arXiv subjects

Robert J. Chapman

Publications and source records attributed to Robert J. Chapman.

At least 19 recordsLinked to original sources

arXiv2026

Cryogenic nonlinear processes in thin-film lithium niobate

Photonic integrated circuits operating at cryogenic temperatures are necessary for many quantum technologies such as quantum transduction, integrated single-photon emitters and detectors, as well as deep-space communication and sensing devices. Thin-film lithium niobate (TFLN) is an emerging platform that is a strong candidate for fully integrated quantum photonics, offering low loss, fast electro-optic reconfigurability, nonlinear quantum light sources, and the ability to host quantum emitters and single-photon detectors. To interface TFLN with technologies that require cryogenic operation, like superconducting single-photon detectors, microwave-to-optical transducers, and solid-state quantum emitters, it is important to study its optical and electrical properties from room temperature down to cryogenic temperatures. Here, we investigate linear and nonlinear photonic devices, including racetrack resonators, Mach-Zehnder modulators and periodically poled waveguides in TFLN using a cryogenic fiber probe station with full temperature control down to 5 K. We quantify a shift in resonances, a 22% increase in electro-optic modulator half-wave voltage, a blue shift of 18 nm for Type-0 phase-matching as well as a red shift of 64 nm for Type-II phase-matching as the sample temperature decreases. Our study of nonlinear processes in a cryogenic environment will contribute towards developing novel devices for inter-platform quantum information processing, secure communication, and enhanced sensing.

physics.optics↗

arXiv2025

Tunable polarization-entangled near-infrared photons from orthogonal GaAs nanowires

Quantum entanglement is a fundamental resource for emerging quantum technologies, enabling secure communication and enhanced sensing. For decades, generating polarization entangled states has been mainly achieved using bulk crystals with spontaneous parametric down conversion (SPDC), preventing scalability and on-chip integration. Miniaturizing the quantum source provides access to more versatility and tunability while enabling an easier integration to other devices, notably necessary for satellite-based quantum communication, and eventually reducing fabrication costs. This challenging task can be achieved with Zinc Blende GaAs nanowires. They already have shown an efficient photon pairs generation via SPDC at 1550 nm. Here we demonstrate that a pair of orthogonal GaAs nanowires constitutes a new nanoscale platform to control the quantum state at telecommunication wavelength, enabling a transition from polarization entangled to separable states as a function of the pump polarization, with fidelities reaching 90%

physics.optics↗

arXiv2025

High-temperature growth of ultra thin NbTiN films on lithium niobate for integrated single photon detection

Lithium niobate-on-insulator (LNOI) is an emerging photonic platform with high potential for scalable quantum information processing due to its strong second-order nonlinearity. However, little progress has been made in developing on-chip single-photon detectors on LNOI. Niobium titanium nitride (NbTiN) superconducting nanowire single-photon detectors (SNSPDs) are a promising candidate for this application. In this work, we use DC reactive magnetron sputtering to grow high-quality NbTiN thin films using an ultra-high vacuum deposition system with a base pressure lower than $2\times 10^{-10}$ mbar. Enabled by the low concentration of background impurities in this system, we investigate the impact of substrate temperature during NbTiN growth. We achieve four nm thick superconducting films with a critical temperature ($T_{c}$) of 12.3 K grown at a substrate temperature of 825 K. We find that the NbTiN films grow in the (111) orientation and evolve from a porous pillar structure when grown at low temperatures to densely packed fibrous grains at higher temperatures. Furthermore, we demonstrate that the increased substrate temperature reduces the oxygen concentration in our films and improves the overall stoichiometry. In addition, we integrate these films with the LNOI platform and investigate the obtained interface quality. Lastly, we fabricate SNSPDs from the NbTiN film on LNOI and characterize the detector performance.

cond-mat.supr-con↗

arXiv2025

Nanodomain poling unlocking backward nonlinear light generation in thin film lithium niobate

