WORKSHOP PROGRAM
Monday 11 August : QMOTORS
TIME

SPEAKER

TITLE TALK

REMARKS

9.00 
9.30 registration




9.30 –
10.15

Peter Hänggi

Brownian motors in physics and biology

Theory

10.15 – 11.00

Stefan Scheidl

QBrownian
motion in ratchet potentials

Theory

Coffee break




11.30 – 12.15

Tomas Novotny

Quantum theory of shuttling instability

Theory

Lunch




Free discussion




15.30 – 16.15

Ken Segall

Fluxon ratchet dynamics in a 1D circular array
of Josephson Junctions

Experiment

16.15 – 17.00

Joël Peguiron

Fewbands quantum ratchets

Theory/Exp

17.00 – wine + cheese
welcome party




Tuesday 12 August: QTHERMODYNAMICS
TIME

SPEAKER

TITLE TALK

REMARKS

9.00 – 9.45

Markus Buttiker

Energy fluctuations, persistent current and
entanglement in the ground state of a system coupled to a bath

Theory

9.45 – 10.30

Tammy Humphrey

Quantum heat engines in the maximum power
regime

Experiment

Coffee break




11.00 – 11.45

Marlan Scully

Radiative
quantum thermodynamics

Theory

11.45 – 12.30

Juan Parrondo

Probabilistic arrow of time

Theory

Lunch




14.0014.30 Poster presentation

(2 minutes and maximum 2 slides)



14.30– 16.00 Poster session (with drinks)




16.00 – 16.30

Jens Siewert

Tunneling rates for ferromagnetic junctions

Theory

16.30 – 17.00

Luigi Amico

Integrable generalizations of Jaynes Cummings
models with counterroutating terms

Theory

Wednesday 13 August:
SYMPOSIUM QDISSIPATION
TIME

SPEAKER

TITLE TALK

REMARKS

9.00 –
9.45

Amir Caldeira

Quantum dissipation and applications

Theory

9.45 –
10.30

Hans Mooij

Superconducting flux qubits

Experiment

Coffee break




11.00 – 11.45

David Haviland

Quantum fluctuation effects in 1D Josephson
junction arrays

Experiment

11.45 – 12.30

Paolo Tombesi

Actively controlling decoherence in quantum
systems

Exp./Theory

Lunch




14.00 –14.45

GertLudwig Ingold

Overdamped Josephson junction and duality:
Between Coulomb blockade and
macroscopic quantum tunneling

Theory

14.45 – 15.30

Giuseppe Falci

Modulation of dephasing due to a spinboson
environment

Theory

Coffee break




16.00 – 16.40

Peter Talkner

Is the dynamics of an open quantum system
always linear?

Theory

16.40 –17.20

Reinhold Egger

Dissipative
quantum dynamics simulations

Theory/
Numerical methods

17.20 –18.00

Jürgen Stockburger

Stochastic Liouvillevon Neuman equations for
nonMarkovian quantum dissipation decoherence and transport

Theory/ numerical methods

18.30 –20.00 Symposium dinner




20.30 –21.30 Colloquium

Herman Grabert

Macroscopic
quantum tunneling in Josephson systems:
From fundamental research to applications


Thursday 14 August:
QCOMPUTING
TIME

SPEAKER

TITLE TALK

REMARKS

9.00 –
9.45

Denis Vion

Steering
the quantum state of an electrical circuit

Experiment

9.45 – 10.30

Irinel Chiorescu

Quantum decoherence for a superconducting flux
qubit

Experiment

Coffee break




11.00 – 11.45

Kouichi Semba

Single shot readout of the flux qubit

Experiment

11.45 – 12.30

Alexander Shnirman

Noise and decoherence in quantum twostate systems:
nonlinear coupling and higherorder effects

Theory

Lunch




Free discussion




16.00 – 16.45

Michael Thorwart

Dephasing
and relaxation in a flux qubit coupled to a noisy detector

Theory/
Numerical methods

16.45 – 17.30

Frank Wilhelm

Engineering decoherence of solid state quantum
bits

Theory

19.00 – Boat excursion
+
Conference dinner




Friday 15 August: QDECOHERENCE AND QDISSIPATION
TIME

SPEAKER

TITLE TALK

REMARKS

9.00 – 9.45

Ulrich Weiss

Nonequilibrium quantum decay, decoherence and noise in
quantum impurity problems

Theory

9.45 – 10.30

Fernando Sols

Quantum electrodynamical fluctuations in the
macroscopic phase of a Josephson link

Theory

Coffee break




11.00 – 11.45

Robert Withney

Berry phase of nonorthogonal resonances

Theory

11.45 – 12.30

Massimo Palma

Berry phase for a spin ½ in a classical
fluctutating field

Theory

Lunch




Free discussion




16.00 – 16.30

Shiro Saito

Multiphoton absorption observed in a
superconducting flux qubit

Experiment

16.30 –17.00

Pasquale Sodano

Quantum macroscopic coherence in Josephson junction
networks with non conventional architectures

Theory

Saturday 16 August: QTHERMODYNAMICS
II
TIME

SPEAKER

TITLE TALK

REMARKS

9.00  9.45

Peter Keefe

Coherent magnetocaloric effect superconductive
heat engine process cycle

Theory

9.45 – 10.30

Ken Sekimoto

Measurement and control in classical mesoscale
thermodynamics

Theory

Coffe break




11.00  11.45

Armen Allahverdyan

Work extraction from mesoscopic systems

Theory

11.45 –10.30 closure



Theory

Closure lunch




POSTERS
AUTHOR

TITLE

Sang Wook Kim

Floquet formalism of quantum pumps.

