arXiv subjects

Franco Nori

Publications and source records attributed to Franco Nori.

At least 523 records · Page 29Linked to original sources

arXiv2014

Lamb-Dicke spectroscopy of atoms in a hollow-core photonic crystal fibre

Unlike photons, which are conveniently handled by mirrors and optical fibres without loss of coherence, atoms lose their coherence via atom-atom and atom-wall interactions. This decoherence of atoms deteriorates the performance of atomic clocks and magnetometers, and also hinders their miniaturisation. Here we report a novel platform for precision spectroscopy. Ultracold strontium atoms inside a kKagome-lattice hollow-core photonic crystal fibre (HC-PCF) are transversely confined by an optical lattice to prevent atoms from interacting with the fibre wall. By confining at most one atom in each lattice site, to avoid atom-atom interactions and Doppler effect, a 7.8-kHz-wide spectrum is observed for the $^1 S_0-{}^3P_1$ (m=0) transition. Atoms singly trapped in a magic lattice in hollow-core photonic crystal fibresHC-PCFs improve the optical depth while preserving atomic coherence time.

physics.atom-ph↗

arXiv2014

Conservation of the spin and orbital angular momenta in electromagnetism

We review and re-examine the description and separation of the spin and orbital angular momenta (AM) of an electromagnetic field in free space. While the spin and orbital AM of light are not separately-meaningful physical quantities in orthodox quantum mechanics or classical field theory, these quantities are routinely measured and used for applications in optics. A meaningful quantum description of the spin and orbital AM of light was recently provided by several authors, which describes separately conserved and measurable integral values of these quantities. However, the electromagnetic field theory still lacks corresponding locally-conserved spin and orbital AM currents. In this paper, we construct these missing spin and orbital AM densities and fluxes that satisfy the proper continuity equations. We show that these are physically measurable and conserved quantities. These are, however, not Lorentz-covariant, so only make sense in the single laboratory reference frame of the measurement probe. The fluxes we derive improve the canonical (non-conserved) spin and orbital AM fluxes, and include a `spin-orbit' term that describes the spin-orbit interaction effects observed in nonparaxial optical fields. We also consider both standard and dual-symmetric versions of the electromagnetic field theory. Applying the general theory to nonparaxial optical vortex beams validates our results and allows us to discriminate between earlier approaches to the problem. Our treatment yields the complete and consistent description of the spin and orbital AM of free Maxwell fields in both quantum-mechanical and field-theory approaches.

physics.optics↗

arXiv2014

PT-Symmetric Phonon Laser

By exploiting recent developments associated with coupled microcavities, we introduce the concept of PT-symmetric phonon laser with balanced gain and loss. This is accomplished by introducing gain to one of the microcavities such that it balances the passive loss of the other. In the vicinity of the gain-loss balance, a strong nonlinear relation emerges between the intracavity photon intensity and the input power. This then leads to a giant enhancement of both optical pressure and mechanical gain, resulting in a highly efficient phonon-lasing action. These results provide a promising approach for manipulating optomechanical systems through PT-symmetric concepts. Potential applications range from enhancing mechanical cooling to designing phonon-laser amplifiers.

quant-ph↗

arXiv2014

Disorder-induced mutation of quasi-normal modes in 1D open systems

We study the relation between quasi-normal modes (QNMs) and transmission resonances (TRs) in one-dimensional (1D) disordered systems. We show for the first time that while each maximum in the transmission coefficient is always related to a QNM, the reverse statement is not necessarily correct. There exists an intermediate state, at which only a part of the QNMs are localized and these QNMs provide a resonant transmission. The rest of the solutions of the eigenvalue problem (denoted as strange quasi-modes) are never found in regular open cavities and resonators, and arise exclusively due to random scatterings. Although these strange QNMs belong to a discrete spectrum, they are not localized and not associated with any anomalies in the transmission. The ratio of the number of the normal QNMs to the total number of QNMs is independent of the type of disorder, and slightly deviates from the constant $\sqrt{2/5}$ in rather large ranges of the strength of a single scattering and the length of the random sample.

cond-mat.dis-nn↗

arXiv2014

Terahertz transition radiation of bulk and surface electromagnetic waves by an electron entering a layered superconductor

