slow heating, effective hamiltonians, prethermalization€¦ · dynamics is described by a...
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Slow heating, effective Hamiltonians, prethermalization
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Kapitza pendulum
Rapidly oscillating suspension point: “inverted” pendulum
Periodic driving à new states, not possible in static systems
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Realization of topological states by driving
Driving à topological Bloch bands Bands with non-zero Chern number (recently measured) Many theory works: Floquet topological insulators, fractional Chern insulators, SPTs Oka, Aoki’09, Lindner, Refael, Galitski’10, Kitagawa, Rudner, Berg, Demler’10, Caysoll, Moessner’12,
Rudner et al’13, Neupert, Grushin’14, many others
Chern number
BUT: Theory mostly limited to single-particle physics This talk: Driven many-body systems
Jotzu et al’14, Aidelsburger et al’14
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Driven many-body systems
NO! Driven MBL systems do not heat up (at high enough driving frequency) Is this the fate of all driven systems?
-Many-body systems heat up to infinite T -- a challenge for “Floquet engineering”
How rapid is heating in driven lattice ergodic systems? -At fast driving, heating is exponentially slow (theorem for spins and fermions)
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Floquet operator F = T exp− i H (t)dt0
T
∫
H (t +T ) = H (t)
F = exp[−iHeffT ]Conventional approach: (low order) Magnus expansion in
Conventional approach: Magnus expansion
T = 1ν
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Floquet operator
Magnus expansion à effective, time-independent Hamiltonian
F = T exp− i H (t)dt0
T
∫
H (t +T ) = H (t)
F = exp[−iHeffT ]
Heff = Heff(0) +Heff
(1) +Heff(2) +...
Heff(0) =
1T
H (t)dt0
T
∫ Heff(1) =
12T
dt1 dt2[H (t1),H (t2 )]0
t1
∫0
T
∫ ,.....
Conventional approach: (low order) Magnus expansion in
Conventional approach: Magnus expansion
T = 1ν
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Floquet operator
Magnus expansion à effective, time-independent Hamiltonian BUT: known to converge only for bounded Hamiltonians PROBLEM: BREAKS DOWN IN MANY-BODY SYSTEMS
F = T exp− i H (t)dt0
T
∫
H (t +T ) = H (t)
F = exp[−iHeffT ]
Heff = Heff(0) +Heff
(1) +Heff(2) +...
Heff(0) =
1T
H (t)dt0
T
∫ Heff(1) =
12T
dt1 dt2[H (t1),H (t2 )]0
t1
∫0
T
∫ ,.....
H (t) T < π
Conventional approach: (low order) Magnus expansion in
Conventional approach: Magnus expansion
T = 1ν
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Floquet operator
Magnus expansion à effective, time-independent Hamiltonian BUT: known to converge only for bounded Hamiltonians PROBLEM: BREAKS DOWN IN MANY-BODY SYSTEMS
F = T exp− i H (t)dt0
T
∫
H (t +T ) = H (t)
F = exp[−iHeffT ]
Heff = Heff(0) +Heff
(1) +Heff(2) +...
Heff(0) =
1T
H (t)dt0
T
∫ Heff(1) =
12T
dt1 dt2[H (t1),H (t2 )]0
t1
∫0
T
∫ ,.....
H (t) T < π
Conventional approach: (low order) Magnus expansion in
Conventional approach: Magnus expansion
T = 1νValidity of Magnus expansion in many-body
systems? Asymptotic expansion? Optimal order of expansion? Rigorous results?
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Heating in lattice systems: qualitative considerations H (t) = H + gV cosωtDriven many-body lattice system
ω >> t,UH = t ci+ci+1 + h.c.+U
i∑ nini+1
i∑
ε(k)
k
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Heating in lattice systems: qualitative considerations H (t) = H + gV cosωtDriven many-body lattice system
1 photon absorbed à create e-h pairs
ω >> t,U
N ~ ωt>>1
H = t ci+ci+1 + h.c.+U
i∑ nini+1
i∑
ε(k)
k
ε(k)
k
ω
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Heating in lattice systems: qualitative considerations H (t) = H + gV cosωtDriven many-body lattice system
1 photon absorbed à create e-h pairs Seems difficult, starting from ground state at weak interaction (V is local!)
