Two-photon interference in the telecom C-band after frequency conversion of photons from remote quantum emitters

Selective active resonance tuning for multi-mode nonlinear photonic cavities

Resonant enhancement of nonlinear photonic processes is critical for the scalability of applications such as long-distance entanglement generation. To implement nonlinear resonant enhancement, multiple resonator modes must be individually tuned onto a precise set of process wavelengths, which requires multiple linearly-independent tuning methods. Using coupled auxiliary resonators to indirectly tune modes in a multi-resonant nonlinear cavity is particularly attractive because it allows the extension of a single physical tuning mechanism, such as thermal tuning, to provide the required independent controls. Here we model and simulate the performance and tradeoffs of a coupled-resonator tuning scheme which uses auxiliary resonators to tune specific modes of a multi-resonant nonlinear process. Our analysis determines the tuning bandwidth for steady-state mode field intensity can significantly exceed the inter-cavity coupling rate g if the total quality factor of the auxiliary resonator is higher than the multi-mode main resonator. Consequently, over-coupling a nonlinear resonator mode to improve the maximum efficiency of a frequency conversion process will simultaneously expand the auxiliary resonator tuning bandwidth for that mode, indicating a natural compatibility with this tuning scheme. We apply the model to an existing small-diameter triply-resonant ring resonator design and find that a tuning bandwidth of 136 GHz ≈ 1.1 nm can be attained for a mode in the telecom band while limiting excess scattering losses to a quality factor of 106. Such range would span the distribution of inhomogeneously broadened quantum emitter ensembles as well as resonator fabrication variations, indicating the potential for the auxiliary resonators to enable not only low-loss telecom conversion but also the generation of indistinguishable photons in a quantum network.

Heralded Three-Photon Entanglement from a Single-Photon Source on a Photonic Chip

In the quest to build general-purpose photonic quantum computers, fusion-based quantum computation has risen to prominence as a promising strategy. This model allows a ballistic construction of large cluster states which are universal for quantum computation, in a scalable and loss-tolerant way without feed forward, by fusing many small n-photon entangled resource states. However, a key obstacle to this architecture lies in efficiently generating the required essential resource states on photonic chips. One such critical seed state that has not yet been achieved is the heralded three-photon Greenberger-Horne-Zeilinger (3-GHZ) state. Here, we address this elementary resource gap, by reporting the first experimental realization of a heralded 3-GHZ state. Our implementation employs a low-loss and fully programmable photonic chip that manipulates six indistinguishable single photons of wavelengths in the telecommunication regime. Conditional on the heralding detection, we obtain the desired 3-GHZ state with a fidelity 0.573±0.024. Our Letter marks an important step for the future fault-tolerant photonic quantum computing, leading to the acceleration of building a large-scale optical quantum computer.

Experimental Quantum Non-Gaussian Coincidences of Entangled Photons

Quantum non-Gaussianity, a more potent and highly useful form of nonclassicality, excludes all convex mixtures of Gaussian states and Gaussian parametric processes generating them. Here, for the first time, we conclusively test quantum non-Gaussian coincidences of entangled photon pairs with the Clauser-Horne-Shimony-Holt-Bell factor S=2.328±0.004 from a single quantum dot with a depth up to 0.94±0.02  dB. Such deterministically generated photon pairs fundamentally overcome parametric processes by reducing crucial multiphoton errors. For the quantum non-Gaussian depth of the unheralded (heralded) single-photon state, we achieve the value of 8.08±0.05  dB (19.06±0.29  dB). Our Letter experimentally certifies the exclusive quantum non-Gaussianity properties highly relevant for optical sensing, communication, and computation.

Ultra-narrow inhomogeneous spectral distribution of telecom-wavelength vanadium centres in isotopically-enriched silicon carbide

Spin-active quantum emitters have emerged as a leading platform for quantum technologies. However, one of their major limitations is the large spread in optical emission frequencies, which typically extends over tens of GHz. Here, we investigate single V4+ vanadium centres in 4H-SiC, which feature telecom-wavelength emission and a coherent S = 1/2 spin state. We perform spectroscopy on single emitters and report the observation of spin-dependent optical transitions, a key requirement for spin-photon interfaces. By engineering the isotopic composition of the SiC matrix, we reduce the inhomogeneous spectral distribution of different emitters down to 100 MHz, significantly smaller than any other single quantum emitter. Additionally, we tailor the dopant concentration to stabilise the telecom-wavelength V4+ charge state, thereby extending its lifetime by at least two orders of magnitude. These results bolster the prospects for single V emitters in SiC as material nodes in scalable telecom quantum networks.

