QCCQI 2008
Quantum/Classical Control in Quantum Information
QUROPE WORKSHOP QUANTUM/CLASSICAL CONTROL IN QUANTUM
INFORMATION: THEORY AND EXPERIMENTS
13-20, September 2008, Otranto (Italy)
Abstracts
CONTRIBUTED PAPERS
A solid-state light-matter interface at the single photon level
Mikael Afzelius, Hugues de Riedmatten, Matthias Staudt,
Christoph Simon, Nicolas Gisin
Group of Applied Physics, University of Geneva
Coherent and reversible mapping of quantum information between
light and matter is an important experimental challenge in quantum
information science. In particular, it is a decisive milestone for
the implementation of quantum networks and quantum repeaters. So
far, quantum interfaces between light and atoms have been
demonstrated with atomic gase, and with single trapped atoms in
cavities.
Here we will present experimental results of coherent and
reversible mapping of a light field with less than one photon per
pulse onto an ensemble of 10^7 atoms naturally trapped in a solid.
This is achieved by coherently absorbing the light field in a
solid-state atomic medium, which has been prepared by spectrally
shaping the optical inhomogeneous transition into an Atomic
Frequency Comb (AFC) [1]. The state of the light is mapped onto
collective atomic excitations on an optical transition and stored
for a pre-programmed time up of to 1μs before being released in a
well defined spatio-temporal mode as a result of a collective
interference due to the AFC. The coherence of the process is
verified by performing an interference experiment with two stored
weak pulses with a variable phase relation. Visibilities of more
than 95% are obtained, which demonstrates the high coherence of
the mapping process at the single photon level. In addition, we
show experimentally that our interface allows one to store and
retrieve light fields in multiple temporal modes.
[1] M. Afzelius, C. Simon, H. de Riedmatten, N. Gisin, Multi-Mode
Quantum
Memory
based
on
Atomic
Frequency
Combs,
arXiv:0805.4164
State selective microwave potentials on atom chips - towards a
controlled phase gate
Pascal Böhi(1,2), Max Riedel(1,2), Johannes Hoffrogge(2), Theodor
W. Hänsch(1,2) and Philipp Treutlein(1,2)
(1)Max-Planck-Institut für Quantenoptik, Garching, Germany
(2)Fakultät für Physik, Ludwig-Maximilians-Universität München,
München, Germany
We present the status of our experiment with microwave near-fields
on atom chips. Microwave near-fields are a key ingredient for atom
chip
applications
such
as
quantum
information
processing,
entanglement of Bose-Einstein condensates, atom interferometry,
the study of Josephson effects and chipbased atomic clocks. We
have integrated miniaturized microwave guiding structures on our
atom chip. The micrometersized structures allow to generate
microwave near-fields with unusually strong gradients. Through
microwave dressing of hyperfine states, these can be used to
create state-selective double-well potentials, which are the basic
building block for a collisional quantum phase gate [1] on the
atom chip.
[1] P. Treutlein et al., Phys. Rev. A 74, 022312 (2006)
Spatial and Spectral Phase Control in Quantum Interferometry
Cristian Bonato (1, 2), Olga Minaeva (1, 3), Alexander V.
Sergienko (1), Bahaa E. A. Saleh (1), Stefano Bonora (2), Paolo
Villoresi (2)
(1) Department of Electrical and Computer Engineering,
Boston University, 8 Saint Mary's Street, Boston (MA) 02215
(2) CNR-INFM LUXOR, Department of Information Engineering,
Via Gradenigo 6/B35131, Padova (Italy)
(3) Department of Physics, Moscow State Pedagogical University
119992 Moscow (Russia)
The study of quantum entanglement has lead to important
applications in the field of quantum information and quantum
metrology. Nonclassical states concurrently entangled in wavevector, frequency and polarization can be generated by means of
the
nonlinear
optical
process
of
spontaneous
parametric
downconversion (SPDC).
Frequency entanglement is at the heart of the even-order
dispersion cancellation effect: only the odd-order dispersion
terms contribute to the intererference pattern in the coincidence
rate when a sample is placed in one arm of a HOM interferometer.
The cancellation of group-velocity-dispersion leads to a reduction
in the broadening of a white-light interference pattern thus
fostering
superior
accuracy
in
position
and
trip-time
measurements. More tools dealing with spectral (dispersion) and
spatial (aberration) phase control may prove to be useful in other
applications.
Here we introduce a new spectral-domain technique that allows to
separate the contribution of even-order and odd-order dispersion
terms in different subregions of a global quantum interference
pattern. This effect is based on the manipulation of the quantum
probability amplitudes of the entangled-photon pairs produced by
SPDC. Selection of specific parameters of our coincidence
interferometer enables us to separate the detection of two nonclassical dispersion cancellation effects in one experimental
setup.
In addition, we experimentally demonstrate a spatial counterpart
of even-order dispersion cancellation, based on the entanglement
of the transverse components of the wave-vectors emitted in SPDC.
In particular, we modulate the spatial phase of entangled photons
in the far-field by a Fourier-domain controller comprising a
deformable mirror. We then feed the photons in a type-II quantum
interferometer, using it as an analysis tool: due to the
correlations between wave-vector and frequencies the interference
pattern in the polarization-temporal domain will be affected by
spatial distortions imparted by the adaptive mirror. We show that
even-order aberrations are cancelled and therefore do not affect
the shape of the dip.
For example, astigmatism, defocus and
spherical aberration are cancelled, while
coma and trefoil are
not.
