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We present our approach for sharing photons and assessing resultant four-photon visibility between two distant parties using concatenated entanglement swapping. In addition we determine the corresponding key generation rate and the quantum bit-error rate. Our model is based on practical limitations of resources, including multipair parametric down-conversion sources, inefficient detectors with dark counts and lossy channels. Through this approach, we have found that a trade-off is needed between experimental run-time, pair-production rate and detector efficiency. Concatenated entanglement swapping enables huge distances for quantum key-distribution but at the expense of low key generation rate.

We model frozen light stored as a spin wave via electromagnetically induced transparency quantum-memory techniques in a Bose-Einstein condensate. The joint evolution of the condensate and the frozen light is typically modeled using coupled Gross-Pitaevskii equations for the two atomic fields, but these equations are only valid in the mean-field limit. Even when the mean-field limit holds for the host condensate, coupling between the host and the spin wave component could lead to a breakdown of the mean-field approximation if the host fluctuations are large compared the mean-field value of the spin wave. We develop a theoretical framework for modeling the corrections to the mean-field theory of a two-component condensate. Our analysis commences with a full second-quantized Hamiltonian for a two-component condensate. The field operators are broken up into a mean-field and a quantum fluctuation component. The quantum fluctuations are truncated to lowest non-vanishing order. We find the transformation diagonalizing the second-quantized approximate Hamiltonian and show that it can be described using the solutions to a system of coupled differential equations.

We show that electromagnetically induced transparency can be used to store coherently and to retrieve light pulses propagating in a one-dimensional waveguide through a line of atoms, thereby realizing light-controlled on-demand quantum memory. Storage can be highly efficient even with just a few atoms despite inter-atomic scattering interference due to the one-dimensional constraint.

We characterize slab and cylindrical waveguides with a dielectric core and a metamaterial cladding (metamaterial-dielectric guide). We use the lossy Drude model to describe the permittivity of the metamaterial and a lossy Lorentz-like model for the permeability. We identify the possible modes and examine various related quantities, such as dispersion and attenuation, for metamaterial-dielectric waveguides. We find that at certain frequencies the modes of the metamaterial-dielectric guides differ from metal-dielectric or dielectric-dielectric guides. Our results also point to the possibility of having a metamaterial-dielectric waveguide with lower loss than a metal-dielectric waveguide.

Weak-light all-optical switches would enhance photonics capabilities and enable important optical quantum information processing tasks, but optical switching based on third-order optical nonlinearity is proving to be elusive and controversial. I discuss strategies to generate large cross-phase modulation by combining double electromagnetically induced transparency, competing atomic transitions, electron population engineering for pulse speed control, and using boundary conditions to confine the transverse field beyond the diffraction limit.

We present a formalism within which, using entangled graph states of prime-dimensional systems, a variety of different secret-sharing schemes (involving both quantum and classical secrets and quantum and classical channels shared between parties) may be unified. We review the explicit protocols we have found for three varieties of secret sharing within this formalism, including some for which the analogous formalism using qubit graph states is not sufficient.

Three-mode (tripartite) squeezed light is important for continuous variable quantum information tasks such as quantum teleportation, state sharing, and generating tripartite continuous variable entangled states, and furthermore is the first nontrivial step towards multipartite squeezed states. In such cases just one two-mode squeezer is used, and the squeezed light is distributed across multiple channels by passive optical elements. Here we show that these three-mode squeezed states are produced by optical networks that can be described as SU(1, 1) transformations, which enables relatively straightforward and elegant calculations of any output state from such squeezing networks for any input state.

Strong optical cross-phase modulation (XPM) for weak fields is tremendously important for optical quantum information (QI) processing and for all-optical switches in classical communication. A sufficiently large XPM would allow the design of deterministic controlled quantum gates for photonic qubits and thus enable universal optical quantum computation. Recently, several proposals have been brought forward to create large XPM using double electromagnetically induced transparency (DEIT) in which two weak signal light pulses travel at equally slow group velocity, but creating DEIT still poses an experimental challenge. We give a brief overview about DEIT and discuss its applications and limitations. A scheme that combines the best features of previous proposals and optimizes the large XPM parameter for DEIT schemes is outlined. Finally we devise a scheme to perform universal quantum information processing, which respects the bound on the achievable nonlinearity and addresses the requirement of quantum error correction.

