An Approximate Solution of the Dynamical Casimir Effect in a Cavity with a Two Level Atom

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1 An Approximate Solution of the Dynamical Casimir Effect in a Cavity with a Two Level Atom arxiv: v [quant-ph] 6 Dec 01 Kazuyuki FUJII and Tatsuo SUZUKI International College of Arts and Sciences Yokohama City University Yokohama, Japan Department of Mathematical Sciences College of Systems Engineering and Science Shibaura Institute of Technology Saitama, Japan Abstract In this paper we treat the so called dynamical Casimir effect in a cavity with a two level atom and give an analytic approximate solution under the general setting. The aim of the paper is to show another approach based on Mathematical Physics to the paper [arxiv : quant-ph)] by A. V. Dodonov and V. V. Dodonov. We believe that our method is simple and beautiful. address : fujii@yokohama-cu.ac.jp address : suzukita@sic.shibaura-it.ac.jp 1

2 Keywords : dynamical Casimir effect; Law s Hamiltonian; detection of photon; two level system of an atom; analytic approximate solution. 1 Introduction In this paper we revisit the so-called dynamical Casimir effect DCE). This means the photon generation from vacuum due to the motionchange) of neutral boundaries, which corresponds to a kind of quantum fluctuation of the electro-magnetic field. This phenomenon is a typical example of interactions between the microscopic and the macroscopic levels and is very fascinating from the point of view of not only pure) Physics but also Mathematical Physics. See for example [1] and its references. Then, how do we detect the photons generated? This is also an important problem. Recently, Dodonov and Dodonov in [] treated this problem by use of a two-level system of an atom in a cavity. They called it the cavity dynamical Casimir effect in the presence of a two-level atom, and gave an effective Hamiltonian by simplifying the Hamiltonian given by Law [3] and adding a two-level system of an atom to it, and constructed an approximate analytical solution. In this paper we treat this problem once more and present another approach to construct an analytic approximate solution under the general setting. We believe that our method is clearer than that of []. Model First of all let us make a brief review of [] within our necessity. In the following we set h = 1 for simplicity. i) Cavity DCE [3], [1]). For this we take the simplest Hamiltonian which is the special case of Law [3] namely, ǫx,t) = ǫt)) H DCE = ωt)a a+iχt) { a ) a } 1)

3 where ωt) is a periodic function depending on the cavity form and χt) is given by χt) = 1 dωt) 4ωt) dt = 1 d 4dt log ωt), and a and a are the cavity photon annihilation and creation operators respectively. Therefore, the physics that we are treating is two-photon generation processes from the vacuum state. ii) Detection [4], [5]). For this we take a two-level system of an atom inserted in the cavity and the Rabi Hamiltonian as interaction H D = Ω σ 3 1+gσ + +σ ) a+a ) = Ω σ 3 1+gσ + a+σ + a +σ a+σ a ) ) where 1 is the identity operator on the Fock space generated by {a,a,n a a} and σ 3 = 1 0, σ + = 0 1, σ = 0 0, 1 = However, to solve this system is very hard. In fact, our aim is to obtain some analytic approximate solution. Therefore, as a rule we replace the Rabi Hamiltonian with the Jaynes- Cummings one H D = Ω σ 3 1+gσ + a+σ a ). 3) Here, let us note the well known su) relations [1/)σ 3,σ + ] = σ +, [1/)σ 3,σ ] = σ, [σ +,σ ] = 1/)σ 3. iii) Total System. By adding 3) to 1) we treat the effective) Hamiltonian like H = Ht) = H DCE + H D = ωt)1 N +iχt)1 { a ) a } + Ω σ 3 1+gσ + a+σ a ). 4) Our aim is to solve the Schrödinger equation i d Ψt) = H Ψt) = Ht) Ψt) 5) dt 3

4 under the general setting. If H is time independent, then the general formal) solution is given by Ψt) = e ith Ψ0). To calculate e ith exactly is another problem which is in general very hard). However, in our case H is time dependent, so solving 5) becomes increasingly difficult. We must give a further approximation to the Hamiltonian. iv) Interaction Picture. From here let us change to the interaction picture. Namely, for V = Vt) we set Φt) = V Ψt) Ψt) = V Φt). 6) Then it is easy to see that the equation 5) can be changed to This is in general called the interaction picture. [] i d dt Φt) = V HV iv d dt V ) Φt). 7) In the following we impose some restrictions on the model. Namely, we take ωt) as in ωt) = ω 0 1+ǫsinηt)) where ω 0, ǫ and η are real constants. We assume that ω 0 > 0, 0 < ǫ 1 and η is large enough. Then ωt) ω 0 and, and here we take V as χt) = V = Vt) = e itη 4 σ 3 e itη N = ǫη cosηt) 41+ǫsinηt)) ǫη 4 cosηt) e itη 4 itη N e itη 4 itη N. 8) Some calculation by use of 8) gives Ĥt) V HV iv d dt V = ω 0 η )1 N +i ǫη 8 1 { a ) a +e iηt a ) e iηt a } + Ω η/ σ 3 1+gσ + a+σ a ) 4