Telecom-band quantum dot technologies for long-distance quantum networks

A future quantum internet is expected to generate, distribute, store and process quantum bits (qubits) over the world by linking different quantum nodes via quantum states of light. To facilitate long-haul operations, quantum repeaters must operate at telecom wavelengths to take advantage of both the low-loss optical fibre network and the established technologies of modern optical communications. Semiconductor quantum dots have thus far shown exceptional performance as key elements for quantum repeaters, such as quantum light sources and spin-photon interfaces, but only in the near-infrared regime. Therefore, the development of high-performance telecom-band quantum dot devices is highly desirable for a future solid-state quantum internet based on fibre networks. In this Review, we present the physics and technological developments towards epitaxial quantum dot devices emitting in the telecom O- and C-bands for quantum networks, considering both advanced epitaxial growth for direct telecom emission and quantum frequency conversion for telecom-band down-conversion of near-infrared quantum dot devices. We also discuss the challenges and opportunities for future realization of telecom quantum dot devices with improved performance and expanded functionality through hybrid integration.

Lead Halide Perovskite Nanocrystals with Low Inhomogeneous Broadening and High Coherent Fraction through Dicationic Ligand Engineering

Lead halide perovskite nanocrystals (LHP NCs) are an emerging materials system with broad potential applications, including as emitters of quantum light. We apply design principles aimed at the structural optimization of surface ligand species for CsPbBr₃ NCs, leading us to the study of LHP NCs with dicationic quaternary ammonium bromide ligands. Through the selection of linking groups and aliphatic backbones guided by experiments and computational support, we demonstrate consistently narrow photoluminescence line shapes with a full-width-at-half-maximum below 70 meV. We observe bulk-like Stokes shifts throughout our range of particle sizes, from 7 to 16 nm. At cryogenic temperatures, we find sub-200 ps lifetimes, significant photon coherence, and the fraction of photons emitted into the coherent channel increasing markedly to 86%. A 4-fold reduction in inhomogeneous broadening from previous work paves the way for the integration of LHP NC emitters into nanophotonic architectures to enable advanced quantum optical investigation.

Triply-resonant sum frequency conversion with gallium phosphide ring resonators

We demonstrate quasi-phase matched, triply-resonant sum frequency conversion in 10.6-µm-diameter integrated gallium phosphide ring resonators. A small-signal, waveguide-to-waveguide power conversion efficiency of 8 ± 1.1%/mW; is measured for conversion from telecom (1536 nm) and near infrared (1117 nm) to visible (647 nm) wavelengths with an absolute power conversion efficiency of 6.3 ± 0.6%; measured at saturation pump power. For the complementary difference frequency generation process, a single photon conversion efficiency of 7.2%/mW from visible to telecom is projected for resonators with optimized coupling. Efficient conversion from visible to telecom will facilitate long-distance transmission of spin-entangled photons from solid-state emitters such as the diamond NV center, allowing long-distance entanglement for quantum networks.

Quantum interference of identical photons from remote GaAs quantum dots

Photonic quantum technology provides a viable route to quantum communication^1,2, quantum simulation³ and quantum information processing⁴. Recent progress has seen the realization of boson sampling using 20 single photons³ and quantum key distribution over hundreds of kilometres². Scaling the complexity requires architectures containing multiple photon sources, photon counters and a large number of indistinguishable single photons. Semiconductor quantum dots are bright and fast sources of coherent single photons^5-9. For applications, a roadblock is the poor quantum coherence on interfering single photons created by independent quantum dots^10,11. Here we demonstrate two-photon interference with near-unity visibility (93.0 ± 0.8)% using photons from two completely separate GaAs quantum dots. The experiment retains all the emission into the zero phonon line-only the weak phonon sideband is rejected; temporal post-selection is not employed. By exploiting quantum interference, we demonstrate a photonic controlled-not circuit and an entanglement with fidelity of (85.0 ± 1.0)% between photons of different origins. The two-photon interference visibility is high enough that the entanglement fidelity is well above the classical threshold. The high mutual coherence of the photons stems from high-quality materials, diode structure and relatively large quantum dot size. Our results establish a platform-GaAs quantum dots-for creating coherent single photons in a scalable way.

