An on-demand source of bright entangled photon pairs is desirable for quantum key distribution (QKD) and quantum repeaters. The leading candidate to generate entangled photon pairs is based on spontaneous parametric down-conversion (SPDC) in a non-linear crystal. However, a fundamental trade-off exists between entanglement fidelity and efficiency in SPDC sources due to multiphoton emission at high brightness, which limits the pair extraction efficiency to 0.1% when operating at near-unity fidelity. Quantum dots in photonic nanostructures can in principle overcome this trade-off; however, the quantum dots that have achieved an en- entanglement fidelity on par with an SPDC source (99%) have poor pair extraction efficiency of 0.01%. Here, we show a measured peak concurrence of 95.3% ± 0.5% and pair extraction efficiency of 0.65% from an InAsP quantum dot in an InP photonic nanowire waveguide. Additionally, we show that an oscillating two-photon Bell state generated by a semiconductor quantum dot can establish a secure key for peer-to-peer QKD while using all generated photon pairs. Using our time-resolved QKD scheme alleviates the need to remove the exciton fine structure splitting.
The development of photonic-based quantum information technologies depends on the availability of devices that consistently, and with high efficiency, deterministically emit identical single photons. Furthermore, a key requirement for the implementation of fiber-based quantum secured communication protocols demands that these sources be compatible with optical fiber networks operating in the low-loss telecom C-band (λ ~ 1550 nm). Semiconductor quantum dot emitters offer on-demand operation at high rates and can be incorporated into photonic structures that allow for high efficiency collection. Through composition engineering of InAs_(x)P_(1-x) dot-in-a-rod (DROD) nanowire quantum dot structures we have previously demonstrated single photon emission from wavelengths of up to the telecom O-band. Here we show how the DROD structure can be modified to shift emission wavelength to the telecom C-band with single-photon purities of g(2)(0) = 0.062. Through further optimization of these structures, we aim to dramatically increase source brightness with the long-term goal of developing scalable and efficient C-band emitting site-selected single-photon sources.
Highly entangled photon sources play a crucial role in advancing the capabilities of quantum networks. In this context, we introduce an advanced scheme aimed at improving entanglement of photons emitted from quantum dots based on the framework proposed by Fognini et al. (2018). We propose a setup with reduced physical footprint which employs one electro-optic modulator strategically to enhance entanglement, mitigating the detrimental effects of fine structure splitting (FSS) observed in quantum dots that contribute to the degradation of entanglement.
With recent developments in the field of quantum computing and cryptography, establishing quantum networks would allow for the implementation of post-quantum cryptographic protocols, distributed quantum computing, and quantum sensor networks. Though, quantum networks require the use of quantum repeaters to preserve the transmitted quantum information over long distances. This work focuses on the implementations of quantum frequency conversion which is used to ensure the signal is of a suitable frequency for transmission between the different optical components in the system.
Single photons and quantum interference between indistinguishable pairs of photons are promising resources in the ongoing development of quantum information technologies. On-demand generation of such photons on a photonic integrated circuit (PIC) is desirable as it can allow for stable operation and device scalability alongside other requisite components. Solid-state two-level emitters—in particular, epitaxial semiconductor quantum dots—have demonstrated to be a good source of single photons, though efficient integration onto PICs remains a challenge. Hybrid integration of such dots into on-chip photonic circuitry can provide a basis for testing practical implementations of quantum communication devices.
In this talk, I will discuss NRC's InP-based nanowire quantum dots and our work integrating these onto silicon nitride integrated photonics. The cryogenic environment poses challenges in the operation of key components such as optical phase shifters, tunable filters, and on-chip detectors. With this in mind, I will review our progress and near-term plans for realizing on-chip quantum information processing. Also examined is our recent work developing nanowire sources that emit in telecom O or C bands—a key requirement for practical long distance quantum communications—and coherent control schemes for optical pumping.
We present an assortment of experiments exploring loading, control, and probing of laser-cooled caesium atoms inside a hollow-core photonic-bandgap fiber.
