Quantum information has reached a stage where real-world applications stimulate an intense research for the implementation of reliable and practical protocols for quantum communication and information processing. The implementation of such protocols, though, requires distributing quantum correlations (entanglement) among a number of degrees of freedom (modes) increasing with the complexity of the task to achieve. In the large-number-of-modes regime, the most promising example is probably one-way quantum computation in which the computation is achieved by applying local measurements to a set of modes initially in a cluster state [1]. However the generation of multipartite entangled states requires experimental configurations whose complexity increases with the number of the modes involved by means of optical devices. In contrast, a practical source should be compact, scalable, and permit to master the quantum properties of the generated states even when the number of modes is very large.
Quantum information has reached a stage where real-world applications stimulate an intense research for the implementation of reliable and practical protocols for quantum communication and information processing. The implementation of such protocols, though, requires distributing quantum correlations (entanglement) among a number of degrees of freedom (modes) increasing with the complexity of the task to achieve. In the large-number-of-modes regime, the most promising example is probably one-way quantum computation in which the computation is achieved by applying local measurements to a set of modes initially in a cluster state [1]. However the generation of multipartite entangled states requires experimental configurations whose complexity increases with the number of the modes involved by means of optical devices. In contrast, a practical source should be compact, scalable, and permit to master the quantum properties of the generated states even when the number of modes is very large.
The exploitation of quantum mechanics has the potential to solve important challenges and \nintroduce novel technologies enabling, among others, powerful computation (which can solve \ncertain problems exponentially faster than classical computers), perfectly secure communications, \nas well as sensing and imaging with precision and resolutions not achievable by classical means. \nIn order to make use of the unique properties of quantum physics, experimental platforms \nthat provide access to, and allow the control of quantum states, need to be developed. In \nparticular, quantum systems composed of multiple parties are required, where each party is in a \ncoherent superposition of states, and where multiples of such parties are in a coherent \nsuperposition with each other (e.g. being entangled). Additionally, in order to make use of such \nentangled quantum systems, it is required that each individual party can be coherently controlled \nand measured. The usability and available quantum resources of the entangled system is related \nto its size, which is proportional to the number of entangled parties, as well as the dimensionality \nof the superposition each party is in. For this reason, there is a imminent demand to achieve, \ninvestigate, and improve the generation and control of large quantum systems, comprised of as \nmany entangled parties as possible, where each party is in an ideally high-dimensional \nsuperposition. \nMany quantum platforms are currently subject to extensive research, such as trapped ions, \nsuperconducting circuits, defect centers in solid-state crystals, mechanical oscillators, and optical \nquantum states (i.e. photons). While all platforms provide distinct advantages (as well as \nchallenges), optical photon states are of particular interest, because they can interact with other \nquantum systems, and can be transmitted over long distances while preserving their quantum \ncoherence without the need for extensive shielding. A large variety of sources for optical \nquantum states has been demonstrated, however most such implementations suffer from high \ncomplexity, ultimately limiting their scalability. Integrated photonics has recently become a \nleading platform for the compact, cost-efficient, and stable generation and processing of optical \nquantum states. However, to date, integrated quantum sources have only been able to generate \nphoton pairs, which operated on single channels, and single polarization modes. Furthermore, in \nthe few realizations of on-chip entanglement, they were limited to two entangled parties, where \neach party was in a two-dimensional superposition. These limitations of current integrated photon sources arise in large part from the concepts currently exploited to generate and control these \nstates. In particular, path-entangled sources are the most commonly used, but increasing the \nquantum state complexity requires a significant, and very challenging, increase in device \ncomplexity. There is therefore an urgent need to develop new concepts that can achieve the onchip \ngeneration of large and complex photon states without increasing source complexity, while \nstill enabling coherent quantum state control and detection. \nIn this work, we aim to address these limitations by making use of multiple frequency and \ntemporal modes within a single spatial waveguide mode (therefore readily compatible with \nstandard optical fibers) of an on-chip microring resonator. This approach is in contrast to more \ncommonly used techniques relying on path entanglement, which exploits multiple spatial modes \nand generates entanglement by means of complex waveguide structures. Working within a single \nspatial mode, we achieve the first demonstration of pure heralded single photons and entangled \nphoton pairs, which are emitted on multiple frequency modes from a single source. We then \ndemonstrate the first generation of multi-photon entangled quantum states on a photonic chip. \nFinally, by making use of the spectral multi-mode nature of optical frequency combs, we achieve \nthe generation of photon pairs in a coherent superposition of multiple frequency modes, \nrepresenting the first generation of high-dimensional entangled states on a photonic chip. \nOur work represents a significant step forward in the generation of complex quantum \nstates on a photonic chip and in a single spatial mode compatible with standard optical fibers. In \naddition, we demonstrated coherent quantum state control and manipulation for all generated \nstates by means of standard telecommunications components, which could in the future be also \nintegrated on single, monolithic chip. Our results indicate that the exploitation of integrated \noptical frequency comb sources for the generation of optical quantum states can provide a \nscalable and practical platform for quantum technologies.