Sang Wook Kim

Decoherence from
chaotic internal dynamics

Henryk Gutman

Compensation of decoherence
from telegraph noise by means of bangbang control

Markus Storcz

Decoherence and gate performance of coupled
solid state qubits

Oleg Jouravlev

Enhanced shot noise in resonant tunnelling via
interacting localised states

A.E. Allahverdyan, R. Balian, Th.M.
Nieuwenhuizen

CurieWeiss model for the quantum measurement process

S. Nicolosi, A.
Napoli, A. Messina

Stationary
entanglement induced by dissipation

ABSTRACTS
MONDAY: QMOTORS
Brownian
motors in physics and biology
Peter Hänggi
University
of Augsburg, Germany
Noise is usually thought of as the enemy of
order rather as a constructive influence. For the phenomena of Stochastic
Resonance and Brownian Motors (and/or molecular motors) [1,2] random noise
plays a beneficial role in enhancing detection and/or facilitating directed
transmission of information in absence of biasing forces. Here, I will focus on
the possibility to rectify noise so that quantum and classical objects can be
directed around on a priori designed routes in biological and physical systems.
In doing so, the energy from the haphazard motion of (quantum) Brownian
particles is extracted to perform useful work against an external load. This
very concept together with first experimental realizations is discussed.
Moreover, new applications that involve the electron transfer in molecular
bridged metallic leads are presented.
[1] R. D. Astumian and P. Hänggi, Brownian Motors, Physics Today, 55 (11): 3339 (2002).
[2] P. Reimann and P. Hänggi, Introduction to the Physics of Brownian
Motors, Appl. Phys. 75: 169178
(2002).
QBrownian motion in ratchet potentials
Stefan Scheidl
University of Koeln, Germany
We investigate
the dynamics of quantum particles in a ratchet potential subject to an ac force
field. We develop a perturbative approach for weak ratchet potentials and force
fields [1]. Within this approach, we obtain an analytic description of dc
current rectification and current reversals. Transport characteristics for
various limiting
cases  such as the classical limit, limit of high or low frequencies, and/or
high temperatures  are derived explicitly. To gain insight into the intricate
dependence of the rectified current on the relevant parameters, we identify
characteristic scales and obtain the response of the ratchet system in
terms of scaling
functions. We pay a special attention to inertial effects and show that they
are often relevant, for example, at high temperatures. We find that the high
temperature decay of the rectified current follows an algebraic law with a
nontrivial exponent, j~T^{17/6}.
[1] S. Scheidl and V.M. Vinokur, Phys. Rev.
B 65, 195305 (2002)
Quantum theory of shuttling instability
Tomas Novotny
Department of
Electronic Structures, Charles University, Prague, Czech Republic
We present a quantum theory for an
electromechanical instability in a NEMS device [1], the singleelectron shuttle
first studied by Gorelik et al. [2]. This device consists of a movable
singleelectron transistor (SET) and exhibits an electromechanical instability
from the tunnelling regime to a shuttle regime in which the SET oscillates and
carries an integer number of electrons per a cycle. We demonstrate that the
characteristic strong correlation between the oscillator motion and the
electron transfer persists even in the quantum regime. The noise generated by
various sources (shot and thermal, both having quantum components) is found to
be very important for the phenomenon. We identify a crossover from the
tunnelling to the shuttling regime using a phase space formulation in terms of
the Wigner function, thus extending the previously found classical results to
the quantum domain. We also discovered a new dynamical regime, where the
shuttling is driven exclusively by the quantum noise.
[1] T. Novotny, A. Donarini, and A.P.
Jauho, Quantum Shuttle in Phase Space,
accepted to Phys. Rev. Lett., (2003).
[2] L. Y. Gorelik
et al, Phys. Rev. Lett. 80 (20), 4526 (1998).
Fluxon ratchet dynamics in a 1D circular array of Josephson
Junctions
Ken Segall, Terry P. Orlando and Juan J. Mazo
Massachussets Institute of Technology, Cambridge, USA
We describe the experiments, data and
analysis related with our work to realize the ratchet effect in a 1D, circular
array of Josephson junctions. We have
fabricated superconducting NbAlOxNb arrays with carefully chosen parameters
designed to realize a ratchet potential for magnetic fluxons (or “kinks”)
trapped in the array. A ratchet
potential can be realized by varying the Josephson critical currents and the
cell inductances in an asymmetric way.
With small junctions and low temperatures, fluxons can behave like
quantum particles and undergo macroscopic quantum tunneling between cells. With highimpedance junctions, the dynamics
are underdamped and the system has two possible states: a zerovoltage state
where the fluxon is pinned in a potential minimum and a finite voltage state
where the fluxon moves throughout the array.
A current in the array applies a force to the fluxon, eventually causing
a transition from the zero voltage state to the running state. Measurements of the critical force required
to move the fluxon, socalled switching current measurements, probe the
transport dynamics of the system. The
ratchet potential causes different switching rates in the two different
directions. A temperaturedependent
crossover in the average switching current is observed, a possible indication
of the onset of quantum tunneling and hence the rectification of quantum
fluctuations. We discuss our device
design, experimental setup and results, and analysis and interpretation of the
crossover.
Fewbands quantum ratchets
Joël Peguiron, Johannes B. Majer, Milena
Grifoni, and Hans Mooij
Delft University, The Netherlands
In the first
part of the talk we investigate
theoretically [1] the rectification of quantum fluctuations by unbiased driving forces in periodic ratchet potentials
sustaining few bands below the barrier. Upon restricting the dynamics to the
lowest M bands, the total systemplusbath Hamiltonian ismapped onto a discrete
tightbinding model containing all the information on the intrawell as well as
the interwell tunneling motion. A closed form for the current in the
incoherent tunneling regime is obtained. We find that in effective single band
ratchets no rectification occurs.
In the second
part of the talk we report on the measured quantum ratchet effect for vortices
in a quasi onedimensional Josephson junction array [2]. In this solid state
device the shape of the vortex potential energy, and consequently the band
structure, can be accurately designed. We find that asymmetric structures
possessing only one band below the barrier do not exhibit current rectification
at low temperatures and low bias currents. The quantum nature of transport is
also revealed in universal/nonuniversal powerlaw dependence of the measured
voltagecurrent characteristic for samples without/with rectification.