5 and we can also use the rotating wave approximation e iηt 0 when η is large enough. As a result we finally obtain the time-independent Hamiltonian see []) like Ĥ = ω 0 η ) 1 N +i ǫη 8 1 { a ) a } + Ω η/ σ 3 1+gσ + a+σ a ). 9) v) Our Target. The aim of this paper is to solve the equation i d Φt) = Ĥ Φt) 10) dt under the general setting and the formal solution is given by Φt) = e itĥ Φ0). 11) Therefore, what we must do in the following is to calculate the term e itĥ explicitly. Before closing this section we must present an important problem. Problem We used approximation a kind of rotating wave approximation) two times. Make an adaptive range or region) of the model clear. 3 Approximate Solution In this section we try to calculate the term e itĥ in 11). For simplicity we set itĥ = X +Y where X = ita, A = ω 0 η ) 1 N +i ǫη 8 1 { a ) a }, Y = itb, B = Ω η/ σ 3 1+gσ + a+σ a ). 1) For our purpose the Zassenhaus formula is useful : Zassenhaus Formula We have an expansion e X+Y = e 1 6 {[[X,Y],Y]+[[X,Y],X]} e 1 [X,Y] e Y e X. 5

6 Note that the formula is a bit different from that of [6]. In this paper we use e X+Y e 1 [X,Y] e Y e X = e t [A,B] e itb e ita. 13) Therefore, let us calculate each term in the following. [I] First, we calculate e ita. If we set α = ω 0 η ), β = 1 ǫη 14) 4 then A in 1) becomes A = α 1 N +i β 1 { a ) a } = 1 {α 1 N +iβ 1 a ) 1 a )}. Here, we use a well known Lie algebraic method, see for example [7]. Namely, if we set K + = 1 a ), K = 1 a, K 3 = 1 N + 1 ) 15) it is easy to see both K + = K, K 3 = K 3 and the su1,1) relations [K 3,K + ] = K +, [K 3,K ] = K, [K +,K ] = K 3 16) by use of the relation [a,a ] = 1. Then, A in 1) can be written as A = 1 = { 1 } 4 α+αk 3 +iβk + K ) 1 4 α+αk 3 +iβk + K ) 1 4 α+αk 3 +iβk + K ) and from this we have e ita = eit 4 α e it{αk 3+iβK + K )} Therefore, we have only to calculate the term e it 4 α e it{αk 3+iβK + K )}. Ut) e it{αk 3+iβK + K )} = e itαk 3+tβK + K ). 6

7 For the purpose we want to look for the following disentangling form Ut) = e ft)k + e gt)k 3 e ht)k 17) with functions ft), gt), ht) f0) = g0) = h0) = 0) 1. The result is as follows. ft) = cosh t α gt) = log cosh ht) = β α 4 +β sinh 4 +β )+ t β α ) t α 4 +β i α α α 4 +β cosh t )+ α 4 +β 4 +β sinh sinh 4 ) +β + t α i α α 4 +β sinh ), t α 4 +β i α sinh α 4 +β ) 4 +β The derivation is analogous to that of [8]. See also the appendix. t α 4 +β ) = ft). t α 4 +β ), 18) [II] Second, we calculate e itb. The result is well known and is given by e itb = exp it = Ω η/ ga ga cost ϕ+g iδ Ω η/ sint ϕ+g ϕ+g ig sint ϕ+g ϕ+g a ig sint ϕ ϕ a cost ϕ+ iδ sint ϕ ϕ 19) where we have set for simplicity. δ Ω η, ϕ δ 4 +g N 1 this is a standard method 7

8 [III] Third, we calculate e t [A,B]. From 1) simple calculation gives where we have set for simplicity. Note that The result is [A,B] = g α σ + a+σ a ) igβσ + a +σ a) 0 α = g a iβa α a iβa 0 g 0 D 0) D 0 e t [A,B] = D = α a+iβa, D = α a iβa ) α [D,D ] = 4 β 1. ) cos g t DD ) 1 sin D g t D D D D 1 sin DD cos ) g t DD D ) g t D D. 1) [IV] As a result, our approximate solution to the equation 5) is given by Ψt) e t [A,B] e itb e ita Ψ0) ) under any initial value Ψ0). If we write Ût) e t [A,B] e itb e ita = U 11 U 1 U 1 U 3) 8