In conclusion, we introduce new quantum optical tools for spatial
and spectral dispersion menagement and control. We believe that
these new physical effects will be useful in quantum metrological
and quantum imaging applications.
Ultrafast manipulation of a tunable flux qubit by pulses:
observation of coherent oscillations, RSFQ control and emerging
strategies
Fabio Chiarello (1), M.G. Castellano (1), P. Carelli (2), C.
Cosmelli (3),
J. Lisenfelf (4), A. Lukashenko (4), S. Poletto (4), G. Torrioli
(1)
and A.V. Ustinov (4)
(1) Istituto di Fotonica e Nanotecnologie - CNR, 00156 ROMA, Italy
(2) Dip. Ingegneria Elettrica, Università dell’Aquila, 67040
Monteluco di Roio, Italy
(3) Dip. Fisica, Università di Roma “La Sapienza”, 00185 Roma,
Italy
(4) Physikalisches Institut, Universitaet at karlsruhe (TH), D76131 Karlsruhe, Germany
We present a particular superconducting flux qubit, the double
SQUID qubit, manipulated with a technique based on the fast
modification of the qubit potential with pulses, in the absence of
microwaves.
This technique has been experimentally tested, and we observed
coherent oscillations with frequencies that can be tuned from
about 6GHz to 25GHz, which are very high values with respect to
similar systems.
The capability to tune the oscillation frequency, the simple
“digital-like” manipulation of the qubit, the good tolerance to
external noise and, in particular,
the very short time required
for a single operation make this system one of the most promising
for quantum computing applications.
We discuss also some development, such as the RSFQ control of the
system (which is particularly suitable for this kind of
manipulation), and the controllable coupling of many qubits.
Numerical optimisation applied to control problems
Pierre Becq de Fouquieres, Sonia G. Schirmer
Centre for Mathematical Sciences,
Wilberforce Road, Cambridge CB3 0WA, United Kingdom
We consider the problem of finding good control pulses for fully
characterised quantum systems. Although pulses motivated by
geometric decomposition are widely used within the experimental
community, applying numerical optimisation techniques to the
problem often leads to better pulses (eg: of shorter total time,
lower total energy, lower peak amplitude).
We study a local update scheme, constrained so as to impose a high
level of smoothness on the optimised pulse. This minimises the
amount of information content in the final pulse, so as to make it
more readily implementable; In particular, its spectrum can be
expected to decay rapidly for large frequencies, so that it can
realistically be implemented by standard frequency domain optical
pulse shaping equipment.
Although, the focus of this work has been unitary gate
engineering, it is immediately applicable to quantum state
preparation, and can be readily generalised to apply to
dissipative systems.
Quantum simulations on a few-qubit system
Miroslav Dobsicek, Goran Johansson, Vitaly Shumeiko, and Goran
Wendin
Chalmers Univ. of Technology, Dept. of Microtechnology and
Nanoscience, MC2
Applied Quantum Physics Laboratory, SE-412 96 Goteborg, Sweden
The design and study of robustness of small testbed applications
currently represents one of the short-term chief goals in the
quantum computing field. We focus on sample quantum simulations
which can be performed with as much as three to four qubits, e.g.
in the next generation of superconducting qubit systems. Recently,
we discussed how to perform quantum phase estimation algorithm in
an iterative manner [1] and herewith reduce the number of required
qubits. Our ongoing work deals with compact mapping from fermionic
systems to qubits in order to reduce the number of qubits even
more. Additionally, we perform classical simulations of designed
quantum circuits, study the effects of noise and discuss some fine
tunning for supercoducting qubits with ZZ-coupling.
[1] Miroslav Dobsicek, Goran Johansson, Vitaly Shumeiko, and Goran
Wendin. Arbitrary accuracy iterative quantum phase estimation
algorithm using a single ancillary qubit: A two-qubit
benchmark. Physical Review A, 76:030306(R), 2007.
Controlling many-body quantum systems via time-periodic forcing
Andre Eckardt
ICFO-The Institute of Photonic Sciences
Av. Canal Olimpic, s/n, E-08860 Castelldefels, Barcelona, Spain
It will be pointed out that time-periodic potential modulations
can be a robust and powerful tool for the manipulation of manybody systems as they are realized experimentally with ultracold
(bosonic) atoms in optical lattice potentials. Such control
schemes are reminiscent of manipulating internal atomic or
molecular degrees of freedom by means of coherent radiation, and
we describe them theoretically by an approach similar to the
dressed atom picture. In their simplest form, off-resonant forcing
is used to effectively modify the tight-binding tunneling matrix
element $-J$ describing the kinetics of these systems. This effect
(including $J\\approx0$ and $J<0$) has recently been measured from
the coherent expansion of a Bose-Einstein condensate in a shaken
lattice
[PRL
99,
220403
(2007)].
We
predict
the
tunnel
modification to survive in the strongly correlated regime,
allowing to induce the transition from a superfluid to a Mott
insulator and back by smoothly switching on and off a kHz drive
[PRL 95, 260404 (2005)]. These phenomena as well as further
control schemes based on time-periodic forcing will be discussed.
Local temperature in quantum thermal states
Artur Garcia-Saez
ICFO-The Institute of Photonic Sciences
We consider parts of quantum spin chains at thermal equilibrium,
focusing on their properties from a thermodynamical perspective.
Under some conditions, it is expected that the description of
blocks of the chain as thermal states with the same temperature as
the whole chain will fail. Specifically, we analyze when the
temperature ceases to be an intensive magnitude by employing the
quantum fidelity as a particularly sensitive figure of merit. Then
we show that the blocks can be considered indeed as thermal states
with a high fidelity, provided an effective local temperature is
properly identified. Such a result originates from typical
properties of reduced sub-systems of energy-constrained Hilbert
spaces. Finally, the relation between local and global temperature
is analyzed as a function of the size of the block and the system
parameters. This allows to single out in details the departure
from the classical behavior in these quantum systems.