A complete theoretical understanding of
our data is missing.
[1] M.
Grifoni, M.S. Ferreira, J. Peguiron and J.B. Majer, Phys. Rev. Lett. 89, 146801 (2002).
[2] J.B. Majer, J. Peguiron, M. Grifoni, M.
Tuesveld and J.E. Mooij, Phys. Rev. Lett. 90,
056802 (2003).
TUESDAY: QTHERMODYNAMICS
Energy fluctuations, persistent current
and entanglement in the ground state of a system coupled to a bath
Markus Buttiker
University of Geneva, Switzerland
It is often
stated that in the ground state a system can not exchange energy with a bath
since neither the system nor the bath can give up energy. In this talk we focus
on simple model problems which permit an exact discussion of their ground state
properties. As a physical illustration we consider a mesoscopic ring with an
inline quantum dot, penetrated by an AharonovBohm flux [1] and capacitatively
coupled to an external resistor. This problem is mapped onto a spin boson
problem with a level spacing that depends on the AharonovBohm flux. We find
that the persistent current decreases and its relative fluctuations away from
the average increase with increasing coupling to the reservoir [2]. We make a
sharp distinction between systems depending on whether or not the system
Hamiltonian commutes (or does not commute) with the total Hamiltonian [3]. The
distinction is illustrated by considering the energy fluctuations in the ground
state of small systems coupled to a bath [3]. We establish connections between
the persistent current, energy fluctuations and the degree of entanglement
between system and bath.
[1]
M. Buttiker and C. A. Stafford, Phys. Rev. Lett. 76, 495 (1996).
[2]
P. Cedraschi, V. V. Ponomarenko, and M. Buttiker, Phys. Rev. Lett. 84, 346 (2000).
[3]
K. E. Nagaev and M. Buttiker, Europhys. Lett. 58, 475 (2002).
Quantum heat engines in the maximum power regime
Tammy
Humphrey (1), Heiner Linke (2)
(1)
UNSW Sydney, Australia
(2) University
of Oregon, Eugene, USA
Models of experimental electron tunneling ratchets [1] predict that such devices have a certain
capability to pump heat, even though at very low efficiency [2]. However,
utilizing "energy filters", such as 1D0D1D resonant tunneling
structures, the transfer of electrons between heat baths can in principle be
made reversible, allowing the theoretical construction of quantum Brownian heat
engines that operate arbitrarily close to Carnot efficiency [3]. As must be the
case, the power goes to zero as Carnot efficiency is approached. It is
therefore interesting to ask: What is the efficiency of a quantum Brownian heat
engine under conditions when it delivers maximum power [4]? This question will be explored both for the
cases of a heat engine and for its reverse realization, the refrigerator. The
results will be compared to those for an endoreversible classical Carnot
engine and for the threelevel amplifier, a laserbased quantum heat engine
[5].
[1] H. Linke et al., Science 286, 2314 (1999)
[2] T. E.
Humphrey et al., Physica E 11, 281
(2001)
[3] T. E.
Humphrey et al., PRL 89, 116801
(2002)
[4] T. E. Humphrey, PhD thesis,
unpublished.
[5] H.E.D. Scovil and E.O. SchulzDuBois,
PRL 2, 262 (1959)
Radiative quantum thermodynamics
Marlan O. Scully
Princeton University, USA
We present here a
quantum Carnot engine in which the atoms in the heat bath are given a small bit
of quantum coherence. The induced quantum coherence becomes vanishingly small
in the hightemperature limit at which we operate and the heat bath is
essentially thermal. However, the phase Φ,
associated with the atomic coherence, provides a new control parameter that can
be varied to increase the temperature of the radiation field and to extract
work from a single heat bath. The deep physics behind the second law of
thermodynamics is not violated; nevertheless, the quantum Carnot engine has
certain features that are not possible in a classical engine.
In a related
problem we study the situation in which ground state atoms are accelerated
through a high Q microwave cavity.
Unruh radiation is produced with an intensity which can exceed the intensity of
ordinary Unruh acceleration radiation in free space by many orders of
magnitude. The cavity field at steady state is described by a thermal density
matrix under most conditions. However, under some conditions gain is possible,
and when the atoms are injected in a regular fashion, the radiation can be
produced in a squeezed state.
Probabilistic arrows of time
Juan MR Parrondo
Univesidad Complutense de Madrid, Spain
We present processes which are not
invariant under time reversal, but slightly differ from the irreversible
processes usually considered in thermodynamics. The inversion of these new
processes can occur with a probability less than one but not negligible, and
they involve a microscopic decrease of entropy
Tunneling rates for ferromagnetic junctions
Jens
Siewert (1),
Giuseppe Falci (2), and Klaus Richter (1)
(1) Institut für Theoretische Physik,
Universität Regensburg, Germany
(2) Dipartimento di Metodologie Fisiche e
Chimiche per l'Ingegneria, Universita’ di Catania, Italy
If a tunneling
electron is coupled to some other degree of freedom, this may give rise to a
zerobias anomaly in the currentvoltage chararcteristics. A prominent example
for this phenomenon is the coupling to the modes of the electromagnetic
environment of the circuit via the electron charge, leading to the socalled P(E)
theory. Here we investigate whether assisted electron tunneling between
itinerant ferromagnets may lead to zerobias anomalies for ferromagnetic tunnel
junctions. We compare our results with
recent experiments.
Integrable generalizations of Jaynes Cummings models with
counterroutating terms
Luigi
Amico,
Universita` di Catania, Italy
We construct models describing interaction between spin and
bosonic degrees of freedom using a quantum inverse scattering procedure. The
form of the coupling constants result to be
restricted in such a way that the corresponding Hamiltonians are
integrable by construction. For a single bosonic mode interacting with a spinS the model we find is a
generalization of the JaynesCummings model including "counterrotating"
operators. Using connections with Gaudin models, the approach is described also
for the case corresponding to manyspins interacting with certain bosonic bath
WEDNESDAY: SYMPOSIUM QDISSIPATION
Quantum Dissipation and Applications
Amir Caldeira
University of Campinas, Brasil
In this talk it is our intention to present the basic ideas of what is
known today as “ Quantum Dissipation” and show
its importance to new problems appearing in different areas of Physics. We shall approach the problem
investigating realistic physical situations where the question of dissipation
is really relevant and then identify the concrete problems to be tackled. Some
examples are; the dissipative quantum tunnelling, the dissipative coherent
tunnelling and the decoherence between wave packets in the classically
accessible region of the phase space of the system. Once we have accomplished
