### Inhalt des Dokuments

- [1]
- © AG Brandes

Das Institut für Theoretische Physik trauert um Prof. Dr. Tobias Brandes. Er verstarb völlig unerwartet am 2.2.2017 im Alter von 50 Jahren. Mit ihm haben wir viel zu früh einen ausserordentlich geschätzten Kollegen und einen herausragenden Hochschullehrer verloren. Seinen Angehörigen gilt unsere tiefe Anteilnahme.

The Institute for Theoretical Physics mourns the decease of Prof. Dr. Tobias Brandes. He suddenly passed away on the 2.2.2017 at the age of 50 years. We have lost an extraordinarily appreciated colleague and an exceptional scientist. May our deepest condolences go to his family and friends.

Condolences [2]

# Kommissarische Leitung PD Dr. Gernot Schaller

- [3]
- © TUB

Gastprofessor

Raum EW 737

Sprechstunde:
**bitte melden Sie sich per Email für einen
online-Termin**

gernot.schaller@tu-berlin.de [4]

Homepage [5]

# 640. WE-Heraeus Seminar

- [6]
- © W.E. Heraeus Stiftung

** Non-Markovianity and Strong Coupling Effects in
Thermodynamics** [7]

**April 10 - April 13, 2017, Bad Honnef,
Germany**

We are glad to announce the 640. W. E. Heraeus Seminar on non-Markovianity and strong coupling effects in thermodynamics. Please find more information in our website [8].

Die Arbeitsgruppe beschäftigt sich zur Zeit stark mit Quantenkohärenz, Quantenoptik, und Vielteilchen-Effekten in niedrigdimensionalen Festkörperstrukturen. Ein wesentlicher Schwerpunkt ist die Schnittstelle zwischen elektronischem Transport und quantenoptischen Effekten. Uns interessiert ein breites Spektrum theoretischer Probleme, die von eher abstrakten Fragestellungen (z.B. der Definition quantenmechanischer Verschränkung in Vielteilchen-Systemen oder im Nichtgleichgewicht) über neue physikalische Modelle und Methoden (z.B. für nanomechanische Systeme) bis hin zu elektronischen, optischen und elastischen Materialeigenschaften (z.B. Phononen) reichen. Dementsprechend kommen verschiedene Techniken zum Einsatz - exakte Lösungen, rein numerische Berechnungen, und häufig Kombinationen analytischer und numerischer Methoden. Die Arbeitsgruppe ist an zahlreichen internationalen Kooperationen beteiligt, u.a. Madrid (R. Aguado, G. Platero, D. Marcos, R. Sanchez), Prag (T. Novotny), Tainan (Y.-N. Chen), Tokyo (N. Lambert).

## Topologically Enforced Bifurcations in Superconducting Circuits

- [9]
- © TUB

The relationship of topological insulators and superconductors and the field of nonlinear dynamics is widely unexplored. To address this subject, we adopt the linear coupling geometry of the Su-Schrieffer-Heeger model, a paradigmatic example for a topological insulator, and render it nonlinearly in the context of superconducting circuits. As a consequence, the system exhibits topologically enforced bifurcations as a function of the topological control parameter, which finally gives rise to chaotic dynamics, separating phases that exhibit clear topological features.

** G.
Engelhardt, M. Benito, G. Platero, and T. Brandes
**

PRL 118, 197702 (2017) [10], arxiv:1611.01467 [11]

## Topological instabilities in ac-driven bosonic systems

- [12]
- © TUB

Under non-equilibrium conditions, bosonic modes can become dynamically unstable with an exponentially growing occupation. On the other hand, topological band structures give rise to symmetry protected midgap states. In this letter, we investigate the interplay of instability and topology. Thereby, we establish a general relation between topology and instability under ac-driving. We apply our findings to create dynamical instabilities which are strongly localized at the boundaries of a finite-size system. As these localized instabilities are protected by symmetry, they can be considered as topological instabilities.

