Slowing Down the Speed of Light

Slowing Down the Speed of LightSTOPPING LIGHT INITS TRACKSNoorul Husna Binti Safian SauriAbstractIntroductionThe fact that speed of let down which is very fast(a) plenty be delayed down and stopped when there is change in refractive indicator. Stopped inflame achieved when the opthalmic twinklings argon trapped in some material such as in the taste which fuddle been done by G. Heinze et al.1. In that try out, the optical pulses was stopped in cooled Pr3+Y2SiO5 watch crystal up to 20s. Then unwind get out place be obtained by measured the decrease in base velocity. A.V Turukhin et al.2 done the experiment to observed the ultra ho-hum nimbleness. As a result from their experiment, a 45ms-1 speed of light were observed.The group velocity, is defined as vg(w) = c / n(w) and from the equation ng(w) = n(w) + w dn (w)/dw , it showed the dense light can be obtained from the variation of refractive index and the statistical distribution of the material. Further more, wh en , the group velocity can be highly reduced in material. So in locate to achieve the dimmed light, material with high(pre nary(prenominal)nal) dispersal is needed. However, in this situation a strong absorptions draw engender a big obstacle. It is because of the fact that higher dispersion gives a higher absorption and lead to a bad data transmission.Technique of Electromagnetically bring on Transp bency (EIT) introduced by researchers give the best solution for the strong absorptions although the EIT does non give 100% transmission. At the kindred conviction, EIT give additional advantages in order to slowed down and stopped the fast speed optical pulses.A solid- call down average is more p tintred in many applications of slow light because it has special properties. In solid- posit strong point, jotic diffusion is non being present, so it allow longer times for optical pulses storage. The slow light techniques play important role in numerous applications for o bject lesson in optical- conversation schema.Theoretical backgroundThe condition of vg Where E is voltaic field and is angular oftenness propagating through a forte. Then the grade of this wave is given by The points of constant phase will play in a velocity ofwhere k is a wavenumber. Then utilize the relation of the phase velocity become. n from the equation is refer to a refrective index and c is the velocity of light in a vacuum. When the field is not considered as monochromatic, that velocity is defined as a group velocity and this velocity shows the gene ration rate of changes in amplitude. Mathematically, group velocity is given by Group velocity besides can be defined as velocity at which elan vital or information is transmitted along a wave. Noticed that group index isthenFrom equation (6), the group index depends on dispersion and slow light can be obtained by making the large.Professor Jakob Khanin and Olga Kocharovskaya were the number 1 physicists that int roduced the theory of EIT. EIT is a phenomenon of quantum optics which permits an mirky medium to be transp arent to the optical light wave while still keeping the strong dispersion needed to generate the slow light. base on Figure 2, when a first light dick is applied to the mirky medium, the light beam is completely absorbed. Then, after a second light is applied to the opaque medium suddenly the medium become transparent.Figure 1 notification when the work beam is off and onThe blood corpuscles experience a free process. When light beam comes in which it resonant with the frequence of the transitions, the atoms will raise up to the higher energy level. This situation describes the normal absorption of light. Then, without any outside(a) radiation, the atom will decay spontaneously and end up in the lower realm. These process depend on the number density of photon,. At first, I study the deuce level nuclear system. From Einstein B coefficients, the transition probabili ty up from ground express 1 to excited state 2 is B12 and from the Einstein A coefficient, the spontaneous transition down from excited state 2 to ground state of 1 is A21. When the light is shining in the system, the number N1 of atom at the state 1 must be equal with the population N2 of atoms in the state 2. Then, the absorption in the system is said to be utter(a) which mean there is no absorption of light beam can be happen. This situation will not be able to kick in the phenomena of EIT from being happen. To make the two level system unbalance, another state is introduce by shining a second light. This is why third level nuclear system is used in the EIT.Figure 2 representation of absorption and spontaneous emissionFigure 3 illustrates the tierce level atomic system interact by a manipulate beam which is a strong optical field and a canvass beam which is a weak optical field. The tercet level system is also called a -system based on its shape. The atoms are consider to have a pair of lower energy states of 1 and 2. The see beam couples states 1 and 3 with frequency while the control beam couples states 2 and 3 with frequency . The two pathway put in and cancel each other and then clear a iniquity states polariton. Dark states means that there is no atoms will be call forth to the excited state and then vanish the absorption of light.Figure 3 representation of three level atomic systemThe expression of one-dimensional susceptibility spectrum explained the how the atomic ensemble respond to the strong resonant control beam and weak probe beam4. The expression is Where be similar to the slackening rate of the coherence, N is the total number of atoms in the sample, is the conflict amid the probe beam frequency and the frequency of the atomic transition (with = 0 show the frequency of the atom-field resonance), is the atom-field coupling constant, and lastly is the Rabi frequency of the control beam. Rabi frequency is defined as , where and E are correspond to the transition atomic number 42 and galvanic field of the control beam.From the susceptibility, the imaginary part,where is the transmission coefficient describes absorptive properties of the medium and L is the length of the medium. Then the real part of the susceptibility,showed the refractive index. Based on figure 4, the various wiggles refractive index whichis green gunstock showed the behaviour of nonlinear when detuning the frequency. The strength of the dispersion which relate to the slow light is determined by the gradient of the refractive index and at the selfsame(prenominal) time the penetrating EIT exhibit the strong nonlinear dispersive behaviour. The sharp peak of the transmission line showed the i rent EIT take place at exact resonance ( ). ). The exact resonance condition achieved when the frequency difference amid the two light beams accurately matches with the frequency separation amongst the two lower states of and .When t he exact resonance condition is not