The radiation–pressure interaction between electromagnetic fields and mechanical resonators can be used to efficiently entangle two light fields coupled to the same mechanical mode. We analyze the performance of this process under realistic conditions, and we determine the effectiveness of the resulting entanglement as a resource for quantum teleportation of continuous-variable light signals over large distances, mediated by concatenated swap operations. We study the sensitiveness of the protocol to the quality factor of the mechanical systems, and its performance in non-ideal situations in which losses and reduced detection efficiencies are taken into account.
The radiation–pressure interaction between electromagnetic fields and mechanical resonators can be used to efficiently entangle two light fields coupled to the same mechanical mode. We analyze the performance of this process under realistic conditions, and we determine the effectiveness of the resulting entanglement as a resource for quantum teleportation of continuous-variable light signals over large distances, mediated by concatenated swap operations. We study the sensitiveness of the protocol to the quality factor of the mechanical systems, and its performance in non-ideal situations in which losses and reduced detection efficiencies are taken into account.
Quantum communication is needed to build powerful quantum computers and establish reliable quantum networks. At its basis lies the ability to generate and distribute entanglement to separate quantum systems, which can be used to run remote quantum operations on them or teleport quantum states from one system to another with the help of classical channels. To this end, it is useful to harness the resource of continuous-variable (CV) entanglement, since it can be efficiently and unconditionally produced by squeezing light in a nonlinear medium and can be easily manipulated, distributed, and measured using standard components. While various aspects of CV-based quantum communication have been successfully demonstrated in the optical domain, some key capabilities, such as entanglement swapping, have been lacking in the microwave domain. Here, we demonstrate three key elements of CV-based microwave quantum communication: (i) a Josephson mixer operating as a nondegenerate two-mode entangler with maximum measured logarithmic negativity E N = 1.5 , (ii) a quantum teleportation apparatus, capable of teleporting vacuum and coherent states with a maximum fidelity of 73%, which exceeds the 50% classical limit and is mainly limited by intermediate losses in the setup, and (iii) an entanglement-swapping system which generates entanglement between two remote noninteracting modes via entanglement-swapping operations applied to input vacuum and coherent states with maximum measured logarithmic negativity E N = 0.53 . Such hardware-efficient CV entanglement building blocks that are based on nondegenerate Josephson mixers could enable wide-ranging applications in modular quantum computation, quantum cryptography, and quantum communication.
We propose a scheme for achieving quantum entanglement between surface-plasmon-polariton (SPP) modes in a hybrid plasmonic-graphene waveguide system. We then demonstrate that the entanglement of two SPP modes can serve as a useful resource for quantum communication of continuous-variable light signals over a long distance, mediated by concatenated swap operations. The teleportation fidelity of an unknown coherent state has strong robustness to loss and imperfect detection at relatively strong effective optical-microwave coupling. Our study opens the way for continuous-variable quantum communication and quantum information based on hybrid plasmonic-graphene systems.
This thesis is concerned with variations on, and improvements to, optical strategies of implementing quantum teleportation. In particular, I investigate four continuous variable teleportation schemes. Two schemes are based upon number-phase entanglement between two coupled harmonic oscillator modes. Joint measurements of number and phase on the coupled states are the basis for the protocols. The effects of phase decoherence on output quality are analysed and an equivalent classical protocol is introduced. The two remaining schemes improve and extend standard continuous variable teleportation. In one, coincidence measurements on the two-mode squeezed vacuum condition the entanglement resource, improving the efficacy of the protocol. In the other, the measurements made by the sender and displacements performed by the receiver are tailored to make more efficient use of the available information. In each case the protocol is described and relevant output
parameters derived. The trends in output quality with respect to input parameters are interpreted and discussed.
We explain how to share photons between two distant parties using concatenated entanglement swapping and assess performance according to the two-photon visibility as the figure of merit. From this analysis, we readily see the key generation rate and the quantum bit error rate as figures of merit for this scheme applied to quantum key distribution (QKD). Our model accounts for practical limitations, including higher-order photon pair events, dark counts, detector inefficiency, and photon losses. Our analysis shows that compromises are needed among the runtimes for the experiment, the rate of producing photon pairs, and the choice of detector efficiency. From our quantitative results, we observe that concatenated entanglement swapping enables secure QKD over long distances but at key generation rates that are far too low to be useful for large separations. We find that the key generation rates are close to both the Takeoka–Guha–Wilde and the Pirandola–Laurenza–Ottaviani–Banchi bounds.
Distributing and sharing entanglement at a distance is a key ingredient in many future quantum communication protocols, however entanglement is a fragile resource and can break down upon interacting with the environment. Within this thesis we present two possible entanglement swapping protocols, and show that these protocols are resilient to small levels of photonic losses. We propose the use of these protocols in quantum communication schemes that require shared entangled qubits, in the form of a Bell state. \nThe input states to our proposed protocols are hybrid entangled states, which are discrete-variable and continuous-variable entangled states. We use the vacuum and \nsingle photon Fock state as our discrete half, which is stationary in our entanglement swapping protocol, whereas the continuous variable half is modelled as travelling through lossy optical fibre before being measured. The first protocol uses coherent states in a superposition as propagating modes in our entanglement swapping setup, \nwhereas the second, more complicated, protocol uses superposed cat states. \nWe model photonic losses by applying a beam-splitter of transmission T to our propagating continuous variable modes, along with an input vacuum state. We also model the more realistic circumstance in which the losses in these two continuous variable modes are not equal. We then detect these continuous variable modes using a vacuum projective measurement and balanced homodyne detection. We also investigate homodyne measurement imperfections and non-ideal outcomes, as well as success probabilities of these measurement schemes. We calculate the entanglement negativity and linear entropy of our final two qubit state, as well as fidelity against the |\\Phi^+> Bell state in the coherent state protocol, and a phase-rotated |\\Phi^+(\\alpha)> Bell state in the cat state regime. \nWe demonstrate that a small amount of loss mismatch does not destroy the overall entanglement, thus demonstrating the physical practicality of this protocol.
We investigate the continuous-variable entanglement swapping protocol in a non-Gaussian setting, with non- Gaussian states employed either as entangled inputs and/or as swapping resources. The quality of the swapping protocol is assessed in terms of the teleportation fidelity achievable when using the swapped states as shared entangled resources in a teleportation protocol. We thus introduce a two-step cascaded quantum communication scheme that includes a swapping protocol followed by a teleportation protocol. The swapping protocol is fed by a general class of tunable non-Gaussian states, the squeezed Bell states, which, by means of controllable free parameters, allows for a continuous morphing from Gaussian twin beams up to maximally non-Gaussian squeezed number states. In the realistic instance, taking into account the effects of losses and imperfections, we show that as the input two-mode squeezing increases, optimized non-Gaussian swapping resources allow for a monotonically increasing enhancement of the fidelity compared to the corresponding Gaussian setting. This result implies that the use of non-Gaussian resources is necessary to guarantee the success of continuous-variable entanglement swapping in the presence of decoherence.