a r X i v :q u a n t -p h /9706041v 1 18 J u n 1997A Linear Quantum Dynamic Theory for Coherent Output of
Bose-Einstain Condensation
C.P.Sun a,e ,J.M.Li b ,H.Zhan c ,Y.X.Miao c ,S.R.Zhao d and G.Xu a
a Institute of Theoretical Physics,Academia Sinica,Beijing 100080,China
b Department of Physics,Peking University,Beijing 100871,China
c Institute of Theoretical Physics,Northeast Normal University,Changchun 130024,China
d Department of Applied Mathematics,University of Western Ontario,London,Ontario,Canada,N6A,5B7
e Department o
f Physics,the Chinese University of Hon
g Kong,Shatin,NT,Hong Kong
ABSTRACT A model for the coherent output coupler of the Bose-Einstein condensed atoms from a trap in the recent MIT experiment (Phys.Rev.Lett.,78(1997)582)is established with a simple many-boson system of two states with linear coupling.Its exact solution for the many-body problem shows a factorization of dynamical evolution process,i.e.,the wave function initially prepared in a direct product of a vacuum state and a coherent state remains in a direct product of two coherent states at any instance in the evolution of the total system.This conclusion always holds even for a system with a ?nite average particle number in the initial state.Its thermodynamical limit can be directly dealt with in the Bogoliubov approximation and manifests that an ideal condensate in the trap will remain in a coherent state after the r.f.interaction while the output-coupler pulse of atoms is also in a coherent state,which means a coherent output of atomic beam to form a macroscopic quantum state in a propagating mode.PACS numbers:03.65,03.75,05.30
1.Introduction
According to de Broglie matter wave theory,all matter in general can behave itself like a light that spreads out in space and combines with another one to form interference pattern.In principle,it is logical to image that a“matter wave laser”can be realized,in analog to a light laser,to produce the output coherent matter wave.Based on such fundamental consideration and stimulated by the experiments observing atomic Boson-Einstein condensate(BEC)in last two years[1-3],a number of e?orts coming from di?erent research groups have been placed on both its experimental setup and theoretical possibilities[4-7].Until the end of last year,a milestone experiment for the matter wave laser of bosonic atoms(also called atom laser)was accomplished by Mewes et al in MIT[8].
In their experiment,they coupled the toms trapped in a state|1 with that untrapped in another state|2 by a sweeping radiation frequency(r.f.)?eld.The r.f.?eld coherently turns the BEC atoms constrained in|1 into a propagating mode of atoms in|2 .The experiment shows that“the condensate which is initially in a coherent state remains in such a state after the r.f.interaction while the output-coupler pulse of atom is also in a coherent state”[8].In fact,a laser of photons above threshold can be understood in terms of the coherent state for its well-de?ned classical analogy with minimum quantum?uctuation in amplitude[9].Similarly,it is reasonable to believe that such a output of atoms in a single coherent state is something like the photon laser.Roughly,we can regard the dynamic process of wave function evolution driven by the r.f.?eld as a“stimulated”process of(massive)matter wave.
In this paper,we?rst use the Bogoliubov approximation[10,11]in thermodynamic limit to give a theoretical analysis for the dynamic lasering process of BEC atoms in the MIT experi-ment.We will show that such an approximation is not actually needed to reach the conclusion about coherent output in the requirement of very large N https://www.doczj.com/doc/c918722100.html,ly,even for the case that the number N c of atoms condensed in the ground state is not very large,thereby the Bogoliubov approximation is violated,the output of?nite atoms is still coherent so long as the system is interatomic interaction free and the atoms in the trap is initially in a coherent state.Theoreti-cally,this is the factorization structure for the evolution of the many-boson model of two state with linear coupling,i.e.,the wave function at any instance remains in a direct product of two coherent states if the system is initially prepared in a direct product of a vacuum state and a coherent state.This fact does not depend on the average number of condensed atoms in the initial state and it implies that the experiment result by Mewes et al is quite steady even for rather?nite N c.
2.A Dynamic Model of Output Coupler via Bogoliubov Approximation
The second quantization of model Hamiltonian for the MIT experiment can be written as
H=ˉhωa b?2b2+ˉhωR b?2b1exp ?i t0ω(τ)dτ +h.c (1)
in terms of the creation and annihilation operators,b?1,b?2,b1and b2,of bosonic atoms for the magnetically trapped state|1 and the untrapped state|2 with level di?erenceˉhωa.The r.f. pulse of varying frequencyω(t)is a sweeping classical electromagnetic?eld,which couples|1 and|2 through the dipole matrix elementˉhωR= V.V is the e?ective mode
volume and ε0is the vacuum permittivity.This many atom system without interatomic interac-tion can be modeled as a linear coupling system of two-oscillators.The main simpli?cation is to ignore the quantized motion of atomic center of mass in the trapped state by an inhomogeneous magnetic ?eld.As pointed out by Mewes et al,this inhomogeneous feature can be neglected for a su?ciently short r.f.pulse.
The MIT experiment is theoretically described as an initial value problem for Sch¨o rdinger equation governed by H with the initial state |ψ(0) =|α=√N c 1is a Glauber coherent state of the operator b 1characterizing N c atoms condensed in the trapped state |1 .No atoms occupy the untrapped state |2 initially.Why does the coherent state |α represent the BEC is still an open question,but there are some reasons enable one to believe it is right,such as the phase locking implied by BEC [12]and the correct average atomic number N c .The additional reason we believe is due to the connection with the Bogoliubov approximation [10,11],which replaces operators b 1and b ?1with a c -number
√N c b ?2exp ?i t
0ω(τ)dτ +h.c
.For the case of very large N c in BEC,the coupling term is very small in comparison with the term of ˉh ωR
√N c b ?2exp ?i t
0ω(τ)dτ +h.c .(2)
This is just the Bogoliubov approximation of the original Hamiltonian H .
Now let’s consider the thermodynamical limit of an in?nite number of atoms in an in?nite volume but with the density ?xed,i.e.,n c
=lim N,V →∞(N c /V )→constant ,the coupling term ˉh ωR ∝1/
√N c =ˉh g √
method calculating wave function developed in[14,15]will be used in this paper to obtain the exact solution for factorized wave function in section4.