Maximizing Noisy Quantum Memory Channels Capacity by
Dynamical Modulation
Goren Gordon and Gershon Kurizki
Department of Chemical Physics, Weizmann Institute of Science,
Rehovot 76100, Israel
Applying selective modulations on transmitted qubits, encoding
classical information, through a quantum noisy memory channel is
shown to be able to drastically increase the channel capacity.
The memory channel, whereby transmitted qubits affect each other's
decoherence through the channel, can be characterized by four
independent parameters, namely the channel decoherence rate
magnitude, asymmetry, cross-decoherence(memory) and effective
temperature.
We analyze the entire parameter space to reveal a non-trivial
interplay between the parameters and their effects on the channel
capacity.
We show that decreased magnitude and effective temperature,
together
with
increased
cross-decoherence
and
decoherence
asymmetry maximize the channel capacity.
Furthermore, a sharp transition from an optimal factorized to
optimal
fully
entangled
basis
for
encoding
the
classical
information is demonstrated as a function of the channel
parameters.
A parameter manifold whereby above it, it is beneficial to encode
the information in a fully entangled (Bell) basis is presented,
and may suggest an important first step in selecting an
experimentally optimal protocol for a dynamically controlled
memory channel.
Cold Ytterbium atoms in High-finesse optical cavities:
Cavity Cooling and Collective Interactions
H. Gothe, M. Cristiani, T. Valenzuela, J. Eschner
ICFO – The Institute of Photonic Sciences, Mediterranean
Technology Park,
08860 Castelldefels (Barcelona), Spain
The quantum behavior of cold atoms interacting with photons
confined in high-finesse cavities has been a subject of rising
interest during the last decade. In particular, new schemes for
laser cooling based on cavity feedback have been extensively
studied both theoretically and experimentally. Furthermore, such
systems are potential building blocks in quantum information
processing, serving for the interconversion between photonic and
atomic quantum states.
Here we present the status of the experimental setup we are
developing at ICFO. This apparatus will be suitable for studying
collective excitation of a cold atomic cloud interacting with the
standing wave of a resonator, with the perspective of using this
system for investigating new cooling mechanisms based on atomcavity interaction, as well as cavity-QED-based atom-photon
interfaces. We recently observed cooling and confinement of 174Yb
atoms in a Magneto-Optical Trap operating on the 1S0→1P1 transition
(λ=399nm, Γ=2π·28 MHz), and we observed the 1S0→3P1 intercombination transition (λ=556nm, Γ=2π·182 kHz) for various
isotopes. At the moment we are stabilising the 556nm laser source,
in order to use this transition for improved cooling and trapping.
At the same time a high-finesse cavity at 556nm is being designed
Spectral Characterisation of SPDC Entangled Photons Sources
Marco Gramegna(1), Giorgio Brida(1), Valentina Caricato(1), Maria
V. Chekhova(2), Mikhail V. Fedorov(3) Marco Genovese(1), Leonid A.
Krivitsky(4), Sergej P. Kulik(2)
(1) INRIM - Istituto Nazionale di Ricerca Metrologica,
Strada delle Cacce 91, 10135 Turin, Italy
(2) Department of Physics, M. V. Lomonosov Moscow State
University
Leninskie Gory, 119992 Moscow, Russia
(3) A. M. Prokhorov General Physics Institute,
Russian Academy of Sciences, Russia
(4) Institut fur Optik, Information und Photonik Max-Planck
Forschungsgruppe,
Universitaet Erlangen-Nurnberg, Guenther-Scharowsky-Str. 1/Bau 24,
91058 Erlangen, Germany
Entangled biphoton states generated via Parametric Down Conversion
(SPDC) stand at the heart of quantum optics and quantum
information,and the two-photon correlations can be investigated
with respect to several variables like polarization, momentum or
frequency, being these both discrete or continuous variables
states.
To explore deeper the entanglement properties in continuous
variables and perform a characterization of SPDC sources in terms
of frequency variables, we report experimental evidence for
creation of biphoton states with high spectral entanglement, under
condition when a femtosecond pulsed pump beam is well shaped to
provide biphoton coincidence spectrum much narrower and singleparticle one much wider than the pump spectrum[1-2], and the
evaluation of the ratio R between the FWHM of the two
distributions, as a measure of the achievable entanglement degree,
being temporal walk-off the physical key factor providing a large
contrast between single- to coincidence distributions.
The design of our experiment considered frequency entangled
biphoton states by e -> o + o type-I SPDC decay, in collinear
degenerate regime, obtained by a 397.5 nm doubled mode-locked
laser. With a beam splitter generating signal and idler channels,
the photons were addressed to two SPADs, with spectral selection
resolution of 0.2 nm for channel.
Two-photon correlations have been investigated by fixing one
monochromator at the maximal transmission wavelength on signal
gate and scanning the one placed in the idler gate to observe the
spectral distribution of single counts and coincidences, showing
experimental
evidence
for
a
large
contrast
between
these
distributions, in comparison also with the spectral properties of
the pump pulse.
The operational method relates the degree of entanglement to R,
approximately equal to the Schmidt number, that corresponds to
experimentally measurable ratio between single particle and
coincidence widths of the relative photon wave packets: the
greater
R,
the
higher
entanglement
between
two
photons.
