Gabriele Bulgarini

Bright single-photon sources in bottom-up tailored nanowires

The ability to achieve near-unity light-extraction efficiency is necessary for a truly deterministic single-photon source. The most promising method to reach such high efficiencies is based on embedding single-photon emitters in tapered photonic waveguides defined by top-down etching techniques. However, light-extraction efficiencies in current top-down approaches are limited by fabrication imperfections and etching-induced defects. The efficiency is further tempered by randomly positioned off-axis quantum emitters. Here we present perfectly positioned single quantum dots on the axis of a tailored nanowire waveguide using bottom-up growth. In comparison to quantum dots in nanowires without waveguides, we demonstrate a 24-fold enhancement in the single-photon flux, corresponding to a light-extraction efficiency of 42%. Such high efficiencies in one-dimensional nanowires are promising to transfer quantum information over large distances between remote stationary qubits using flying qubits within the same nanowire p–n junction.

Avalanche amplification of a single exciton in a semiconductor nanowire

Researchers investigate the internal gain of InAsP quantum dots embedded in an InP nanowire by performing photocurrent measurements down to the single-photon regime. The resulting gain ( > 104) is a significant step towards single-shot electrical read-out of an exciton qubit state for the transfer of quantum information between flying and stationary qubits.

Ultraclean Emission from InAsP Quantum Dots in Defect-Free Wurtzite InP Nanowires

We report on the ultraclean emission from single quantum dots embedded in pure wurtzite nanowires. Using a two-step growth process combining selective-area and vapor-liquid-solid epitaxy, we grow defect-free wurtzite InP nanowires with embedded InAsP quantum dots, which are clad to diameters sufficient for waveguiding at λ ~ 950 nm. The absence of nearby traps, at both the nanowire surface and along its length in the vicinity of the quantum dot, manifests in excitonic transitions of high spectral purity. Narrow emission line widths (30 μeV) and very-pure single photon emission with a probability of multiphoton emission below 1% are achieved, both of which were not possible in previous work where stacking fault densities were significantly higher.

Nanowire waveguides launching single photons in a Gaussian mode for ideal fiber coupling

Quantum communication as well as integrated photonic circuits require single photons propagating in a well-defined Gaussian mode. However, tailoring the emission mode to a Gaussian remains an unsolved challenge for solid-state quantum emitters due to their random positioning in the host material or photonic structure. Here, we overcome these limitations by embedding a semiconductor quantum dot in a tapered nanowire waveguide. Owing to the deterministic positioning of the emitter in the waveguide, we demonstrate a Gaussian emission profile in the far field. Hence, we further couple the emission into a single-mode optical fiber with a record efficiency of 93%, thereby addressing a major hurdle for practical implementation of single photon sources in emerging photonic technologies.

Spontaneous emission control of single quantum dots in bottom-up nanowire waveguides

Nanowire waveguides with controlled shape are promising for engineering the collection efficiency of quantum light sources. We investigate the exciton lifetime in individual InAsP quantum dots, perfectly positioned on-axis of InP nanowire waveguides. We demonstrate control over the quantum dot spontaneous emission by varying the nanowire diameter in e-beam patterned arrays, which modifies the coupling efficiency of the emitter to the fundamental waveguide mode. The spontaneous emission rate is inhibited by a factor of 12 in thin nanowires compared to nanowires with optimized waveguide diameter. From the measured inhibition factor, we determine a high radiative yield exceeding 92% in bottom-up grown nanowires. V C 2012 American Institute of Physics .[ http://dx.doi.org/10.1063/1.3694935] Nanowire waveguides enable the efficient collection of single photons emitted from solid-state sources with the promise to reach efficiencies larger than 90%. High collection efficiency has been demonstrated for self-assembled quantum dots in top-down etched nanowires, 1 single quantum dots in bottom-up grown nanowires, 2 and color-centers in etched diamond nanowires. 3 One of the key parameters in designing a waveguide structure is the ratio between the nanowire diameter, D, and the emission wavelength, k. Under optimum waveguide conditions (D/k ¼0.22 for GaAs (Ref. 4) and 0.23 for InP (Ref. 2) nanowires), emitted photons are funneled into the fundamental waveguide mode confined in the nanowire. As a result, the emission is very directional and can be efficiently collected with limited numerical aperture objectives. 4 In contrast, at low D/k ratios, optical modes are no longer confined in the nanowire and the source only couples to a continuum of non-guided radiative modes yielding a non-directional emission pattern and inefficient collection of the light. Bleuse et al. 5 demonstrated that the spontaneous emis

Design of broadband high-efficiency superconducting-nanowire single photon detectors