Nonlinear frequency conversion offers powerful capabilities for applications in telecommunications, signal processing, and computing. Thin-film lithium niobate (TFLN) has emerged as a promising integrated photonics platform due to its strong electro-optic effect and second-order nonlinearity, which can be exploited through periodic poling. However, conventional poling techniques in x-cut TFLN are limited to minimum period sizes on the order of microns, preventing the efficient generation of interactions involving counter-propagating waves. Here we report scalable periodic poling of x-cut TFLN with periods down to 215 nm and realize devices for counter- and back-propagating phase matching. We estimate conversion efficiencies of 1474 $\%$/W/cm$^2$ and 45 $\%$/W/cm$^2$ respectively, and measuring sum frequency generation we confirm that the nonlinear generation takes place in the desired direction. We report spontaneous parametric down conversion for the counter-propagating and, for the first time, for a backward propagating device. This technological advance provides the control of domain geometry in TFLN with an unprecedented precision and leads into the generation of photon pairs with spatial and spectral properties tailored for quantum signal processing, quantum computing and metrology.

physics.optics↗

arXiv2025

Scalable quantum interference in integrated lithium niobate nanophotonics

Photonics has emerged as one of the leading platforms for the implementation of real-world-applicable quantum technologies, enabling secure communication, enhanced sensing capabilities, as well as resolving previously intractable computational challenges. However, to harness the full potential of the photonics platform, several engineering feats need to be accomplished, among those is the quest for a scalable source of pure single photons. While single photon sources can be implemented in a variety of different ways, integrated lithium niobate stands out as a prime contender for a monolithic quantum photonics platform, given its second-order nonlinearity and proven classical scalability. Despite the extensive effort put into developing the platform, integrating suitable photon pair sources remains a hurdle limiting the scalability of quantum photonic systems in lithium niobate. We engineer three-wave-mixing in a nanophotonic lithium niobate device, integrating multiple near-perfect spectrally separable heralded single photon sources. By mixing photons generated via the developed sources, we show bosonic interference between indistinguishable photons, a crucial interaction for many photonic quantum computing protocols. This demonstration of the first proof-of-principle multi-source interference in integrated lithium niobate contributes to developing a truly scalable quantum photonics platform.

quant-ph↗

arXiv2025

Counter-propagating spontaneous parametric down-conversion source in lithium niobate on insulator

Quantum photonic technologies rely on the ability to generate, manipulate, and interfere indistinguishable single photons on a scalable platform. Among the various approaches, spontaneous parametric down-conversion (SPDC) remains one of the most widely used methods for generating entangled or pure photon pairs. However most integrated SPDC sources relying on co-propagating geometries have a limited purity of heralded photons, or require lossy filtering. Type-2 SPDC processes can produce pure separable photons but typically suffer from lower efficiency and added complexity due to polarisation management. Here we show the first integrated counter-propagating photon-pair source on lithium niobate on insulator, where signal and idler photons are generated in opposite directions. The counter-propagating geometry leads to spectrally uncorrelated photon pairs without spectral filtering. The joint spectral intensity measurements and unheralded $g^{(2)}$ correlations, yield purities of (92$\pm$3)%. Interference between two independent sources achieves heralded visibilities of (71$\pm$3)%, confirming the scalability of the platform. These results establish a new route toward integrated, high-purity, and tunable photon sources. The demonstrated counter-propagating geometry offers a scalable solution for quantum photonic networks.

quant-ph↗

arXiv2025

Programmable Bell State Generation in an Integrated Thin Film Lithium Niobate Circuit

Entanglement is central to quantum technologies such as cryptography, sensing, and computing. Photon pairs generated via nonlinear optical processes are excellent for preparing entangled states due to their long coherence times and compatibility with fiber optic networks. Steady progress in nanofabrication has positioned lithium niobate-on-insulator (LNOI) as a leading platform for monolithic integration of photon pair sources into optical circuits, leveraging its strong second-order nonlinearity. Here, we present a reconfigurable photonic integrated circuit on LNOI, which combines two on-chip photon pair sources with programmable interferometers, enabling generation of entangled states. The pair sources achieve a source brightness of 26 MHz nm$^{-1}$ mW$^{-1}$ while maintaining a coincidence-to-accidental ratio above 100. We successfully interfere the two sources with $99 \pm 0.7$ % visibility, demonstrating the indistinguishability required for producing entanglement on-chip. We show preparation of any of the maximally entangled Bell states with fidelity above 90 % verified by quantum state tomography. These results establish LNOI as a compelling, scalable platform to explore integrated quantum photonic technologies enabled by high-brightness sources of entangled quantum states.

