We review recent works in optical signal shaping and advanced characterization techniques within the framework of nonlinear fiber propagation. Specifically, we focus on the development of characterization methods based on the dispersive Fourier transform to monitor incoherent spectral broadening processes with enhanced resolution and sensitivity. In this framework, we also discuss recent studies of modulation instability in a noise-driven regime. Paired with suitable optical monitoring techniques, we show that controlled coherent optical seeding can be leveraged via several machine learning approaches to tailor and optimize incoherent spectral broadening dynamics.
Today’s quantum technology relies on the realization of large-scale non-classical systems in practical formats to enable quantum-accelerated computing, secure communications and enhanced sensing. Optical on-chip quantum frequency combs, characterized by many equidistantly spaced frequency modes, allow the storage of large amounts of quantum information and together with control mechanisms can provide practical large-scale quantum systems. In this contribution, we present recent advances on the controlled generation and use of quantum frequency combs for information processing. First, we demonstrate an electrically-pumped laser-integrated quantum light source of two- and high-dimensional maximally entangled photons. We exploit a hybrid InP-SiN approach which allows to include a filter, a gain section and a parametric photon pair source in a single system. Second, we demonstrate the generation of high-dimensional bi-photon quantum frequency combs with tunable entropies by exploiting a novel excitation technique and spectral filtering. Using this, we reveal unidirectional bosonic quantum walks, asymmetric energy transfer, and directional entanglement transport.
We demonstrate the first fully on-chip electrically-pumped laser-integrated quantum photonic source, which consists of a laser cavity, an InP gain section, and a Vernier filtering stage. This sub-millimeter-sized hybrid chip generates high-quality, low-noise (CAR ~80) entangled photon states with a remarkably high pair detection rate (~620 Hz) over four resonant modes within the C-band. We confirmed the realization of high-dimensional entangled states with fidelity of ~ 0.99 and visibility of ~ 96%. This fully integrated, practical, and field-deployable quantum light source brings scalability to applications ranging from quantum processing circuits, quantum internet to quantum satellite systems.
Quantum technologies harness nonclassical features of particles, here, photons, to develop novel, efficient, and precise devices for information processing applications. Superposition, entanglement, as well as the coherent manipulation of quantum states are at the heart of the second quantum revolution (quantum 2.0) which targets the development of secure cryptographic systems, complex computation protocols, and more. Emerging quantum architectures rely on the realistic implementation of photonic schemes which are scalable, resource-efficient, and compatible with CMOS technologies as well as fiber networks. This work demonstrates current schemes utilized for time-/frequency-bin entanglement generation and processing by leveraging existing telecommunications and integrated photonics infrastructures.
We generate high-dimensional bi-photon quantum frequency combs with tunable entropies exploiting the second-order nonlinearity of a periodically-poled lithium niobate waveguide through a novel pumping and filtering scheme. Using these quantum states with varying degrees of entanglement, we demonstrate unidirectional bosonic quantum walks, asymmetric energy transfer, and entanglement transport. Our non-maximally entangled quantum states can serve as excellent testbeds for several computational protocols. Moreover, we achieve the steering of the directionality in a scalable format, which enables a new control mechanism for quantum walks as well as novel modification means of joint probability distributions.
Spectral Hong-Ou-Mandel effect is experimentally revealed between a thermal field and a heralded state. We demonstrate that multi-photon components in heralded state get engaged in quantum interference which is manifested in improved visibilities for certain mean photon numbers of the thermal field. A theoretical relationship is derived for the visibility, showing its compound dependency on the mean photon number per pulse period within the thermal field and the thermal part of the heralded state. Relying on pulsed excitation, we demonstrate an experimental approach for time-efficient acquisition of visibility. The results of this research are of potential application to high-speed remote secret key-sharing and combating the security gaps arising from multi-photon contamination.
