Universal quantum computation using optical coherent states is studied. A teleportation scheme for a coherent-state qubit is developed and applied to gate operations. This scheme is shown to be robust to detection inefficiency.PACS number(s); 03.67. 03.67.Lx,

Genuinely quantum states of a harmonic oscillator may be distinguished from their classical counterparts by the Glauber-Sudarshan P-representation -a state lacking a positive P-function is said to be nonclassical. In this paper, we propose a general operational framework for studying nonclassicality as a resource in networks of passive linear elements and measurements with feed-forward. Within this setting, we define new measures of nonclassicality based on the quantum fluctuations of quadratures, as well as the quantum Fisher information of quadrature displacements. These lead to fundamental constraints on the manipulation of nonclassicality, especially its concentration into subsystems, that apply to generic multi-mode non-Gaussian states. Special cases of our framework include no-go results in the concentration of squeezing and a complete hierarchy of nonclassicality for single mode Gaussian states.

We propose a realistic optimal strategy for continuous-variable teleportation in a realistic situation. We show that any imperfect quantum operation can be understood by a combination of an asymmetrically-decohered quantum channel and perfect apparatuses for other operations. For the asymmetrically-decohered quantum channel, we find some counter-intuitive results; teleportation does not necessarily get better as the channel is initially squeezed more. We show that decoherenceassisted measurement and transformation may enhance the fidelity for the asymmetrically mixed quantum channel. PACS number(s); 03.65. Bz, 03.67, 42.50.Dv Quantum teleportation is one of the important manifestations of quantum mechanics. In particular, quantum teleportation of continuous variable states [1] has attracted a great deal of attention because of a high detection efficiency, handy manipulation of continuous variable states [2][3][4], and possibility of application to highquality quantum communication. Two kinds of protocols have been suggested for continuous variable teleportation; one utilizes the entanglement between quadraturephase variables [2] and the other between the photonnumber sum and the relative phase [5]. Both the protocols employ a squeezed two-mode vacuum for the quantum channel. In this paper, we report how to optimize the quantum teleportation of quadrature-phase variables when the quantum channel and experimental conditions are not perfect.There are a few obstacles which make the teleportation of quadrature-phase variables imperfect. The perfect quantum teleportation is possible only by a maximally-entangled quantum channel, i.e., by an infinitely squeezed state which is unphysical as it incurs the infinite energy. Moreover, when the quantum channel is exposed to the real world, it is influenced by the environment, which turns the pure squeezed state into a mixture and deteriorates the entanglement property. To maximize the channel entanglement, purification protocols for continuous variable states have been suggested by Parker et al.[6] for partially-entangled pure states and by Duan et al. [7] for mixed Gaussian states. However, the theoretical suggestions have not been realized by experiment. Further, there are other obstacles in experiment such as imperfect detection efficiency at the sending station and imperfect unitary transformation at the receiving station. We show that the imperfect conditions may be absorbed into the imperfect quantum channel while other apparatuses are treated perfect, and find the optimization condition for the teleportation under a given experimental condition. We show that blindly maximizing the initial entanglement of the quantum channel does not necessarily bring about the best teleportation.Two modes a and b of the squeezed vacuum are distributed, respectively, to a sending and a receiving station. At the sending station, the original unknown state is entangled with the field mode a of the quantum channel by a 50/50 beam splitter. Two conjugate quadrature variables are measure...

A state in a d-dimensional Hilbert space can be simulated by a state defined in a different dimension with high fidelity. We assess how faithfully such an approximated state can perform quantum protocols using an example of the squeezed coherent superposition state, which was recently experimentally generated.

The superposition principle is at the heart of quantum mechanics and at the root of many paradoxes arising when trying to extend its predictions to our everyday world. Schrödinger's cat [1] is the prototype of such paradoxes and here, in contrast to many others, we choose to investigate it from the operational point of view. We experimentally demonstrate a universal strategy for producing an unambiguously distinguishable type of superposition, that of an arbitrary pure state and its orthogonal. It relies on only a limited amount of information about the input state to first generate its orthogonal one. Then, a simple change in the experimental parameters is used to produce arbitrary superpositions of the mutually orthogonal states. Constituting a sort of Schrödinger's black box, able to turn a whole zoo of input states into coherent superpositions, our scheme can produce arbitrary continuous-variable optical qubits, which may prove practical for implementing quantum technologies and measurement tasks.