quant-ph↗

arXiv2024

Time-bin entangled Bell state generation and tomography on thin-film lithium niobate

Optical quantum communication technologies are making the prospect of unconditionally secure and efficient information transfer a reality. The possibility of generating and reliably detecting quantum states of light, with the further need of increasing the private data-rate is where most research efforts are focusing. The physical concept of entanglement is a solution guaranteeing the highest degree of security in device-independent schemes, yet its implementation and preservation over long communication links is hard to achieve. Lithium niobate-on-insulator has emerged as a revolutionising platform for high-speed classical telecommunication and is equally suited for quantum information applications owing to the large second-order nonlinearities that can efficiently produce entangled photon pairs. In this work, we generate maximally entangled quantum states in the time-bin basis using lithium niobate-on-insulator photonics at the fibre optics telecommunication wavelength, and reconstruct the density matrix by quantum tomography on a single photonic integrated circuit. We use on-chip periodically-poled lithium niobate as source of entangled qubits with a brightness of 242 MHz/mW and perform quantum tomography with a fidelity of 91.9+-1.0 %. Our results, combined with the established large electro-optic bandwidth of lithium niobate, showcase the platform as perfect candidate to realise fibre-coupled, high-speed time-bin quantum communication modules that exploit entanglement to achieve information security.

quant-ph↗

arXiv2024

Polarization-entangled photon pair generation from an epsilon-near-zero metasurface

Polarization-entangled photon pair sources are essential for diverse quantum technologies, such as quantum communication, computation, and imaging. However, the generation of complex polarization-entangled quantum states has long been constrained by the available nonlinear susceptibility tensor of natural nonlinear crystals, necessitating a cumbersome and intricate setup for additional coherent superposition or post-selection. In this study, we introduce and experimentally demonstrate a nanoscale polarization-entangled photon pair source utilizing an artificially-engineered metamaterial platform. This platform is based on a plasmonic metasurface that is strongly coupled to an epsilon-near-zero (ENZ) material. By precisely engineering resonances at both pump and signal/idler wavelengths, and leveraging the field enhancement provided by the ENZ effect, the photon pair generation efficiency of the 68-nm-thick metasurface is significantly boosted. More notably, the ENZ metasurface platform facilitates versatile manipulation of the system's anisotropic second-order nonlinear susceptibility tensor, enabling direct control over the polarization states of the photon pairs, which leads to the generation of a polarization-entangled Bell state without the need for additional components. Our approach opens a new avenue for the simultaneous photon pair generation and quantum state engineering in a compact platform.

physics.optics↗

arXiv2024

Programmable quantum circuits in a large-scale photonic waveguide array

Over the past decade, integrated quantum photonic technologies have shown great potential as a platform for studying quantum phenomena and realizing large-scale quantum information processing. Recently, there have been proposals for utilizing waveguide lattices to implement quantum gates, providing a more compact and robust solution compared to discrete implementation with directional couplers and phase shifters. We report on the first demonstration of precise control of single photon states on an $11\times 11$ continuously-coupled programmable waveguide array. Through electro-optical control, the array is subdivided into decoupled subcircuits and the degree of on-chip quantum interference can be tuned with a maximum visibility of 0.962$\pm$0.013. Furthermore, we show simultaneous control of two subcircuits on a single device. Our results demonstrate the potential of using this technology as a building block for quantum information processing applications.

quant-ph↗

arXiv2024

Extremely high extinction ratio electro-optic modulator via frequency upconversion to visible wavelengths