that, we will introduce several examples of applications of the previously
developed techniques to superconductivity, magnetism and optics. The relevance
of these questions to problems related to quantum computation will be briefly
touched upon.
Superconducting flux qubits
Hans Mooij
Delft University of Technology, The Netherlands
We study a
superconducting flux qubit that consists of a small ring with three Joesphson
junctions. When biased at about a half flux quantum through the ring it has two
quantum states with opposite circulating persistent current. Transitions
between these states are induced by resonant microwave signals. With continuous
radiation, the level splitting can be determined spectroscopically. From such
measurements the occurrence of superpositions of the macroscopic states could
be determined. More recently, we studied coherent quantum dynamics of a
threejunction flux qubit. Rabi oscillations were observed with periods down to
1 ns over a time up to 500 ns. Two and three pulse experiments were performed
to establish the dephasing time. Also, spectroscopic measurements were
performed on two coupled qubits. New experiments are in progress, aimed at a
reduction of dephasing and improved fidelity of the readout.
Quantum fluctuation effects in 1D Josephson junction
arrays
David Haviland,
Royal Institute of Technology (KTH),
Stockholm, Sweden
One dimensional arrays of small
capacitance Josephson junctions form an interesting system for the study of
quantum fluctuation effects in electronic circuits. The arrays can be modeled as a transmission line with an
impedance that can exceed the quantum resistance. In this case a Coulomb blockade results, in spite of the large
Josephson coupling energy and the low impedance of the environment. The arrays can be fabricated in a SQUID
geometry to allow for tuning of the Josephson coupling energy. Experiments will
be described which investigate the Coulomb blockade regime in these arrays, and
the use these arrays as a tunable electrodynamic environment for the study of
tunneling in a single, small capacitance Josephson junction.
Actively controlling decoherence in quantum systems
Paolo Tombesi,
Universita` di Camerino, Italy
Some methods to control
decoherence, by actively acting on the quantum system, proposed in the last
years, will be discussed. They are quantum feedback control, dynamical decoupling and stochastic modulation of system
parameters.
Overdamped Josephson junction and duality:
Between Coulomb blockade and
macroscopic quantum tunneling
GertLudwig
Ingold,
Universität
Augsburg, Germany
For ultrasmall
Josephson junctions in the presence of an Ohmic environment, the socalled P(E)theory predicts a
zero bias anomaly. In particular, for weak environmental resistance, the
current should diverge as the external
voltage approaches zero. It turns out, however, that this result applies
only at sufficiently high voltages and thus describes only a part of the
currentvoltage characteristics which exhibits a finite peak as a remnant of
the dc Josephson effect. Exploiting results of U. Weiss and others on the
duality of an overdamped particle in a periodic potential, a global
understanding of the currentvoltage characteristics is obtained and it is
found that two different physical mechanisms are at work. For sufficiently small
external resistance, the transport at low voltages is determined by macroscopic
quantum tunneling while the high voltage behavior is dominated by Coulomb
blockade. It is proposed that a clear distinction between the two mechanisms
and thus a verification of this scenario can be achieved by measuring both flux
noise and charge noise.
[1] G.L. Ingold and H. Grabert, Phys. Rev.
Lett. 83, 3721 (1999)
[2] H. Grabert and G.L. Ingold, Europhys.
Lett. 58, 429 (2002)
Modulation of dephasing due to a spinboson environment
(1)
Elisabetta Paladino, (2) Maura Sassetti and
(1) Giuseppe Falci
(1)Universita` di Catania, Italy
(2) Universita` di Genova, Italy
We study the
reduced dynamics of a spin (qubit) coupled to a spinboson environment in the
case of pure dephasing. We derive formal exact expressions which can be cast in
terms of exact integrodifferential master equations. We present results for a
spinboson environment with Ohmic dissipation at finite
temperatures. For the special value of the Ohmic damping strength K=1/2 the reduced dynamics is found in
analytic form. For K << 1 we
discuss the possibility of modulating the effect of the spinboson environment
on the qubit. In particular we study the effect of the crossover to a slow
environment dynamics, which
may be triggered by changing both the
temperature and the systemenvironment coupling.
Is the dynamics of an open quantum system always linear ?
Peter
Talkner, and
Karen Fonseca Romero
Universität Augsburg, Germany
The linearity of the dynamics of an open
quantum system generally is thought to be a simple consequence of the well
established linearity of the Schroedinger equation for the full system that
comprises the considered system interacting with its environment. Combined with
the linearity of the partial trace over the full density matrix that gives the
reduced density matrix of the considered open system, the linearity of the
reduced dynamics appears to follow as a trivial statement. This argument,
however, overlooks the fact that in order to apply the unitary dynamics of the
total system the knowledge of the state of the total system is required. Hence,
at least once in the beginning, when the process is started a density matrix of
the full system has to be assigned to each possible initial density matrix of
the reduced system. We construct this mapping for one of the simplest possible
compound systems under the assumption of an experimentally feasible preparation
procedure, and show that in general it is not linear and that, consequently,
the dynamics of an open quantum system can generally not be represented by a
linear mapping.
Dissipative quantum dynamics simulations
Reinhold
Egger
Universität Düsseldorf, Germany
In this talk I will discuss some ideas
related to realtime Monte Carlo simulations for dissipative quantum systems.
Two recent applications will be discussed in detail: (1) Lowtemperature
electron transfer dynamics within a spinboson description, and (2) the problem
of resonant tunneling through a double barrier structure in a nanotube.
Stochastic Liouvillevon Neuman
equations for nonMarkovian quantum
dissipation, decoherence and transport
Jürgen T. Stockburger (1) and Hermann Grabert (2)
(1) Universität Stuttgart, Germany,
(2) AlbertLudwigsUniversität,
Freiburg, Germany,
Memory effects inherent in the dynamics of open quantum
systems have so far severely
restricted the range of applicable methods other than perturbation theory. A recent approach using stochastic
Liouvillevon Neuman (SLN) equations [1] formally eliminates quantum
memory effects by transforming them
into correlations of cnumber noise forces acting on the open system. This
