We report on a source of ultranarrow-band photon pairs generated by widely nondegenerate cavity-enhanced spontaneous down-conversion. The source is designed to be compatible with Pr(3+) solid state quantum memories and telecommunication optical fibers, with signal and idler photons close to 606 nm and 1436 nm, respectively. Both photons have a spectral bandwidth around 2 MHz, matching the bandwidth of Pr(3+) doped quantum memories. This source is ideally suited for long distance quantum communication architectures involving solid state quantum memories.

Quantum correlations between long lived quantum memories and telecom photons that can propagate with low loss in optical fibers are an essential resource for the realization of large scale quantum information networks. Significant progress has been realized in this direction with atomic and solid state systems. Here, we demonstrate quantum correlations between a telecom photon and a multimode on-demand solid state quantum memory. This is achieved by mapping a correlated single photon onto a spin collective excitation in a Pr 3+ :Y2SiO5 crystal for a controllable time. The stored single photons are generated by cavity enhanced spontaneous parametric down conversion (SPDC) and heralded by their partner photons at telecom wavelength. These results represent the first demonstration of a multimode on-demand solid state quantum memory for external quantum states of light. They provide an important resource for quantum repeaters and pave the way for the implementation of quantum information networks with distant solid-state quantum nodes.

We report on experiments demonstrating the reversible mapping of heralded single photons to long lived collective optical atomic excitations stored in a Pr 3+ :Y2SiO5 crystal. A cavity-enhanced spontaneous down-conversion source is employed to produce widely non-degenerate narrow-band (≈ 2 MHz) photon-pairs. The idler photons, whose frequency is compatible with telecommunication optical fibers, are used to herald the creation of the signal photons, compatible with the Pr 3+ transition. The signal photons are stored and retrieved using the atomic frequency comb protocol. We demonstrate storage times up to 4.5 µs while preserving non-classical correlations between the heralding and the retrieved photon. This is more than 20 times longer than in previous realizations in solid state devices, and implemented in a system ideally suited for the extension to spin-wave storage.PACS numbers: 03.67. Hk,42.50.Gy,42.50.Md Many protocols in quantum information science rely on the efficient and reversible interaction between photons and matter [1]. The interaction lays the basis for the realization of quantum memories for light and of their application, e.g. in quantum repeaters [2,3]. Possible choices for the system used to store light are single atoms in cavities [4], cold or hot atomic gases [5-13], or rare earth (RE) doped solid state systems [14]. Thanks to the weak interaction between the optical active ions and the environment, RE doped crystals offer, when cryogenically cooled, the long optical and spin coherence times typical of atomic systems, free of the drawbacks deriving from atomic motion [15]. Moreover they possess the benefits of the solid state systems, such as strong interaction with light, allowing for efficient storage of photons [16,17] and prospect for integrated devices. Furthermore, their inhomogeneously broadened absorption lines can be tailored in appropriate structures, like atomic frequency combs (AFCs), to enable storage protocols with remarkable properties (e.g. temporal or frequency multiplexing) [18][19][20][21][22][23][24].Single photon level weak coherent pulses [25,26] and qubits [18,27,28] have been stored in the excited state of rare-earth doped crystals using the AFC scheme. This has recently been extended to the ground state, in the regime of a few photons per pulse [29]. The storage of non-classical light generated by spontaneous parametric down-conversion (SPDC) has also been demonstrated and enabled entanglement between one photon and one collective optical atomic excitation in a crystal [30,31], entanglement between two crystals [32], and single photon qubit storage [33,34]. However, the mapping of nonclassical light using AFC in rare earth doped crystals was obtained so far only in systems with two ground state levels, thus inherently limited to the optical coherence and not directly extendable to spin-wave storage.On the contrary, Pr 3+ or Eu 3+ doped crystals have the required level structure for spin-wave storage [22,23,29].In particular, Pr 3+ :Y 2 SiO 5 is one of the optical ...

