Time reversal of Bose-Einstein condensates

Using Gross-Pitaevskii equation, we study the time reversibility of Bose-Einstein condensates (BEC) in kicked optical lattices, showing that in the regime of quantum chaos, the dynamics can be inverted from explosion to collapse. The accuracy of time reversal decreases with the increase of atom interactions in BEC, until it is completely lost. Surprisingly, quantum chaos helps to restore time reversibility. These predictions can be tested with existing experimental setups.     

用菲涅耳理论研究衍射光路可逆性

 用菲涅耳衍射理论研究了光栅及双光栅成像系统衍射光路的可逆性。首先用菲涅耳衍射理论分析单片光栅衍射光的复振幅分布,根据其复振幅分布的相位关系研究其可逆性;再根据单片光栅衍射光路所具有的结论来分析双光栅衍射成像效应的衍射光路可逆性,得到了光栅衍射光路及双光栅成像系统光路具有部分可逆性;应用衍射光路的部分可逆性诠释了双光栅成像效应的本质及光栅的汇合光谱特性是色散光谱特性的逆效应,并根据衍射光路具有部分可逆性的特性实现了图像处理。     

Irreversible coupling by use of dissipative optics

The time reversibility of optical propagation impedes the definite performance of many optical devices, such as couplers, polarization converters, etc. We suggest a novel concept in which we use media with loss and gain, thus breaking the time-reversal characteristics, to achieve a unidirectional optical mode interference and coupling, which is a desirable feature for light-wave circuits. Using a matched periodic modulation of both the index of refraction and loss (gain) of the medium, we implement a spatially single sideband perturbation, which breaks the symmetry to allow only a unidirectional energy transfer from mode m to mode n of the optical structure. We elaborate on this phenomenon in coupling between two modes of a multimode optical waveguide.     

Fiber-optic sensors for the estimation of oxygen gradients within biofilms on metals

This paper presents an optical approach to measure the oxygen gradients in biofilms grown on stainless-steel 316L. The sensor is constructed by immobilizing an oxygen-quenchable fluorophore at the tip of an optical fiber. The response time and reversibility of fiber-optic sensor have been analyzed. By mounting the optical fiber using a computerized micromanipulator, the vertical profiles of oxygen are measured with 10 μm depth resolution across biofilms on metal coupons. Steep concentration gradients of oxygen have been observed within the biofilm, both perpendicular and parallel to the metal surface. Optical response of the biofilm grown metal surface is found to be a consistent measure of the oxygen profiles within biofilms.     

BR-D96N薄膜共线全息图像存储实验研究

摘要:     利用一种生物光致变色材料—基因突变菌紫质BR-D96N薄膜的光致变色特性,实现了可擦写式共线全息图像存储实验.对于光密度为3.0的BR-D96N薄膜,在700 mW/cm2的记录光强下（波长为632.8 nm,物光、参考光光强比为约1:1.2）,全息图建立的最佳曝光时间约为3s,最佳再现光强约为50 mW/cm2,全息图寿命约为10 min.实验表明：共线全息存储技术光学系统简单,系统体积小,受存储环境的影响低,并能够实现高密度存储|同时也实验验证了BR-D96N薄膜具有响应速度快,感光灵敏度高,擦写次数高,稳定耐用,使用方便等优点,可以作为一种较灵敏的可擦写共线全息记录介质.     

Lead-insensitive fiber optic pH sensor and performance under bending

We have designed and tested a lead-insensitive fiber optic pH sensor that is based on the absorption of an indicator dye immobilized in a polymer bead. The fiber optic pH sensor uses two wavelengths; one senses the change in absorption of the indicator dye and the other provides a reference signal. In a view to desired time modulation and the dual wavelength time division multiplexing, we used light emitting diodes for sources, a phototransistor for the detector, and electronics process the detected signal. The sensor has demonstrated fast response, repeatable calibration, reversibility, and stability. This inexpensive device may also be used during bending of the optical fiber for applications in the field.     

Universally balanced photonic interferometers

A new general class of optical interferometers is proposed, and the physical principle of their operation is explained. They split the spectrum entering one input port among the interferometer arms in an arbitrarily chosen wavelength- and/or time-dependent manner but guarantee broadband constructive interference into a single output port by symmetry. The design relies only on time reversibility of Maxwell's equations and a phase condition that holds for lossless, reflectionless four-ports. As an application, a new Vernier scheme is proposed to multiply the tuning and free spectral range of microphotonic add-drop filters. It provides effective suppression of both the amplitude and the phase response of unwanted resonant passbands.     

Importance of reversibility in the quantum formalism

In this Letter I stress the role of causal reversibility (time symmetry), together with causality and locality, in the justification of the quantum formalism. First, in the algebraic quantum formalism, I show that the assumption of reversibility implies that the observables of a quantum theory form an abstract real C^{?} algebra, and can be represented as an algebra of operators on a real Hilbert space. Second, in the quantum logic formalism, I emphasize which axioms for the lattice of propositions (the existence of an orthocomplementation and the covering property) derive from reversibility. A new argument based on locality and Soler's theorem is used to derive the representation as projectors on a regular Hilbert space from the general quantum logic formalism. In both cases it is recalled that the restriction to complex algebras and Hilbert spaces comes from the constraints of locality and separability.     