ω >> t,U
N ~ ωt>>1
H = t ci+ci+1 + h.c.+U
i∑ nini+1
i∑
ε(k)
k
ε(k)
k
ω
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Heating in lattice systems: qualitative considerations H (t) = H + gV cosωtDriven many-body lattice system
1 photon absorbed à create e-h pairs Seems difficult, starting from ground state at weak interaction (V is local!)
ω >> t,U
N ~ ωt>>1
H = t ci+ci+1 + h.c.+U
i∑ nini+1
i∑
ε(k)
k
ε(k)
k
ω
E.g.: Doublon decay in large-U Hubbard model is slow (exp+perturbation th.)
Strohmaier et al’10; Sensarma et al’10
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General bound for the heating rate
Heating rate (golden rule) Γβ (ω)∝ g
2 e−βEη η ' V ηη,η '∑
2δ(ω − (Eη ' −Eη ))
H = hii∑ V = Vi
i∑
Driven many-body lattice system H (t) = H + gV cosωtLocality:
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General bound for the heating rate
Heating rate (golden rule)
Theorem:
Γβ (ω)∝ g2 e−βEη η ' V ηη,η '∑
2δ(ω − (Eη ' −Eη ))
Γβ (ω) ≤C1 exp −ω
C2 hi
$
%&&
'
())
H = hii∑ V = Vi
i∑
Fundamental reason: locality of quantum dynamics
Driven many-body lattice system H (t) = H + gV cosωtLocality:
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General bound for the heating rate
Heating rate (golden rule)
Theorem:
Γβ (ω)∝ g2 e−βEη η ' V ηη,η '∑
2δ(ω − (Eη ' −Eη ))
Γβ (ω) ≤C1 exp −ω
C2 hi
$
%&&
'
())
H = hii∑
EXPONENTIALLY SLOW HEATING AT FAST DRIVING
V = Vii∑
Fundamental reason: locality of quantum dynamics
Driven many-body lattice system H (t) = H + gV cosωtLocality:
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General bound for the heating rate
Heating rate (golden rule)
Theorem:
Γβ (ω)∝ g2 e−βEη η ' V ηη,η '∑
2δ(ω − (Eη ' −Eη ))
Γβ (ω) ≤C1 exp −ω
C2 hi
$
%&&
'
())
H = hii∑
EXPONENTIALLY SLOW HEATING AT FAST DRIVING General: holds for any initial state and for strong interactions
V = Vii∑
Fundamental reason: locality of quantum dynamics
Driven many-body lattice system H (t) = H + gV cosωtLocality:
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The idea of the proof 1) Consider the case of a local operator V. Rewrite
A(ω) ~ µ V ηη,µ∑
2δ(ω − (Eµ −Eη )) =
µ [V,H ] ηω 2
η,µ∑
2
δ(ω − (Eµ −Eη )) =
=µ [V,H,H,...,H ] η
ω 2kη,µ∑
2
δ(ω − (Eµ −Eη ))
k commutators
2) Use locality of H to bound the commutators
[V,H,H,...,H ] <Ckk!3) Choose optimal 4) Bound: 5) For global driving, use Lieb-Robinson bounds to estimate “cross-terms” involving with large
k* ≈ωCe
A(ω)< e−κω κ =2ce
Vi,Vj | i− j |
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General bound for the heating rate
Heating rate (golden rule)
Theorem:
Γβ (ω)∝ g2 e−βEη η ' V ηη,η '∑
2δ(ω − (Eη ' −Eη ))
Γβ (ω) ≤C1 exp −ω
C2 hi
$
%&&
'
())
H = hii∑
EXPONENTIALLY SLOW HEATING AT FAST DRIVING General: holds for any initial state and for strong interactions
V = Vii∑
Fundamental reason: locality of quantum dynamics
Driven many-body lattice system H (t) = H + gV cosωtLocality:
DA, Huveneers, De Roeck, arXiv:1507.01474
Result can be made fully non-perturbative (in driving strength and not only in interaction!) à Controlled version of high-frequency expansion
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Driven many-body systems: non-perturbative results
Very long heating times What governs dynamics at intermediate times?
