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 . 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 . Finally, I will discuss a recent proposal for an all optical experimental implementation of complex quantum networks , 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 field’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.