In this paper several designs to maximize the absorption efficiency of superconducting-nanowire single-photon detectors are investigated. Using a simple optical cavity consisting of a gold mirror and a SiO2 layer, the absorption efficiency can be boosted to over 97%: this result is confirmed experimentally by the realization of an NbTiN-based detector having an overall system detection efficiency of 85% at 1.31 μm. Calculations show that by sandwiching the nanowire between two dielectric Bragg reflectors, unity absorption (>99.9%) could be reached at the peak wavelength for optimized structures. To achieve broadband high efficiency, a different approach is considered: a waveguide-coupled detector. The calculations performed in this work show that, by correctly dimensioning the waveguide and the nanowire, polarization-insensitive detectors absorbing more than 95% of the injected photons over a wavelength range of several hundred nm can be designed. We propose a detector design making use of GaN/AlN waveguides, since these materials allow lattice-matched epitaxial deposition of Nb(Ti)N films and are transparent on a very wide wavelength range.

Single-photon detectors combining high efficiency, high detection rates, and ultra-high timing resolution

Single-photon detection with high efficiency, high time resolution, low dark counts, and high photon detection rates is crucial for a wide range of optical measurements. Although efficient detectors have been reported before, combining all performance parameters in a single device remains a challenge. Here, we show a broadband NbTiN superconducting nanowire detector with an efficiency exceeding 92%, over 150 MHz photon detection rate, and a dark count rate below 130 Hz operated in a Gifford-McMahon cryostat. Furthermore, with careful optimization of the detector design and readout electronics, we reach an ultra-low system timing jitter of 14.80 ps (13.95 ps decoupled) while maintaining high detection efficiencies (>75%).

Far field emission profile of pure wurtzite InP nanowires

We report on the far field emission profile of pure wurtzite InP nanowires in comparison to InP nanowires with predominantly zincblende crystal structure. The emission profile is measured on individual nanowires using Fourier microscopy. The most intense photoluminescence of wurtzite nanowires is collected at small angles with respect to the nanowire growth axis. In contrast, zincblende nanowires present a minimum of the collected light intensity in the direction of the nanowire growth. Results are explained by the orientation of electric dipoles responsible for the photoluminescence, which is different from wurtzite to zincblende. Wurtzite nanowires have dipoles oriented perpendicular to the nanowire growth direction, whereas zincblende nanowires have dipoles oriented along the nanowire axis. This interpretation is confirmed by both numerical simulations and polarization dependent photoluminescence spectroscopy. Knowledge of the dipole orientation in nanostructures is crucial for developing a wide range of photonic devices such as light-emitting diodes, photodetectors, and solar cells.

High absorption efficiency and polarization-insensitivity in superconducting-nanowire single-photon detectors

The performance of superconducting-nanowire single-photon detectors depends on the efficiency of light absorption in the ultrathin (3-8 nm) superconducting nanowire. In this work, we will discuss two approaches to boost light absorption: coupling the nanowire to the evanescent field propagating in a waveguide and enclosing the nanowire in an optical cavity. The latter method is the most widely used, but it is intrinsically very sensitive to the polarization of light. To overcome this issue, we propose some innovative cavity designs which make use of high-index (n >2) dielectrics. With this technique, highly-efficient polarization-insensitive devices can be easily implemented.

Bright single-photon sources based on self-aligned quantum dots in tapered nanowire waveguides

Single quantum dots embedded in tapered nanowire waveguides have emerged as leading candidates for designing high efficiency single-photon and entangled photon sources, with efficiencies exceeding 90%. Here we have developed a bottom-up growth approach that allows for independent control of boththe quantum dot size, and position, as well as the nanowire shape. Importantly, by design, the single quantum dot is always found perfectly on the nanowire axis. By integrating a gold mirror at the base of a tapered nanowire waveguide we obtain a 20-fold enhancement in the single-photon flux in comparison to no waveguide. The 20-fold enhancement is accompanied by a shortening of the exciton lifetime as the quantum emitter couples to the fundamental waveguide mode with an increased rate. Finally, the optical quality of the emitter is drastically improved by removing the nanowire stacking faults in the vicinity of the quantum dot. As a result, we demonstrate very pure single-photon emission with a probability of multi-photon emission below 1%, and an emission line width that is reduced by at least an order of magnitude (<30 μeV) as compared to when stacking faults were present in the nanowire (as high as 10-100 per micron). The demonstrated brightness of our single-photon source (42 % efficiency), combined with the very pure single photon emission and high spectral purity is encouraging in development of future quantum technologies based on nanowires, such as interfacing remote quantum bits or constructing a secure quantum network.