Applications In Quantum Photonics Research Articles

Optical Properties of Vanadium in 4 H Silicon Carbide for Quantum Technology

We study the optical properties of tetravalent vanadium impurities in 4H silicon carbide (4H SiC). Emission from two crystalline sites is observed at wavelengths of 1.28 \mum and 1.33 \mum, with optical lifetimes of 163 ns and 43 ns. Group theory and ab initio density functional supercell calculations enable unequivocal site assignment and shed light on the spectral features of the defects. We conclude with a brief outlook on applications in quantum photonics.

Role of the high vacuum in the precise control of index contrasts and index profiles of LiNbO3 waveguides fabricated by high vacuum proton exchange

Precise control and reproducibility of optical waveguides fabrication are extremely important for efficient and compact devices for a wide range of applications in either classical or quantum photonics. In this way, waveguides fabricated in lithium niobate (LN) crystals offer one of the most established platforms in modern photonics. Here we experimentally validate the excellent control and reproducibility of the index contrast and index profile of optical waveguides fabricated on Z-cut lithium niobate (LN) substrates by the recently proposed High Vacuum Proton Exchange (HiVacPE) technique. Based on the known chemical reactions involved in the refractive index variation mechanism we explain why the high vacuum, having a drying effect, is the key feature to obtain highly reproducible HiVacPE waveguides.

Ultrafast Carrier Redistribution in Single InAs Quantum Dots Mediated by Wetting-Layer Dynamics

Individual epitaxial semiconductor quantum dots (QDs) have been extensively considered as an ``artificial atom'' platform for quantum optics and photonics applications. The QD carrier dynamics responsible for ultimate device performance is indeed complex, due in part to rich interaction with the wetting layer's two-dimensional carrier reservoir. The authors investigate this interaction with time-resolved experiments and rate-equation modeling, showing that these analyses are important for understanding the limitations of single-photon photoluminescence emission, improving lasers and fast optical modulators, and developing next-generation ultrafast all-optical switches.

Very Large and Reversible Stark-Shift Tuning of Single Emitters in Layered Hexagonal Boron Nitride

Combining solid state single photon emitters (SPE) with nanophotonic platforms is a key goal in integrated quantum photonics. In order to realize functionality in potentially scalable elements, suitable SPEs have to be bright, stable, and widely tunable at room temperature. In this work we show that selected SPEs embedded in a few layer hexagonal boron nitride (hBN) meet these demands. In order to show the wide tunability of these SPEs we employ an AFM with a conductive tip to apply an electrostatic field to individual hBN emitters sandwiched between the tip and an indium tin oxide coated glass slide. A very large and reversible Stark shift of $(5.5 \pm 3)\,$nm at a zero field wavelength of $670\,$nm was induced by applying just $20\,$V, which exceeds the typical resonance linewidths of nanodielectric and even nanoplasmonic resonators. Our results are important to further understand the physical origin of SPEs in hBN as well as for practical quantum photonic applications where wide spectral tuning and on/off resonance switching are required.

Single-photon sources with quantum dots in III–V nanowires

Abstract Single-photon sources are one of the key components in quantum photonics applications. These sources ideally emit a single photon at a time, are highly efficient, and could be integrated in photonic circuits for complex quantum system designs. Various platforms to realize such sources have been actively studied, among which semiconductor quantum dots have been found to be particularly attractive. Furthermore, quantum dots embedded in bottom-up-grown III–V compound semiconductor nanowires have been found to exhibit relatively high performance as well as beneficial flexibility in fabrication and integration. Here, we review fabrication and performance of these nanowire-based quantum sources and compare them to quantum dots in top-down-fabricated designs. The state of the art in single-photon sources with quantum dots in nanowires is discussed. We also present current challenges and possible future research directions.

Optimizing the spectro-temporal properties of photon pairs from Bragg-reflection waveguides

Bragg-reflection waveguides (BRWs) fabricated from AlGaAs provide an interesting nonlinear optical platform for photon-pair generation via parametric down-conversion (PDC). In contrast to many conventional PDC sources, BRWs are made of high refractive index materials and their characteristics are very sensitive to the underlying layer structure. First, we show that the design parameters like the phase matching wavelength and the group refractive indices of the interacting modes can be reliably controlled even in the presence of fabrication tolerances. We then investigate how these characteristics can be taken advantage of when designing quantum photonic applications with BRWs. We especially concentrate on achieving a small differential group delay between the generated photons of a pair and then explore the performance of our design when realizing a Hong–Ou–Mandel interference experiment or generating spectrally multi-band polarization entangled states. Our results show that the versatility provided by engineering the dispersion in BRWs is important for employing them in different quantum optics tasks.