We report on a source of heralded narrowband (»3 MHz) single photons compatible with solid-state spin-wave quantum memories based on praseodymium doped crystals. Widely non-degenerate narrow-band photon pairs are generated using cavity enhanced down conversion. One photon from the pair is at telecom wavelengths and serves as heralding signal, while the heralded single photon is at 606 nm, resonant with an optical transition of Pr 3+:Y 2 SiO 5 . The source offers a heralding efficiency of 28% and a generation rate exceeding 2000pairs mW −1 in a single-mode. The single photon nature of the heralded field is confirmed by a direct antibunching measurement, with a measured antibunching parameter down to 0.010(4). Moreover, we investigate in detail photon cross-and autocorrelation functions proving non-classical correlations between the two photons. The results presented in this paper offer prospects for the demonstration of single photon spin-wave storage in an on-demand solid state quantum memory, heralded by a telecom photon.

We demonstrate frequency-bin entanglement between ultra-narrowband photons generated by cavity enhanced spontaneous parametric down conversion. Our source generates photon pairs in widely non-degenerate discrete frequency modes, with one photon resonant with a quantum memory material based on praseodymium doped crystals and the other photon at telecom wavelengths. Correlations between the frequency modes are analyzed using phase modulators and narrowband filters before detection. We show high-visibility two photon interference between the frequency modes, allowing us to infer a coherent superposition of the modes. We develop a model describing the state that we create and use it to estimate optimal measurements to achieve a violation of the Clauser-Horne (CH) Bell inequality under realistic assumptions. With these settings we perform a Bell test and show a significant violation of the CH inequality, thus proving the entanglement of the photons. Finally we demonstrate the compatibility with a quantum memory material by using a spectral hole in the praseodymium (Pr) doped crystal as spectral filter for measuring highvisibility two-photon interference. This demonstrates the feasibility of combining frequency-bin entangled photon pairs with Pr-based solid state quantum memories.

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Heterodyne laser phase measurements in the Laser Interferometer Space Antenna (LISA) are degraded by the phase fluctuations of the onboard clocks, resulting in unacceptable sensitivity performance levels of the interferometric data. The current scheme for cancellation of the clock phase noise requires 1 GHz modulation of the ranging laser beams and additional interspacecraft clock recovery heterodyne phase measurements. Here, we report experimental results for an alternative approach to clock noise cancellation based on modified second generation time-delay interferometry (TDI) with optical frequency combs (OFCs). The use of OFCs in the LISA scheme allows simultaneous cancellation of both laser and clock noises, and would eliminate the need for 1 GHz laser modulations and associated demodulation detections. Two Mach-Zehnder interferometers with acousto-optic modulators were used to simulate two LISA arms with Doppler shifts and time delays. With a self-referenced OFC locked to the laser providing the clock signal, we achieve simultaneous suppression of laser and clock noises by 7 and 1.5 orders of magnitudes, respectively, down to the setup noise floor. Based on a numerical analysis, we further show that the noise suppression performance of the OFC-based TDI scheme can meet the LISA mission requirements with an ample margin.

Event synchronisation is a ubiquitous task, with applications ranging from 5G technology to industrial automation and smart power grids. The emergence of quantum communication networks will further increase the demands for synchronisation in optical and electronic domains, thus incurring a significant resource overhead, e.g. through the use of ultra-stable clocks or additional synchronisation lasers. Here we show how temporal correlations of energy-time entangled photons may be harnessed for synchronisation in quantum networks. We achieve stable synchronisation jitter < 50 ps with as few as 36 correlated detection events per 100 ms and demonstrate feasibility in realistic high-loss link scenarios. In contrast to previous work, this is accomplished without any external timing reference and only simple crystal oscillators. Our approach replaces the optical and electronic transmission of timing signals with classical communication and computer-aided postprocessing. It can be easily integrated into a wide range of quantum communication networks and could pave the way to future applications in entanglement-based secure time transmission.