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We present a device-independent protocol to test if a given black-box measurement device is entangled, that is, has entangled eigenstates. Our scheme involves three parties and is inspired by entanglement swapping; the test uses the Clauser-Horne-Shimony-Holt Bell inequality, checked between each pair of parties. In the case where all particles are qubits, we characterize quantitatively the deviation of the measurement device from a perfect Bell-state measurement.

Single-photon entangled states, i.e., states describing two optical paths sharing a single photon, constitute the simplest form of entanglement. Yet they provide a valuable resource in quantum information science. Specifically, they lie at the heart of quantum networks, as they can be used for quantum teleportation, swapped, and purified with linear optics. The main drawback of such entanglement is the difficulty in measuring it. Here, we present and experimentally test an entanglement witness allowing one to say whether a given state is path entangled and also that entanglement lies in the subspace, where the optical paths are each filled with one photon at most, i.e., refers to single-photon entanglement. It uses local homodyning only and relies on no assumption about the Hilbert space dimension of the measured system. Our work provides a simple and trustworthy method for verifying the proper functioning of future quantum networks. Motivations.-Quantum networks [1] provide broad capabilities, ranging from long distance quantum communication at large scales [2,3], to the simulation of quantum many-body systems [4] in tabletop implementations. Remarkable progresses have been made in practice [5][6][7] and experimental capabilities are now advancing into a domain of rudimentary functionality for quantum nodes connected by quantum channels [8][9][10][11]. Surprisingly, the task of checking that a newly implemented quantum network performs well remains nontrivial.In the past decade, a great number of architectures based on atomic ensembles and linear optics have been proposed [12]. We now know that quantum networks based on singlephoton entanglement [13], i.e., entangled states of the form

The generation of ultra-narrowband, pure and storable single photons with widely tunable wave shape is an enabling step toward hybrid quantum networks requiring interconnection of remote disparate quantum systems. It allows interaction of quantum light with several material systems, including photonic quantum memories, single trapped ions and opto-mechanical systems. Previous approaches have offered a limited tuning range of the photon duration of at most one order of magnitude. Here we report on a heralded single photon source with controllable emission time based on a cold atomic ensemble, which can generate photons with temporal durations varying over three orders of magnitude up to 10 μs without a significant change of the readout efficiency. We prove the nonclassicality of the emitted photons, show that they are emitted in a pure state, and demonstrate that ultra-long photons with nonstandard wave shape can be generated, which are ideally suited for several quantum information tasks.

The ability to coherently control mechanical systems with optical fields has made great strides over the past decade, and now includes the use of photon counting techniques to detect the nonclassical nature of mechanical states. These techniques may soon be used to perform an optomechanical Bell test, hence highlighting the potential of cavity optomechanics for device-independent quantum information processing. Here, we propose a witness which reveals optomechanical entanglement without any constraint on the global detection efficiencies in a setup allowing one to test a Bell inequality. While our witness relies on a well-defined description and correct experimental calibration of the measurements, it does not need a detailed knowledge of the functioning of the optomechanical system. A feasibility study including dominant sources of noise and loss shows that it can readily be used to reveal optomechanical entanglement in present-day experiments with photonic crystal nanobeam resonators.

