The Remarkable Bose-Hubbard Dimer

Transcription

1 The Remarkable Bose-Hubbard Dimer David K. Campbell, Boston University Winter School, August 2015 Strongly Coupled Field Theories for Condensed Matter and Quantum Information Theory International Institute of Physics, Natal = 0.5 =1.5 =5

2 Prolog: Axioms for Lecturers Kac Axiom: Tell the truth, nothing but the truth, but NOT the whole truth. Weisskopf Axiom It is better to uncover a little than to cover a lot. But we all tend to follow the Rubbia lemma: TTN: What was the remark? Faster

3 Bottom Lines Temporal decoherence of BEC is critical for potential applications. For a simple system (BEC dimer ~ Bosonic Josephson Junction=BJJ) we show that coherence is enhanced near fixed points of a classical analog system, including self-trapped fixed points but is supressed near separatrix We do this by introducing global phase space (GPS) portraits of quantum observables including 1) the condensate fraction ; and 2) entanglement. We compare and contrast standard Gross-Pitaevski equation (GPE) (mean field) with Bose-Hubbard model (fully quantum) and show that much of the observed behavior can be understood by studying the phase space of a related classical nonlinear dynamical system, BUT We predict novel quantum effects, possibly observable in near-term experiments: 1) introducing dissipation (atom loss), we find surprising result that coherence and entanglement can be enhanced by dissipation; and 2) we predict quantum tunneling between selftrapped fixed points in the system

4 Outline The Big Picture Ultra-cold atoms (Bose-Einstein Condensates and Degenerate Fermions) key experimental breakthroughs Some comments on BECs Specific Background for talk BECs in mean field description: continuum Gross-Pitaevski Eqn BECs in Optical Lattices: discrete GPE ~ Discrete Nonlinear Schrödinger Eqn. BEC dimer= Bose Josephson Junction (BJJ) Comparison of mean field results with experiments The Bose Hubbard Model and quantum effects Global Phase space study of quantum vs. mean field without and with dissipation Tunneling between self-trapped regions in the BH dimer Summary and Conclusions References

5 810 nk Ultracold atoms: Fermions vs. Bosons 510 nk 240 nk TTN: Which is which? 7 Li Bosons 6 Li Fermions NB: T c for BEC and T F for Fermions depend on densities, masses, interactions. Typically scales as 1/M, for 87 Rb atoms T c ~ 300 nk. Figure from Truscott, A., K. Strecker, W. McAlexander, G. Partridge, and R. G. Hulet, Science 291, (2001).

6 Bosons vs. Fermions Figure from Duchon PhD thesis

7 Key Experimental Breakthroughs Optical force on neutral atoms via AC Stark effect (induced polarization): can change sign of force by using red-or bluedetuning of laser from dipole resonance. Magneto-optical traps Optical molasses means of slowing velocity (=cooling) atoms (Nobel Prize 1997) Evaporative cooling Controlling size and sign of interactions via Feshbach resonances Together methods can lower effective temperatures to nanokelvin levels and offer exquisite control of interactions: Good short summary in B. demarco and D.S. Jin, Science (1999). Review in I. Bloch et al., Rev Mod Phys 80, 885 (2008).

8 Background: BEC Bose Einstein condensation Macroscopic occupancy of single quantum state by large number of identical bosons Predicted in 1925 by Einstein for non-interacting bosons Observed in 1995 by two groups, Wieman/Cornell and Ketterle: Nobel Prize in 2001

9 Einstein s Original Manuscript

10 Modeling BEC BEC is described by a quantum wavefunction Φ(x,t). In the context of cold atomic vapors trapped in an external potential (typically combined magnetic confinement and optical potential) in mean field approximation Φ(x,t) obeys the Gross-Pitaevskii equation (GPE) Nonlinear dynamicists recognize this immediately as a variant of the Nonlinear Schrödinger equation, here in 3D and with an external potential more on this later. The interaction term g 0 describes s-wave scattering of atoms In 1D case (realizable depending on external potential) we expect (continuum) solitons among excitations.

