Instantaneous frequency measurement system based on quantum dash mode-locked laser.

We present the theory and experimental results of a microwave photonic (MWP) filter based instantaneous frequency measurement system. A quantum dash mode-locked laser is used as an optical frequency comb source. With up to 41 flat comb lines and a real-time feedback loop for comb shaping, a set of MWP filters with linear frequency responses for either linear unit or dB unit are experimentally demonstrated. The maximum measurement frequency can be up to 20 GHz limited by the available test-and-measurement instruments. By using one MWP filter, the root-mean-square error is 51∼66 MHz, which can be improved to 42.2 MHz for linear unit, and 30.7 MHz for dB unit by using two MWP filters together.

Performance of quantum-dash mode-locked lasers (QD-MLLDs) for high-capacity coherent optical communications.

We investigate the capabilities and limitations of quantum-dash mode-locked lasers (QD-MLLDs) as optical frequency comb sources in coherent optical communication systems. We demonstrate that QD-MLLDs are on par with conventional single-wavelength narrow linewidth laser sources and can support high symbol rates and modulation formats. We manage to transmit 64 quadrature amplitude modulation (QAM) signals up to 80 GBd over 80 km of standard single-mode fiber (SSMF), which highlights the distinctive phase noise performance of the QD-MLLD. Using a 38.5 GHz (6 dB bandwidth) silicon photonic (SiP) modulator, we achieve a maximum symbol rate of 104 GBd with 16QAM signaling and a maximum net rate of 416 Gb/s per carrier in a single polarization setup and after 80 km-SSMF transmission. We also compare QD-MLLD performance with commercial narrow-linewidth integrable tunable laser assemblies (ITLAs) and explore their potential for use as local oscillators (LOs) and signal carriers. The QD-MLLD has 45 comb lines usable for transmission at a frequency spacing of 25 GHz, and an RF linewidth of 35 kHz.

Photonic beamforming using a quantum-dash optical frequency comb source.

We demonstrate photonic beamforming using a quantum-dash (QD) optical frequency comb (OFC) source. Thanks to the 25 GHz free spectral range (FSR) and up to 40 comb lines available from the QD OFC, we can implement phased antenna arrays (PAAs) with directional radiation and scanning. We consider two types of PAAs: a uniform linear array (ULA) and a uniform planar array (UPA). By selecting different comb lines with a programmable optical filter, we can tune the FSR of the OFC source and realize a discrete scanning function. We evaluate the beam squint of the ULAs, and the results show that we can achieve broadband operation. Finally, we show that we can achieve both directional radiation and scanning simultaneously using the UPA.

Performance Investigations of InAs/InP Quantum-Dash Semiconductor Optical Amplifiers with Different Numbers of Dash Layers.

We present here a performance comparison of quantum-dash (Qdash) semiconductor amplifiers (SOAs) with three, five, eight, and twelve InAs dash layers grown on InP substrates. Other than the number of Qdash layers, the structures were identical. The eight-layer Qdash SOA gave the highest amplified spontaneous emission power (4.3 dBm) and chip gain (26.4 dB) at 1550 nm, with a 300 mA CW bias current and at 25 °C temperature, while SOAs with fewer Qdash layers (for example, three-layer Qdash SOA), had a wider ASE bandwidth (90 nm) and larger 3 dB gain saturated output power (18.2 dBm) in a shorter wavelength range. The noise figure (NF) of the SOAs increased nearly linearly with the number of Qdash layers. The longest gain peak wavelength of 1570 nm was observed for the 12-layer Qdash SOA. The most balanced performance was obtained with a five-layer Qdash SOA, with a 25.4 dB small-signal chip gain, 15.2 dBm 3 dB output saturated power, and 5.7 dB NF at 1532 nm, 300 mA and 25 °C. These results are better than those of quantum well SOAs reported in a recent review paper. The high performance of InAs/InP Qdash SOAs with different Qdash layers shown in this paper could be important for many applications with distinct requirements under uncooled scenarios.

Dynamic Strain Modulation of a Nanowire Quantum Dot Compatible with a Thin-Film Lithium Niobate Photonic Platform.

The integration of indistinguishable single photon sources in photonic circuits is a major prerequisite for on-chip quantum applications. Among the various high-quality sources, nanowire quantum dots can be efficiently coupled to optical waveguides because of their preferred emission direction along their growth direction. However, local tuning of the emission properties remains challenging. In this work, we transfer a nanowire quantum dot onto a bulk lithium niobate substrate and show that its emission can be dynamically tuned by acousto-optical coupling with surface acoustic waves. The purity of the single photon source is preserved during the strain modulation. We further demonstrate that the transduction is maintained even with a SiO2 encapsulation layer deposited on top of the nanowire acting as the cladding of a photonic circuit. Based on these experimental findings and numerical simulations, we introduce a device architecture consisting of a nanowire quantum dot efficiently coupled to a thin-film lithium niobate rib waveguide and strain-tunable by surface acoustic waves.