Preliminary measurements valued R=153, more larger than 1
(separable states), showing good agreement with theory.
It will be shown how to increase this value compensating for
spatial-frequency chirp of the pump pulse and a study of the the
behavior of entanglement degree as a linear function of the
crystals length.
[1] Yu.M. Mikhailova, P.A. Volkov, M.V.Fedorov,
ph/0801.0689v1 (2008)
[2] M.V. Fedorov, et al., P.R.L., 99, 063901 (2007)
arxiv:quant-
Perfet state transfer in quantum spin chains
Giulia Gualdi
Dipartimento di Fisica, Università di Camerino
We investigate the most general conditions under which a finite
long-range interacting spin chain achieves unitary fidelity and
the shortest transfer time in transmitting an unknown input qubit.
At the same time we gain a deeper insight into system dynamics,
that allows us to identify an ideal system involving sender and
receiver only. However, this two-spin ideal chain is unpractical
due to the rapid decrease of the coupling strength with the
distance. Therefore, we propose an optimization scheme for
approaching the ideal behaviour, while keeping the interaction
strength still reasonably high. The procedure
is scalable with
the size of the system and straightforward to implement.
Spin Squeezing on the Cesium Clock Transition
N. Kjaergaard, P. Windpassinger, J. Appel, D. Oblak, U. Hoff, and
E. Polzik
QUANTOP, Niels Bohr Institute, University of Copenhagen,
Copenhagen, Denmark
When an ensemble of N independent particles is prepared in a
coherent
superposition
of
two
internal
quantum
states,
a
projective measurement of the population difference will have a
variance of N. This so-called projection noise is a current
limitation to the precision of atomic clocks. In recent
experiments on dipole trapped ensembles of Cs atoms in an equal
supersition of the clock states we have observed the quantum
projection noise by interrogations using off-resonant probe laser
light. The population difference between the clock states is
measured via the state dependent phase shift of probe light as
recorded in a Mach Zehnder interferometer. Since the dispersive
light-atom interaction has a quantum nondemolition measurement
character we can use the information gained when applying a probe
pulse of light to predict the outcome of a subsequent measurement
beyond the standard quantum limit. Hence a reduction or
"squeezing" of the population difference is encountered. Since a
two-level quantum system is equivalent to a spin 1/2 particle this
is referred to a pseudo-spin squeezing. The observation of
squeezing implies that the particles in the ensemble are nonclassically correlated (entangled). When taking into account
decoherence resulting from spontaneously scattered probe photons
our experiments show about 3 dB of spectroscopically relevant
squeezing (noise reduction) .
Continous Variable Entanglement Distribution
Natalia Korolkova1 , Tomas Tyc1,2, Ladislav Mista1,3, David Menzies1,
Gary Sinclair1
1University
of St. Andrews, Scotland, UK
University, Brno, Czech Republic
3Palacky University, Olomouc, Czech Republic
e-mail:[email protected]
2Masaryk
We address different aspects of long-distance quantum
communication using infinitely – dimensional, continuous-variable
quantum systems. With examples of several schemes, we discuss the
physics underlying entanglement distribution over large distances
and tailoring of quantum systems for this purpose. This work also
responds to the quest for experimentally feasible building
elements for optical quantum repeater.
First, we discuss entanglement concentration scheme for
infinite-dimensional quantum systems based on non-linear crossKerr coupling of the one part of two-mode squeezed vacuum and an
ancillary coherent state. We then show how the whole family of
such entanglement concentration protocols can be derived using the
framework and concept of weak quantum measurements. Next, we
modify this scheme for the quantum state engineering purposes.
Both schemes can be implemented in the same experimental setting.
We propose an experiment that employs the cross-Kerr effect to
create highly non-classical non-Gaussian states of light via
interaction of two coherent beams in an atomic medium exhibiting
electromagnetically-induced transparency, subsequent measurement
on one beam and feed-forward on the other. The resultant states
are highly non-classical states of electromagnetic field an d
exhibit negativity of their Wigner function has a distinctly
pronounced
“crescent”
shape
specific
for
the
Kerr-type
interactions, which so far was not demonstrated experimentally. We
show that creating and detecting such states should be possible
with the present technology using electromagnetically induced
transparency in a four-level atomic system in N-configuration.
Finally, we address the question of quantum information
distribution in general. As a development from the earlier work of
Cubitt et al for qubits [1], we demonstrate the possibility to
distribute entanglement without sending entanglement in infinitedimensional systems. Remarkably, for mixed quantum states one can
entangle two distant modes by sending a separable mode. This can
be done using experimentally feasible Gaussian states and
operations involving single-mode squeezed states, correlated
displacements and beam splitters, dispensing with the CNOT gates
of the qubit case. The distributed entanglement is distillable and
therefore can be used for quantum communication.
The
proposed
schemes
prepare
the
ground
for
better
understanding and engineering of optical quantum networks,
continous-variable
cryptography
and
other
entanglement-based
communication protocols using light modes and/or atomic ensembles.
References
[1] T.S. Cubitt et al, Phys. Rev. Lett. 91, 037902 (2003).
Sideband Transitions for Quantum Information Processing in Circuit
Quantum Electrodynamics
P. J. Leek (1), P. Maurer (1), S. Filipp (1), M. G\"oppl (1), M.
Baur (1), L. Steffen (1), R. Bianchetti (1), J. Fink (1), A. Blais
(2), A. Wallraff (1)
(1) Department of Physics, ETH Z\"urich, CH-8093, Z\"urich,
Switzerland.
