Aurelian Isar - Decoherence And Transition From Quantum To Classical In Open Quantum Systems

Decoherence and transition from quantum
to classical in open quantum systems

Aurelian Isar

Department of Theoretical Physics
National Institute of Physics and Nuclear Engineering
Bucharest-Magurele, Romania
isar@theory.nipne.ro

Faculty of Science and Mathematics, Nis, 18 October 2011

Content

- theory of OQS based on q. dyn. semigs (Lindblad)
- master eq. for h.o. interacting with an environment (thermal
bath)
¨
- Schrodinger gen-zed uncertainty f.
- q. and thermal ﬂuctuations
- q. decoherence (QD) and degree of QD
- decoherence time
- summary

Aurelian Isar Decoherence and transition from quantum to classical in ope

Introduction

- quantum - classical transition and classicality of q. ss - among
the most interesting problems in many ﬁelds of physics
- 2 conditions - essential for classicality of a q. s.:
a) quantum decoherence (QD)
b) classical correlations (CC): Wigner f. has a peak which
follows the classical eqs. of motion in phase space with a good
degree of approx. (q. state becomes peaked along a class.
trajectory)
- Classicality: emergent property of OQSs (both main features
– QD and CC – strongly depend on the interaction between s.
and its external E)
- necessity and sufﬁciency of both QD and CC as conditions of
classicality - subject of debate
- they do not have an universal character (not nec. for all
physical models)


Content of the talk

- Theory of OQS (q. dynamical semigroups)
- Partic. case: h.o.
- QD and CC for a h. o. interacting with an E (thermal bath) in
the framework of the theory of OQS
- degree of QD and CC and the possibility of simultaneous
realization of QD and CC
- true quantum - classical transition takes place (classicality -
temporary phenomenon)
- tdeco - of the same scale with time when q. and thermal
ﬂuctuations become comparable
- summary and further development (q. ﬁdelity - in the context
of CV approach to QIT)


Open systems

- the simplest dynamics for an OS which describes an
irreversible process: semigroup of transformations introducing
a preferred direction in time (characteristics for dissipative
processes)
- in Lindblad axiomatic formalism of introducing dissipation in
quantum mechanics, the usual von Neumann-Liouville eq.
ruling the time evolution of closed q. ss is replaced by the
following Markovian master eq. for the density operator ρ(t) in
¨
the Schrodinger rep.:

d Φt (ρ)
= L(Φt (ρ))
dt


Lindblad theory (1)

- Φt - the dynamical semigroup describing the irreversible time
evolution of the open system and L is the inﬁnitesimal generator
of Φt
- fundamental properties are fulﬁlled (positivity, unitarity,
Hermiticity)
- the semigroup dynamics of the density operator which must
hold for a quantum Markov process is valid only for the
weak-coupling regime, with the damping λ typically obeying the
inequality λ ω0 , where ω0 is the lowest frequency typical of
reversible motions


Lindblad theory of OQS

- Lindblad axiomatic formalism is based on quantum
dynamical semigroups ( complete positivity property is fulﬁlled)
- irreversible time evolution of an open system is described by
the following general q. Markovian master equation for the
density operator ρ(t):

d ρ(t) i 1
= − [H, ρ(t)] + ([Vj ρ(t), Vj† ] + [Vj , ρ(t)Vj† ])
dt 2
j

- H - Hamiltonian of the system
- Vj , Vj† - operators on the Hilbert space of H (they model the
environment)


Master equation for damped h.o.

- V1 and V2 - linear polynomials in q and p (equations of motion
as close as possible to the classical ones) and H - general
quadratic form

µ 1 2 mω 2 2
H = H0 + (qp + pq), H0 = p + q
2 2m 2

dρ i
= − [H0 , ρ]
dt
i i
− (λ + µ)[q, ρp + pρ] + (λ − µ)[p, ρq + qρ]
2 2
Dpp Dqq Dpq
− 2 [q, [q, ρ]] − 2 [p, [p, ρ]] + 2 ([q, [p, ρ]] + [p, [q, ρ]])


Diffusion and dissipation coeffs
- fundamental constraints Dpp > 0, Dqq > 0,

2 λ2 2
Dpp Dqq − Dpq ≥
4

- when the asymptotic state is a Gibbs state
H0 H0
ρG (∞) = e− kT /Tre− kT ,

λ+µ ω λ−µ ω
Dpp = mω coth , Dqq = coth ,
2 2kT 2 mω 2kT
ω
Dpq = 0, (λ2 − µ2 ) coth2 ≥ λ2 , λ > µ
2kT
- fundamental constraint is a necessary condition for the
generalized uncertainty relation
2
2
σqq (t)σpp (t) − σpq (t) ≥
4

Evolution Eq. in coordinate rep.

