 secret bits per second across distances of up to 1000 km.

Extended quantum networks are based on quantum repeaters that often rely on the distribution of entanglement in an efficient and heralded fashion over multiple network nodes. Many repeater architectures require multiplexed sources of entangled photon pairs, multiplexed quantum memories, and photon detection that distinguishes between the multiplexed modes. Here we demonstrate the concurrent employment of (1) spectrally multiplexed cavity-enhanced spontaneous parametric down-conversion in a nonlinear crystal; (2) a virtually-imaged phased array that enables mapping of spectral modes onto distinct spatial modes for frequency-selective detection; and (3) a cryogenically-cooled Tm³⁺:LiNbO₃ crystal that allows spectral filtering in an approach that anticipates its use as a spectrally-multiplexed quantum memory. Through coincidence measurements, we demonstrate quantum correlations between energy-correlated photon pairs and a strong reduction of the correlation strength between all other photons. This constitutes an important step towards a frequency-multiplexed quantum repeater.

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.

Here, we discuss our experimental efforts toward building an alignment-free, long-lived, and efficient cavity-enhanced quantum memory in a thulium-doped crystal. A significant step forward for creating efficient quantum memories with long optical storage times.

The creation of well-understood structures using spectral hole burning is an important task in the use of technologies based on rare-earth ion-doped crystals. We apply a series of different techniques to model and improve the frequency dependent population change in the atomic level structure of thulium yttrium gallium garnet (Tm:YGG). In particular we demonstrate that, at zero applied magnetic field, numerical solutions to frequency-dependent three-level rate equations show good agreement with spectral hole-burning results. This allows us to predict spectral structures given a specific hole-burning sequence, the underpinning spectroscopic material properties, and the relevant laser parameters. This enables us to largely eliminate power-dependent hole broadening through the use of adiabatic hole-burning pulses. Although this system of rate equations shows good agreement at zero field, the addition of a magnetic field results in unexpected spectral diffusion proportional to the induced Tm ion magnetic-dipole moment and average magnetic-field strength, which, through the quadratic Zeeman effect, dominates the optical spectrum over long timescales. Our results allow optimization of the preparation process for spectral structures in a large variety of rare-earth ion-doped materials for quantum memories and other applications.

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.

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.

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.

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.

The geometric and dynamic phases have competing effects as far as the scattering of light from an inhomogeneous anisotropic optical medium is concerned. If fine-tuned appropriately, these effects can completely cancel each other for a chosen spin component while having an additive effect on the orthogonal component. Here, we show a manifestation of extraordinary spin-selective modes in the Fourier spectrum of a Gaussian beam transmitted through an anisotropic disordered medium. We realize the concept by using a twisted nematic liquid-crystal-based spatial light modulator with random gray-level distributions for an incident Gaussian beam.

Quantum communication is a secure way to transfer quantum information and to communicate with legitimate parties over distant places in a network. Although communication over a long distance has already been attained, technical problem arises due to unavoidable loss of information through the transmission channel. Quantum repeaters can extend the distance scale using entanglement swapping and purification scheme. Here we demonstrate the working of a quantum repeater by the above two processes. We use IBM’s real quantum processor ‘ibmqx4’ to create two pair of entangled qubits and design an equivalent quantum circuit which consequently swaps the entanglement between the two pairs. We then develop a novel purification protocol which enhances the degree of entanglement in a noisy channel that includes combined errors of bit-flip, phase-flip and phase-change error. We perform quantum state tomography to verify the entanglement swapping between the two pairs of qubits and working of the purification protocol.