Franco Nori

Parity–time-symmetric whispering-gallery microcavities

It is now shown that coupled optical microcavities bear all the hallmarks of parity–time symmetry; that is, the system’s dynamics are unchanged by both time-reversal and mirror transformations. The resonant nature of microcavities results in unusual effects not seen in previous photonic analogues of parity–time-symmetric systems: for example, light travelling in one direction is resonantly enhanced but there are no resonance peaks going the other way.

Hybrid quantum circuits: Superconducting circuits interacting with other quantum systems

Hybrid quantum circuits combine two or more physical systems, with the goal of harnessing the advantages and strengths of the different systems in order to better explore new phenomena and potentially bring about novel quantum technologies. This article presents a brief overview of the progress achieved so far in the field of hybrid circuits involving atoms, spins and solid-state devices (including superconducting and nanomechanical systems). How these circuits combine elements from atomic physics, quantum optics, condensed matter physics, and nanoscience is discussed, and different possible approaches for integrating various systems into a single circuit are presented. In particular, hybrid quantum circuits can be fabricated on a chip, facilitating their future scalability, which is crucial for building future quantum technologies, including quantum detectors, simulators, and computers.

Superconducting Circuits and Quantum Information

Superconducting circuits can behave like atoms making transitions between two levels. Such circuits can test quantum mechanics at macroscopic scales and be used to conduct atomic-physics experiments on a silicon chip.

Spin–orbit interactions of light

This Review article provides an overview of the fundamental origins and important applications of the main spin–orbit interaction phenomena in modern optics that play a crucial role at subwavelength scales. Light carries both spin and orbital angular momentum. These dynamical properties are determined by the polarization and spatial degrees of freedom of light. Nano-optics, photonics and plasmonics tend to explore subwavelength scales and additional degrees of freedom of structured — that is, spatially inhomogeneous — optical fields. In such fields, spin and orbital properties become strongly coupled with each other. In this Review we cover the fundamental origins and important applications of the main spin–orbit interaction phenomena in optics. These include: spin-Hall effects in inhomogeneous media and at optical interfaces, spin-dependent effects in nonparaxial (focused or scattered) fields, spin-controlled shaping of light using anisotropic structured interfaces (metasurfaces) and robust spin-directional coupling via evanescent near fields. We show that spin–orbit interactions are inherent in all basic optical processes, and that they play a crucial role in modern optics.

Observation of the dynamical Casimir effect in a superconducting circuit

One of the most surprising predictions of modern quantum theory is that the vacuum of space is not empty. In fact, quantum theory predicts that it teems with virtual particles flitting in and out of existence. Although initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences—for instance, producing the Lamb shift of atomic spectra and modifying the magnetic moment of the electron. This type of renormalization due to vacuum fluctuations is now central to our understanding of nature. However, these effects provide indirect evidence for the existence of vacuum fluctuations. From early on, it was discussed whether it might be possible to more directly observe the virtual particles that compose the quantum vacuum. Forty years ago, it was suggested that a mirror undergoing relativistic motion could convert virtual photons into directly observable real photons. The phenomenon, later termed the dynamical Casimir effect, has not been demonstrated previously. Here we observe the dynamical Casimir effect in a superconducting circuit consisting of a coplanar transmission line with a tunable electrical length. The rate of change of the electrical length can be made very fast (a substantial fraction of the speed of light) by modulating the inductance of a superconducting quantum interference device at high frequencies (>10 gigahertz). In addition to observing the creation of real photons, we detect two-mode squeezing in the emitted radiation, which is a signature of the quantum character of the generation process.

Loss-induced suppression and revival of lasing

Controlling and reversing the effects of loss are major challenges in optical systems. For lasers, losses need to be overcome by a sufficient amount of gain to reach the lasing threshold. In this work, we show how to turn losses into gain by steering the parameters of a system to the vicinity of an exceptional point (EP), which occurs when the eigenvalues and the corresponding eigenstates of a system coalesce. In our system of coupled microresonators, EPs are manifested as the loss-induced suppression and revival of lasing. Below a critical value, adding loss annihilates an existing Raman laser. Beyond this critical threshold, lasing recovers despite the increasing loss, in stark contrast to what would be expected from conventional laser theory. Our results exemplify the counterintuitive features of EPs and present an innovative method for reversing the effect of loss. Introducing loss into a coupled optical system can result in an enhancement of the optical properties. [Also see Perspective by Schwefel] Achieving gain despite increasing loss When energy is pumped into an optically active material, the buildup (or gain) of excitations within the material can reach a critical point where the emission of coherent light, or lasing, can occur. In many systems, however, the buildup of the excitations is suppressed by losses within the material. Overturning conventional wisdom that loss is bad and should be minimized, Peng et al. show that carefully tweaking the coupling strength between the various components of a coupled optical system can actually result in an enhancement of the optical properties by adding more loss into the system (see the Perspective by Schwefel). The results may provide a clever design approach to counteract loss in optical devices. Science, this issue p. 328; see also p. 304