G. Engelhardt, M. Benito, G. Platero, T. Brandes

PRL 117, 045302 (2016) [13], arXiv:1512.07653 [14]

## Topological Bogoliubov excitations in inversion-symmetric systems of interacting bosons

- [15]
- © TUB

On top of the mean-field analysis of a Bose-Einstein condensate, one typically applies the Bogoliubov theory to analyze quantum fluctuations of the excited modes. Therefore, one has to diagonalize the Bogoliubov Hamiltonian in a symplectic manner. In our article, we investigate the topology of these Bogoliubov excitations in inversion-invariant systems of interacting bosons in one dimension. We analyze, how the condensate influences the topology of the Bogoliubov excitations. Analogously to the fermionic case, we here establish a symplectic extension of the polarization characterizing the topology of the Bogoliubov excitations and link it to the eigenvalues of the inversion operator at the inversion-invariant momenta. We also demonstrate at an instructive, but experimentally feasible example that this quantity is also related to edge states in the excitation spectrum.

G. Engelhardt and T. Brandes

Phys. Rev. A**91** 053621 (2015),
[16]arXiv:1503.02503 [17]

## Time-delayed feedback control of the Dicke–Hepp–Lieb superradiant quantum phase transition

- [18]
- © TUB

We apply the time-delayed Pyragas control scheme to the dissipative Dicke model via a modulation of the atom-field-coupling. The feedback creates an infinite sequence of non-equilibrium phases with fixed points and limit cycles in the primary superradiant regime. We analyse this Hopf bifurcation scenario as a function of delay time and feedback strength and determine analytical conditions for the phase boundaries.

W. Kopylov, C. Emary, E. Schöll, and T. Brandes

New J. Phys. **17** 013040 (2015),
[19]arXiv:1403.0620 [20]

## Nonequilibrium relaxation transport of ultracold atoms

- [21]
- © TUB

We analyze the equilibration process between two either fermionic or bosonic reservoirs containing ultracold atoms with a fixed total number of particles that are weakly connected via a few-level quantum system. We allow for both the temperatures and particle densities of the reservoirs to evolve in time. Subsequently, linearizing the resulting equations enables us to characterize the equilibration process and its time scales in terms of equilibrium reservoir properties and linear-response transport coefficients. Additionally, we investigate the use of such a device as particle transistor or particle capacitor and analyze its efficiency.

F. G.-Marcos, G. Platero, C. Nietner, G. Schaller, and T.
Brandes

Phys.Rev.A **90** 033614 (2014), [22]arXiv:1407.2804
[23]

## Quantum Criticality and Dynamical Instability in the Kicked-Top Model

- [24]
- © TUB

We investigate precursors of critical behavior in the quasienergy spectrum due to the dynamical instability in the kicked top. Using a semiclassical approach, we analytically obtain a logarithmic divergence in the density of states, which is analogous to a continuous excited state quantum phase transition in undriven systems. We propose a protocol to observe the cusp behavior of the magnetization close to the critical quasienergy.

V. M. Bastidas, P. Perez-Fernandez, M. Vogl, T. Brandes

Phys.Rev.Lett. [25]** 112**, arXiv:1308.5640
[26]

## Transport with ultracold atoms at constant density

- [27]
- © TUB

We investigate the transport through a few-level quantum system described by a Markovian master equation with temperature- and particle-density-dependent chemical potentials. From the corresponding Onsager relations we extract linear response transport coefficients in analogy to the electronic conductance, thermal conductance, and thermopower. Considering ideal Fermi and Bose gas reservoirs, we observe steady-state currents against the thermal bias as a result of the nonlinearities introduced by the constraint of a constant particle density in the reservoirs. Most importantly, we find signatures of the onset of Bose-Einstein condensation in the transport coefficients.