∂ρ i ∂2 ∂2 imω 2 2
= ( 2− )ρ − (q − q 2 )ρ
∂t 2m ∂q ∂q 2 2
1 ∂ ∂
− (λ + µ)(q − q )( − )ρ
2 ∂q ∂q
1 ∂ ∂
+ (λ − µ)[(q + q )( + ) + 2]ρ
2 ∂q ∂q
Dpp ∂ ∂ 2
− 2
(q − q )2 ρ + Dqq ( + ) ρ
∂q ∂q
∂ ∂
−2iDpq (q − q )( + )ρ
∂q ∂q


Fokker-Planck Eq. for Wigner f.

∂W p ∂W ∂W
=− + mω 2 q
∂t m ∂q ∂p
∂ ∂
+(λ + µ) (pW ) + (λ − µ) (qW )
∂p ∂q
∂2W ∂2W ∂2W
+Dpp + Dqq + 2Dpq
∂p 2 ∂q 2 ∂p∂q


Physical signification

- first two terms generate a purely unitary evolution (usual
Liouvillian evolution)
- third and forth terms - dissipative (damping effect: exchange
of energy with environment)
- last three terms: noise (diffusive) (fluctuation effects)
- Dpp : diffusion in p + generates decoherence in q: it reduces
the off-diagonal terms, responsible for correlations between
spatially separated pieces of the wave packet
- Dqq : diffusion in q + generates decoherence in p
- Dpq : ”anomalous diffusion” term - does not generate
decoherence)


Initial Gaussian wave function

- correlated coherent state (CCS) or squeezed CS (special
class of pure states, which realizes equality in generalized
uncertainty relation)

1 1
Ψ(q) = ( )4
2πσqq (0)

1 2i i
× exp[− (1 − σpq (0))(q − σq (0))2 + σp (0)q],
4σqq (0)
δ mω r
σqq (0) = , σpp (0) = 2)
, σpq (0) = √
2mω 2δ(1 − r 2 1 − r2


Parameters and variances

- δ - squeezing parameter (measures the spread in the initial
Gaussian packet), r , |r | < 1 - correlation coefﬁcient at time
t =0
- for δ = 1, r = 0 CCS - red Glauber coherent state - σqq and
σpp denote the dispersion (variance) of the coordinate and
momentum, respectively, and σpq denotes the correlation
(covariance) of the coordinate and momentum
- in the case of a thermal bath
ω mω ω
σqq (∞) = coth , σpp (∞) = coth ,
2mω 2kT 2 2kT
σpq (∞) = 0


Density matrix

1 1 1 q+q
< q|ρ(t)|q >= ( ) 2 exp[− ( − σq (t))2
2πσqq (t) 2σqq (t) 2

σ(t) iσpq (t) q + q
− 2σ
(q − q )2 + ( − σq (t))(q − q )
2 qq (t) σqq (t) 2
i
+ σp (t)(q − q )] − general Gaussian form

- thermal bath, t → ∞ ( stationary solution)

mω 1 mω (q + q )2
< q|ρ(∞)|q >= ( ) 2 exp{− [
π coth 4 coth

+(q − q )2 coth ]}, ≡ ω/2kT


Quantum decoherence (QD)

- irreversible, uncontrollable and persistent formation of q.
correlations ( entanglement) of the s. with its environment
(interference between different states are negligible - decay
(damping) of off-diagonal elements representing coherences
between q. states below a certain level, so that density matrix
becomes approximately diagonal)
- strongly depends on the interaction between s. and
environment (an isolated s. has unitary evolution and
coherences of states are not lost – pure states evolve in time
only to pure states)


Role of QD

- an isolated system has an unitary evolution and the
coherence of the state is not lost – pure states evolve in time
only to pure states
- loss of coherence can be achieved by introducing an
interaction between the system and environment: an initial pure
state with a density matrix (containing nonzero off-diagonal
terms) can non-unitarily evolve into a ﬁnal mixed state with an
approx. diagonal density matrix
- in QI processing and computation we are interested in
understanding the speciﬁc causes of QD: to prevent
decoherence from damaging q. states and to protect the
information stored in these states from the degrading effect of
the interaction with the environment


Degree of quantum decoherence

Σ = (q + q )/2, ∆ = q − q ,
1 σ(t) σpq (t)
α = 2σqq (t) , γ = 2 2 σqq (t) , β = σqq (t)

α
ρ(Σ, ∆, t) = exp[−αΣ2 − γ∆2 + iβΣ∆]
π

(for zero initial mean values of q and p)
- representation-independent measure of the degree of QD :
√
ratio of the dispersion 1/ 2γ of the off-diagonal element to the
dispersion 2/α of the diagonal element

δQD (t) = (1/2) α/γ = /2 σ(t)


¨
Schrodinger gen-zed uncert. f.

