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Entanglement of the spin of an electron in a semiconductor quantum dot with a single photon is reported, and verified by means of time-resolved frequency downconversion to a telecommunications wavelength; this process is an essential requirement for future quantum networks. Quantum spin, on the dot Future quantum networks will combine ideally stationary quantum bits (qubits), such as single electron spins, with 'flying' qubits, which are photons that transfer quantum states between distant qubits. It has therefore been a long-standing challenge in the field of quantum computation and communication to couple a single electron spin to a single photon in a solid-state platform. Two groups working independently have now achieved that goal, by demonstrating entanglement between a photon and a single electron spin trapped in a semiconductor 'quantum dot' structure. The quantum dot acts as the stationary node. This achievement is a small step towards eventual implementation of quantum networks that can support long-distance quantum communication. Long-distance quantum teleportation and quantum repeater technologies require entanglement between a single matter quantum bit (qubit) and a telecommunications (telecom)-wavelength photonic qubit 1 , 2 , 3 , 4 , 5 . Electron spins in III–V semiconductor quantum dots are among the matter qubits that allow for the fastest spin manipulation 6 , 7 and photon emission 8 , 9 , but entanglement between a single quantum-dot spin qubit and a flying (propagating) photonic qubit has yet to be demonstrated. Moreover, many quantum dots emit single photons at visible to near-infrared wavelengths, where silica fibre losses are so high that long-distance quantum communication protocols become difficult to implement 10 . Here we demonstrate entanglement between an InAs quantum-dot electron spin qubit and a photonic qubit, by frequency downconversion of a spontaneously emitted photon from a singly charged quantum dot to a wavelength of 1,560 nanometres. The use of sub-10-picosecond pulses at a wavelength of 2.2 micrometres in the frequency downconversion process provides the necessary quantum erasure to eliminate which-path information in the photon energy. Together with previously demonstrated indistinguishable single-photon emission at high repetition rates 11 , 12 , the present technique advances the III–V semiconductor quantum-dot spin system as a promising platform for long-distance quantum communication.

In the past, long-distance free-space quantum communication experiments could only be implemented at night. During the daytime, the bright background sunlight prohibits quantum communication in transmission under conditions of high channel loss over long distances. Here, by choosing a working wavelength of 1,550 nm and developing free-space single-mode fibre-coupling technology and ultralow-noise upconversion single-photon detectors, we have overcome the noise due to sunlight and demonstrate free-space quantum key distribution over 53 km during the day. The total channel loss is ∼48 dB, which is greater than the 40 dB channel loss between the satellite and ground and between low-Earth-orbit satellites. Our system thus demonstrates the feasibility of satellite-based quantum communication in daylight. Moreover, given that our working wavelength is located in the optical telecom band, our system is naturally compatible with ground fibre networks and thus represents an essential step towards a satellite-constellation-based global quantum network. The feasibility of satellite-assisted quantum communication is demonstrated by a field test on the ground. To supress noise due to sunlight the wavelength of 1,550 nm is chosen, and spectrum and spatial filtering technology developed.

Unconventional, special-purpose machines may aid in accelerating the solution of some of the hardest problems in computing, such as large-scale combinatorial optimizations, by exploiting different operating mechanisms than those of standard digital computers. We present a scalable optical processor with electronic feedback that can be realized at large scale with room-temperature technology. Our prototype machine is able to find exact solutions of, or sample good approximate solutions to, a variety of hard instances of Ising problems with up to 100 spins and 10,000 spin-spin connections.

Second-order nonlinear optical processes convert light from one wavelength to another and generate quantum entanglement. Creating chip-scale devices to efficiently control these interactions greatly increases the reach of photonics. Existing silicon-based photonic circuits utilize the third-order optical nonlinearity, but an analogous integrated platform for second-order nonlinear optics remains an outstanding challenge. Here we demonstrate efficient frequency doubling and parametric oscillation with a threshold of tens of micro-watts in an integrated thin-film lithium niobate photonic circuit. We achieve degenerate and non-degenerate operation of the parametric oscillator at room temperature and tune its emission over one terahertz by varying the pump frequency by hundreds of megahertz. Finally, we observe cascaded second-order processes that result in parametric oscillation. These resonant second-order nonlinear circuits will form a crucial part of the emerging nonlinear and quantum photonics platforms. Here, the authors demonstrate a chip-scale device that realizes a comprehensive set of resonant second order nonlinear processes including optical parametric oscillation with a threshold power of 70 microwatts.

The quantum noise of light, attributed to the random arrival time of photons from a coherent light source, fundamentally limits optical phase sensors. An engineered source of squeezed states suppresses this noise and allows phase detection sensitivity beyond the quantum noise limit (QNL). We need ways to use quantum light within deployable quantum sensors. Here we present a photonic integrated circuit in thin-film lithium niobate that meets these requirements. We use the second-order nonlinearity to produce a squeezed state at the same frequency as the pump light and realize circuit control and sensing with electro-optics. Using 26.2 milliwatts of optical power, we measure (2.7 ± 0.2)% squeezing and apply it to increase the signal-to-noise ratio of phase measurement. We anticipate that photonic systems like this, which operate with low power and integrate all of the needed functionality on a single die, will open new opportunities for quantum optical sensing. Squeezed light allows for quantum-enhanced, sub-shot-noise sensing, but its generation and use on a chip has so far remained elusive. Here, the authors fill this gap by demonstrating a thin-film lithium-niobate-based integrated quantum optical sensor, which beats shot-noise-limited SNR by ~ 4%.