3.Coherent Output via Factorization
The next step is to prove|Φ(t) is still a coherent state evolving from its vacuum state|0 2. It is quite clear that the Bogoliubov approximation Hamiltonian(2)is just a forced harmonic oscillator(FHO).It is well-known that the displacement property of a quantized FHO results dynamically in a coherent state for the initial vacuum state.By considering the case ofω(τ) independent of time,i.e.,ω(τ)=ω,the solution of the Hamiltonian(2)can be found in some standard textbook[16]|Φ(t) =|?α(t) with
?α(t)=ωR
√
? e i?t?1 e?iωa t,(4)
Namely,the total wave function in the BEC with large N c can be factorized in the direct product of two coherent states|ψ(t) =|α ?|?α(t) .The second components implies the output-couple pulse of the atomic beam is in the coherent state,which“can be regarded as a pulsed atomic laser”.
Notice again that the above theoretical discussion about the output-coupler for atomic laser is only with the help of a highly-simpli?ed many-boson model of two states with linear coupling rather than the complicated nonlinear one,such as the system of Gross-Petaevskii equation. Because the atomic number of the coherent output in the untrapped state
ψ(t)|b?2b2|ψ(t) =|?α|2=2ω2R
n c ∞0exp iπn c
very large N c is not necessary so long as the atoms are prepared in a coherent state|α even with rather?nite average atom number N c= α|b?2b2|α .The coherent state is only a special
Poisson superposition of in?nite number states with marched phasesΘn=nθ,n=1,2,....
The second reason to consider the exact solution concerns the analogy in the photon case.In the theory of light laser,a su?cient strong light?eld can be treated as a classical electromagnetic ?eld and it can be regarded as a coherent state above the threshold[9].In this sense,the quantum e?ect mainly results from the quantum?uctuation,which is demonstrated by a weak light?eld. Therefore,for the matter wave of massive particles,the in?nite number case only corresponds to the classical analogy of photon?eld.Then,we need to study the case corresponding to the quantum?uctuation of the weak light?eld,for which the Bogoliubov approximation is broken down.
According to[14,15],the exact solution of the wave function of the linear system in the Schr¨o dinger picture can be easily obtained from that of the canonical operators,such as b1(t) and b2(t),in Heisenberg picture.In the case ofω(τ)independent of time,by solving a system of one-order di?erential equations resulting from the Heisenberg equations of b1(t)and b2(t),the exact expressions of these solutions are
b1(t)=α1(t)b1(0)+α2(t)b2(0),(7)
b2(t)=β1(t)b1(0)+β2(t)b2(0)(8)
where
β1(t)=?ωR
?2+4ω2R e iω+t?e iω?t e?iωt,(9)
β2(t)=
1
?2+4ω2R ω+e iω+t?ω?e iω?t
e?iωt,(10)
α1(t)=
1
?2+4ω2R ω+e iω?t?ω?e iω+t
,(11)
α2(t)=?ωR
?2+4ω2R e iω+t?e iω?t ,(12)
ω±=1
?2+4ω2R .(13)
From the property of the evolution operator,U(t),of the total system,i.e.,bα(t)=U(t)?bα(0)U(t), (α=1,2),the initial state|ψ(0) =|α=
√
2|α|2
∞ n=0αn
2|α|2
∞ n=0αn
That is just a direct product of two coherent states
|ψ(t) =|αα1(?t) ?|αα2(?t)
=| 2?t(cos(?t)+i sin(?t)cosθ) ?|?i 2?t sinθsin(?t)e?iωt (15) where tgθ=2ωR/?,where?=
2ˉh n c
?t?ωt (16)
2
of the bosonic?eld,?φ(x)= k
?φ(x)2 ? ?φ(x) 2do not depend on N c.This means the relative uncertainty of the amplitude,?φ/ ?φ(x) ,will approach zero only in the thermodynamics
limit:N c,V→∞,but N c/V→constant.Therefore,for?nite N c,the quantum?uctuation √
?φ∝
,but N c sinθ∝g√
2
N c 1and no atoms occupy the untrapped state|2 ,the spin-like dynamics of Rabi oscillation will create a factorized wave function|ψ(t) =|α(t) 1?|β(t) 2,which
is a product of two coherent states.Even for the case without ideal Bose-Einstein condensation
√
that N c is not su?ciently large and thus|α=
N c 1is coherent states.However,it seems very di?cult to prepare?nite numbers of atoms in a coherent state duo to the massive feature of atoms.The spin-like dynamics of Rabi oscillation will only create an entanglement evolution state|ψ(t) = i,j|αj(t) 1?|βi(t) 2for an arbitrary initial components|ψ(0) 1other than a coherent state.Only in BEC case that the?xed phase and amplitude of macroscopic wave function are de?ned and thus to form a Glauber coherent state, this entanglement state can be completely factorized.
Such a factorized structure of wave function?rst pointed out by Mewes et al is very crucial to realize their experiment,but it dynamical origins mainly depend on the linearity of coupling. For the case with an interatomic interaction in the trapped or untrapped states,which roughly is of the form b?i b?i b i b i,the corresponding Heisenberg equations are no longer linear and thus this factorized structure will be broken down.Therefore,the su?ciently weak interaction should be required for a coherent output in practical system.In fact,Ballagh et al[18]have modeled a theory for the evolution of two component BEC by generalizing the Gross-Pitaevskii equation.
Their theory introduces a non-linear element resulting from the interatomic interaction and shows a rich variety of phenomena of nonlinear e?ect such as the localization of atom in the untrapped state due to the nonlinear coherent coupling.
Notice that an interference experiment was also carried out by Andrews et al in the same research group at MIT[19]to examine the coherence of atoms in BEC by overlapping two expanding condensates?ying from the double well magnetically-trapped potential when this trap is adiabatically switched o?.The next step following this experiment was immediately accomplished to test the coherence of the output couplers rather than a pair of expanding atomic sources.They also have shown that a similar high-contrast regular pattern of interference.Based on an intuitional understanding of the MIT“atomic laser”in terms of coherent states described in this paper,a many–body quantum theory for the interference of output couplers can be present in a forecoming paper as a space-dependent generalization of present results.The non-vanishing order parameter of quantum system,concerning a symmetry broken involving the atomic number conservation,can be obtained for the description of regular pattern of stripes in these interference experiments.