Single-photon entanglement is one of the primary resources for quantum networks, including quantum repeater architectures. Such entanglement can be revealed with only local homodyne measurements through the entanglement witness presented in Morin et al (2013 Phys. Rev. Lett. 110 130401). Here, we provide an extended analysis of this witness by introducing analytical bounds and by reporting measurements confirming its great robustness with regard to losses. This study highlights the potential of optical hybrid methods, where discrete entanglement is characterized through continuous-variable measurements.B where A and B are two spatial modes sharing a delocalized single-photon, has recently been proposed and experimentally tested [11]. It relies only on homodyne detections (i.e., on continuous quadrature measurements and not on photon counting) and offers significant advantages relative to other witnessing methods [12][13][14][15]. Indeed, unlike most steering experiments [16], it does not require postselection and does not assume knowledge of the underlying Hilbert space dimension. Also, in contrast with other entanglement witnesses [17], it specifically identifies the entanglement present in the singlephoton subspace. Finally, the measurements are only operated locally on the entangled modes, an important feature if applied to large-scale networks [18,19].The witness presented in [11] was built up on numerical arguments. In the present work, we extend its analysis by means of analytical calculations. The aim is to gain insight into the properties of the witness with respect to various practical imperfections. In particular, we theoretically and experimentally investigate its robustness with regard to channel loss or, equivalently, to imperfect single-photon states used as the initial resource for entanglement generation. We demonstrate that even for a large admixture of vacuum, our witness reveals the presence of entanglement, confirming its suitability for use in realistic networks and entanglement distribution protocols where losses are inherent.The paper is organized as follows. Section 2 gives an overview of the single-photon entanglement witness based on local homodyne measurements. Then, in the case where the state only contains vacuum and single-photon components (i.e., the state lies within a qubit subspace) the witness parameter is evaluated and compared to the separable bound. Symmetric and asymmetric channels are considered. In section 3, multiphoton components, which are critical in experimental realizations, are taken into account. We show, in particular, how the witness is extended to this realistic case by experimentally bounding the Hilbert space, and we then derive the effect of losses in the communication channels. This study leads to several expressions for the separable bound. The setup and experimental results are presented in section 4, and we give our conclusions in section 5. Principle of the witnessThis section presents the principle of the single-photon entanglement witness, which relies...

We want to certify in a black box scenario that two parties simulating the teleportation of a qubit are really using quantum resources. If active compensation is part of the simulation, perfect teleportation can be faked with purely classical means. If active compensation is not implemented, a classical simulation is necessarily imperfect: in this case, we provide bounds for certification of quantumness using only the observed statistics. The usual figure of merit, namely the average fidelity of teleportation, turns out to be too much of a coarse-graining of the available statistical information in the case of a black-box assessment.Introduction.-Shortly after the milestone paper that introduced quantum teleportation [1], the question was asked of which deviation from the ideal case one can tolerate while still claiming that proper quantum effects are being observed. Popescu proved that, if Alice and Bob share no entanglement, the average fidelity for the teleportation of unknown qubit states is bounded byF = 2

Spontaneous Raman processes in cold atoms have been widely used in the past decade for generating single photons. Here, we present a method to optimise their efficiencies for given atomic coherences and optical depths. We give a simple and complete recipe that can be used in present-day experiments, attaining near-optimal single photon emission. IntroductionOn-demand single photon sources are appealing ingredients for many quantum information tasks. Examples include the distribution of entanglement over long distances using quantum repeaters or quantum communications with security guarantees which remain valid, independent of the details of the actual implementation [1, 2]. These tasks necessitate stringent purity and efficiency requirements on the performance of the single photon sources used. Techniques based on spontaneous Raman processes in cold atoms are among the most advanced single-photon sources with such characteristics. The basic principle is to use an ensemble of three-level atoms in a Λ-configuration and two pulsed laser fields (see figure 1(a)). The first write pulse-the write control field-off-resonantly excites one transition, which can spontaneously produce a frequency-shifted photon-the write photon field-along the second transition through a Raman process. Since all the interacting atoms participate in the process, and there is no information about which atom emitted the photon, the detection of this write photon heralds the existence of a single delocalised excitation across the sample-an atomic spin wave. Once the spin wave has been prepared, the atomic sample is ready to be used as a source, and a second pulse-the read control field-along the second transition performs a conversion of the atomic spin wave into a second photon-the read photon field. If the duration of the process is short enough with respect to the atomic coherence times, and the optical depth of the sample sufficiently high, then the read photon is emitted efficiently in a well defined mode and the protocol provides a viable single photon source.Such sources have been at the core of numerous experiments during the last decade following the seminal paper of Duan, Lukin, Cirac and Zoller [3], showing how they could be used for long-distance quantum communication based on quantum repeater architectures (for reviews, see [4][5][6][7]). Recently, they have been used as quantum memories with storage times up to 200 ms [8,9] or as a source producing pure single photons with a temporal duration that can be varied over up to 3 orders of magnitude while maintaining constant efficiencies [10]. We stress that the efficiency of such a source is a critical parameter for the implementation of efficient quantum repeater architectures. While very high efficiencies of∼90% are essential, a reduction of the source efficiency by 1% can reduce the repeater distribution rate by 10%-20%, depending on the specific architecture [4].Several solutions can be envisioned to ensure high efficiencies. One solution relies on the use of an optical cavity to...