(2) D\'partement de Physique, Universit\'e de Sherbrooke,
Sherbrooke,
Qu\'ebec, J1K 2R1 Canada.
The
preparation
of
multiple
qubit
entangled
states
and
implementation of universal two-qubit quantum gates are important
milestones in the development of a quantum information processor.
Such tasks may be carried out by making use of sideband
transitions with a harmonic mode that is dispersively coupled to
multiple qubits. Such a scheme has been employed successfully in
ion traps, in which quanta of information are interchanged between
individual ions and collective vibrational modes of the ions in
the trap [1]. In circuit QED, in which superconducting qubits are
strongly coupled to the harmonic modes of a microwave resonator
[2], such sideband transitions are also possible [3]. In
superconducting qubits, the use of sidebands may have advantages
over direct coupling methods since qubits and resonator can remain
at all times decoupled and at fixed frequencies chosen for optimal
coherence. In this talk I will report on recent spectroscopic and
time resolved experiments involving driving of sidebands between
transmon style qubits [4] and a resonator in circuit QED, and
discuss potential use of such transitions for computational tasks.
[1]
[2]
[3]
[4]
H\"affner et al., Nature 438, 643 (2005).
Wallraff et al., Nature 431, 162 (2004).
Wallraff et al., Phys. Rev. Lett. 99, 50501 (2007).
Koch et al., Phys. Rev. A, 76, 42319 (2007).
QLib - A Matlab Package for Quantum Information Theory
Calculations
with Applications
Shai Machnes
Tel-Aviv University, Israel
Developing intuition about quantum information theory problems is
difficult, as is verifying or ruling-out of hypothesis
We present a Matlab package intended to provide the QIT community
with a new and powerful tool-set for quantum information theory
calculations. The package covers most of the "QI textbook" and
includes novel parametrization of quantum objects and a robust
optimization mechanism. New ways of re-examining well-known
results is demonstrated.
QLib is designed to be further developed and enhanced by the
community and is available for download at www.qlib.info
Photonic crystal defect cavities coupled to N-V centres in diamond
Luca Marseglia (1), A. C. Stanley-Clarke(2), J.P. Harrison(1), R.
Gibson(1),
Y.-L. D. Ho(1), Jeremy O’Brien(2), J.G. Rarity(1)
(1)Department of Electrical & Electronic Engineering, University
of Bristol,
Merchant Ventures Building, Woodland Road, Bristol BS8 1UB, UK
(2)H.H. Wills Physics Laboratory, University of Bristol, Tyndall
Avenue, Bristol BS8 1TL, UK
The nitrogen-vacancy (N-V) defect in ultra-pure diamond shows
great promise for the implementation of qubits for quantum
computing. The N-V defect is a three level system which could
behave as an efficient room temperature source of single photons
at a wavelength of 637 nm. The defect has a ground state spin that
can be addressed optically, hence an efficient coupling of light
to this transition required. Here we aim to place the N-V centre
into a cavity at the centre of a suspended slab photonic crystal
structure made from hexagonal array of cylindrical air holes. The
light is confined by distributed Bragg reflection in the plane of
periodicity (xy) and by total internal reflection in the
perpendicular plane (z). The work consists of both modelling the
photonic crystal to optimise the parameters and fabricating the
structures using focussed ion beam milling.
We calculated the photonic band gap of the photonic crystal
structure and then modelled it using finite difference domain
methods. The photonic crystal structure considered was a modified
defect cavity (M3). Starting with a hexagonal array of holes, the
central three are filled and the diameters and positions of the
holes surrounding the cavity are modified, as shown in Figure 1.
To design high quality factor cavities (Q>10,000) we simulate the
photonic crystal structure and vary the parameters to maximise Q.
The parameters calculated were the correct lattice constant which
allows the photonic crystal slab to be resonant with the 637 nm
wavelength emission, the sizes of the holes (both “normal” and
modified), and the shifts in position.
Having simulated photonic crystal structure cavities M3 we have
begun
to
investigate
fabrication
using
focused
ion
beam
lithography (FIB), obtaining triangular lattice structures as seen
in figure 1. We will be assessing the damage due to the etching
process using a variety of optical diagnostics. The eventual aim
then will be to locate the position of a suitable single N-V
centre using a scanning confocal microscope and etch a cavity
around it. Modification of the emission properties due to the
cavity will then be investigated, again making use of the scanning
confocal microscope.
Quantum spin models with trapped electrons
Irene Marzoli
Università di Camerino, Dipartimento di Fisica
62032 Camerino, Italy
I will discuss a scheme to design and control an effective spinspin interaction starting from a system of trapped electrons. The
insertion of a magnetic field gradient, combined to the Coulomb
interaction, enables an effective J-coupling between the particles
in the array. The resulting system may be regarded as an
artificial molecule, suitable for nuclear magnetic resonance (NMR)
quantum computation. This analogy suggests to use techniques,
similar to the refocusing schemes of NMR, to better design and
control the effective spin-spin interaction. Our proposal relies
on the application of appropriate sequences of electromagnetic
pulses, alternated to periods of free evolution, to engineer an
effective spin Hamiltonian. The final goal is to reproduce notable
quantum spin systems, such as Ising and XY models.
Static and Dynamic Controls in Virtual Photonic Quantum Circuits
Hideaki Matsueda
Kochi University, Emeritus Professor,
230-28 Nagatani-cho, Iwakura, Sakyo-ku, Kyoto 606-0026, Japan
The quantum information controling may have two aspects, i.e.
satatic and dynamic.
Control by some built-in guiding structures
is an example of the former,
and by abrupt operations such as
nonadiabatic bang-bang pulses is the latter.
