Effective quantum efficiency in the pulsed homodyne detection of a n-photon state

Homodyne detection can be used to perform measurements on various quantum states of the light, such as conditional single photon states produced by parametric fluorescence processes. In the pulsed regime, the time and frequency overlap between the single photon wave packet and the local oscillator field plays a crucial role. We show in this paper that this overlap can be characterized by an effective quantum efficiency, which is explicitly calculated in various situations of experimental interest. Received 27 July 2000 and Received in final form 29 November 2000     

Calculation of quantum correlation spectra using the regression theorem

We present a simple method, based on the quantum regression theorem, to calculate the quantum correlation spectra for two optical beams in the linearized fluctuation regime. As an application, we discuss the dynamical instability, the squeezing spectra and the QND properties of a crossed Kerr-type dispersive model. Received 30 August 1999 and Received in final form 4 July 2000     

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     

Environment-induced nonclassical behaviour

We analyze the transient nonclassical behaviour of a single-mode field whose interaction with an environment is governed by the quantum optical master equation. Our analytic method makes use of the generalized characteristic function of the field state. First, we find a time at which all squeezing effects disappear by decoherence regardless of the initial state of the mode. In the case of an input even coherent state, an unusual modification of higher-order squeezing at low values of thermal mean occupancy transferred to the field is found and discussed. For the same initial state, we also perform a comprehensive analysis of the mixing process during the interaction with the reservoir. We prove that a maximum in the evolution of the 2-entropy of the attenuated mode exists on condition that its initial mean photon number exceeds the mean occupancy of the reservoir. This transient mixing enhancement can be considered as a quantum effect of the initial state on the mode damping. Received 22 April 1999 and Received in final form 2 November 1999     

Detecting quantum signatures of optical fields by ultrasmall Josephson junctions

We prove that a mesoscopic Josephson junction, irradiated with a quantum superposition of two -out of phase optical coherent states, exhibits an experimentally observable sensitivity to the quantum coherences of the field state. Received 1 June 1999 and Received in final form 30 July 1999     

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.     

Quantum structures in traveling-wave spontaneous parametric down-conversion

We analyze theoretically spatial structures appearing in the far diffraction zone of the electromagnetic field emitted in the cavityless parametric down-conversion. We investigate in detail the spatial correlation functions of intensity and demonstrate the existence of strong quantum correlations between the regions of the far field symmetrical with respect to the optical axis. Our simplified model allows us to obtain analytical results for some limiting cases. We demonstrate that in the limit of small diffraction and ideal quantum efficiency of photodetection the noise reduction in the photocurrent difference between symmetrical regions in the far diffraction field becomes complete at zero frequency of photocurrent fluctuations. Received 5 February 2001 and Received in final form 11 April 2001     

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     

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     

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     

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     

Quantum effects in the interaction of the exciton with a leaky quasi-mode cavity

We have studied quantum effects in the interaction of the exciton with a leaky quasi-mode cavity field. When the exciton is initially prepared in a superposition state which exhibits holes in its photon-number distribution, whereas the cavity field initially is in the vacuum state, it is found that there exists an energy exchange between the exciton and the cavity field. The exciton and the cavity field may exhibit sub-Poissonian distributions and quadrature squeezings. It is shown that there does not exist a violation of the Cauchy-Schwartz inequality, which means that the correlation between the exciton and the cavity field is classical. Received 25 November 2000 and Received in final form 1st January 2001     

Triggered single photons and entangled photons from a quantum dot microcavity

Current quantum cryptography systems are limited by the attenuated coherent pulses they use as light sources: a security loophole is opened up by the possibility of multiple-photon pulses. By replacing the source with a single-photon emitter, transmission rates of secure information can be improved. We have investigated the use of single self-assembled InAs/GaAs quantum dots as such single-photon sources, and have seen a tenfold reduction in the multi-photon probability as compared to Poissonian pulses. An extension of our experiment should also allow for the generation of triggered, polarization-entangled photon pairs. The utility of these light sources is currently limited by the low efficiency with which photons are collected. However, by fabricating an optical microcavity containing a single quantum dot, the spontaneous emission rate into a single mode can be enhanced. Using this method, we have seen 78% coupling of single-dot radiation into a single cavity resonance. The enhanced spontaneous decay should also allow for higher photon pulse rates, up to about 3 GHz. Received 8 July 2001 and Received in final form 25 August 2001     

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     

Steady-state populations of a driven three-level atom in a broad-band squeezed vacuum

A theoretical study is made of the steady-state populations of a three-level atom in a ladder configuration, driven by a superposition of a monochromatic laser wave with a broad-band squeezed vacuum. The master equation for the system and the atomic Bloch equations are derived. The steady-state populations are calculated numerically and shown graphically as functions of two-photon detuning for various cases of the squeezed vacuum. It is shown that, the atomic populations depend strongly on the relative phase of the driving field and the squeezed vacuum. When the phase matching condition is fulfilled, there will be a strong two-photon resonant absorption from the squeezed vacuum, a characteristic different from absorption of photons from a classical field. Received: 28 May 1997 / Revised and Accepted: 10 December 1997     

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     

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     

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     

On the observation of decoherence with a movable mirror

Following almost a century of debate on possible “independent of measurement" elements of reality, or “induced" elements of reality - originally invoked as an ad-hoc collapse postulate, we propose a novel line of interference experiments which may be able to examine the regime of induced elements of reality. At the basis of the proposed experiment, lies the hypothesis that models of “induced" elements of reality should exhibit symmetry breaking within quantum evolution. The described symmetry experiment is thus aimed at being able to detect and resolve spatial symmetry breaking signatures. The proposed experiment stands at the edge of present day technological abilities and will be, so we believe, realizable in the near future. Received 2 December 1999 and Received in final form 6 April 2000     

Dichromatic squeezing generation

We propose a novel scheme for the joint generation of two squeezed beams at arbitrary frequencies ω ₁ and ω ₂. The scheme consists of two successive steps, both involving nonlinear interactions in χ⁽²⁾ crystals. The dynamics of the setup is analyzed both quantum mechanically and classically within the parametric approximation. An experimental implementation involving the fundamental and the harmonics of a Nd:YAG laser pulse, and β-BaB ₂ O ₄ nonlinear crystals is suggested. Received 17 May 2000 and Received in final form 9 October 2000