The Approach Towards Equilibrium in a Reversible Ising Dynamics Model: An Information-Theoretic Analysis Based on an Exact Solution

We study the approach towards equilibrium in a dynamic Ising model, the Q2R cellular automaton, with microscopic reversibility and conserved energy for an infinite one-dimensional system. Starting from a low-entropy state with positive magnetisation, we investigate how the system approaches equilibrium characteristics given by statistical mechanics. We show that the magnetisation converges to zero exponentially. The reversibility of the dynamics implies that the entropy density of the microstates is conserved in the time evolution. Still, it appears as if equilibrium, with a higher entropy density is approached. In order to understand this process, we solve the dynamics by formally proving how the information-theoretic characteristics of the microstates develop over time. With this approach we can show that an estimate of the entropy density based on finite length statistics within microstates converges to the equilibrium entropy density. The process behind this apparent entropy increase is a dissipation of correlation information over increasing distances. It is shown that the average information-theoretic correlation length increases linearly in time, being equivalent to a corresponding increase in excess entropy.     

基于Nafion-结晶紫传感膜的光纤湿度传感器研究

通过研究基于荧光和可见光吸收的两种湿度传感方法,从数种湿度分子探针中优选出结晶紫为分子识别器,包埋于Nafion溶胶中,制备成基于可见光吸收原理的光纤化学湿度传感膜。该传感膜与电荷耦合二极管阵列检测器等构成的光纤湿度传感器,于640 nm波长处对30%～100%范围内的相对湿度(relative humidity, RH)具有较快的响应时间(＜2 min)、较高的灵敏度(≤5%RH)、选择性和良好的可逆性(RSD≤2.6%)。     

An optical fibre pH sensor based on dye doped mesostructured silica

This paper describes the synthesis and optical pH sensing properties of a pH indicator dye activated mesostructured silica coated optical fibre. The pH-sensitive dye bromothymol blue was incorporated into the mesostructure sol precursor and thin films/coatings were used for pH-sensing applications without prior removal of the surfactant. The pH sensitivity range and operating wavelength were analysed by absorption measurements on layer dip-coated on glass slides. The pH sensing properties of coated optical fibre were tested by evanescent wave method. Experiments were carried out by using a series of solutions of known pH. No leaching was observed (except at very high pH) and an excellent reversibility was obtained between pH=2 and 5.     

Time-reversibility violation and isolated nuclear resonances

It is pointed out that measurements of differential cross-sections in nuclear reactions proceeding via an isolated resonance can provide in principle a test for time reversibility.     

Information symmetries in irreversible processes

We study dynamical reversibility in stationary stochastic processes from an information-theoretic perspective. Extending earlier work on the reversibility of Markov chains, we focus on finitary processes with arbitrarily long conditional correlations. In particular, we examine stationary processes represented or generated by edge-emitting, finite-state hidden Markov models. Surprisingly, we find pervasive temporal asymmetries in the statistics of such stationary processes. As a consequence, the computational resources necessary to generate a process in the forward and reverse temporal directions are generally not the same. In fact, an exhaustive survey indicates that most stationary processes are irreversible. We study the ensuing relations between model topology in different representations, the process's statistical properties, and its reversibility in detail. A process's temporal asymmetry is efficiently captured using two canonical unifilar representations of the generating model, the forward-time and reverse-time ε-machines. We analyze example irreversible processes whose ε-machine representations change size under time reversal, including one which has a finite number of recurrent causal states in one direction, but an infinite number in the opposite. From the forward-time and reverse-time ε-machines, we are able to construct a symmetrized, but nonunifilar, generator of a process--the bidirectional machine. Using the bidirectional machine, we show how to directly calculate a process's fundamental information properties, many of which are otherwise only poorly approximated via process samples. The tools we introduce and the insights we offer provide a better understanding of the many facets of reversibility and irreversibility in stochastic processes.     

Efficient electrochromic device based on sol–gel prepared WO<Subscript>3</Subscript> films

Tungsten oxide (WO₃) films were prepared on indium–tin oxide (ITO) glass by sol–gel method. The influence of annealing temperature on the structural, morphological, optical, electrochemical, and electrochromic properties has been investigated. The film annealed at 250 °C with an amorphous structure exhibits a noticeable electrochromic performance, such as the highest optical modulation of 58.5 % at 550 nm, high electrochemical stability, and excellent reversibility (Q _b/Q _c?=?96.3 %). An electrochromic (EC) device based on WO₃/NiO complementary structure shows improved performance. It exhibits high optical transmittance modulation of 62 % at 550 nm, excellent cycling stability, and relatively fast electrochromic response time (10 s for coloration and 19 s for bleaching).     