C. Nietner, G. Schaller, T. Brandes

Phys.Rev.A **89** 013605 (2014), [28]arXiv:1309.3488
[29]

## Thermodynamics of a physical model implementing a Maxwell demon

- [30]
- © TUB

We present a physical implementation of a Maxwell demon which consists of a conventional single electron transistor (SET) capacitively coupled to another quantum dot detecting its state. Altogether, the system is described by stochastic thermodynamics. We identify the regime where the energetics of the SET is not affected by the detection, but where its coarse-grained entropy production is shown to contain a new contribution compared to the isolated SET. This additional contribution can be identified as the information flow generated by the “Maxwell demon” feedback in an idealized limit.

P. Strasberg, G. Schaller, T. Brandes, and M. Esposito

*Phys. Rev. Lett. 110, 040601 (2013), [31]
arXiv:1210.5661 [32]*

## Criticality in transport through the quantum Ising chain

- [33]
- © TUB

We consider thermal transport between two reservoirs coupled by a quantum Ising chain as a model for non-equilibrium physics induced in quantum-critical many-body systems. By deriving rate equations based on exact expressions for the quasiparticle pairs generated during the transport, we observe signatures of the underlying quantum phase transition in the steady-state energy current already at finite reservoir temperatures.

M. Vogl, G. Schaller, T. Brandes

Phys. Rev. Lett **109**, 240402 (2012) [34]

arXiv:1208.5989 [35]

## Non-equilibrium Quantum Phase Transitions in the Dicke Model

- [36]
- © TUB

We establish a set of nonequilibrium quantum phase transitions in the Dicke model by considering a monochromatic nonadiabatic modulation of the atom-field coupling. For weak driving the system exhibits a set of side-bands which allow the circumvention of the no-go theorem which otherwise forbids the occurence of superradiant phase transitions. At strong driving we show that the system exhibits a rich multistable structure and exhibits both first- and second-order nonequilibrium quantum phase transitions.

V. M. Bastidas, C. Emary, B. Regler, T. Brandes

Physical
Review Letters **108**, 043003 (2012) [37]

arXiv:1108.2987
[38]

## Dynamics of interacting transport qubits

- [39]
- © Copyright??

We investigate the electronic transport through two parallel double quantum dots coupled both capacitively and via a perpendicularly aligned charge qubit. The presence of the qubit leads to a modification of the coherent tunnel amplitudes of each double quantum dot. We study the influence of the qubit on the electronic steady-state currents through the system, the entanglement between the transport double quantum dots, and the back-action on the charge qubit. We use a Born-Markov secular quantum master equation for the system. The obtained currents show signatures of the qubit. The stationary qubit state may be tuned and even rendered pure by applying suitable voltages. In the Coulomb diamonds, it is also possible to stabilize pure entangled states of the transport double quantum dots.

C. Nietner, G. Schaller, C. Pöltl, and T. Brandes

*Physical Review B 85, 245431 (2012)*
[40]

arXiv:1204.2978 [41]

## Limit-Cycles and Chaos in the Current Through a Quantum Dot

- [42]
- © Copyright??

We investigate non-linear magneto-transport through a single level quantum dot where the electron spin is coupled to a large, external (pseudo-)spin via an anisotropic exchange interaction. We find regimes where the average current through the dot displays self-sustained oscillations that reflect the limit-cycles and chaos and map the dependence of this behaviour on magnetic field strength and the tunnel coupling to the external leads.

C. Lopez-Monis, C. Emary, G. Kiesslich, G. Platero, and T.
Brandes

*Physical Review B 85, 045301 (2012)*
[43]

arXiv:1104.3995 [44]

## Adiabaticity in semiclassical nanoelectromechanical systems

- [45]
- © Copyright??