allows the use of equations of motion where FeynmanVernon path integrals used
to be the only known exact formalism
for the reduced dynamics. Using Gaussian identities, the path integral description is transformed into the elementary
linear SLN equation
where x (t) and n(t) are coloured noise
forces whose correlation functions
reflect the quantum statistical correlations of the environment interacting with the open system. Stochastic
averaging of samples over the distribution of x (t) and n(t) yields the physical
reduced density matrix. Many equivalent stochastic processes with this property exist; the linear SLN
equation can be transformed into the process
where the persample expectation value
is introduced to yield a process which conserves trr for each
sample. This nonlinear SLN equation shows an evident similarity to classical
Langevintype dynamics, to which it
reduces in the classical limit of vanishing Planck’s constant. Both equations are separable into stochastic Schrödinger equations through
the ansatz
for the stochastic sample. The use of
equations of motion in numerical computation circumvents the dynamical sign problem inherent in the
MonteCarlo integration of realtime
path integrals. Algorithms based on SLN equations are applicable for times far beyond the transient timescales at
which pathintegral based algorithms break down. Applications to the dynamics of bound systems as well as transport
in periodic structures will be discussed, including numerical results.
[1] J.T.
Stockburger and H. Grabert,
Phys. Rev. Lett., 88, 170407 (2002)
COLLOQUIUM
Enhancement of Macroscopic Quantum Tunneling by LandauZener
Transitions
Hermann
Grabert and
Joachim Ankerhold
AlbertLudwigsUniversität
Freiburg, Germany
Motivated by recent realizations of qubits
with a readout by macroscopic quantum tunneling in a Josephson junction, we
study the problem of barrier
penetration in presence of coupling to a spin ½ system. It is shown that when
the diabatic potentials for fixed spin intersect in the barrier region, Landau Zener transitions
lead to an enhancement of the tunneling rate. The effect of these spin flips in
imaginary time is in agreement with experimental observations.
THURSDAY: QCOMPUTATION
Steering the Quantum State of an Electrical Circuit
G. Ithier, E. Collin, A.
Aassime, A. Cottet, D. Vion, P. Joyez, P.F. Orfila, H. Pothier, C. Urbina, M.H. Devoret and D.
Esteve.
Quantronics group, SPEC,
CEASaclay, Gif sur Yvette Cedex,
France
We have designed,
fabricated, and operated a superconducting electrical circuit, the quantronium,
that can be i) prepared in any quantum superposition of its two lowest energy
eigenstates and ii) measured by projection onto one of these eigenstates. This
circuit implements a quantum bit (qubit). It is based on the Cooper pair box
[1], a device which combines charging and Josephson effects. In our design [2],
the qubit can be decoupled from its measuring circuitry during preparation of a
quantum state so that relaxation and random dephasing are minimum. We present
experimental results demonstrating quantum coherent driven and free evolutions
of the qubit, including spin echoes. Decoherence sources are discussed.
[1] V. Bouchiat et al, PhysicaScripta, 76, 165 (1998).
[2] D.Vion et al, Science 296 (2002).
Quantum Coherence of a Superconducting Flux Qubit
Irinel Chiorescu, Yasunobu Nakamura, Cees Harmans and Hans Mooij
Delft University
of Technology, Delft, The Netherlands
We show that a superconducting flux qubit,
consisting of a closed loop interrupted
by three Josephson junctions, behaves according to the laws of quantum
mechanics when separated sufficiently from external degrees of freedom. The two states of such a
micronsize superconducting ring contain billions of Cooper pairs. From a ground
state in which all the Cooper pairs circulate in one direction, application of
resonant microwave pulses can excite the system to a state where all pairs move
oppositely, allowing us to have control over the coherent superposition of
these two states. Under strong microwave driving it was possible to induce
hundreds of coherent oscillations. Moreover, multiple pulses can be used to
create quantum operation sequences. With a twopulse sequence we performed
Ramsey interference experiments yielding a decoherence time of about 20 ns
whereas the spinecho time (threepulse sequence) was measured to be about 30
ns. The qubit readout is done by means of switchingevent measurements with an
attached SQUID revealing quantumstate oscillations with about 60% efficiency.
The SQUID has a hysteretic currentvoltage characteristic and is in direct
contact with the qubit loop. The mutual coupling is relatively large due to the
shared kinetic and geometric inductances of the joint part enhancing the qubit
signal. The sample fabrication is done by means of ebeam lithography and metal
evaporation using the nanotechnology facilities located in our campus (DIMES,
TU Delft).
Single shot readout of the fluxqubit
Kouichi Semba
NTT Basic Research Laboratories, Atsugishi, Kanagawa,
Japan
We
have succeeded in single shot readout of a superconducting fluxqubit. We
adopted a design of Josephson persistentcurrent qubit which consists of three Josephson junctions arranged in a
superconducting loop made of aluminum [1]. An underdamped dcSQUID, a quantum
detector, is configured outside of the qubit which couples inductively with the
qubit. From the spectroscopy measurement, we obtained the qubit energy
dispersion with an avoided crossing at flux
bias of p
and also an expected energy separation D of the relevant
two quantum levels. The switching current distribution of the SQUID detector
obtained by single shot measurement showed clear crossover behavior from the
classical LandauZener type tunneling to the cshaped crossing of a
quantum system, as we change the samples which have larger D. We have
obtained a switching histogram of the SQUID in which two peaks corresponding
the ground state and the excited state of the qubit separated clearly within
this cshaped crossing region [2].
[1] J. E. Mooij
et al., Science 285, 1036 (1999).
[2] S. Saito et al., in the proceedings of
the MQC2 Conference, Napoli, Kluwer Academic Plenum Publishers 2002.
Noise and decoherence in quantum twostate
systems: nonlinear coupling and higherorder effects
Yuriy Makhlin (1,2),
Gerd Schoen (1,3) and Alexander
Shnirman (1,4)
(1) Universität Karlsruhe, Germany
(2) Landau
Institute for Theoretical Physics, Moscow, Russia
(3) Forschungszentrum
Karlsruhe, Institut für Nanotechnologie, Germany
(3) Theoretical
Division, Los Alamos National Laboratory, USA
Motivated by
recent experiments with Josephsonjunction circuits we reconsider decoherence
effects in quantum twolevel systems (TLS). On one hand, the experiments
demonstrate the importance of $1/f$ noise, on the other hand, by operating at
symmetry points one can suppress noise effects in linear order. We, therefore,
analyze noise sources with a variety of power spectra, with linear or quadratic
