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Exercise 3.23. The quantum statistics of particles have, according to the above,
a profound in¬‚uence on the form of the Green™s function. Show that, for the case
of non-interacting fermions at zero temperature, the Fermi surface is manifest in
the time-ordered Green™s function, Eq. (3.61), according to (say) in the canonical
ensemble,
1
G0 (E, p) = , (3.137)
E ’ p + iδ sign(|p| ’ pF )
where δ = 0+ , and the sign-function, sign(x) = θ(x)’θ(’x) = x/|x|, is plus or minus
one depending on the sign of the argument. The grand canonical case corresponds
to the substitution p ’ ξp = p ’ μ.
3.5. Summary 77


Exercise 3.24. For N non-interacting bosons in a volume V at zero temperature,
they all occupy the lowest energy level corresponding to the label p = 0. In the ¬eld
operator, ψ(x) = ξ0 + ψ (x), the creation operator for the lowest energy level is
√ †
singled out, ξ0 = a0 / V , and ξ0 and ξ0 can, for a non-interacting system in the

thermodynamic limit, be regarded as c-numbers, [ξ0 , ξ0 ] = 1/V .
Show that the time ordered Green™s function for non-interacting bosons in the
ground state, |¦N = (N !)’1/2 (a† )N |0 , is given by
0

G0 (x, t, x , t ) = G(0) (t ’ t ) + G0 (x, t, x , t ) , (3.138)

where

G(0) (t ’ t ) = ’i ¦N |T (ξ0 (t) ξ0 (t ))|¦N (3.139)
is speci¬ed at t = t + 0 by iG(0) (0’ ) = n, where n = N/V is the density of the
bosons, and G0 (x, t, x , t ) is speci¬ed by its Fourier transform
1
G0 (E, p) = (3.140)
E’ + iδ
p

corresponding to G-lesser vanishing for the ¬eld ψ , or equivalently, the density of the
bosons is solely provided by the occupation of the lowest energy level. The presence
of a Bose“Einstein condensate at low temperatures thus leads to such modi¬cations
for boson Green™s function expressions.18


As mentioned, perturbation theory and diagrammatic summation schemes are
the main tools in unraveling the e¬ects of interactions on the equilibrium properties
of a system. This has been dealt with in textbooks mainly using the imaginary-
time formalism (which we will discuss in Section 5.7.1), and unfortunately most
numerously in the so-called Matsubara technique. This technique, which is based on
a purely mathematical feature, lacks physical transparency. A main purpose of this
book is to show that the real-time technique, which is based on the basic feature of
quantum dynamics, has superior properties in terms of physical insight. Furthermore,
there is no need to delve into equilibrium theory Feynman diagrammatics since it will
be a simple corollary of the general real-time non-equilibrium theory we now turn to
develop.


3.5 Summary
In this chapter we have shown that by transforming to the Heisenberg picture, the
quantum dynamics of a many-body system can be described by the time development
of the ¬eld operator in the Heisenberg picture. The measurable physical quantities
of a system were thus expressed in terms of strings of Heisenberg operators weighted
18 Ifthe ground state of a system of interacting bosons has no condensed phase, standard zero-
temperature perturbation theory can not be applied. In the opposite case, the zero-momentum ¬elds
can be treated as external ¬elds. This leads to additional vertices in the Feynman diagrammatics
as encountered in Section 10.6.
78 3. Quantum dynamics and Green™s functions


with respect to a state, generally a mixture described by a statistical operator. The
dynamics of such systems are therefore described in terms of the correlation functions
of ¬eld operators, the Green™s functions of the theory.
In thermal equilibrium, the ¬‚uctuation“dissipation relation leads to simpli¬cation
as all the one-particle or two-point Green™s functions can be expressed in terms of the
spectral weight function. Di¬erent schemes pertaining to equilibrium can be devised
for calculating equilibrium Green™s functions but we shall not entertain them here as
they will be simple corollaries of the general non-equilibrium theory presented in the
next chapter.
The equations of motion for Green™s functions of interacting quantum ¬elds in-
volve ever increasing higher-order correlation functions. The rest of the book is
devoted to the study and calculation of Green™s functions for non-equilibrium states
using diagrammatic and functional methods. We therefore turn to develop the for-
malism necessary for obtaining information about the properties of systems when
they are out of equilibrium.
4