τ * ~ eCω
t ≤ τ *
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Driven many-body systems: non-perturbative results
Very long heating times What governs dynamics at intermediate times?
τ * ~ eCω
t ≤ τ *
Dynamics is described by a quasi-conserved effective Hamiltonian
H* = H0 +1ωH1 +
1ω 2 H2 +...+
1ω n Hn n ~ Cω >>1
Conservation breaks down only at exponentially long times t ~ τ *
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Driven many-body systems: non-perturbative results
Very long heating times What governs dynamics at intermediate times?
τ * ~ eCω
t ≤ τ *
H* = H0 +1ωH1 +
1ω 2 H2 +...+
1ω n Hn n ~ Cω >>1
Conservation breaks down only at exponentially long times t ~ τ *
t
Prethermalization
Heating to
0 τ *
T =∞
Dynamics is described by a quasi-conserved effective Hamiltonian
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Driven many-body systems: non-perturbative results
Very long heating times What governs dynamics at intermediate times?
τ * ~ eCω
t ≤ τ *
H* = H0 +1ωH1 +
1ω 2 H2 +...+
1ω n Hn n ~ Cω >>1
Conservation breaks down only at exponentially long times t ~ τ *
t
Prethermalization
Heating to
0 τ *
T =∞
Applies to: driven systems of spins and fermions with local interactions
Rapidly driven many-body systems show a long prethermalization regime
Dynamics is described by a quasi-conserved effective Hamiltonian
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Driven many-body systems: idea of the approach
ψ(t) = Q̂(t) ϕ(t)1)“Gauge transformation”: Q̂+Q̂ = IQ̂(t +T ) = Q̂(t)
Stroboscopic evolution not affected, but Hamiltonian changed:
H '(t) = Q̂+H (t)Q̂− iQ̂+ dQ̂dt
(1)
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Driven many-body systems: idea of the approach
ψ(t) = Q̂(t) ϕ(t)1)“Gauge transformation”: Q̂+Q̂ = IQ̂(t +T ) = Q̂(t)
Stroboscopic evolution not affected, but Hamiltonian changed:
H '(t) = Q̂+H (t)Q̂− iQ̂+ dQ̂dt
2) Goal: choose to minimize the driving term Result: Using locality, can find for that decreases driving term exponentially, by 3) Obtain the quasi-conserved quantity using (1)
Q̂
Q̂ = eTΩ1+T2Ω2+..+T
nΩn
e−Cω
H*
n ~ Cω
(1)
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Numerics on finite-size systems
Kick protocol: UF = e−iH0te−iH1t
Err
or, e
xpan
sion
ord
er n
H0 = hi∑ si
ziz + Jsi
zizsi+1
ziz
H1 = Jsizixsi+1
x
i∑t -driving period
-Confirmed that this is an asymptotic expansion
Prosen model ‘97
n* =Cω The bound is saturated
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Implications
1) Broad prethermalization regime in driven many-body systems 2) Optimal order of the Magnus expansion 3) “Floquet topological insulators” can be very long-lived 4) Suggests ways of experimental preparation of “Floquet fractional Chern insulators” and other correlated states in driven systems OTHER APPLICATIONS: -Bounds of heating in bosonic lattice systems -Some rigorous results about quasi-adiabatic behavior in Floquet systems
n* ~ Cω
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Part II: MBL in periodically driven systems
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Kicked rotor: localization in a periodically driven system
U = e−iH0T0e−2iVFloquet operator H0 =12∂2
∂ϑ 2 V = K cosϑ
rotor kick
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Kicked rotor: localization in a periodically driven system
U = e−iH0T0e−2iVFloquet operator H0 =12∂2
∂ϑ 2 V = K cosϑ
rotor kick
Mapping onto an effective Anderson model
On-site energy
U ψ = e−iϑ ψ
tan EnT0 −ϑ2
"
#$
%
&' Hopping operator tnm = n tan V̂( ) mtnm
