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On the right wavelength Quantum communication with photonic quantum bits (qubits) could be conveniently achieved using telecom optical fibres, but the memory and processing elements of a quantum network — typically alkaline atoms — operate at shorter wavelengths. Tanzilli et al . now report the development of an interface that enables qubits to be transferred between these different wavelengths, while preserving their essential quantum properties. Such an interface could prove useful in quantum information networks based on stationary atomic memories and travelling photonic qubits. Quantum communication requires the transfer of quantum states 1 , or quantum bits of information (qubits), from one place to another. From a fundamental perspective, this allows the distribution of entanglement and the demonstration of quantum non-locality over significant distances 2 , 3 , 4 , 5 , 6 . Within the context of applications, quantum cryptography offers a provably secure way to establish a confidential key between distant partners 7 . Photons represent the natural flying qubit carriers for quantum communication, and the presence of telecommunications optical fibres makes the wavelengths of 1,310 nm and 1,550 nm particularly suitable for distribution over long distances. However, qubits encoded into alkaline atoms that absorb and emit at wavelengths around 800 nm have been considered for the storage and processing of quantum information 8 , 9 . Hence, future quantum information networks made of telecommunications channels and alkaline memories will require interfaces that enable qubit transfers between these useful wavelengths, while preserving quantum coherence and entanglement 9 , 10 , 11 . Here we report a demonstration of qubit transfer between photons of wavelength 1,310 nm and 710 nm. The mechanism is a nonlinear up-conversion process, with a success probability of greater than 5 per cent. In the event of a successful qubit transfer, we observe strong two-photon interference between the 710 nm photon and a third photon at 1,550 nm, initially entangled with the 1,310 nm photon, although they never directly interacted. The corresponding fidelity is higher than 98 per cent.

Shaping single-mode operation in high-power fibers requires a precise knowledge of the gain-medium optical properties. This requires precise measurements of the refractive index differences (Δ n ) between the core and the cladding of the fiber. We exploit a quantum optical method based on low-coherence Hong-Ou-Mandel interferometry to perform practical measurements of the refractive index difference using broadband energy-time entangled photons. The precision enhancement reached with this method is benchmarked with a classical method based on single photon interferometry. We show in classical regime an improvement by an order of magnitude of the precision compared to already reported classical methods. Strikingly, in the quantum regime, we demonstrate an extra factor of 4 on the precision enhancement, exhibiting a state-of-the-art Δ n precision of 6 × 10 −7 . This work sets the quantum photonics metrology as a powerful characterization tool that should enable a faster and reliable design of materials dedicated to light amplification.

A new kind of correlated photon-pair source based on a waveguide integrated on a periodically poled lithium niobate substrate is reported. Using a pump laser of a few mu W at 657 nm, photon-pairs are generated/degenerated at 1314 nm. Detecting approximately 1500 coincidences per second, a conversion rate of 10 super(-6) pairs per pump photon can be inferred, which is four orders of magnitude higher than that obtained with previous bulk sources. These results are very promising for the realization of sources for quantum communication and quantum metrology experiments requiring a high signal-to-noise ratio or working with more than one photon-pair at a time.

Quantum networks involve entanglement sharing between multiple users. Ideally, any two users would be able to connect regardless of the type of photon source they employ, provided they fulfill the requirements for two-photon interference. From a theoretical perspective, photons coming from different origins can interfere with a perfect visibility, provided they are made indistinguishable in all degrees of freedom. Previous experimental demonstrations of such a scenario have been limited to photon wavelengths below 900 nm, unsuitable for long distance communication and suffered from low interference visibility. We report two-photon interference using two disparate heralded single photon sources, which involve different nonlinear effects, operating in the telecom wavelength range. The measured visibility of the two-photon interference is 80 ± 4%, which paves the way to hybrid universal quantum networks.