Converting single photons from an InAs/GaAs quantum dot into the ultraviolet: preservation of second-order correlations

Wavelength conversion at the single-photon level is required to forge a quantum network from distinct quantum devices. Such devices include solid-state emitters of single or entangled photons, as well as network nodes based on atoms or ions. Here we demonstrate the conversion of single photons emitted from a III-V semiconductor quantum dot at 853 nm via sum frequency conversion to the wavelength of the strong transition of Yb⁺ ions at 370 nm. We measure the second-order correlation function of both the unconverted and the converted photon and show that the single-photon character of the quantum dot emission is preserved during the conversion process.

Dimension-Dependent Phenomenological Model of Excitonic Electric Dipole in InGaAs Quantum Dots

Permanent electric dipole is a key property for effective control of semiconductor quantum-dot-based sources of quantum light. For theoretical prediction of that, complex geometry-dependent quantum simulations are necessary. Here, we use k·p simulations of exciton transition in InGaAs quantum dots to derive a simple geometry-dependent analytical model of dipole. Our model, discussed here, enables reasonably good estimation of the electric dipole, caused in quantum dot by the elastic strain, including an externally induced one. Due to its apparent simplicity, not necessitating elaborate and time-consuming simulations, it might after experimental verification serve as a preferred choice for experimentalists enabling them to make quick estimates of built-in and induced electric dipole in quantum dots.

Optical charge injection and coherent control of a quantum-dot spin-qubit emitting at telecom wavelengths

Solid-state quantum emitters with manipulable spin-qubits are promising platforms for quantum communication applications. Although such light-matter interfaces could be realized in many systems only a few allow for light emission in the telecom bands necessary for long-distance quantum networks. Here, we propose and implement an optically active solid-state spin-qubit based on a hole confined in a single InAs/GaAs quantum dot grown on an InGaAs metamorphic buffer layer emitting photons in the C-band. We lift the hole spin-degeneracy using an external magnetic field and demonstrate hole injection, initialization, read-out and complete coherent control using picosecond optical pulses. These results showcase a solid-state spin-qubit platform compatible with preexisting optical fiber networks.

Quantum-dot-based deterministic photon-emitter interfaces for scalable photonic quantum technology

The scale-up of quantum hardware is fundamental to realize the full potential of quantum technology. Among a plethora of hardware platforms, photonics stands out: it provides a modular approach where the main challenges lie in the construction of high-quality building blocks and in the development of methods to interface the modules. The subsequent scale-up could exploit mature integrated photonics foundry technology to produce small-footprint quantum processors of immense complexity. Solid-state quantum emitters can realize a deterministic photon-emitter interface and enable key quantum photonic resources and functionalities, including on-demand single- and multi-photon-entanglement sources, as well as photon-photon nonlinear quantum gates. In this Review, we use the example of quantum dot devices to present the physics of deterministic photon-emitter interfaces, including the main photonic building blocks required to scale up, and discuss quantitative performance benchmarks. While our focus is on quantum dot devices, the presented methods also apply to other quantum-emitter platforms such as atoms, vacancy centres, molecules and superconducting qubits. We also identify applications within quantum communication and computing, presenting a route towards photonics with a genuine quantum advantage.