We compare the semiclassical description of
nanoelectromechanical systems (NEMS) within and beyond the adiabatic
approximation. We consider a NEMS model that contains a single phonon
(oscillator) mode linearly coupled to an electronic few-level system
in contact with external particle reservoirs (leads). Using
Feynman-Vernon influence functional theory, we derive a Langevin
equation for the oscillator trajectory that is nonperturbative in the
system-leads coupling. A stationary electronic current through the
system generates nontrivial dynamical behavior of the oscillator, even
in the adiabatic regime. The “ backaction” of the oscillator onto
the current is studied as well. For the two simplest cases of one and
two coupled electronic levels, we discuss the differences between the
adiabatic and the nonadiabatic regime of the oscillator dynamics.

Anja Metelmann and Tobias Brandes

Physical Review B 84, 155455
(2011) [46]

arXiv:1107.3762 [47]

## Probing the Power of an electronic Maxwell's Demon

- [48]
- © Copyright??

We suggest that a single-electron transistor
continuously monitored by a quantum point contact may function as a
Maxwell demon when closed-loop feedback operations are applied as
time-dependent modifications of the tunneling rates across its
junctions. The device may induce a current across the single-electron
transistor even when no bias voltage or thermal gradient is applied.
For different feedback schemes, we derive effective master equations
and compare the induced feedback current and its fluctuations as well
as the generated power. Provided that tunneling rates can be modified
without changing the transistor level, the device may be implemented
with current technology.

Gernot Schaller, Clive Emary, Gerold Kiesslich, and Tobias
Brandes

Physical Review B 84, 085418 (2011) [49]

arXiv:1106.4670 [50]

## Counting statistics of collective photon transmissions

- [51]
- © Copyright??

We theoretically study cooperative effects in the
steady-state transmission of photons through a medium of N radiators.
Using methods from quantum transport, we find a cross-over in scaling
from N to N^2 in the current and to even higher powers of N in the
higher cumulants of the photon counting statistics as a function of
the tunable source occupation. The effect should be observable for
atoms confined within a nano-cell with a pumped optical cavity as
photon source.

Malte Vogl, Gernot Schaller, and Tobias Brandes

Ann. Phys.
(N.Y.) **326**, 2827 (2011) [52]

arXiv:1010.4416
[53]

## Charge Qubit Purification by an Electronic Feedback Loop

- [54]
- © Copyright??

We propose the manipulation of an isolated qubit by a simple instantaneous closed-loop feedback scheme in which a time-dependent electronic detector current is directly back-coupled into qubit parameters. As specific detector model we employ a capacitively coupled single-electron transistor. We demonstrate the stabilization of pure delocalized qubit states above a critical detector-qubit coupling. This electronic purification is independent of the initial qubit state and is accomplished after few electron jumps through the detector. Our simple scheme can be used for the efficient and robust initialization of solid-state qubits in quantum computational algorithms at arbitrary temperatures.

Gerold Kiesslich, Gernot Schaller, Clive Emary, and Tobias
Brandes

Physical Review Letters **107**, 050501 (2011) [55]

arXiv:1102.3771 [56]

## Quantum Equilibration under Constraints and Transport Balance

- [57]
- © Copyright??

For open quantum systems coupled to a thermal bath
at inverse temperature β, it is well known that under the Born,
Markov, and secular approximations, the system density matrix will
approach the thermal Gibbs state with the bath inverse temperature β.
We generalize this to systems where there exists a conserved quantity
(e.g., the total particle number), where for a bath characterized by
inverse temperature β and chemical potential μ, we find
equilibration of both temperature and chemical potential. For
couplings to multiple baths held at different temperatures and
different chemical potentials, we identify a class of systems that
equilibrates according to a single hypothetical average but in general
nonthermal bath, which may be exploited to generate desired nonthermal
states. Under special circumstances, the stationary state may again be
described by a unique Boltzmann factor. These results are illustrated
by several examples.

Gernot Schaller

Physical Review E 83, 031111 (2011) [58]

arXiv:1010.3094 [59]

## Semi-classical dynamics of nano-electromechanical systems

- [60]
- © Copyright??