Non-equilibrium theory

In this chapter we will develop the general formalism necessary for dealing with
non-equilibrium situations. We ¬rst formulate the non-equilibrium problem, and
discuss why the standard method applicable for the study of ground state properties
fails for arbitrary states. We then introduce the closed time path formulation, and
construct the perturbation theory for the closed time path or contour ordered Green™s
function valid for non-equilibrium states. The diagrammatic perturbation theory in
the closed time path formulation is then formulated, and generic types of interaction
are considered.1


4.1 The non-equilibrium problem
Let us consider an arbitrary physical system described by the Hamiltonian H. Since
we consider non-equilibrium quantum ¬eld theory, the Hamiltonian acts on the multi-
particle state space introduced in the ¬rst chapter, consisting of products of multi-
particle spaces for the species involved. The generic non-equilibrium problem can be
construed as follows: far in the past, prior to time t0 , the system can be thought
of as having been brought to the equilibrium state characterized by temperature T .
The state of the system is thus at time t0 described by the statistical operator2

e’H/kT
ρ(H) = , (4.1)
Tr(e’H/kT )

where Tr denotes the trace in the multi-particle state space of the physical system
in question. At times larger than t = t0 , a possible time-dependent mechanical
perturbation, described by the Hamiltonian H (t), is applied to the system. The
1 In this chapter we follow the exposition given in reference [3].
2 We can also imagine and treat the case where the particles in the system are coupled to particle
reservoirs described by their chemical potentials as this is simply included by tacitly understanding
that single-particle energies are measured relative to their chemical potentials, H ’ H ’ s μs Ns ,
i.e. shifting from the canonical to the grand canonical ensemble. In fact, in actual calculations it is
the more convenient choice, as discussed in Sections 2.5 and 3.4.



79
80 4. Non-equilibrium theory


total Hamiltonian is thus

H(t) = H + H (t) . (4.2)

The simplest non-equilibrium problem is concerned with the calculation of some
average value of a physical quantity A at times t > t0 . The state of the system is
evolved to
ρ(t) = U (t, t0 ) ρ(H) U † (t, t0 ) , (4.3)
where (recall Eq. (3.7))
t
U (t, t ) = T e’ dt H(t)
i ¯¯
(4.4)
t


is the evolution operator, and the average value of the quantity of interest is thus

A(t) = Tr(ρ(t) A) , (4.5)

where A is the operator representing the physical quantity in question in the Schr¨dinger
o
picture. Transforming to the Heisenberg picture

A(t) = Tr(ρ(H) AH (t)) = AH (t) , (4.6)

where, as discussed in Section 3.1.2, AH (t) denotes the operator representing the
physical quantity in question in the Heisenberg picture with respect to H(t), and we
have chosen the reference time in Eq. (3.17) to be t0 . The average value is typically
a type of quantity of interest for a macroscopic system, i.e. a system consisting of a
huge number of particles. For example, for the average (probability) density of the
particle species described by the quantum ¬eld ψ, we have according to Eq. (2.28),3

n(x, t) = ψH (x, t) ψH (x, t) . (4.7)