Quantum optics has been, from its beginning, a driving force both for the exploration of fundamental limits of the quantum world and for conceiving seminal ideas and applications of the so-called quantum technologies. The last 20 years have seen a rapid development of ideas and proof-of-principle experiments involving the fields of quantum communication, quantum computation, quantum metrology and quantum simulation. The so called continuous variable (CV) approach to quantum optics, which uses many photon in collective states, and continuous observables like the quadratures of the electric field to encode information, has many interesting properties, especially from a communications perspective. It is inherently broadband and compatible with standard telecom infrastructures. Moreover, in CV, entanglement, one of the fundamental resource of quantum optics, can be generated deterministically. One of the main challenges for all quantum information technologies is scalability, being able to generate and manipulate a large number of quantum resources to achieve practical tasks efficiently. One approach to solve the issue of scalability is to use highly multimode quantum states. The quantum description of the electromagnetic field associate each photon (particle of light) with a mode (way for light to propagate). In a mutlimodal approach, we look at the quantum state bases and optical modes bases conjointly and tailor quantum fields not only in given modes, but also optimize the spatio-temporal shapes of the modes in which the state is defined. This opens wide perspectives for treating complex quantum states. In particular, using ultra-fast pulses of light which contain many temporal/spectral modes, we are able to generate large and complex entangled states of light using simple resources. In this thesis, we used an optical parametric oscillator pumped synchronously (SPOPO) with a frequency comb to generate multimode squeezed vacuum states which can be used to form cluster states: a large collection of modes (the nodes) entangled to each other in a certain way. These state are the basic resource for Measurement Based Quantum Computation (MBQC). This set-up has the advantage to generate entanglement in many mode with a single device. It is also highly tunable. Indeed, by tuning the spectrum of the OPO pump with a pulse shaper, one can tailor the properties of the generated quantum state. In this work, we focus on the optimization of the pump spectral shape to generate specific states. Using simulations based on Machine Learning Algorithms (MLA), we find optimal pump profile for typical target states. We then implement those shapes on the experimental set-up and measure the resulting states using multipixel homodyne detection. We also study intra-cavity dispersion effects. Dispersion inside the SPOPO cavity is indeed one of the main factor that limits the number of entangled modes in the generated quantum states. A systematic study of dispersion effects is therefore necessary to model the SPOPO output accurately. This works paves the way toward a fully tunable device that can be optimized in real time to generate specific quantum resources tailored for specific tasks.
The next generation of telecommunication networks will rely on the transmission of complex quantum states to enable secure and transformative information processing, utilizing entanglement and superposition. Cluster states - multipartite entangled states that retain entanglement under local measurements - are a vital resource for quantum networking applications such as blind photonic quantum computing, quantum state teleportation and all-photonic quantum repeaters. However, the transmission of cluster states over optical fiber has remained elusive with previous approaches. Here, we demonstrate the first transmission of a four-qubit cluster state over 25 km of single-mode fiber by using a two-photon multi-level time-bin encoding. We directly generate the state by exploiting coherent control of a parametric generation process, rendering a resource-intensive controlled-phase gate obsolete. To enable efficient and reconfigurable projective measurements on the multi-level time-bin encoded state, we introduce chirped-pulse modulation and implement the first time-bin beam splitter, allowing us to certify genuine multipartite entanglement and to demonstrate one-way computing operations. Our approach enables the transmission of complex quantum states over long-distance fibers, permitting the implementation of multipartite protocols and laying the foundation for large-scale quantum resource networks.
High-dimensional entangled states of light provide novel possibilities for quantum information, from fundamental tests of quantum mechanics to enhanced computation and communication protocols. In this context, the frequency degree of freedom is particular attractive thanks to its robustness to propagation in optical fibers and its capability to convey large scale of quantum information into a single spatial mode. This provides a strong incentive for the development of efficient and scalable methods for the generation and the manipulation of frequency-encoded quantum states. Nonlinear parametric processes are powerful tools to generate such states, but up to now the manipulation of the generated frequency states has been carried out mostly by post-manipulation, which demands complex and bulk-like experimental setups. Direct production of on-demand frequency-states at the generation stage, and if possible using a chip-based source, is crucial in view of practical and scalable applications for quantum information technologies. Here we use an integrated semiconductor chip to engineer the wavefunction and exchange statistics of frequency-entangled photon pairs directly at the generation stage, without post-manipulation. Tuning the pump spatial intensity allows to produce frequency-anticorrelated, correlated and separable states, while tuning the spatial phase enables to switch between symmetric and antisymmetric spectral wavefunctions, leading respectively to bosonic and fermionic behaviors of the photons. We also demonstrate the generation of non-Gaussian entanglement in the continuous variables formed by the frequency and time degrees of freedom of the photon pairs. These results, obtained at room temperature and telecom wavelength, and with a chip-based source, open promising perspectives for the quantum simulation of fermionic problems with photons on an integrated platform, as well as for communication and computation protocols exploiting antisymmetric high-dimensional quantum states.