Quantum State Diffusion Theory and a Quantum Jump Experiment

Abstract

We use a recent stochastical diffusion model of quantum evolution to represent the evolution of a three-level quantum system undergoing quantum jumps. This is possible because the continuous change in the quantum state in this diffusion model is so rapid that it appears to be instantaneous in comparison with the time between transitions. Experimental data from a study of the intermittent fluorescence of a single trapped ²⁴Mg⁺ ion and equivalent theoretical data are shown to be strikingly similar. Statistical comparisons of the data are also made.     

Can the kicked top be realized?

Abstract

Collections of identical two-level atoms can give rise to (quantum) chaotic behaviour if non-resonantly coupled to a resonator mode and periodically driven. Observation of such chaos would require a new generation of experiments on microwave superradiance, or optical variants thereof which would exploit the strong coupling characteristic of very small cavities. Similarly, collections of identical three-level atoms non-resonantly coupled to two cavity modes could provide ‘SU(3) laboratories’, capable of realizing the semiclassical and classical limits of SU(3) dynamics, both integrable and chaotic. Some of the more interesting modes of behaviour of such systems are discussed.     

Quantum Jump Studies Using the 5d2

Abstract

Quantum jumps arising from excitation on the magnetic dipole allowed 5d²D_3/2–5d²D_5/2 transition of a single Ba⁺ ion have been observed from the changes in the ion's strong 493 nm fluorescence. Studies of the quantum jump dark and bright periods were performed for variations in the weak- and strong-transition laser intensities. The observations made on this four-level system were found to be well described by the available three-level quantum jump theory.     

Creating quantum entanglement between multi-atom Dicke states and two cavity modes

We present a scheme to create quantum entanglement between multi-atom Dicke states and two cavity modes by passing N three-level atoms in Λ configuration through a resonant two-mode cavity one by one. We further show that such a scheme can be used to generate arbitrary two-mode N-photon entangled states, arbitrary superposition of Dicke states, and a maximal entangled state of Dicke states. These states may find applications in the demonstration of quantum non-locality, high-precision spectroscopy and quantum information processing.     

Back-action-induced Squeezed Light in a Detuned Quantum Non-demolition Scheme

Abstract

We describe the experimental observation and theoretical interpretation of a squeezing effect which occurs through the coupling of two light beams in a three-level atomic system. The origin of this effect can be attributed to the transfer of the intensity fluctuations of one beam to the phase fluctuations of the other one, followed by an optical feedback onto the intensity of the initial beam. The first step of the information transfer is similar to the one which occurs in a quantum non-demolition (QND) measurement of the intensity. The feedback effect is obtained through mixing of the phase and intensity quadratures, due to the detuning of the optical cavity which contains the nonlinear medium. Therefore the information obtained by the QND measurement is used to correct the intensity fluctuations of the signal beam by a build-in mechanism, which does not require any use of external electronics.     

Measurement of beam intensity of electrically charged particles in superconductive accelerators using SQM

This paper is aimed at theoretical aspects of the application of superconducting quantum magnetometers in measurement of beam intensity of electrically charged particles in superconductive accelerators. The parameters of antenna system, which picks up the magnetic field induced by unidirectional component of the accelerated particles current, are examined. From the analysis it follows that for accelerators with a beam trajectory radius of the order of10 ¹ m the spectral sensitivity limit is of the order of (10 ²–10³) elementary particles/(cycleHz ^1/2).     

The vertical beam quality of GaInP/AlGaInP strained multiple quantum well laser

Abstract

The waveguided mode of the visible GaInP/AlGaInP compressive strained multiple quantum well laser is calculated by using transfer matrix method. On the basis of the nonparaxial vectorial moment theory of light beam propagation, the vertical (perpendicular to junction plane) optical beam quality factor M ²? of the waveguided mode is shown to be smaller than unity. This result may be useful for the design of semiconductor lasers and analysis of nonparaxial beam propagating characteristics.     

Magnetism in ultracold quantum gases

Abstract

We study the static and dynamic magnetic properties of ultracold quantum gases, in particular the spinor physics of F = 1 and F = 2 Bose-Einstein condensates of ⁸⁷Rb atoms. Our data lead to the conclusion, that the F = 2 ground state of ⁸⁷Rb is polar, while we find the F = 1 ground state to be ferromagnetic. The dynamics of spinor systems is linked to an interplay between coherent mean-field interactions, losses and interactions with atoms in the thermal cloud. Within this rich parameter space we observe indications for coherent spinor dynamics and novel thermalization regimes.     

