I will present recent experimental results giving a direct
evidence of superfluidity in a quasi two-dimensional Bose gas
by observing the scissors collective excitation. We are able to
identify the superfluid and thermal phases inside the gas and
locate the boundary at which the BKT crossover occurs, through
a novel local correlation analysis relying purely on dynamical
criteria.
Noise is the current main obstacle in the path to useful
quantum devices. Quantum coherence, the key to the
miracles of quantum information processing (QIP) tasks, is
also its Achilles' heel : It is extremely fragile and
susceptible to the adverse effects of noise. Many
noise-mitigation strategies have been pro- posed and used
in the QIP community, from simple-to-implement dynamical
decoupling techniques, to harder-to-achieve quantum error
correction, and ultimately, fault-tolerant quantum
computation. Here, I introduce some basics of noise
mitigation, partly focusing on the current efforts in the
community towards the implementation of quantum computers,
and discuss some of my recent work on the subject.
The so-called information-thermodynamics link created by a
thought experiment of Szilard became a core of the modern orthodoxy in
the field of quantum information and resources theory in quantum
thermodynamics. We remind existing objections against standard
interpretation of the Szilard engine operation and illustrate them by
two quantum models : particle in a box with time-dependent thin
potential barrier and the spin-boson model. The consequences of the
emerging superselection rules for thermodynamics and foundations of
quantum mechanics are discussed. The role of non-ergodic systems as
information carriers and the thermodynamic cost of stability and
accuracy of information processing is briefly discussed and compared
to the generally accepted Landauer's principle.
Your prankster friend gave you a box into which, he says,
there is a quantum system. He asks you to hold the box for him, and
not to ruin the fragile quantum system that is inside. But you do not
trust him and want to find out if he is telling the truth or not. How
would you ascertain that the system within your friend's box is indeed
genuinely quantum ? As preposterous as this situation might sound, it
is not far from conditions routinely found in quantum labs : the
direct revelation of the non-classical properties of a system is often
either too disruptive for the system itself (if you measure it, you
ruin it !), or simply technically difficult to realise (the system
might be difficult to access, just like the one in your friend's box).
In this talk I will illustrate a scheme based on quantum communication
and the theory of quantum correlations, that allows you to "certify"
the quantum nature of an inaccessible system. I will show how, besides
its fundamental interest, the scheme is prone to verification in a
number of experimental settings, including quantum
opto-mechanics. Finally, I will conjecture that it can be used as a
Trojan horse to investigate the possible quantum nature of gravity -
for which I will describe a recent proposal for an experiment - and
biological processes.
The work presented in this talk is based on the following papers
1. T. Krisnanda, M. Zuppardo, M. Paternostro, and T. Paterek,
Phys. Rev. Lett. 119, 120402 (2017).
2. S. Bose et al., Phys. Rev. Lett. 119, 240401 (2017)
[see also Synopsis in Physics :
https ://physics.aps.org/synopsis-for/10.1103/PhysRevLett.119.240402].
3. T. Krisnanda, C. Marletto, V. Vedral, M. Paternostro, and T. Paterek,
arXiv :1711.06485 (2017).
The "entanglement problem" is to decide whether a given
quantum state of a composite system is entangled over a
chosen partition or not. We show that it can be mapped to
the "truncated moment problem" studied in mathematics, for
which recently a complete solution was found in the sense
of a necessary and sufficient condition. It gives rise to
a hierarchy of semi-definite programs corresponding to
state extensions with polynomial constraints, and the
positive-partial-transpose criterion as a first step, that
generalizes and unifies on an abstract level previous
approaches such as the Doherty-Parrilo-Spedalieri
hierarchy. Flat extensions play a crucial role and are a
systematic ingredient that allows us to prove separability
of a state and obtain its explicit decomposition into a
convex sum of product states. The approach is very
flexible and general. It can accomodate naturally missing
experimental data, symmetries, and subsystems of different
dimensions.
1. Fabian Bohnet-Waldraff, Daniel Braun, Olivier Giraud,
Entanglement and the truncated moment problem,
Phys. Rev. A 96, 032312 (2017).
