Optical-to-electrical conversion, which is the basis of the operation of optical detectors, can be linear or nonlinear. When high sensitivities are needed, single-photon detectors are used, which operate in a strongly nonlinear mode, their response being independent of the number of detected photons. However, photon-number-resolving detectors are needed, particularly in quantum optics, where n-photon states are routinely produced. In quantum communication and quantum information processing, the photon-numberresolving functionality is key to many protocols, such as the implementation of quantum repeaters 1 and linear-optics quantum computing 2 . A linear detector with single-photon sensitivity can also be used for measuring a temporal waveform at extremely low light levels, such as in longdistance optical communications, fluorescence spectroscopy and optical time-domain reflectometry. We demonstrate here a photon-number-resolving detector based on parallel superconducting nanowires and capable of counting up to four photons at telecommunication wavelengths, with an ultralow dark count rate and high counting frequency.Among the approaches proposed so far for photon-numberresolving (PNR) detection (Table 1) are detectors based on charge integration or field-effect transistors 3-5 , which are, however, affected by long integration times, leading to bandwidths of ,1 MHz. Transition edge sensors 6 operate at 100 mK and show long response times (several microseconds). Approaches based on photomultipliers 7 and avalanche photodiodes, such as the visiblelight photon counter 8,9 , two-dimensional arrays of avalanche photodiodes 10,11 and time-multiplexed detectors 12,13 are not sensitive or are plagued by high dark count rates (DKs) and long dead times in the telecommunication spectral windows. Arrays of single-photon detectors (SPDs) also involve complex readout schemes 11 or separate contacts, amplification and discrimination 14. The parallel nanowire detector (PND) presented here significantly outperforms these approaches in terms of simplicity, sensitivity, speed and multiplication noise.The basic structure of the PND comprises the parallel connection of N superconducting nanowires, each connected in series to a resistor R 0 (Fig.

The recent progress in integrated quantum optics has set the stage for the development of an integrated platform for quantum information processing with photons, with potential applications in quantum simulation. Among the different material platforms being investigated, direct-bandgap semiconductors and particularly gallium arsenide (GaAs) offer the widest range of functionalities, including single-and entangled-photon generation by radiative recombination, low-loss routing, electro-optic modulation and single-photon detection. This paper reviews the recent progress in the development of the key building blocks for GaAs quantum photonics and the perspectives for their full integration in a fully-functional and densely integrated quantum photonic circuit.

A linear-optics quantum computer 5 requires hundreds to thousands of single-photon components including sources, detectors and interferometers, which is obviously only feasible in an integrated circuit.Even the small-scale circuits needed in quantum repeaters 2 would greatly benefit from monolithic integration in view of the improved stability and coupling efficiency attainable in a chip. A very large experimental research activity has been dedicated to the development of single-photon sources based on III-V semiconductors 10 , in view of large-scale integration, and to passive quantum circuits based on silica-onsilicon 6 and on laser-micromachined glass 8,9 , but a clear approach towards a fully integrated photonic network including sources and detectors has not been proposed. This is in large part due to the complexity of most single-photon detector technologies -for example, the complex device structures associated to avalanche photodiodes are not easily compatible with the integration of low-loss waveguides and even less of sources. Transition-edge sensors may be suited for integration 11 , but they are plagued by very slow response times (leading to maximum counting rates in the tens of kHz range) and require cooling down to <100 mK temperatures. Here we propose a platform for the full integration of quantum photonic components on the same chip. It is based on the mature III-V semiconductor technology and comprises ( Fig. 1(a) ) waveguide single-photon sources based on InAs quantum dots (QDs), GaAs/AlGaAs ridge waveguides, Mach-Zehnder interferometers using directional couplers or multimode-interference couplers, and 3 waveguide detectors based on superconducting nanowires. Efficient single-photon emission from QDs in a waveguide can be obtained by using photonic crystals (PhCs), e.g. in a cavity side-coupled to a waveguide 12 or using the slow-light regime in PhC waveguides 13 , and the photons can then be transferred to ridge waveguides using tapers. Photons emitted by distinct QDs can be made indistinguishable by using electric fields to control the exciton energy 14 . The high index contrast available in the GaAs/AlGaAs system allows circuits with short bending radii, therefore more compact than in the silica platform 6 , while the large electrooptic coefficient of GaAs enables compact modulators operating at GHz frequencies. In this letter we report the key missing component, a single-photon detector integrated with GaAs waveguides. Our waveguide single-photon detectors (WSPDs) are based on the principle of photon-induced hot-spot creation in ultranarrow superconducting NbN wires, which is also used in nanowire superconducting single-photon detectors 15 (SSPDs) and can provide ultrahigh sensitivity at telecommunication wavelengths, high counting rates, broad spectral response and high temporal resolution due to low jitter values. In our design (see Fig. 1(b)), the wires are deposited and patterned on top of a GaAs ridge waveguide, in order to sense the evanescent field on the surface. Four NbN nanowi...