According to the
Hermiticity of quantum formalism, it is natural to regard the
basic flow of the quantum computing as the spontaneous evolution
of wave functions through a quantum circuit (QC), entangling and
disentangling themselves on the way.
The entangling may be
thought
of
as
the
manifestation
of
multipole-multipole
interactions (MMIs) assisted by virtual photons (VPHs), and the
disentangling may be triggered by an external operations that
abruptly put the set of basis toward which the wave functions
should collapse.
Another example is the dynamic suppression of
decoherence by sharp and strong bang-bang pulses.
The VPHs are guided along the pre-fabricated nano-structured
paths having features finer than their range determined by the
time-energy uncertainty principle, where the separations between
devices, polarization axies of the alignments, and the energy gaps
should be designed for purposeful and may be one-way manner
guiding. The MMI is assumed to be started by pumping photons of
which number is just sufficient to excite only limited number of
sites, e.g. only one site out of two sites, and no electron
transfer will occur while the energy is transferred, so that it
goes spontaneously or adiabatically not only throughout the range,
but also over temporal differences, even realizing retrospective
interconnections.
In this paper, the coherent MMIs of various origines are
graphically compared with respect to the energy, lifetime, and
range, on the basis of our spectroscopic data, and a Minkowski
type space-time diagram is illustrated for some major MMIs,
visualizing the difference due to materials. Finally, speculative
prospects are discussed including the retrospective entangling
action which may bring a nanotech version of a time machine
working within the uncertainty time slot, and a relatively
adiabatical dynamic control for the near future.
Spin chain quantum state transfer at the quantum speed limit
M. Murphy, S. Montangero, V. Giovannetti, T. Calarco
(1) Institute für QIV, Universität Ulm, Albert-Einstein-Allee 11,
89081 Ulm, Deutschland
(2) Scuola Normale Superiore, P.zza dei Cavalieri 7, 56126 Pisa,
Italy
Quantum state population transfer through spin chains may provide
a viable means of propagating information within a quantum
computer. We study here the controlled propagation of a quantum
state along a spin chain by the application of a moving parabolic
magnetic potential. Using optimal control techniques we find that
optimisation of the control parameters for both the magnetic field
strength and the moving speed of the parabolic potential yields
very high fidelity of population transfer along the spin chain for
short times and large numbers of spin sites. Furthermore, we show
that the optimal control allows us to achieve the maximum speed of
state transfer as allowed by the quantum speed limit.
Two-photon nonlinearity in one atom cavity QED
I. Schuster, A. Kubanek, A. Fuhrmanek, T. Puppe, P.W.H. Pinkse,
K. Murr and G. Rempe
Max-Planck-Institut for Quantum Optics, Hans-Kopfermann-Str. 1,
D-85748 Garching, Germany
Cavity quantum electrodynamics explores fundamental processes of
light-matter interaction at the level of single atomic and
photonic quanta and could provide a wealth of new applications.
While microwave experiments have long explored effects with
several intracavity photons, optical experiments have focussed on
the weak excitation regime, with typically not more than one
photon in the cavity. Here we report on a laser spectroscopy
experiment which probes the energy-level structure of a strongly
coupled atom-cavity system at higher excitation, with a single
atom dipole-trapped inside a high-finesse optical cavity. A new
resonance is observed at a frequency distinctively different from
those studied in previous cavity QED experiments.
We show that the new resonance results from a multi-photon
transition between the anharmonically spaced energy levels. This
is direct spectroscopic proof of the quantum nature of the
combined atom-cavity system.
It is also found that the response of the system is nonlinear in
the laser intensity.
This occurs in a regime where conventional optical nonlinearities
are highly suppressed.
The investigation of nonlinear quantum optics with just a single
atom opens up new avenues for the controlled generation of novel
multi-photon states.
[1] I. Schuster, A. Kubanek, A. Fuhrmanek, T. Puppe, P.W.H.
Pinkse, K. Murr and G. Rempe, Nature Physics 4, 382 (2008)
Quantum State Preparation in Micro-traps
Brian o'Sullivan, Thomas Busch
Ultra Cold Quantum Gases Group, Physics Department,
University College Cork, Cork - Ireland
Reaching high fidelities while not compromising fast time scales
is one of the most important criteria any realistic technique for
quantum information processing must deliver. While this usually
requires
good
control
over
many
experimental
parameters,
alternative adiabatic techniques are often known that can make
life easier in the laboratory.
Here we investigate techniques for state preparation of single
atoms in systems of spatially separated micro-traps. For such
systems it was recently shown that an analogue to the celebrated
three-level STIRAP technique in optics can be constructed,
allowing for high fidelity atomic transport, EIT and CPT. As
spatial atom-optical systems contain various additional degrees of
freedom as compared to optical systems, they hold a large promise
for developing new and exciting techniques.
In our system we consider a four trap diamond arrangement in two
dimensions and show that an atom trapped initially in a single
trap can be transferred into an arbitrary, but well defined,
spatial superposition state. This process requires only control
over individual trapping frequencies and a STIRAP type positioning
sequence
of the traps. We will show that that process does not only allow
for large fidelities when carried out perfectly, but is also
robust against many experimental uncertainties.
Conservation Laws Limit Quantum Control Accuracy
Masanao Ozawa
Graduate School of Information Science,
Nagoya University, chikusa-ku, Nagoya 464-8601, JAPAN
The threshold theorem of quantum error correction ensures faulttolerant quantum computing, if every component can be controlled
within a constant threshold accuracy. However, the threshold is so
demanding that the realizability problem of scalable quantum
computers is reduced to the problem of controllability under such
a stringent accuracy requirement rather than solved in principle.
