(Dynamical) quantum typicality: What is it and what are its
Transcription
(Dynamical) quantum typicality: What is it and what are its
(Dynamical) quantum typicality: What is it and what are its physical and computational implications ? Jochen Gemmer University of Osnabrück, Kassel, May 13th, 2014 J.Gemmer quantum typicality Outline Thermal relaxation in closed quantum systems? Typicality in a nutshell Numerical experiment: model, observables and results Typicality in formulas Spin transport in the Heisenberg chain Eigenstate thermalization hypothesis J.Gemmer quantum typicality Thermal relaxation in closed quantum systems? Why it should occur: We see it in system we assume to be closed. Why it should not occur: There are issues with the underlying theory: Quantum Mechanics (Non-eq.) Thermodynamics autonomous dynamics of a few macrovariables attractive fixed point, equilibrium often describable by master equations, Fokker-Planck equations, stochastic processes, etc. autonomous dynamics of the wave function (number of parameters: insane) no attractive fixed point (Schroedinger equation) Schroedinger equation is no rate equation Quantum systems that explicitly exhibit relaxation but are not of the “small system + large bath” type appear to be rare in the literature. To cut it short: Why and how do two cups of coffee thermalize each other? J.Gemmer quantum typicality Typicality in a nutshell The naive view on relaxation i.e. 2nd law of thermodynamcis: QM: ρˆ0 = |ψihψ| evolves into 1 − Hˆ 1ˆ ˆ e kT or ρˆeq ≈ δ( H − E) Z Z ρˆeq = problem: invariance of Von Neuman-entropy traditional cure: open quantum systems ⇒ this requires: environment special structure of structure of environment: large, broad band (oscillator) bath adequate weak coupling, applicability of projection techniques specific initial states: factorizing, thermal bath..... etc. Typicality: ρˆ0 = |ψihψ| but does not evolve into ˆ hψ|A(t)|ψi ρˆeq = 1ˆ ˆ 1 − Hˆ e kT , ρˆeq = δ( H − E) Z Z evolves into ˆ ≈ Tr{ˆ ρeq A} ˆ ............ Can this be true? for many (all?) A J.Gemmer quantum typicality Numerical experiment: model and observables R L spin-model Heisenberg-type Hamiltonian: A ladder with anisotropic, XXZ -type couplings which are strong along the legs and weak along the rungs: X ˆ = Jij (ˆ σxi σ ˆxj + σ ˆyi σ ˆyj + 0.6 σ ˆzi σ ˆzj ), H ij Jij = 1 for solid lines, Jij = κ = 0.2 for dotted lines and Jij = 0 otherwise. Total number of spins: NP= 32. The z-component of total magnetization Sz = i σ ˆzi is conserved We analyze: “magnetization difference” xˆ ! X l X r σ ˆz σ ˆz − xˆ = l ∈L r ∈R eigenvalues of xˆ within the subspace of vanishing total magnetization, Sz = 0: X = −16, −14, ....., +16. J.Gemmer quantum typicality Numerical experiment: results (Phys. Rev. E, 89, 042113, (2014) xˆ: z-magnetization difference between legs PX (t): probability to find a certain X H. de Raedt, K. Michielsen (Juelich)) time shifted hˆ x (t)i <x(t)> 12 1 0.8 0.6 <x(0)>=2 <x(0)>=4 <x(0)>=6 <x(0)>=8 <x(0)>=10 <x(0)>=12 8 4 0.4 100 Px(t) 0.2 80 0 15 0 0 60 10 5 40 0 -5 -15 0 |ψX (0)i = e t 100 150 ˆ x P(S ˆ z = 0)|ωi, P (remark: taking this picture took 6h on 65 000 CPU’s. Thanks to: J.Gemmer 12 σ ² (t) data from solving the Schroedinger equation (N = 32) for two pure, partially random initial states: ˆ2 −αH 50 variances of x 20 -10 x t 8 <x(0)>=0 <x(0)>=2 <x(0)>=4 <x(0)>=6 <x(0)>=8 <x(0)>=10 <x(0)>=12 typical variance 4 00 quantum typicality 50 t 100 150 Typicality in formulas (Phys. Rev. Lett., 102, 110403 (2009), Phys. Rev. Lett., 86, 1927 (2001)) “static typicality” ˆ < A >:= Tr{A}/d : expectation value of the maximally mixed state |ωi uniform random states sampled according to the unitary invariant measure ˆ HA[hω|A|ωi] =< A > ˆ HV[hω|A(t)|ωi] = 1 < A2 > − < A >2 d +1 In a high dimensional Hilbert space almost all possible states feature very similar expectation values for observables with bound spectra. ⇒ It is no surprise to find these expectation values eventually. “dynamical typicality” p ˆ → B(t) ˆ |ψi := ρˆd |ωi: “taylored”, non-uniform random states, A ˆ ˆ ρ} HA[hψ|B(t)|ψi] = Tr{B(t)ˆ ˆ HV[hψ|B(t)|ψi] ≤ ˆ 2 (t)ˆ Tr{B ρ} deff deff : inverse of the largest eigenvalue of ρˆ, interpreted as “effective dimension” ⇒ Very many different pure states exhibit very similar dynamics of expectation values J.Gemmer quantum typicality Typicality in formulas “correlation functions” p |ψi := ρˆd |ωi: “taylored”, non-uniform random states, ˆ →C ˆ (t) := 1 (B(t) ˆ B ˆ +B ˆ B(t)) ˆ A 2 ˆ (t)|ψi] = Tr{C ˆ (t)ˆ HA[hψ|C ρ} ˆ (t)|ψi] ≤ HV[hψ|C ˆ 2 (t)ˆ Tr{C ρ} deff What to do practically? ˆ Compute |ψ(t)i := e −i Ht/~ p ˆ ˆ ρ} hψ(t)|B|ψ(t)i ≈ Tr{B(t)ˆ ˆ ˆ ρˆd |ωi , |ψ ′ (t)i := e −i Ht/~ B p ρˆd|ωi ˆ ′ (t)i] ≈ Tr{ 1 (B(t) ˆ B+ ˆ B ˆ B(t))ˆ ˆ Re[hψ(t)|B|ψ ρ} 2 ˆ ¯ 2 ˆ ¯ 2 ˆ ˆ If ρˆ is of “exponential form”, e.g., ρˆ ∝ e −β(H−E ) −α(B−B) or ρˆ ∝ e −β H−αB than everything can be done based on “pure state propagation” J.Gemmer quantum typicality Spin transport in the anisotropic Heisenberg chain 2 (a) L=33 (β=0) norm (a.u.) 0.05 0 (a) ∆=1.0 (β=0) -1 [34,36] [t1,t2] 0 100 tJ -2 10 -1 (b) ∆=1.5 (β=0) 10 -2 10 [20,21] 0.05 [35] 0 1/30 1/20 1/10 1/L 2 -3 10 tJ 20 0.1 0.05 0 J.Gemmer odd fit (β=0) 10 0 even (b) Drude weight (β=0) 0 C(t1,t2) / J C(t) / J 2 L ∆=0.5 ∆=1.0 ∆=1.5 2 L=18 (ED) L=18 L=20 L=22 L=24 L=26 L=28 L=30 (k=0) L=32 (k=0) L=33 (k=0) tDMRG C(t1,t2) / J C(t) / J 2 10 ∆=0.5 ∆=1.0 ∆=1.5 0.1 C(t) / J Linear response ⇒ conductivity from current autocorrelation function ˆ → Jˆ here: infinite temperature, i.e., ρˆ ∝ ˆ 1, B ˆ J} ˆ C (t) ∝ Tr{J(t) (PRL., 112, 120601, (2014) ) 0 quantum typicality Bethe Ansatz lower bound ED, fit RK, L=30 RK, L→∞ tDMRG 1 0.5 anisotropy ∆ 1.5 Eigenstate thermalization hypothesis (ETH) ˆ that are close in energy feature ETH: Eigenstates of some Hamiltonian H ˆ that are close to each other. expectation values of some observable B ˆ ˆ En ≈ Em → hn|B|ni ≈ hm|B|mi Jochen’s formulation: “Eigenstates belong to the set of typical states” If ETH applies: - Expecation values from microncanonical ensembles are close to expecation values of individual eigenstates - Initial state independent equilibration: X ˆ ρnm Bmn e i (En −Em ) t hB(t)i = n,m If the oscillating terms behave like “white noise” for τ large enough ˆ )i = hB(τ X ETH ρnn Bnn ≈ Bnn n But does ETH apply? Have to check by exact diagonalization, or....... J.Gemmer quantum typicality ETH check for the Heisenberg ladder (Phys. Rev. Lett., 112, 130403, (2014)) ˆ := xˆ2 : width of the magnetization distribution A X 2 ˆ ˆ ¯ := ρnn Ann ≈ hψ|A|ψi ρˆ ∝ e −α(H−U) A n ¯ 2 := Σ +A 2 X ˆ ′ (τ )i] ≈ Re[hψ(τ )|A|ψ ρnn A2nn n (a) L=7 (d) ED 3 2 Ann 6 Ann L=7 1 J|| 0 J 0 0.03 0.02 -1/2 (deff) L=10 0.01 0 ETH applies! L A -4 L=9 L=8 (c) L=10 3 0 6 t1 0 (b) L=9 3 6 180 Σ 0 t J|| 0 3 γ(t) Ann 6 -2 U / J|| 0 A±Σ d=2 2 J.Gemmer L 2L ˆ was a random Σ scales approx like H matrix. quantum typicality The End more people involved in this: R. Steinigeweg, A. Khodja, H. Niemeyer, D. Schmidtke, C. Gogolin .... Thank you for your attention! The talk itself as well as the mentioned papers may be found on our webpage. J.Gemmer quantum typicality