Long-lived storage of single photons under the form of atomic excitations is at the foundation of long-distance entanglement distribution in quantum networks. To mitigate decoherence effects induced by the environment, rephasing of the hyperfine coherences using microwave pulses have been implemented in a variety of single-emitter and ensemble-based solid-state systems. However, the demonstration of storage of single photons in an absorptive quantum memory including such spin rephasing mechanism remains elusive. In this work, we show non-classical storage of telecom-heralded single photons in a Pr^3+:Y_2SiO_5 rare-earth ion doped crystal quantum memory using the atomic frequency comb (AFC) spin-wave protocol combined with a XY4 spin rephasing sequence. Long-lived AFC photon echoes are first observed in the classical regime for storage times of up to approximately 3 ms. We then demonstrate non-classical correlations between heralding photons and stored signal photons generated by a cavity-enhanced parametric photon-pair source for storage times of up to 180 μs and with measured cross-correlation values as high as 4.6(4). Together with the capacity of Pr^3+:Y_2SiO_5 QMs to support highly efficient and multiplexed storage, this result represents a significant step towards scalable long-distance quantum repeater links.

A promising platform for quantum information research relies on cavity coupled atomic spin-waves, enabling efficient operations such as quantum memories, quantum light generation and entanglement distribution. In this work, we study the strong coupling between non-classical collective spin excitations generated by Raman scattering in a cold 87Rb atomic ensemble, and a single cavity mode. We report on an intracavity spin wave to single photon conversion efficiency of up to chi=0.75 +/- 0.02 in the quantum domain, as evidenced by a violation of the Cauchy-Schwarz inequality. Our work establishes a relationship between the retrieval of an atomic spin wave in the non-classical regime and the vacuum Rabi splitting. We show that this relationship emerges within the efficiency spectrum, and we finally provide the optimal operational conditions to achieve the maximum intrinsic retrieval efficiency. Our data is well reproduced by simulations based on optical Bloch equations. This work deepens the understanding of cavity-enhanced spin wave readout and its potential applications.

In the NISQ era, there is a need for resource-efficient proof-of-principle experiments that can be built up to genuine utility. Single-qubit classifiers (SQCs) are small-scale hybrid quantum-classical machines capable of performing a basic machine learning task: classifying data. In principle, these can be scaled up to many-qubit quantum classifiers capable of quantum computational advantage. Another type of quantum advantage is enabled by blind quantum computation (BQC), wherein a client may run delegated quantum computations on an untrusted server with information-theoretic security. In this paper, we develop a framework and propose a prototype experiment for a SQC where it is known to the server that a classification is being performed, but the data and outcome stay hidden, i.e., it performs partially-blind SQC (PB-SQC). This can be integrated into a quantum network to deliver quantum-secured classifications to remote clients; we study this for a heterogeneous quantum network link in which entanglement is shared between a server and a client equipped with a multiplexed solid-state quantum memory using entanglement swapping. The framework we develop for PB-SQC on this setup is tested in a simulation with realistic hardware parameters on a real-world credit card transaction fraud database with classification outcomes approaching those of its equivalent classical deep-belief network. In addition, we show how a two-qubit classifier (TQC) instead of a SQC enables verification of the computation. These results pave the way towards a short- to mid-term quantum network offering use-case-ready quantum applications.

In this talk, we demonstrate storage of heralded single photons in a dynamically decoupled spin-wave quantum memory for up to 180 microseconds and on-demand telecom-heralded entanglement of two memories with up to 15 temporal modes.

We report on the realization of an efficient solid-state spin-wave quantum memory, with on-demand readout, using the full atomic frequency comb (AFC) scheme in a Pr^{3+}:Y_{2}SiO_{5} crystal embedded in an impedance-matched cavity. We demonstrate operation at the single-photon level by storing weak coherent states with an efficiency up to (40±2)% and a signal-to-noise ratio of 14 for an input photon number of 0.42 photons per pulse. We also investigated the enhancement of the incoherent noise due to the impedance-matched cavity and characterized the quantum memory performance, showing a two-way transfer from excited to spin states and back of up to 83%. Finally, we confirmed the quantum nature of our memory by storing nonclassical states of light, i.e., a heralded single photon from a nondegenerate spontaneous parametric down-conversion source, and achieved nonclassical correlations between the heralding and the stored-and-retrieved photon. These results demonstrate that impedance-matched AFC spin-wave quantum memories with on-demand readout can be used for experiments involving the storage of photonic quantum states. They also open the door to solid-state on-demand quantum memories with very high efficiencies, serving as a key resource for quantum networks and quantum repeaters.