In order to figure out how fundamental laws set a limit for the
elementary gate operations, we consider here the accuracy limit
induced by conservation laws. The idea that conservation laws
limit quantum control goes back to the works of Wigner, Araki, and
Yanase (the WAY theorem) stating that observables not-commuting
with additively conserved quantity cannot be measured precisely
[1]. Recently, the WAY theorem has been reformulated in the modern
framework of measurement theory to obtain various quantitative
generalizations [2, 3] derived from the universally valid
reformulation
of
the
uncertainty
principle
on
the
noisedisturbance trade-off [4, 5, 6], and applied them to quantum
limits of the accuracy of elementary gate operations under the
angular momentum conservation law obeyed by the interaction
between the computational qubits and the controller, including the
atom-field interaction described by a Jaynes-Cummings model. The
inevitable error probability has been shown to be inversely
proportional to the variance of the controller\'s conserved
quantity for the CNOT gate [7, 8], the Hadamard gate [3], and the
NOT gate [9], while the SWAP gate obeys no constraint. In this
talk, these considerations will be extended to multiqubit gates
such as the Toffoli gate, the Fredkin gate, and general controlled
unitary gates.
This work was supported in part by the SCOPE project of MIC and
the CREST project of JST.
[1] H. Araki and M. M. Yanase, Phys. Rev. 120, 622 (1960).
[2] M. Ozawa, Phys. Rev. Lett. 88, 050402 (2002).
[3] M. Ozawa, Int. J. Quant. Inf. 1, 569 (2003).
[4] M. Ozawa, Phys. Rev. A 67, 042105 (2003).
[5] M. Ozawa, Ann. Phys. (N.Y.) 311, 350 (2004).
[6] M. Ozawa, J. Opt. B: Quantum Semiclass. Opt. 7, S672 (2005).
[7] M. Ozawa, Phys. Rev. Lett. 89, 057902 (2002).
[8] M. Ozawa, Phys. Rev. Lett. 91, 089802 (2003).
[9] T. Karasawa and M. Ozawa, Phys. Rev. A 75, 032324 (2007).
Spin chains as quantum wires: maximising information transfer
speed and fidelity though minimal control
Peter J. Pemberton-Ross (1), Sonia G. Schirmer (1)
(1) Department of Applied Maths and Theoretical Physics,
University of Cambridge, Wilberforce Road,
Cambridge, CB3 0WA, United Kingdom
Spin chains have been proposed as quantum wires for information
transfer
in solid state quantum architectures. We show that huge gains in
both
transfer speed and fidelity are possible using a minimalist
control
approach that relies only a single, local, on-off switch actuator.
We
show that effective switching time sequences can be determined
using a
simple optimization technique that lends itself to closed-loop
direct
laboratory optimization for both ideal chains and disordered
chains.
MaxLik-based photon statistics reconstruction by on/off detection
Giorgio Brida(1), Marco Genovese(1), Marco Gramegna(1), Alice
Meda(1), Stefano Olivares(2), Matteo G. A. Paris(2), Fabrizio
Piacentini(1), Paolo Traina(1)
(1)Istituto Nazionale di Ricerca Metrologica (INRIM),
Strada delle Cacce 91, 10135 Torino, Italy
(2) Dipartimento di Fisica dell'Università di Milano,
Via Celoria 16, 20133 Milano (MI), Italy
The reconstruction of the photon distribution of one or more modes
of radiation plays a crucial role in fundamental quantum optics,
and finds relevant applications in quantum communication, imaging
and spectroscopy.
Few years ago, a new method for reconstructing the photon
statistics of optical states was developed and demonstrated. It is
based on the maximum-likelihood (MaxLik) applied to an on/off
detection with variable quantum efficiency scheme.
Here we present the results obtained at the INRIM labs by applying
this method (and its derivations) to several optical fields, both
monopartite[1] and bipartite[2], all generated via spontaneous or
stimulated Parametric Down-Conversion (PDC): from the heralded
photon regime to the multi-photon one, all the results are in good
agreement with the
values above 99%.
theoretical
predictions,
showing
fidelity
[1] G. Zambra et al., Phys. Rev. Lett. 95, 063602 (2005);
[2] G. Brida, M. Genovese, M.G.A. Paris, F. Piacentini,
Lett., Vol. 31, Issue 23, (2006);
Opt.
Quantum simulation in Ion traps and BECs
A. Retzker, R.C. Thompson, D.M. Segal, M.B. Plenio, J. I. Cirac,
B. Reznik
Imperial College London,SW7 2PE, UK.
Max-Planck-Institut f¨ur Quantenoptik,
Hans-Kopfermann-Str. 1, 85748 Garching, Germany.
Department of Physics and Astronomy, Tel-Aviv
University, Tel Aviv 69978, Israel
The field of quantum information processing has dramatically
evolved in the past decade.
Various systems have been proposed for the realization of a
quantum computer.
Despite an enormous experimental progress, due to the high degree
of precision that is required, the realization of a large scale
quantum computer is still not within reach. Recently it was
recognized that quantum simulators are less demanding than quantum
computers. In this talk I will describe proposals for quantum
simulations in Ion traps and BECs. It will be shown that the
radial degree of freedom of strings of trapped ions in the quantum
regime may be prepared and controlled accurately through the
variation of the external trapping potential while at the same
time its properties are measurable with high spatial and temporal
resolution. This provides a new test-bed giving access to static
and dynamical properties of the physics of quantum-many-body
systems and quantum phase transitions that are hard to simulate on
classical computers. Furthermore, it allows for the creation of
double well potentials with experimentally accessible tunnelling
rates and with applications in testing the foundations of quantum
physics and precision sensing.