λnλm
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Kicked rotor: localization in a periodically driven system
U = e−iH0T0e−2iVFloquet operator H0 =12∂2
∂ϑ 2 V = K cosϑ
rotor kick
Mapping onto an effective Anderson model
On-site energy
U ψ = e−iϑ ψ
tan EnT0 −ϑ2
"
#$
%
&' Hopping operator tnm = n tan V̂( ) mtnm
λnλm
Quasi-random à dynamical localization! Hopping decays with | n−m |
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Driven many-body systems: ergodic vs MBL
L spins +local driving V̂ U = e−iH0T0e−2iVMapping onto an effective hopping problem; sites=eigenstates of H0
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Driven many-body systems: ergodic vs MBL
L spins +local driving V̂ U = e−iH0T0e−2iVMapping onto an effective hopping problem; sites=eigenstates of H0
ergodic
Hopping
Effective level spacing
Δ ~ 1D~ 12L
tnm ~1D~ 12L/2
MBL
Δ ~ 12L
tnm ~e−L/ξ
2L
H0 H0
(from ETH)
tnm >> ΔNonlocal hopping delocalization
tnm << Δ localization
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Periodically driven many-body localized systems
1) Construct Floquet Hamiltonian iteratively (different from Magnus!) Show convergence and MBL at high driving frequency 2) Argue delocalization at low driving frequency
H (t) = HMBL + gV (t) many-body localized HMBL
ω driving frequency
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Periodically driven many-body localized systems
1) Construct Floquet Hamiltonian iteratively (different from Magnus!) Show convergence and MBL at high driving frequency 2) Argue delocalization at low driving frequency 3) Establish phase diagram (fixed disorder, interactions, and g)
H (t) = HMBL + gV (t) many-body localized HMBL
ω driving frequency
ω
Delocalized
MBL
0 ωc
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Periodically driven many-body localized systems
1) Construct Floquet Hamiltonian iteratively (different from Magnus!) Show convergence and MBL at high driving frequency 2) Argue delocalization at low driving frequency 3) Establish phase diagram (fixed disorder, interactions, and g) 4) Driving: probe MBL (little/no heating), access MBL-ergodic transition
H (t) = HMBL + gV (t) many-body localized HMBL
ω driving frequency
ω
Delocalized
MBL
0 ωc
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Driven MBL systems: numerical results
1) Spectral statistics: transition Between Poisson and Wigner-Dyson 2) Area-law for eigenstates in MBL phase 3) Log-growth of entanglement 4) Constructed a set of local integrals of motion
Kicked spin chain
Confirms analytical theory
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Iterative procedure and its convergence
i ddtU(t) = H (t)U(t)Evolution operator:
Decompose U(t) = P(t)e−iHeff t P(t +T ) = P(t)
P+(t) H (t)− i ddt
"
#$
%
&'P(t) = Heff
Solve for iteratively, gradually eliminating time-dependent terms Perturbation theory in
P(t)
gν<<1 g2
νW<<1
Quasi-local Floquet Hamiltonian, which itself is MBL
Convergence criteria
Higher orders: combinatorics equivalent to time-independent MBL problem
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Low frequency: delocalization via Landau-Zener transitions
Change
Consider instantaneous eigenstates of
H (λ) = HMBL + gV (λ)
λ =tT
Multi-level Landau-Zener problem
Diabatic crossing: not dangerous “Intermediate” crossings: the state get mixed, delocalization
At low frequency, many intermediate/adiabatic crossings à delocalization DA, De Roeck, Huveneers, arXiv:1412.4752 See also: Khemani, Nandkishore, Sondhi’14 (local ramp)