P. L. Knight

The Jaynes-Cummings Model

Abstract The Jaynes-Cummings model (JCM), a soluble fully quantum mechanical model of an atom in a field, was first used (in 1963) to examine the classical aspects of spontaneous emission and to reveal the existence of Rabi oscillations in atomic excitation probabilities for fields with sharply defined energy (or photon number). For fields having a statistical distributions of photon numbers the oscillations collapse to an expected steady value. In 1980 it was discovered that with appropriate initial conditions (e.g. a near-classical field), the Rabi oscillations would eventually revive, only to collapse and revive repeatedly in a complicated pattern. The existence of these revivals, present in the analytic solutions of the JCM, provided direct evidence for discreteness of field excitation (photons) and hence for the truly quantum nature of radiation. Subsequent study revealed further non-classical properties of the JCM field, such as a tendency of the photons to antibunch. Within the last two years it ha...

MULTIPARTICLE GENERALIZATION OF ENTANGLEMENT SWAPPING

We generalize the procedure of entanglement swapping to obtain a scheme for manipulating entanglement in multiparticle systems. We describe how this scheme allows to establish multiparticle entanglement between particles belonging to distant users in a communication network through a prior distribution of singlets followed by only local measurements. We show that this scheme can be regarded as a method of generating entangled states of many particles and compare it with existing schemes using simple quantum computational networks. We highlight the practical advantages of using a series of entanglement swappings during the distribution of entangled particles between two parties. Applications of multiparticle entangled states in cryptographic conferencing and in reading messages from more than one source through a single measurement are also described.

Proposal for teleportation of an atomic state via cavity decay

We show how the state of an atom trapped in a cavity can be teleported to a second atom trapped in a distant cavity simply by detecting photon decays from the cavities. This is a rare example of a decay mechanism playing a constructive role in quantum information processing. The scheme is comparatively easy to implement, requiring only the ability to trap a single three level atom in a cavity.

Purification via entanglement swapping and conserved entanglement

We investigate the purification of entangled states by local actions using a variant of entanglement swapping. We show that there exists a measure of entanglement which is conserved in this type of purification procedure. @S1050-2947~99!06807-9# PACS number~s!: 03.67.2a The resource of entanglement @1# has many useful applications in quantum information processing, such as secret key distribution @2#, teleportation @3#, and dense coding @4#. Polarization entangled photons have been used to demonstrate both dense coding @5# and teleportation @6# in the laboratory. Teleportation has also been realized using pathentangled photons @7# and entangled electromagnetic field modes @8#. Accompanying the practical applications of entanglement are some useful schemes for entanglement manipulation which may help in the distribution of entanglement between distant parties. One such scheme is entanglement swapping @9,10#, which enables one to entangle two quantum systems that have never interacted directly with each other; we have discussed how this may be used in constructing a quantum telephone exchange @10#. Recently, entanglement swapping has been demonstrated experimentally @11#. There exists yet another useful manipulation of entanglement in which local actions and classical communication are used by two distant parties to distill a certain number of shared Bell states from a larger number of shared but less entangled states. When the initial shared but less entangled states are pure, this manipulation is termed as entanglement concentration @12,13#, while for the more general case of the initial shared states being mixed, the process is termed as entanglement purification or distillation @14,15#. The importance of such a scheme in the distribution of entanglement is obvious as Bell pairs are essential for the implementation of quantum communication schemes with perfect fidelity. Curiously, such an important procedure remains to be realized in an experiment. In this paper, we will show that a simple variant of the entanglement swapping scheme can be viewed as a type of entanglement concentration procedure and has a physical realization with polarization entangled photons. Moreover, we show that there exists a certain measure of entanglement which remains conserved on average in this type of entanglement concentration. Note that in this paper we will often use the terms ‘‘entanglement concentration’’ and ‘‘entanglement purification’’ in an interchangeble manner, though what we demonstrate is, in the strict sense, only the concentration of pure shared entanglement.

Entanglement by a beam splitter: Nonclassicality as a prerequisite for entanglement

A beam splitter is a simple, readily available device which can act to entangle output optical fields. We show that a necessary condition for the fields at the output of the beam splitter to be entangled is that the pure input states exhibit nonclassical behavior. We generalize this proof for arbitrary (pure or impure) Gaussian input states. Specifically, nonclassicality of the input Gaussian fields is a necessary condition for entanglement of the field modes with the help of a beam splitter. We conjecture that this is a general property of beam splitters: Nonclassicality of the inputs is a necessary condition for entangling fields in a beam splitter.

Quantum computing using dissipation to remain in a decoherence-free subspace

We propose a new approach to the implementation of quantum gates in which decoherence during the gate operations is strongly reduced. This is achieved by making use of an environment induced quantum Zeno effect that confines the dynamics effectively to a decoherence-free subspace.

CAVITY-LOSS-INDUCED GENERATION OF ENTANGLED ATOMS

Original article can be found at: http://pra.aps.org/ Copyright American Physical Society DOI : 10.1103/PhysRevA.59.2468

Entanglement induced by a single-mode heat environment

A thermal field, which frequently appears in problems of decoherence, provides us with minimal information about the field. We study the interaction of the thermal field and a quantum system composed of two qubits and find that such a chaotic field with minimal information can nevertheless entangle the qubits which are prepared initially in a separable state. This simple model of a quantum register interacting with a noisy environment allows us to understand how memory of the environment affects the state of a quantum register.

Laser-induced continuum structure

Atomic and molecular continuum states may be structured by embedding a bound state into a previously unstructured continuum by a dressing interaction. Autoionizing resonances, in which a high lying discrete state is mixed by configuration interaction into a photoionization continuum, is the best known example in atomic physics. We discuss the process by which a dressing laser can embed a low-lying atomic state into a flat continuum to produce a tunable resonance of adjustable width. Such laser-induced continuum structures (LICS) behave in many ways like autoionizing states. We discuss how such states are formed and how they are probed in photoionization. Saturation properties, including the formation of stabilized dressed states and consequent population trapping, are reviewed. The effects of laser phase fluctuations on the interference phenomena responsible for LICS are included in both semiclassical Wiener-Levy and fully-quantum mechanical theories of multiplicative stochastic processes. We demonstrate how the laser fluctuations can lead to a dephasing of the LICS process and a destruction of population trapping, although these may be restored if the dressing and probing lasers have mutually correlated fluctuations. We also show that additional photoionization rates induced by strong dressing and probe laser fields can cause the population trapped in otherwise stable dressed states to decay away. Finally we discuss the complicating effects of competing transitions, laser pulse shapes and so on of relevance to the experimental observation of LICS.

Subsystem Purity as an Enforcer of Entanglement

We show that entanglement can always arise in the interaction of an arbitrarily large system in any mixed state with a single qubit in a pure state. This small initial purity is enough to enforce entanglement even when the total entropy is close to maximum. We demonstrate this feature using the Jaynes-Cummings interaction of a two-level atom in a pure state with a field in a thermal state at an arbitrarily high temperature. We find the time and temperature variation of a lower bound on the amount of entanglement produced and study the classical correlations quantified by the mutual information.