Multiphoton Quantum Optics and Quantum State Engineering

We present a review of theoretical and experimental aspects of multiphoton quantum optics. Multiphoton processes occur and are important for many aspects of matter-radiation interactions that include the efficient ionization of atoms and molecules, and, more generally, atomic transition mechanisms; system-environment couplings and dissipative quantum dynamics; laser physics, optical parametric processes, and interferometry. A single review cannot account for all aspects of such an enormously vast subject. Here we choose to concentrate our attention on parametric processes in nonlinear media, with special emphasis on the engineering of nonclassical states of photons and atoms. We present a detailed analysis of the methods and techniques for the production of genuinely quantum multiphoton processes in nonlinear media, and the corresponding models of multiphoton effective interactions. We review existing proposals for the classification, engineering, and manipulation of nonclassical states, including Fock states, macroscopic superposition states, and multiphoton generalized coherent states. We introduce and discuss the structure of canonical multiphoton quantum optics and the associated one- and two-mode canonical multiphoton squeezed states. This framework provides a consistent multiphoton generalization of two-photon quantum optics and a consistent Hamiltonian description of multiphoton processes associated to higher-order nonlinearities. Finally, we discuss very recent advances that by combining linear and nonlinear optical devices allow to realize multiphoton entangled states of the electromnagnetic field, that are relevant for applications to efficient quantum computation, quantum teleportation, and related problems in quantum communication and information.Comment: 198 pages, 36 eps figure

Quantum information with continuous variables

Quantum information is a rapidly advancing area of interdisciplinary research. It may lead to real-world applications for communication and computation unavailable without the exploitation of quantum properties such as nonorthogonality or entanglement. We review the progress in quantum information based on continuous quantum variables, with emphasis on quantum optical implementations in terms of the quadrature amplitudes of the electromagnetic field.Comment: accepted for publication in Reviews of Modern Physic

Nonlinear Single-photon Generation for Photonic Quantum Technology

Single photons are the smallest indivisible quanta of light, canonically described by quantum mechanics. By carefully controlling the interaction of single photons, exquisite non-classical phenomena can be observed. Mature photonic chip technology has recently emerged as an ideal platform for quantum information processing using single photons. However, generating single photons efficiently on-chip remains a fundamental challenge. One solution is to harness the intrinsic nonlinearity available in certain photonic materials for nonlinear photon generation directly in on-chip waveguides themselves. This work examines nonlinear photon generation in two key material platforms. The first is chalcogenide glass. Chalcogenide, while highly nonlinear, is amorphous and thus has broadband Raman noise. In this study the Raman noise is characterised at the single-photon level to find an intrinsic minima, which is then targeted for low-noise photon generation using an engineered waveguide. The second platform is silicon. As silicon is complementary metal-oxide-semiconductor (CMOS) fabrication compatible, it is congruent with mass production. Thus, in this study, photon-pair generation is first shown in a compact photonic crystal, before combining two monolithic sources using active multiplexing. This thesis presents significant progress towards a key goal of the field – on- demand photon generation in a fully integrated photonic quantum processor

Hydrogenated amorphous silicon photonics

Silicon Photonics is quickly proving to be a suitable interconnect technology for meeting the future goals of on-chip bandwidth and low power requirements. However, it is not clear how silicon photonics will be integrated into CMOS chips, particularly microprocessors. The issue of integrating photonic circuits into electronic IC fabrication processes to achieve maximum flexibility and minimize complexity and cost is an important one. In order to maximize usage of chip real estate, it will be advantageous to integrate in three-dimensions. Hydrogenated-amorphous silicon (a-Si:H) is emerging as a promising material for the 3-D integration of silicon photonics for on-chip optical interconnects. In addition, a-Si:H film can be deposited using CMOS compatible low temperature plasma-enhanced chemical vapor deposition (PECVD) process at any point in the fabrication process allowing vertical stacking of optical interconnects. In this thesis we demonstrate a-Si:H as a high performance alternate platform to crystalline silicon, enabling backend integration of optical interconnects in a hybrid photonic-electronic network-on-chip architecture. High quality passive devices are fabricated on a low-loss a-Si:H platform enabling wavelength division multiplexing schemes. We demonstrate a broadband all-optical modulation scheme based on free-carrier absorption effect, which can enable compact electro-optic modulators in a-Si:H. Furthermore, we comprehensively characterize the optical nonlinearities in a-Si:H and observe that a-Si:H exhibits enhanced nonlinearities as compared to crystalline silicon. Based on the enhanced nonlinearities, we demonstrate low-power four-wave mixing in a-Si:H waveguides enabling high-speed all-optical devices in an a-Si:H platform. Finally, we demonstrate a novel data encoding scheme using thermal and all-optical tuning of silicon waveguides, increasing the spectral efficiency in an interconnect link. Looking forward, we shall also discuss some of the challenges that still need to be overcome to realize an integrated a-Si:H based photonic link

