Quantum Quenches in Extended Systems
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1 Quantum Quenches in Extended Systems Spyros Sotiriadis 1 Pasquale Calabrese 2 John Cardy 1,3 1 Oxford University, Rudolf Peierls Centre for Theoretical Physics, Oxford, UK 2 Dipartimento di Fisica Enrico Fermi, Università di Pisa, Pisa, Italy 3 All Souls College, Oxford, UK The Capri Spring School on Transport in Nanostructures Capri, March 29 - April 5, 2009.
2 Outline Introduction
3 Outline Introduction Linearly coupled oscillators (free fields) Simple harmonic oscillator Coupled oscillators
4 Outline Introduction Linearly coupled oscillators (free fields) Simple harmonic oscillator Coupled oscillators Anharmonic coupled oscillators (interacting field theory) Self-consistent approximation Time evolution
5 Outline Introduction Linearly coupled oscillators (free fields) Simple harmonic oscillator Coupled oscillators Anharmonic coupled oscillators (interacting field theory) Self-consistent approximation Time evolution Conclusions - Open Problems
6 Outline Introduction Linearly coupled oscillators (free fields) Simple harmonic oscillator Coupled oscillators Anharmonic coupled oscillators (interacting field theory) Self-consistent approximation Time evolution Conclusions - Open Problems
7 What is a Quantum Quench? A definition An instantaneous change in the parameters that determine the dynamics of an isolated quantum system.
8 What is a Quantum Quench? A definition An instantaneous change in the parameters that determine the dynamics of an isolated quantum system. instantaneous: change done in a time interval much shorter than any characteristic time scale of the model.
9 What is a Quantum Quench? A definition An instantaneous change in the parameters that determine the dynamics of an isolated quantum system. instantaneous: change done in a time interval much shorter than any characteristic time scale of the model. dynamics: a parameter of the hamiltonian of the system.
10 What is a Quantum Quench? A definition An instantaneous change in the parameters that determine the dynamics of an isolated quantum system. instantaneous: change done in a time interval much shorter than any characteristic time scale of the model. dynamics: a parameter of the hamiltonian of the system. isolated: no connection to the environment - the system evolves unitarily (i.e. under quantum mechanical laws).
11 What is a Quantum Quench? A definition An instantaneous change in the parameters that determine the dynamics of an isolated quantum system. instantaneous: change done in a time interval much shorter than any characteristic time scale of the model. dynamics: a parameter of the hamiltonian of the system. isolated: no connection to the environment - the system evolves unitarily (i.e. under quantum mechanical laws). Motivation Simple out-of-equilibrium quantum system.
12 Outline Introduction Linearly coupled oscillators (free fields) Simple harmonic oscillator Coupled oscillators Anharmonic coupled oscillators (interacting field theory) Self-consistent approximation Time evolution Conclusions - Open Problems
13 Simple harmonic oscillator Before (t < 0): H 0 = 1 2 p ω2 0x 2 After (t > 0): H = 1 2 p ω2 x 2
14 Simple harmonic oscillator Before (t < 0): H 0 = 1 2 p ω2 0x 2 ω 0 ω After (t > 0): H = 1 2 p ω2 x 2 Ψ 0 Assume that initially the system lies in the ground state Ψ 0 of H 0. After the quench Ψ 0 acts as a source of excitations.
15 Simple harmonic oscillator Before (t < 0): H 0 = 1 2 p ω2 0x 2 ω 0 ω After (t > 0): H = 1 2 p ω2 x 2 Ψ 0 Assume that initially the system lies in the ground state Ψ 0 of H 0. After the quench Ψ 0 acts as a source of excitations. Heisenberg equations of motion ẍ + ω 2 x = 0
16 Simple harmonic oscillator Before (t < 0): H 0 = 1 2 p ω2 0x 2 ω 0 ω After (t > 0): H = 1 2 p ω2 x 2 Ψ 0 Assume that initially the system lies in the ground state Ψ 0 of H 0. After the quench Ψ 0 acts as a source of excitations. Heisenberg equations of motion ẍ + ω 2 x = 0 Solution sin ωt x(t) = x(0)cos ωt + p(0) ω
17 Use CCR [x, p] = i and the initial expectation values Ψ 0 x 2 (0) Ψ 0 = 1/2ω 0 Ψ 0 p 2 (0) Ψ 0 = ω 0 /2 Ψ 0 x(0)p(0) + p(0)x(0) Ψ 0 = 0
18 Use CCR [x, p] = i and the initial expectation values to find Ψ 0 x 2 (0) Ψ 0 = 1/2ω 0 Ψ 0 p 2 (0) Ψ 0 = ω 0 /2 T {x(t 1 )x(t 2 )} = Ψ 0 x(0)p(0) + p(0)x(0) Ψ 0 = 0 = (ω ω 0) 2 4ω 2 ω 0 cos ω(t 1 t 2 ) + ω2 ω 2 0 4ω 2 ω 0 cos ω(t 1 + t 2 ) + 1 2ω e iω t 1 t 2
19 Use CCR [x, p] = i and the initial expectation values to find Ψ 0 x 2 (0) Ψ 0 = 1/2ω 0 Ψ 0 p 2 (0) Ψ 0 = ω 0 /2 T {x(t 1 )x(t 2 )} = Ψ 0 x(0)p(0) + p(0)x(0) Ψ 0 = 0 = (ω ω 0) 2 4ω 2 ω 0 cos ω(t 1 t 2 ) + ω2 ω 2 0 4ω 2 ω 0 cos ω(t 1 + t 2 ) + 1 2ω e iω t 1 t 2 Feynman part (no quench).