Scalable Generation and Detection of on-Demand W States in Nanophotonic Circuits.

Quantum physics phenomena, entanglement and coherence, are crucial for quantum information protocols, but understanding these in systems with more than two parts is challenging due to increasing complexity. The W state, a multipartite entangled state, is notable for its robustness and benefits in quantum communication. Here, we generate eight-mode on-demand single-photon W states, using nanowire quantum dots and a silicon nitride photonic chip. We demonstrate a reliable and scalable technique for reconstructing the W state in photonic circuits using Fourier and real-space imaging, supported by the Gerchberg-Saxton phase retrieval algorithm. Additionally, we utilize an entanglement witness to distinguish between mixed and entangled states, thereby affirming the entangled nature of our generated state. The study provides a new imaging approach of assessing multipartite entanglement in W states, paving the way for further progress in image processing and Fourier-space analysis techniques for complex quantum systems.

Tuning the Liquid-Vapour Interface of VLS Epitaxy for Creating Novel Semiconductor Nanostructures.

Controlling the morphology and composition of semiconductor nano- and micro-structures is crucial for fundamental studies and applications. Here, Si-Ge semiconductor nanostructures were fabricated using photolithographically defined micro-crucibles on Si substrates. Interestingly, the nanostructure morphology and composition of these structures are strongly dependent on the size of the liquid-vapour interface (i.e., the opening of the micro-crucible) in the CVD deposition step of Ge. In particular, Ge crystallites nucleate in micro-crucibles with larger opening sizes (3.74-4.73 µm2), while no such crystallites are found in micro-crucibles with smaller openings of 1.15 µm2. This interface area tuning also results in the formation of unique semiconductor nanostructures: lateral nano-trees (for smaller openings) and nano-rods (for larger openings). Further TEM imaging reveals that these nanostructures have an epitaxial relationship with the underlying Si substrate. This geometrical dependence on the micro-scale vapour-liquid-solid (VLS) nucleation and growth is explained within a dedicated model, where the incubation time for the VLS Ge nucleation is inversely proportional to the opening size. The geometric effect on the VLS nucleation can be used for the fine tuning of the morphology and composition of different lateral nano- and micro-structures by simply changing the area of the liquid-vapour interface.

Position-Controlled Telecom Single Photon Emitters Operating at Elevated Temperatures.

A key resource in quantum-secured communication protocols are single photon emitters. For long-haul optical networks, it is imperative to use photons at wavelengths compatible with telecom single mode fibers. We demonstrate high purity single photon emission at 1.31 µm using deterministically positioned InP photonic waveguide nanowires containing single InAsP quantum dot-in-a-rod structures. At excitation rates that saturate the emission, we obtain a single photon collection efficiency at first lens of 27.6% and a probability of multiphoton emission of g(2)(0) = 0.021. We have also evaluated the performance of the source as a function of temperature. Multiphoton emission probability increases with temperature with values of 0.11, 0.34, and 0.57 at 77, 220 and 300 K, respectively, which is attributed to an overlap of temperature-broadened excitonic emission lines. These results are a promising step toward scalably fabricating telecom single photon emitters that operate under relaxed cooling requirements.

Nuclear magnetic resonance diffraction with subangstrom precision.

We have combined ultrasensitive force-based spin detection with high-fidelity spin control to achieve NMR diffraction (NMRd) measurement of ~2 million [Formula: see text]P spins in a [Formula: see text] volume of an indium-phosphide (InP) nanowire. NMRd is a technique originally proposed for studying the structure of periodic arrangements of spins, with complete access to the spectroscopic capabilities of NMR. We describe two experiments that realize NMRd detection with subangstrom precision. In the first experiment, we encode a nanometer-scale spatial modulation of the z-axis magnetization of [Formula: see text]P spins and detect the period and position of the modulation with a precision of <0.8 Å. In the second experiment, we demonstrate an interferometric technique, utilizing NMRd, to detect an angstrom-scale displacement of the InP sample with a precision of 0.07 Å. The diffraction-based techniques developed in this work extend the Fourier-encoding capabilities of NMR to the angstrom scale and demonstrate the potential of NMRd as a tool for probing the structure and dynamics of nanocrystalline materials.

Unity yield of deterministically positioned quantum dot single photon sources.

