Thesis (Ph.D.) - University of St Andrews, April 2009.

Quantum control of many body atomic spins is often pursued in the context of an atom-light quantum interface, where a quantized light field acts as a "quantum bus" that can be used to entangle distant atoms. One key challenge is to improve the coherence of the atom-light interface and the amount of atom-light entanglement it can generate, given the constraints of working with multilevel atoms and optical fields in a 3D geometry. We have explored new ways to achieve this, through rigorous optimization of the spatial geometry, and through control of the internal atomic state. Our basic setup consists of a quantized probe beam passing through an atom cloud held in a dipole trap, first generating spin-probe entanglement through the Faraday interaction, and then using backaction from a measurement of the probe polarization to squeeze the collective atomic spin. The relevant figure of merit is the metrologically useful spin squeezing determined by the enhancement in the resolution of rotations of the collective spin, relative to the commonly used spin coherent state. With an optimized free-space geometry, and by using a 2-color probe scheme to suppress tensor light shifts, we achieve 3(2) dB of metrologically useful spin squeezing. We can further increase atom-light coupling by implementing internal state control to prepare spin states with larger initial projection noise relative to the spin coherent state. Under the right conditions this increase in projection noise can lead to stronger measurement backaction and increased atom-atom entanglement. With further internal state control the increased atom-atom entanglement can then be mapped to a basis where it corresponds to improved squeezing of, e.g., the physical spin-angular momentum or the collective atomic clock pseudospin. In practice, controlling the collective spin of N ~ 10⁶ atoms in this fashion is an extraordinarily difficult challenge because errors in the control of individual atoms tend to be highly correlated. By employing precise internal state control, we have prepared and detected projection noise limited "cat" states (which have initial projection noise that is larger by a factor of 2f = 8 for Cs relative to the spin coherent state) and estimate that we can generate up to 6.0(5) dB of metrologically useful spin squeezing, demonstrating the advantage of using the internal atomic structure as a resource for ensemble control.

Over the last few decades, quantised excitations of the electromagnetic field have proven to be an ideal system with which to investigate and harness quantum optical phenomena. The techniques developed have enabled fundamental tests of quantum mechanics as well as practical applications in quantum metrology and quantum information processing. Advancing to larger-scale entangled quantum systems will open up new regimes of quantum many-body physics, allowing us to probe the limits of quantum mechanics and enabling truly quantum-enhanced technologies. However, moving towards this goal will require further experimental and theoretical innovations. The work described in this thesis focuses on several different aspects of optical quantum information, but are ultimately all linked by this long-term aim. The first part of this thesis describes a novel method for strain-based active control of quantum optical circuits and a new method for the characterisation of high efficiency detectors. Building on this, I discuss in detail two different fields of quantum optics that stand to benefit from these techniques. I initially consider quantum-enhanced metrology, including work aimed towards demonstrating a truly better-than-classical phase measurement, and a theoretical exploration of multiple-phase estimation. Finally, I focus on linear-optical quantum information processing, exploring in detail the use of time-frequency encodings for quantum computing.

xvii, 163 p. ; ill. (some col.) A print copy of this title is available from the UO Libraries, under the call number: SCIENCE QC446.2.C85 2008 / We present experimental and theoretical studies of a hemispherical, high-solid-angle external optical micro-cavity strongly coupled to nanoscale optical centers for cavity-quantum electrodynamics (QED) strong coupling and efficient single-photon sources. Implementations of single-photon sources based on various optical centers have been reported in the last three decades. The need for efficient single-photon sources, however, is still a major challenge in the context of quantum information processing. In order to efficiently produce single photons single optical centers are coupled to a resonant high-finesse optical micro-cavity. A cavity can channel the spontaneously emitted photons into a well-defined spatial mode and in a desired direction to improve the overall efficiency, and can alter the spectral width of the emission. It can also provide an environment where dissipative mechanisms are overcome so that a pure-quantum-state emission takes place. We engineered a hemispherical optical micro-cavity that is comprised of a planar distributed Bragg reflector (DBR) mirror, and a concave dielectric mirror having a radius of curvature 60 μm. Nanoscale semiconductor optical centers (quantum dots) are placed at the cavity mode waist at the planar mirror and are located at an antinode of the cavity field to maximize the coherent interaction rate. The three-dimensional scannable optical cavity allows both spatial and spectral selection to ensure addressing single optical centers. This unique micro-cavity design will potentially enable reaching the cavity-QED strong-coupling regime and realize the deterministic production of single photons. This cavity can also be operated with a standard planar dielectric mirror replacing the semiconductor DBR mirror. Such an all-dielectric cavity may find uses in atomic cavity-QED or cold-atom studies. We formulated a theory of single-photon emission in the cavity-QED strong-coupling regime that includes pure dipole dephasing and radiative decay both through the cavity mirror and into the side directions. This allows, for the first time, full modeling of the emission quantum efficiency, and the spectrum of the single photons emitted into the useful output mode of the, cavity. / Adviser: Michael G. Raymer