One(CPS)of t he authors wish to express his sincere thanks to K.Young for inviting him to visit the Chinese University of Hong Kong as a C.N.Yang’s Fellow.The work is supported in part by the NSF of China.
References
1M.H.Anderson,J.R.Ensher,M.R.Matthews, C.E.Wieman and E.A.Cornell, Science,269(1995)198.
2C.C.Bradley,C.A.Sackett,J.J.Tollett and R.G.Hulet,Phys.Rev.Lett.,75(1995)1687. 3K.B.Davis,M.-O.Mewes,M.R.Andrews,N.J.van Druten,D.S.Durfee,D.M.Kurn and W.Ketterle,Phys.Rev.Lett.,75(1995)3969.
4.H.Wiseman and M.Collett,Phys.Lett.A.,202(1995),246
5R.Spreeuw,T.Pfau,U.Janicke,and M.Wilkens,Europhys.Lett.,32(1995),469
6M.Holland et al.,Phys.Rev.A.,54(1996),R1757.
7A.Guzman,M.Moore,and P.Meystre,Phys.Rev.A53(1996),977
8M.Mewes,M.R.Andrews,D.S.Durfee,D.M.Kurn,C.G.Townsend and W.Ketterle, Phys.Rev.Lett.,78(1997)582.
9R.Loudon,The Quantum Theory of Light,Oxford Univ.Press,1983,Chapters4and7
10N.N.Bogoliubov,J.Phys.(USSR),11(1947)23.
11A.L.Fetter and J.D.Walecka,Quantum Theory of Many-Particle System,McGraw-Hill,1971
12P.Nozi`e res,in Bose-Einstein Condensation,A.Gri?n et al(eds),Cambridge Univ.
Press,1995,pp.15-30.
13C.P.Sun,Phys.Rev.,A48(1994),898
14L.H.Yu and C.P.Sun,Phys.Rev.,A49(1994)592.
15C.P.Sun and L.H.Yu,Phys.Rev.,A51(1995)1845.
16W.H.Louisell,Quantum Statistical Properties of Radiation,John Wiley&Sons,New York,1973,pp.203-207.
17J.Rubbmark,M.M.Kash,M.G.Littman and D.Kleppner,Phys.Rev.,A23(1981)3107. 18R.J.Ballagh,K.Burnett and T.F.Scott,Phys.Rev.Lett.,78(1997)1607.
19M.R.Andrews,C.G.Townsend,H.J.Miesner,D.S.Durfee,D.M.Kurn and W.
Ketterle,Science,275(1997)637.
第一章 ?1、人类面临的能源挑战的两大矛盾是什么？ ?能源的有限供给与经济发展日益扩大的能源需求之间的矛盾。 ?能源的大量使用与环境形势日益加剧之间的矛盾。 ?2、对未来能源的要求是什么？ ?未来将被广泛使用的新能源中，首先在能源总量上必须能满足人类对能源的整体需求水平。其次，将完全替代传统能源的新能源会遍布人类生活的每一个角落。 ?3、有可能成为人类社会的未来能源有哪些？ ?从目前的发展态势看，氢能、太阳能、地热能及核能(聚变)将成为未来人类社会的主流能源。其中氢能包括:可燃冰、页岩气、煤层气、生物质能等。 ?4、能源和材料有什么关系？ ?能源的发展离不开材料的支撑； ?世界能源消耗在持续增长，80%的能源来自于化石燃料，从长远来看，需要没有污染和可持续发展的新型能源来代替所有化石燃料，未来的清洁能源包括氢能、太阳能、地热能、核聚变能等。而解决能源问题的关键是能源材料的突破，无论是提高燃烧效率以减少资源消耗，还是开发新能源及利用可再生能源都与材料有着极为密切的关系 5、什么是合同能源管理？ ?合同能源管理是一种新型的市场化节能机制。其实质就是以减少的能源费用来支付节能项目全部成本的节能业务方式。这种节能投资方式允许客户用未来的节能收益为工厂和设备升级，以降低运行成本；或者节能服务公司以承诺节能项目的节能效益、或承包整体能源费用的方式为客户提供节能服务。 第二章 1、何为能源？什么是二次能源？ ?所谓能源，是指能够直接或经过转换而获取某种能量的自然资源 ?二次能源是由一次能源经过加工或转换得到的其他种类和形式的能源，包括煤气、焦炭、汽油、柴油、电力、蒸汽、核电等。一次能源无论经过几次转换所得到的另一种能源都被称为二次能源。二次能源比一次能源的利用更为有效、更为清洁、更为方便。 2、地球变暖的原因到底是什么？ ?人类使用能源一天所放出的热量 ＝0.1 ?1016kJ； ?地球一天从太阳获得的热量 ＝1500 ?1016kJ。 ?太阳射向地球的光约1/3被云层、冰粒和空气分子反射回去； ?约25％穿过大气层时暂时被大气吸收起到增温作用，但以后又返回到太空； ?其余的大约37％则被地球表面吸收。这些被吸收的太阳辐射能大部分在晚间又重新发射到天空。 ?如果这部分热量遇到了阻碍，不能全部被反射出去，地球表面的温度就会升高。3、大气环境的三大问题包括哪些具体内容？ 酸雨，臭氧层破坏，温室效应 4、什么是可持续发展？可持续发展的深刻内涵是什么？ 指在保护环境的条件下既满足当代人的需求，又不损害后代人的需求的发展模式。
上个世纪的飞雪和溪流迟子建去年深冬，在回故乡的慢行列车上，我遇见了两个老者。他们一胖一瘦，愉快地交谈。其中的一个说，四十多年前的一个夜晚，他驾着手推车，从山上拉烧柴回家。走到半程时，天飘起了雪花。雪越下越大，到了一个三岔路口时，他习惯地上了一条路。然而走了一会儿，他发现那路越走越生，于是掉转车头，又回到岔路口。雪花纷纷扬扬的，天又黑，他分辨不出南北东西了，于是凭着直觉，又踏上了一条路。可是他越走越心虚，因为那条路似乎也是陌生的，他害怕了，又一次回到岔路口。深夜时，家人寻来了。他这才知道，他第一次踏上的路，是正确的。只不过因为雪太大，改变了路的风貌。那人说：“谁能相信，我让雪花给迷了路呢！要是搁现在，可能吗？”他指着车窗外的森林说：“看看，这雪一年比一年小，风一年比一年大，这还叫大兴安岭吗？”透过车窗，我看见稀疏的林地上，覆盖着浅浅的积雪，枯黄的蒿草在风中舞动。而在雪大的年份，那些蒿草会被雪深深地埋住，你是看不到的。天虽然仍是蓝的，可因为雪少得可怜，那幅闪烁的冬景给人残破不堪的感觉。而这样的景象，在大兴安岭，自新世纪以来，是越来越司空见惯了。我想起童年在小山村的时候，每逢冬天来临，老天就会分派下一项活儿，等着我们小孩子来接收，那就是扫雪。那个年代的雪，真是恋人间啊！常常是三天一小场，十天一大场，很少碰到一个月没有雪的时候。雪会大到什么程度呢？有的时候，它闷着头下了一夜，清晨起来，你无法出去抱柴了，因为大雪封门了。这个时候，就得慢慢地推门，让它渐渐透出缝隙，直到能伸出笤帚，一点点地掘开雪，门才会咧开嘴，将满院子的白雪推进你的视野，有如献给你一个明朗的笑。那个年代，不光是雪多，溪流也是多的。夏天，我们常到山上玩，渴了，随时捧山间的溪水来喝。溪水清冽甘甜，带着草木的清香，我喝的这世上最好的水，就是大兴安岭的溪水。那时植被好，雨水丰沛，因而溪流纵横。女孩们夏天洗衣服，爱到溪水旁。省了挑水，可以洗个透彻。