Continuous variable remote state preparation and teleportation are analysed using Wigner functions in phase space. We suggest a remote squeezed state preparation scheme between two parties sharing an entangled twin beam, where homodyne detection on one beam is used as a conditional source of squeezing for the other beam. The scheme also works with noisy measurements, and provides squeezing if the homodyne quantum efficiency is larger than 50%. The phase space approach is shown to provide a convenient framework to describe teleportation as a generalized conditional measurement, and to evaluate relevant degrading effects, such the finite amount of entanglement, the losses along the line and the nonunit quantum efficiency at the sender location.
Continuous variable remote state preparation and teleportation are analysed using Wigner functions in phase space. We suggest a remote squeezed state preparation scheme between two parties sharing an entangled twin beam, where homodyne detection on one beam is used as a conditional source of squeezing for the other beam. The scheme also works with noisy measurements, and provides squeezing if the homodyne quantum efficiency is larger than 50%. The phase space approach is shown to provide a convenient framework to describe teleportation as a generalized conditional measurement, and to evaluate relevant degrading effects, such the finite amount of entanglement, the losses along the line and the nonunit quantum efficiency at the sender location.
Continuous-variable quantum key distribution (CV-QKD) is a leading technology for secure communication, whose security guarantees fundamentally rely on the quantum nature of the transmitted states and the measurement process. However, real-world detectors invariably suffer from limitations, such as inefficiency, which affects the overall security and performance of the system. Here, we investigate the impact of detector efficiency on remote state preparation in the context of entanglement-based CV-QKD. Specifically, by using the Wigner function formalism and starting from a two-mode squeezed vacuum state, we derive the analyticalform of the quantum state of one mode conditioned on a homodyne measurement performed on the other mode. Our analysis explicitly contrasts the ideal case of unit efficiency (η=1), which prepares a pure, displaced squeezed state, with the non-ideal scenario. For η < 1, we demonstrate that the preparation result is a mixedstate characterized by increased noise and degraded squeezing. We calculate the exact analytical expressions for the conditional state’s variance and mean displacement, quantifying their dependence on both the efficiency η and the initial squeezing parameter.
We suggest a general scheme for continuous variable quantum-state engineering based on conditional measurements carried out on entangled twin beam of radiation. Realistic detection schemes such as on/off photodetection, homodyne detection, and joint measurement of two-mode quadratures are analyzed in detail. Imperfections of the apparatuses, such as nonunit quantum efficiency and finite resolution, are taken into account. We show that conditional on/off photodetection provides a reliable scheme to verify nonclassicality, whereas conditional homodyning represents a tunable and robust source of squeezed light. We also describe optical continuous variable teleportation as a conditional measurement, and evaluate the degrading effects of finite amount of entanglement, decoherence due to losses, and nonunit quantum efficiency.
Remote state preparation enables one to create and manipulate a quantum state based on the shared entanglement between distant nodes. Here, we experimentally demonstrate remote preparation and manipulation of squeezed light. By performing a homodyne projective measurement on one mode of the continuous variable entangled state at Alice's station, a squeezed state is created at Bob's station. Moreover, rotation and displacement operations are applied on the prepared squeezed state by changing the projective parameters on Alice's state. We also show that the remotely prepared squeezed state is robust against loss and N - 1 squeezed states can be remotely prepared based on an N-mode continuous variable Greenberger-Horne-Zeilinger-like state. Our results verify the entanglement-based model used in security analysis of quantum key distribution with continuous variables and have potential application in remote quantum information processing.
ABSTRACT We propose a setup for teleportation of continuous quantum variables which is optimized for teleportation ofsqueezed coherent states. Our scheme is a modified version of the original setup proposed by Braunstein andKimble [Phys. Rev. Lett. 80, 869, 1998] and it involves an unbalanced beam splitter on the sender's side and asingle mode squeezer on receiver's side. We show that the quality of the teleportation can be evaluated by meansof a fidelity of entanglement swapping and we prove that our modified setup and the original Braunstein-Kimblesetup are equivalent with respect to this fidelity measure.Keywords: Quantum teleportation, squeezed states, completely positive maps, fidelity 1. INTRODUCTION Quantum teleportation, a disembodied transfer of a quantum state between two distant parties, is perhaps themost remarkable and exciting development in the rapidly growing field of quantum information processing. Intheir seminal paper, Bennett et al.1 proposed a protocol for teleportation of a single qubit from the sender Aliceto the receiver Bob. The information about the teleported state is sent to Bob via dual classical and quantumEPR channels. The latter is established by a maximally entangled state shared by Alice and Bob.Recently, there has been much interest in quantum information processing with continuous quantum van-ables. Here, the focus is on systems with infinite dimensional Hubert spaces such as quantized electromagneticfield. Teleportation of continuous quantum variables was first considered by Vaidman,2 and an experimentallyfeasible setup was suggested by Braunstein and Kimble.3 Subsequent experiment4 demonstrated that theteleportation of a single mode of optical field is fully within the reach of current technology. Since then, manyauthors discussed various aspects of the teleportation,5'° including fidelity of teleportation of various quantumstates and influence of losses on the degradation of the quantum channel.Besides teleportation, many other protocols developed originally in the framework of discrete quantum van-ables, have been successfully formulated also for continuous variables. These include quantum cloning,11'3quantum
Quantum communication protocols based on nonclassical correlations can be more efficient than known classical methods and offer intrinsic security over direct state transfer. In particular, remote state preparation aims at the creation of a desired and known quantum state at a remote location using classical communication and quantum entanglement. We present an experimental realization of deterministic continuous-variable remote state preparation in the microwave regime over a distance of 35 cm. By employing propagating two-mode squeezed microwave states and feedforward, we achieve the remote preparation of squeezed states with up to 1.6 dB of squeezing below the vacuum level. Finally, security of remote state preparation is investigated by using the concept of the one-time pad and measuring the von Neumann entropies. We find nearly identical values for the entropy of the remotely prepared state and the respective conditional entropy given the classically communicated information and, thus, demonstrate close-to-perfect security.
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.