A Bell test is a randomized trial that compares experimental observations against the philosophical worldview of local realism 1 , in which the properties of the physical world are independent of our observation of them and no signal travels faster than light. A Bell test requires spatially distributed entanglement, fast and high-efficiency detection and unpredictable measurement settings 2 , 3 . Although technology can satisfy the first two of these requirements 4 – 7 , the use of physical devices to choose settings in a Bell test involves making assumptions about the physics that one aims to test. Bell himself noted this weakness in using physical setting choices and argued that human ‘free will’ could be used rigorously to ensure unpredictability in Bell tests 8 . Here we report a set of local-realism tests using human choices, which avoids assumptions about predictability in physics. We recruited about 100,000 human participants to play an online video game that incentivizes fast, sustained input of unpredictable selections and illustrates Bell-test methodology 9 . The participants generated 97,347,490 binary choices, which were directed via a scalable web platform to 12 laboratories on five continents, where 13 experiments tested local realism using photons 5 , 6 , single atoms 7 , atomic ensembles 10 and superconducting devices 11 . Over a 12-hour period on 30 November 2016, participants worldwide provided a sustained data flow of over 1,000 bits per second to the experiments, which used different human-generated data to choose each measurement setting. The observed correlations strongly contradict local realism and other realistic positions in bipartite and tripartite 12 scenarios. Project outcomes include closing the ‘freedom-of-choice loophole’ (the possibility that the setting choices are influenced by ‘hidden variables’ to correlate with the particle properties 13 ), the utilization of video-game methods 14 for rapid collection of human-generated randomness, and the use of networking techniques for global participation in experimental science. The BIG Bell Test, which used an online video game with 100,000 participants worldwide to provide random bits to 13 quantum physics experiments, contradicts the Einstein–Podolsky–Rosen worldview of local realism.

Disposing of simple and efficient sources for photonic states with non-classical photon statistics is of paramount importance for implementing quantum computation and communication protocols. In this work, we propose an innovative approach that drastically simplifies the preparation of non-Gaussian states as compared to previous proposals, by taking advantage from the multiplexing capabilities offered by modern quantum photonics tools. Our proposal is inspired by iterative protocols, where multiple resources are combined one after the other for obtaining high-amplitude complex output states. Here, conversely, a large part of the protocol is performed in parallel, by using a single projective measurement along a mode which partially overlaps with all the input modes. We show that our protocol can be used to generate high-quality and high-amplitude Schr\"odinger cat states as well as more complex states such as error-correcting codes. Remarkably, our proposal can be implemented with experimentally available resources, highlighting its straightforward feasibility.

Photon statistical measurements on a semiconductor microlaser, obtained using single-photon counting techniques, show that a newly discovered spontaneous pulsed emission regime possesses superthermal statistical properties. The observed spike dynamics, typical of small-scale devices, is at the origin of an unexpected discordance between the probability density function and its representation in terms of the first moments, a discordance so far unnoticed in all devices. The impact of this new dynamics is potentially large, since coincidence techniques are presently the sole capable of characterizing light emitted by nanolasers.

The complex internal atomic structure involved in radiative transitions has an effect on the spectrum of fluctuations (noise) of the transmitted light. A degenerate transition has different properties in this respect than a pure two-level transition. We investigate these variations by studying a certain transition between two degenerate atomic levels for different choices of the polarization state of the driving laser. For circular polarization, corresponding to the textbook two-level atom case, the optical spectrum shows the characteristic Mollow triplet for strong laser drive, while the corresponding noise spectrum exhibits squeezing in some frequency ranges. For a linearly polarized drive, corresponding to the case of a multilevel system, additional features appear in both optical and noise spectra. These differences are more pronounced in the regime of a weakly driven transition: whereas the two-level case essentially exhibits elastic scattering, the multilevel case has extra noise terms related to spontaneous Raman transitions. We also discuss the possibility to experimentally observe these predicted differences for the commonly encountered case where the laser drive has excess noise in its phase quadrature.