Generation of superpositions of coherent states on a circle

We propose a simple method to obtain a superposition of coherent states on a circle, including Schr?dinger cat states as a special case, via conditional measurement of the state of three level atoms interacting with a one mode cavity field. In the low amplitude limit, very good approximation of Fock states can also be generated in this way. Received: 8 October 1998 / Accepted: 30 October 1998     

Eigenstates of boson creation operator

We examine the existence of right-hand eigenstates (or eigenkets) of the boson creation operator a and determine their coordinate and their Bargmann representation. The eigenkets of the creation operator are ultrasingular and cannot be considered as a limiting case of normalizable states. Applications of these eigenstates as auxiliary states for purposes of representation of states by path integrals over coherent states are discussed. A completeness relation for coherent states on paths through the complex plane is derived and special examples of its application are considered. Received 9 March 2001 and Received in final form 13 June 2001     

Spontaneous emission from a three-level atom embedded in an anisotropic photonic crystal

We study the spontaneous emission properties of a V-type three-level atom embedded in a photonic crystal with the anisotropic dispersion relation. We show that the localized field can disappear and the diffusion field can become intense in some regions. This originates from no singularity of the density of states. The quantum interference leads to oscillatory, quasi-oscillatory or complete decay behavior of population. The complete decay can also be realized in certain condition without depending on the initial state. Received 9 April 2000 and Received in final form 1st August 2000     

Atomic-dipole squeezing for arbitrarily prepared atom and field in multi-photon interaction

Summary The atomic-dipole squeezing condition for arbitrarily prepared atomic and field states in multi-photon Jaynes-Cummings model was derived. Three special cases were discussed. For the field initially in a vacuum state and a certain eigenstate of the Susskind-Glogower phase operator, the analytical dipole squeezing conditions were obtained and the maximum squeezing values were calculated. For a squeezed vacuum input field, the dipole squeezing behaviours under the influence of the field squeezing parameterr and the phased atom were numerically treated.     

Nonclassical properties and algebraic characteristics of negative binomial states in quantized radiation fields

We study the nonclassical properties and algebraic characteristics of the negative binomial states introduced by Barnett recently. The ladder operator formalism and displacement operator formalism of the negative binomial states are found and the algebra involved turns out to be the SU(1,1) Lie algebra via the generalized Holstein-Primarkoff realization. These states are essentially Perelomov's SU(1,1) coherent states. We reveal their connection with the geometric states and find that they are excited geometric states. As intermediate states, they interpolate between the number states and geometric states. We also point out that they can be recognized as the nonlinear coherent states. Their nonclassical properties, such as sub-Poissonian distribution and squeezing effect are discussed. The quasiprobability distributions in phase space, namely the Q and Wigner functions, are studied in detail. We also propose two methods of generation of the negative binomial states. d 32.80.Pj Optical cooling of atoms; trapping Received 8 May 1999 and Received in final form 8 November 1999     

Continuous variables quantum switch teleportation using two-mode squeezed light

We propose a novel quantum switch teleportation with a continuous variable, which can teleport a quantum state to two different receivers alternatively, using a pair of two-mode squeezed lights as the quantum switching to manipulate the transmission route. In this scheme, the EPR entangled beams shared by sender and receivers are produced by mixing a pair of two-mode squeezed lights on one beamsplitter and separating them by using a polarizing beam splitter. The teleportation capability of this system is examined by the criteria proposed by Ralph and Lam [#!ralph!#] from a small signal quantum optical point of view. Received 20 May 2002 / Received in final form 29 July 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: zhangyun@crl.go.jp     

Statistical properties of a solvable three-boson squeeze operator model

In the present paper we introduce a new squeeze operator, which is related to the time-dependent evolution operator for Hamiltonian representing mutual interaction between three different modes. Squeezing phenomenon as well as the variances of the photon-number sum and difference are considered. Moreover, Glauber second-order correlation function is discussed, besides the quasiprobability distribution function and phase distribution for different states. The joint photon-number distribution is also reported. Received 29 March 2000 and Received in final form 20 September 2000     

