It is already known that one can always find a set of measurements on any two-qubit entangled state that will lead to a violation of the CHSH inequality. We provide an explicit state in terms of the angle between Alice's choice of measurements and the angle between Bob's choice of measurements, such that the CHSH inequality is always violated provided Alice's or Bob's choice of inputs are not collinear. We prove that inequalities with a corresponding Bell operator written as a linear combination of tensor products of Pauli matrices, excluding the identity, will generate the most nonlocal correlations using maximally entangled states in our experiment. From this result and a proposition from Horodecki et. al., we are able to construct the state that generates these optimal correlations. To achieve this state in a lab, one party must rotate their qubit using the orthogonal operation we provide and also rotate their Bloch sphere such that all their measurements lie in the same plane. We provide a comprehensive study of how Bell inequalities change when experiments introduce error via imperfect detection efficiency. The original cases of perfect efficiency are covered first and then a more realistic approach, when inefficient detectors are used, will follow. It is shown that less entanglement is needed to demonstrate more nonlocality in some Clasure-Horne-Shimony-Holt (CHSH) experiments when detector inefficiency is introduced. An example of this is shown for any given specific set of measurements in the CHSH Bell experiment. This occurs when one party has a detector of efficiency for each choice of input and the other party makes projective measurements. The efficiency can be pushed down to fifty percent while still violating the CHSH inequality, and for the experimental set-up illustrated, there is more nonlocality with less entanglement. Furthermore, it is shown that if the first party has an imperfect detector for only one choice of inputs rather than two, the efficiency can be brought down arbitrarily close to zero percent while still violating the CHSH inequality. Historically, nonlocality and entanglement were viewed as two equivalent resources, but recently this equality has come under question; these results further support this fundamental difference. Further more, we introduce Mermin's game in the case of relaxed conditions. The original constraints were that when the detectors in separate labs of a two-qubit experiment are in the same setting, then the results should be the same. We require that the outcomes are the same at least part of the time, given by some epsilon variable. Initially, one could find a maximum violation of one-fourth by allowing to parties to share the singlet state and have measurement settings one-hundred and twenty degrees apart from one another. By allowing some epsilon error in the perfect correlations regime, one can find a maximum violation of minus one plus the square root of two using the singlet state and measurement inputs that achieve Tsirelson's bound for the CHSH experiment. The reason is that we show Mermin's inequality is technically the CHSH inequality "in disguise", but with using constraints the CHSH experiment does not use. We derive Mermin's inequality under new conditions and give the projective measurements needed to violate maximally.

No / A system consisting of two Josephson qubits coupled through a quantum monochromatic electromagnetic field mode of a resonant tank circuit is studied. It is shown that for certain values of the parameters, it can be used as an entangling gate, which entangles the two qubits whilst the electromagnetic field remains disentangled. The gate operates with decent fidelity to a gate and could form the basis for initial experimental investigations of coupled superconducting qubits.

Nesta tese abordamos o problema de transferência e manipulação de informação quântica em sistemas dissipativos. Inicialmente apresentamos uma técnica para construir, dentro de redes bosônicas dissipativas, canais livres de decoerência (CLD): um grupo de modos normais de osciladores com taxas de amortecimento efetivas nulas. Verificamos que os estados protegidos dentro do CLD definem subespaços livres de decoerência (SLD) quando mapeados de volta para a base dos osciladores naturais da rede. Portanto, a nossa técnica para obter canais protegidos formados por modos normais é uma forma alternativa para construir SLD, que oferece vantagens em relação ao método convencional. Nosso protocolo permite o cálculo de todos os estados da rede protegidos de uma só vez, assim como leva naturalmente ao conceito de subespaço quase livre de decoerência (SQLD), dentro do qual um estado de superposição é quase completamente protegido. O conceito de SQLD, é mais fraco do que a dos SLD, pode proporcionar um mecanismo mais manejável para controlar decoerência. Em seguida desenvolvemos um protocolo para transferência quase perfeita de estados de poláriton de um sistema emissor para um receptor, separados espacialmente, ambos acoplados por um canal de transmissão não ideal que é modelado por uma rede de cavidades dissipativas. Esse protocolo consiste no acoplamento dispersivo entre o estado de poláriton preparado no emissor com os modos normais da rede que forma o canal, o que possibilita que o estado tunele para o receptor. Após a obtenção de um Hamiltoniano efetivo para o acoplamento entre o emissor e receptor, calculamos a fidelidade para a transferência de alguns estados de poláriton, por exemplo, estados tipo gato de Schrödinger. Mostramos que as taxas de decaimento da fidelidade são proporcionais a cooperatividade, parâmetro esse que avalia a relação entre a taxa de dissipação e o acoplamento efetivo. Analisamos a dependência da fidelidade e do tempo de transferência em relação à topologia da rede. Por fim, propomos o mecanismo de tunelamento não local para transferência de estados bosônicos e fermiônicos com alta fidelidade. Demonstramos que a incoerência decorrente das não idealidades quânticas do canal