We investigate the dynamics of a single phonon (oscillator) mode linearly coupled to an electronic few-level system in contact with external particle reservoirs (leads). A stationary electronic current through the system generates nontrivial dynamical behavior of the oscillator. Using Feynman-Vernon influence functional theory, we derive a Langevin equation for the oscillator trajectory that is nonperturbative in the system-leads coupling and from which we extract effective oscillator potentials and friction coefficients. For the two simplest cases of a single and two coupled electronic levels, we discuss various regimes of the oscillator dynamics.

R. Hussein, A. Metelmann, P. Zedler, and T. Brandes *Phys.
Rev. B 82, 165406 (2010) [61]*

*arXiv:1006.2076 [62]*

## Feedback Control of Quantum Transport

- [63]
- © Copyright??

The current through nanostructures like quantum dots can be stabilized by a classical feedback loop that continuously adjusts system parameters as a function of the number of tunnelled particles n. At large times, the feedback loop freezes the fluctuations of n, which leads to highly accurate, continuous single particle transfers. For the simplest case of feedback acting simultaneously on all system parameters, we show how to reconstruct the original full counting statistics from the frozen distribution.

Tobias Brandes*Phys. Rev. Lett. 105, 060602 (2010)
[64]*

*arXiv:1005.0018 [65]*

## Low-dimensional detector model for full counting statistics: Trajectories, Back-action, and Fidelity

- [66]
- © Copyright??

We study the combined counting statistics of two
capacitively coupled transport channels. In particular, we examine the
conditions necessary for utilizing one channel as detector sensitive
to the occupation of the other. A good detector fidelity may be
achieved in a bistable regime when the tunneling rates through the two
channels are vastly different -- even when the physical back-action of
the detector on the probed channel is large. Our methods allow to
estimate the error of charge counting detectors from time-resolved
current measurements -- which have been obtained in recent experiments
-- alone.

Gernot Schaller, Gerold Kiesslich, and Tobias Brandes*Phys. Rev. B 82, 041303(R) (2010) [67]*

*arXiv:1004.2604 [68]*

## Distinguishing quantum and classical transport through nanostructures

- [69]
- © TUB

We consider the question of how to distinguish
quantum from classical transport through nanostructures. To address
this issue we have derived two inequalities for temporal correlations
in nonequilibrium transport in nanostructures weakly coupled to leads.
The first inequality concerns local charge measurements and is of
general validity; the second concerns the current flow through the
device and is relevant for double quantum dots. Violation of either of
these inequalities indicates that physics beyond that of a classical
Markovian model is occurring in the nanostructure.

Neill
Lambert, Clive Emary, Yueh-Nan Chen, Franco Nori*Phys. Rev.
Lett. 105 176801 (2010) [70]arXiv:1002.3020
[71]Featured in Technology Review [72]*

## Counting Statistics in Multistable Systems

- [73]
- © TUB

Using a generic model for stochastic transport
through a single quantum dot that is modified by the Coulomb
interaction of environmental (weakly coupled) quantum dots, we derive
general properties of the full counting statistics for multi-stable
Markovian transport systems. We study the temporal crossover from
multi-modal to broad uni-modal distributions depending on the initial
mixture, the long-term asymptotics and the divergence of the cumulants
in the limit of a large number of transport branches. These findings
demonstrate that the counting statistics of a single resonant level
may be used to probe background charge configurations.

Gernot Schaller, Gerold Kiesslich, and Tobias Brandes*Phys.
Rev. B 81, 205305 (2010)* [74] Or

*arXiv:0912.2887*

[75]

## Magnetotunneling into Fock-Darwin-like quantum dot states

- [76]
- © Copyright??