Today’s quantum technology relies on the realization of large-scale non-classical systems in practical formats to enable quantum-accelerated computing, secure communications and enhanced sensing. Optical on-chip quantum frequency combs, characterized by many equidistantly spaced frequency modes, allow the storage of large amounts of quantum information. The combination with control techniques, using accessible state-of-the-art telecommunications infrastructure, can constitute a powerful frequency-domain quantum circuit with new functionalities and represents an approach towards realizing practical large-scale controllable quantum systems. In this presentation, we will review approaches for the efficient realization of quantum frequency combs in on-chip waveguide structures and micro-resonators. We will show their applicability for the realization of quantum systems with considerably enhanced complexity, particularly generating and manipulating on-chip multi-photon and high-dimensional quantum states as well as discrete high-dimensional cluster states, laying at the basis of measurement based-quantum computing. Building on this, the realization of frequency-domain Hong-Ou-Mandel interference of independent photons, fundamental to quantum information processing, as well as an outlook on frequency-domain circuits will be discussed.
We review our work on implementing integrated QFC sources based on microring resonators for on-chip generation of two- and multi-photon time-bin entangled states, d-level frequency-entangled photon pairs, and multipartite d-level cluster states. We also present our recent progress on telecom-compatible, scalable, time-entangled two-photon qubits using on-chip Mach-Zehnder interferometers (MZI) in combination with spiral waveguides. Both approaches are highly cost-effective, efficient, and practical, since we coherently manipulate the time and frequency modes through standard fiber-linked components that are compatible with off-the-shelf telecommunications infrastructures. Our work paves the way for robust sources and processors of complex photon states for future quantum technologies.
We demonstrate two-photon interference and polarization entanglement at 2090 nm, constituting a crucial leap towards
free-space mid-infrared quantum communication systems in a spectral region with high atmospheric transparency and
reduced solar background.
Quantum-enhanced optical technologies operating within the 2- to 2.5-μm spectral region have the potential to revolutionize emerging applications in communications, sensing, and metrology. Currently, sources of entangled photons are available at visible, near-infrared and telecom wavelengths but are strongly underdeveloped at longer wavelengths. Here, using custom-designed lithium niobate crystals for spontaneous parametric down-conversion and tailored superconducting nanowire single-photon detectors, we demonstrate two-photon interference and polarization-entangled photon pairs at 2090 nm. These results open the 2- to 2.5-μm mid-infrared window for the development of optical quantum technologies such as quantum key distribution in next-generation mid-infrared fiber communication systems and future Earth-to-satellite communications.
The heterogeneous integration of AlGaAs-on-insulator (AlGaAs-OI) has proven to be a highly efficient material platform for nonlinear photonics. When AlGaAs is bonded to silica, there is a large difference in refractive index which results in waveguides with a strong modal confinement and enhanced nonlinearities. This, in combination with low linear and nonlinear losses, makes AlGaAs-OI an ideal platform for exploring nonlinear phenomena. In this talk we present recent progress on the design, fabrication and testing of AlGaAs-OI waveguides for efficient second harmonic generation, supercontinuum generation and the creation of photon pairs.
A key challenge in today’s quantum science is the realization of large-scale complex non-classical systems to enable e.g. ultra-secure communications, quantum-enhanced measurements, and computations faster than classical approaches. Optical frequency combs represent a powerful approach towards this, since they provide a very high number of temporal and frequency modes which can result in large-scale quantum systems. Here, we discuss the recent progress on the realization of integrated quantum frequency combs and reveal how their use in combination with on-chip and fiber-optic telecommunications components can enable quantum state control with new functionalities, yielding unprecedented capability.
Optical square wave sources are particularly important for applications in high speed signal processing and optical communications. In most realizations, optical square waves are generated by electro-optic modulation, dispersion engineering of mode-locked lasers, polarization switching, or by exploiting optical bi-stability and/or optical delayed feedback in semiconductor diode lasers, as well as vertical-cavity surface-emitting lasers (VCSELs). All such configurations are bulky and cause significant timing jitters. Here we demonstrate the direct generation of optical square waves from a polarization-maintaining figure-eight nonlinear amplifying loop mirror (NALM) configuration that uses an embedded high index glass micro-cavity as the nonlinear element. Such a NALM mimics the behavior of a saturable absorber and has been used to reach passive mode-locking of pico- and even nano-second pulses. In our method, the NALM, including a high-Q micro-ring resonator, acts as an ultra-narrowband spectral filter and at the same time provides a large nonlinear phase-shift. Previously we have demonstrated that such a configuration enables sufficient nonlinear phase-shifts for low-power narrow-bandwidth (~100 MHz FWHM) passive mode-locked laser operation. Here we demonstrate the switching of stable optical square wave pulses from conventional mode-locked pulses by adjusting the cavity properties. In addition, the square wave signal characteristics, such as repetition rate and pulse duration, can be also modified in a similar fashion. The source typically produces nanosecond optical square wave pulses with a repetition rate of ~ 120 MHz at 1550nm. In order to verify the reach of our approach, we compare our experimental results with numerical simulations using a delay differential equation model tailored for a figure-eight laser.