The average density is seen to be equal to the equal-time and equal-space value of
the G-lesser Green™s function, G< , introduced in Section 3.3

n(x, t) = “iG< (x, t, x, t) , (4.8)

where upper (lower) sign is for bosons and fermions, respectively.
If ¬‚uctuations are of interest or importance we encounter higher order correlation
functions, generically according to Section 2.1, then appears the trace over products
of pairs of Heisenberg ¬eld operators for particle species weighted by the initial
state. If one is interested in the probability that a certain sequence of properties are
measured at di¬erent times, one encounters arbitrary long products of Heisenberg
operators.4 Since physical quantities are expressed in terms of the quantum ¬elds of
the particles and interactions in terms of their higher-order correlations, of interest
are the correlation functions, the so-called non-equilibrium Green™s functions.
Owing to interactions, memory of the initial state of a subsystem is usually rapidly
lost. We shall not in practice be interested in transient properties but rather steady
3 Possible
spin degrees of freedom are suppressed, or imagined absorbed in the spatial variable.
4 See
chapter 1 of reference [1] for a discussion of such probability connections or histories with
a modern term.
4.2. Ground state formalism 81


states, where the dependence on the initial state is lost, and the time dependence is
governed by external forces. Initial correlations can be of interest in their own right,
even in many-body systems.5 In fact, for all of the following, the statistical operator
in the previous formulae, say Eq. (4.6), could have been chosen as arbitrary. This
would lead to additional features which we point out as we go along, and in practice
each case then has to be dealt with on an individual basis.
The equation of motion for the one-particle Green™s function leads to an in¬nite
hierarchy of equations for correlation functions containing ever increasing numbers of
¬eld operators, describing the correlations between the particles set up in the system
by the interactions and external forces. In order to calculate the e¬ects of interac-
tions, we now embark on the construction of perturbation theory and the diagram-
matic representation of non-equilibrium theory starting from the canonical formalism
presented in the ¬rst chapter. But ¬rst we describe why the zero temperature, i.e.
ground state, formalism is not capable of dealing with general non-equilibrium situ-
ations, before embarking on ¬nding the necessary remedy, and eventually construct
non-equilibrium perturbation theory and its corresponding diagrammatic represen-
tation.


4.2 Ground state formalism
To see the need for the closed time path description consider the problem of pertur-
bation theory. The Hamiltonian of a system

H = H0 + H (i) (4.9)

consists of a term quadratic in the ¬elds, H0 , describing the free particles, and a
complicated term, H (i) , describing interactions.
Constructing perturbation theory for zero temperature quantum ¬eld theory, i.e.
the system is in its ground state |G , only the time-ordered Green™s function
† †
G(x, t, x , t ) = ’i T (ψH (x, t) ψH (x , t )) = ’i G|T (ψH (x, t) ψH (x , t ))|G
(4.10)
needs to be considered. Here ψH (x, t) is the ¬eld operator in the Heisenberg picture
with respect to H for one of the species of particles described by the Hamiltonian.6
The time-ordered Green™s function contains more information than seems necessary
for calculating mean or average values, since for times t < t it becomes the G-lesser
Green™s function
G(x, t, x , t ) = G< (x, t, x , t ) (4.11)
5 All transient e¬ects for the above chosen initial condition are of course included. Whether this
choice is appropriate for the study of transient e¬ects depends on the given physical situation.
6 For a reader not familiar with zero temperature quantum ¬eld theory, no such thing is required.

It will be a simple corollary of the more powerful formalism presented in Section 4.3.2, and devel-
oped to its ¬nal real-time formalism presented in Chapter 5. The reason for the usefulness of the
time ordering operation is to be expected remembering the crucial appearance of time-ordering in
the evolution operator. Also, under the governing of the time-ordering symbol, operators can be
commuted without paying a price except for the possible quantum statistical minus signs in the
case of fermions.
82 4. Non-equilibrium theory


and thereby are all average values of physical quantities speci¬ed once the time-
ordered Green™s function is known for t < t . However, a perturbation theory involv-
ing only the G-lesser Green™s function can not be constructed.
The time-ordered Green™s function can, instead of being expressed in terms of the
¬eld operator ψH (x, t), i.e. in the Heisenberg picture with respect to H, be expressed
in terms of the ¬eld operators ψH0 (x, t), the Heisenberg picture with respect to H0
or the so-called interaction picture,

