Single photons and entangled photon pairs are a key resource of many quantum secure communication and quantum computation protocols, and non-Poissonian sources emitting in the low-loss wavelength region around 1,550 nm are essential for the development of fibre-based quantum network infrastructure. However, reaching this wavelength window has been challenging for semiconductor-based quantum light sources. Here we show that quantum dot devices based on indium phosphide are capable of electrically injected single photon emission in this wavelength region. Using the biexciton cascade mechanism, they also produce entangled photons with a fidelity of 87 ± 4%, sufficient for the application of one-way error correction protocols. The material system further allows for entangled photon generation up to an operating temperature of 93 K. Our quantum photon source can be directly integrated with existing long distance quantum communication and cryptography systems, and provides a promising material platform for developing future quantum network hardware.

Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fiber-optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into one-dimensional optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realize QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs ''droplet-epitaxy'' of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at ∼900 nm, and metal-organic vapor-phase epitaxy on InP emitting at ∼1550 nm, with mean FSS 4× smaller than for Stranski-Krastanow QDs.

We demonstrate a new hot-carrier photovoltaic cell based on the resonant tunnelling of hot electrons from a narrow-band-gap semiconductor to a wider-band-gap semiconductor. Hot electrons are photogenerated at a variety of wavelengths in a GaAs absorber followed by resonant tunnelling through a double-barrier quantum well into an AlGaAs collector, forming an energy-selective interface in the centre of the device. We show theoretically the presence of a tunnel current from the absorber to the collector under illumination, offering a method to extract carriers from a hot-electron distribution at zero bias. We experimentally demonstrate a hot-carrier photovoltaic cell based on this concept. Two features of its measured current-voltage characteristic, namely the peak to valley current ratio and the current peak voltage, are shown to vary with the wavelength of illumination in a way that clearly demonstrates hot-carrier extraction.

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 telecom wavelength band around 1550 nm, where the photon coherence time has to be 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 even under non-resonant excitation, more than a factor two longer than values reported for shorter wavelength quantum dots under similar conditions. We demonstrate that these coherence times enable near-optimal interference with a C-band 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 fidelity reaching 88.3±4%. 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.

The growth of InAs Quantum Dots (QDs) on InP(100) via droplet epitaxy in a Metal Organic Vapour Phase Epitaxy (MOVPE) reactor is studied. Formation of Indium droplets is investigated with varying substrate temperature, and spontaneous formation of nanoholes is observed for the first time under MOVPE conditions. Indium droplets are crystallized into QDs under Arsenic flow at different temperatures. For temperatures greater than 500ºC, a local etching takes place in the QD vicinity, showing an unexpected morphology which is found to be strongly dependent on the Received: (( ))Revised: (( )) Published online: (( ))

This is a repository copy of Coherence in single photon emission from droplet epitaxy and Stranski-Krastanov quantum dots in the telecom C-band.

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Zinc Nitride (Zn3N2) films were grown by DC sputtering of a Zn target in a N2 plasma under a variety of different growth conditions, which resulted in the deposition of films with variable compositions. The as deposited films exhibited a polycrystalline Zn3N2 structure, which was converted to a ZnO-based structure after several weeks of ambient exposure. Zn3N2 films that were N-poor exhibited electrical properties indicative of a natively doped semiconductor and reached a minimum carrier concentration in the order of 1018 cm−3 at compositions, which approached the stoichiometric ratio of Zn3N2. A maximum carrier mobility of 33 cm2 V−1 s−1 was obtained in N-rich films due to an improved microstructure. The Zn3N2 films had an optical band gap of 1.31–1.48 eV and a refractive index of 2.3–2.7. Despite a wide range of Zn3N2 samples examined, little variation of its optical properties was observed, which suggests that they are closely related to the band structure of this material. In contrast to the as grown films, the oxidized film had a band gap of 3.44 eV and the refractive index was 1.6–1.8, similar to ZnO and Zn(OH)2.