We report on a platform for the production of single photon devices with a fabrication yield of 100%. The sources are based on InAsP quantum dots embedded within position-controlled bottom-up InP nanowires. Using optimized growth conditions, we produce large arrays of structures having highly uniform geometries. Collection efficiencies are as high as 83% and multiphoton emission probabilities as low as 0.6% with the distribution away from optimal values associated with the excitation of other charge complexes and re-excitation processes, respectively, inherent to the above-band excitation employed. Importantly, emission peak lineshapes have Lorentzian profiles indicating that linewidths are not limited by inhomogeneous broadening but rather pure dephasing, likely elastic carrier-phonon scattering due to a high phonon occupation. This work establishes nanowire-based devices as a viable route for the scalable fabrication of efficient single photon sources and provides a valuable resource for hybrid on-chip platforms currently being developed.

Reconfigurable microwave photonic filter based on a quantum dash mode-locked laser.

We demonstrate a reconfigurable microwave photonic (MWP) filter using a quantum dash (QDash) mode-locked laser (MLL) that can generate an optical frequency comb (OFC) with ∼50 comb lines and a free spectral range of 25 GHz. Thanks to the large number of comb lines, the MWP filter responses can be easily programmed by tailoring the OFC spectrum. We implement MWP filter responses with Gaussian, sinc, flat-top, and multiple peaks, as well as demonstrate that tuning of the central frequency. We achieve a minimum 3 dB bandwidth of ∼100 MHz for a sinc-shaped MWP filter, while the maximum out-of-band rejection can be up to ∼30 dB with Gaussian apodization. Our results show that the QDash-MLL is a promising OFC source for developing integrated and reconfigurable MWP filters.

Magnetic tuning of tunnel coupling between InAsP double quantum dots in InP nanowires.

We study experimentally and theoretically the in-plane magnetic field dependence of the coupling between dots forming a vertically stacked double dot molecule. The InAsP molecule is grown epitaxially in an InP nanowire and interrogated optically at millikelvin temperatures. The strength of interdot tunneling, leading to the formation of the bonding-antibonding pair of molecular orbitals, is investigated by adjusting the sample geometry. For specific geometries, we show that the interdot coupling can be controlled in-situ using a magnetic field-mediated redistribution of interdot coupling strengths. This is an important milestone in the development of qubits required in future quantum information technologies.

InAs/InP quantum dot mode-locked laser with an aggregate 12.544 Tbit/s transmission capacity.

Chip-scale optical frequency comb sources are ideal compact solutions to generate high speed optical pulses for applications in wavelength division multiplexing (WDM) and high-speed optical signal processing. Our previous studies have concentrated on the use of quantum dash based lasers, but here we present results from an InAs/InP quantum dot (QDot) C-band passively mode-locked laser (MLL) for frequency comb generation. By using this single-section QDot-MLL we demonstrate an aggregate line rate of 12.544 Tbit/s 16QAM data transmission capacity for both back-to-back (B2B) and over 100-km of standard single mode fiber (SSMF). This finding highlights the viability for InAs/InP QDot lasers to be used as a low-cost optical source for large-scale networks.

Optical fibre-based single photon source using InAsP quantum dot nanowires and gradient-index lens collection.

We present a compact, fibre-coupled single photon source using gradient-index (GRIN) lenses and an InAsP semiconductor quantum dot embedded within an InP photonic nanowire waveguide. A GRIN lens assembly is used to collect photons close to the tip of the nanowire, coupling the light immediately into a single mode optical fibre. The system provides a stable, high brightness source of fibre-coupled single photons. Using pulsed excitation, we demonstrate on-demand operation with a single photon purity of 98.5% when exciting at saturation in a device with a source-fibre collection efficiency of 35% and an overall single photon collection efficiency of 10%. We also demonstrate "plug and play" operation using room temperature photoluminescence from the InP nanowire for room temperature alignment.

InAs/InP quantum dash buried heterostructure mode-locked laser for high capacity fiber-wireless integrated 5G new radio fronthaul systems.

We have developed and experimentally demonstrated a highly coherent and low noise InP-based InAs quantum dash (QDash) buried heterostructure (BH) C-band passively mode-locked laser (MLL) with a pulse repetition rate of 25 GHz for fiber-wireless integrated fronthaul 5G new radio (NR) systems. The device features a broadband spectrum providing over 46 equally spaced highly coherent and low noise optical channels with an optical phase noise and integrated relative intensity noise (RIN) over a frequency range of 10 MHz to 20 GHz for each individual channel typically less than 466.5 kHz and -130 dB/Hz, respectively, and an average total output power of ∼50 mW per facet. Moreover, the device exhibits low RF phase noise with measured RF beat-note linewidth down to 3 kHz and estimated timing jitter between any two adjacent channels of 5.5 fs. By using this QDash BH MLL device, we have successfully demonstrated broadband optical heterodyne based radio-over-fiber (RoF) fronthaul wireless links at 5G NR in the underutilized spectrum of around 25 GHz with a total bit rate of 16-Gb/s. The device performance is experimentally evaluated in an end-to-end fiber-wireless system in real-time in terms of error vector magnitude (EVM) and bit error rate (BER) by generating, transmitting and detecting 4-Gbaud 16-QAM RF signals over 0.5-m to 2-m free-space indoor wireless channel through a total length of 25.22 km standard single mode fiber (SSMF) with EVM and BER under 8.4% and 2.9 × 10-5, respectively. The intrinsic characteristics of the device in conjunction with its system transmission performance indicate that QDash BH MLLs can be readily used in fiber-wireless integrated systems of 5G and beyond wireless communication networks.

