Here we show Fourier-limited as well as elastically scattered photons at telecom wavelengths from InAs/InP quantum dots. Indistinguishability measurements over 25 km of fibre further highlight the potential of this system for quantum network applications.

Control over the shape and polarization of the beam emitted by a laser source is important in applications such as optical communications, optical manipulation and high-resolution optical imaging. In this paper, we present the inverse design of monolithic whispering-gallery nanolasers which emit along their axial direction with a tailored laser beam shape and polarization. We design and experimentally verify three types of submicron cavities, each one emitting into a different laser radiation mode: an azimuthally polarized doughnut beam, a radially polarized doughnut beam and a linearly polarized Gaussian-like beam. The measured output laser beams yield a field overlap with respect to the target mode of 92%, 96%, and 85% for the azimuthal, radial, and linearly polarized cases, respectively, thereby demonstrating the generality of the method in the design of ultracompact lasers with tailored beams.

We propose a novel semi-empirical approach for mapping the performance of InP quantum dots passively monolithic two-section mode-locked lasers (MLLs). A prediction of the mode-locking regimes for varying reverse-bias conditions and saturable absorber to gain section length ratios has been determined for 3mm InP MLLs.

Five mesa p(+)-i-n(+) photodiodes (each of 0.126 mm(2) area) were investigated as conversion devices for X-ray-voltaics, in order to understand the comparative effects between different semiconductor materials, device structures, and X-ray incident power, and to explore them for use in future radioisotope microbatteries. Three semiconductor materials (AlInP, InGaP, and GaAs) and three i layer thicknesses of one material, AlInP (2 mu m, 6 mu m, and 10 mu m), were investigated under the illumination of various controlled X-ray incident powers. The highest short circuit current was achieved with the GaAs device due to its thickest active layer and highest linear absorption coefficients at the most numerous incident X-ray photon energies (Mo K alpha at 17.48 keV; Mo K beta at 19.6 keV). The highest open circuit voltage was achieved with the 10 mu m AlInP device due to its widest bandgap (cf. InGaP and GaAs) and its highest short circuit current (cf. the 2 mu m and the 6 mu m AlInP devices). The greatest output X-ray power was recorded with the InGaP device due to its highest fill factor (i.e. relatively low series and high shunt resistance) compared to the rest of the devices, although the GaAs device had the highest theoretical output X-ray power. A method for selecting the most suitable semiconductor material and device structure of conversion devices for radioisotope microbatteries (for exhibiting the highest power output) is presented considering the incident X-ray spectrum, while highlighting the importance of non-ideal device effects (reduced charge collection efficiency, increased series resistance, and reduced shunt resistance).

An AlInP 3 × 3 pixel monolithic array was fabricated from a p+-i-n+ structure wafer (6 μm thick i layer) grown by metalorganic vapour phase epitaxy; each pixel (square mesa photodiode) had an area of 200 μm × 200 μm. The pixels were initially electrically characterized and were then subjected to illumination by X-ray and γ-ray photons of energies ≤88 keV while each of them was in turn coupled to a custom-made, low noise, charge-sensitive preamplifier. The photon counting X-/γ-ray spectroscopic performance of the pixels was investigated by obtaining and analysing 55Fe X-ray, 241Am X-/γ-ray, and 109Cd X-/γ-ray spectra; the energy resolution (Full Width at Half Maximum), when the array and preamplifier were operated at the maximum investigated temperature (100 °C), was 1.54 keV ± 0.08 keV at 5.9 keV, 1.58 keV ± 0.08 keV at 22.16 keV, and 1.57 keV ± 0.08 keV at 59.54 keV. This work sets the agenda for future development of an AlInP photon counting X-/γ-ray spectroscopic imager for uncooled operation in high temperature environments.