洗衣服的时候，蝴蝶和蜻蜓在你眼前飞来飞去的，它们的翅膀有时会温柔地触着你的脸；而溪水中呢，不仅浸泡着衣服，还浸泡着树和云的影子，好像它们嫌自己不干净，要你帮着洗一洗似的。大兴安岭的河流，到了冬天都封冻了。柔软的水遇到零下三四十度的严寒，哪有不僵的呢？可母亲告诉我，我们家在设计队住的时候，后山上有一道泉水，冬天是不冻的。母亲说，我们后来搬家了，所以那道泉水在那座山上，究竟活了多少个冬天，她是不知道的。大兴安岭的开发，使林木资源日渐匮乏，小时候常见的参天大树，好像都被老天召走，做了另一个世界晚祷的蜡烛，难觅踪影了。而那如丰富的神经一样遍布大地的溪流，也悄然消逝了。我已故的爱人，他曾天真地对我说：“大兴安岭全境人口不过五十多万，我看不如把所有的人口都迁出去，异地安置，做到真正的封山。几十年后，树茂盛了，溪水也充沛了，中国会留下最好的一片原始森林。”可我知道，这样的想法，无论是在他生前还是死后，都是“天上的想法”。我怀念上个世纪故乡的飞雪和溪流。我幻想着，有一天，它们还会在新世纪的曙光中，带着重回人间的喜悦，妖娆地起舞和歌唱。（有删改）文中写到了上个世纪大兴安岭的飞雪和溪流，请结合全文用简洁的语言分别概括其特点。(4分) 飞雪：雪大，雪多。溪流：多（溪流纵横，遍布大地），清冽甘甜，有些经冬不冻。文章第一段写两个老者的对话，这样写有何用意？请简要分析。（6分）⑴内容上，回忆了曾经的大兴安岭雪大的特点，表现了作者对上个世纪大兴安岭飞雪的怀念之情；⑵结构和手法上，与后文现在残破不堪的冬景形成鲜明对比，为回忆上个世纪大兴安岭飞雪和溪流做铺垫；⑶效果上，起到吸引读者的作用。本文是围绕“大兴安岭的飞雪和溪流”展开的，请梳理全文的行文思路。（6分）⑴开篇借老者之口讲述雪夜迷路的故事，突出了上个世纪大兴安岭雪大的特点，与现在残破不堪的冬景形成鲜明对比。⑵回忆起上个世纪雪和溪流的景象。⑶回到现实，说明人们对大兴安岭的开发、对林木资源的破坏是这一改变的根源。⑷作者怀念上个世纪大兴安岭飞雪和溪流，憧憬它们重回人间。请结合全文分条归纳，作者主要流露出了哪些情感。（6分）⑴怀念、惋惜之情；⑵憧憬、期盼之情；⑶对人们破坏林木资源的批判，对封山
(一) 假定你是红星中学初三学生李华。你的美国朋友Jim在给你的邮件中提到他对中国新近出现的一种共享单车“mobike”很感兴趣，并请你做个简要介绍。请你给Jim回信，内容包括： 1. 这种单车的使用方法(如：APP查看车辆、扫码开锁等); 2. 这种单车的优势; 3. 你对这种单车的看法。 注意：1. 词数不少于80; 2. 开头和结尾已给出，不计入总词数。 提示词：智能手机smartphone, 二维码the QR code 参考范文 Dear Jim, I’m writing to tell you more about the new form of sharing bike mobike mentioned in your latest letter. It’s very convenient to use if you have a smartphone. What you do is find a nearest mobikethrough the APP, scan the QR code on the bike, and enjoy your trip. Compared to other forms of sharing bike, the greatest advantage of mobike is that you can easily find one and never worry about where to park it. It is becoming a new trend as a means of transportation, which relieves the traffic pressure and does good to the environment as well. Hope to ride a mobike with you in China. Yours, Li Hua (二) 最近很多大城市都投放了共享单车(shared bikes)，比如摩拜单车(Mobike)、Ofo共享单车等。由于它们方便停放，骑车也能起到锻炼身体的作用，作为代步工具很受大家欢迎。但是，各地也出现了很多毁车现象，比如刮掉车上的二维码(QR code)、上私锁等。 你对这种现象怎么看?你对共享单车公司有什么建议吗?写一篇符合逻辑的英语短文，80词左右。 参考词汇：bike-sharing companies 共享单车公司，Mobike 和Ofo 是两家共享单车公司，convenience 方便，register登记 参考范文 The shared bikes like Mobike and Ofo bring great convenience to people. You needn’t lock them by simply using your smart phone. They can take you where the subway and bus don’t go. And they can be left anywhere in public for the next user. However, bad things happen. Some people damage the QR code on the bike, or use their own lock, which causes trouble to other users. In my opinion, it’s difficult to turn these people’s ideas in a short time. Therefore, bike-sharing companies like Mobike and Ofo need to do something. For example, those who damage the bike should pay for their actions. Also, because people use their real name toregister as a user, it’s a good way to connect to one’s personal credit. In the end, what I want to say is to take good care of public services. (三) 共享单车(bicycle sharing)已成为时下最热的话题之一，请你就这一话题写一篇短文。内容须包括三方面：1. 共享单车蓬勃发展，成为社会热潮;2. 共享单车带来便利，但也存在问题;3. 我对解决问题的建议。 参考范文 Bicycle Sharing With the development of technology, bicycle sharing comes into people's lives. It becomes more and more popular and much news reported it. At the same time, we should see that there are some problems caused by bicycle sharing. On one side, bicycle sharing makes it very convenient of people traveling. You can find a bicycle anywhere at any time when you want to go out for a cycling, and the price of one trip is very low. It can save time for people. On the other side, its management is not perfect. Even kids can open the lock and ride the bicycle, there is no doubt that such behavior is very dangerous.