Intensity modulators are fundamental components for integrated photonics. From near infrared to visible spectral ranges, they find applications in optical communication and quantum technologies. In particular, they are required for the control and manipulation of atomic systems such as atomic clocks and quantum computers. Typical integrated electro-optic modulators operating at these wavelengths show high bandwidth and low voltage operation, but their extinction ratios are moderate. Here we present an integrated thin-film lithium niobate electro-optic modulator operating in the C-band, which uses a subsequent periodically poled waveguide to convert the modulated signal from 1536 nm to 768 nm using second harmonic generation. We demonstrate that the upconverted signal retains the characteristics of the modulated input signal, reaching a measured high bandwidth of 40 GHz. Due to the nature of the nonlinear process, it exhibits, with respect to the fundamental signal, a doubled extinction ratio of 46 dB, which is the highest recorded for near-infrared light on this platform.

physics.optics↗

arXiv2024

On-chip quantum interference between independent lithium niobate-on-insulator photon-pair sources

Generating and interfering non-classical states of light is fundamental to optical quantum information science and technology. Quantum photonic integrated circuits provide one pathway towards scalability by combining nonlinear sources of non-classical light and programmable circuits in centimeter-scale devices. The key requirements for quantum applications include efficient generation of indistinguishable photon-pairs and high-visibility programmable quantum interference. Here, we demonstrate a lithium niobate-on-insulator (LNOI) integrated photonic circuit that generates a two-photon path-entangled state, and a programmable interferometer for quantum interference. We generate entangled photons with $\sim2.3\times10^8$ pairs/s/mW brightness and perform quantum interference experiments on the chip with $96.8\pm3.6\%$ visibility. LNOI is an emerging photonics technology that has revolutionized high-speed modulators and efficient frequency conversion. Our results provide a path towards large-scale integrated quantum photonics including efficient photon-pair generation and programmable circuits for applications such as boson sampling and quantum communications.

quant-ph↗

arXiv2024

Polarization dynamics of solid-state quantum emitters

Quantum emitters in solid-state crystals have recently attracted a lot of attention due to their simple applicability in optical quantum technologies. The polarization of single photons generated by quantum emitters is one of the key parameters that play a crucial role in the applications, such as quantum computation that uses the indistinguishability of photons. However, the degree of single photon polarization is typically quantified using time-averaged photoluminescence intensity of single emitters, which provides limited information about the dipole properties in solids. In this work, we use single defects in hexagonal boron nitride and nanodiamond as efficient room-temperature single photon sources to reveal the origin and the temporal evolution of dipole orientation in solid-state quantum emitters. The angle of excitation and emission dipoles relative to the crystal axes are determined experimentally and then calculated using density functional theory, which results in characteristic angles for every specific defect that can be used as an efficient tool for defect identification and understanding their atomic structure. Moreover, the temporal polarization dynamics reveal a strongly modified linear polarization visibility that depends on the excited state decay time of individual excitation. This effect can be traced back potentially to the excitation of excess charges in the local crystal environment. Understanding such hidden time-dependent mechanisms can further be used to improve the performance of polarization-sensitive experiments, in particular that of quantum communication with single photon emitters.

quant-ph↗

arXiv2023

Experimental graybox quantum system identification and control

Understanding and controlling engineered quantum systems is key to developing practical quantum technology. However, given the current technological limitations, such as fabrication imperfections and environmental noise, this is not always possible. To address these issues, a great deal of theoretical and numerical methods for quantum system identification and control have been developed. These methods range from traditional curve fittings, which are limited by the accuracy of the model that describes the system, to machine learning methods, which provide efficient control solutions but no control beyond the output of the model, nor insights into the underlying physical process. Here we experimentally demonstrate a "graybox" approach to construct a physical model of a quantum system and use it to design optimal control. We report superior performance over model fitting, while generating unitaries and Hamiltonians, which are quantities not available from the structure of standard supervised machine learning models. Our approach combines physics principles with high-accuracy machine learning and is effective with any problem where the required controlled quantities cannot be directly measured in experiments. This method naturally extends to time-dependent and open quantum systems, with applications in quantum noise spectroscopy and cancellation.