Optical Quality of InAs/InP Quantum Dots on Distributed Bragg Reflector Emitting at 3rd Telecom Window Grown by Molecular Beam Epitaxy

We present an experimental study on the optical quality of InAs/InP quantum dots (QDs). Investigated structures have application relevance due to emission in the 3rd telecommunication window. The nanostructures are grown by ripening-assisted molecular beam epitaxy. This leads to their unique properties, i.e., low spatial density and in-plane shape symmetry. These are advantageous for non-classical light generation for quantum technologies applications. As a measure of the internal quantum efficiency, the discrepancy between calculated and experimentally determined photon extraction efficiency is used. The investigated nanostructures exhibit close to ideal emission efficiency proving their high structural quality. The thermal stability of emission is investigated by means of microphotoluminescence. This allows to determine the maximal operation temperature of the device and reveal the main emission quenching channels. Emission quenching is predominantly caused by the transition of holes and electrons to higher QD's levels. Additionally, these carriers could further leave the confinement potential via the dense ladder of QD states. Single QD emission is observed up to temperatures of about 100 K, comparable to the best results obtained for epitaxial QDs in this spectral range. The fundamental limit for the emission rate is the excitation radiative lifetime, which spreads from below 0.5 to almost 1.9 ns (GHz operation) without any clear spectral dispersion. Furthermore, carrier dynamics is also determined using time-correlated single-photon counting.

Heralded Nondestructive Quantum Entangling Gate with Single-Photon Sources

Heralded entangling quantum gates are an essential element for the implementation of large-scale optical quantum computation. Yet, the experimental demonstration of genuine heralded entangling gates with free-flying output photons in linear optical system, was hindered by the intrinsically probabilistic source and double-pair emission in parametric down-conversion. Here, by using an on-demand single-photon source based on a semiconductor quantum dot embedded in a micropillar cavity, we demonstrate a heralded controlled-NOT (CNOT) operation between two single photons for the first time. To characterize the performance of the CNOT gate, we estimate its average quantum gate fidelity of (87.8±1.2)%. As an application, we generated event-ready Bell states with a fidelity of (83.4±2.4)%. Our results are an important step towards the development of photon-photon quantum logic gates.

Quantum cryptography with highly entangled photons from semiconductor quantum dots

Semiconductor quantum dots are capable of emitting polarization entangled photon pairs with ultralow multipair emission probability even at maximum brightness. Using a quantum dot source with a fidelity as high as 0.987(8), we implement here quantum key distribution with an average quantum bit error rate as low as 1.9% over a time span of 13 hours. For a proof of principle, the key generation is performed with the BBM92 protocol between two buildings, connected by a 350-m-long fiber, resulting in an average raw (secure) key rate of 135 bits/s (86 bits/s) for a pumping rate of 80 MHz, without resorting to time- or frequency-filtering techniques. Our work demonstrates the viability of quantum dots as light sources for entanglement-based quantum key distribution and quantum networks. By increasing the excitation rate and embedding the dots in state-of-the-art photonic structures, key generation rates in the gigabits per second range are in principle at reach.

Proposal for noise-free visible-telecom quantum frequency conversion through third-order sum and difference frequency generation

Quantum frequency conversion (QFC) between the visible and telecom is a key to connect quantum memories in fiber-based quantum networks. Current methods for linking such widely separated frequencies, such as sum/difference frequency generation and four-wave mixing Bragg scattering, are prone to broadband noise generated by the pump laser(s). To address this issue, we propose to use third-order sum/difference frequency generation (TSFG/TDFG) for an upconversion/downconversion QFC interface. In this process, two long wavelength pump photons combine their energy and momentum to mediate frequency conversion across the large spectral gap between the visible and telecom bands, which is particularly beneficial from the noise perspective. We show that waveguide-coupled silicon nitride microring resonators can be designed for efficient QFC between 606 and 1550 nm via a 1990 nm pump through TSFG/TDFG. We simulate the device dispersion and coupling, and from the simulated parameters, estimate that the frequency conversion can be efficient (${\gt}80 \%$) at 50 mW pump power. Our results suggest that microresonator TSFG/TDFG is promising for compact, scalable, and low-power QFC across large spectral gaps.