A 2 × 2 square pixel In0.5Ga0.5P p+-i-n+ mesa photodiode array was fabricated and investigated for its suitability in photon counting X-ray and γ-ray spectroscopy. Each pixel, which had an area of 200μm × 200μm and a 5μm thick i layer, was coupled to a low-noise charge-sensitive preamplifier and standard onwards readout electronics to form an X-ray and γ-ray photon counting spectrometer. The pixels were illuminated in turn with an 55Fe radioisotope X-ray source, an 241Am radioisotope X-ray and γ-ray source, and a 109Cd radioisotope X-ray and γ-ray source. The mean value (across all pixels) of the best energy resolution (Full Width at Half Maximum, FWHM) at 20 °C was 770 eV ± 30 eV at 5.9 keV, 840 eV ± 20 eV at 22.16 keV, and 870 eV ± 30 eV at 59.54 keV. The spectroscopic response of one of the pixels was then investigated at temperatures up to 100 °C; noise analysis was performed and the different noise contributions were identified. The FWHM at 100 °C was 1.29 keV ± 0.04 keV at 5.9 keV, 1.32 keV ± 0.06 keV at 22.16 keV, 1.34 keV ± 0.06 keV at 59.54 keV, and 1.43 keV ± 0.08 keV at 88.03 keV. The results indicate that the detector did not suffer from incomplete charge collection, and that the spectrometer had better energy resolution at 100 °C than any other multi-pixel radiation spectrometer so far reported.

The ability of two photons to interfere lies at the heart of many photonic quantum networking concepts and requires that the photons are indistinguishable with sufficient coherence times to resolve the interference signals. However, for solid-state quantum light sources, this can be challenging to achieve as they are in constant interaction with noise sources in their environment. Here, we investigate the noise sources that affect InAs/InP quantum dots emitting in the telecom C-band by comparing their behavior on a wetting layer for Stranski–Krastanov grown quantum dots with a nearly wetting layer-free environment achieved with the droplet epitaxy growth mode. We show that the droplet epitaxy growth mode is beneficial for a quiet environment, leading to 96% of exciton transitions having a coherence time longer than the typical detector resolution of 100 ps, even under non-resonant excitation. We also show that the decay profile indicates the presence of slow dephasing processes, which can be compensated for experimentally. We finally conduct Hong–Ou–Mandel interference measurements between subsequently emitted photons and find a corrected two-photon interference visibility of 98.6 ± 1.6% for droplet-epitaxy grown quantum dots. The understanding of the influence of their surroundings on the quantum optical properties of these emitters is important for their optimization and use in future quantum networking applications.

The development of new x-ray and gamma-ray spectrometers based on AlInP photodiodes with increased quantum detection efficiency and improved energy resolution is reported. The spectroscopic responses of two AlInP p(+)-i-n(+) mesa photodiodes (10 mu m i layer, the thickest so far reported) were investigated at photon energies from 4.95 to 88.03keV; the detectors and preamplifier were operated at 30 degrees C. Energy resolutions (full width at half maximum) of 750 +/- 40eV and 850 +/- 30eV at 4.95keV were achieved with the two detectors. The energy resolution deteriorated with increasing photon energy; this was in accordance with the increasing Fano noise with energy and suggested negligible incomplete charge collection noise across the photon energy range investigated. The measured voltage output of each spectrometer was found to be linear as a function of incident x-ray photon energy. The count rate (measured at 8.63keV) was also found to linearly increase with incoming x-ray photon flux for the investigated spectrometers. These results, which were obtained using the thickest AlInP photodiodes produced so far, suggest that AlInP detectors are highly promising candidates for future uncooled x-ray and gamma-ray spectrometers.

The development of new x-ray and γ-ray spectrometers based on AlInP photodiodes with increased quantum detection efficiency and improved energy resolution is reported. The spectroscopic responses of two AlInP p+–i–n+ mesa photodiodes (10 μm i layer, the thickest so far reported) were investigated at photon energies from 4.95 to 88.03 keV; the detectors and preamplifier were operated at 30 °C. Energy resolutions (full width at half maximum) of 750 ± 40 eV and 850 ± 30 eV at 4.95 keV were achieved with the two detectors. The energy resolution deteriorated with increasing photon energy; this was in accordance with the increasing Fano noise with energy and suggested negligible incomplete charge collection noise across the photon energy range investigated. The measured voltage output of each spectrometer was found to be linear as a function of incident x-ray photon energy. The count rate (measured at 8.63 keV) was also found to linearly increase with incoming x-ray photon flux for the investigated spectrometers. These results, which were obtained using the thickest AlInP photodiodes produced so far, suggest that AlInP detectors are highly promising candidates for future uncooled x-ray and γ-ray spectrometers.

Quantum light emitting diodes are important for many secure quantum networking applications. Here, we address some remaining practical challenges, and present progress in improving the electrical frequency, fiber compatibility and optical coherence of these devices.