Entangled photons are an important resource for quantum optics. Quantum dots are a source of on-demand and highly entangled photon pairs at a high repetition rate. However, fine structure splitting (FSS) in the biexciton-exciton cascade causes the photons to be emitted in a time-dependant state instead of an ideal Bell state. Current techniques to remove the FSS include applying a strain, electric, or magnetic field and require post-processing of the quantum dots which reduces device yield. We use a novel all-optical approach implemented by emulating a fast-rotating half-wave plate in a Lithium Niobate waveguide using a electro-optic modulator. This method allows us to frequency shift single photons and produce a time-independant entangled photon.
Quantum dots (QDs) embedded inside indium-phosphide (InP) nanowires have the potential to be bright, on-demand sources of perfect polarization-entangled photon pairs fabricated with near-unity yields. However, to date very high degrees of entanglement have not yet been measured from such devices. By performing quantum state tomography with state-of-the-art superconducting nanowire single-photon detectors (SNSPDs) and two-photon resonant excitation of the QD, we show that these sources are indeed capable of producing near-unity entangled photon pairs. We measure a raw peak fidelity of 97.5% +/- 0.8% and a lifetime-weighted fidelity of 0.94% +/- 0.04%. These results conclusively demonstrate that the majority of the degradation from unity-measured entanglement fidelity in earlier studies was not due to spin-spin dephasing from the large 9/2 nuclear spin of indium. These results solidify InP nanowire QDs as a promising platform for future quantum photonics applications.
In this talk, we highlight our progress towards scalable quantum-tech solutions. We demonstrate hybrid integration of single InAs quantum dots on a Si3N4 optical waveguide platform and show that the hybridization process does not degrade the single emitter properties, and can even enhance them.
We describe the development and applications of a single-photon source based on a quantum dot embedded in a semiconductor nanowire, which can be precision-tuned to emit ∼1ns long photons at wavelengths that match the transitions of caesium D1 line. We discuss interfacing such single-photon source with atomic ensembles and present our experimental results demonstrating a new method of tuning the emission of the quantum dot by condensing inert gas (N2) on the nanowire. Next, we describe how these single photons at ∼895nm can be efficiently converted to wavelength suitable for satellite QKD links (∼794 nm) and optical fiber links (∼1469 nm) using a laser-cooled atomic ensemble that is loaded and confined inside a hollow-core optical fiber. Lastly, we inroduce our proposal of integrating the semiconductor nanowire with a lensed fiber to create a compact single-photon source with improved photon-collection efficiency compared to conventional setups.
We describe the experimental progress and the challenges of integrating a single photon source based on quantum dots embedded in semiconductor nanowires with a cold-atom experiment in which laser-cooled caesium atoms are loaded and confined inside a hollow-core micro-structured optical fiber. We focus in particular on wavelength conversion of the photons between 895nm and wavelengths suitable for satellite links (~794nm).
Quantum communication applications require a scalable approach to integrate bright sources of entangled photon-pairs in complex on-chip quantum circuits. Currently, the most promising sources are based on III/V semiconductor quantum dots. However, complex photonic circuitry is mainly achieved in silicon photonics due to the tremendous technological challenges in circuit fabrication. We take the best of both worlds by developing a new hybrid on-chip nanofabrication approach. We demonstrate for the first time on-chip generation, spectral filtering, and routing of single-photons from selected single and multiple III/V semiconductor nanowire quantum emitters all deterministically integrated in a CMOS compatible silicon nitride photonic circuit.
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.
Normally, the larger refractive index contrast of silicon-on-insulator (SOI) photonics used for transporting highly
confined optical modes is not available in compound semiconductor systems because the optically active layer rests upon
an epitaxial support layer having a similar refractive index. Here, a semiconductor-under-insulator (SUI) technology for
compound semiconductor membrane photonic circuitry is presented. It will be shown that such a technology can
facilitate the transport of highly confined optical modes in compound semiconductor systems and is anticipated to be a
critical part of future scalable quantum photonics applications.