coupling,
which are
longitudinal or transverse relative to the eigenbasis of the unperturbed
Hamiltonian. Manipulations of the quantum state of the TLS define
characteristic time scales. We discuss the consequences for relaxation and
dephasing processes.
Dephasing and relaxation in a flux qubit coupled to a
noisy detector
Michael Thorwart (1,2), Elisabetta
Paladino (3), Milena Grifoni (2)
(1) Universität
Düsseldorf, Germany
(2) Delft
University of Technology, The
Netherlands
(3) Dipartimento di Metodologie Fisiche e
Chimiche per l'Ingegneria, Universita`di Catania, Italy
We investigate
the dynamics of a quantummechanical twolevel system which is coupled to
harmonic oscillator representing the detector. The detector itself is damped
due to the interaction with its environment. This model system currently
receives considerable interest as it finds an application in experimental
realizations of a condensed matter qubit (flux qubit). It is attractive also because the model can be
mapped into a spinboson problem with a spectral density of the bath having a
resonance at the detector frequency. By using the numerically exact technique
of the quasiadiabatic propagator pathintegral, we calculate the dynamics of
the damped twolevel system and investigate the dependence of the dephasing and
relaxation rates on the various system parameters. A comparison with the
conventional rates for a weak coupling to an Ohmic bath reveals important
differences. At low temperatures and weak damping, we develop a threelevel
approximation to find analytical results for the dephasing rates. We apply the model to the specific device of
the flux qubit coupled to a dcSQUID which has been realized in the Delft
Quantum Transport group lead by J. Mooij.
Engineering decoherence of solid state quantum bits
Frank
K. Wilhelm
LudwigMaximiliansUniversität,
München, Germany
In order to realize solid state quantum
bits, it is crucial to minimize the decoherence from the coupling to the
numerous degrees of freedom of the solid state environment and the outside world.
I am going to outline, how to engineer the decoherence properties in different
ways, motivated by results of statistical physics and / or quantum information
theory. One way is to shape the spectrum of the environment by filtering out
the relevant frequencies, which leads to a spin boson model with a structured
bath. If more than one bath is present, such in the case of a double quantum
dot charge qubit, decoherence is related to an electron flow through the
device. We show that the decoherence can be controlled through the voltage
between the reservoirs and surprisingly is minimal at a finite voltage, i.e.,
out of equilibrium. Finally, I exemplify how general ideas on engineering
decoherence from quantum information research can be implemented using quantumstatistical
methods and will benchmark their effectiveness when applied to realistic models
of superconducting quantum bits. Examples will be dynamical decoupling and
decoherencefree subspaces. I will show that the requirements of engineering
decoherence differ between operation and measurement of a qubit and propose a
model that implements detectordominated measurements with long relaxation
times using a spin Boson model with a peaked spectral density.
FRIDAY: RELAXATION AND
DEPHASING IN QSYSTEMS
Nonequilibrium quantum
decay, decoherence and noise in quantum impurity problems
Ulrich
Weiss
Universität Stuttgart, Germany
In quantum impurity models (QIMs), few or many
quantum degrees of freedom are coupled to a field. Prominent representatives
are the spinboson model and the Schmid model. QIMs have attracted a great deal
of interest recently, because the underlying physics is nontrivial and the
models are manageable technically despite their essentially nonperturbative
nature. In addition, they have a multitude of experimental applications,
including the Kondo effect, quantum dots, dissipative quantum mechanics,
tunneling in quantum wires, nanotubes and fractional quantum Hall devices. In
this talk recent progress in understanding nonequilibrium transport,
statistical fluctuations, and decoherence in these models  obtained upon
combining results from the thermodynamic Bethe ansatz with those of the more
rigorous Keldysh approach  is reported. At zero temperature, exact results in
analytic form covering the full range from weak to strong tunneling have been
found. It is also shown that all the QIMs are closely related and the
respective reasons are given.
Quantum electrodynamical fluctuations in the macroscopic
phase of a Josephson link
Fernando Sols, Heiner Kohler and Paco Guinea
Universidad Autonoma de Madrid, Spain
We study
the equilibrium dynamics of the relative phase in a Josephson link
taking into account the fluctuations of the electrodynamic vacuum. The photons
act as a superohmic heat bath on the relative Cooper pair number and thus,
indirectly, on the macroscopic phase difference f. This leads to an enhancement of the
mean square <f ^{2}> that adds to the spread due to
the Coulomb interaction carried by the longitudinal electromagnetic field. We
also include the coupling to the electronic degrees of freedom due to
quasiparticle tunnelling. The simultaneous inclusion of both the radiation
field fluctuations and quasiparticle tunnelling leads to a novel type of
particlebath Hamiltonian in which the particle couples with its position and
its momentum to two independent bosonic heat baths. We study the interplay
between the two mechanisms in the present context and find interference
contributions to the phase quantum fluctuations. We explore the observability
of the QED effects discussed here.
Berry phase of
nonorthogonal resonances
Robert S. Whitney
Theoretical Physics, University of
Oxford,United Kingdom
We investigate the dynamics of the resonances
of a system coupled to its environment. The nonHermitian nature of the
system's evolution (once one has traced out the environmental degrees of
freedom) means that these resonances need not be orthogonal to each other. We
systematically calculate the properties of the resonances of a spinhalf weakly
coupled to a bath of oscillators (the biased spinboson model), and show that
indeed they are not orthogonal. We then investigate the dynamics of these
spinresonances when the spinHamiltonian is (a) static and (b) varied
adiabatical slowly. The nonorthogonal nature of the resonances manifests
itself clearly in the dynamics of spinstate. In case (b) the spinresonances
exhibit a Berry phase which has a rather complicated geometric interpretation,
different from naive expectations.
Berry phase for a spin ½ in a classical fluctutating field
Massimo
Palma
Dipartimento di Tecnologie
dell'Informazione, Università degli studi di Milano, Crema, Italy
Berry phases and
related geometrical phases have received renewed interest in recent years due
to several
proposal for
their use in the implementation of quantum computing gates. Such interest is