The development and characterization of a prototype temperature tolerant (capable of operation up to at least 100 degrees C) particle counting electron spectrometer with an AlInP detector is reported. This is the first time that the response of an AlInP detector to electrons (beta- particles) has been reported. The detector was a custom made circular mesa (200 mu m diameter) Al0.52In0.48P p(+)-i-n(+) (2 mu m i layer) photodiode; this was coupled to a custom made low-noise charge-sensitive preamplifier and otherwise standard readout electronics. The detector was electrically characterized and the spectrometer was investigated for its response to illumination from a(63)Ni radioisotope beta(-) particle source over the temperature range 100 degrees C-20 degrees C. The absorbed electron energy within the active region (i layer) of the AlInP detector and the expected to be detected spectra were calculated using Monte Carlo simulations. Comparisons between the simulated and measured spectra indicated that the response of the spectrometer was in agreement with the Monte Carlo model. Future generations of electron spectrometers of this type are expected to be useful for space science missions where the instrumentation would be subject to high temperatures and intense radiation (e.g. to study the radiolytic processes in comets close to perihelion). In order to inform development of future generations of AlInP electron detectors for such applications, the response of the prototype instrument to illumination with solar wind electrons was modelled within the experimentally verified energy range of the detector; avenues of future development to improve AlInP detector performance, identified from the presently reported results, was investigated and discussed.

The impact ionization characteristics of (Al x Ga 1-x ) 0.52 In 0.48 P have been studied comprehensively across the full composition range. Electron and hole impact ionization coefficients ( α and β, respectively) have been extracted from avalanche multiplication and excess noise data for seven different compositions and compared to those of Al x Ga 1-x As. While both α and β initially decrease gradually with increasing bandgap, a sharp decrease in β occurs in (Al x Ga 1-x ) 0.52 In 0.48 P when x > 0.61, while α decreases only slightly. α and β decrease minimally with further increases in x and the breakdown voltage saturates. This behavior is broadly similar to that seen in Al x Ga 1-x As, suggesting that it may be related to the details of the conduction band structure as it becomes increasingly indirect in both alloy systems.

A practical way to link separate nodes in quantum networks is to send photons over the standard telecom fibre network. This requires sub-Poissonian photon sources in the wavelength band around 1550 nm, with photon coherence times sufficient to enable the many interference-based technologies at the heart of quantum networks. Here, we show that droplet epitaxy InAs/InP quantum dots emitting in the telecom C-band can provide photons with coherence times exceeding 1 ns under low power non-resonant excitation, and demonstrate that these coherence times enable near-optimal interference with a C-band polarisation-encoded laser qubit, with visibilities only limited by the quantum dot multiphoton emission. Using entangled photons, we further show teleportation of such qubits in six different bases with average postselected fidelity reaching 88.3 ± 4.0%. Beyond direct applications in long-distance quantum communication, the high degree of coherence in these quantum dots is promising for future spin-based telecom quantum network applications.

This letter reports on InP/GaInP quantum dot mode-locked lasers emitting in the 730 nm wavelength region, extending the spectral range of previously reported monolithic mode-locked edge-emitting lasers. Modal gain and absorption measurements were used to identify a relatively broad spectrum which is utilised to support passive mode-locking in a monolithically integrated two-section ridge laser. The conditions for mode-locking were explored by varying the current to the gain section and reverse bias to the absorber section. For a total cavity length of 3 mm, the shortest pulse train observed was 6 ps in duration with a repetition rate of 12.55 GHz.

A prototype In 0.53 Ga 0.47 As p + -i-n + x-ray photodiode, fabricated from material grown by metalorganic vapour phase epitaxy, was investigated as a novel detector of x-rays. The detector was connected to a custom low-noise charge sensitive preamplifier and standard readout electronics to produce an x-ray spectrometer. The detector and preamplifier were operated at a temperature of 233 K (−40 °C). An energy resolution of 1.18 keV ± 0.06 keV Full Width at Half Maximum at 5.9 keV was achieved. This is the first time InGaAs (GaInAs) has been shown to be capable of spectroscopic photon counting x-ray detection.