We theoretically study the transition radiation of bulk and surface electromagnetic waves by an electron crossing an interface between a layered superconductor and an isotropic dielectric. We assume that the direction of the electron motion and the orientation of the superconducting layers are perpendicular to the interface. We derive the analytical expressions for the strongly anisotropic radiation fields and for the time-integrated energy fluxes of bulk and oblique surface electromagnetic waves (OSWs). We show that the OSWs with frequencies close to the Josephson plasma frequency $ω_J$ provide the main contribution to the OSWs energy flux. Moreover, for frequencies close to the Josephson plasma frequency, the spectral density of the OSWs radiation diverges at some critical value of the azimuth angle $φ$. At the angles $φ=0$ and $φ=90^o$, the radiation field has a transverse magnetic polarization. We have also studied the Cherenkov radiation by the electron escaping from the layered superconductor and show that this radiation is almost monochromatic. A remarkable feature of the Cherenkov radiation in a layered superconductor is that, contrary to the isotropic case, the Cherenkov radiation distinctly manifests itself in the angular dependence of the radiation energy flux.

cond-mat.supr-con↗

arXiv2014

Toroidal qubits: naturally-decoupled quiet artificial atoms

The requirements of quantum computations impose high demands on the level of qubit protection from perturbations; in particular, from those produced by the environment. Here we propose a superconducting flux qubit design that is naturally protected from ambient noise. This decoupling is due to the qubit interacting with the electromagnetic field only through its toroidal moment, which provides an unusual qubit-field interaction.

quant-ph↗

arXiv2014

Superposition principle for nonlinear Josephson plasma waves in layered superconductors

We show that a specific superposition principle is valid for \emph{nonlinear} Josephson plasma waves in layered superconductors. We study theoretically the reflection and transmission of terahertz waves through a finite-size superconducting slab placed inside a rectangular waveguide with ideal-metal walls. We assume that the superconducting layers are parallel to the waveguide axis. We show that there exist two specific mutually-orthogonal polarizations for waves which, in spite of the nonlinearity, reflect and transmit through the superconductor \emph{independently}. The wave of the first polarization causes a strong shielding current along the crystallographic \textbf{ab}-plane of the superconductor. Therefore, this wave reflects nearly completely from the superconductor and excites only an evanescent mode inside it. The wave of the other polarization does not contain the electric field component parallel to both the sample surface and the crystallographic \textbf{ab}-plane, and excites much weaker shielding currents. Therefore, it partially reflects and partially transmits through the sample. Moreover, this wave excites the nonlinear mode in the layered superconductor, and the transmission coefficient of the superconductor depends on the amplitude of the incident wave of this polarization.On the basis of the discussed superposition principle, we suggest a new method for solving nonlinear problems of waves interaction in layered superconductors. Namely, it is reasonably to represent incident, reflected, and transmitted waves of any polarizations as superpositions of the modes with the two specific polarizations considered here, and then solve the problem separately for these modes. We apply this method to the case of nonlinear interaction and mutual transformation of the transverse electric and transverse magnetic modes in layered superconductors.

cond-mat.supr-con↗

arXiv2014

Dynamical topological quantum computation using spin pulse control in the Heisenberg model

Hamiltonian engineering is an important approach for quantum information processing, when appropriate materials do not exist in nature or are unstable. So far there is no stable material for the Kitaev spin Hamiltonian with anisotropic interactions on a honeycomb lattice (A. Kitaev, Annals of Physics vol 321, 2 (2006)), which plays a crucial role in the realization of both Abelian and non-Abelian anyons. Here, we show how to dynamically realize the Kitaev spin Hamiltonian from the conventional Heisenberg spin Hamiltonian using a pulse-control technique. By repeating the same pulse sequence, the quantum state is dynamically preserved. The effects of the spin-orbit interaction and the hyperfine interaction are also investigated.