é quase totalmente contornada pelo mecanismo de tunelamento que possibilita um processo de transferência de alta fidelidade. Aplicamos esse mecanismo para transferência e processamento de informações entre múltiplos circuitos quântico (CQs) não ideais. Um conjunto de saídas é simultaneamente acoplado ao conjunto correspondente de entradas de outro QC espacialmente separado do primeiro, através de um único canal quântico não ideal. Mostramos que além da transferência de estados, podemos realizar operações logicas entre qubits distantes e gerar uma pletora de estados quânticos emaranhados. / In this thesis we address the problem of transfer and manipulation of quantum information in dissipative systems. First we present a technique to build, within a dissipative bosonic network, decoherence-free channels (DFCs): a group of normal-mode oscillators with null effective damping rates. We verify that the states protected within the DFC define the wellknown decoherence-free subspaces (DFSs) when mapped back into the natural network oscillators. Therefore, our technique to build protected normal-mode channels turns out to be an alternative way to build DFSs, which offers advantages over the conventional method. It enables the computation of all the network-protected states at once, as well as leading naturally to the concept of the decoherence quasi-free subspace (DQFS), inside which a superposition state is quasi-completely protected against decoherence. The concept of the DQFS, weaker than that of the DFS, may provide a more manageable mechanism to control decoherence. Finally, as an application of the DQFSs, we show how to build them for quasi-perfect state transfer in networks of coupled quantum dissipative oscillators. Then we present a scheme for quasi perfect transfer of polariton states from a sender to a spatially separated receiver, both composed of high-quality cavities filled by atomic samples. The sender and the receiver are connected by a nonideal transmission channel the data bus modelled by a network of lossy empty cavities. In particular, we analyze the influence of a large class of data-bus topologies on the fidelity and transfer time of the polariton state. Moreover, we also assume dispersive couplings between the polariton fields and the data-bus normal modes in order to achieve a tunneling-like state transfer. Such a tunneling-transfer mechanism, by which the excitation energy of the polariton effectively does not populate the data-bus cavities, is capable of attenuating appreciably the dissipative effects of the data-bus cavities. After deriving a Hamiltonian for the effective coupling between the sender and the receiver, we show that the decay rate of the fidelity is proportional to a cooperativity parameter that weigh the cost of the dissipation rate against the benefit of the effective coupling strength. The increase of the fidelity of the transfer process can be achieved at the expense of longer transfer times. We also show that the dependence of both the fidelity and the transfer time on the network topology for distinct regimes of parameters. It follows that the data-bus topology can be explored to control the time of the state-transfer process. Finally we propose the nonlocal tunneling mechanism for high-fidelity state transfer between distant parties. We apply this mechanism for highfidelity information transfer and processing between remote multi-branch nonideal quantum circuits (QCs). We show that in addition to the transfer of states, we can perform logic operations between distant qubits and generate a plethora of entangled quantum states.

Nesta tese abordamos o problema de transferência e manipulação de informação quântica em sistemas dissipativos. Inicialmente apresentamos uma técnica para construir, dentro de redes bosônicas dissipativas, canais livres de decoerência (CLD): um grupo de modos normais de osciladores com taxas de amortecimento efetivas nulas. Verificamos que os estados protegidos dentro do CLD definem subespaços livres de decoerência (SLD) quando mapeados de volta para a base dos osciladores naturais da rede. Portanto, a nossa técnica para obter canais protegidos formados por modos normais é uma forma alternativa para construir SLD, que oferece vantagens em relação ao método convencional. Nosso protocolo permite o cálculo de todos os estados da rede protegidos de uma só vez, assim como leva naturalmente ao conceito de subespaço quase livre de decoerência (SQLD), dentro do qual um estado de superposição é quase completamente protegido. O conceito de SQLD, é mais fraco do que a dos SLD, pode proporcionar um mecanismo mais manejável para controlar decoerência. Em seguida desenvolvemos um protocolo para transferência quase perfeita de estados de poláriton de um sistema emissor para um receptor, separados espacialmente, ambos acoplados por um canal de transmissão não ideal que é modelado por uma rede de cavidades dissipativas. Esse protocolo consiste no acoplamento dispersivo entre o estado de poláriton preparado no emissor com os modos normais da rede que forma o canal, o que possibilita que o estado tunele para o receptor. Após a obtenção de um Hamiltoniano efetivo para o acoplamento entre o emissor e receptor, calculamos a fidelidade para a transferência de alguns estados de poláriton, por exemplo, estados tipo gato de Schrödinger. Mostramos que as taxas de decaimento da fidelidade são proporcionais a cooperatividade, parâmetro esse que avalia a relação entre a taxa de dissipação e o acoplamento efetivo. Analisamos a dependência da fidelidade e do tempo de transferência em relação à topologia da rede. Por fim, propomos o mecanismo de tunelamento não local para transferência de estados bosônicos e fermiônicos com alta fidelidade. Demonstramos que a incoerência decorrente das não idealidades quânticas do canal é quase totalmente contornada pelo mecanismo de tunelamento que possibilita um processo de transferência de alta fidelidade. Aplicamos esse mecanismo para transferência e processamento de informações