ap-5-10.dvi

Acta Polytechnica Vol. 50 No. 5/2010

Exceptional Points and Dynamical Phase Transitions

I. Rotter

Abstract

In the framework of non-Hermitianquantumphysics, the relationbetweenexceptionalpoints, dynamicalphase transitions
and the counterintuitivebehavior of quantumsystems at high level density is considered. The theoretical results obtained
for open quantumsystems andprovenexperimentally someyears ago onamicrowave cavity,may explain environmentally
induced effects (including dynamical phase transitions), which have been observed in various experimental studies. They
also agree (qualitatively) with the experimental results reported recently in PT symmetric optical lattices.

Keywords: non-Hermitian quantum physics, dynamical phase transitions, exceptional points, open quantum systems,
PT-symmetric optical lattices.

Many years ago, Kato [1] introduced the notation
exceptional points for singularities appearing in the
perturbation theory for linear operators. Consider a
family of operators of the form

T(ς)= T(0)+ ςT ′ (1)

where ς is a scalar parameter, T(0) is the unper-
turbed operator and ςT ′ is the perturbation. Then
the number of eigenvalues of T(ς) is independent of ς
with the exception of some special values of ς where
(at least) two eigenvalues coalesce. These special val-
ues of ς are called exceptional points. An example is
the operator

T(ς)=

(
1 ς

ς −1

)
. (2)

In this case, the two values ς = ± i give the same
eigenvalue 0. According to Kato, not only the num-
ber of eigenvalues but also the number of eigenfunc-
tions is reduced at the exceptional point.
Operators of the type (2) appear in the descrip-

tion of physical systems, for example in the theory of
open quantum systems [2]. Here, the function space
of the system consisting of well localized states is
embedded in an extended environment of scattering
wavefunctions. Due to this embedding, the Hamilto-
nian of the system is non-Hermitian. The interaction
between two neighboring levels is given by a 2 × 2
symmetric Hamiltonian that describes the two-level
system with the unperturbed energies �1 and �2 and
the interaction ω between the two levels,

H(ω)=

(
�1 ω

ω �2

)
. (3)

The operators (2) and (3) are, indeed, of the same
type.

In the following, we will discuss the role played
by exceptional points in physical systems. It will be
shown that they influence not only resonance states
but also discrete states lying beyond the energywin-
dow coupled directly to the environment. Further-
more (and most important), they are responsible for
the appearance of dynamical phase transitions occur-
ring in the regime of overlapping resonances.
In an open quantum system, two states can inter-

act directly (corresponding to a first-order term) as
well as via an environment (described by a second-
order term) [2]. Here, we consider the case that
the direct interaction is contained in the energies �k
(k = 1,2). This means that the �k are considered
to be eigenvalues of a non-Hermitian Hamilton op-
erator H0 which contains both the direct interaction
V between the two states and also the coupling of
each of the two individual states to the environment
of scattering wavefunctions. Then ω contains exclu-
sively the coupling of the two states via the envi-
ronment. This allows one to study environmentally
induced effects in open quantum systems in a very
clear manner.
The eigenvalues of the operator H(ω) are

ε1,2 =
�1 + �2
2

± Z ; Z =
1
2

√
(�1 − �2)2 +4ω2 . (4)

The physical meaning of Re(Z) is the well-known
level repulsion occurring at small (mostly real) ω
while Im(Z) is related to width bifurcation. The two
eigenvalue trajectories cross when Z =0, i.e. when

�1 − �2
2ω

= ± i . (5)

At thesecrossing points, the twoeigenvaluescoalesce,

ε1 = ε2 ≡ ε0 . (6)

Acta Polytechnica Vol. 50 No. 5/2010

The crossing points may therefore be called excep-
tional points. They have a nontrivial topological
structure. For details and for a reference to the ex-
perimental proof, see the review [2].
The eigenfunctions of the non-Hermitian opera-

tor (3) are biorthogonal,

〈φ∗k |φl〉 = δk,l . (7)