Previously, an Al0.52In0.48P p(+)-i-n(+) spectroscopic photon counting X-ray photodiode with 2 mu m thick i layer (200 pm diameter) was shown to suffer from energy-dependent incomplete charge collection noise (Lioliou et al., 2019). Subsequent measurements on a larger (400 mu m diameter) Al0.52In0.48P p(+)-i-n(+) photodiode (reported here) revealed the presence of even greater incomplete charge collection noise. Given these findings, an expectation would have been that thicker Al0.52In0.48P structures (which would be required for efficient absorption of all but the softest X-rays) would have a greater incomplete charge collection noise contribution, thus suggesting that thick Al0.52In0.48P photodiodes may be of limited practicality as high performance detectors for photon counting X-ray spectroscopy. However, two new Al0.52In0.48P p(+)-i-n(+)photodiodes (with 6 mu m i layers) were fabricated from material grown by the same technique (metalorganic vapour phase epitaxy) in the same reactor, and are now shown here to exhibit no signs of detectable incomplete charge collection noise under the illumination of X-ray photons of energy 4.95 keV to 21.17 keV. As such, now that greater experience has been built with Al0.52In0.48P, concerns about incomplete charge collection noise in X-ray detectors made from the material appear to have been unwarranted; the path towards thick Al0.52In0.48P X-ray detectors is now clear.

Teleportation is a fundamental concept of quantum mechanics with an important application in extending the range of quantum communication channels via quantum relay nodes. To be compatible with real-world technology such as secure quantum key distribution over fibre networks, such a relay node must operate at GHz clock rates and accept time-bin encoded qubits in the low-loss telecom band around 1550 nm. Here, we show that InAs/InP droplet epitaxy quantum dots with their sub-Poissonian emission near 1550 nm are ideally suited for the realisation of this technology. To create the necessary on-demand photon emission at GHz clock rates, we develop a flexible pulsed optical excitation scheme, and demonstrate that the fast driving conditions are compatible with a low multiphoton emission rate. We show further that, even under these driving conditions, photon pairs obtained from the biexciton cascade show an entanglement fidelity close to 90\%, comparable to the value obtained under cw excitation. Using asymetric Mach Zehnder interferometers and our photon source, we finally construct a time-bin qubit quantum relay able to receive and send time-bin encoded photons, and demonstrate mean teleportation fidelities of $0.82\pm0.01$, exceeding the classical limit by nearly 10 standard deviations.

Quantum networks are essential for realising distributed quantum computation and quantum communication. Entangled photons are a key resource, with applications such as quantum key distribution, quantum relays, and quantum repeaters. All components integrated in a quantum network must be synchronised and therefore comply with a certain clock frequency. In quantum key distribution, the most mature technology, clock rates have reached and exceeded 1GHz. Here we show the first electrically pulsed sub-Poissonian entangled photon source compatible with existing fiber networks operating at this clock rate. The entangled LED is based on InAs/InP quantum dots emitting in the main telecom window, with a multi-photon probability of less than 10% per emission cycle and a maximum entanglement fidelity of 89%. We use this device to demonstrate GHz clocked distribution of entangled qubits over an installed fiber network between two points 4.6km apart.

Two circular Al0.6Ga0.4As p+-i-n+ 2 µm i layer spectroscopic x-ray avalanche photodiodes (one 200 µm diameter and one 400 µm diameter) were made from a structure produced by metalorganic vapour phase epitaxy. The capacitances and currents of the detectors as functions of applied bias were measured, and 55Fe x-ray (Mn Kα = 5.9 keV; Mn Kβ = 6.49 keV) spectra were accumulated at 20 °C (293 K). Improved energy resolutions (measured as the full width at half maximum of the 5.9 keV peak) with increased applied reverse bias were observed with both detectors. In part, the improvement was attributed to avalanche multiplication. Energy resolutions of 630 eV ± 40 eV and 730 eV ± 50 eV were achieved with the 200 µm detector at an applied reverse bias of 38 V and the 400 µm detector at an applied reverse bias of 40 V, respectively. It is the first time Al0.6Ga0.4As has been demonstrated as capable of photon counting x-ray spectrometry. Measurements to determine the average electron-hole pair creation energy in Al0.6Ga0.4As were made; the results suggested a value of 4.97 eV ± 0.12 eV at 25 °C ± 1 °C (298 K ± 1 K). This value was then used to refine the apparent relationship between bandgap energy and electron-hole pair creation energy as defined by the Bertuccio–Maiocchi–Barnett relationship. AlxGa1-xAs x-ray photodiodes of this type are anticipated to be of benefit for future space missions, including those to explore the surfaces of the inner planets (e.g. Mercury and Venus) and the moons of Jupiter and Saturn.