Europium-doped nanocrystals constitute a promising material for a scalable future quantum computing platform. Long-lived nuclear spin states could serve as qubits addressed via coherent optical transitions. In order to realize an efficient spin-photon interface, we couple the emission from a single nanoparticle to a fiber-based microcavity under cryogenic conditions. The spatial and spectral tunability of the cavity permits us to place individual nanoparticles in the cavity, to measure the inhomogeneous linewidth of the ions, and to show a multi-modal Purcell-enhancement of two transition in Eu3+. A halving of the free-space lifetime to 1.0 ms is observed, corresponding to a 140-fold enhancement of the respective transition. Furthermore, we observe a narrow optical linewidth of 3.3 MHz for a few-ion ensemble in the center of the inhomogeneous line. The results represent an important step towards the efficient readout of single Eu3+ ions, a key requirement for the realization of single-ion-level quantum processing nodes in the solid state.

The exploitation of multimodality in different degrees of freedom is one of the most promising ways to increase the rate of heralded entanglement between distant quantum nodes. In this paper, we realize a spatially-multiplexed solid-state quantum memory array with ten individually controllable spin-wave memory cells featuring on-demand read-out and temporal multiplexing. By combining spatial and temporal multiplexing, we store weak coherent pulses at the single-photon level in up to 250 spatio-temporal modes, with an average signal-to-noise ratio of 10(2). We perform a thorough characterization of the whole system, including its multiplexing and demultiplexing stage. We verify that the memory array exhibits low cross-talk even at the single-photon level. The measured performance indicates readiness for storing non-classical states and promises a speed-up in entanglement distribution rates.

We present an architecture for remotely connecting cavity-coupled trapped ions via a quantum repeater based on rare-earth-doped crystals. The main challenge for its realization lies in interfacing these two physical platforms, which produce photons with a typical temporal mismatch of one or two orders of magnitude. To address this, we propose an efficient protocol that enables custom temporal reshaping of single-photon pulses while preserving purity. Our approach is to modify a commonly used memory protocol, called atomic frequency comb, for systems exhibiting inhomogeneous broadening like rare-earth-doped crystals. Pertaining to a growing interest in hybrid quantum information systems designed to exploit the distinct advantages of diverse physical components, our results offer a viable solution for uniting quantum processing nodes with a quantum repeater backbone.

Quantum frequency conversion is a widely used technique to interface atomic systems with the telecom band in order to facilitate propagation over longer distances in fiber. Here, we demonstrate the difference-frequency conversion from 606 nm to 1552 nm of microsecond-long weak coherent pulses at the single-photon level compatible with Pr3+:Y2SiO5 quantum memories, with a high signal-to-noise ratio (SNR). We use a single-step difference-frequency-generation process with a continuous-wave pump at 994 nm in a periodically poled lithium niobate (MgO:ppLN) waveguide and ultranarrow spectral filtering down to a bandwidth of 12.5 MHz. With this setup, we achieve the conversion of weak coherent pulses of duration up to 13.6 mu s with a device efficiency of about 25 % and an SNR >460 for 10-mu s-long pulses containing one photon on average. This SNR is large enough to enable a high-fidelity conversion of qubits emitted from an emissive quantum memory based on Pr3+:Y2SiO5 and to realize an interface with quantum processing nodes based on narrow-linewidth cavity-enhanced trapped ions.

In this talk, we report on recent progress towards the realization of quantum repeaters based on rare-earth doped crystal quantum memories (QM) and cavity -enhanced SPDC photon-pair sources. We first present our demonstration of on-demand telecom-heralded entanglement between multimode solid-state quantum memories with high rate and fully adjustable recall time. Then, we show how we successfully established longdistance non-classical correlations between a telecom photon and a QM in a field-deployed experiment in the city of Barcelona.

The ability to distribute heralded entanglement between distant matter nodes is a primitive for the implementation of large-scale quantum networks. Some of the most crucial requirements for future applications include high heralding rates at telecom wavelengths, multiplexed operation, and on-demand retrieval of stored excitations for synchronization of separate quantum links. Despite tremendous progress in various physical systems, the demonstration of telecom-heralded entanglement between quantum nodes featuring both multiplexed operation and on-demand retrieval remains elusive. In this work, we combine narrow band parametric photon-pair sources and solid-state quantum memories based on rare-earth doped crystals to demonstrate telecom-heralded entanglement between spatially separated spin-wave quantum memories with fully adjustable recall time and temporal multiplexing of 15 modes. In a first experiment, the storage in the spin state is conditioned on the entanglement heralding. We take advantage of the control over readout pulse phase to achieve feedforward conditional phase shifts on the stored photons depending on which heralding detector clicked. We exploit this effect to double the entanglement heralding rate for a given quantum state up to 510 counts/s, with an associated detection rate of 0.32 counts/s and measured positive concurrence by up to 6 standard deviations. In a second experiment, we simulate the communication time of a long-distance link by implementing an unconditional storage scheme with a dead time of 100 mu s. We take advantage of temporal multiplexing to increase the entanglement rates by a factor of 15 with respect to single mode storage, reaching a value of 22 counts/s per heralding detector. These results establish our architecture as a prime candidate for the implementation of scalable high-rate quantum network links.