This article reviews recent progress in quasi-phasematched χ ( 2 ) nonlinear nanophotonics, with a particular focus on dispersion-engineered nonlinear interactions. Throughout this article, we establish design rules for the bandwidth and interaction lengths of various nonlinear processes, and provide examples for how these processes can be engineered in nanophotonic devices. In particular, we apply these rules towards the design of sources of non-classical light and show that dispersion-engineered devices can outperform their conventional counterparts. Examples include ultra-broadband optical parametric amplification as a resource for measurement-based quantum computation, dispersion-engineered spontaneous parametric downconversion as a source of separable biphotons, and synchronously pumped nonlinear resonators as a potential route towards single-photon nonlinearities.

Amorphous films and coatings are rapidly growing in importance. Yet, there is a dearth of high-quality structural data on sub-micron films. Not understanding how these materials assemble at atomic scale limits fundamental insights needed to improve their performance. Here, we use grazing-incidence x-ray total scattering measurements to examine the atomic structure of the top 50–100 nm of Ta 2 O 5 films; mirror coatings that show high promise to significantly improve the sensitivity of the next generation of gravitational-wave detectors. Our measurements show noticeable changes well into medium range, not only between crystalline and amorphous, but also between as-deposited, annealed and doped amorphous films. It is a further challenge to quickly translate the structural information into insights into mechanisms of packing and disorder. Here, we illustrate a modeling approach that allows translation of observed structural features to a physically intuitive packing of a primary structural unit based on a kinked Ta-O-Ta backbone. Our modeling illustrates how Ta-O-Ta units link to form longer 1D chains and even 2D ribbons and how doping and annealing influences formation of 2D order. We also find that all the amorphousTa 2 O 5 films studied in here are not just poorly crystalline but appear to lack true 3D order.

The solvolysis of poly(ethylene-terephthalate) (PET) is one of the most researched areas in chemical recycling. In this study PET aminolysis with isophorondiamine has been done - in opposition to recent trends - without excess reagent and the raw reaction product was further used without purification. The aminolysis product was thoroughly characterized with nuclear magnetic resonance (NMR) spectroscopy. Isophoronediamine was used as a solvent to prepare amine crosslinker solutions for epoxy resins. The effect of the concentration on the cross-linking reaction and thermomechanical properties were investigated. The curing reaction was found to be significantly accelerated by the presence of the aminolysis product. Both the ethylene-glycol and the terephthal-amide-diamines have a catalytic effect on the reaction. The glass transition temperature decreased with increasing concentrations of the cross-linker solutions due to the decreasing cross-link density. Thus, raw aminolysis products can be utilized for epoxy curing and are advantageous in modifying slower curing cycloaliphatic cross linker systems.

Practical quantum communication between remote quantum memories rely on single photons at telecom wavelengths. Although spin-photon entanglement has been demonstrated in atomic and solid-state qubit systems, the produced single photons at short wavelengths and with polarization encoding are not suitable for long-distance communication, because they suffer from high propagation loss and depolarization in optical fibres. Establishing entanglement between remote quantum nodes would further require the photons generated from separate nodes to be indistinguishable. Here, we report the observation of correlations between a quantum-dot spin and a telecom single photon across a 2-km fibre channel based on time-bin encoding and background-free frequency downconversion. The downconverted photon at telecom wavelengths exhibits two-photon interference with another photon from an independent source, achieving a mean wavepacket overlap of greater than 0.89 despite their original wavelength mismatch (900 and 911 nm). The quantum-networking operations that we demonstrate will enable practical communication between solid-state spin qubits across long distances. Quantum communication requires quantum correlations between the information processing units and the information carrying units. Here, the authors use time-bin encoding and frequency downconversion to telecom wavelengths to achieve kilometre-scale spin-photon correlations.

Light absorption measurements of nominally undoped congruent lithium niobate crystals (CLN) as well as 5 mol% magnesium-oxide-doped lithium niobate (MgO:LN) crystals were performed in the light wavelength range of 350 to 800 nm. Absorption spectra reveal that—besides iron (Fe) impurities—chromium (Cr) impurities of less than 0.5 wt. ppm concentration contribute significantly to the total optical absorption in the CLN crystals with a maximum of 0.035 cm −1 around 500 nm. The axial distribution of Cr within a CLN boule is examined, revealing that the bottom part of the boule contains less Cr and therefore light absorption is reduced as well. In the case of the MgO:LN crystals, Cr impurities also contribute significantly to the total optical absorption, which is on the order of 0.025 cm −1 for ordinarily polarized light and 0.015 cm −1 for extraordinarily polarized light around 500 nm.