Terahertz electromagnetic radiation is extremely useful for numerous applications such as imaging and spectroscopy. Therefore, it is highly desirable to have an efficient table-top emitter covering the 1-to-30-THz window whilst being driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source which relies on tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photo-induced spin currents, the inverse spin-Hall effect and a broadband Fabry-Pérot resonance. Guided by an analytical model, such spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1-to-30-THz range. Our novel source outperforms laser-oscillatordriven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz-field amplitude, flexibility, scalability and cost. IntroductionThe terahertz (THz) window, loosely defined as the frequency range from 0.3 to 30 THz in the electromagnetic spectrum, is located between the realms of electronics and optics 1,2 . As this region coincides with many fundamental resonances of materials, THz radiation enables very selective spectroscopic insights into all phases of matter with high temporal 3,4 and spatial 5,6,7,8 resolution. Consequently, numerous applications in basic research 3,4 , imaging 5 and quality control 8 have emerged.To fully exploit the potential of THz radiation, energy-efficient and low-cost sources of ultrashort THz pulses are required. Most broadband table-top emitters are driven by femtosecond laser pulses that generate the required THz charge current by appropriately mixing the various optical frequencies 9,10 . Sources made from solids usually consist of semiconducting or insulating structures with naturally or artificially broken inversion symmetry. When the incident photon energy is below the semiconductor band gap, optical rectification causes a charge displacement that follows the intensity envelope of the incident pump pulse 9,10,11,12,13,14,15,16,17 . For above-band-gap excitation, the response is dominated by a photocurrent 18,19,20,21,22,23,24 with a temporally step-like onset and, thus, generally smaller bandwidth than optical rectification 9 . Apart from rare exceptions 14 , however, most semiconductors used are polar 1,2,12,13,15,16,17,21,22 and strongly attenuate THz radiation around optical phonon resonances, thereby preventing emission in the so-called Reststrahlen band located between ~1 and 15 THz.The so far most promising sources covering the full THz window are photocurrents in transient gas plasmas 9,10,25,26,27,28,29 . The downside of this appealing approach is that the underlying ionization process usually requires amplified laser pulses with high threshold energies on the order of 0....

Quantum networks consisting of quantum memories and photonic interconnects can be used for entanglement distribution [1,2], quantum teleportation [3] and distributed quantum computing [4]. Remotely connected two-node networks have been demonstrated using memories of the same type: trapped ion systems [5], quantum dots [6] and nitrogen vacancy centers [6,7]. Hybrid systems constrained by the need to use photons with the native emission wavelength of the memory, have been demonstrated between a trapped ion and quantum dot [8] and between a single neutral atom and a Bose-Einstein Condensate [9]. Most quantum systems operate at disparate and incompatible wavelengths to each other so such two-node systems have never been demonstrated. Here, we use a trapped 138Ba + ion and a periodically poled lithium niobate (PPLN) waveguide, with a fiber coupled output, to demonstrate 19% end-to-end efficient quantum frequency conversion (QFC) of single photons from 493 nm to 780 nm. At the optimal signal-to-noise operational parameter, we use fluorescence of the ion to produce light resonant with the 87 Rb D 2 transition. To demonstrate the quantum nature of both the unconverted 493 nm photons and the converted photons near 780 nm, we observe strong quantum statics in their respective second order intensity correlations. This work extends the range of intra-lab networking between ions and networking and communication between disparate quantum memories.A quantum network may be established by interfering photons emitted by quantum memories. Connecting different types of quantum memories for hybrid networking requires overcoming the disparate photon wavelengths emitted by each quantum memory. Given advances in modularity in trapped ion and neutral atom architectures, a hybrid system with modular inter-connectivity is advantageous. In the case of photons emitted from trapped ions (with fiber attenuations of 70 dB/km at 369 nm for Yb + and 50 dB/km at 493 nm for Ba + ) generating photons in, or converting photons to, the near-infra-red range (with a fiber attenuation of 3.5 dB/km) would sig-nificantly extend the networking range between trapped ions and provide the ability to match the wavelength of another quantum memory.Trapped ions [10] are an excellent candidate for elementary logical units [11] of a network as many prerequisite components have been shown, including: modularity for photon generation and detection [5,11,12], quantum computation [13,14] and excellent single photon emission properties [15]. In this work, we overcome a challenge to extending the networking range of trapped ions by frequency converting the ion light. We demonstrate the conversion of 493 nm photons, emitted from a single barium ion, to a 780 nm wavelength resonant with the D 2 transition in neutral 87 Rb. This conversion provides the two-fold benefit of paving the way for neutralion hybrid networking and communication as well as extending the networking capability of barium ions from 100s of meters to several kilometres allowing for both an ion-ion and ne...

Advances in the distribution of quantum information will likely require entanglement shared across a hybrid quantum network [1][2][3]. Many entanglement protocols require the generation of indistinguishable photons between the various nodes of the network [4,5]. This is challenging in a hybrid environment due to typically large differences in the spectral and temporal characteristics of single photons generated in different systems [1]. Here we show, for the first time, quantum interference between photons generated from a single atomic ion and an atomic ensemble, located in different buildings and linked via optical fibre. Trapped ions are leading candidates for quantum computation and simulation with good matter-to-photon conversion [6][7][8][9][10][11][12][13]. Rydberg excitations in neutral-atom ensembles show great promise as interfaces for the storage and manipulation of photonic qubits with excellent efficiencies [14][15][16][17]. Our measurement of high-visibility interference between photons generated by these two, disparate systems is an important building block for the establishment of a hybrid quantum network.Recently, Rydberg atoms have proven to be a useful tool in the field of quantum information. The strong optical nonlinearity exhibited by neutral-atom Rydberg ensembles enables the construction of single-photon sources [15], gates [16], and transistors [17]. Strong light-matter interactions make them well suited as quantum memories [14], and for implementing quantum repeaters [18,19]. Furthermore, arrays of Rydberg atoms are a powerful new platform for quantum simulation [20, 21]. The continued success of trapped-ion systems in quantum computation [6,7], simulation [8, 9], and communication [12] owes to their long coherence and trapping lifetimes [10], high fidelity operations [11], and ease of generating ion-photon entanglement [12, 13].Given the wide-ranging applications of both platforms, future efforts in quantum information will benefit from the construction of remote hybrid atomic-ensemble-ion networks. Flying photonic qubits provide an excellent means arXiv:1907.04387v1 [quant-ph]