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Register a new userConstructing finite dimensional codes with optical continuous variables
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We show how a qubit can be fault-tolerantly encoded in the infinite-dimensional Hilbert space of an optical mode. The scheme is efficient and realizable with present technologies. In fact, it involves two travelling optical modes coupled by a cross-Kerr interaction, initially prepared in coherent states, one of which is much more intense than the other. At the exit of the Kerr medium, the weak mode is subject to a homodyne measurement and a quantum codeword is conditionally generated in the quantum fluctuations of the intense mode.
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We study the impact of finite-size effects on the key rate of continuous-variable (CV) measurement-device-independent (MDI) quantum key distribution (QKD). Inspired by the parameter estimation technique developed in [Rupert textit{et al.} Phys. Rev. A textbf{90}, 062310 (2014)]~we adapt it to study CV-MDI-QKD and, assuming realistic experimental conditions, we analyze the impact of finite-size effects on the key rate. We find that, increasing the block-size, the performance of the protocol converges towards the ideal one, and that block-sizes between $10^{6}$ and $10^{9}$ data points can already provide a key rate $sim10^{-2}$ bit/use over metropolitan distances.
We describe a continuous-variable scheme for simulating the Kitaev lattice model and for detecting statistics of abelian anyons. The corresponding quantum optical implementation is solely based upon Gaussian resource states and Gaussian operations, hence allowing for a highly efficient creation, manipulation, and detection of anyons. This approach extends our understanding of the control and application of anyons and it leads to the possibility for experimental proof-of-principle demonstrations of anyonic statistics using continuous-variable systems.
In this thesis we study the finite-size analysis of two continuous-variables quantum key distribution schemes. The first one is the one-way protocol using Gaussian modulation of thermal states and the other is the measurement-device-independent protocol. To do so, we adopt an efficient channel parameter estimation method based on the assumption of the Gaussian variables and the central limit theorem introduced by Ruppert et al. [Phys. Rev. A 90, 062310 (2014)]. Furthermore, we present a composable security analysis of the measurement device independent protocol for coherent attacks with a channel parameter estimation that is not based on the central limit theorem. We also investigated, in the asymptotic regime, an asymmetric situation for the authenticated parties against the eavesdropper caused by fast-fading channels. Here we assume that the eavesdropper has the full control of the communication channel and can instantaneously change its transmissivity in every use of it. We assumed the simple model of a uniform fading and addressed the cases of one-way protocols, continuous-measurement device-independent protocol in symmetric configuration and its star network extension for three users. Finally, we extended the asymptotic study of the one-way protocols using an arbitrary number of phase-encoded coherent states assuming a thermal loss channel without using a Gaussian approximation.
By coherently combining advantages while largely avoiding limitations of two mainstream platforms, optical hybrid entanglement involving both discrete and continuous variables has recently garnered widespread attention and emerged as a promising idea for building heterogenous quantum networks. Different from previous results, here we propose a new scheme to remotely generate hybrid entanglement between discrete-polarization and continuous-quadrature optical qubits heralded by two-photon Bell state measurement. As a novel nonclassical light resource, we further utilize it to discuss two examples of ways -- entanglement swapping and quantum teloportation -- in which quantum information processing and communications could make use of this hybrid technique.
We generalize the operational quasiprobability involving sequential measurements proposed by Ryu {em et al.} [Phys. Rev. A {bf 88}, 052123] to a continuous-variable system. The quasiprobabilities in quantum optics are incommensurate, i.e., they represent a given physical observation in different mathematical forms from their classical counterparts, making it difficult to operationally interpret their negative values. Our operational quasiprobability is {em commensurate}, enabling one to compare quantum and classical statistics on the same footing. We show that the operational quasiprobability can be negative against the hypothesis of macrorealism for various states of light. Quadrature variables of light are our examples of continuous variables. We also compare our approach to the Glauber-Sudarshan $mathcal{P}$ function. In addition, we suggest an experimental scheme to sequentially measure the quadrature variables of light.
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