20 Use CCR [x, p] = i and the initial expectation values to find Ψ 0 x 2 (0) Ψ 0 = 1/2ω 0 Ψ 0 p 2 (0) Ψ 0 = ω 0 /2 T {x(t 1 )x(t 2 )} = Ψ 0 x(0)p(0) + p(0)x(0) Ψ 0 = 0 = (ω ω 0) 2 4ω 2 ω 0 cos ω(t 1 t 2 ) + ω2 ω 2 0 4ω 2 ω 0 cos ω(t 1 + t 2 ) + 1 2ω e iω t 1 t 2 Feynman part (no quench). (t 1 + t 2 )-dependent part that breaks time invariance.
21 Linearly coupled oscillators (free fields) H = 1 2 d d r [π 2 + ( φ) 2 + m 2 φ 2 ] = 1 2 πk 2 + ω2 k φ2 k Set of independently evolving momentum modes with ω 2 k = k2 + m 2. k
22 Linearly coupled oscillators (free fields) H = 1 2 d d r [π 2 + ( φ) 2 + m 2 φ 2 ] = 1 2 πk 2 + ω2 k φ2 k Set of independently evolving momentum modes with ω 2 k = k2 + m 2. Quantum quench of the mass (or energy gap) from m 0 to m. k
23 Linearly coupled oscillators (free fields) H = 1 2 d d r [π 2 + ( φ) 2 + m 2 φ 2 ] = 1 2 πk 2 + ω2 k φ2 k Set of independently evolving momentum modes with ω 2 k = k2 + m 2. Quantum quench of the mass (or energy gap) from m 0 to m. The propagator is the Fourier transform of last expression. The (t 1 + t 2 )-dependent part d d k deik rm2 m 2 0 (2π) 4ωk 2ω cos ω k (t 1 + t 2 ) 0k decays for large times as (t 1 + t 2 ) d/2 cos m(t 1 + t 2 ) if m > 0 or even exponentially if m = 0 and d = 3. k
24 The propagator becomes stationary for large times.
25 The propagator becomes stationary for large times. Any local observable tends to a stationary value in the thermodynamic limit due to quantum interference between the infinite set of independent momentum modes.
26 Comparison to the thermal propagator The stationary part of the quench propagator in momentum space is (ω k ω 0k ) 2 4ω 2 k ω 0k cos ω k (t 1 t 2 ) + 1 2ω k e iω k t 1 t 2
27 Comparison to the thermal propagator The stationary part of the quench propagator in momentum space is (ω k ω 0k ) 2 4ω 2 k ω 0k cos ω k (t 1 t 2 ) + 1 2ω k e iω k t 1 t 2 Compare to the thermal or Matsubara propagator in real time 1 ω k (e βω cos ω k k (t 1 t 2 ) + 1 e iω k t 1 t 2 1) 2ω k
28 Comparison to the thermal propagator The stationary part of the quench propagator in momentum space is (ω k ω 0k ) 2 4ω 2 k ω 0k cos ω k (t 1 t 2 ) + 1 2ω k e iω k t 1 t 2 Compare to the thermal or Matsubara propagator in real time 1 ω k (e βω cos ω k k (t 1 t 2 ) + 1 e iω k t 1 t 2 1) 2ω k The two expressions are equal if β satisfies the condition { tanh βω k 4 = ω k /ω 0k if m < m 0 ω 0k /ω k if m > m 0
29 Comparison to the thermal propagator The stationary part of the quench propagator in momentum space is (ω k ω 0k ) 2 4ω 2 k ω 0k cos ω k (t 1 t 2 ) + 1 2ω k e iω k t 1 t 2 Compare to the thermal or Matsubara propagator in real time 1 ω k (e βω cos ω k k (t 1 t 2 ) + 1 e iω k t 1 t 2 1) 2ω k The two expressions are equal if β satisfies the condition { tanh βω k 4 = ω k /ω 0k if m < m 0 ω 0k /ω k if m > m 0 The propagator exhibits thermal-like behaviour (thermalization) with β 4/m 0 for m 0 m, k.