Vortices in a stirred Bose-Einstein condensate

Abstract

We stir with a focused laser beam a Bose-Einstein condensate of ⁸⁷Rb atoms confined in a magnetic trap. We observe the formation of a single vortex for a stirring frequency exceeding a critical value. At larger rotation frequencies we produce states of the condensate for which up to eleven vortices are simultaneously present. We present measurements of the decay of a vortex array once the stirring laser beam is removed.     

Special foundation lecture: A personal view

Abstract

I (try to) review some of my work in theoretical quantum optics done over some 45 years. Highlights include discovery of ?optical solitons‘ as solutions to the nonlinear Maxwell-Bloch (MB) systems of equations as reported at the 1st National QE Conference, also at Owens Park, Manchester, in 1973. Bose-Einstein condensation (BEC) in magnetic traps at temperatures T ～ 10^?9K achieved in 1995 is described by forms of the quantum nonlinear Schrödinger (NLS) equation closely related to the quantized MB equations. It may be possible to see quantum soliton solutions of the quantum attractive NLS equation in one-dimensional (d = 1) magnetic traps holding the Bose condensed metal vapour ⁷Li. More generally such d = 1 quantum solitons may be significant to modern long-distance optical fibre communication, and perhaps to ?quantum information‘.     

The orientational phase transition at a vortex in3He-B

The orientational phase transition in the vicinity of a single vortex in³He-B is studied. It is the phase transition from a uniformn-texture withn parallel to the magnetic field and the vortex line to ann-texture that is nonuniform near the vortex. The problem of the instability of the the uniformn-texture is equivalent to the quantum mechanical two-dimensional problem of a bound state in a field with an attractive potential 1/r ². The orientational phase transition at a vortex array is also considered. In the limit of large vortex density the orientational phase transition transforms to the phase transition studied by Gongadze et al. The theoretical results are compared with the observed phase transition at a vortex in³He-B.     

Intracavity Optical Bistability Driven by Squeezed Light from a Parametric Amplifier

Abstract

We describe the theory of optical bistability when atoms are collectively excited within the cavity of a parametric oscillator. Both optical bistability and parametric amplification can squeeze significantly the cavity-field quantum noise. When they are coupled together we find significant changes both on the mean value bistability and on the spectrum of squeezing as the parametric coupling increases. These are calculated directly from the appropriate master equation for the density matrix using a quantum distribution function (positive P) to develop Fokker—Planck and Ito equations.     

Generation of quantum photon states in an active microcavity trap

Abstract

The active microcavity is adopted as an efficient source of non-classical light. By this device, excited by a mode-locked laser at a rate of 100 MHz, single photons are generated over a single field mode with a non-classical sub-Poissonian distribution. The process of adiabatic recycling within a multistep Franck–Condon molecular optical-pumping mechanism, characterized in our case by a quantum efficiency close to one, implies a pump self-regularization process leading to a striking n-squeezing effect. Moreover, the microactivity has been adopted as active beam splitter in a novel Hanbury-Brown–Twiss configuration for the radiation taking place over the two output mirrors. By a replication of the basic single-atom excitation process a beam of quantum photon |n〉 states (Fock states) can be created. The new process may represent a significant advance in the modern fields of basic quantum-mechanical investigation, quantum communication and quantum cryptography.     

Escape rate of surface state electrons on liquid helium below 1 K

The escape rate of surface state electrons on liquid helium was measured in the temperature interval 0.56<T<1.0 K for electron areal density below 5×10⁷ cm^–2, where the rate is independent of density. It is found that the escape rate is dominated by the scattering from the helium gas atoms in the vapor phase above 0.8 K, while it is dominated by that from ripplons (surface wave quanta) below 0.8 K. Quantitative agreement is obtained between our experiment and the theoretical calculation by Nagano et al.Preliminary results were published inSurface Sci. 98, 17 (1980).     