Preparation of Fock states and quantum entanglement via stimulated Raman adiabatic passage using a four-level atom

We study the behaviour of an atom-cavity system exposed to a stimulated Raman adiabatic passage (STIRAP) process in a four-level system, with a coupling scheme which generate two degenerate dark states. We find that the non-adiabatic interaction of the two dark states guarantees that the cavity Fock states can always be generated by both intuitively and counterintuitively ordered pulses. Furthermore, we propose a method to entangle two atoms. Depending on the ordering of the pulses two orthogonal entangled states can be prepared. Since these entangled states do not have component of the excited states included, the technique is robust against the detrimental consequences of spontaneous emission. Received 20 March 2001     

Evolution of cat states in a dissipative parametric amplifier: decoherence and entanglement

The evolution of the Schr?dinger-cat states in a dissipative parametric amplifier is examined. The main tool in the analysis is the normally ordered characteristic function. Squeezing, photon-number distribution and reduced factorial moments are discussed for the single- and compound-mode cases. Also the single-mode Wigner function is demonstrated. In addition to the decoherence resulting from the interaction with the environment (damped case) there are two sources which can cause such decoherence in the system even if it is completely isolated: these are the decay of the pump and the relative phases of the initial cat states. Furthermore, for the damped case there are two regimes, which are underdamped and overdamped. In the first (second) regime the signal mode or the idler mode “collapses" to a statistical mixture (thermal field). Received 25 September 2001 / Received in final form 30 May 2002 Published online 13 December 2002 RID="a" ID="a"e-mail: sebaweh@awalnet.net.sa RID="b" ID="b"Joint Laboratory of Optics of Palacky University and Institute of Physics, Academy of Sciences of the Czech Republic, 17. listopadu 50, 772 07 Olomouc, Czech Republic.     

A single atom-based generation of Bell states of two cavities

A new conditional scheme for generating Bell states of two spatially separated high-Q cavities is reported. Our method is based on the passage of one atom only through the two cavities. A distinctive feature of our treatment is that it incorporates from the very beginning the unavoidable presence of fluctuations in the atom-cavity interaction times. The possibility of successfully implementing our proposal against cavity losses and atomic spontaneous decay is carefully discussed. Received 31 July 2001 and Received in final form 9 October 2001     

Direct experimental test of non-separability and other quantum techniques using continuous variables of light

We present schemes for the generation and evaluation of continuous variable entanglement of bright optical beams and give a brief overview of a variety of optical techniques and quantum communication applications on this basis. A new entanglement-based quantum interferometry scheme with bright beams is suggested. The performance of the presented schemes is independent of the relative interference phase which is advantageous for quantum communication applications. Received 1st August 2001     

Quantum state engineering using conditional measurement on a beam splitter

State preparation via conditional output measurement on a beam splitter is studied, assuming the signal mode is mixed with a mode prepared in a Fock state and photon numbers are measured in one of the output channels. It is shown that the mode in the other output channel is prepared in either a photon-subtracted or a photon-added Jacobi polynomial state, depending upon the difference between the number of photons in the input Fock state and the number of photons in the output Fock state onto which it is projected. The properties of the conditional output states are studied for coherent and squeezed input states, and the probabilities of generating the states are calculated. Relations to other states, such as near-photon-number states and squeezed-state-excitations, are given and proposals are made for generating them by combining the scheme with others. Finally, effects of realistic photocounting and Fock-state preparation are discussed. Received: 17 March 1998 / Revised and Accepted: 8 May 1998     

Series solution to the laser-ion interaction in a Raman-type configuration

The Raman interaction of a ultracold ion trapped with two travelling wave lasers is studied analytically with series solutions, in the absence of the rotating wave approximation (RWA) and without restrictions of both the Lamb-Dicke limit and the weak excitation regime. The comparison is made between our solutions and those obtained under the RWA in order to demonstrate the validity region of the RWA. As a practical example, the preparation of Schr?dinger-cat states is proposed beyond the weak excitation regime, using our calculations. Received 12 March 2001 and Received in final form 16 October 2001     