Random-access quantum memories may offer computational advantages for quantum computers and networks. In this paper, we advance arrays of solid-state quantum memories toward their usage as randomaccess quantum memory. We perform quantum storage of path and time-bin qubits implemented with weak coherent states at the single-photon level, in an array of ten temporally multiplexed memory cells with controllable addressing. The qubits can be stored in arbitrary combinations of memory cells, from which they are read out on demand. We find average fidelities of 95 & thorn;2-2 % for path qubits and 91 & thorn;2 qubits. The measured fidelities violate the classical bounds for both encodings and for all ten cells. We also sequentially store a time-bin qubit in two different memory cells, maintain both qubits simultaneously in the array, and perform a collective readout. The individual control paired with high storage fidelity represents a significant advance toward a solid-state random-access quantum memory for quantum repeaters and photonic quantum processors.

We demonstrate the storage of path and time-bin qubits in solid-state quantum memories. Our system stores up to 250 modes with faint laser pulses and validates performance using single photons from a nonlinear source.

Quantum key distribution (QKD) leverages the principles of quantum mechanics to enable secure secret key exchange. Continuous-variable QKD (CV-QKD), utilizing coherent states and coherent detection, has emerged as a promising method, easily integrable into existing telecom networks. In this work, we study, through numerical simulation, the coexistence of a CV-QKD channel with a classical channel capable of up to 1 Gb/s. We first analyze the performance of the quantum channel when the classical channel emits a pure tone, by varying the frequency gap between both channels and adjusting their power ratios. Then, by maintaining a fixed electrical separation of 500 MHz, we study QPSK modulation with root-raised-cosine filtering in the classical channel. In this scenario, for several signal-to-noise ratios, we reduce crosstalk by optimizing filter properties such as roll-off factor, symbol rate and number of filter taps.

We report our progress towards the demonstration of telecom-heralded matter-matter entanglement between solid-state, on-demand and remote quantum memories based on rare-earth—doped crystals.

We investigate the interaction between nonclassical light with a tunable multiphoton component and a highly nonlinear medium based on cold Rydberg atoms. The nonclassical field emitted by a DLCZ quantum memory is stored using Rydberg electromagnetically induced transparency, experiencing strong nonlinear response due to the dipole blockade. We show that the storage efficiency in the Rydberg ensemble decreases as a function of the multiphoton strength of the input field, as a result of the nonlinearity. We also show that the autocorrelation function g^{(2)}(0) of the retrieved field after storage in the Rydberg state is considerably reduced, leading to the first demonstration of single photon filtering with nonclassical input light. Finally, we develop a simple simulation that allows us to model the effect of our medium on the input state. This work is a step towards matter-mediated photon-photon interactions with nonclassical light.

Generating indistinguishable photons from independent nodes is an important challenge for the development of quantum networks. In this work, we demonstrate the generation of highly indistinguishable single photons from two dissimilar atomic quantum nodes. One node is based on a fully blockaded cold Rydberg ensemble and generates on-demand single photons. The other node is a quantum repeater node based on a DLCZ quantum memory and emits heralded single photons after a controllable memory time that is used to synchronize the two sources. We demonstrate an indistinguishability of ${94.6 \pm 5.2 \%}$ for a temporal window including ${90\%}$ of the photons. This advancement opens new possibilities for interconnecting quantum repeater and processing nodes with high fidelity Bell-state measurement without sacrificing its efficiency.

We demonstrate the generation of highly-indistinguishable single photons from two independent quantum nodes. Node 1 is a quantum repeater node based on a quantum memory and Node 2 is a fully blockaded cold Rydberg ensemble.

Quantum Frequency Conversion (QFC) is a widely used technique to interface atomic systems with the telecom band in order to facilitate propagation over longer distances in fiber. Here we demonstrate the difference-frequency conversion from 606 nm to 1552 nm of microsecond-long weak coherent pulses at the single photon level compatible with Pr^3+:Y_2SiO_5 quantum memories, with high-signal to noise ratio. We use a single step difference frequency generation process with a continuous-wave pump at 994 nm in a MgO:ppLN-waveguide and ultra-narrow spectral filtering down to a bandwidth of 12.5 MHz. With this setup, we achieve the conversion of weak coherent pulses of duration up to 13.6 μ s with a device efficiency of about 25 signal-to-noise ratio >460 for 10 μ s-long pulses containing one photon on average. This signal-to-noise ratio is large enough to enable a high-fidelity conversion of qubits emitted from an emissive quantum memory based on Pr^3+:Y_2SiO_5 and to realize an interface with quantum processing nodes based on narrow-linewidth cavity-enhanced trapped ions.

We report the first cavity-enhanced on-demand atomic frequency comb spin-wave quantum memory. We used a Pr 3+ :Y 2 SiO 5 crystal embedded in an impedance-matched cavity to achieve a device efficiency of 38 % for weak coherent pulses.