entre múltiplos circuitos quântico (CQs) não ideais. Um conjunto de saídas é simultaneamente acoplado ao conjunto correspondente de entradas de outro QC espacialmente separado do primeiro, através de um único canal quântico não ideal. Mostramos que além da transferência de estados, podemos realizar operações logicas entre qubits distantes e gerar uma pletora de estados quânticos emaranhados. / In this thesis we address the problem of transfer and manipulation of quantum information in dissipative systems. First we present a technique to build, within a dissipative bosonic network, decoherence-free channels (DFCs): a group of normal-mode oscillators with null effective damping rates. We verify that the states protected within the DFC define the wellknown decoherence-free subspaces (DFSs) when mapped back into the natural network oscillators. Therefore, our technique to build protected normal-mode channels turns out to be an alternative way to build DFSs, which offers advantages over the conventional method. It enables the computation of all the network-protected states at once, as well as leading naturally to the concept of the decoherence quasi-free subspace (DQFS), inside which a superposition state is quasi-completely protected against decoherence. The concept of the DQFS, weaker than that of the DFS, may provide a more manageable mechanism to control decoherence. Finally, as an application of the DQFSs, we show how to build them for quasi-perfect state transfer in networks of coupled quantum dissipative oscillators. Then we present a scheme for quasi perfect transfer of polariton states from a sender to a spatially separated receiver, both composed of high-quality cavities filled by atomic samples. The sender and the receiver are connected by a nonideal transmission channel the data bus modelled by a network of lossy empty cavities. In particular, we analyze the influence of a large class of data-bus topologies on the fidelity and transfer time of the polariton state. Moreover, we also assume dispersive couplings between the polariton fields and the data-bus normal modes in order to achieve a tunneling-like state transfer. Such a tunneling-transfer mechanism, by which the excitation energy of the polariton effectively does not populate the data-bus cavities, is capable of attenuating appreciably the dissipative effects of the data-bus cavities. After deriving a Hamiltonian for the effective coupling between the sender and the receiver, we show that the decay rate of the fidelity is proportional to a cooperativity parameter that weigh the cost of the dissipation rate against the benefit of the effective coupling strength. The increase of the fidelity of the transfer process can be achieved at the expense of longer transfer times. We also show that the dependence of both the fidelity and the transfer time on the network topology for distinct regimes of parameters. It follows that the data-bus topology can be explored to control the time of the state-transfer process. Finally we propose the nonlocal tunneling mechanism for high-fidelity state transfer between distant parties. We apply this mechanism for highfidelity information transfer and processing between remote multi-branch nonideal quantum circuits (QCs). We show that in addition to the transfer of states, we can perform logic operations between distant qubits and generate a plethora of entangled quantum states.

Quantum cryptography is arguably the most successfully applied area of quantum information theory. In this work, We invsetigate the role of quantum indistinguishability in random number generation, quantum temporal correlations, quantum nonlocality and counterfactuality for quantum cryptography. We study quantum protocols for key distribution, and their security in the conventional setting, in the counterfactual paradigm, and finally also in the device-independent scenario as applied to prepare-and-measure schemes. We begin with the interplay of two essential non-classical features like quantum indeterminism and quantum indistinguishability via a process known as bosonic stimulation is discussed. It is observed that the process provides an efficient method for macroscopic extraction of quantum randomness. Next, we propose two counterfactual cryptographic protocols, in which a secret key bit is generated even without the physical transmission of a particle. The first protocol is semicounterfactual in the sense that only one of the key bits is generated using interaction-free measurement. This protocol departs fundamentally from the original counterfactual key distribution protocol in not encoding secret bits in terms of photon polarization. We discuss how the security in the protocol originates from quantum single-particle non-locality. The second protocol is designed for the crypto-task of certificate authorization, where a trusted third party authenticates an entity (e.g., bank) to a client. We analyze the security of both protocols under various general incoherent attack models. The next part of our work includes study of quantum temporal correlations. We consider the use of the Leggett-Garg inequalities for device-independent security appropriate for prepare-and-measure protocols subjected to the higher dimensional attack that would completely undermine standard BB84. In the last part, we introduce the novel concept of nonlocal subspaces constructed using the graph state formalism, and propose their application for quantum information splitting. In particular, we use the stabilizer formalism of graph states to construct degenerate Bell operators, whose eigenspace determines the nonlocal subspace, into which a quantum secret is encoded and shared among an authorized group of agents, or securely transmitted to a designated secret retriever. The security of our scheme arises from the monogamy of quantum correlations. The quantum violation of the Bell-type inequality here is to its algebraic maximum, making this approach inherently suitable for the device-independent scenario.