Wet granular materials

Most studies on granular physics have focused on dry granular media, with no liquids between the grains. However, in geology and many real world applications (e.g. food processing, pharmaceuticals, ceramics, civil engineering, construction, and many industrial applications), liquid is present between the grains. This produces inter-grain cohesion and drastically modifies the mechanical properties of the granular media (e.g. the surface angle can be larger than 90 degrees). Here we present a review of the mechanical properties of wet granular media, with particular emphasis on the effect of cohesion. We also list several open problems that might motivate future studies in this exciting but mostly unexplored field. Contents PAGE 1. Introduction 2  1.1. Granular physics and wet granular media 2  1.2. What is different from dry granular media? 3 2. Wet granular media: Grains with liquid and air 5  2.1. Cohesion between two spheres 5  2.1.1. Meniscus and suction 5  2.1.2. Liquid bridge between two spheres 6  2.2. Wet granular media with various liquid content 7  2.2.1. Four states of liquid content: pendular, funicular, capillary, and slurry state 7  2.2.2. Liquid content and suction 8  2.2.2.1. Measurement of suction in granular media 8  2.2.2.2. Relation between the liquid content and suction 10 3. Mechanical properties 12  3.1. Granular cohesion in the static and quasi-static regimes 12  3.1.1. Compaction of wet granular media 12  3.1.2. Angle of repose for small amounts of liquid 13  3.1.2.1. Experiments 13  3.1.2.2. Cohesion and angle of repose 14  3.1.3. Tensile, compression, and shear tests for intermediate and large liquid content 19  3.1.3.1. Test methods and Mohr circle 19  3.1.3.2. Tests in the pendular state 23  3.1.3.3. Tests with intermediate liquid content 24  3.1.3.4. Tests in soil mechanics: relatively large amounts of liquid 25  3.2. Dynamical behaviour 27  3.2.1. Dynamics in the pendular state 28  3.2.1.1. Avalanches in rotating drums 28  3.2.1.2. Vibrated wet granular media 29  3.2.1.3. Segregation 31  3.2.2. Shear experiments for various liquid content 32  3.2.3. Dynamics of wet granular media: practical applications 35  3.2.3.1. Agglomeration processing: grains bound by liquid 35  3.2.3.2. Geological events 35 4. Summary and open questions 35  4.1. Effect of the liquid content on quasi-static behaviour 35  4.2. Open problems 36  4.2.1. Jamming 37  4.2.2. Statistical mechanics approach 39  4.2.3. Arches and contact-force fluctuations 39  4.2.4. Simple experimental set-ups to study the dynamics of wet granular media 39  4.2.5. Numerical simulations 40  4.2.6. Mechanical properties of snow 40 5. Conclusion 41 Acknowledgements 41 References 41

QuTiP 2: A Python framework for the dynamics of open quantum systems

Abstract We present version 2 of QuTiP, the Quantum Toolbox in Python. Compared to the preceding version [J.R. Johansson, P.D. Nation, F. Nori, Comput. Phys. Commun. 183 (2012) 1760.], we have introduced numerous new features, enhanced performance, and made changes in the Application Programming Interface (API) for improved functionality and consistency within the package, as well as increased compatibility with existing conventions used in other scientific software packages for Python. The most significant new features include efficient solvers for arbitrary time-dependent Hamiltonians and collapse operators, support for the Floquet formalism, and new solvers for Bloch–Redfield and Floquet–Markov master equations. Here we introduce these new features, demonstrate their use, and give a summary of the important backward-incompatible API changes introduced in this version. Program Summary Program title: QuTiP: The Quantum Toolbox in Python Catalog identifier: AEMB_v2_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEMB_v2_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: GNU General Public License, version 3 No. of lines in distributed program, including test data, etc.: 33625 No. of bytes in distributed program, including test data, etc.: 410064 Distribution format: tar.gz Programming language: Python. Computer: i386, x86-64. Operating system: Linux, Mac OSX. RAM: 2+ Gigabytes Classification: 7. External routines: NumPy, SciPy, Matplotlib, Cython Catalog identifier of previous version: AEMB_v1_0 Journal reference of previous version: Comput. Phys. Comm. 183 (2012) 1760 Does the new version supercede the previous version?: Yes Nature of problem: Dynamics of open quantum systems Solution method: Numerical solutions to Lindblad, Floquet–Markov, and Bloch–Redfield master equations, as well as the Monte Carlo wave function method. Reasons for new version: Compared to the preceding version we have introduced numerous new features, enhanced performance, and made changes in the Application Programming Interface (API) for improved functionality and consistency within the package, as well as increased compatibility with existing conventions used in other scientific software packages for Python. The most significant new features include efficient solvers for arbitrary time-dependent Hamiltonians and collapse operators, support for the Floquet formalism, and new solvers for Bloch–Redfield and Floquet–Markov master equations. Restrictions: Problems must meet the criteria for using the master equation in Lindblad, Floquet–Markov, or Bloch–Redfield form. Running time: A few seconds up to several tens of hours, depending on size of the underlying Hilbert space.

Colloquium: The physics of Maxwell's demon and information

Maxwells demon was born in 1867 and still thrives in modern physics. He plays important roles in clarifying the connections between two theories: thermodynamics and information. Here, we present the history of the demon and a variety of interesting consequences of the second law of thermodynamics, mainly in quantum mechanics, but also in the theory of gravity. We also highlight some of the recent work that explores the role of information, illuminated by Maxwells demon, in the arena of quantum information theory.

Electronic properties of mesoscopic graphene structures: Charge confinement and control of spin and charge transport

This brief review discusses electronic properties of mesoscopic graphene-based structures. These allow controlling the confinement and transport of charge and spin; thus, they are of interest not only for fundamental research, but also for applications. The graphene-related topics covered here are: edges, nanoribbons, quantum dots, pn-junctions, pnp-structures, and quantum barriers and waveguides. This review is partly intended as a short introduction to graphene mesoscopics.