30 The propagator exhibits thermal-like behaviour (thermalization) with β 4/m 0 for m 0 m, k. The effective temperature β eff (k) is momentum dependent - expected for free fields since the different momentum modes thermalize independently. Comparison to the thermal propagator The stationary part of the quench propagator in momentum space is (ω k ω 0k ) 2 4ω 2 k ω 0k cos ω k (t 1 t 2 ) + 1 2ω k e iω k t 1 t 2 Compare to the thermal or Matsubara propagator in real time 1 ω k (e βω cos ω k k (t 1 t 2 ) + 1 e iω k t 1 t 2 1) 2ω k The two expressions are equal if β satisfies the condition { tanh βω k 4 = ω k /ω 0k if m < m 0 ω 0k /ω k if m > m 0
31 Outline Introduction Linearly coupled oscillators (free fields) Simple harmonic oscillator Coupled oscillators Anharmonic coupled oscillators (interacting field theory) Self-consistent approximation Time evolution Conclusions - Open Problems
32 Anharmonic coupled oscillators (interacting field theory) How is this behaviour affected by interactions?
33 Anharmonic coupled oscillators (interacting field theory) How is this behaviour affected by interactions? Rapidly switch on a φ 4 -interaction term at t = 0 as well ( 1 H = d d r 2 π ( φ) m2 φ ) 4 λφ4
34 Anharmonic coupled oscillators (interacting field theory) How is this behaviour affected by interactions? Rapidly switch on a φ 4 -interaction term at t = 0 as well ( 1 H = d d r 2 π ( φ) m2 φ ) 4 λφ4 The 2-point correlation function in the interaction picture is ( ) Ψ 0 T {φ(0, t 1 )φ(r, t 2 )exp i dt H int (t ) } Ψ 0 C
35 Anharmonic coupled oscillators (interacting field theory) How is this behaviour affected by interactions? Rapidly switch on a φ 4 -interaction term at t = 0 as well ( 1 H = d d r 2 π ( φ) m2 φ ) 4 λφ4 The 2-point correlation function in the interaction picture is ( ) Ψ 0 T {φ(0, t 1 )φ(r, t 2 )exp i dt H int (t ) } Ψ 0 ATTENTION! Time integration must be done along the appropriate Schwinger-Keldysh contour C since the interaction is not switched on and off adiabatically as usual in QFT. t = 0 C C t 1 t 2
36 Loop correction and the Hartree-Fock approximation Let s try to apply perturbation theory. First order Feynman diagram: loop diagram (renormalized) r 1,t 1 ( d d (ω0k ω k ) 2 k 4ωk 2ω (m2 0 ) m2 ) 0k 4ωk 2ω cos 2ω k t 0k t' r 2,t 2
37 Loop correction and the Hartree-Fock approximation Let s try to apply perturbation theory. First order Feynman diagram: loop diagram (renormalized) r 1,t 1 ( d d (ω0k ω k ) 2 k 4ωk 2ω (m2 0 ) m2 ) 0k 4ωk 2ω cos 2ω k t 0k t' r 2,t 2 As before the time dependent part decreases with time.
38 Loop correction and the Hartree-Fock approximation Let s try to apply perturbation theory. First order Feynman diagram: loop diagram (renormalized) r 1,t 1 ( d d (ω0k ω k ) 2 k 4ωk 2ω (m2 0 ) m2 ) 0k 4ωk 2ω cos 2ω k t 0k t' r 2,t 2 As before the time dependent part decreases with time. QUESTION: Can we ignore this part for large times?
39 Loop correction and the Hartree-Fock approximation Let s try to apply perturbation theory. First order Feynman diagram: loop diagram (renormalized) r 1,t 1 ( d d (ω0k ω k ) 2 k 4ωk 2ω (m2 0 ) m2 ) 0k 4ωk 2ω cos 2ω k t 0k t' r 2,t 2 As before the time dependent part decreases with time. QUESTION: Can we ignore this part for large times? If yes, what is the effect of a constant loop to the correlation function?
40 Loop correction and the Hartree-Fock approximation Let s try to apply perturbation theory. First order Feynman diagram: loop diagram (renormalized) r 1,t 1 ( d d (ω0k ω k ) 2 k 4ωk 2ω (m2 0 ) m2 ) 0k 4ωk 2ω cos 2ω k t 0k t' r 2,t 2 As before the time dependent part decreases with time. QUESTION: Can we ignore this part for large times? If yes, what is the effect of a constant loop to the correlation function? To keep only the loop and neglect other more complicated diagrams is the Hartree-Fock approximation.
41 Resummation using Dyson equation The 1st order correction to the correlation function assuming a constant loop, increases for large times.