11 BECs in Optical Lattice Counter-propagating laser pulses create standing wave that interacts via AC Stark effect with neutral atoms, so that the BEC experiences a periodic potential of the form: U L (x,y) is transverse confining potential, λ is the laser wavelength (typically ~850 nm) and z is the direction of motion. Image from I. Bloch Nature (2008)

12 BECs in Optical Lattices Quantum system in periodic potential just as in solid state. Can describe in terms of (extended) Bloch wave functions or (localized) Wannier wave functions, centered on the wells of the potential (but allowing tunneling between the wells Expanding the GPE in terms of Wannier functions leads to the Discrete Nonlinear Schrodinger Equation (DNLSE) for the wave function amplitudes. In normalization we shall later use Comes from Hamiltonian (λ=2u/j)

13 BEC Dimer: Physical Realizations and Models Single coherent BEC in a double well potential or two internal (hyperfine) states with resonant interactions in single well. Analogous to two-site dimer system and therefore called BEC dimer but also to Josephson Junction therefore called Bosonic Josephson Junction (BJJ). Parameters: number of particles in BEC (N) and in each of two wells /states (N 1 and N 2 ), z= N 1 -N 2, phase difference between two condensates ( ϕ = ϕ 1 ϕ 2 ), resonant coupling/tunneling between two wells (J), interaction of Bosons within condensate (U); single parameter in simple models Λ = (NU)/J Two Models: Gross-Pitaevskii equation (GPE): corresponds to integrable classical dynamical system* in (z, ϕ) for this case Bose-Hubbard Hamiltonian (BHH) : fully quantum * * NB: Quantum dimer also integrable by Bethe Ansatz: solution not useful for calculating physical observables

14 Different Physical realizations of Dimer Two-well optical lattice M. Albiez et al.,phys. Rev. Lett. 95, (2005) Two internal atomic states ( 87 Rb) T. Zibold et al. Phys. Rev. Lett. 105, (2010)

15 Mean-Field (GPE) Eqn for Dimer Hamiltonian becomes Operators replaced with c-numbers Let Ψ j = b j e iθj, where b j and θ j are real. Define z = [b 1 2 b 2 2 ] / N = [N 1 N 2 ] / N, ϕ = θ 2 θ 1 The Hamiltonian becomes

16 Equivalent Nonlinear Dynamical System TTN: What kind of dynamical system is this? Answer: a simple one degree of freedom (=>integrable) dynamical like a pendulum but with length dependent on its momentum. Hamilton s equations become z = 1 z 2 sinϕ ϕ = Λz + zcosϕ 1 z 2

17 Equivalent Nonlinear Dynamical System Fixed Points? Work out on blackboard Further analysis of FPs shows for Λ < 1, two stable FPs, (z,φ)= (0,0) and (0,π) then a bifurcation at Λ = 1 and for Λ > 1, three stable FPs, (z,φ)=(0,0), (z +, π), and (z -, π) z +/- = +/- (Λ 2 1) 1/2 /Λ +/- 1 for Λ =>

18 Images of Global Phase Space Λ = 0.5 Λ= 1.5 Λ=5

19 Comparison to Experiments! Next slides show how dynamics of the dimer/bjj observed in experiments follows (or does not follow) classical phase portraits of this system.

20 Experimental Observations Bifurcation at Λ = 1 T. Zibold et al. Phys. Rev. Lett. 105, (2010)

21 Experimental Observations Existence of Z c for self-trapping: work out on board Z below Z c (Λ) Z above Z c (Λ) M. Albiez et al, Phys. Rev. Lett (2005)

22 Experimental Observations Here Z c ~ ½ => Λ~15 M. Albiez et al, Phys. Rev. Lett (2005)

23 Experimental Observations Full Global Phase Space plots NB: Each point is different experiment! T. Zibold et al., PRL (2010)