motivated by the belief that geometric quantum gates should exhibit an intrinsic fault tolerance in the presence of
external noise. Such belief is based on the heuristic argument that being Berry
phases geometrical in their nature, i.e. proportional to the area spanned in
parameter space, any fluctuating perturbation of zero average should indeed
average out. Although this argument seems convincing to the best of our
knowledge it has not been quantitatively probed so far. In particular, although
several authors have investigated aspects of Berry phases in the presence of
quantum external noise, we are not aware of any in which the effect of classical noise in a simple
model of qubit, namely a spin 1/2 interacting with an external classical field
with a fluctuating component has been analyzed. This is precisely the aim of
this talk. For such system the effects of classical fluctuations in the control
parameter on both geometric and dynamic phases is studied and their impact on
dephasing analyzed. We will explicitly show that in the adiabatic limit
dephasing is due to fluctuations of the dynamical phase.
Multiphoton absorption observed in a
superconducting flux qubit
Shiro Saito(1,5),
Hirotaka Tanaka(1,5), Michael Thorwart(1,2,3), Hayato Nakano(1,5),
Masahito Ueda(1,4,5), Kouich Semba(1,5), and Hideaki
Takayanagi(1,5)
(1) NTT Basic Research
Laboratories, NTT Corporation, Japan
(2) University of Duesseldorf, Germany
(3) Delft University
of Technology, The Netherlands
(4) Department of
Physics, Tokyo Institute of Technology, Japan
(5) CREST, Japan
Science and Technology Corporation, Japan
We have observed
multiphoton absorption peaks and dips in the magneticfield dependence of
switching current in a superconducting flux qubit system. Our qubit consists of
an aluminum loop with three Josephson junctions and inductively couples to a
dcSQUID as a switching detector[1]. The Josephson energy of the largest
junction in the qubit and the qubit energy splitting at the degeneracy point
are 350 GHz and 0.86 GHz, respectively. We have achieved strong coupling
between the qubit and an RF control line by using an onchip strip line.
Spectroscopy measurements were performed by applying a continuous microwave to
the qubit at fixed frequencies and by sweeping the magnetic fields[2]. We used
a triangular wave of 140Hz as a bias current to measure the switching current
of the dcSQUID. Up to three resonant
peaks and dips
were observed in the magnetic field dependence of the switching current at
fixed microwave frequencies. The width of these multiphoton absorption peaks
is described by the Bessel functions derived from realtime pathintegral
expressions[3]. The peak amplitudes are well explained by a phenomenological
model based on the Bloch equation[4].
[1] J. E. Mooij et al., Science 285,
1036 (1999).
[2] C. H. van
der Wal et al., Science 290, 773
(2000).
[3] L. Hartmann
et al., Phys. Rev. E 61, R4687
(2000).
[4] M. C.
Goorden, Master thesis, TU Delft (2002); M. C. Goorden and F. K. Wilhelm,
condmat/0305467.
Quantum Macroscopic Coherence in Josephson Junction Networks
with Non Conventional Architectures
Pasquale
Sodano,
Dipartimento di Fisica e Sezione
I.N.F.N, Università di Perugina, Italy
We shall focus on the interesting
properties emerging in Josephson networks with nonconventional architectures
showing, by means of explicit examples, how the network’s topology and geometry
may either lead to novel and unexpected coherent phenomena or be responsible
for taming de coherence in quantum Josephson devices. We shall also comment on
some network’ s geometries leading to remarkable connections with gauge
theories with discrete gauge groups.
SATURDAY: QTHERMODYNAMICS II
.
Coherent magnetocaloric
effect superconductive heat engine process cycle
Peter Keefe
24405 Gratiot Avenue, Eastpointe,
Michigan 48021 USA
A quantum motor is proposed in which a
mesoscopic (coherence range size) superconductor in a macroquantum state is
closed cycled in magnetic field  temperature space (HT space), the cycle
processes including a magnetocaloric magnetization, a magnetocaloric
demagnetization accompanied by the Meissner effect, and a heat influx. The
first order adiabatic phase transitions of the magnetocaloric processes occur
without the appearance of an intermediate state, resulting in movements of the T coordinate of the superconductor in HT
space. These T coordinate movements
involve interaction of the quantum mechanically condensed superelectron regime
with the normal regime devoid of collective system interaction, the result being
an overall lowering of the entropy of the superconductor. When these T coordinate movements in HT
space are combined with the Meissner effect which positively moves the H coordinate in HT space, the net effect
is a process cycle, unanticipated by Carnot theoretics, which transforms
ambient heat energy into work in a manner consistent with the First Law, but,
as a result of the macroquantum state of the superconductor, inconsistent with
traditional formulations of the Second Law.
.
Measurement and control in classical mesoscale
thermodynamics
Ken Sekimoto
University of Strasbourg, France
Small systems
require careful consideration of the operation of measurement and control, even on the scales where the quantum
interferences are unimportant. With a recently developed method of mesoscale
thermodynamics ("stochastic
energetics"), we will discuss about a kind of complementarity relation of
the
freeenergy measurement of mesoscale
systems, and also about two sources of intrinsic irreversibilities related to
the operation of making thermal contact with a heat bath.
Work extraction from mesoscopic systems.
A.E. Allahverdyan
University of Amsterdam, The Netherlands
Thermodynamics teaches that maximal work extractable from a
state is governed by its energy and entropy. In mesoscopic physics this bound
is usually not reachable. The maximal work extraction compatible with quantum
mechanics (``ergotropy'') is derived and related with the property of
majorization: states more ordered with respect to it, may provide more
work. Several scenarios of
workextraction are discussed, contrasting the thermodynamical intuition, e.g.
states with larger entropy may produce more work. Work extracted in a closed cycle from a twotemperature
mesoscopic system cannot exceed the Carnot bound.
POSTERS:
Floquet formalism of quantum pumps.
Sang Wook Kim
Max Planck
Institut, Dresden, Germany
A quantum pump is a device that
generates a dc current at zero bias potential through cyclic change of system
parameters. I would like to present Floquet formalism for quantum pumps, which
allows us to explore nonadiabatic as well as adiabatic regimes [1]. The issue
of the Pauli blocking factor is discussed during the derivation of the current
expression of the quantum pumps in the Floquet formalism [2]. Magnetic field
inversion symmetry in quantum pumps with discrete symmetries obtained from
Floquet formalism is also presented [3].
[1] S. W. Kim, Phys. Rev. B, 66, 235304 (2002)