中英文资料对照外文翻译 基于网络共享的无线传感网络设计 摘要：无线传感器网络是近年来的一种新兴发展技术，它在环境监测、农业和公众健康等方面有着广泛的应用。在发展中国家，无线传感器网络技术是一种常用的技术模型。由于无线传感网络的在线监测和高效率的网络传送，使其具有很大的发展前景，然而无线传感网络的发展仍然面临着很大的挑战。其主要挑战包括传感器的可携性、快速性。我们首先讨论了传感器网络的可行性然后描述在解决各种技术性挑战时传感器应产生的便携性。我们还讨论了关于孟加拉国和加利 尼亚州基于无线传感网络的水质的开发和监测。 关键词：无线传感网络、在线监测 1.简介 无线传感器网络，是计算机设备和传感器之间的桥梁，在公共卫生、环境和农业等领域发挥着巨大的作用。一个单一的设备应该有一个处理器，一个无线电和多个传感器。当这些设备在一个领域部署时，传感装置测量这一领域的特殊环境。然后将监测到的数据通过无线电进行传输，再由计算机进行数据分析。这样，无线传感器网络可以对环境中各种变化进行详细的观察。无线传感器网络是能够测量各种现象如在水中的污染物含量，水灌溉流量。比如，最近发生的污染涌流进中国松花江，而松花江又是饮用水的主要来源。通过测定水流量和速度，通过传感器对江水进行实时监测，就能够确定污染桶的数量和流动方向。 不幸的是，人们只是在资源相对丰富这个条件下做文章，无线传感器网络的潜力在很大程度上仍未开发，费用对无线传感器网络是几个主要障碍之一，阻止了其更广阔的发展前景。许多无线传感器网络组件正在趋于便宜化（例如有关计算能力的组件），而传感器本身仍是最昂贵的。正如在在文献[5]中所指出的，成功的技术依赖于
重庆市农业机械化校 2018 年（春） 《新能源汽车概论》期末考试题答案
一、选择 1、以下属于新能源的是（A） A 柴油、B 太阳能、C 地热能、D 风能 2、不可再生资源是（D） A 波浪能、B 潮汐能、C 海流能、D 煤炭 3、以下汽车，不属于电动汽车的是（D） A 混合动力汽车、B 纯电动汽车、C 燃料电池汽车、D 乙醇汽车 4、根据储能机理不同，再生制动能量回收系统回收能量的方法也不同，下列不属于这三种储能方式的是：D A、飞轮储能 B、液压储能 C、电化学储能 D、电子储能 5、汽车在城市中行驶，制动能量占总驱动能量的比重是（A） 。 A、50% B、70% C、30% D、20% 6、具有再生制动能量回收系统的电动汽车，一次充电续驶里程可以增加：B A、5%~15% B、10%~30% C、30%~40% D、40%~50% 7、下列不属于电池故障级别信息的是（C） 。 A、尽快维修 B、立即维修 C、电池报废 D、电池寿命 8、下列不属于电池成组后会出现的问题的是（D） 。 A、过充/过放 B、温度过高 C、短路或漏电 D、充电过慢 9、不是电动汽车用电池的主要性能指标的是（D） 。 A、电压 B、内阻 C、容量和比容量 D、流量 10、动力电池组的总电压可以达到（C） 。 A、36~88V B、420~500V C、90~400V D、12~35V 二、判断 1、混合动力汽车也称复合动力汽车，但是只有一个动力源。× 2、新能源又称非常规能源，是指刚开始开发利用或正在积极研究、有待推广的能源。√ 3、燃料电池化学反应产物主要是电能和水，还有极度多的二氧化碳和氮氧化物。 × 4、氢燃料汽车的动力系统是在传统内燃机的基础上加以改进后制成的。√ 5、 不可外接充电型混合动力汽车是指被设计成在正常使用情况下从车载燃料中获取全部能量的混合动力汽车。 √ 6、混合动力指装备有两种具有不同特点驱动装置的汽车。√ 7、为避免发动机的怠速及低负荷工况，以减小油耗，故发动机不工作，仅电机利用其高速大转矩的特性单独 使车辆起步。× 8、在电池失效报废后，可直接将动力电池丢弃。× 9、在动力电池日常维护工作中，要做到日常管理的周到、细致和规范性。√ 10、电动汽车的安全包括人身安全与系统安全。√ 三、填空 1、太阳能的利用主要有三种：光热转换、光电转换以及光化学转换。 2、混合动力汽车分为串联、并联、串并联三种结构形式。 3、按照电机相对于燃油发动机的功率比大小分为重度混合型、中度混合型、轻度混合型以及微混合型混合动 力汽车。
苏雪林《溪水》阅读答案 导读：溪水 苏雪林 ⑴我们携着手走进林子，溪水漾着笑窝，似乎欢迎我们的双影。这道溪流，本来温柔得像少女般可爱，但不知何时流入深林，她的身体便被囚禁在重叠的浓翠中间。 ⑵早晨时她不能更向玫瑰色的朝阳微笑，深夜时不能和娟娟的月儿谈心，她的chè莹晶的眼波，渐渐变成忧郁的深蓝色，时时凄咽着忧伤的调子，她是如何的沉闷呵!在夏夜的时候。 ⑶几番秋雨之后，溪水涨了几篙;早diāo的梧楸，飞尽了翠叶;黄金色的晓霞，从杈桠树隙里，深入溪中;泼靛的波面，便泛出彩虹似的光。 ⑷现在，水恢复从前活泼和快乐了，一面疾忙的向前走着，一面还要和沿途遇见的落叶、枯枝……淘气。 ⑸一张小小的红叶儿，听了狡狯的西风劝告，私下离开母校出来顽玩，走到半路上，风偷偷儿的溜走了，他便一交跌在溪水里。 ⑹水是怎样的开心呵，她将那可怜的失路的小红叶儿，推推挤挤的推到一个xuán涡里，使他滴滴溜溜的打圆转儿;那叶向前不得，向后不能，急得几乎哭出来;水笑嘻嘻的将手一松，他才一溜烟的逃走了。 ⑺水是这样欢喜捉弄人的，但流到坝塘边，她自己的磨难也来
了。你记得么?坝下边不是有许多大石头，阻住水的去路。 ⑻水初流到水边时，还是不经意的涎着脸撒嗔痴的要求石头放行，但石头却像没有耳朵似的，板着冷静的面孔，一点儿不理。于是水开始娇嗔起来了，拼命向石头冲突过去;冲突激烈时，浅碧的衣裳袒开了，漏出雪白的胸臂，肺叶收放，呼吸极其急促，发出怒吼的声音来，缕缕银丝头发，四散飞起。 ⑼辟辟拍拍，温柔的巴掌，尽打在石头皱纹深陷的颊边，---- 她真的发怒了，不是儿嬉。 ⑽谁说石头是始终顽固的呢，巴掌来得狠了，也不得不低头躲避。于是谁得安然度过难关了。 ⑾她虽然得胜了，然而弄得异常疲倦，曳了浅碧的衣裳去时， 我们还听见她继续的chuǎn息声。 ⑿我们到这树林中来，总要到这坝塘边参观水石的争执，一坐总要一两个钟头。 1.文章着重刻画了溪水流经途中的哪两件事?表现出溪水怎样的 性格特点?请根据文意简要回答。 2.下列句子生动形象，富有表现力，请加以赏析。 1）.一张小小的红叶儿，听了狡狯的西风劝告，私下离开母校出来顽玩，走到半路上，风偷偷儿的溜走了，他便一交跌在溪水里。 2）.于是水开始娇嗔起来了，拼命向石头冲突过去;冲突激烈时，浅碧的衣裳袒开了，漏出雪白的胸臂，肺叶收放，呼吸极其急促，发