When the distance between the two individual states
is large and they do not overlap, they are almost or-
thogonal in the standard manner, 〈φ∗k |φl〉 ≈ 〈φk |φl〉.
In approaching the exceptional point, however, they
become linearly dependent,

φcr1 → ± i φ
cr
2 φ

cr
2 → ∓i φ

cr
1 . (8)

Hence, thephases of the eigenfunctions φk of thenon-
Hermitian Hamilton operator (3) are not rigid. A
quantitative measure for phase rigidity is the value

rk ≡
〈φ∗k |φk〉
〈φk |φk〉

(9)

which varies between 1 at large distance of the
states and 0 at the exceptional point. Further de-
tails, including the experimentalproof of relations (8)
and (9), are discussed in the review [2].
One of the most interesting differences between

Hermitian and non-Hermitian quantum physics is
surely the fact that the phases of the eigenfunctions
of the Hamiltonian are rigid (rk = 1) in the first
case while they may vary according to (9) in the
second case [2]. It is possible therefore that the
wavefunction of one of the two states aligns with the
scatteringwavefunction of the environmentwhile the
other state decouples (more or less) fromthe environ-
ment. This phenomenon, called resonance trapping,
is caused by Im(Z) in (4), i.e. by width bifurcation.
It starts near (or at) the crossing (exceptional) point
under the influence of the continuum of scattering
wavefunctions. This means that the non-Hermitian
quantum physics is able to describe environmentally
induced effects, for example spectroscopic redistribu-
tion processes inducedby themixing of the states via
the continuum of scattering wavefunctions, which is
described by ω in (3).
Another feature involved in non-Hermitian quan-

tumphysics is the appearance of nonlinearities in the
neighborhood of exceptional points [2]. For exam-
ple, the S matrix at a double pole (corresponding to
an exceptional point) in the two-level one-continuum
case reads

S =1 − 2i
Γ0

E − E0 + i2Γ0
−

Γ20
(E − E0 + i2Γ0)2

(10)

where the notation (6) is used and ε0 ≡ E0 −
i

2
Γ0.

At the exceptional point, the cross section vanishes

due to interferences. The minimum is washed out in
the neighborhood of the double pole, however, the
resonance is broader than a Breit-Wigner resonance
according to (10).
Further studies have shown that the effects dis-

cussed above by means of the toy model (3) survive
when the full problem in the whole function space
with many levels is considered. This means that,
when the level density is high and the individual res-
onances overlap, Hamilton operators of type (3) and
the exceptional points related to themplayan impor-
tant role for the dynamics of the system. Mainly two
types of phenomena are caused by the exceptional
points in physical systems. The two phenomena con-
dition each other (for details see [2]).
First, the spectroscopy of discrete and resonance

states is strongly influenced by exceptional points.
Both types of states are eigenstates of a non-
Hermitianmany-level Hamilton operator being anal-
ogous to (3). They differ by the boundary condi-
tions. The states are discrete (corresponding to an
infinitely long lifetime) when their energy is beyond
the window coupled to the continuum of scattering
wavefunctions. The states are resonant (correspond-
ing, in general, to a finite lifetime) when their en-
ergy is inside the window coupled to the continuum
of scattering wavefunctions. Accordingly, the excep-
tional points influence the behavior not only of the
resonance states but also of the discrete states. For
example, the avoided crossing of discrete states can
be traced back to an exceptional point and, further-
more, themixing of discrete states aroundanavoided
crossing of levels is shown to arise from the existence
of an exceptionalpoint in the continuumof scattering
wavefunctions.
Discrete states have been well described in the

framework of conventional quantum mechanics for
verymanyyears. TheHamiltonian isHermitianwith
effective forces thatarenot calculated in the standard
theory. The effective forces simulate (at least partly)
theprincipal value integralwhicharises fromthe cou-
pling to other states via the continuum (denoted by
ω in (3)). The phases of the eigenfunctions are rigid
(rk = 1), the discrete states avoid crossing and the
topological phase of the diabolic point is the Berry
phase. Due to rk = 1, the Schrödinger equation is
linear, and the levels are mixed (entangled) in the
whole parameter range of avoided level crossings. At
the critical point, the mixing is maximal (1 : 1).
Resonance states are well described when quan-