24 Experimental Observations View on Bloch sphere T. Zibold et al. Phys. Rev. Lett. 105, (2010)

25 Full Quantum Dynamics Bose-Hubbard Model hopping interaction (repulsive in our case) Atoms in a double-well optical trap Two spin states of atoms trapped in one well Observables of interest: Single-particle density matrix (Condensate Fraction/ purity) Entanglement measure

26 Observables and Results We study the time evolution of quantum observables: discuss two today 1) condensate fraction C(z,ϕ) λ max /N 1, λ max = largest eigenvalue of single particle density matrix 2) entanglement E <a 1* a 2 > 2 -<a 1* a1a 2* a 2 >, E > 0 for entangled state Results Both C(z,ϕ) and E generally track classical orbits in GPS C(z,ϕ) decreases dramatically near classical separatrix, remains large at classical fixed point and especially in self-trapped region=> regular motion enhances coherence, chaotic motion destroys coherence Symmetry z => - z of classical equations is broken by quantum dynamics Ridge of enhanced coherence exists in quantum but not in LD or GPE (classical) or dynamics Overlap of initial coherent state with eigenfunctions of BHH is minimal along separatrix, high near fixed points

27 Classical phase space, quantum dynamics H. Hennig et al., PRA 86, (R) (2012)

28 Condensate Fraction at T=1 sec Λ = 1.5 Note considerable variation in C(z,ϕ,T=1) and how it generally follows contours of classical dynamics Constant energy contours Separatrix Orbits around FP (0,0): have <z>=<ϕ>=0 z=>-z symmetry broken Orbits around FP+ and FP- are macroscopic self-trapped states, <z> 0, <ϕ>=π π-phase orbits still exist, <z>=0, <ϕ>=π

29 Results for C of Dimer Λ = 5.0 GPE= classical gives C=1 Separatrix, C falls rapidly nearby: decoherence enhanced ϕ = 0.85 Separatrix, C falls, then recovers as FP(0,0) controls dynamics ϕ = 0.0 Ridge of increased C, pure quantum effect

30 Initial State overlap with Eigenfunctions Color code shows A(ϕ,z) max n <ϕ,z E n >, where E n is an eigenstate of the BH dimer Λ = 5.0: Λ = 1.5 Conclusion: localization in phase space (FPs) maintains coherence, delocalization (separatrix) induces decoherence.

31 What do quantum orbits really look like? Large Λ, near fixed point, green is classical orbit

32 Dissipationless Quantum Dynamics TTN: What do you see? ANS: Two frequencies

33 Quantum orbits, interesting features thick orbits two frequencies in EPR, condensate fraction

34 Explain two frequencies: High frequency High frequency is mean-field (semiclassical) Conversion to seconds Data from power spectrum of EPR:

35 Low-frequency oscillations Low frequency quantum revival In the limit J/U = 0, for any state and : But revivals seen also for J/U > 0 (or, Λ < ) Greiner et al., Nature (2002)

36 Summary: Dynamics near the FP High frequency is mean-field (semiclassical) Low frequency is a quantum revival: But how far from the fixed point is this a good picture?

37 Low-Λ anomaly: Husimi density TTN: What do you notice for Λ~ 1? ANS: Looks like quantum tunneling between STFPs!!

38 Projections as Interpretation of Two Frequencies Energy eigenstate expansion: Projection onto most important states: Three eigenstates produce beats in observables: Replacing the full state with the projection should work well where,

39 How informative is the projection? 2 states: only fast motion 3 states: fast and slow motion

40 How informative is the projection?

41 How informative is the projection?

42 How informative is the projection? For z = 0.95, Φ = π the contributions of three highest eigenstates are a 0 = , a 1 = , a 2 = so that a a a 2 2 = And eigenfrequency differences fit with observed frequencies in numerics almost to within 3 %

43 Dissipation in a BEC dimer Focused electron beam removes atoms from one of the wells Gericke et al., Nature Phys. 4, 949 (2008) Open quantum system: quantum jump method Dalibard et al., PRL 68(5) 580 (1992) Garraway and Knight, PRA 49(2) 1266 (1994) Witthaut et al., PRA 83(6) 1 (2011)

44 Dissipation-induced coherence

45 Dissipative Quantum Dynamics Λ = 5

46 Results: phase space TTN: What did dissipation do? Ans: Drove system near fixed point!

47 Results: condensate fraction

48 Results: EPR entanglement

49 What about tunneling for Λ ~ 1? Classical approximation poor for Λ close to 1; can we observe quantum tunneling in experiment???