Complex optical quantum states based on entangled photons are essential for investigations of fundamental physics and are the heart of applications in quantum information science. Recently, integrated photonics has become a leading platform for the compact, cost-efficient, and stable generation and processing of optical quantum states. However, onchip sources are currently limited to basic two-dimensional (qubit) two-photon states, whereas scaling the state complexity requires access to states composed of several (<2) photons and/or exhibiting high photon dimensionality. Here we show that the use of integrated frequency combs (on-chip light sources with a broad spectrum of evenly-spaced frequency modes) based on high-Q nonlinear microring resonators can provide solutions for such scalable complex quantum state sources. In particular, by using spontaneous four-wave mixing within the resonators, we demonstrate the generation of bi- and multi-photon entangled qubit states over a broad comb of channels spanning the S, C, and L telecommunications bands, and control these states coherently to perform quantum interference measurements and state tomography. Furthermore, we demonstrate the on-chip generation of entangled high-dimensional (quDit) states, where the photons are created in a coherent superposition of multiple pure frequency modes. Specifically, we confirm the realization of a quantum system with at least one hundred dimensions. Moreover, using off-the-shelf telecommunications components, we introduce a platform for the coherent manipulation and control of frequencyentangled quDit states. Our results suggest that microcavity-based entangled photon state generation and the coherent control of states using accessible telecommunications infrastructure introduce a powerful and scalable platform for quantum information science.
The on-chip generation of optical quantum states will enable accessible advances for quantum technologies. We demonstrate that integrated quantum frequency combs (based on high-Q microring resonators made from a CMOS-compatible, high refractive-index doped-glass platform) can enable the generation of pure heralded single photons, cross-polarized photon pairs, as well as bi- and multi-photon entangled qubit states over a broad frequency comb covering the S, C, L telecommunications band, with photon frequencies corresponding to standard telecommunication channels spaced by 200 GHz.
Exploiting a self-locked, intra-cavity excitation configuration, a highly-stable source of multiplexed heralded single photons is demonstrated, operating continuously for several weeks with less than 5% fluctuations. The photon bandwidth of 110 MHz is compatible with quantum memories, and high photon purity was confirmed through single-photon auto-correlation measurements. In turn, by simultaneously exciting two orthogonal polarization mode resonances, we demonstrate the first realization of type-II spontaneous FWM (in analogy to type-II spontaneous parametric down-conversion), allowing the direct generation of orthogonally-polarized photon pairs on a chip.
By using a double-pulse excitation, we demonstrate the generation of time-bin entangled photon pairs. We measure qubit entanglement with visibilities above 90%, enabling the implementation of quantum information processing protocols. Finally, the excitation field and the generated photons are intrinsically bandwidth-matched due to the resonant characteristics of the ring cavity, enabling the multiplication of Bell states and the generation of a four-photon time-bin entangled state. We confirm the generation of this four-photon entangled state through four-photon quantum interference.
We present the first systematic study on the emission polarization of coherent random lasing (RL). Employing a single-shot spectro-polarimeter the complete polarization state and the degree of polarization could be recovered for individual RL modes. It turned out that the polarization of RL modes is strongly dependent on the pump polarization, which can be explained with the dipolar nature of the dye molecules. Furthermore, by using a highly viscous solvent for the laser-dye and a linear pump polarization, it was possible to force all random-lasing modes into just the same linear polarization.
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