In this thesis, we are concerned with continuous-variable quantum optical state engineering protocols. Such protocols are designed to repair or enhance the nonclassical features of a given state. In particular, we build a weak measurement model of Gaussian entanglement concentration of the two mode squeezed vacuum state. This model allows the simultaneous description of all possible ancilla system variations. In addition, it provides an explanation of the Gaussian-preserving property of these protocols while providing a success criterion which links all of the degrees of freedom on the ancilla. Following this, we demonstrate the wider application of weak measurements to quantum optical state engineering by showing that they allow probabilistic noiseless amplifi cation of photon number. We then establish a connection between weak measurements and entanglement concentration as a fundamental result of weak measurements on entangled probes. After this, we explore the trade-off between Gaussian and non-Gaussian operations in the preparation of non-Gaussian pure states. In particular, we suggest that an operational cost for an arbitrary non-Gaussian pure state is the largest Fock state required for its approximate preparation. We consider the extent to which this non-Gaussian operational cost can be reduced by applying unitary Gaussian operations. This method relies on the identification of a minimal core state for any target non-Gaussian pure state.

Long-term storage of quantum information has diverse applications in quantum information science. This work presents an experimental realization of quantum memories with lifetimes greater then 0.1 s. The memories are based on cold rubidium atoms confined in one-dimensional optical lattices. First realization of lattice-based quantum memory and entanglement between a light field and a spin wave is presented in Chapter II. Chapter III describes two different methods (two-photon and magnetic) of compensation for inhomogeneous differential light shifts between the memory levels due to optical trapping potentials, and demonstration of entanglement between a telecom-band light field and a light-shift compensated memory qubit. Highly excited Rydberg atoms present a unique platform for study of strongly correlated systems and quantum information, because of their enormous dipole moments and consequent strong, long-range interactions. In the experiment described in Chapter IV single collective Rydberg excitations are created in a cold atomic gas. After a variable storage period the excitations are converted into light. As the principal quantum number n of the Rydberg level is increased beyond ~ 70, no more than a single excitation is retrieved from the entire mesoscopic ensemble of atoms. In Chapter V, by spatially selective conversion of the spin wave into a light field, we demonstrate that Rydberg-level interactions create long-range correlations of collective atomic excitations. These results hold promise for studies of dynamics and disorder in many-body systems with tunable interactions and for scalable quantum information networks. Chapter VI presents initial observations of coherent many-body Rabi oscillations between the ground level and a Rydberg level using several hundred cold rubidium atoms. The strongly pronounced oscillations indicate a nearly complete excitation blockade of the entire mesoscopic ensemble by a single excited atom.

Quantum information is a highly interesting and fast emerging field that involves processing information encoded into quantum systems and their subsequent use in various information tasks. The use of quantum resources such as superposition and entanglement have shown to enhance information processing capabilities beyond classical means in a number of communication, information and computation tasks. In this thesis, we have used single photons to study the advantage of d-level quantum systems (qudits) for a communication task commonly known as random access codes (RACs). A successful experimental demonstration of quantum random access codes (QRACs) with four dimensions is realized to demonstrate that the higher dimensional QRACs not only outperform the classical RACs but also provide an advantage over their quantum bit (qubit) counterparts. QRACs are also studied in regards to two specific applications: certification of true randomness and for testing the non-classicality of quantum systems. A method for increased certification of generated randomness is realized for the former and a successful experimental demonstration of a test of non-classicality with arbitrarily low detection efficiency is provided for the latter. This is followed by an implementation of a QRAC in a one-path communication network consisting of preparation, transformation and measurement devices. We have shown that the distributed QRAC provides optimal success probabilities for a number of tasks. Moreover, a novel quantum protocol for the solution to the problem of dining cryptographers and anonymous veto voting is also presented. This single photon transmission based protocol provides an efficient solution, which is experimentally demonstrated for a 3-party description. Lastly, Nitrogen-Vacancy (NV) center in diamond is studied as a potential resource for single photon emission and two methods to enhance the photon collection efficiency are successfully explored. Due to this enhancement, single photons from an NV center may also be used in similar single quantum system based communication experiments. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript. Paper 5: Manuscript.</p>

Secure communication is a topic of great importance. The goal of our research reported here is to develop a scheme of secure communication system using coherence modulation at the photon counting level. When operated at the photon counting level, coherence modulation can provide quantifiably secure binary transmission between two entities, security being based on the nonclonability of photons. The objectives of the research are as follows: Show that interferometer-based coherence modulation can provide physics guaranteed secure binary transmission where each bit is associated with a countable number of photons. Review the operation of conventional high-light-level coherence modulation communication and then show, by means of example, that through the reduction of light intensities to photon-counting levels the system can be made secure. Analyze a particular attack on the system to demonstrate the quantifiability of the scheme’s security. Implement a version of the BB84 quantum key distribution protocol by slightly modifying our scheme in order to achieve a greater security. Identify a variety of issues related to hardware and the challenges of implementing our scheme in the real world. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection

Submitted by Erika Demachki (erikademachki@gmail.com) on 2014-09-05T19:52:53Z No. of bitstreams: 2 Bastos, Wellison Peixoto - 2011.pdf: 686060 bytes, checksum: ca3480442dd033c3262ee9c2c3216240 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2014-09-05T19:52:53Z (GMT). No. of bitstreams: 2 Bastos, Wellison Peixoto - 2011.pdf: 686060 bytes, checksum: ca3480442dd033c3262ee9c2c3216240 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2011-03-04 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Taking advantage of the Faraday rotations that occur in a photonic cristal placed in an optical cavity with low quality factor, we proposed two schemes to obtain the swapping of entanglement of atomic states, useful in quantum communication and quantum computation. They employ three-level atoms in a -configuration, a linearly polarized photon source, a single detector, and a quarter wave plate. Three (four) cavities are used in the first (second) scheme. An additional scheme was also proposed to obtain controlled teleportation of superposition states, partial controlled teleportation of entangled states and controlled teleportation of entanglement. In all schemes we have included the imperfections that affect the system, such as transmission and coupling of photons in optical components, the fraction of photons with a desired polarization, the quantum efficiency of single photon detection, the effective solid angle where the photon are collected, and the rate of emitted photons by the source. Under these realistic conditions we estimate the success probability of each proccess, including the time spent for its realization. / Aproveitando as rotações de Faraday que ocorrem em um cristal fotônico colocado em uma cavidade óptica com baixo fator de qualidade, propusemos dois esquemas para obter a troca de emaranhamento de estados atômicos, útil em comunicação quântica e computação quântica. Eles empregam átomos de três níveis em uma configuração, uma fonte de fótons com polarização linear, um único detector, e uma placa de quarto de onda. Três (quatro) cavidades são usadas no primeiro (segundo) esquema. Um método adicional foi também proposto para obter teletransporte controlado de estados de superposição, teletransporte parcial controlado de estados emaranhados e teletransporte controlado de estados emaranhados. Em todos os esquemas incluímos as imperfeições que afetam o sistema, tais como a transmissão e acoplamento de fótons em componentes ópticos, a fração de fótons com uma polarização desejada, a eficiência quântica da detecção de um único fóton, o ângulo sólido efetivo em que o fótons são coletados e a taxa de fótons emitidos pela fonte. Sob estas condições realistas, estimamos a probabilidade de sucesso de cada processo, incluindo o tempo gasto para sua realização.

This project is a comprehensive investigation into the application of the exchange interaction, particularly with the realization of the SWAP^1/n quantum operator, in quantum information processing. We study the generation, characterization and application of entanglement in such systems. Given the non-commutativity of neighbouring SWAP^1/n gates, the mathematical study of combinations of these gates is an interesting avenue of research that we have explored, though due to the exponential scaling of the complexity of the problem with the number of qubits in the system, numerical techniques, though good for few-qubit systems, are found to be inefficient for this research problem when we look at systems with higher number of qubits. Since the group of SWAP^1/n operators is found to be isomorphic to the symmetric group Sn, we employ group-theoretic methods to find the relevant invariant subspaces and associated vector-states. Some interesting patterns of states are found including onedimensional invariant subspaces spanned by W-states and the Hamming-weight preserving symmetry of the vectors spanning the various invariant subspaces. We also devise new ways of characterizing entanglement and approach the separability problem by looking at permutation symmetries of subsystems of quantum states. This idea is found to form a bridge with the entanglement characterization tool of Peres-Horodecki's Partial Positive Transpose (PPT), for mixed quantum states. We also look at quantum information taskoriented 'distance' measures of entanglement, besides devising a new entanglement witness in the 'engle'. In terms of applications, we define five different formalisms for quantum computing: the circuit-based model, the encoded qubit model, the cluster-state model, functional quantum computation and the qudit-based model. Later in the thesis, we explore the idea of quantum computing based on decoherence-free subspaces. We also investigate ways of applying the SWAP^1/n in entanglement swapping for quantum repeaters, quantum communication protocols and quantum memory.