Thermal stability of emission from single InGaAs/GaAs quantum dots at the telecom O-band

Single-photon sources are key building blocks in most of the emerging secure telecommunication and quantum information processing schemes. Semiconductor quantum dots (QD) have been proven to be the most prospective candidates. However, their practical use in fiber-based quantum communication depends heavily on the possibility of operation in the telecom bands and at temperatures not requiring extensive cryogenic systems. In this paper we present a temperature-dependent study on single QD emission and single-photon emission from metalorganic vapour-phase epitaxy-grown InGaAs/GaAs QDs emitting in the telecom O-band at 1.3 μm. Micro-photoluminescence studies reveal that trapped holes in the vicinity of a QD act as reservoir of carriers that can be exploited to enhance photoluminescence from trion states observed at elevated temperatures up to at least 80 K. The luminescence quenching is mainly related to the promotion of holes to higher states in the valence band and this aspect must be primarily addressed in order to further increase the thermal stability of emission. Photon autocorrelation measurements yield single-photon emission with a purity of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${g}_{50K}^{(2)}\left(0\right)=0.13$$\end{document}g50K(2)0=0.13 up to 50 K. Our results imply that these nanostructures are very promising candidates for single-photon sources at elevated (e.g., Stirling cryocooler compatible) temperatures in the telecom O-band and highlight means for improvements in their performance.

Quantum dot-based broadband optical antenna for efficient extraction of single photons in the telecom O-band

Long-distance fiber-based quantum communication relies on efficient non-classical light sources operating at telecommunication wavelengths. Semiconductor quantum dots are promising candidates for on-demand generation of single photons and entangled photon pairs for such applications. However, their brightness is strongly limited due to total internal reflection at the semiconductor/vacuum interface. Here we overcome this limitation using a dielectric antenna structure. The non-classical light source consists of a gallium phosphide solid immersion lens in combination with a quantum dot nanomembrane emitting single photons in the telecom O-band. With this device, the photon extraction is strongly increased in a broad spectral range. A brightness of 17% (numerical aperture of 0.6) is obtained experimentally, with a single photon purity of g⁽²⁾(0)=0.049±0.02 at saturation power. This brings the practical implementation of quantum communication networks one step closer.

Advanced technologies for quantum photonic devices based on epitaxial quantum dots

Quantum photonic devices are candidates for realizing practical quantum computers and networks. The development of integrated quantum photonic devices can greatly benefit from the ability to incorporate different types of materials with complementary, superior optical or electrical properties on a single chip. Semiconductor quantum dots (QDs) serve as a core element in the emerging modern photonic quantum technologies by allowing on-demand generation of single-photons and entangled photon pairs. During each excitation cycle, there is one and only one emitted photon or photon pair. QD photonic devices are on the verge of unfolding for advanced quantum technology applications. In this review, we focus on the latest significant progress of QD photonic devices. We first discuss advanced technologies in QD growth, with special attention to droplet epitaxy and site-controlled QDs. Then we overview the wavelength engineering of QDs via strain tuning and quantum frequency conversion techniques. We extend our discussion to advanced optical excitation techniques recently developed for achieving the desired emission properties of QDs. Finally, the advances in heterogeneous integration of active quantum light-emitting devices and passive integrated photonic circuits are reviewed, in the context of realizing scalable quantum information processing chips.

Quantum Beat between Sunlight and Single Photons

We demonstrate fourth-order quantum beat between sunlight and single photons from a quantum dot. With a fast time-resolved detection system, we observed high-visibility quantum beat between the independent photons of different frequencies from the two astronomically separated light sources. The temporal dynamics of the beat oscillation indicate the coherent behavior of the interfering photons, and the raw visibility of two-photon interference shows violation of the classical limit with a frequency mismatch of three-times the line width.

Carbon Nanotube Color Centers in Plasmonic Nanocavities: A Path to Photon Indistinguishability at Telecom Bands

Indistinguishable single photon generation at telecom wavelengths from solid-state quantum emitters remains a significant challenge to scalable quantum information processing. Here we demonstrate efficient generation of "indistinguishable" single photons directly in the telecom O-band from aryl-functionalized carbon nanotubes by overcoming the emitter quantum decoherence with plasmonic nanocavities. With an unprecedented single-photon spontaneous emission time down to 10 ps (from initially 0.7 ns) generated in the coupling scheme, we show a two-photon interference visibility at 4 K reaching up to 0.79, even without applying post selection. Cavity-enhanced quantum yields up to 74% and Purcell factors up to 415 are achieved with single-photon purities up to 99%. Our results establish the capability to fabricate fiber-based photonic devices for quantum information technology with coherent properties that can enable quantum logic.