tum theory is extended by including the environ-
ment of scatteringwavefunctions into the formalism.
The Hamiltonian is, in general, non-Hermitian and
ω in (3) is complex since it contains both the prin-
cipal value integral and the residuum arising from
the coupling to other states via the continuum. The
phases of the eigenfunctions are, in general, not rigid

Acta Polytechnica Vol. 50 No. 5/2010

corresponding to (9) with 0 ≤ rk ≤ 1. This can
be seen in the skewness of the basis. The resonance
states can cross in the continuum (at the exceptional
point) and the topological phase of the crossingpoint
is twice theBerryphase. When rk < 1 (regimeof res-
onance overlappingwith avoided level crossings), the
Schrödinger equation is nonlinear and the levels are
mixed (entangled) in the parameter range in which
the resonances overlap. The parameter range shrinks
to one point when the levels cross, i.e. when rk → 0
and (8) is approached.
Secondly, a dynamical phase transition is induced

by exceptional points in the regime of overlapping
resonances. Such a phase transition is environmen-
tally induced and occurs due to width bifurcation.
The number of localized states is reduced since a few
resonance states align to the scattering states of the
environment and cease to be localized. By this, the
dynamical phase transition destroys the relation be-
tween localized states below and above the critical
regime in which the resonances overlap.
The twophases are characterizedby the following

properties. In oneof thephases, the discrete andnar-
row resonance states have individual spectroscopic
features. Here, the real parts (energies) of the eigen-
value trajectories avoid crossing while the imaginary
parts (widths) can cross. As a function of increasing
(but small) coupling strength between system and
environment, the number of localized states does not
change and the widths of the resonance states in-
crease, as expected. Here, the exceptional points are
of minor importance.
In the other phase, the narrow resonance states

are superimposedwith a smooth backgroundand the
individual spectroscopic features are lost. The nar-
row resonance states appear due to resonance trap-
ping, i.e. as a consequenceof the alignmentof a small
number of resonance states to the environment (for
details see [2]). Here, the real parts (energies) of the
eigenvalue trajectories of narrow(trapped) resonance
states can cross with those of the broad (aligned)
states since they exist at different time scales. The
narrow resonance states show a counterintuitive be-
havior: with increasing (strong) coupling strength
between system and environment, the widths of the
narrow (trapped) states decrease. Furthermore, the
number of trapped resonance states is smaller than
the number of individual (basic) states. This means,
that the number of localized states is reduced when
the (complex) interaction ω in (3) is sufficiently large.
This phase results from the spectroscopic redistribu-
tion processes caused by exceptional points.
The transition region between the two phases is

the regime of overlapping resonances. Here, short-
livedand long-livedresonancestates coexist, i.e. they
arenot clearly separated fromoneanother in the time
scale. Innuclearphysics, this regime is describedwell

by the doorway picture. According to this picture,
the long-lived states are decoupled from the contin-
uumwhile thedoorwaystates are coupled toboth the
continuum and the long-lived states. In the transi-
tion region, the cross section is enhanced due to the
(partial) alignment of some states (of the doorway
states)with the scattering states of the environment.
It is interesting to see that the system behaves

according to expectations only at low level density.
Here, the resonance states are characterized by their
individual spectroscopic properties and their num-
ber does not change by varying a parameter. After
passing the transition regime with overlapping reso-
nances by further variation of the parameter, the be-
havior of the system becomes counterintuitive: the
narrow resonance states decouple more or less from
the continuum of scattering wavefunctions and the
number of localized states decreases. The decoupling
increases with increasing coupling strength between
system and environment. This counterintuitive be-
haviorwas proven experimentally, some years ago, in
a study on a microwave cavity [3].
Recently, a dynamical phase transition and the