quant-ph↗

arXiv2014

Magnetoelectric Effects in Local Light-Matter Interactions

We study the generic interaction of a monochromatic electromagnetic field with bi-isotropic nanoparticles. Such an interaction is described by dipole-coupling terms associated with the breaking of dual, P- and T-symmetries, including the chirality and the nonreciprocal magnetoelectric effect. We calculate absorption rates, radiation forces, and radiation torques for the nanoparticles and introduce novel characteristics of the field quantifying the transfer of energy, momentum, and angular-momentum in these interactions. In particular, we put forward the concept of 'magnetoelectric energy density', quantifying the local PT-symmetry of the field. Akin to the 'super-chiral' light suggested recently for sensitive local probing of molecular chirality [Phys. Rev. Lett. 104, 163901 (2010); Science 332, 333 (2011)], here we describe a complex field for sensitive probing of the nonreciprocal magnetoelectric effect in nanoparticles or molecules.

physics.optics↗

arXiv2014

Giant Negative Mobility of Janus Particles in a Corrugated Channel

We numerically simulate the transport of elliptic Janus particles along narrow two-dimensional channels with reflecting walls. The self-propulsion velocity of the particle is oriented along either their major (prolate) or minor axis (oblate). In smooth channels, we observe long diffusion transients: ballistic for prolate particles and zero-diffusion for oblate particles. Placed in a rough channel, prolate particles tend to drift against an applied drive by tumbling over the wall protrusions; for appropriate aspect ratios, the modulus of their negative mobility grows exceedingly large (giant negative mobility). This suggests that a small external drive suffices to efficiently direct self-propulsion of rod-like Janus particles in rough channels.

physics.bio-ph↗

arXiv2014

Theory of macroscopic quantum tunneling with Josephson-Leggett collective excitations in multi-band superconducting Josephson junctions

Collective excitations reveal fundamental properties and potential applications of superconducting states. We theoretically study macroscopic quantum tunneling (MQT) in a Josephson junction composed of multi-band superconductors, focusing on a phase mode induced by inter-band fluctuations: the Josephson-Leggett (JL) collective excitation mode. Using the imaginary-time path-integral method, we derive a formula for the MQT escape rate for low-temperature switching events. We clarify that the JL mode has two major effects on the MQT: (i) the zero-point fluctuations enhance the escape rate, and (ii) the quantum dissipation induced by the couplings to the gauge-invariant phase difference suppresses the quantum tunneling. We show that the enhancement exceeds the suppression for a wide range of junction parameters. This enhancement originates from the single-mode interaction between the tunneling variable and the inter-band fluctuations.

cond-mat.supr-con↗

arXiv2014

Enhanced interferometry using squeezed thermal states and even or odd states

We derive a general expression of the quantum Fisher information for a Mach-Zehnder interferometer, with the port inputs of an \emph{arbitrary} pure state and a squeezed thermal state. We find that the standard quantum limit can be beaten, when even or odd states are applied to the pure-state port. In particular, when the squeezed thermal state becomes a thermal state, all the even or odd states have the same quantum Fisher information for given photon numbers. For a squeezed thermal state, optimal even or odd states are needed to approach the Heisenberg limit. As examples, we consider several common even or odd states: Fock states, even or odd coherent states, squeezed vacuum states, and single-photon-subtracted squeezed vacuum states. We also demonstrate that super-precision can be realized by implementing the parity measurement for these states.

quant-ph↗

arXiv2014

What is- and what is not- Electromagnetically-Induced-Transparency in Whispering-Gallery-Microcavities

Electromagnetically-induced-transparency (EIT) and Autler-Townes splitting (ATS) are two prominent examples of coherent interactions between optical fields and multilevel atoms. They have been observed in various physical systems involving atoms, molecules, meta-structures and plasmons. In recent years, there has been an increasing interest in the implementations of all-optical analogues of EIT and ATS via the interacting resonant modes of one or more optical microcavities. Despite the differences in their underlying physics, both EIT and ATS are quantified by the appearance of a transparency window in the absorption or transmission spectrum, which often leads to a confusion about its origin. While in EIT the transparency window is a result of Fano interference among different transition pathways, in ATS it is the result of strong field-driven interactions leading to the splitting of energy levels. Being able to tell objectively whether a transparency window observed in the spectrum is due to EIT or ATS is crucial for clarifying the physics involved and for practical applications. Here we report a systematic study of the pathways leading to EIT, Fano, and ATS, in systems of two coupled whispering-gallery-mode (WGM) microtoroidal resonators. Moreover, we report for the first time the application of the Akaike Information Criterion discerning between all-optical analogues of EIT and ATS, and clarifying the transition between them.