Tailoring the Geometry of Bottom-Up Nanowires: Application to High Efficiency Single Photon Sources.

For nanowire-based sources of non-classical light, the rate at which photons are generated and the ability to efficiently collect them are determined by the nanowire geometry. Using selective-area vapour-liquid-solid epitaxy, we show how it is possible to control the nanowire geometry and tailor it to optimise device performance. High efficiency single photon generation with negligible multi-photon emission is demonstrated using a quantum dot embedded in a nanowire having a geometry tailored to optimise both collection efficiency and emission rate.

Multiplexed Single-Photon Source Based on Multiple Quantum Dots Embedded within a Single Nanowire.

Photonics-based quantum information technologies require efficient, high emission rate sources of single photons. Position-controlled quantum dots embedded within a broadband nanowire waveguide provide a fully scalable route to fabricating highly efficient single-photon sources. However, emission rates for single-photon devices are limited by radiative recombination lifetimes. Here, we demonstrate a multiplexed single-photon source based on a multidot nanowire. Using epitaxially grown nanowires, we incorporate multiple energy-tuned dots, each optimally positioned within the nanowire waveguide, providing single photons with high efficiency. This linear scaling of the single-photon emission rate with number of emitters is demonstrated using a five-dot nanowire with an average multiphoton emission probability of <4% when excited at saturation. This represents the first ever demonstration of multiple single-photon emitters deterministically incorporated in a single photonic device and is a major step toward achieving GHz single-photon emission rates from a scalable multi-quantum-dot system.

Passively mode-locked quantum dash laser with an aggregate 5.376 Tbit/s PAM-4 transmission capacity.

This paper presents an InAs/InP quantum dash (QD) C-band passively mode-locked laser (MLL) with a channel spacing of 34.224 GHz. By using this QD-MLL we demonstrate an aggregate 5.376 Tbit/s PAM-4 data transmission capacity both for back-to-back (B2B) and over 25-km of standard single mode fiber (SSMF). This represents the first demonstration of QD-MLL acting as error-free operation at an aggregate data transmission capacity of 5.376 Tbit/s for some filtered individual channels. This finding highlights the viability for InAs/InP QD lasers to be used as a low-cost optical source for data center networks.

Controlled integration of selected detectors and emitters in photonic integrated circuits.

Integration of superconducting nanowire single-photon detectors and quantum sources with photonic waveguides is crucial for realizing advanced quantum integrated circuits. However, scalability is hindered by stringent requirements on high-performance detectors. Here we overcome the yield limitation by controlled coupling of photonic channels to pre-selected detectors based on measuring critical current, timing resolution, and detection efficiency. As a proof of concept of our approach, we demonstrate a hybrid on-chip full-transceiver consisting of a deterministically integrated detector coupled to a selected nanowire quantum dot through a filtering circuit made of a silicon nitride waveguide and a ring resonator filter, delivering 100 dB suppression of the excitation laser. In addition, we perform extensive testing of the detectors before and after integration in the photonic circuit and show that the high performance of the superconducting nanowire detectors, including timing jitter down to 23 ± 3 ps, is maintained. Our approach is fully compatible with wafer-level automated testing in a cleanroom environment.

Nanowire-based sources of non-classical light.

Sources of quantum light that utilize photonic nanowire designs have emerged as potential candidates for high efficiency non-classical light generation in quantum information processing. In this review we cover the different platforms used to produce nanowire-based sources, highlighting the importance of waveguide design and material properties in achieving optimal performance. The limitations of the sources are identified and routes to optimization are proposed. State-of-the-art nanowire sources are compared to other solid-state quantum emitter platforms with regard to the key metrics of single photon purity, indistinguishability and entangled-pair fidelity to maximally entangled Bell states. We also discuss the unique ability of the nanowire platform to incorporate multiple emitters in the same optical mode and consider potential applications. Finally, routes to on-chip integration are discussed and the challenges facing the development of a nanowire-based scalable architecture are presented.