We study theoretically the magnetotunneling
transport through quantum dots formed by diffusion of charged
manganese interstitials in the vicinity of a GaAs quantum well
[Phys.Rev.Lett. **101** 226807 (2008)]. In particular, we examine
the lateral matrix elements between Landau subbands in the contact and
Fock-Darwin-like states of an individual dot at high magnetic fields.
We explicitly demonstrate the effect of spatial deformation of the dot
on the wave function's overlap. The comparison with measured data
suggests a selection rule similar to angular momentum conservation for
tunneling into perfect Fock-Darwin states.

Gerold
Kiesslich*Phys. Status Solidi (Rapid Research Letters)
4, 139 (2010)* [77]

## Entanglement and parametric resonance in driven quantum systems

- [78]
- © TUB

We study the entanglement dynamics in the
externally-driven single-mode Dicke model in the thermodynamic limit,
when the field is in resonance with the atoms. We compute the
correlations in the atoms-field ground state by means of the density
operator that represents the pure state of the universe and the
reduced density operator for the atoms, which results from taking the
partial trace over the field coordinates. As a measure of bipartite
entanglement, we calculate the linear entropy, from which we analyze
the entanglement dynamics. In particular, we found a strong relation
between the stability of the dynamical parameters and the reported
entanglement.

Victor M. Batsidas, John H. Reina, and Tobias
Brandes

Phys. Rev. A **81**, 012316 (2010) [79] Or
arXiv:0904.2411 [80]

## The speed of Markovian relaxation towards the ground state

- [81]
- © Copyright??

For sufficiently low reservoir temperatures, it is known that open quantum systems subject to decoherent interactions with the reservoir relax toward their ground state in the weak coupling limit. Within the framework of quantum master equations, this is formalized by the Born-Markov-secular (BMS) approximation, where one obtains the system Gibbs state with the reservoir temperature as a stationary state. When the solution to some problem is encoded in the (isolated) ground state of a system Hamiltonian, decoherence can therefore be exploited for computation. The computational complexity is then given by the scaling of the relaxation time with the system size n. We study the relaxation behavior for local and nonlocal Hamiltonians that are coupled dissipatively with local and nonlocal operators to a bosonic bath in thermal equilibrium. We find that relaxation is generally more efficient when coherences of the density matrix in the system energy eigenbasis are taken into account. In addition, the relaxation speed strongly depends on the matrix elements of the coupling operators between initial state and ground state. We show that Dicke superradiance is a special case of our relaxation models and can thus be understood as a coherence-assisted relaxation speedup.

Malte Vogl, Gernot Schaller and Tobias Brandes

*Phys. Rev. A*** 81** 012102 (2010)
[82] and

Virtual Journal of Nanoscale Science & Technology 21(3) and

Virtual Journal of Quantum Information 10(1),

or *arXiv:0906.4684* [83]

## Universal oscillations in counting statistics

- [84]
- © TUB

Noise is a result of stochastic processes that
originate from quantum or classical sources. Higher-order cumulants of
the probability distribution underlying the stochastic events are
believed to contain details that characterize the correlations within
a given noise source and its interaction with the environment, but
they are often difficult to measure. Here we report measurements of
the transient cumulants 〈〈n^m〉〉 of the
number n of passed charges to very high orders (up to m = 15) for
electron transport through a quantum dot. For large m, the cumulants
display striking oscillations as functions of measurement time with
magnitudes that grow factorially with m. Using mathematical properties
of high-order derivatives in the complex plane we show that the
oscillations of the cumulants in fact constitute a universal
phenomenon, appearing as functions of almost any parameter, including
time in the transient regime. These ubiquitous oscillations and the
factorial growth are system-independent and our theory provides a
unified interpretation of previous theoretical studies of high-order
cumulants as well as our new experimental data.