50 Summary of Main Results Thus far Results for 1) condensate fraction C(z,ϕ) λ max /N 1, λ max = largest eigenvalue of single particle density matrix and entanglement E <a 1* a 2 > 2 -<a 1* a1a 2* a 2 >, E > 0 for entangled state Both C(z,ϕ) and E generally track classical orbits in GPS C(z,ϕ) decreases dramatically near classical separatrix, remains large at classical fixed point and especially in self-trapped region=> regular motion enhances coherence, chaotic motion destroys coherence Overlap of initial coherent state with eigenfunctions of BHH is minimal along separatrix, high near fixed points Quantum orbits show two frequencies: one semiclassical, the other quantum revival ; interpretation in terms of three leading eigenfrequencies Dissipation can enhance coherence for both C and E Predict tunneling between self-trapped FPs for Λ ~ 1 + ε

51 Husimi function vs Time Behavior of Husimi function vs. time: tunneling between selftrapped fixed points is clearly seen: Λ = 1.1, J= 10 Hz

52 Interpretation? Near the self-trapped fixed points (STFP) we have seen that only two states could explain most of dynamics: does this work here? YES! Energy difference between highest two states in BH model corresponds very closely to tunneling frequency: think of symmetric and anti-symmetric states in simple 1D QM. Can work this out in more detail with semi-classical approach to tunneling: full details in T. Pudlik et al. Phys.Rev A 90, (2014).

53 Probability of states vs. NB

54 Tunneling frequency vs. NB

55 Observation in experiment? Prediction of tunneling frequency vs Λ for experimental parameters of T. Zibold et al. Phys Rev Lett (2010) (U= 2π (x) 0.063Hz, N=500

56 The Dimer and beyond For dimer, key challenge is experimental observation of tunneling between self-trapped fixed points. For spatially extended systems, Intrinsic Localized Modes / Discrete Breathers play role of self-trapped states: may serve as means of maintain quantum coherence in large optical lattices or macromolecules.

57 ILMs/DBs in Spatially extended systems Results of Ng et al (Geisel group) confirm for extended systems (M ~ 300) that DBs are not formed at small Λ (=λn) but are readily formed at large Λ. Calculations are within the mean field/gpe model Fig 2 from Ng et al., New J. Phys 11, (2009).

58 Bottom Lines Redux In BEC dimer=bjj coherence effects (condensate fraction., entanglement) are enhanced near fixed points of a classical analog system, including self-trapped fixed points but are suppressed near separatrix. Introducing dissipation (atom loss), we find surprising result that coherence and entanglement can be enhanced! We predict quantum tunneling between self-trapped fixed points in the system. We speculate that similar effects may occur in extended optical lattices due for formation of Intrinsic Localized Modes/Discrete Breathers.

59 Questions?

60 References Excellent review article on theory and experiments, I. Bloch et al. Rev. Mod. Phys. 80, (2008). H. Hennig, D. Witthaut, and DKC, Global phase space of coherence and entanglement in a double-well Bose- Einstein Condensate, Phys. Rev. A 86, (R) (2012). T. Pudlik, H. Hennig, D. Witthaut, and DKC Dynamics of entanglement in a dissipative Bose-Hubbard Dimer, Phys. Rev. A 88, (2013). T. Pudlik, H. Hennig, D. Witthaut, and DKC Tunneling in the self-trapped regime of a two-well Bose-Einstein condensate Phys. Rev. A 90, (2014)

61 END