Quantum imaging,quantum lithography and the uncertainty principle

One of the most surprising consequences of quantum mechanics is the entanglement of two or more distant particles. Even though we still have questions in regard to fundamental issues of the entangled quantum systems, quantum entanglement has started to play important roles in practical applications. Quantum imaging is one of the hot topics. Quantum imaging has many interesting features which are useful for different applications. For example, quantum imaging can be nonlocal, which is useful for secure two-dimensional information transfer. Quantum imaging can reach a much higher spatial resolution comparing with classical imaging, even beyond the diffraction limit, which is useful for lithography and other microsystem fabrication technology. It is not a violation of the uncertainty principle, however, a quantum mechanical multi-particle phenomenon. Received 30 August 2002 / Received in final form 11 November 2002 Published online 4 February 2003     

Multimode squeezed states produced by a confocal parametric oscillator

We investigate the spatial behaviour of the quantum-noise-reduction spectrum of the vacuum state emitted by a degenerate optical parametric oscillator below threshold. In view of possible experimental implementations, we consider a mode-degenerate resonator and two matching lenses and show that, for the realistic case of a finite-width pump, significant level of squeezing can be observed in a very small region of the beam. Received 25 September 2002 Published online 11 February 2003     

Collective atomic spin squeezing and control

Using a quantum theory for an ensemble of two- or three-level atoms driven by electromagnetic fields in an optical cavity, we show that the various spins associated with the atomic ensemble can be squeezed. Two kinds of squeezing are obtained: on the one hand self-spin squeezing when the input fields are coherent ones and the atomic ensemble exhibits a large non-linearity; on the other hand squeezing transfer when one of the incoming fields is squeezed. Received 14 August 2001 and Received in final form 7 November 2001     

Generation of superpositions of coherent states of the motion of a trapped ion

We propose a method for preparing superpositions of coherent states of the motion of an ion in an anisotropic two-dimensional trap, in which the ion is tightly bound in the y direction. In the scheme the ion is excited by two resonant laser beams with equal amplitude, propagating along the x and y directions, respectively. In the Dicke-Lamb limit, an initial coherent state of the ion motion can be converted into a superposition of several coherent states on a circle through the laser-ion interactions and state-selective measurements on the ion. Received: 30 May 1997 / Revised: 29 July 1997 / Accepted: 22 October 1997     

Quantum state transfer from light beams to atomic ensembles

We show the equivalence between an ensemble of two-level atoms driven by a squeezed vacuum field, and a harmonic oscillator coupled to a squeezed field. We give the conditions for optimal squeezing transfer from the field to the atomic ensemble. We show that EPR-type correlations are created between the atomic ensemble and the incoming field. Received 23 January 2001     

Effect of the initial phase of the field in ionization by ultrashort laser pulses

We report on observable new features related to ionization of atoms by laser pulses of only few cycles and some intensity. We show that for particular photo-electron energies, the angular distribution becomes asymmetric and that this asymmetry is related to the initial phase of the field. Received: 4 November 1997/Revised: 21 January 1998/Accepted: 23 February 1998     

Squeezed two-mode lasers without and with inversion

On the basis of the nondegenerate quantum-beat laser model, we introduce a coherent field which drives the transition between the upper lasing level and an auxiliary level. We demonstrate that such a four-level system can produce squeezed two-mode laser without and with inversion. When the laser is operated well above threshold, the intensity fluctuation in the average mode is reduced below the shot noise with an optimum Mandel parameter Q=- 1/2. At the same time, the noises in the relative amplitude and the relative phase drop to their vaccum noise levels. Furthermore, regardless of inversion, noninversion, and transition between inversion and noninversion, the optimum Mandel Q parameter of Q=- 1/2 is retained when the system operates well above threshold. A simple physical explanation of the squeezing mechanism for two-mode squeezing is given. Received: 22 December 1997 / Revised: 25 March 1998 / Accepted: 9 September 1998