Quantum dynamics in low dimensional isolated systems Anatoli
- Slides: 32
Quantum dynamics in low dimensional isolated systems. Anatoli Polkovnikov, Boston University Roman Barankov Claudia De Grandi Vladimir Gritsev AFOSR Joint Atomic Physics Colloquium, 02/27/2008
Cold atoms: (controlled and tunable Hamiltonians, isolation from environment) 1. Equilibrium thermodynamics: Quantum simulations of equilibrium condensed matter systems 2. Quantum dynamics: Coherent and incoherent dynamics, integrability, quantum chaos, …
In the continuum this system is equivalent to an integrable Kd. V equation. The solution splits into non-thermalizing solitons Kruskal and Zabusky (1965 ).
Qauntum Newton Craddle. (collisions in 1 D interecating Bose gas – Lieb-Liniger model) T. Kinoshita, T. R. Wenger and D. S. Weiss, Nature 440, 900 – 903 (2006) No thermalization in 1 D. Fast thermalization in 3 D. Quantum analogue of the Fermi-Pasta. Ulam problem.
Cold atoms: (controlled and tunable Hamiltonians, isolation from environment) 1. Equilibrium thermodynamics: Quantum simulations of equilibrium condensed matter systems 2. Quantum dynamics: Coherent and incoherent dynamics, integrability, quantum chaos, … 3. = 1+2 Nonequilibrium thermodynamics?
Adiabatic process. Assume no first order phase transitions. Adiabatic theorem: “Proof”: then
Adiabatic theorem for isolated systems. Integrable systems: density of excitations Alternative (microcanonical) definition: In a cyclic adiabatic process the energy of the system does not change. This implies absence of work done on the system and hence absence of heating. General expectation: EB(0) is the energy of the state adiabatically connected to the state A.
Adiabatic theorem in quantum mechanics Landau Zener process: In the limit 0 transitions between different energy levels are suppressed. This, for example, implies reversibility (no work done) in a cyclic process.
Adiabatic theorem in QM suggests adiabatic theorem in thermodynamics: 1. Transitions are unavoidable in large gapless systems. 2. Phase space available for these transitions decreases with d. Hence expect Is there anything wrong with this picture? Hint: low dimensions. Similar to Landau expansion in the order parameter.
More specific reason. Equilibrium: high density of low-energy states -> • strong quantum or thermal fluctuations, • destruction of the long-range order, • breakdown of mean-field descriptions, Dynamics -> population of the low-energy states due to finite rate -> breakdown of the adiabatic approximation.
This talk: three regimes of response to the slow ramp: A. Mean field (analytic) – high dimensions: B. Non-analytic – low dimensions C. Non-adiabatic – lower dimensions
Example: crossing a QCP. gap t, 0 tuning parameter Gap vanishes at the transition. No true adiabatic limit! How does the number of excitations scale with ? A. P. 2003
Possible breakdown of the Fermi-Golden rule (linear response) scaling due to bunching of bosonic excitations. Bogoliubov Hamiltonian: Hamiltonian of Goldstone modes: superfluids, phonons in solids, (anti)ferromagnets, … In cold atoms: start from free Bose gas and slowly turn on interactions.
Zero temperature regime: Energy Assuming the system thermalizes at a fixed energy
Finite Temperatures d=1, 2 Non-adiabatic regime! d=2; d=1; d=3 Artifact of the quadratic approximation or the real result?
Numerical verification (bosons on a lattice). Nonintegrable model in all spatial dimensions, expect thermalization. Use the fact that quantum fluctuations are weak in the SF phase and expand dynamics in the effective Planck’s constant:
T=0. 02
Thermalization at long times.
2 D, T=0. 2
Another Example: loading 1 D condensate into an optical lattice or merging two 1 D condensates (work in progress with R. Barankov and C. De Grandi) Relevant sine Gordon model:
Results: K=2 corresponds to a SF-IN transition in an infinitesimal lattice (H. P. Büchler, et. al. 2003) K=0 – massive bosons, regime C – K=1 – Tonks regime (free fermions): Strong indications for regime C at finite temperatures.
Expansion of quantum dynamics around classical limit. Classical (saddle point) limit: (i) Newtonian equations for particles, (ii) Gross-Pitaevskii equations for matter waves, (iii) Maxwell equations for classical e/m waves and charged particles, (iv) Bloch equations for classical rotators, etc. Questions: What shall we do with equations of motion? What shall we do with initial conditions? Challenge : How to reconcile exponential complexity of quantum many body systems and power law complexity of classical systems?
Partial answers. Leading order in : equations of motion do not change. Initial conditions are described by a Wigner “probability’’ distribution: G. S. of a harmonic oscillator: Quantum-classical correspondence: ; Semiclassical (truncated Wigner approximation):
Summary of the semiclassical approximation: • Expectation value is substituted by the average over the initial conditions. • Exact for harmonic theories! • Not limited by low temperatures! • Asymptotically exact at short times. Beyond the semiclassical approximation. Quantum jump. Each jump carries an extra factor of 2.
Example (back to FPU problem). m = 10, = 1, = 0. 2, L = 100 Choose initial state corresponding to initial displacement at wave vector k = 2 /L (first excited mode). Follow the energy in the first excited mode as a function of time.
Classical simulation
Classical + semiclassical simulations
Classical + semiclassical simulations
Similar problem with bosons in an optical lattice. Prepare and release a system of bosons from a single site. Little evidence of thermalization in the classical limit. Strong evidence of thermalization in the quantum and semiclassical limits.
Many-site generalization 60 sites, populate each 10 th site.
Conclusions. Three generic regimes of a system response to a slow ramp: A. Mean field (analytic): B. Non-analytic C. Non-adiabatic Many open challenging questions on nonequilibrium quantum dynamics. Cold atoms should be able to provide unique valuable experiments.
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