On-chip generation and characterization of quantum light

Technologies based on quantum mechanics promise to revolutionize the collection, processing and communication of information. However, due to the fragility of quantum coherence, complex quantum states can only exist in highly isolated and stable environments. One suitable environment is that of a quantum photonic chip. Quantum integrated photonics seeks to generate, process and detect complex quantum states inside a photonic chip. This thesis presents theory and experimental verification of novel approaches for the integration of various functionalities into quantum photonic chips in a scalable way. As such, this thesis encompasses a broad area of physics including quantum optics and nonlinear photonics. The results presented in this thesis have applications in the areas of quantum enhanced measurement, communication and information processing. In particular we develop the theory and experimentally demonstrate flexible on-chip sources of spatially entangled photons, the state of which can be reconfigured alloptically. We show how such techniques could enable the realization of simple cluster state quantum computing algorithms using spatially encoded two-photon states. Furthermore, we suggest new and practical approaches for the efficient characterization of mass produced nonlinear quantum photonic chips. Finally we develop and experimentally demonstrate a scalable method for the full quantum state tomography of multi-photon states on-chip. Importantly this technique only requires a linearly increasing number of single photon detectors relative to the number of photons in the state being characterized, and is also highly compatible with on-chip single photon detectors

Tunable generation of entangled photons in a nonlinear directional coupler

The on-chip integration of quantum light sources has enabled the realization of complex quantum photonic circuits. However, for the practical implementation of such circuits in quantum information applications it is crucial to develop sources delivering entangled quantum photon states with on-demand tunability. Here we propose and experimentally demonstrate the concept of a widely tunable quantum light source based on spontaneous parametric down-conversion in a nonlinear directional coupler. We show that spatial photon-pair correlations and entanglement can be reconfigured on-demand by tuning the phase difference between the pump beams and the phase mismatch inside the structure. We demonstrate the generation of split states, robust N00N states, various intermediate regimes and biphoton steering. This fundamental scheme provides an important advance towards the realization of reconfigurable quantum circuitry

Reconfigurable cluster state generation in specially poled nonlinear waveguide arrays

We present an approach for generating cluster states on-chip, with the state encoded in the spatial component of the photonic wave function. We show that for spatial encoding, a change of measurement basis can improve the practicality of cluster-state algorithm implementation and demonstrates this by simulating the Grover's search algorithm. Our state generation scheme involves shaping the wave function produced by spontaneous parametric down-conversion in on-chip waveguides using specially tailored nonlinear poling patterns. Furthermore, the form of the cluster state can be reconfigured quickly by driving different waveguides in the array.We acknowledge funding from the Australian Research Council (ARC) through Projects No. DP160100619, No. DP190100277, and No. DE180100070

Synthetic photonic lattice for single-shot reconstruction of frequency combs

We formulate theoretically and demonstrate experimentally an all-optical method for reconstruction of the amplitude, phase and coherence of frequency combs from a single-shot measurement of the spectral intensity. Our approach exploits synthetic frequency lattices with pump-induced spectral short- and long-range couplings between different signal components across a broad bandwidth of of hundreds GHz in a single nonlinear fiber. When combined with ultra-fast signal conversion techniques, this approach has the potential to provide real-time measurement of pulse-to-pulse variations in the spectral phase and coherence properties of exotic light sources.Comment: 15 pages, 4 figure

Scalable on-chip quantum state tomography

Quantum information systems are on a path to vastly exceed the complexity of any classical device. The number of entangled qubits in quantum devices is rapidly increasing, and the information required to fully describe these systems scales exponentially with qubit number. This scaling is the key benefit of quantum systems, however it also presents a severe challenge. To characterize such systems typically requires an exponentially long sequence of different measurements, becoming highly resource demanding for large numbers of qubits. Here we propose and demonstrate a novel and scalable method for characterizing quantum systems based on expanding a multi-photon state to larger dimensionality. We establish that the complexity of this new measurement technique only scales linearly with the number of qubits, while providing a tomographically complete set of data without a need for reconfigurability. We experimentally demonstrate an integrated photonic chip capable of measuring two- and three-photon quantum states with statistical reconstruction fidelity of 99.71%. npj Quantum Information (2018) 4:19 ; doi:10.1038/s41534-018-0063-We acknowledge support by the Australian Research Council (ARC) (DP130100135, DP160100619 and DE180100070); Erasmus Mundus (NANOPHI 2013 5659/002-001); Alexander von Humboldt-Stiftung; Australia-Germany Joint Research Co-operation Scheme of Universities Australia; German Academic Exchange Service (project 57376641), and the Deutsche Forschungsgemeinschaft (grants SZ 276/12-1 and BL 574/13-1)

Scalable on-chip quantum state tomography

We formulate a method of quantum tomography that scales linearly with the number of photons and involves only one optical transformation. We demonstrate it experimentally for twophoton entangled states using a special photonic chi

Inline detection and reconstruction of multiphoton quantum states

Integrated single-photon detectors open new possibilities for monitoring inside quantum photonic circuits. We present a concept for the inline measurement of spatially encoded multiphoton quantum states, while keeping the transmitted ones undisturbed. We theoretically establish that by recording photon correlations from optimally positioned detectors on top of coupled waveguides with detuned propagation constants, one can perform robust reconstruction of the -photon density matrix describing amplitude, phase, coherence, and quantum entanglement. We report proof-of-principle experiments using classical light, which emulates the single-photon regime. Our method opens a pathway towards practical and fast inline quantum measurements for diverse applications in quantum photonics.Australian Research Council (ARC) (DP160100619); Australia-Germany Joint Research Cooperation Scheme; Erasmus+ (NANOPHI 2013 5659/002-001); Alexander von Humboldt-Stiftung; Deutsche Forschungsgemeinschaft (DFG) (BL 574/13-1, SZ 276/12-1, SZ 276/15-1, SZ 276/ 20-1, SZ 276/9-1)

Letter - Direct characterization of a nonlinear photonic circuit's wave function with laser light

Integrated photonics is a leading platform for quantum technologies including nonclassical state generation1, 2, 3, 4, demonstration of quantum computational complexity5 and secure quantum communications6. As photonic circuits grow in complexity, full quantum tomography becomes impractical, and therefore an efficient method for their characterization7, 8 is essential. Here we propose and demonstrate a fast, reliable method for reconstructing the two-photon state produced by an arbitrary quadratically nonlinear optical circuit. By establishing a rigorous correspondence between the generated quantum state and classical sum-frequency generation measurements from laser light, we overcome the limitations of previous approaches for lossy multi-mode devices9, 10. We applied this protocol to a multi-channel nonlinear waveguide network and measured a 99.28±0.31% fidelity between classical and quantum characterization. This technique enables fast and precise evaluation of nonlinear quantum photonic networks, a crucial step towards complex, large-scale, device production.This work was supported by the Australian Research Council (ARC) under the Grants DP140100808 and DP160100619, the Centre of Excellence for Ultrahigh bandwidth Devices for Optical Systems (CUDOS), Centre of Excellence for Quantum Computation and Communication Technology (CE170100012), and the Griffith University Research Infrastructure Program. BH and PF are supported by the Australian Government Research Training Program Scholarship. ANP acknowledges partial support from the Russian Ministry of Education and Science project 3.1365.2017/4.6