9 then each component is given by ) U 11 = e it 4 {cos α g t DD cost ϕ+g iδ sint ) ϕ+g ϕ+g ) 1 ig sin g t DD DD D sint ϕ ϕ a }e ft)k + e gt)k 3 e ht)k, ) U 1 = e it 4 { igcos α g t DD sint ϕ+g a + ϕ+g ) 1 sin g t DD D cost DD ϕ+ iδ sint ϕ )}e ft)k + e gt)k 3 e ht)k, ϕ { U 1 = e it 4 α 1 D D sin g t D D )D cost ϕ+g iδ sint ) ϕ+g ϕ+g ) igcos g t sint ϕ D D a }e ft)k + e gt)k 3 e ht)k, ϕ ) U = e it 4 {ig α 1 D D sin g t D D This is our main result. D sint ϕ+g a + ϕ+g ) cos g t D D cost ϕ+ iδ sint ϕ )}e ft)k + e gt)k 3 e ht)k. ϕ 4 Closing Remarks In this paper we treated the model by A. V. Dodonov and V. V. Dodonov and presented another approach based on Mathematical Physics and obtained some analytic approximate solution. Details with applications and developments will be published separately. We have neglected Dissipation for example, the Cavity loss) in this paper, which is not realistic. If we take dissipation into consideration the model will become very complicated. For example, see our papers [8], [9], [10] and [11], [1] thelast two are highly recommended). Then, to obtain an analytic approximate solution will become increasingly difficult. Further study and new ideas are needed. Appendix 9

10 A note is added. In the text we must calculate Ut) = e it{αk 3+iβK + K )} = e itαk 3+tβK + K ). From this we have the differential equation d dt Ut) = { iαk 3 +βk + K )}Ut). On the other hand, under the ansatz 17) : Ut) = e ft)k + e gt)k 3 e ht)k ) some calculation gives d { ) } dt Ut) = f ġf +ḣe g f )K + + ġ ḣe g f K 3 + ḣe g)k Ut) where we have used f = df, etc for simplicity. Therefore, by comparing two equations we obtain dt f ġf +ḣe g f = β ġ ḣe g f = iα ḣe g = β = f ġf βf = β ġ +βf = iα ḣe g = β = f +iαf +βf = β ġ +βf = iα ḣ = βe g. The equation f +iαf +βf = β is a famous) Riccati equation of general type. If we can solve the equation we have the solutions like ft) = gt) = ht). See 18) as these solutions. References [1] V. V. Dodonov : Current status of the Dynamical Casimir Effect, Physica Scripta, 8 010), arxiv : [quant-ph]. [] A. V. Dodonov and V. V. Dodonov : Approximate analytical results on the cavity Casimir effect in the presence of a two level atom, Phys. Rev. A 85 01), arxiv : [quant-ph]. 10

11 [3] C. K. Law : Effective Hamiltonian for the radiation in a cavity with a moving mirror and a time varying dielectric medium, Phys. Rev. A ), 309. [4] W. P. Schleich : Quantum Optics in Phase Space, WILEY VCH, Berlin, 001. [5] E. T. Jaynes and F. W. Cummings : Comparison of Quantum and Semiclassical Radiation Theories with Applications to the Beam Maser, Proc. IEEE, ), 89. [6] C. Zachos : Crib Notes on Campbell-Baker-Hausdorff expansions, unpublished, 1999, see [7] K. Fujii : Introduction to Coherent States and Quantum Information Theory, quant-ph/ [8] K. Fujii : Quantum Damped Harmonic Oscillator, A book-chapter of Quantum Mechanics, Paul Bracken Ed.), ISBN , InTech, arxiv: [quant-ph]. [9] R. Endo, K. Fujii and T. Suzuki : General Solution of the Quantum Damped Harmonic Oscillator, Int. J. Geom. Meth. Mod. Phys, 5 008), 653, arxiv : [quant-ph]. [10] K. Fujii and T. Suzuki : General Solution of the Quantum Damped Harmonic Oscillator II : Some Examples, Int. J. Geom. Meth. Mod. Phys, 6 009), 5, arxiv : [quant-ph]. [11] K. Fujii and T. Suzuki : An Approximate Solution of the Jaynes Cummings Model with Dissipation, Int. J. Geom. Methods Mod. Phys, 8 011), 1799, arxiv : [math-ph]. [1] K. Fujii and T. Suzuki : An Approximate Solution of the Jaynes Cummings Model with Dissipation II : Another Approach, Int. J. Geom. Methods Mod. Phys, 9 01), , arxiv : [math-ph]. 11