Quantum metrology of solid-state single-photon sources using photon-number-resolving detectors

Quantum-light sources based on semiconductor quantum dots (QDs) are promising candidates for many applications in quantum photonics and quantum communication. Important emission characteristics of such emitters, namely the single-photon purity and photon indistinguishability, are usually assessed via time-correlated measurements using standard ‘click’ detectors in Hanbury Brown and Twiss or Hong-Ou-Mandel (HOM-) type configurations. In this work, we employ a state-of-the-art photon-number-resolving (PNR) detection system based on superconducting transition-edge sensors (TESs) to directly access the photon-number distribution of deterministically fabricated solid-state single-photon sources. Offering quantum efficiencies close to unity and high energy resolution, our TES-based two-channel detector system allows us to analyse the quantum optical properties of a QD-based non-classical light source. In particular, it enables the direct observation of the two-particle Fock-state resulting from interference of quantum mechanically indistinguishable photons in HOM-experiments. Additionally, comparative measurements reveal excellent quantitative agreement of the photon-indistinguishabilities obtained with PNR ((90 ± 7)%) and standard click ((90 ± 5)%) detectors. Our work thus demonstrates that TES-based detectors are perfectly suitable for the quantum metrology of non-classical light sources and higlights appealing prospects for the efficient implementation of quantum information tasks based on multi-photon states.

Coherent interaction of orthogonal polarization modes in a photonic crystal nanofiber cavity.

We show that coherent interaction between two sets of multiple resonances leads to exotic resonant effects, such as Fano-type resonances, optical analogue of electro-magnetically induced transparency, and avoided crossing between modes, under different coupling regimes. We experimentally demonstrate such resonant effects in a photonic crystal nanofiber cavity using two sets of cavity modes with orthogonal polarizations. The interaction between the modes arises due to intra-cavity polarization mixing. The observed line shapes are reproduced using a multiple-mode interaction model. Such spectral characteristics may further enhance the capabilities of the nanofiber cavity as a fiber-in-line platform for nanophotonics and quantum photonics applications.

Inline detection and reconstruction of multiphoton quantum states

Integrated single-photon detectors open new possibilities for monitoring inside quantum photonic circuits. We present a concept for the in-line measurement of spatially-encoded multi-photon quantum states, while keeping the transmitted ones undisturbed. We theoretically establish that by recording photon correlations from optimally positioned detectors on top of coupled waveguides with detuned propagation constants, one can perform robust reconstruction of the density matrix describing the amplitude, phase, coherence and quantum entanglement. We report proof-of-principle experiments using classical light, which emulates single-photon regime. Our method opens a pathway towards practical and fast in-line quantum measurements for diverse applications in quantum photonics.

Single-photon emitters in van der Waals materials

Solid-state sources of single-photon emitters are highly desired for scalable quantum photonic applications, such as quantum communication, optical quantum information processing, and metrology. In the past year, great strides have been made in the characterization of single defects in wide-bandgap materials, such as silicon carbide and diamond, as well as single molecules, quantum dots, and carbon nanotubes. More recently, single-photon emitters in layered van der Waals materials attracted tremendous attention, because the two-dimensional (2D) lattice allows for high photon extraction efficiency and easy integration into photonic circuits. In this review, we discuss recent advances in mastering single-photon emitters in 2D materials, electrical generation pathways, detuning, and resonator coupling towards use as quantum light sources. Finally, we discuss the remaining challenges and the outlooks for layered material-based quantum photonic sources.

High-efficiency shallow-etched grating on GaAs membranes for quantum photonic applications

We have designed and fabricated a shallow-etched grating on gallium arsenide nanomembranes for efficient chip-to-fiber coupling in quantum photonic integrated circuits. Experimental results show that the grating provides a fiber-coupling efficiency of >60%, a greatly suppressed back reflection of <1% for the designed wavelength of 930 nm, and a 3-dB bandwidth of >43 nm. Highly efficient single-photon collection from embedded indium arsenide quantum dots to an optical fiber was realized with the designed grating, showing an average six-fold increase in the photon count compared to commonly used circular gratings, offering an efficient interface for on-chip quantum information processing.