Resolving the temporal evolution of line broadening in single quantum emitters

Light emission from solid-state quantum emitters is inherently prone to environmental decoherence, which results in a line broadening and in the deterioration of photon indistinguishability. Here we employ photon correlation Fourier spectroscopy (PCFS) to study the temporal evolution of such a broadening in two prominent systems: GaAs and In(Ga)As quantum dots. Differently from previous experiments, the emitters are driven with short laser pulses as required for the generation of high-purity single photons, the time scales we probe range from a few nanoseconds to milliseconds and, simultaneously, the spectral resolution we achieve can be as small as ∼ 2µeV. We find pronounced differences in the temporal evolution of different optical transition lines, which we attribute to differences in their homogeneous linewidth and sensitivity to charge noise. We analyze the effect of irradiation with additional white light, which reduces blinking at the cost of enhanced charge noise. Due to its robustness against experimental imperfections and its high temporal resolution and bandwidth, PCFS outperforms established spectroscopy techniques, such as Michelson interferometry. We discuss its practical implementation and the possibility to use it to estimate the indistinguishability of consecutively emitted single photons for applications in quantum communication and photonic-based quantum information processing.

Entanglement between a Diamond Spin Qubit and a Photonic Time-Bin Qubit at Telecom Wavelength

We report on the realization and verification of quantum entanglement between a nitrogen-vacancy electron spin qubit and a telecom-band photonic qubit. First we generate entanglement between the spin qubit and a 637 nm photonic time-bin qubit, followed by photonic quantum frequency conversion that transfers the entanglement to a 1588 nm photon. We characterize the resulting state by correlation measurements in different bases and find a lower bound to the Bell state fidelity of ≥0.77±0.03. This result presents an important step towards extending quantum networks via optical fiber infrastructure.

Spectral noise in frequency conversion from the visible to the telecommunication C-band

We report a detailed study of the noise properties of a visible-to-telecom photon frequency converter based on difference frequency generation (DFG). The device converts 580 nm photons to 1541 nm using a strong pump laser at 930 nm, in a periodically poled lithium niobate ridge waveguide. The converter reaches a maximum device efficiency of 46 % (internal efficiency of 67%) at a pump power of 250 mW. The noise produced by the pump laser is investigated in detail by recording the noise spectra both in the telecom and visible regimes and measuring the power dependence of the noise rates. The noise spectrum in the telecom is very broadband, as expected from previous work on similar DFG converters. However, we also observe several narrow dips in the telecom spectrum, with corresponding peaks appearing in the 580 nm noise spectrum. These features are explained by sum frequency generation of the telecom noise at wavelengths given by the phase-matching condition of different spatial modes in the waveguide. The proposed noise model is in good agreement with all the measured data, including the power dependence of the noise rates, both in the visible and telecom regimes. These results are applicable to the class of DFG converters where the pump laser wavelength is in between the input and target wavelength.

Quantum Frequency Conversion of a Quantum Dot Single-Photon Source on a Nanophotonic Chip

Single self-assembled InAs/GaAs quantum dots are promising bright sources of indistinguishable photons for quantum information science. However, their distribution in emission wavelength, due to inhomogeneous broadening inherent to their growth, has limited the ability to create multiple identical sources. Quantum frequency conversion can overcome this issue, particularly if implemented using scalable chip-integrated technologies. Here, we report the first demonstration of quantum frequency conversion of a quantum dot single-photon source on a silicon nanophotonic chip. Single photons from a quantum dot in a micropillar cavity are shifted in wavelength with an on-chip conversion efficiency ≈ 12 %, limited by the linewidth of the quantum dot photons. The intensity autocorrelation function g ( 2 ) ( τ ) for the frequency-converted light is antibunched with g ( 2 ) ( 0 ) = 0.290 ± 0.030 , compared to the before-conversion value g ( 2 ) ( 0 ) = 0.080 ± 0.003 . We demonstrate the suitability of our frequency conversion interface as a resource for quantum dot sources by characterizing its effectiveness across a wide span of input wavelengths (840 nm to 980 nm), and its ability to achieve tunable wavelength shifts difficult to obtain by other approaches.