[2] S. W. Kim, Phys. Rev. B (in press) [condmat/0210485]
[3] S. W. Kim, Phys. Rev. B (in press) [condmat/0212409]
Decoherence from chaotic internal dynamics
Sang Wook Kim
Max Planck
Institut, Dresden, Germany
I would like to present that the
classicalquantum correspondence of center of mass motion in two coupled
deltakicked rotors is enhanced by the entanglement of the center of mass
motion to the internal degree of freedom. The observed correspondence can be
attributed to the decoherence generated from chaotic internal dynamics with a
few degree of freedom.
H.K. Park and S. W. Kim, Phys. Rev. 67,
060102(R) (2003)
Decoherence and gate performance of coupled solid state
qubits
M.J. Storcz and F.K.Wilhelm
LudwigMaximiliansUniversität, München,Germany
Solid state
quantum bits are promising candidates for the realization of a scalable quantum computer. However, they are
usually strongly limited by decoherence due to the many extra degrees of
freedom of a solid state system. We
investigate a system of two solid state qubits that are coupled via type s_{z(i)} s_{z (j)} of coupling. This kind of setup is typical for pseudospin
solidstate quantum bits such as charge or flux systems. We evaluate
decoherence properties and gate quality factors
in the presence
of a common and two uncorrelated baths coupling to s_{z} , respectively. We show
that at low temperatures, uncorrelated baths do degrade the gate quality
more severely. In particular, we show
that in the case of a common bath, optimum gate performance of a CPHASE gate
can be reached at very low temperatures, because our type of coupling commutes
with the coupling to the decoherence, which makes this type of coupling
attractive as compared to previously studied proposals with s_{y(i)} s_{y(j)} coupling. Although less pronounced, this advantage also applies to
the CNOT gate. For superconducting flux qubits it appears to be relatively
easy, to implement a tunable coupling between the qubits, if one uses tunable Josephson junctions. We evaluate
possible coupling strengths and show how much extra decoherence is induced by
the subgap conductance of a switchable
junction. In the light of these results, we evaluate several options of using
intrinsically shunted junctions and show that based on stateofthe art
numbers, Josephson field effect transistors and highT_{c} junctions as pshifters would be a good
option, whereas the use of magnetic junctions as pshifters severely limits
quantum coherence.
Compensation of decoherence from telegraph noise by means of
bangbang control
H. Gutmann,
W.M. Kaminsky, S.Lloyd and F.K. Wilhelm
LudwigMaximiliansUniversität, München,Germany
With growing
success in isolating solidstate qubits from external noise sources, the
origins of decoherence inherent of the material start to play a relevant role.
One representative example are charged impurities in the disordered substrate
or junction material, which produce typical telegraph noise and can hence be
modeled as bistable fluctuators [1,2]. In order to demonstrate the possibility
of the active suppression of the disturbance from a single fluctuator, we theoretically implement an elementary
bangbang control protocol [3,4]. We simulate numerically the random walk of
the qubit state on the Bloch sphere with and without bangbang compensation
scheme and compare it with analytical results we receive by use of appropriate
Langevin equations in the longtime limit. Hereby we find out, that the
deviation of the pure random walk is scaled down approximatively by the ratio
of the bangbang period and the typical flipping time of the bistable
fluctuator.; therefore we expect the bangbang control working as a highpass
filter on the spectrum of noise sources. This indicates, how the influence of
1/fnoise ubiquitous to the solid
state world could be reduced. To describe non perfect bangbang pulses and evaluate
their extra dissipating influence on the qubit we also derive two generic
random walk models, which can be solved analytically. So we build a realistic
picture, how effective our idealized bangbang scheme might be in practise and
which the technical limitations are for this concept in fighting decoherence
generated by material defects.
[1] H. Gassmann, F. Marquardt and C. Bruder, Phys. Rev. E 66,
041111 (2002).
[2] E. Paladino, L. Faoro, G. Falci and R. Fazio, Phys. Rev. Lett. 88, 228304
(2002).
[3] S. Lloyd,
Phys. Rev. A 62, 022108 (2000).
[4] S.Lloyd and
L.Viola, Phys. Rev. A 65, 010101
(2001).
Enhanced shot noise in resonant
tunnelling via interacting localised states
Oleg Youravlev,
Yu. V. Nazarov
Dept. of NanoScience, Delft University of
Technology, The Netherlands
In a variety of
mesoscopic systems shot noise is seen to be suppressed in comparison with its
Poisson value. In this work we observe a considerable enhancement of shot noise
in the case of resonant tunnelling via localised states. We present a model of
correlated transport through two localised states which provides both a
qualitative and quantitative description of this effect.
CurieWeiss model for the quantum
measurement process
A.E. Allahverdyan, R. Balian and Th.M. Nieuwenhuizen
University of Amsterdam, The Netherlands
A Hamiltonian
model is solved, which satisfies all
requirements for a realistic ideal quantum measurement. The system S is a spin1/2, whose zcomponent is measured through coupling
with an apparatus A=M+B,
consisting of a magnet M formed by a
set of N >> 1 spins with
quartic infiniterange Ising interactions, and a phonon bath B at temperature T. Initially A is in a
metastable paramagnetic phase. The process involves several timescales.
Without being much affected, A first
acts on S, whose state collapses in a
very brief time. The mechanism differs from the usual decoherence. Soon after
its irreversibility is achieved. Finally
the field
induced by S on M, which may take two opposite values with probabilities given by
Born's rule, drives A into its up or down ferromagnetic
phase. The overall final state involves the expected correlations between the
result registered in M and the state
of S. The measurement is thus
accounted for by standard quantum statistical mechanics and its specific
features arise from the macroscopic size of the apparatus.
Stationary
entanglement induced by dissipation
S.
Nicolosi, A.
Napoli, A. Messina
INFM, MIUR and
Dipartimento di Scienze Fisiche ed Astronomiche, Palermo, Italy
The exact
dynamics of 2 twolevel dipoledipole interacting atoms coupled to a common
electromagnetic bath and closely located inside a lossy cavity is reported.
Stationary radiation trapping effects are found and very transparently
interpreted in the context of our approach. We prove that initially injecting
one excitation only in the 2 atoms cavity system, loss mechanisms
asymptotically drive the matter sample toward a stationary maximally entangled
state. The role played by the closeness of the 2 atoms with respect to such a
cooperative behaviour is brought to light and carefully discussed.