Mid/far-infrared few-cycle-pulse emission via resonant mixing in semiconductor heterostructures

Abstract

Mid/far-infrared emission from a semiconductor multiple quantum well structure under femtosecond optical pulse excitation is studied. It is shown that resonant nonlinear-wave mixing in the quantum wells can be used for the generation of ultra-short mid/far-infrared pulses with a duration of a few cycles or even a single cycle. Explicit analytical formulas for the mid/far-infrared radiation field and polarization in a simple three-level model of a quantum well are presented and compared with numerical simulations. The power of the mid/far-infrared emission and the down conversion efficiency of the resonant nonlinear-wave mixing are discussed.     

The Energy Dependence of Roton Quantum Evaporation from Superfluid 4He

We have measured the energy dependence of roton quantum evaporation. A transiently heated cavity filled with superfluid ⁴ He generates R⁺ and R^– rotons. The emitted rotons are collimated and arrive at the free liquid surface with a narrow range of incident angles. We detect two beams of evaporated atoms, one due to R⁺ rotons and the other due to R^– rotons. Our numerical simulation of roton and atom trajectories, using an evaporation probability of 1, yields two angular distributions of atom flux which are similar to our experimental results, but do have systematic differences which we attribute to the evaporation probability. The ratio of the observed signal to the computed value at each bolometer position gives the relative roton quantum evaporation probability as a function of roton energy. We find that this probability increases with roton energy, except perhaps for low energy R^– rotons. We compare these results with theoretical predictions.     

Compression and localization of an atomic cloud in a time dependent optical lattice

We analyze a method of compressing a cloud of cold atoms by dynamic control of a far off-resonance optical lattice. We show that by reducing the lattice spacing either continuously or in discrete steps while cooling the atoms with optical molasses large compression factors can be achieved. Particle motion in the time-dependent lattice is studied numerically using a three-dimensional semiclassical model. Two experimentally realistic models are analyzed. In the first we continuously vary the lattice beam angles to compress atoms initially in a Gaussian distributed cloud with standard deviation of 250 µm into a single site of a two-dimensional lattice of area A ～ 35 × 35λ², with λ the wavelength of the lattice beams. This results in an optical depth for an on-resonant probe beam >80 which is an increase by a factor of about 1800 compared to the uncompressed cloud. In the second approach we use a discrete set of lattice beam angles to decrease the spatial scale of the cloud by a factor of 500, and localize a few atoms to a single lattice site with an area A ? λ².     

Application of nondiffracting Bessel beams for shaping of surface metal microstructures

Abstract

A novel method of laser-controlled shaping of metal microstructures based on the processes of metal atoms adsorption on the surface of crystalline substrate and simultaneous control of photostimulated desorption of atoms by spatially modulated nondiffracting laser beam illumination is presented. The experiments were performed for sodium atoms deposition to the sapphire substrate, which was illuminated by Bessel beam at 532 nm wavelength and 2 W/cm² intensity. Experiments showed that the optical pattern was well reproduced in the sodium deposits thus creating the annularly microstructured metal film with few tens nanometre thickness.     

A Fast Pulsed Source of Ballistic R− Rotons in Superfluid 4He

Superfluid ⁴ He is unique in having well-defined excitations (R ^– rotons) with momentum oppositely directed to their velocity. If a beam of R ^– rotons can be produced, it could be unambiguously detected by quantum evaporation because the atoms will emerge in the opposite quadrant to that for atoms evaporated by R ⁺ rotons and phonons. Previous work shows that a heated metal film which is immersed in superfluid ⁴ He only creates phonons and R ⁺ rotons. A sponge-like heater does appear to produce R ^– rotons but, because it has a long time constant, it cannot be used in time of flight studies. We have developed a source that produces fast pulses of R ^– rotons suitable for time of flight measurements. The method uses interactions between R ⁺ rotons to create R ^– rotons, so a transient high density of R ⁺ rotons in a small confined volume is needed. The source appears to operate as we expect from a model of the evolution of the R ⁺ and R ^– roton populations.     

Cooperative Atomic Behaviour and Oscillator Formation in a Squeezed Vacuum

Abstract

Time-dependent (numerical) results are presented for super-radiant behaviour in the Dicke model of N _a = 2, 3 atoms in a broad band squeezed vacuum. This concerns the fluctuations and the intensity of the fluorescent radiation as well as the atomic population inversion of the system with atoms initially in an atomic coherent state. In the steady state, and in the N _a → ∞, we show that the ‘atomic’ Dicke model behaves like a ‘giant quantum oscillator’, in which the number of excited atoms asymptotically approaches the average number of photons in the resonant mode of the squeezed vacuum, just as in the thermally driven case.