achieved, the interposition is not ideal the medium become absorbing5.Figure 5 represents the prolongation of light in the EIT medium. At the beginning, the light pulse is outside the medium and all the atoms are in the ground states( 1). The front edge of the light pulse then enters the medium and is quickly decelerated. The back edge of the light pulse propagates with vacuum speed c because it is still outside of the medium. Hence, upon incoming into the cell, the spatial extent of the light pulse is compressed by the ratio of , while its peak amplitude remains same. The energy of the light pulse is obviously much smaller when it is inside the medium. The photons are being expended to create the coherence between states of 1 and 2 , or in other words, to flip the atomic wrenchs, with the excess energy carried away by the control field. Then, the wave of flipped spins outright propagates together with the light pulse. The atoms are therefore strongly coupled to the photons which is in the light pulse, with an associated quasiparticle called a dark-state polarition6. The quasiparticle is a combination of photons and spins pique. The spatial extent increases again when the light pulse exits the medium and then the atoms return back to their sure ground state. However, a delayed of the whole light pulse occurred bywhere L is the length of the medium. In the dark-state polarition, when the control field is adiabatically switched off, the coupled excitation is converted into a pure atomic excitation, that is, the probe field is stopped7. Then, to restore the probe field, the control field is switched on. A storage time of more than a second were achieved in this way.Figure 4 Spectrum of transmission and refractive index corresponding to EIT. This figure is taken from abduce 4.. Figure 4 spatial compression occurred when the light beam enters the medium and at the same time the photons are changed into a atomic (spin excitation). This figure is taken from reference 5.In the situation when the decay rate between state and is negligible, the propagation of probe field can be describe by the electric field operator,where the sum is over the free-space of photonic modes with wave vectors k and bosonic operator 8. A collective atomic operators,averaged over small is used to describe the properties of the medium but at position z, macroscopic volumes containing particles 6.Specifically, the operator is used to describes the polarisation of atom oscillating at an optical frequency, whereas the operator decribes a low-frequency spin wave. Assumed that the control field is strong and it is treated classically. The atomic growth is conducted by a set of Heisenberg equation which is , where is represent the atom-field interaction Hamiltonian and 8. From these equations, the probe field is assuming to be weak and that and change in time slowly. To leading order in the probe field , find thatThe H eisenberg equation, draw the evolution of the probe field. A new quantum field which is a superposition principle of photonics and spin-wave components is introduced in order to attained the solution for the equations (15),(16) and (17).Furthermore, the new quantum field, must obeys the equation of motion,The equation (21) describes a shape-preserving propagation with velocity which is proportional to the magnitude of its photonic component.Moreover, EIT is also related to the Stark effect and Zeeman set up9. Stark effects is the splitting of energy level due to the present of electric field while Zeeman effects is the splitting of energy level due to the present of magnetic field. If a volume is considered to be filled by a hydrogen atom and a DC field is applied to the volume, the state will splits into three states with specific energies. By follow the notation ,the states degenerates while the states is generates which the state mixes with the states. The states are Di sscussionIn this section, I will disscuss about the implementation of EIT to observe the slow light and stopping.One of the technique used to induce transparency in opaque medium is Coherent population trapping or CPT. In CPT, the atoms are forced into a coherent superposition of Zeeman or hyperfine states that are strongly coupled to the light via Raman transition10. The definition of superposition state is the state which is stable against absorption from the radiation of electromagnetic field. Generally, CPT is more refer to the plan of two fields closely equal to the Rabi frequency. The experiment done by Alexander et al. 11 used the concept of CPT to observe the slow light in a hot rubidium atoms. The main idea in the experiment is to obtain the narrow EIT resonance.Currently, the quantum mechanics is used for a fundamental of communication and computation. To carry out these ideas, information should be encoded in delicate quantum states, uniform a single-photon states, and then manipulated without being destroyed. It is fact that photons are the fastest and be a simplest carriers of quantum information. However, they are not easy to localize and process.EIT has already had a large impact on the field of optical science. One of the possible application is used in optical buffering for the optical communication. From figure , there is an switch in optical router. The switch can only deal with one data packet at a time 10. So if two data packet arrive at the same time, the hitting of data packet will occur and the overall flow of the data packet will be slow down. Furthermore, the information carried by the two data packet will be loss. To avoid these problems, a slow light medium is activated in one of the branches in order to delay the velocity of data packet. As a result, the flow of information will be speed up because no collision between two data packet happened.References1 G. Heinze, a. Rudolf, F. Beil, and T. Halfmann, repositing of images in atomic coherences in a rare-earth-ion-doped solid, Phys. Rev. A, vol. 81, no. 1, p. 011401, Jan. 2010.2 a. Turukhin, V. Sudarshanam, M. Shahriar, J. Musser, B. Ham, and P. Hemmer, Observation of Ultraslow and Stored Light Pulses in a Solid, Phys. Rev. Lett., vol. 88, no. 2, p. 023602, Dec. 2001.3 L. Hau, S. Harris, Z. Dutton, and C. Behroozi, Light speed reduction to 17 metres per second in an ultracold atomic gas, Nature, vol. 397, no. February, pp. 594598, 1999.4 M. D. Lukin, Colloquium Trapping and manipulating photon states, vol. 75, no. April, 2003.5 M. Lukin and A. Imamolu, Controlling photons using electromagnetically induced transparency, Nature, pp. 273276, 2001.6 M. Fleischhauer and M. Lukin, Dark-State Polaritons in Electromagnetically Induced Transparency, Phys. Rev. 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Lett., vol. 82, no. 26, pp. 52295232, Jun. 1999.12 A. L. G. Robert W. Boyd, Daniel J. Gauthier, Applications of Slow Light in Telecommunications, Opt. Photonics News, vol. 17, no. April, p. 18, 2006.