[标签:标题] 篇一：关于汽车的英语作文 好的 Nowadays, with the rapid improvement of people’s living standards, cars have become an indispensable part of people's lives,so that more and more people have a car of their own, especially in cities. It brings some benefits for us but also causes many problems at the same time. For one thing,there’s no doubt that cars provide much convenience for people to go where they want to quickly and easily. Especially on weekday,driving a car can save a lot of time for us to go to work.When some places are too far away from our home, driving our own car is also convenient, we can go wherever we want. However,for another, too many cars will lead to the pressure of public transport, a series of problems will appear.First of all,it will bring about more air pollution,a large amount of polluted air given off by cars do great harm to our health.What’s more, as the existing roads are not so wide for the increasing number of cars,undoubtedly,traffic jams will become more and more serious. Last but not least, cars also place burden on the public facilities in providing more parking lots. As far as I am concerned,everything has its advantages and disadvantages. It’s high time that effective action must be token to limit the ever growing number of cars, the government should take measures to control the air pollution from the cars. Some roads should be widened and more new roads should be constructed. Only in this way，will people benefit from the popularity of cars. 坏的 Nowadays, with the rapid improvement of people's living standards, cars have become an indispensable part of people's lives,so that more and more people have a car of their own, especially in cities.It brings some benefit for us but also causes many problems at the same time. For one thing,it's no doubt that that cars provide much convenience for people to go where they want to quickly and easily. Especially on weekday,driving a car can save a lot of time for us to go to work.When some places are too far away from our home, driving our own car is also convenient, we can go wherever we want. However,for another, too many cars will lead to the pressure of public transport, a series of problems will appear.First of all,it will bring about more air pollution,a large amount of polluted air given off by cars do great harm to our health .What's more, as the existing roads are not so wide for the increasing number of cars,undoubtedly,traffic jams will become more and more serious. Last but not least, cars also place burden on the public facilities in providing more parking lots. As far as I am concerned,everything has its advantages and disadvantages. It's high time that effective action must be token to limit the ever growing number of cars, the government should take measures to control the air pollution from the cars. Some roads should be widened and more new roads should be constructed. Only in this way,will people benefit from the popularity of cars. 篇二：雅思作文高分范文：私家车的利与弊 智课网IELTS备考资料 雅思作文高分范文：私家车的利与弊
Wireless sensor network monitoring system design Kang yi-mei,Zhao lei,Hu jiang,Yang en-bo (Study on Beijing University of Aeronautics and Astronautics) Summary: A car wireless sensor network monitoring system based on IEEE 802.15.4 and ZigBee standards. With universal wireless sensor networks, expansion of the scope of monitoring and monitoring functions for in-car system, car data acquisition and condition monitoring of equipment status and the necessary equipment control, topology control, topology query functions. Keywords: wireless sensor networks; monitoring system