We look at the problem of verifying optical entanglement for two types of states relevant to quantum information processing. One type occurs in Hong-Ou-Mandel interference and is relevant to quantum computing. The other type is time frequency entanglement which is useful for quantum key distribution. For these types of states the conventional methods of entanglement verification do not work well, and we develop new criteria and methods to verify entanglement of such states. Explicitly, one method takes into account the possible multimode character of two photons, while the other method takes into account the missing data that occur due to the finite range of detectors. This dissertation includes previously published and unpublished co-authored material.

The creation of optical states of light that are quantum mechanical in nature in optical fibers is discussed and demonstrated experimentally. Specifically, two- photon states created by spontaneous four-wave-mixing in commercially available single-mode, birefringent fibers are studied. When creating photon states of light, it is important to verify the created states are of the proper photon number distribution and free of noise. We detail a method for combining thresholding, non-number resolving detectors to characterize the photon number distribution created via SFWM and a procedure to quantify the noise sources present in the process. Frequency translation in optical fibers with two-photon states is discussed and experimental considerations are presented.

Nesse trabalho utilizamos um modelo analítico para avaliar o desempenho de estruturas semicondutoras contendo pontos quânticos que servem de base para a fabricação de fotodetectores que operam na faixa do infravermelho. O desempenho desses dispositivos foram avaliados através da corrente no escuro e da detectividade. Os trabalhos existentes na literatura, baseados neste modelo, não consideram a de­ pendência da estrutura eletrônica do ponto quântico com suas dimensões. Desta forma, neste trabalho, analisamos o comportamento da corrente no escuro e da de­tectividade em função de vários parâmetros que definem a estrutura da amostra, levando em consideração as dimensões dos QDs. Nossos resultados mostraram quais parâmetros devemos ajustar para fazer fotodetectores: (1) que contenham a maior densidade de QDs com dimensões compatíveis com a energia de ionização desejada; (2) que maximizam o desempenho do dispositivo e (3) minimizam o ruído do mesmo. / In this work we used an analytical model to calculate the dark current and the detectivity of infrared photodetectors based on InAs quantum dots semiconductor heterostructures. The existing works reported in the literature based on this analytical model do not take into account the electronic structure of the QD in the calculations. In this way, in the present work, we took into account the QD dimensions when we analized the dependence of the dark current and the detectivity on the parameters which define the sample structure. Our findings show which parameters must be adjusted in order to obtain photodetectors with: (1) the larger density of QDs with dimensions compatible with the wanted ionization energy; (2) that maximize the performance; (3) and that minimize the noise of the devices.

Fazendo uso da equivalência entre um potencial não-local e um potencial dependente da velocidade, mostramos como construir um potencial não-local capaz de gerar estados comprimidos. No contexto do modelo de Jaynes-Cummings que descreve a interação quântica de um único átomo de dois níveis com um único modo do campo eletromagnético quantizado, mostrando uma nova característica no comportamento dinâmico da inversão atômica, quando o campo é preparado em um estado coerente comprimido par. Usando o modelo de Jaynes-Cummings propomos um método para estudar o estado quântico e a variância do operador de fase, através da medida da distribuição de momento de átomos espalhados, proveniente da interação de um feixe atômico com um único modo do campo eletromagnético via interação de dois fótons. / Exploring the equivalence between a non-local and a velocity-dependent potential we show how to construct a non-local potential, which is able to generate squeezed states. In the context of the Jaynes-Cummings model we describe the quantum interaction of a single two-level atom with a single mode of a quantum electromagnetic field, showing a new feature in the dynamical behavior of atomic inversion when the electromagnetic field is prepared in an even squeezed coherent state. By using the Jaynes-Cummings model, a method to study quantum state and variance of the phase operator is proposed. The method consists in measuring the momentum distribution of atoms resulting from the interaction of a prepared atomic beam with a single mode electromagnetic field via two-photon interaction.