42 Resummation using Dyson equation The 1st order correction to the correlation function assuming a constant loop, increases for large times. Perturbation theory not directly applicable - Resummation of Feynman diagrams required.
43 Resummation using Dyson equation The 1st order correction to the correlation function assuming a constant loop, increases for large times. Perturbation theory not directly applicable - Resummation of Feynman diagrams required. Dyson equation G(k; t 1, t 2 ) = G 0 (k; t 1, t 2 ) + dt G 0 (k; t 1, t)σ(t)g(k; t, t 2 ) C G G t 2 t 2 G t G 2 0 G 0 t = + t 1 t 1 t 1 Σ is the self-energy insertion assumed to tend to a constant as t Σ(t) = λ d d k G(k, t)
44 Result: for large times the correlation function is the same as in the free problem, but with a mass shift to be determined self-consistently m 2 eff = m2 + λ d d k G(m eff ) t
45 Result: for large times the correlation function is the same as in the free problem, but with a mass shift to be determined self-consistently m 2 eff = m2 + λ d d k G(m eff ) t m eff 10 8 d = d = 3 d = λ Figure: Numerical solution to the self-consistency equation for m = 0.
46 Time evolution Alternative application of Hartree-Fock approximation: Substitute the interaction term as φ 4 φ 2 (t) φ 2
47 Time evolution Alternative application of Hartree-Fock approximation: Substitute the interaction term as φ 4 φ 2 (t) φ 2 i.e. the hamiltonian becomes quadratic but with a time dependent mass to be determined self-consistently m 2 eff (t) = m2 + λ d d k φ 2 k (t)
48 Time evolution Alternative application of Hartree-Fock approximation: Substitute the interaction term as φ 4 φ 2 (t) φ 2 i.e. the hamiltonian becomes quadratic but with a time dependent mass to be determined self-consistently m 2 eff (t) = m2 + λ d d k φ 2 k (t) Methods of solution: 1. Quasi-adiabatic approximation: ṁ eff /m 2 eff 1.
49 Time evolution Alternative application of Hartree-Fock approximation: Substitute the interaction term as φ 4 φ 2 (t) φ 2 i.e. the hamiltonian becomes quadratic but with a time dependent mass to be determined self-consistently m 2 eff (t) = m2 + λ d d k φ 2 k (t) Methods of solution: 1. Quasi-adiabatic approximation: ṁ eff /m 2 eff Numerics
50 Time evolution Alternative application of Hartree-Fock approximation: Substitute the interaction term as φ 4 φ 2 (t) φ 2 i.e. the hamiltonian becomes quadratic but with a time dependent mass to be determined self-consistently m 2 eff (t) = m2 + λ d d k φ 2 k (t) Methods of solution: 1. Quasi-adiabatic approximation: ṁ eff /m 2 eff Numerics m eff t
51 Outline Introduction Linearly coupled oscillators (free fields) Simple harmonic oscillator Coupled oscillators Anharmonic coupled oscillators (interacting field theory) Self-consistent approximation Time evolution Conclusions - Open Problems
52 Conclusions
53 Conclusions Effective thermalization after a quantum quench.
54 Conclusions Effective thermalization after a quantum quench. Thermalization also occurs in many other cases: Massless 1d theories (free or interacting). Quench of the speed of sound instead of the mass. Lattice models.
55 Conclusions Effective thermalization after a quantum quench. Thermalization also occurs in many other cases: Massless 1d theories (free or interacting). Quench of the speed of sound instead of the mass. Lattice models. Other problems also studied: Spatially inhomogeneous initial state. Thermal initial state.
56 Conclusions Effective thermalization after a quantum quench. Thermalization also occurs in many other cases: Massless 1d theories (free or interacting). Quench of the speed of sound instead of the mass. Lattice models. Other problems also studied: Spatially inhomogeneous initial state. Thermal initial state. Open Problems
57 Conclusions Effective thermalization after a quantum quench. Thermalization also occurs in many other cases: Massless 1d theories (free or interacting). Quench of the speed of sound instead of the mass. Lattice models. Other problems also studied: Spatially inhomogeneous initial state. Thermal initial state. Open Problems Interacting problem: beyond Hartree-Fock?
58 Conclusions Effective thermalization after a quantum quench. Thermalization also occurs in many other cases: Massless 1d theories (free or interacting). Quench of the speed of sound instead of the mass. Lattice models. Other problems also studied: Spatially inhomogeneous initial state. Thermal initial state. Open Problems Interacting problem: beyond Hartree-Fock? Quantum quench through a critical point?
59 References S. Sotiriadis and J. Cardy. Inhomogeneous quantum quenches. J. Stat. Mech S. Sotiriadis, J. Cardy and P. Calabrese. Quantum quench from a thermal initial state. arxiv: S. Sotiriadis and J. Cardy. Quantum quench in interacting non-integrable models: A self-consistent approximation. (in preparation).
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