2. Fabian Bohnet-Waldraff, Daniel Braun, Olivier Giraud,
Partial transpose criteria for symmetric states,
Phys. Rev. A 94, 042343 (2016).
3. O. Giraud, D. Braun, D. Baguette, T. Bastin, and J. Martin,
Tensor tepresentation of spin states, Phys. Rev. Lett. 114, 080401 (2015).
Quantum turbo codes (QTC) are easier to construct than their quantum
LDPC counterparts as thanks the freedom in the choice of code
parameters such as codelength, rate or memory size. The minimum
distance of QTC can be designed unbounded (polynomial or
sub-logarithmic in the codelength). However, the performance
analysis of QTC shows that the error probability under iterative
decoding is strictly positive. It was shown by Abbara and Tillich in
2013 that the error probability can be largely reduced by using the
turbo code construction with two stages. Here we extend this result
and consider a general multi-stage construction. By density evolution
analysis over the erasure channel, we show that an arbitrarily small
decoding erasure probability E can be achieved, when using the
multi-stage construction with \(\log(E)\) stages.
Light is an excellent classical and quantum information
carrier. Optical qubits can be transmitted over long distances by
optical fibers, manipulated by means of linear or non-linear devices
and conveniently interfaced with matter. Traditionally, quantum
information (QI) can be encoded according to two dif- ferent
modalities, naturally stemming from wave-particle duality. In
discrete-variable (DV) approach, qubits are defined in a
two-dimensional space encoded in discrete-spectrum observables. In
parallel, in- formation can be encoded in continuous variables (CV),
for example the amplitude and the phase of the electromagnetic field,
leading to infinite dimensional spaces. For both DV and CV regimes,
the interface of quantum technologies with classical telecommunication
infrastructure is a major advantage towards the realization of future
reliable and user-friendly quantum communication networks. In this
talk I will present our recent results on this challenging topic. I
will conclude with a description of our current projects and future
work.
We propose to realize a quantum simulator of spin arrays,
based on laser-trapped circular Rydberg atoms. The atoms are protected
from spontaneous emission decay, reaching lifetimes in the minute
range. A defect-free chain of 40 atoms can be prepared thanks to an
innovative technique, that bears re- semblance with evaporative
cooling, based on van der Waals interaction between the atoms. This
strong dipole-dipole interaction emulates spin-1/2 XXZ Hamiltonian,
all parameters of which are experimentally tunable over a wide
range. The chain dynamics can be followed over one second,
corresponding to more than \(10^4\) interaction cycles. The final state of
each spin can be individually measured, and any spin-correlations
between any atoms of the chain can be recovered. This enables the
observation of adiabatic evolutions through quantum phase
transitions, of sudden quenches, and fast modulations of the
interaction parameters. The proposed circular-Rydberg-atom quantum
simulator should open the way towards the simulations of systems and
of their dynamics beyond the grasp of classical computation.
Building reliable and scalable qubits is a huge challenge
to experimental physicists and engineers. Another, a bit
less known, challenge is how to compute with these
qubits. That is : quantum programming. While quantum
algorithmics has been studied for more than 30 years,
quantum programming is still an unexplored area, with lots
of conceptual and technological challenges, for which
advanced numerical techniques are needed. After a brief
introduction to the concepts of quantum programming, we
will provide an overview and live demo of the Atos QLM,
one of the world most advanced research platforms in
quantum programming.
The discovery in the mid 1980s that quantum mechanics
provides resources for performing computational tasks beyond reach of
classical Von Neumann machines triggered an intense research of the
quantum bits suitable for making a quantum computer. In the domain of
electrical circuits, superconducting quantum bits based on Josephson
junctions are presently the most advanced platform for quantum
information processing. I will describe the single Cooper pair box
circuit whose transmon version is now ubiquitously used for making
superconducting quantum bits. I will explain the basic operation of a
minimal quantum processor demonstrating the quantum speed-up of an
elementary instance of a quantum algorithm. I will explain the
challenges faced for developing a scalable platform fitted with
quantum error correction. Given they constitute a major roadblock,
other routes are also considered. In Quantronics, we propose to use
microscopic entities with superior quantum coherence, namely impurity
spins in solids, that we couple to superconducting circuits. I will
present the results obtained in the control of a small number of
electronic spins.