0.52 V mm ITO-based Mach-Zehnder modulator in silicon photonics

Electro-optic modulators transform electronic signals into the optical domain and are critical components in modern telecommunication networks, RF photonics, and emerging applications in quantum photonics, neuromorphic photonics, and beam steering. All these applications require integrated and voltage-efficient modulator solutions with compact form factors that are seamlessly integrable with silicon photonics platforms and feature near-CMOS material processing synergies. However, existing integrated modulators are challenged to meet these requirements. Conversely, emerging electro-optic materials heterogeneously and monolithically integrated with Si photonics open up a new avenue for device engineering. Indium tin oxide (ITO) is one such compelling material for heterogeneous integration in Si exhibiting formidable electro-optic effect characterized by unity-order index change at telecommunication frequencies. Here we overcome these limitations and demonstrate a monolithically integrated ITO electro-optic modulator based on a Mach Zehnder interferometer featuring a high-performance half-wave voltage and active device length product of VπL = 0.52 V mm. We show that the unity-strong index change enables a 30 μm-short π-phase shifter operating ITO in the index-dominated region away from the epsilon-near-zero point for reduced losses. This device experimentally confirms electrical phase shifting in ITO enabling its use in applications such as compact phase shifters, nonlinear activation functions in photonic neural networks, and phased array applications for LiDAR.

Direct writing of single germanium vacancy center arrays in diamond

Color centers in diamond are promising solid-state qubits for scalable quantum photonics applications. Amongst many defects, those with inversion symmetry are of an interest due to their promising optical properties. In this work, we demonstrate a maskless implantation of an array of bright, single germanium vacancy (GeV) centers in diamond. Employing the direct focused ion beam technique, single GeV emitters are engineered with the spatial accuracy of tens of nanometers. The single GeV creation ratio reaches as high as 53% with the dose of 200 Ge+ ions per spot. The presented fabrication method is promising for future nanofabrication of integrated photonic structures with GeV emitters as a leading platform for spin-spin interactions.

Design and Simulations of an Electrically Injected Bragg-Reflection-Waveguide Photon-Pair Source Near 1.3-Micrometer Wavelength

Photon-pair sources are the key devices for quantum photonic applications; most of them are optically pumped. Here we present our design of an electrically injected photon-pair source based a Bragg-reflection-waveguide (BRW) GaInP/AlGaInP quantum well laser and internal spontaneous parametric down conversion, targeting at the wavelengths near 1.3 μm for signal and idler photons. We establish a model that allows us to simulate the major characteristics of the device. Our work shows the feasibility to extend the demonstration of the electrically injected BRW photon-pair source at 1.57 μm [Boitier et al. , “Electrically injected photon-pair source at room temperature,” Physical Review Letters , vol. 112, no. 18, 2014, Art. no. 183901] to another technologically interested wavelength.

Photonic welding points for arbitrary on-chip optical interconnects

Abstract Photonic integrated circuits (PICs) are an ideal platform for chip-scale computation and communication. To date, the integration density remains an outstanding problem that limits the further development of PIC-based photonic networks. Achieving low-loss waveguide routing with arbitrary configuration is crucial for both classical and quantum photonic applications. To manipulate light flows on a chip, the conventional wisdom relies on waveguide bends of large bending radii and adiabatic mode converters to avoid insertion losses from radiation leakage and modal mismatch, respectively. However, those structures usually occupy large footprints and thus reduce the integration density. To overcome this difficulty, this work presents a fundamentally new approach to turn light flows arbitrarily within an ultracompact footprint. A type of “photonic welding points” joining two waveguides of an arbitrary intersecting angle has been proposed and experimentally demonstrated. These devices with a footprint of less than 4 μm2 can operate in the telecommunication band over a bandwidth of at least 140 nm with an insertion loss of less than 0.5 dB. Their fabrication is compatible with photonic foundry processes and does not introduce additional steps beyond those needed for the waveguides. Therefore, they are suitable for the mass production of PICs and will enhance the integration density to the next level.

Deterministic coupling of quantum emitters in WSe2 monolayers to plasmonic nanocavities.

We discuss coupling of site-selectively induced quantum emitters in exfoliated monolayers of WSe2 to plasmonic nanostructures. Gold nanorods of 20 nm-240 nm size, which are arranged in pitches of a few micrometers on a dielectric surface, act as seeds for the formation of quantum emitters in the atomically thin materials. We observe characteristic narrow-band emission signals from the monolayers, which correspond well with the positions of the metallic nanopillars with and without thin dielectric coating. Single photon emission from the emitters is confirmed by autocorrelation measurements, yielding g2(τ = 0) values as low as 0.17. Moreover, we observe a strong co-polarization of our single photon emitters with the frequency matched plasmonic resonances, as a consequence of light-matter coupling. Our work represents a significant step towards the scalable implementation of coupled quantum emitter-resonator systems for highly integrated quantum photonic and plasmonic applications.