Electronic transition and the metallization effect in the BiFeO<Subscript>3</Subscript> crystal at high pressures

The optical absorption spectra from bismuth ferrite (BiFeO₃) have been studied at high pressures up to 60 GPa in diamond anvil cells. An electronic transition at which the energy of the optical absorption edge decreases sharply from ～1.5 eV to zero has been observed at room temperature in a pressure range of 45–55 GPa. This indirectly indicates a insulator-metal transition. The observed electronic transition correlates with the recently revealed structural and magnetic transitions induced by high pressures in this crystal. The behavior of the optical absorption edge with decreasing the pressure is completely reversible in correlation with the reversibility of the magnetic transition. The “smearing” of the structural transition in pressure is caused by thermal fluctuations between the high-spin state and low-spin state of the Fe³⁺ ions near the transition.     

Ultraviolet radiation- and γ radiation-induced color centers in germanium-doped silica glass and fibers

Induced optical losses and paramagnetic Ge(n) centers were investigated in germanium-doped silica glass and optical fibers after γ and ultraviolet (UV) irradiation. It was found that both types of irradiation created similar effects. By means of selective UV irradiation, Ge (1 and 3) centers were identified in optical absorption spectra, presumably as induced bands centered at 4.4 eV and 6.2 eV, respectively. Moreover, photobleaching of Ge(1) centers took place under 266-nm wavelength excitation. In optical fibers no difference was observed between γ- and UV-induced loss spectra in the wavelength range from 480 nm to 1,900 nm. Partial reversibility of the photocoloration was observed. For comparison, the coloration effects were studied in glass prepared by means of modified chemical vapor deposition (MCVD) vapor-phase axial deposition (VAD), and plasma-activated chemical vapor deposition (PCVD) techniques.     

New glassy liquid crystals for optical data-storage applications

Low-molar-mass liquid crystals consisting of a binary nematic mixture of two-naphthylester which exhibit an anisotropic glassy state at room temperature have recently proven to be a suitable material for erasable optical data-storage applications. Using the holographic-grating technique, it is shown that more than 1000 all-optical write, read and erase cycles can be realized without remarkable loss of reversibility. The recording intensities are of the order of 100 W/cm² and optical erasure can be achieved with about twice that intensity. The times for recording and erasure of the optical gratings range on the millisecond scale and the stored gratings persist several months with a diffraction efficiency of typically 1 %. The spatial resolution has been proven down to 2 µm so far. The obtained results are compared with other reversible recording materials.     

Fractional generalization of Fick's law: a microscopic approach

In the study of transport in inhomogeneous systems it is common to construct transport equations invoking the inhomogeneous Fick law. The validity of this approach requires that at least two ingredients be present in the system. First, finite characteristic length and time scales associated with the dominant transport process must exist. Second, the transport mechanism must satisfy a microscopic symmetry: global reversibility. Global reversibility is often satisfied in nature. However, many complex systems exhibit a lack of finite characteristic scales. In this Letter we show how to construct a generalization of the inhomogeneous Fick law that does not require the existence of characteristic scales while still satisfying global reversibility.     

Variable focal lens controlled by an external voltage: An application of electrowetting

We use electrocapillarity in order to change the contact angle of a transparent drop, thus realizing a lens of variable focal length (B. Berge, J. Peseux, Patent deposited in Grenoble France, October 8th 1997, numéro d'enregistrement national 97 12781). The key point is the application of gradients of wettability, which control the shape of the drop edge, in our case a centered circle of variable radius. The quality and reversibility of the lens are surprisingly good. The optical power variation can be 5 to 10 times the one of the human eye, for a comparable diameter, with a typical response time of 0.03 s and a dissipated power of a few mW. Received 1 December 1999     

Ultrashort-laser-pulse-driven rewritable phase-change optical recording in Sb-based films

This paper reviews the work we have carried out over the last years on the development of ultrashort-laser-pulse-driven, rewritable, phase-change optical memories. The materials we have tailored for this application are non-stoichiometric, Sb-rich amorphous thin films, which can be crystallized upon irradiation with ultrashort laser pulses, showing a large optical contrast upon transformation. This result makes them very promising for the development of rewritable phase-change optical memories under ultrashort pulses, since the reversibility of the process has also been demonstrated. Adequate control of the heat-flow conditions has allowed us to achieve a full transformation time faster than 400 ps for crystallization/amorphization using 30-ps pulses. The crystallization threshold fluence has been found to decrease upon irradiation with pulses shorter than 800 fs, thus suggesting the relevance of high-electronic-excitation-induced processes in the amorphous-to-crystalline phase transition. This has been confirmed by the observation of an ultrafast, non-thermal phase transition occurring 200–300 fs after the arrival of the laser pulse at the surface, for fluences above the crystallization threshold. The presence of this transient phase conditions the final state induced therefore enabling the applicability of this material as a rewritable recording medium using femtosecond pulses. Received: 11 October 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +34-91/564-5557, E-mail: J.Solis@io.cfmac.csic.es