Introduction In order to satisfy the people to car safety, handling and comfort requirements, vehicle integrated with more and more electronic system .At present, car electronic equipment is widely used 16 or 32-bit microprocessor control. Creating in-vehicle monitoring system based on IEEE 802.15.4 and ZigBee standard for wireless sensor networks, designed to achieve a more optimized wireless sensor networks, the progressive realization of the network of automotive systems, intelligent and controllable to provide high-Car System security. System design In this paper, the existing vehicle system, the data transmission mode is extended to the wireless transmission mode, the realization of a star network data acquisition system. And can place each data acquisition node of the acquired data is transmitted to the gateway, the gateway through the serial port to upload data to the host computer, in the host data real-time waveform display, and method of database to preserve, for the follow-up data processing. The application of system object is composed of a temperature sensor, pressure sensor, speed sensor, speed sensor, a current sensor, pressure sensor, sensor subsystem. The purpose of this design is to use a monitoring host machine end to the detection of multiple target environment, taking into account the access data throughput and software system complexity, using time-division multiplexing way, one by one on the net terminal collecting point of control and data acquisition. As shown in Figure 1, the system is divided into 3 parts: Vehicle Monitoring Center, gateway and mobile sensor node. Gateway is the whole vehicle system core, and all vehicular sensor node communication. Vehicle monitoring center to the gateway sends a control command by the gateway, the control command is converted to an RF signal and sent to the vehicle sensor node. When the vehicle sensor nodes to transmit data, gateway into the data reception state, and upload data to the monitoring center for further processing. In addition, car between sensor nodes cannot communicate with each other. The monitoring center of the monitoring software and gateway in RS232standard interface for communication. Vehicle sensor node life cycle is active and dormant periods. Nodes in the active phase of the completion of data acquisition, data sent to the gateway, receiving and
新能源汽车概述A卷以 及答案 TTA standardization office【TTA 5AB- TTAK 08- TTA 2C】
7.为了充分利用电池电量，应当尽可能的让电池多放电，保持较深的放电深度，有利于蓄电池的使用寿命。（） 8.汽车排放的污染物主要有一氧化碳、碳氢化合物、氮氧化合物和微粒等，并主要由油箱和化油器等地方排出。（） 9.发动机润滑油在发动机各摩擦表面只起润滑作用。（） 10.汽车行驶系一般由承载式车身、前后车桥、车轮和前后悬架等组。（） 三、选择题（每题1 题，共20分） 1. ( ) A、~2V B、~ C 12~15V 2. 具有极强竞争力的是 A、铅酸蓄电池B C、干电池 D 3.下列不是电动汽车用电池的主要 性能指标的是( ) A、电压 B、内阻 C、容量和 比容量 D、流量 4.能量管理系统是电动汽车的智能 核心，其英文表示为( ) A、ABS B、BMS C、ECU D、DOD 5.动力电池组的总电压可以达到( ) A、36~88V B、420~500V C、 90~400V D、12~35V 6.铅酸蓄电池、锂离子蓄电池、镍 氢蓄电池的最佳工作温度是( ) A、25~40度 B、0~10度 C、 45~80度 D、88~100度 甘肃交通职业技术学院2017——2018学年第二学期期末考试试卷（附页） 7.动力电池的组的英文表示为( ) A、PACK B、BATTERY C、ELECTRIC D、CAR 8.下列不属于电池故障级别信息的 是( ) A、尽快维修 B、立即维修 C、