ψ(x) e’
i i
H0 (t’tr ) H0 (t’tr )
ψH0 (x, t) = e (4.12)
as they are related according to the unitary transformation
ψH (x, t) = U † (t, tr ) ψH0 (x, t) U (t, tr ) , (4.13)
where
t ¯ (i ) ¯
U (t, tr ) = T e’i dt HH (t)
(4.14)
tr 0


is the evolution operator in the interaction picture (leaving out for brevity an index
to distinguish it from the full evolution operator exp{’iH(t ’ tr )}) and

HH0 (t) = eiH0 (t’tr ) H (i) e’iH0 (t’tr ) .
(i)
(4.15)
This is readily seen by noting that the expression on the right-hand side in Eq. (4.13)
satis¬es the ¬rst-order in time di¬erential equation
‚ψ(x, t)
i = [ψ(x, t), H] , (4.16)
‚t
the same equation satis¬ed by the ¬eld ψH (x, t), and at the reference time tr , the two
operators are seen to coincide (coinciding with the ¬eld in the Schr¨dinger picture,
o
ψ(x)).
Transforming to the interaction picture, and using the semi-group property of the
evolution operator, U (t, t ) U (t , t ) = U (t, t ),7 and the relation U † (t, t ) = U (t , t),
the time ordered Green™s function can be expressed in the form

= ’i U † (t, tr )ψH0 (x, t)U (t, t )ψH0 (x , t )U (t , tr ) θ(t ’ t )
G(x, t, x , t )

± i U † (t , tr )ψH0 (x , t )U (t , t)ψH0 (x, t)U (t, tr ) θ(t ’ t) (4.17)

which can also be expressed on the form (tm denotes max{t, t })

G(x, t, x , t ) = ’i U † (tm , tr )T ψH0 (x, t)ψH0 (x , t )U (tm , tr ) (4.18)

since the time-ordering symbol places the operators in the original order.8
7 For
a detailed discussion of the evolution operator and the Heisenberg and interaction pictures
we refer to chapter 2 of reference [1].
8 In fact the operator identity

† †
T (ψH (x, t) ψH (x , t )) = U † (tm , tr )T ψH 0 (x, t) ψH 0 (x , t ) U (tm , tr )
is valid since only transformation of operators was involved, and nowhere is advantage taken of the
averaging with respect to the state in question.
4.2. Ground state formalism 83


Usually, say in a scattering experiment realized in a particle accelerator, only
transitions from an initial state in the far past are of interest so that the reference
time is chosen in the far past, tr = ’∞, and inserting 1 = U (tm , ∞)U (∞, tm ) after
U † gives9

G(x, t, x , t ) = ’i U † (∞, ’∞)T (ψH0 (x, t) ψH0 (x , t ) U (∞, ’∞)) . (4.19)

If the average is with respect to the ground state of the system, one can make use
of the trick of adiabatic switching, i.e. the interaction is assumed turned on and o¬
adiabatically, say by the substitution HH0 (t) ’ e’ |t| HH0 (t). The non-interacting
(i) (i)

(non-degenerate) ground state |G0 , H0 |G0 = E0 |G0 , is evolved by the full adiabatic
evolution operator U into the normal ground state of the interacting system at time
t = 0, |G = U (0, ’∞)|G0 . The on the evolution operator indicates that the
interaction is turned on and o¬ adiabatically. In perturbation theory it can then be
shown, that in the limit of ’ 0, the true interacting ground state at time t = 0
is obtained modulo a phase factor that is obtained from the limiting expression of
turning the interaction on and o¬ adiabatically, (the Gell-Mann“Low theorem [4]),10

U (∞, ’∞) |G0 = eiφ |G0 eiφ = G0 |U (∞, ’∞)|G0 .
, (4.20)

As a consequence, the time-ordered Green™s function, Eq. (4.10), can be expressed
in terms of the non-interacting ground state and the ¬elds in the interaction picture
according to