counterintuitive behavior at strong coupling between
system and environment has been observed experi-
mentally also in P T symmetric optical lattices. At
small loss, the transmission through the system de-
creases with increasing loss according to expecta-
tions. With further increasing loss, however, the P T
symmetry breaks and the transmission is enhanced
[4, 5, 6]. An interpretation of these results from the
point of view of a dynamical phase transition can
be found in [7]. Here, also the difference between
the discrete states in P T symmetric systems and the
bound states in the continuum (resonance stateswith
vanishing decay width) in open symmetric quantum
systems with overlapping resonances is discussed.
Dynamical phase transitions in other systems

are observed experimentally. They are discussed
in [2, 7, 8]. Common to all of them is that the dy-
namical phase transition takes place in the regime of
overlapping resonances. As a result, a few states are
aligned to the scattering states of the environment
while the remaining ones (long-lived trapped reso-
nance states) are (almost) decoupled from the con-
tinuum of scattering wavefunctions.
The results discussed in the present paper can be

summarized as follows. Exceptional points play an
important role in the dynamics of quantum systems.
They are responsible, e.g., for the appearance of dy-
namical phase transitions in the regime of overlap-
ping resonances. In approaching exceptional points
by varying a parameter, some states align with the
states of the environment by trapping almost all the
other resonance states. Such a process can be de-
scribed in non-Hermitian quantum physics since the
phases of the eigenfunctions of the Hamiltonian are

Acta Polytechnica Vol. 50 No. 5/2010

not rigid, 1 ≥ rk ≥ 0. However, it cannot be de-
scribed in conventional Hermitian quantum theory
with fixed phases of the eigenfunctions, rk =1. Due
to the alignment of some states to the states of the
environment, physical processes such as transmission
may be enhanced in a comparably large parameter
range. The alignment increases with increasing cou-
pling strength between system and environment and
causes a behavior of the system at high level den-
sity which is counterintuitive at first glance. Fur-
ther theoretical and experimental studies in this field
will broaden our understanding of quantummechan-
ics. Moreover, the results are expected to be of great
value for applications.

References

[1] Kato, T.: Peturbation Theory for Linear Opera-
tors. Springer Berlin, 1966.

[2] Rotter, I.: A non-Hermitian Hamilton opera-
tor and the physics of open quantum systems,
J. Phys. A 42 (2009) 153001 (51pp), and refer-
ences therein.

[3] Persson, E., Rotter, I., Stöckmann, H. J.,
Barth, M.: Observation of resonance trapping in
an open microwave cavity, Phys. Rev. Lett. 85
(2000) 2478–2481.

[4] Guo, A., Salamo, G. J., Duchesne, D., Moran-
dotti, R., Volatier-Ravat, M., Aimez, V., Sivi-
loglou, G. A., Christodoulides, D. N.: Observa-
tion of PT-symmetry breaking in complex opti-
calpotentials,Phys. Rev. Lett.103 (2009)093902
(4 pp).

[5] Rüter, C. E., Makris, G., El-Ganainy, R.,
Christodoulides, D. N., Segev, M., Kip, D.: Ob-
servation of parity-time symmetry in optics, Na-
ture Physics 6 (2010), 192–195.

[6] Kottos, T.: Broken symmetry makes light work,
Nature Physics 6 (2010), 166–167.

[7] Rotter, I.: Environmentally induced effects and
dynamical phase transitions in quantum systems,
J. Opt. 12 (2010) 065701 (9 pp).

[8] Müller,M., Rotter, I., Phase lapses in open quan-
tumsystemsandthenon-HermitianHamiltonop-
erator, Phys. Rev. A 80 (2009) 042705 (14 pp).

Prof. Ingrid Rotter
E-mail: rotter@pks.mpg.de
Max-Planck-Institut für Physik komplexer Systeme
D-01187 Dresden, Germany