We prove that continuous variable quantum information processing via Gaussian preserving operations, with Gaussian input states plus a (highly nonclassical) Fock number state, and subject to homodyne measurements and feedforward, can be efficiently simulated on a classical computer. This result reinforces the importance of the cubic phase state as an offline resource for universal continuous variable quantum information processing. We study the practical realization of this state by determining the Wigner function for an approximate cubic phase state prepared via two-mode squeezing with postselection on the signal field by displacing and counting photons in the idler field.

Quantum teleportation should be able to teleport states or parts of states in arbitrarily large Hilbert spaces, which will require concatenation of quantum teleportation processes that operate for a fixed dimension. We introduce the concept of concatenated quantum teleportation and obtain exact expression for teleportation fidelities for two simple cases. We also derive the conditions when concatenated teleportation can be considered as "quantum".

We study the quantum to classical transition in bipartite entangled systems in which one system is continuously coupled to a measurement apparatus as in the von Neumann model of quantum measurement. As an example, we study the open system dynamics of a particle in a harmonic well whose motion in the well is coupled to the internal spin. This system provides a rich illustration of the quantum to classical transition in weakly measured coupled systems. We analyze and derive conditions for which the dual constraints of strong localization/small noise required for the quantum-classical transition are satisfied for both regular and chaotic dynamics. We also study the dynamics of bipartite entanglement in the regime where classical trajectories emerge in the measurement record. Our analysis shows the surprising result that bipartite entanglement can persist in the classical limit.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Triggered single-photon sources produce the vacuum state with non-negligible probability, but produce a much smaller multiphoton component. We describe a method for increasing the probability of a single photon via a chain of beam splitters. This method has the drawbacks that it introduces a significant multiphoton component, and it can not be used to increase the probability for a single photon above 1/2. Part of the reason for these drawbacks is the incoherence in the photon sources. If the photon sources produced pure states rather than incoherent superpositions, it would be possible to obtain a perfect single photon output. The method for incoherent inputs is robust against detector inefficiencies, but sensitive to dark counts and the multiphoton component in the input.

We demonstrate a multipartite protocol that utilizes entanglement to securely distribute and reconstruct aquantum state. A secret quantum state is encoded into a tripartite entangled state and distributed to threeplayers. By collaborating together, a majority of the players can reconstruct the state, whilst the remainingplayer obtains nothing. This (2 ,3) threshold quantum state sharing scheme is characterized in terms of Þdelity(F ), signal transfer ( T ) and reconstruction noise ( V ). We demonstrate a Þdelity averaged over all reconstructionpermutations of 0 .73 ± 0.04, a level achievable only using quantum resources.Keywords: quantum information, quantum network, quantum state sharing, quantum cryptography, quantumsecret sharing, entanglement, optical parametric ampliÞer 1. INTRODUCTION In conventional cryptography, secret sharing is a powerful technique which enables a dealer to securely distributeinformation to multiple players (the recipients) who are not all necessarily trustworthy. Secret sharing techniqueshave many present-day applications involving the management of cryptographic keys in information networkssuch as the internet, telecommunication systems and distributed computers. An important class of secret sharingprotocols is ( k,n ) threshold secret sharing

We discuss sharing quantum secrets via optical interferometry and squeezing. A secret quantum state for a single-mode field is encoded into a multimode field as an entangled state and distributed to a set of players so that certain subsets can decode the secret states, and others cannot learn anything about the state. In particular, we discuss the (k,n)-threshold scheme for optics and specifically the (2,3) scheme. An arbitrary (k,n)-threshold scheme can be achieved with no more than two single-mode squeezers.

We present an experimental scheme to perform continuous variable (2,3) threshold quantum secret sharing on the quadratures amplitudes of bright light beams. It requires a pair of entangled light beams and an electro-optic feedforward loop for the reconstruction of the secret. We examine the efficacy of quantum secret sharing in terms of fidelity, as well as the signal transfer coefficients and the conditional variances of the reconstructed output state. We show that, in the ideal limit, perfect secret reconstruction is possible. We discuss two different definitions of quantum secret sharing: the sharing of a quantum secret and the sharing of a classical secret with quantum resources.

We study collective spontaneous emission from arbitrary distributions of N two-state atoms using quantum trajectory theory and without an a priori single-mode assumption. Assuming a fully excited initial state, we calculate the angular distribution of the average integrated intensity. We investigate the dependence of the angular distribution of emission on the geometry of the atomic distribution. The formalism is developed around an unravelling of the master equation in terms of source mode quantum jumps. A modified boson approximation is made to treat the many-atom case, where it is found that strong directional superradiance occurs for a few hundred to a few thousand atoms. In order to illustrate important differences between our model and single-mode models we consider shot-to-shot intensity fluctuations and angular correlations in the emitted intensity.