Nonlinear Single-photon Generation for Photonic Quantum Technology

Single photons are the smallest indivisible quanta of light, canonically described by quantum mechanics. By carefully controlling the interaction of single photons, exquisite non-classical phenomena can be observed. Mature photonic chip technology has recently emerged as an ideal platform for quantum information processing using single photons. However, generating single photons efficiently on-chip remains a fundamental challenge. One solution is to harness the intrinsic nonlinearity available in certain photonic materials for nonlinear photon generation directly in on-chip waveguides themselves. This work examines nonlinear photon generation in two key material platforms. The first is chalcogenide glass. Chalcogenide, while highly nonlinear, is amorphous and thus has broadband Raman noise. In this study the Raman noise is characterised at the single-photon level to find an intrinsic minima, which is then targeted for low-noise photon generation using an engineered waveguide. The second platform is silicon. As silicon is complementary metal-oxide-semiconductor (CMOS) fabrication compatible, it is congruent with mass production. Thus, in this study, photon-pair generation is first shown in a compact photonic crystal, before combining two monolithic sources using active multiplexing. This thesis presents significant progress towards a key goal of the field – on- demand photon generation in a fully integrated photonic quantum processor

Atom-light interfaces for quantum information processing

The emergence of quantum physics from the page to the lab and the world at large is an exciting development of recent years. The prospects of absolutely secure communication and efficient simulation of physical systems have spurred great human effort into understanding these possibilities and turning them into realities. Photons are the most easily manipulated quantum particles and are a promising candidate for implementing these technologies. Limitations of photons include the difficulty of keeping objects that move at the speed of light, and producing strong interactions between particles that do not normally interact. The work presented in this thesis is motivated by the possibility of overcoming these limitations. The ability to faithfully store and reproduce a quantum state is essential for many quantum information technologies. Quantum memories for light have been developed over the last two decades to provide this ability. The group at the Australian National University developed the gradient echo memory (GEM): A quantum state of light can be controllably stored and released from an atomic ensemble by the use of additional optical fields and magnetic field gradients. This scheme was previously shown to preserve the quantum characteristics of the light. We used the GEM scheme with a cold rubidium ensemble to create the first optical memory that simultaneously beat the no-cloning limit, a benchmark for many of the technologies relying on quantum memories, and the loss rate for a delay line composed of optical fibre. We also created an analogue to a pulsed optical resonator using GEM with a warm rubidium vapour. This was done by replacing the circulating optical field of a resonator with light stored in the memory, and replacing the coupling of light into and out of that circulating mode with storage and recall from the memory. The bandwidth and repetition rate of this resonator were rapidly tunable as they were controlled by external optical and magnetic fields. We worked on implementing GEM with strings of thousands of atoms strongly coupled to the evanescent field of an optical nanofibre. This raised new possibilities for creating a true random access memory that would allow a more flexible use of the multi-mode capacity of GEM. We developed the theory for a novel type of stationary light in the gradient echo memory. Our stationary light scheme relies on the destructive interference of counter-propagating optical fields throughout the memory. The optical intensity scales with optical depth, as with other forms of stationary light. However, as the destructive interference could be set up over a much greater distance, more of the optical depth is available for generating stationary light. Finally, we studied how a control-phase gate for single-photon optical states could be implemented using a nonlinear interaction with stationary light. The stationary light generated by one state modulates the phase of another state stored in the memory. The second state modifies the stationary light, also producing a back-action on the first state and generating the required cross-phase shift