A scheme for the study of methods for detecting Unruh-like
acceleration radiation effects in a
Bose-Einstein condensate in a 1+1 dimensional setup will be
descried. In particular, the dispersive effects of the Bogoliubov
spectrum on the ideal case of exact thermalization will be
explained.
Single-ion single-photon interaction
F. Rohde, C. Schuck, N. Piro, M. Almendros, A. Haase, M. Hennrich,
F. Dubin, M. Mitchell, J. Eschner
Institut de Ciencies Fotoniques, Mediterranean Technology Park,
08860 Castelldefels, Barcelona, Spain
The controlled interaction of individual atoms and photons is an
important building block for transferring quantum information
between distant nodes of a quantum network. We study the
absorption of single photons by a single Calcium ion, using
heralded photons generated by a spontaneous parametric downconversion source whose emission is tailored to coincide with the
20MHz bandwidth of the atomic resonance.
Measurement of high fidelity single-qubit gates in circuit-QED
using gate randomization
L. Tornberg (1), J. M. Chow (2), Jens Koch (2), Jay Gambetta (3),
Lev S. Bishop (2), M. H. Devoret (2), S. M. Girvin (2), and R. J.
Schoelkopf (2)
(1) Microtechnology and Nanoscience, MC2, Chalmers University of
Technology,
SE-412 96 Gothenburg, Sweden
(2) Departments of Physics and Applied Physics, Yale University,
New Haven, Connecticut 06520, USA
(3) Institute for Quantum Computing and Department of Physics and
Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1,
Canada
The realization of quantum computers relies upon the ability to
perform elementary qubit gates with sufficiently low error
probability. Standard methods for determining such gate fidelities
include both state and process tomography, which are sensitive to
errors in the state initialization and measurement. In addition,
the cost in terms of experimental resources scales poorly with the
number of qubits for both these methods. In this work, we measure
the fidelity of single-qubit gates in circuit-QED using a
randomization procedure suggested in [1], previously implemented
in trapped ions [2].
This scheme avoids the difficulties
associated with tomography by applying long sequences of randomly
chosen gates, which effectively averages the fidelity of an
operation over all initial states. Because of this, exact state
preparation and measurement are no longer critical issues.
Implementing this procedure on a superconducting transmon qubit,
we demonstrate single-qubit gate fidelities of 0.98 only limited
by relaxation and higher level excitations. To validate the
results of the randomization procedure, we compare with the gate
fidelity obtained using process tomography and with the fidelity
measure proposed by Lucero et. al.[3].
[1] J. Emerson et. al., J. Opt. B: Quantum Semiclass.
(2005) S347-S352
[2] E. Knill, et. al., Phys. Rev. A., 77, 012307 (2008)
[3] E. Lucero et. al., arXiv:0802.0903
Opt.
7
Enhance the performance of decoy-state quantum key distribution
with parametric down-conversion source
Qin Wang and Anders Karlsson
Department of Microelectronics and Applied Physics, KTH-The Royal
Institute of Technology, KTH, Electrum 229, SE-164 40 Kista,
Sweden
We study the behavior of decoy state quantum key distribution
systems using conditionally prepared down-conversion source,
analyzing such a source running in a mode of operation with either
a thermal photon number distribution or a Poisson photon number
distribution. By comparing both modes of operations to the threeintensity proposal of Wang et al's, to the one-intensity proposal
of Adachi et al's, and to currently demonstrated faint-laser pulse
system, we demonstrate that a down-conversion source in a
Poissonian mode of operation can largely enhance the performance
of decoy state quantum key distribution system. This makes downconversion based decoy state system an interesting alternative to
current technological implementations based on faint laser pulses.
Spin Squeezing on the Cs Clock Transition
P. Windpassinger, D. Oblak, U. Busk Hoff, J. Appel, N. Kjærgaard,
and E. S. Polzik
QUANTOP, Niels Bohr Institute, University of Copenhagen,
Copenhagen, Denmark
Quantum nondemolition probing of a collective atomic (pseudo)-spin
is a powerful instrument in quantum information processing and
control. We present a method for non-destructive probing on the
clock transition of laser-cooled, dipole trapped Cs atoms. The
phase shift imposed by the atomic sample on an off-resonant probe
laser beam is determined with a Mach-Zehnder interferometer. In
the setup, the measurement accuracy of the population difference
of the two clock states (the pseudo-spin component) has reached an
accuracy which is limited only by the quantum noise of light (shot
noise) and of the atoms (projection noise)[1]. The observation of
correlations between two consecutive non-destructive measurements
on the same ensemble allow us to infer the degree of pseudo-spin
squeezing of the clock state populations.
Due to the non-destructive probing we can follow the evolution of
the population difference of the Cs-atom clock states online when
subjected to
microwave fields. This allows us to observe Rabi oscillations on
the clock transition over an extended period of time, which should
yield a significant improvement of the signal-to-noise ratio
compared to the traditional fluorescence-based destructive
probing.
Further, the results of detailed studies of the effect of probeinduced inhomogeneous light-shift and of the destructive probeinduced spontaneous photon scattering and its influence on spin
squeezed state are discussed [2].
[1] P. Windpassinger et. al, Phys. Rev. Lett. 100, 103601 (2008)
[2] P. Windpassinger et. al, New J. Phys. 10, 053032 (2008)