电池报废 D、电池寿命 9.下列不属于电池成组后出现的问题的是( ) A、过充/过放 B、温度过高 C、短路或漏电 D、充电过慢 10.国家开展节能宣传和教育，将节能知识纳入国民教育和培训体系，普及节能科学知识，增强国民的节能意识，提倡( )的消费方式。 A、清洁型 B、循环型 C、节约型 D、环保型 11.节约资源是我国的基本国策。国家实施( )的能源发展战略。A、开发为主，合理利用B、利用为主，加强开发 C、开发和节约并举，把开发放在首位D、节约和开发并举，把节约放在首位 12.铅酸蓄电池用的电解液是( ) A、KOH B、H2SO4 C、NH4CL D、有机溶液 13.铅酸蓄电池用的正极材料是( ) A、锌 B、铅板 C、铝 D、Ni(OH)2 14.根据储能机理不同，再生制动能量回收系统回收能量的方法也不同，下列不属于这三种储能方式的是( ) A、飞轮储能 B、液压储能 C、电化学储能 D、电子储能 15.热继电器在三相异步电动机电路中的作用是( ) A、欠压保护 B、过载保护 C、短路保护 D、过压保护 16.直流电动机的主磁场是( ) A、主磁极产生的磁场 B、电枢电流产生的磁场 C、换向极产生的磁场 D、以上三者的合成 17.以下电动汽车对充电装置的要求不包括( ) A、安全 B、质量大 C、经济 D、效率高 18.最小转弯半径、最小离地间隙、接近角、离去角、前悬、后悬是汽车( )的技术参数。
溪流的阅读答案 集团公司文件内部编码：（TTT-UUTT-MMYB-URTTY-ITTLTY-
①我愈来愈爱着生我养我的土地了。 ②就像山地里纵纵横横的沟岔一样，就像山地里有着形形色色的花木一样，我一写山同，似乎思路就开了，文笔也活了。 ③甚至觉得，我的生命，我的笔命，就是那山溪哩。虽然在莽莽的山的世界里，它只是那么柔得可怜，细得伤感的一股儿水流。 ④我常常这么想：天上的雨落在地上，或许会成洪波，但它来自云里；溪是有根的，它凉凉地扎在山峰之下。人都说山是庄严的，几乎是死寂的，其实这是错了。它最有着内涵，最有着活力；那山下一定是有着很大很大的海的，永远在蕴涵的感情，永远是不安宁，表现着的，恐怕便是这小溪了。 ⑤或许，是从小草的根下一个泡儿一个泡儿冒出来的。但是，太阳晒不干、黄风刮不跑的。天性是那么晶莹，气息是那么清新；它一出来，便宣告了它的生命，寻着自己的道路要流动了。 ⑥正因为寻着自己的道路，它的步伐是艰辛的。然而，它从石板上滑下，便有了自己的铜的韵味的声音；它从石崖上跌落，便有了自己白练般的颜色，它回旋在穴潭之中，便有了自己叵不可测的深沉。
⑦它终于慢慢地大起来了，要走更远的道儿；它流过了石川，流过了草地，流过了竹林，它要拜访所有的山岭，叩问每一次石头，有时会突然潜入河床的沙石之下去了呢。（A于是，轻风给了它的柔情，鲜花给了它的芬芳，竹林给了它的凉绿，那多情的游鱼，那斑斓的卵石，也给它增添了美的色彩。） ⑧它在流着，流着。它要流到啊里去呢？我想，山既然给了它的生命，它该是充实的，富有的；（B或许，它是做一颗露珠儿去滋润花瓣，深入到枝叶里了，使草木的绿素传送）；或许，它竟能掀翻一坯污泥，拔脱了一从腐根呢。那么，让它流去吧，山地这么大，这么复杂，只要它流，它探索，它就有了自己的路子。 ⑨我是这么想的，我提醒着我，我鼓励着我，我便将它写成了淡淡的文字，聊作这本小书的小序了。 1.作者笔下的溪流有什么特点？（一排空） 2.简要归纳文中小溪的成长过程。（一排空） 3.细读6、7段，说说这两段有什么思想含义。（一排空） 4.作者为自己的书作序，写的却是溪流，这有什么用意？(一排空） 5.这篇文章有什么特色？（两排空） ⒈以时间为序，从小溪的过去写起，写小溪的现在，再想象它的未来。
基于无线传感器网络的桥梁安全检测系统 摘要 根据桥梁监测无线传感器网络技术的桥梁安全监测系统，以实现方案的安全参数的需要；对整个系统的结构和工作原理的节点集、分簇和关键技术，虽然近年来在无线传感器网络中，已经证明了其潜在的提供连续结构响应数据进行定量评估结构健康，许多重要的问题，包括网络寿命可靠性和稳定性、损伤检测技术，例如拥塞控制进行了讨论。 关键词：桥梁安全监测；无线传感器网络的总体结构；关键技术 1 阻断 随着交通运输业的不断发展，桥梁安全问题受到越来越多人的关注。对于桥梁的建设与运行规律，而特设的桥梁检测的工作情况，起到一定作用，但是一座桥的信息通常是一个孤立的片面性，这是由于主观和客观因素，一些桥梁安全参数复杂多变[1]。某些问题使用传统的监测方法难以发现桥梁存在的安全风险。因此长期实时监测，预报和评估桥梁的安全局势，目前在中国乃至全世界是一个亟待解决的重要问题。 桥梁安全监测系统的设计方案，即通过长期实时桥跨的压力、变形等参数及测试，分析结构的动力特性参数和结构的评价科关键控制安全性和可靠性，以及问题的发现并及时维修，从而确保了桥的安全和长期耐久性。 近年来，桥梁安全监测技术已成为一个多学科的应用，它是在结构工程的传感器技术、计算机技术、网络通讯技术以及道路交通等基础上引入现代科技手段，已成为这一领域中科学和技术研究的重点。 无线传感器网络技术，在桥梁的安全监测系统方案的实现上，具有一定的参考价值。 无线传感器网络（WSN）是一种新兴的网络科学技术是大量的传感器节点，通过自组织无线通信，信息的相互传输，对一个具体的完成特定功能的智能功能的协调的专用网络。它是传感器技术的一个结合，通过集成的嵌入式微传感器实时监控各类计算机技术、网络和无线通信技术、布式信息处理技术、传感以及无线发送收集到的环境或各种信息监测和多跳网络传输到用户终端[2]。在军事、工业和农业，环境监测，健康，智能交通，安全，以及空间探索等领域无线传感器网络具有广泛应用前景和巨大的价值。 一个典型的无线传感器网络，通常包括传感器节点，网关和服务器，如图1
苏雪林《溪水》阅读题的答案 我们携着手走进林子，溪水漾着笑涡，似乎欢迎我们的双影。这道溪流，本来温柔得像少女般可爱，但不知何时流入深林，她的身体便被囚禁在重叠的浓翠中间。 早晨时她不能更向玫瑰色的朝阳微笑，夜深时不能和娟娟的月儿谈心，她的明澈莹晶的眼波，渐渐变成忧郁的深蓝色，时时凄咽着忧伤的调子，她是如何的沉闷呵！在夏天的时候。几番秋雨之后，溪水涨了几篙；早凋的梧楸，飞尽了翠叶；黄金色的晓霞，从杈枒树隙里，深入溪中；泼靛的波面，便泛出彩虹似的光。 现在，水恢复从前活泼和快乐了，一面疾忙的向前走着，一面还要和沿途遇见的落叶、枯枝??淘气。 一张小小的红叶儿，听了狡狯的西风劝告，私下离开母校出来游玩，走到半路上，风偷偷儿溜走了，他便一交跌在溪水里。 水是怎样的开心呵，她将那可怜的失路的小红叶儿，推推挤挤的推到一个漩涡里，使他滴滴溜溜的打圆转儿；那叶向前不得，向后不能，急得几乎哭出来；水笑嘻嘻的将手一松，他才一溜烟的逃走了。
水是这样欢喜捉弄人的，但流到坝塘边，她自己的魔难也来了。你记得么？坝下边不是有许多大石头，阻住水的去路？水初流到石边时，还是不经意的涎着脸撒娇撒痴的要求石头放行，但石头却像没有耳朵似的，板着冷静的面孔，一点儿不理。于是水开始娇嗔起来了，拼命向石头冲突过去；冲突激烈时，浅碧的衣裳袒开了，露出雪白的胸臂，肺叶收放，呼吸极其急促，发出怒吼的声音来，缕缕银丝头发，四散飞起。 噼噼啪啪，温柔的巴掌，尽打在石头皱纹深陷的颊边，──她真的怒了，不是儿戏。谁说石头是始终顽固的呢？巴掌来得狠了，也不得不低头躲避。于是水安然渡过难关了。她虽然得胜了，然而弄得异常疲倦，曳了浅碧的衣裳去时，我们还听见她断续的喘息声。我们到树林中来，总要到这坝塘边参观水石的争执，一坐总是一两个钟头。 1.这是一首优美的散文，作者赋予溪水以少女般的性格。请你说说文章先写溪水______________； 再写溪水______________；最后写溪水_________________。（3分） 2.结合文章内容，说说作者笔下的溪水具有怎样的个性特征？（4分）