G0 |T (ψH0 (x, t) ψH0 (x , t ) U (∞, ’∞))|G0
G(x, t, x , t ) = ’i . (4.21)
G0 |U (∞, ’∞)|G0

In the next section we will show that the arti¬ce of turning the interaction on
and o¬ adiabatically is not needed when using the closed time path formulation
and generalizing time-ordering to contour-ordering, and it can also be avoided by
using functional methods as in Chapter 9, and plays no role in the non-equilibrium
formalism. In describing a scattering experiment, adiabatic switching is of course an
innocent initial and ¬nal boundary condition as the particles are then free.11
Since the Gell-Mann“Low theorem fails for states other than the ground state,
and thus even for an equilibrium state at ¬nite temperature, we are in general stuck
9 In fact as an operator identity
† †
T (ψH (x, t) ψH (x , t )) = U † (∞, ’∞)T (ψH 0 (x, t) ψH 0 (x , t ) U (∞, ’∞)).

10 Clearly, it is important that no dissipation or irreversible e¬ects takes place. Contrarily, in
statistical physics, reduced dynamics is the main interest, i.e. certain degrees of freedom are left
unobserved and emission and absorption takes place, technically partial traces occurs.
11 As will become clear from the following sections, the denominator in Eq. (4.21) is diagrammati-

cally the sum of all the vacuum diagrams that therefore cancel all the disconnected diagrams in the
numerator, and one obtains the standard connected Feynman diagrammatics for the time-ordered
Green™s function for a system at zero temperature such as is relevant in, say, QED. In QED one
works with the so-called scattering matrix or S-matrix, S(∞, ’∞), de¬ned in terms of the full
evolution operator, S(t, t ) = eiH 0 t U (t, t )e’iH 0 t , so that the matrix elements of the S-matrix are
expressed in terms of the free-particle states.
84 4. Non-equilibrium theory


with the operator U † (∞, ’∞) inside the averaging in Eq. (4.19) and Eq. (4.18). At
¬nite temperatures and a fortiori for non-equilibrium states, a perturbation theory
involving only one kind of a real-time Green™s functions can not be obtained. In
order to construct a single object which contains all the dynamical information we
shall follow Schwinger and introduce the closed time path formulation [5].


4.3 Closed time path formalism
Let us return to the non-equilibrium situation of Section 4.1 where the dynamics is
determined by a time dependent Hamiltonian H(t) = H + H (t), where H is the
Hamiltonian for the isolated system of interest and H (t) is a time-dependent pertur-
bation acting on it. The unitary transformation relating operators in the Heisenberg
pictures governed by the Hamiltonians H(t) and H, respectively, is speci¬ed by the
unitary transformation
t
’i ¯ ¯
OH (t) = V † (t, t0 ) OH (t) V (t, t0 ) ,
dt HH (t)
V (t, t0 ) = T e (4.22)
t0



and

UH (t, t0 ) = e’
i
H(t’t0 )
HH (t) = UH (t, t0 ) H (t) UH (t, t0 ) , (4.23)
and we have chosen t0 as reference time where the two pictures coincide. This
relation between the two pictures is obtained by ¬rst comparing both pictures to the
Schr¨dinger picture obtaining
o
† †
OH (t) = UH (t, t0 ) UH (t, t0 ) OH (t) UH (t, t0 ) UH (t, t0 ) , (4.24)

where
’i dt H(t)
t ¯¯
UH (t, t0 ) = T e (4.25)
t0



is the evolution operator corresponding to the Hamiltonian H(t). Then one notes

that V (t, t0 ) and UH (t, t0 ) UH (t, t0 ) satisfy the same ¬rst-order in time di¬erential
equation and the same initial condition. We have thus obtained Dyson™s formula

V (t, t0 ) = UH (t, t0 ) UH (t, t0 ) (4.26)

or explicitly

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