quant-ph↗

arXiv2023

Programmable high-dimensional Hamiltonian in a photonic waveguide array

Waveguide lattices offer a compact and stable platform for a range of applications, including quantum walks, topological effects, condensed matter system simulation, and classical and quantum information processing. In such lattices, the Hamiltonian's hopping and on-site terms determine the optical evolution, which can be engineered using waveguide spacing and refractive index profile. While waveguide lattices have been realized in various photonic platforms, these devices have always been static and designed for specific applications. We present a programmable waveguide array in which the Hamiltonian terms can be electro-optically tuned to implement various Hamiltonian continuous-time evolutions on a single device. We used a single array with 11 waveguides in lithium niobate, controlled via 22 electrodes, to perform a range of experiments that realized the Su-Schriffer-Heeger model, the Aubrey-Andre model, and Anderson localization, which is equivalent to over 2500 static devices. Our architecture's micron-scale local electric fields independently control waveguide coupling coefficients and effective indices, which overcomes cross-talk limitations of thermo-optic phase shifters in other platforms such as silicon, silicon-nitride, and silica. Electro-optic control allows for ultra-fast and more precise reconfigurability with lower power consumption, and with quantum input states, our platform can enable the study of multiple condensed matter quantum dynamics with a single device.

physics.optics↗

arXiv2023

Time-bin entanglement at telecom wavelengths from a hybrid photonic integrated circuit

Mass-deployable implementations for quantum communication require compact, reliable, and low-cost hardware solutions for photon generation, control and analysis. We present a fiber-pigtailed hybrid photonic circuit comprising nonlinear waveguides for photon-pair generation and a polymer interposer reaching 68dB of pump suppression and photon separation with >25dB polarization extinction ratio. The optical stability of the hybrid assembly enhances the quality of the entanglement, and the efficient background suppression and photon routing further reduce accidental coincidences. We thus achieve a 96(-8,+3)% concurrence and a 96(-5,+2)% fidelity to a Bell state. The generated telecom-wavelength, time-bin entangled photon pairs are ideally suited for distributing Bell pairs over fiber networks with low dispersion.

quant-ph↗

arXiv2023

Fabrication of low-loss III-V Bragg-reflection waveguides for parametric down-conversion

Entangled photon pairs are an important resource for quantum cryptography schemes that go beyond point-to-point communication. Semiconductor Bragg-reflection waveguides are a promising photon-pair source due to mature fabrication, integrability, large transparency window in the telecom wavelength range, integration capabilities for electro-optical devices as well as a high second-order nonlinear coefficient. To increase performance we improved the fabrication of Bragg-reflection waveguides by employing fixed-beam-moving-stage optical lithography, low pressure and low chlorine concentration etching, and resist reflow. The reduction in sidewall roughness yields a low optical loss coefficient for telecom wavelength light of alpha_reflow = 0.08(6)mm^(-1). Owing to the decreased losses, we achieved a photon pair production rate of 8800(300)(mW*s*mm)^(-1) which is 15-fold higher than in previous samples.

physics.optics↗

arXiv2023

Quantum logical controlled-NOT gate in a lithium niobate-on-insulator photonic quantum walk

Quantum computers comprise elementary logic gates that initialize, control and measure delicate quantum states. One of the most important gates is the controlled-NOT, which is widely used to prepare two-qubit entangled states. The controlled-NOT gate for single photon qubits is normally realized as a six-mode network of individual beamsplitters. This architecture however, utilizes only a small fraction of the circuit for the quantum operation with the majority of the footprint dedicated to routing waveguides. Quantum walks are an alternative photonics platform that use arrays of coupled waveguides with a continuous interaction region instead of discrete gates. While quantum walks have been successful for investigating condensed matter physics, applying the multi-mode interference for logical quantum operations is yet to be shown. Here, we experimentally demonstrate a two-qubit controlled-NOT gate in an array of lithium niobate-on-insulator waveguides. We engineer the tight-binding Hamiltonian of the six evanescently-coupled single-mode waveguides such that the multi-mode interference corresponds to the linear optical controlled-NOT unitary. We measure the two-qubit transfer matrix with $0.938\pm0.003$ fidelity, and we use the gate to generate entangled qubits with $0.945\pm0.002$ fidelity by preparing the control photon in a superposition state. Our results highlight a new application for quantum walks that use a compact multi-mode interaction region to realize large multi-component quantum circuits.

quant-ph↗