We describe two different approaches to growing precisely positioned InP nanowires on InP wafers. Both of these
approaches utilize the selective area growth capabilities of Chemical Beam Epitaxy, one using the Au catalysed Vapour-Liquid-Solid (VLS) growth mode, the other being catalyst-free. Growth is performed on InP wafers which are first
coated with 20 nm of SiO2. These are then patterned using e-beam lithography to create nanometer scale holes in the
SiO2 layer to expose the InP surface. For the VLS growth Au is then deposited into the holes in the SiO2 mask layer
using a self-aligned lift-off process. For the catalyst-free growth no Au is deposited. In both cases the deposition of InP
results in the formation of InP nanowires. In VLS growth the nanowire diameter is controlled by the size of the Au
particle, whereas when catalyst-free the diameter is that of the opening in the SiO2 mask. The orientation of the
nanowires is also different, <111>B when using Au particles and <111>A when catalyst-free. For the catalysed growth
the effect of the Au particle can be turned off by modifying growth conditions allowing the nanowire to be clad,
dramatically enhancing the optical emission from InAs quantum dots grown inside the nanowire.
In this paper, we present experimental results from site-selected single quantum dots that have
undergone a number of intermixing process steps via rapid thermal annealing. We show that the
intermixing process blueshifts the dot's emission spectrum without affecting the linewidth as well as
decreasing its biexciton binding energy and s-p shell spacing. The anisotropic exchange splitting is
shown to have undergone a sign inversion implying that the splitting had gone through zero.
Intermixing provides another nanoengineering tool for the design of scalable solid-state photon and
entangled photon pair sources.
Dan Dalacu, Khaled Mnaymneh, Vera Sazonova, Philip Poole, Geof Aers, Ross Cheriton, Mike Reimer, Jean Lapointe, Pawel Hawrylak, Marek Korkusiński, Eugene Kadantsev, Robin Williams
Optoelectronic devices based on single, self-assembled semiconductor quantum dots are attractive for applications
in secure optical communications, quantum computation and sensing. In this paper we show how it is possible
to dictate the nucleation site of individual InAs/InP quantum dots using a directed self-assembly process, to
control the electronic structure of the nucleated dots and also how to control their coupling to the optical field by
locating them within the high field region of a photonic crystal nanocavity. For application within fiber networks,
these quantum dots are targeted to emit in the spectral region around 1550 nm.
Site-selective epitaxy and standard electron beam lithography techniques are employed to spatially couple small InAs/InP quantum dot ensembles to 2D photonic crystal membrane cavities. The small InAs quantum dot ensembles, consisting of just a few dots, are localized to areas 100x100nm2 at predetermined positions dictated by a nanotemplate consisting of InP pyramids. The dots are embedded in a 2D membrane using a planarization growth step and single missing-hole defect cavities are fabricated in the membrane with the defect sites centered on the dot ensembles. This spatially couples the ensembles to the χ-dipole mode of the cavities. Emission from the cavities shows the expected mode structure, with quality factors of 2000.
A monolithically integrated asymmetric graded index (GRIN) waveguide structure for coupling light into high index contrast waveguides is described. When analyzed in terms of its waveguide modes, the GRIN coupler is shown to be a multimode interference (MMI) device. The design parameters and tolerances are calculated for quadratic index profile and uniform index amorphous silicon (a-Si) GRIN couplers optimized for coupling light into silicon-on-insulator waveguides. Calculations of coupling efficiencies into 0.5 μm SOI waveguides show that asymmetric GRIN couplers operate over a very wide wavelength range with low polarization dependence, and fabrication requires lithographic resolution of only ±1 μm. Experimental results are presented for a 3 μm thick single layer a-Si coupler integrated with a 0.8 μm SOI waveguide. The measured variation of coupling efficiency with coupler length is in agreement with theory, with an optimal coupling length of 15 μm.
Polarization-resolved second-harmonic spectra are obtained from the resonant modes of a two-dimensional planar
photonic crystal microcavity patterned in a free-standing InP slab. The photonic crystal microcavity is comprised
of a single missing-hole defect in a hexagonal photonic crystal host formed with elliptically-shaped holes. The
cavity supports two orthogonally-polarized resonant modes split by 60 cm-1. Sum-frequency data are reported
from the nonlinear interaction of the two coherently excited modes, and the polarization dependence is explained
in terms of the nonlinear susceptibility tensor of the host InP.