Self-sustained oscillators are oscillators that do not
require external forces to develop oscillations. However, the
emergence of synchronization in quantum systems is widely explored. In
a recent study, we showed that strong entrainment is possible if a
self-sustained oscillator is coupled to a squeezing Hamiltonian, even
if the squeezing.
Networks can range from the massive Internet to
microscopic metabolic networks in a cell, from networks of social
media users to neural connections in the brain. The immense success in
the description of classical complex systems as networks has gradually
led to the study of of their quantum counterpart. In this talk I will
focus on Hamiltonian models describing complex networks of quantum
harmonic oscillators. I will first show that such systems are very
useful for investigating the properties of open quantum systems,
namely quantum systems interacting with an environment [1]. This
framework considers one of the nodes as the open system and the other
nodes of the network as part of the environment. I will show that,
changing the properties of the network, it is possible to engineer ad
hoc open quantum dynamics by modifying the spectral density of the
environment. This is particularly relevant in connection to quantum
technologies where understanding and modelling environmental noise is
crucial to realise robust and scalable commercial quantum
devices. With a change in perspective to the complementary view point,
the node forming the open quantum system can be seen as a local probe
from which one can extract certain properties of the
network. Remarkably, we show that global properties can be mapped into
the time evolution of the probe hence, measuring the latter one, one
can extract them [1, 3]. I will focus in particular on the ability to
measure the connectivity of the network by local probing [3]. Finally,
I will discuss a recent proposal for an all optical experimental
implementation of complex quantum networks [2], and also discuss very
recent ongoing work on routing quantum information using such
networks.
1. J. Nokkala, F. Galve, R. Zambrini, S. Maniscalco,
Complex quantum networks as structured environments :
engineering and probing,
J. Piilo, Sci. Rep. 6, 26861 (2016).
2. J. Nokkala, F. Arzani, F. Galve, R. Zambrini, S. Maniscalco, J. Piilo,
N. Treps, V. Parigi, Reconfigurable optical implementation of
quantum complex networks, New J. Phys. 20, 053024 (2018).
3. J. Nokkala, S. Maniscalco, Local probe for connectivity
and coupling strength in quantum complex networks,
J. Piilo, Sci. Rep. 8, 13010
(2018).
The paraxial propagation of a quasimonochromatic quantum light
field in a dispersive and nonlinear dielectric medium is
considered. In this all-optical platform, the space
propagation of the fieldâ€™s envelope may be mapped onto the
time evolution of a quantum fluid of interacting photons. The
resulting quantum many-body system constitutes a particular
class of quantum fluids of light and presently attracts a
growing interest as a powerful tool for quantum simulation. I
will review recent theoretical and experimental progresses in
this rapidly emerging research field, including investigations
on superfluidity, elementary excitations, disorder, quantum
quenches, pre-thermalization, thermalization, and
Bose-Einstein condensation.
We provide an interpretation of entanglement based on
classical correlations between measurement outcomes of complementary
properties for composite quantum systems. We start with the bipartite
case and discuss in particular what classical correlations in the
measurements of these complementary properties tell us about the
quantum correlations of the state of the system under
consideration. We show that states that exhibit correlations for
complementary observables beyond a certain threshold value are
entangled. We also show that, surprisingly, bipartite separable states
with quantum correlations exhibit smaller correlations for
complementary observables with respect to classical states. We use
mutual information as a measure of classical correlations, but we
conjecture that the first result holds also for other measures
(e.g. the Pearson correlation coefficient or the sum of conditional
probabilities). We extend this approach to multipartite systems and
introduce new measures of multipartite quantum correlations based on
classical correlations of complementary outcomes. We show how these
measures, based on the classical mutual information, can be used to
detect high-dimensional tripartite entanglement by using only a few
local measurements. We finally discuss the use of complementarity and
entropic uncertainty relations to certify steering properties in
multipartite systems.