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Javier Garcia-Frias, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

Quantum communication promises to outperform its classical counterparts and enable protocols previously impossible. Specifically, quantum key distribution (QKD) allows a cryptographic key to be shared between distant parties with provable security. Much work has been performed on theoretical and experi- mental aspects of QKD, and the push is on to make it commercially viable and integrable with existing technologies. To this end I have performed simulations and experiments on QKD and other quantum protocols in regimes previously unexplored. The first experiment involves QKD via distributed entanglement through the standard telecommunications optical fibre network. I show that entanglement is preserved, even when the photons used are a shorter wavelength than the design of the optical fibre calls for. This surprising result is then used to demonstrate QKD over installed optical fibre, even with co-propagating classical traffic. Because the quantum and classical signals are sufficiently separated in wavelength, little cross-talk is observed, leading to high compatibility between this type of QKD and existing telecommunications infrastructure. Secondly, I demonstrate the key components of fully-modulated decoy-state QKD over the highest-loss channel to date, using a novel photon source based on weak coherent (laser) pulses. This system has application in a satellite uplink of QKD, which would enable worldwide secure communication. The uplink allows the complex quantum source to be kept on the ground while only simple receivers are in space, but suffers from high link loss due to atmospheric turbulence, necessitating the use of specific photon detectors and highly tailored photon pulses. My results could be applied in a near term satellite mission.

Although mathematical cryptography has been widely used, its security has only been proven under certain assumptions such as the computational power of opponents. As an alternative, quantum communication, in particular quantum key distribution (QKD) can get around unproven assumptions and achieve unconditional security. However, the key generation rate of practical QKD systems is limited by device imperfections, excess noise from the quantum channel, limited rate of true random-number generation, quantum entanglement preparation, and/or post-processing efficiency. This dissertation contributes to improving the performance of quantum communication systems. First, it proposes a new continuous-variable QKD (CVQKD) protocol that loosens the efficiency requirement on post-processing, a bottleneck for long-distance CVQKD systems. It also demonstrates an experimental implementation of the proposed protocol. To achieve high rates, the CVQKD experiment uses a continuous-wave local oscillator (CWLO). The excess noise caused by guided acoustic-wave Brillioun scattering (GAWBS) is avoided by a frequency-shift scheme, resulting in a 32 dB noise reduction. The statistical distribution of the GAWBS noise is characterized by quantum tomography. Measurements show Gaussian statistics upto 55 dB of dynamical range, which validates the security calculations in the proposed CVQKD protocol. True random numbers are required in quantum and classical cryptography. A second contribution of this thesis is that it experimentally demonstrates an ultrafast quantum random-number generator (QRNG) based on amplified spontaneous emission (ASE). Random numbers are produced by a multi-mode photon counting measurement on ASE light. The performance of the QRNG is analyzed with quantum information theory and verified with NIST standard random-number test. The QRNG experiment demonstrates a random-number generation rate at 20 Gbits/s. Theoretical studies show fundamental limits for such QRNGs. Quantum entanglement produced in nonlinear optical processes can help to increase quantum communication distance. A third contribution is the research on nonlinear optics of graphene, a novel 2D material with unconventional physical properties. Based on a quantum-dynamical model, optical responses of graphene are derived, showing for the first time a link between the complex linear optical conductivity and the quantum decoherence. Nonlinear optical responses, in particular four-wave mixing, is studied for the first time. The theory predicts saturation effects in graphene and relates the saturation threshold to the ultrafast quantum decoherence and carrier relaxation in graphene. For the experimental part, four-wave mixing in graphene is demonstrated. Twin-photon production in graphene is under investigation.

Quantum communication promises to outperform its classical counterparts and enable protocols previously impossible. Specifically, quantum key distribution (QKD) allows a cryptographic key to be shared between distant parties with provable security. Much work has been performed on theoretical and experi- mental aspects of QKD, and the push is on to make it commercially viable and integrable with existing technologies. To this end I have performed simulations and experiments on QKD and other quantum protocols in regimes previously unexplored. The first experiment involves QKD via distributed entanglement through the standard telecommunications optical fibre network. I show that entanglement is preserved, even when the photons used are a shorter wavelength than the design of the optical fibre calls for. This surprising result is then used to demonstrate QKD over installed optical fibre, even with co-propagating classical traffic. Because the quantum and classical signals are sufficiently separated in wavelength, little cross-talk is observed, leading to high compatibility between this type of QKD and existing telecommunications infrastructure. Secondly, I demonstrate the key components of fully-modulated decoy-state QKD over the highest-loss channel to date, using a novel photon source based on weak coherent (laser) pulses. This system has application in a satellite uplink of QKD, which would enable worldwide secure communication. The uplink allows the complex quantum source to be kept on the ground while only simple receivers are in space, but suffers from high link loss due to atmospheric turbulence, necessitating the use of specific photon detectors and highly tailored photon pulses. My results could be applied in a near term satellite mission.