physics.optics↗

arXiv2014

Tunneling spectrum of a pinned vortex with a robust Majorana state

We study a heterostructure which consists of a topological insulator and a superconductor with a hole. The hole pins a vortex. The system supports a robust Majorana fermion state bound to the vortex core. We investigate the possibility of using scanning tunneling spectroscopy (i) to detect the Majorana fermion in the proposed setup and (ii) to study excited states bound to the vortex core. The Majorana fermion manifests itself as a magnetic-field dependent zero-bias anomaly of the tunneling conductance. Optimal parameters for detecting Majorana fermions have been obtained. In the optimal regime, the Majorana fermion is separated from the excited states by a substantial gap. The number of zero-energy states equals the number of flux quanta in the hole; thus, the strength of the zero-bias anomaly depends on the magnetic field. The lowest energy excitations bound to the core are also studied. The excited states spectrum differs from the spectrum of a typical Abrikosov vortex, providing additional indirect confirmation of the Majorana state observation.

cond-mat.str-el↗

arXiv2014

Extraordinary momentum and spin in evanescent waves

Momentum and spin represent fundamental dynamical properties of quantum particles and fields. In particular, propagating optical waves (photons) carry momentum and longitudinal spin determined by the wave vector and circular polarization, respectively. Here we show that exactly the opposite can be the case for evanescent optical waves. A single evanescent wave possesses a spin component, which is independent of the polarization and is orthogonal to the wave vector. Furthermore, such a wave carries a momentum component, which is determined by the circular polarization and is also orthogonal to the wave vector. We show that these extraordinary properties reveal a fundamental Belinfante's spin momentum, known in field theory and unobservable in propagating fields. We demonstrate that the transverse momentum and spin push and twist a probe Mie particle in an evanescent field. This allows the observation of `impossible' properties of light and of a fundamental field-theory quantity, which was previously considered as `virtual'.

physics.optics↗

arXiv2014

Classical Field Approach to Quantum Weak Measurements

By generalizing the quantum weak measurement protocol to the case of quantum fields, we show that weak measurements probe an effective classical background field that describes the average field configuration in the spacetime region between pre- and post-selection boundary conditions. The classical field is itself a weak value of the corresponding quantum field operator and satisfies equations of motion that extremize an effective action. Weak measurements perturb this effective action, producing measurable changes to the classical field dynamics. As such, weakly measured effects always correspond to an effective classical field. This general result explains why these effects appear to be robust for pre- and post-selected ensembles, and why they can also be measured using classical field techniques that are not weak for individual excitations of the field.

quant-ph↗

arXiv2014

Optomechanical-like coupling between superconducting resonators

We propose and analyze a circuit that implements a nonlinear coupling between two superconducting microwave resonators. The resonators are coupled through a superconducting quantum interference device (SQUID) that terminates one of the resonators. This produces a nonlinear interaction on the standard optomechanical form, where the quadrature of one resonator couples to the photon number of the other resonator. The circuit therefore allows for all-electrical realizations of analogs to optomechanical systems, with coupling that can be both strong and tunable. We estimate the coupling strengths that should be attainable with the proposed device, and we find that the device is a promising candidate for realizing the single-photon strong-coupling regime. As a potential application, we discuss implementations of networks of nonlinearly-coupled microwave resonators, which could be used in microwave-photon based quantum simulation.

quant-ph↗

arXiv2014

Arbitrary control of coherent dynamics for distant qubits in a quantum network

We show that the coherent coupling of atomic qubits at distant nodes of a quantum network, composed of several cavities linked by optical fibers, can be arbitrarily controlled via the selective pairing of Raman transitions. The adiabatic elimination of the atomic excited states and photonic states leads to selective qubit-qubit interactions, which would have important applications in quantum information processing. Quantum gates between any pair of distant qubits and parallel two-qubit operations on selected qubit pairs can be implemented through suitable choices of the parameters of the external fields. The selective pairing of Raman transitions also allows the generation of spin chains and cluster states without the requirement that the cavity-fiber coupling be smaller than the detunings of the Raman transitions.

quant-ph↗