Max Lewandowski. Axel Pelster. March 12, 2012

Transcription

1 Primordial Models for Dissipative Bose-Einstein Condensates Max Lewandowski Institut für Physik und Astronomie, Universität Potsdam Axel Pelster Hanse-Wissenschaftskolleg, Delmenhorst March 12, 2012

2 Experiment Introduction 87 Rb-BEC of 10 5 particles at T = 80 nk Anisotropic harmonic trap Ω = 2π 170 Hz, Ω = 2π 13 Hz Focussed electron beam of Gaussian shape with width w = FWHM 8 ln 2 = 100 nm H. Ott et al., Nature Phys. 4, 949 (2008)

3 Complex potential Gross-Pitaevskii-equation with complex potential i [ ] t Ψ(r, t) = 2 2M + V R(r) + iv I (r) + g Ψ(r, t) 2 Ψ(r, t) Real part: harmonic trapping V R (r) = 1 2 M [ Ω 2 (x 2 + y 2 ) + Ω 2 z2] ) Imaginary part: Gaussian beam V I (r) = C exp ( x 2 +y 2 2w 2 C = σ tot I 2 e = 1, J, σ 2πw 2 tot = 100a 0, I = 20 na [ ] Non-stationary probability density ρ(x, t) = exp 2 E I(t t 0 ) ρ(x, t 0 )

4 Complex potential square well Complex potential square well Quantization conditions Simplications for primordial model One-dimensional BEC, extension L = R = 640 µm No interaction g = 0 (Schrödinger equation) Square-well approximation of V R and V I Separation into area 1, 2 and 3 V R (x) = { 0, x < L, x > L V I (x) = { C, x < w 0, x > w

5 Quantization conditions Complex potential square well Quantization conditions Dimensionless variables (extension L normalized to π 2 ) ε = E, c = 2 π 2 2M(2L) 2 C, χ = π 2 π 2 2 2M(2L) 2 Experiment: ω = , c = x L, ω = π w 2 L Quantization conditions for symmetric and antisymmetric states 0 = [( ε s cot ω π ) ε ] s + ( ε 2 s + ic tan ω ) ε s + ic 0 = [( ε a cot ω π ) ε ] a ( ε 2 a + ic cot ω ) ε a + ic lim ε = lim ε = c 0 ω 0 m2 ; lim ε = m 2 ic, ω π 2 m N

6 Energies Densities Limit of large dissipation General solutions of quantization conditions ε I non-positive (damping) Correct limit ε(c = 0) = m 2 lim ε I = 0 k-states c lim ε I linear = n-states c n-states most damped Corresponds to ω 0, ω π 2 m N describes limit c = 0, n, k N for c Figure: ω = adjoining k-states (different parity) fuse to one state (same k)

7 Limit of small and big waist Energies Densities Limit of large dissipation Figure: ω = Figure: ω = 1.57 π 2 ω 0: only k-states; antisymmetric states ε a m 2 for all c; ε s tends to ε a ; ε s I reveals deep minimum ω π 2 : only n-states; all states ε m2 ic for all c

8 Interchange of states Introduction Energies Densities Limit of large dissipation Figure: ω = 0.3 Figure: ω = 0.4 } ω = 0.3 : m = 1 n = 1; m = 3 k = 1 ω crit ω = 0.4 : m = 1 k = 1; m = 3 n = 1 Continuous transfer of k- and n-states between ω = 0 and ω = π 2

9 Densities Introduction Energies Densities Limit of large dissipation Lowest energy levels for ω = 0.6 n-states tend to inside (area 2), k-states to outside (area 1, 3) Corresponds to ω = 0, ω = π 2 (areas vanish: states disappear) k-states: no central maximum no symmetric states Three independent wells develop Area 1 and 3 equivalent always 2 k-states fuse

10 Limit of large dissipation Energies Densities Limit of large dissipation n-states: tend to center, strongly damped k-states: tend to borders, less damped } plausible results Ansatz: ε k I = 0 for k-states and εn I = c for n-states for c Limit for real part via quantization conditions (in original variables) E n R = 2 π 2 2M(2w) 2 n2, E k R = 2 π 2 2M(L w) 2 k2 Energies of real square well potentials with the width of each area Two k-states have same energy (area 1 and 3 have equal width)

11 Possible improvements Introduction Possible improvements Taking more dimensions, only two areas ( r < w and r > w) Considering interaction g 0, derive Gross-Pitaevskii-Equation with complex potential via variational ansatz Harmonic trap V R = 1 2 MΩ2 x 2 instead of square well Next order Taylor approximation of V I, harmonic approximation ( V I (x) = C 1 x 2 ) w 2 Actually no upper border for ω Nearly same results for harmonic approximation

12 Harmonic potential Introduction Possible improvements Nearly same results for harmonic approximation Figure: ω = 1.0

13 Possible improvements Possible assignments of numerical solutions

14 Non-hermitian dynamics Possible improvements Non-hermitian Hamilton operator H = 2 d 2 2M + V dx 2 R (x) + iv I (x) Complex energy eigenvalues E = E R + ie I Time evolution exhibits non-stationary probability density [ ] 2 ρ(x, t) = exp E I(t t 0 ) ρ(x, t 0 ) Continuity equation includes drain of probability since V I (x) 0 ρ(x, t) + t x j(x, t) = 2 V I(x)ρ(x, t)