Orientador: José Antonio Roversi / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-18T22:02:19Z (GMT). No. of bitstreams: 1 GonzalezHenao_JulioCesar_M.pdf: 3018273 bytes, checksum: 8a7504d4a51886c5befb08df936170c0 (MD5) Previous issue date: 2011 / Resumo: Nesta dissertação estudamos um sistema formado por dois íons de dois níveis aprisionados em cavidades ópticas acopladas, onde analisamos a transferência de estados de dois qubits a partir dos graus de liberdade (de movimento unidimensional e dos estados internos) de um dos íons para o outro. Para o acoplamento entre as cavidades consideramos dois mecanismos diferentes: (1) onde acoplamento é dado pela sobreposição dos campos (acoplamento direto) e (2) via fibra óptica. Em ambos casos foram utilizadas aproximações onde participam apenas um ou dois modos normais coletivos na transferência do estado. Tambem observamos que , na aproximação com dois modos normais, uma melhor transferência dos qubits no sistema acoplado por fibra óptica, quando é comparado com o sistema acoplado diretamente. Por último, estudamos, na aproximação com um só modo normal de transferência, os efeitos de reservatórios térmicos sobre a transferência dos qubits no sistema de íons aprisionados em cavidades acopladas via fibra óptica. Tais reservatórios foram representados pelos processos de emissão espontânea e flutuações na armadilha a temperaturas T = 0. Os resultados mostram que os reservatórios geram perdas na fidelidade da transferência dos qubits / Abstract: In this work we studied a system consisting of a pair of two-level trapped ions in coupled optics cavities, where we analyze the transfer of states of two qubits from the degrees of freedom (of movement and of internal states) of one ion to another. For the coupling between the cavities we consider two different mechanisms: One where the coupling is given by the overlap of the fields (direct coupling) and one optical fiber. In both cases we use aproximations in which only one or two collective normal modes are envolved in the transfer of the state. From these calculations we have noted that in the aproximations with two normal modes a better transfer of the qubit in the system coupled by optical fiber, compared with the system connected directly. Finally, we studied the effects of thermal reservoirs in system of trapped ions in cavities coupled via optical fiber in the proximity of the one normal mode transfer. These reservoirs represent the processes of spontaneous emission and fluctuations in the trap for T = 0 / Mestrado / Física / Mestre em Física

This thesis studies continuous variable mixed states with the aim of better understanding the fundamental behaviour of quantum correlations in such states, as well as searching for applications of these correlations. I first investigate the interesting phenomenon of discord increase under local loss and explain the behaviour by considering the non-orthogonality of quantum states. I then explore the counter-intuitive result where entanglement can be created by a passive optical beamsplitter, even if the input states are classical, as long as the input states are part of a larger globally nonclassical system. This result emphasises the importance of global correlations in a quantum state, and I propose an application of this protocol in the form of quantum dense coding. Finally, I develop a quantum digital signature protocol that can be described entirely using the continuous variable formalism. Quantum digital signatures provide a method to ensure the integrity and provenance of a message using quantum states. They follow a similar method to quantum key distribution (QKD), but require less post-processing, which means they can sometimes be implemented over channels that are inappropriate for QKD. The method I propose uses homodyne measurement to verify the signature, unlike previous protocols that use single photon detection. The single photon detection of previous methods is designed to give unambiguous results about the signature, but this comes at the cost of getting no information much of the time. Using homodyne detection has the advantage of giving results all the time, but this means that measurement results always have some ambiguity. I show that, even with this ambiguity, the signature protocol based on homodyne measurement outperforms previous protocols, with the advantage enhanced when technical considerations are included. Therefore this represents an interesting new direction in the search for a practical quantum digital signature scheme.