Christian
Flindt, Christian Fricke, Frank Hohls, Tomas Novotny, Karel Netocny,
Tobias Brandes, and Rolf J. Haug *Proc. Natl. Acad. Sci.
U.S.A. 106, 10116 (2009) [85]* or arXiv:0901.0832
[86]

## Three-level mixing and dark states in transport through quantum dots

- [87]
- © TUB

We consider theoretically the transport through
the double quantum dot structure of the recent experiment of Payette
et al. [Phys. Rev. Lett. 102, 026808 (2009)] and calculate stationary
current and shot noise. Three-level mixing gives rise to a pronounced
current-suppression effect, the character of which changes markedly
with bias direction. We discuss these results in connection with the
dark states of coherent population trapping in quantum dots.

Clive Emary, Christina Pöltl, and Tobias Brandes*Phys.
Rev. B* ** 80**, 235321 (2009) [88] or
arXiv:0908.33935 [89]

## Transport Statistics of interacting double dot systems: Coherent and Non-Markovian effects

- [90]
- © TUB

We formalize the derivation of a generalized
coarse-graining n-resolved master equation by introducing a virtual
detector counting the number of transferred charges in single-electron
transport. Our approach enables the convenient inclusion of coherences
and Lamb shift in counting statistics. As a Markovian example with
Lindblad-type density matrices, we consider the Born-Markov-secular
(BMS) approximation which is a special case of the non-Markovian
dynamical coarse-graining approach. For illustration we consider
transport through two interacting levels that are either serially or
parallelly coupled to two leads held at different chemical potentials.
It is shown that the coherences can strongly influence the
(frequency-dependent) transport cumulants: in the serial case the
neglect of coherences would lead to unphysical currents through
disconnected conductors. Interference effects in the parallel setup
can cause strong current suppression with giant Fano factors and
telegraphlike distribution functions of transferred electrons, which
is not found without coherences. We demonstrate that with finite
coarse-graining times coherences are automatically included and,
consequently, the shortcomings of the BMS approximation are
resolved.

Gernot Schaller, Gerold Kiesslich, and Tobias
Brandes*Phys. Rev. B* ** 80**,
245107 (2009) [91] or arXiv:0908.3620 [92]

## Single spin transport spectroscopy: Current blockade and spin decay

- [93]
- © TUB

We present a theory of a single-electron
transistor exchange-coupled to a localized spin. We show how to gain
detailed quantitative knowledge about the attached spin such as spin
size, exchange coupling strength, Landé g-factor, and spin decay time
T1 by utilizing a robust blockade phenomenon of dc magnetotransport
with accompanying noise enhancement. Our studies are of particular
relevance to spin-resolved scanning ingle-electron transistor
microscopy, electronic transport through nanomagnets, and the effect
of hyperfine interaction on transport electrons by surrounding nuclear
spins.

Gerold Kiesslich, Gernot Schaller, Clive Emary, and Tobias
Brandes*Appl. Phys. Lett. 95, 152104
(2009)* [94] or arXiv:0906.1986 [95]

## Two-Particle Dark State in the Transport through a Triple Quantum Dot

- [96]
- © TUB

We study transport through a triple quantum dot in
a triangular geometry with applied bias such that both singly- and
doubly-charged states participate. We describe the formation of
electronic dark states—coherent superpositions that block current
flow—in the system, and focus on the formation of a two-electron
dark state. We discuss the conditions under which such a state forms
and describe the signatures that it leaves in transport properties
such as the differential conductance and shot noise.

Christina Pöltl, Clive Emary, and Tobias Brandes*Phys.
Rev. B 80, 115313 (2009) [97]* or arXiv:0905.2087
[98]

## Weak coupling approximations in non-Markovian transport

- [99]
- © TUB

We study the transport properties of the
Fano-Anderson model with non-Markovian effects, which are introduced
by making one tunneling-rate energy dependent. We show that the
non-Markovian master equation may fail if these effects are strong. We
evaluate the stationary current, the zero-frequency current noise, and
the occupation dynamics of the resonant level by means of a quantum
master-equation approach within different approximation schemes and
compare the results to the exact solution obtained by the scattering
theory and Green’s functions.