2
σ(t) ≡ σqq (t)σpp (t) − σpq (t)

2 1
σ(t) = {e−4λt [1 − (δ + ) coth + coth2 ]
4 δ(1 − r 2 )
1 ω 2 − µ2 cos(2Ωt)
+e−2λt coth [(δ + − 2 coth )
δ(1 − r 2 ) Ω2
1 µ sin(2Ωt) 2r µω(1 − cos(2Ωt))
+(δ − 2)
) + √ ]
δ(1 − r Ω Ω2 1 − r 2
+ coth2 }
- underdamped case (ω > µ, Ω2 ≡ ω 2 − µ2 )


Limit of long times

2
σ(∞) = coth2 ,
4
ω
δQD (∞) = tanh ,
2kT
- high T :
ω
δQD (∞) =
2kT


Discussion of QD (1)

- QD increases with t and T , i.e. the density matrix becomes
more and more diagonal and the contributions of the
off-diagonal elements get smaller and smaller
- the degree of purity decreases and the degree of mixedness
increases with t and T
- for T = 0 the asymptotic (ﬁnal) state is pure and δQD reaches
its initial maximum value 1
- a pure state undergoing unitary evolution is highly coherent: it
does not lose its coherence, i.e. off-diagonal coherences never
vanish and there is no QD


Discussion of QD (2)

- the considered system interacting with the thermal bath
manifests QD - dissipation promotes quantum coherences,
whereas fluctuation ( diffusion) reduces coherences and
promotes QD; the balance of dissipation and fluctuation
determines the final equilibrium value of δQD
- the quantum system starts as a pure state (Gaussian form)
and this state remains Gaussian, but becomes a quantum
mixed state during the irreversible process of QD


Decoherence time scale

- in the case of a thermal bath
2 ω
tdeco = , ≡
(λ + µ)mωσqq (0) coth 2kT

where we have taken (q − q )2 of the order of the initial
dispersion in coordinate σqq (0)
- the decoherence time scale tdeco is very much shorter than
the relaxation time → in the macroscopic domain QD occurs
very much faster than relaxation
- tdeco is of the same order as the time when thermal
ﬂuctuations overcome q. ﬂuctuations


Q. and thermal fluctuations

- when t trel ≈ λ−1 (relaxation time, which governs the rate
of energy dissipation), the particle reaches equilibrium with the
environment
- σ(t) is insensitive to λ, µ, δ and r and approaches
2
σ BE = 4 coth2 ( Bose-Einstein relation for a system of bosons
in equilibrium at temperature T )
2
- in the case of T = 0, σ0 = 4 - q. Heisenberg relation (limit
of pure q. fluctuations)
- at high T (T ω/k), σ MB = ( kT )2 - Maxwell - Boltzmann
ω
distribution for a s. approaching a classical limit (limit of pure
thermal fluctuations)


Figures (1)

1
0.75 3
∆QD
0.5
0.25 2.5
0
0 2 C
2
4 1.5
t 6
8

Figure: δQD on T (C ≡ coth ω/2kT ) and t
(λ = 0.2, µ = 0.1, δ = 4, r = 0).


Figures (2)
a
-4

-2

0 q

2

4
0.2
Ρ
0.1
0
-4 -2 0 2 4
q’

Figure: ρ in q− representation (λ = 0.2, µ = 0.1, δ = 4, r = 0) at
t = 0.


Figures (3)
b
-4

-2

0 q

2

0.3 4

Ρ 0.2
0.1
0
-4 -2 0 2 4
q’

t → ∞ and C = 3.


Figures (4)
c

-5

0 q

5

0.3
Ρ 0.2
0.1
0
-5 0 5
q’

t → ∞ and C = 20.


Classical correlations

classical correlations - the s. should have, with a good approx.,
an evolution according to classical laws: this implies that the
Wigner f. has a peak along a classical trajectory (there exist CC
between the canonical variables of coordinate and momentum)
- of course, the correlation between the canonical variables,
necessary to obtain a classical limit, should not violate
Heisenberg uncertainty principle, i.e. the position and
momentum should take reasonably sharp values, to a degree in
concordance with the uncertainty principle.


General Gaussian Wigner f.

- most gen. mixed squeezed states described by Wigner f. of
Gauss. form with 5 real parameters

1 1
W (p, q) = √ exp{− [σpp (q − σq )2 + σqq (p − σp )2
2π σ 2σ

−2σpq (q − σq )(p − σp )]}
- for σ > 2 /4 → mixed quantum states
- for σ = 2 /4 → pure correlated coherent states


Summary

- we have studied QD with the Markovian Lindblad Eq. for an
one-dimensional h. o. in interaction with a thermal bath in the
framework of the theory of OQS based on q. dyn. semigs
- the s. manifests a QD which increases with t and T , i.e. the
density matrix becomes more and more diagonal at higher T
(loss of q. coherence); at the same time the degree of purity
decreases and the degree of mixedness increases with T
¨
- q. and thermal ﬂuctuations in gen-zed Schrodinger
uncertainty f.


Aurelian Isar - Decoherence And Transition From Quantum To Classical In Open Quantum Systems

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