Overcoming the limitations of direct transmission of continuous-variable quantum information

We investigate the difficulties associated with the transmission of quantum information and correlation through lossy channels, and we propose solutions to compensate the losses in different contexts. The distribution of quantum information and quantum correlation over distances relevant for telecommunication, enables the distribution of secure encryption keys and experimental tests of non-locality. In this work we use optical photons as information carriers, and we employ the theory of quantum optics to perform theoretical investigations into the distribution of quantum information and correlation.

We propose a Bell test employing an all-optical setup with multiple parties, and using a probabilistic entanglement swap. The Bell test is designed to be within reach of current experimental capabilities, and the design of the setup is restricted to standard quantum optical elements. The parties have dichotomic inputs and outputs and we test the W3ZB inequality, and the associated linear inequalities. The experiment uses displacement-based measurements. The Bell inequality violation is robust against transmission losses, with violations being possible for transmissions as low as 10% for the channels connecting the parties. We furthermore investigate the robustness of the violation toward phase-, amplitude, and dark-count noise.

We then propose a repeater based on light-matter entangled states. We investigate how a two-mode squeezed state that has undergone transmission loss, can be purified and amplified using an array of noiseless amplifiers. The noiseless amplifiers consist of a light-matter entangled state, and the purified state is transferred directly from an optical mode to an atomic quantum memory. We present two applications, one is the creation of entangled qubit registers, and the second is the formation of a quantum repeater. We calculate secret key rates for the repeater, and find conditions under which the repeater can violate the PLOB bound. We then perform a thorough analysis of the sensitivity of the scheme toward relevant experimental errors.

We then present unpublished calculations on the generation of Gottesman-Kitaev-Preskill (GKP) states using an interaction between a collection of qubits and an oscillator. We present a probabilistic scheme based on an interaction between a single qubit and an oscillator. We show that GKP states are fixed points of the oscillator for this interaction, and therefore result naturally. We then present two deterministic schemes employing certain quadrature operators defined for a set of qubits. The first of these schemes uses a measurement to collapse the oscillator into a GKP state. The second scheme enables the formation of a GKP state without using a projective measurement. All of the schemes require that the oscillator is initially in a squeezed state. Both of the deterministic schemes produce GKP states, with the width of the gaussian peaks of the GKP state, decreasing exponentially in the number of used qubits.