Philipp Zedler, Gernot
Schaller, Gerold Kiesslich, Clive Emary, and Tobias Brandes

Phys.
Rev. B 80, 045309 (2009) [100] or *arXiv:0902.2118* [101]

## Noise enhancement due to quantum coherence in coupled quantum dots

- [102]
- © Copyright??

We show that the intriguing observation of noise
enhancement in the charge transport through two vertically coupled
quantum dots can be explained by the interplay of quantum coherence
and strong Coulomb blockade. We demonstrate that this novel mechanism
for super-Poissonian charge transfer is very sensitive to decoherence
caused by electron-phonon scattering as inferred from the measured
temperature dependence.

G. Kiesslich, E. Schöll, T.
Brandes, F. Hohls and R.J. Haug*Phys. Rev. Lett.
99, 206602 (2007)* [103]

## Adiabatic preparation without Quantum Phase Transitions

- [104]
- © Copyright??

Many physically interesting models show a quantum
phase transition when a single parameter is varied through a critical
point, where the ground state and the first excited state become
degenerate. When this parameter appears as a coupling constant, these
models can be understood as straight-line interpolations between
different Hamiltonians H_{I} and H_{F}. For
finite-size realizations however, there will usually be a finite
energy gap between ground and first excited state. By slowly changing
the coupling constant through the point with the minimum energy gap
one thereby has an adiabatic algorithm that prepares the ground state
of H_{F} from the ground state of H_{I}. The adiabatic
theorem implies that in order to obtain a good preparation fidelity
the runtime τ should scale with the inverse energy gap and
thereby also with the system size. In addition, for open quantum
systems not only nonadiabatic but also thermal excitations are likely
to occur. It is shown that—using only local Hamiltonians—for the
one-dimensional quantum Ising model and the cluster model in a
transverse field the conventional straight-line path can be replaced
by a series of straight-line interpolations, along which the
fundamental energy gap is always greater than a constant independent
on the system size. The results are of interest for adiabatic quantum
computation since strong similarities between adiabatic quantum
algorithms and quantum phase transitions exist.

G.
Schaller

Physical Review A **78**, 032328, (2008)
[105] and

Virtual Journal of Nanoscale Science & Technology
**18**(14) and

Virtual Journal of Quantum
Information **8**(10),

e-print:[0807.2516]
[106]

## Measuring the Entanglement between Double Quantum Dot Charge Qubits

- [107]
- © TUB

We present a scheme for creating and measuring
entanglement between two double quantum dot charge qubits in a
transport setup in which voltage pulses can modify system parameters.
Detection of entanglement is performed via the construction of a Bell
inequality with current correlation measurements. An essential feature
is the use of the internal dynamics of the qubits as the constituent
electrons tunnel into the leads to give the single-particle rotations
necessary for the Bell measurement.*C. Emary *

Phys. Rev. B 80, 161309(R) (2009) [108], arXiv:0905.2269
[109]

## Frequency-dependent counting statistics in interacting nanoscale conductors

- [110]
- © TUB

We present a formalism to calculate
finite-frequency current correlations in interacting nanoscale
conductors. We work within the n-resolved density matrix approach and
obtain a multi-time cumulant generating function that provides the
fluctuation statistics, solely from the spectral decomposition of the
Liouvillian. We apply the method to the frequency-dependent third
cumulant of the current through a single resonant level and through a
double quantum dot. Our results, which show that deviations from
Poissonian behaviour strongly depend on frequency, demonstrate the
importance of finite-frequency higher-order cumulants in fully
characterizing interactions.

C. Emary, D. Marcos,
R. Aguado, and T. Brandes*Phys. Rev. B 76,
161404(R) (2007),*

*[cond-mat/0703781]*[111]

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d/AG_Brandes/Bilder/Plots/Ising_stepwise_10.gif

d/AG_Brandes/Bilder/Plots/fevol_Cmax_T1.jpg

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