Conversion between signals in the microwave and optical domains is of great interest both for classical telecommunication, as well as for connecting future superconducting quantum computers into a global quantum network. For quantum applications, the conversion has to be both efficient, as well as operate in a regime of minimal added classical noise. While efficient conversion has been demonstrated with several approaches using mechanical transducers, they have so far all operated with a substantial thermal noise background. Here, we overcome this limitation and demonstrate coherent conversion between GHz microwave signals and the optical telecom band with a thermal background of less than one phonon. We use an electro-opto-mechanical device, that couples surface acoustic waves driven by a resonant microwave signal to an optomechanical crystal featuring a 2.7 GHz mechanical mode. By operating at Millikelvin temperatures, we can initialize the mechanical mode in its quantum groundstate, which allows us to perform the transduction process with less than one quantum of added thermal noise. We further verify the preservation of the coherence of the microwave signal throughout the transduction process. * These authors contributed equally to this work. † s.groeblacher@tudelft.nl arXiv:1812.07588v1 [quant-ph]

Nature © Macmillan Publishers Ltd 19988 radius apertures following the standard prescription 28 . Additional corrections were made as follows: (1) +0.05 mag to correct for the so-called 'long exposure' effect 29 ; (2) −0.04 mag, which is the appropriate colour term to convert to standard Cousin's I magnitudes for red stars with V Ϫ I Ϸ 1:5 (ref. 28); and (3) −0.04 mag to allow for the expected foreground extinction 30 . We estimate that the combined uncertainty from the zero-point calibration and the various correction terms amounts to 0.06 mag. Clearly extended sources were removed from the catalogue by restricting our analysis to sources with a DAOPHOTsharpness parameter Ϫ 0:6 Ͻ s Ͻ 0:4. A similar procedure was also followed for the HDF control field. Finally, we created 12 test data sets in which 265 simulated stars were added to the real data frames. The input magnitudes of the simulated stars covered the range from 23.5 to nearly 30. We then processed these frames in the same way as the original data, and produced a matrix relating the recovered magnitudes and detection efficiency to the input magnitudes of the simulated stars. From these simulations we estimate that the catalogue is ϳ80% complete at I ¼ 27:9, and use this as the limiting magnitude in our analysis.

We demonstrate simultaneous lasing at two well-separated wavelengths in self-assembled InAs quantum-dot lasers, via ground-state (GS) and excited-state (ES) transitions. This effect is reproducible and strongly depends on the cavity length. By a master-equation model, we attribute it to incomplete clamping of the ES population at the GS threshold.

A new class of hybrid systems that couple optical, electrical and mechanical degrees of freedom in nanoscale devices is under development in laboratories worldwide. These nano-opto-electromechanical systems (NOEMS) offer unprecedented opportunities to dynamically control the flow of light in nanophotonic structures, at high speed and low power consumption. Drawing on conceptual and technological advances from cavity optomechanics, they also bear the potential for highly efficient, low-noise transducers between microwave and optical signals, both in the classical and quantum domains. This Progress Article discusses the fundamental physical limits of NOEMS, reviews the recent progress in their implementation, and suggests potential avenues for further developments in this field.2

Second-order optical nonlinearities in materials are of paramount importance for optical wavelength conversion techniques, which are the basis of new high-resolution spectroscopic tools. Semiconductor technology now makes it possible to design and fabricate artificially asymmetric quantum structures in which optical nonlinearities can be calculated and optimized from first principles. Extremely large second-order susceptibilities can be obtained in these asymmetric quantum wells. Moreover, properties such as double resonance enhancement or electric field control will open the way to new devices, such as fully solid-state optical parametric oscillators.