Observation of Fourier transform limited lines in hexagonal boron nitride

Single defect centers in layered hexagonal boron nitride (hBN) are promising candidates as single photon sources for quantum optics and nanophotonics applications. However, until today spectral instability hinders many applications. Here, we perform resonant excitation measurements and observe Fourier-Transform limited (FL) linewidths down to $\approx 50$ MHz. We investigate optical properties of more than 600 quantum emitters (QE) in hBN. The QEs exhibit narrow zero-phonon lines (ZPL) distributed over a spectral range from 580 nm to 800 nm and with dipole-like emission with high polarization contrast. The emission frequencies can be divided into four main regions indicating distinct families or crystallographic structures of the QEs, in accord with ab-initio calculations. Finally, the emitters withstand transfer to a foreign photonic platform - namely a silver mirror, which makes them compatible with photonic devices such as optical resonators and paves the way to quantum photonics applications including quantum commmunications and quantum repeaters.

Man-made synthesis of ultrafine photoluminescent nanodiamonds containing less than three silicon-vacancy colour centres

Silicon-vacancy (SiV) centred diamond exhibits near infrared emission at 738 nm with narrower zero-phonon line and shorter lifetime comparative with nitrogen-vacancy (NV) centre, considered as a promising room-temperature single-photon source. However, unlike NV centred nanodiamonds easily obtained by detonation, man-made synthesis of ultrafine SiV nanodiamonds is a key challenge. Here, we provide a controllably method for synthesizing nanodiamonds with one or few SiV emitters by reducing their size to the near minimum via oxygen plasma treatments. The size of nanodiamond is uniformly reduced from 11 to 1.7 nm under different treatment durations. It is found that nanodiamonds of a size between 2.1 and 4 nm contain fewer than three emitters. Size dependence of microstructure and photoluminescence suggests that SiV centres exist in the outer shell of nanodiamonds with diameter larger than ∼2 nm. This material has significant applications in nanophotonics platform, quantum communications and quantum photonics.

(Invited) III-Nitride UV-Visible Integrated Photonics for Quantum and Biomedical Applications

III-nitride semiconductors have drawn considerable attentions for its wide success in photonic devices including light-emitting diodes, laser diodes, solar cells, and photodetectors. In addition to discrete devices, recent studies on III-nitride waveguides have now opened up new opportunities in integrated photonics for biochemical sensing, beam steering, nonlinear optics, and quantum photonics applications in the UV and visible spectral region. For example, due to their wide bandgap, low material dispersion, and active integration capability, III-nitride materials are particularly attractive for integrated nonlinear optics applications, including second harmonic generation, comb generation, and parametric down conversion. In this talk, I will first present our recent studies on the fundamental nonlinear optical properties of GaN, namely, two photon absorption coefficient, three photon absorption coefficient, and Kerr nonlinear refractive index, and discuss their impacts on the device performance. I will also discuss on-going research at ASU on AlN nanophotonic ring resonators for second order harmonic generation of picosecond UV coherence light, as well as supercontinuum generation from UV to IR. The presented III-nitride integrated photonics platform would enable a wide rangeof applications in biochemical sensing, quantum information, and optical communications.

Silicon-Vacancy Centers in Ultra-Thin Nanocrystalline Diamond Films.

Color centers in diamond have shown excellent potential for applications in quantum information processing, photonics, and biology. Here we report the optoelectronic investigation of shallow silicon vacancy (SiV) color centers in ultra-thin (7–40 nm) nanocrystalline diamond (NCD) films with variable surface chemistry. We show that hydrogenated ultra-thin NCD films exhibit no or lowered SiV photoluminescence (PL) and relatively high negative surface photovoltage (SPV) which is ascribed to non-radiative electron transitions from SiV to surface-related traps. Higher SiV PL and low positive SPV of oxidized ultra-thin NCD films indicate an efficient excitation—emission PL process without significant electron escape, yet with some hole trapping in diamond surface states. Decreasing SPV magnitude and increasing SiV PL intensity with thickness, in both cases, is attributed to resonant energy transfer between shallow and bulk SiV. We also demonstrate that thermal treatments (annealing in air or in hydrogen gas), commonly applied to modify the surface chemistry of nanodiamonds, are also applicable to ultra-thin NCD films in terms of tuning their SiV PL and surface chemistry.