We review the use of the oxide cladding stress induced photoelastic effect to eliminate the modal birefringence in silicon-on-insulator (SOI) ridge waveguide components, and highlight characteristics particular to high index contrast (HIC) systems. The birefringence in planar waveguides has its origin in the electromagnetic boundary conditions at the waveguide boundaries, and can be further modified by the presence of stress in the materials. It is shown that geometrical constraints imposed by different design and fabrication considerations become increasingly difficult to satisfy with decreasing core sizes. On the other hand, with typical stress levels of -100 MPa to -400 MPa (compressive) in SiO2 used as the upper cladding, the effective indices are altered up to the order of 10-3 for ridges with heights ranging from 1 μm to 5 μm. We demonstrate that the stress can be effectively used to balance the geometrical birefringence. Birefringence-free operation is achieved for waveguides with otherwise large birefringence by using properly chosen thickness and stress of the upper cladding layer. This allows the waveguide cross-section profiles to be optimized for design criteria other than zero-birefringence. Since the index changes induced by the stress are orders of magnitude smaller than the waveguide core/cladding index contrast, changes in the mode profiles are insignificant and the associated mode mismatch loss is negligible. We study the stress-induced effects in two parallel waveguides of varying spacing, to emulate the condition in directional couplers and ring-resonators. In the arrayed waveguide grating (AWG) demultiplexers fabricated in the SOI platform, we demonstrated the reduction of the birefringence from 1.3x10-3 (without the upper cladding) to below 1x10-4 across the spectral band by using a 0.6 μm oxide upper cladding with a stress of -320 MPa (compressive). Design options for relaxed dimensional tolerance and improved coupler performance made available by using stress engineering are discussed.
Coupling light into and out of small high index contrast waveguides is a fundamental challenge to implementing practical microphotonic waveguide and photonic crystal devices. Previous approaches include three-dimensional tapers, inverse taper waveguides, and grating based couplers. We propose and describe a much simpler coupler based on a short length of graded index (GRIN) material deposited on top of a silicon-on-insulator (SOI) microphotonic waveguide. The GRIN coupler has a refractive index that decreases from the index of silicon at the waveguide-coupler interface, to an optimized value at the coupler surface. Beam propagation method calculations are used to evaluate the coupling efficiency from a 4 μm thick coupler section to the fundamental mode of a 0.5 μm thick SOI waveguide. Coupling efficiencies are compared for couplers with smoothly varying quadratic index profiles and with one, two and three index steps. Coupling efficiencies of 75% (1.3 dB) or better are predicted using a three step GRIN structure with indices ranging from n=3.30 to 3.41 (Si). This index range is easily accessed using a-Si layers deposited by PECVD at varying deposition conditions, or by using composite digital alloys of high and low index films. With this method, microphotonic waveguide couplers can be designed and fabricated using only PECVD deposition and one patterning etch step with very modest tolerances. Efficiency increases to 87% (0.6 dB) when the index range of the 3-step coupler is extended to 3.0.
The high index contrast of silicon-on-insulator (SOI) enables the scaling down of planar waveguide components into the microphotonic regime, but has the unwanted consequence of inducing a large TE-TM polarization birefringence. For ridge waveguides this birefringence can be reduced to an acceptable level by using appropriate ridge dimensions [1], but with decreasing Si thickness the required fabrication tolerances quickly become too narrow to implement this solution. Components with slab waveguide regions such as echelle grating demultiplexers cannot be made polarization independent using this method. An alternate approach is to introduce a polarization compensation region in the combiner section of an AWG or echelle grating to eliminate the polarization dependent wavelength shift[2]. In its original implementation, the compensator is fabricated by changing the local waveguide thickness. The resulting birefringence correction can be sensitive to errors in etch depth, and the mode mismatch between compensator and slab waveguide sections is a source of approximately 1 dB extra insertion loss (IL) for SOI demultiplexers with Si waveguide thickness in the range from 2 to 5 m.
We describe a new compensator structure for the SOI platform using a buried low index layer, in this case SiO2 sandwiched between the Si waveguide layer and another Si cap layer. Such a silicon-oxide-silicon (SOS) compensator on SOI can eliminate the TE-TM wavelength shift of an SOI AWG or echelle grating demultiplexer without introducing a significant mode mismatch between the compensator and slab waveguide sections. A demultiplexer with an SOS compensator has almost 1 dB lower insertion loss of an equivalent device with an etched compensator. The SOS compensator is easily implemented using standard oxide and a-Si or polysilicon deposition techniques. In this paper we present calculations and experimental results on the effective birefringence compensation, PDL and IL of SOS compensators in SOI waveguide demultiplexers.
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