arXiv subjects

Franco Nori

Publications and source records attributed to Franco Nori.

At least 559 records · Page 31Linked to original sources

arXiv2013

Non-Markovian Dynamics Impact on the Foundations of Statistical Mechanics

The foundations of statistical mechanics, namely how equilibrium hypothesis emerges microscopically from quantum theory, is explored through investigating the environment-induced quantum decoherence processes. Based on the recent results on non-Markovian dynamics [Phys. Rev. Lett. 109, 170402 (2012)], we find that decoherence of quantum states manifests unexpected complexities. Indeed, an arbitrary given initial quantum state, under the influence of different reservoirs, can evolve into four different steady states: thermal, thermal-like, quantum memory and oscillating quantum memory states. The first two steady states \textit{de facto} provided a rigorous proof how the system relaxes to thermal equilibrium with its environment. The latter two steady states, with strong non-Markovian effects, will maintain the initial state information and not reach thermal equilibrium, which is beyond the conventional wisdom of statistical mechanics.

quant-ph↗

arXiv2013

Entanglement swapping and testing quantum steering into the past via collective decay

We propose a scheme to realize entanglement swapping via superradiance, entangling two distant cavities without a direct interaction. The successful Bell-state-measurement outcomes are performed naturally by the electromagnetic reservoir, and we show how, using a quantum trajectory method, the non-local properties of the state obtained after the swapping procedure can be verified by the steering inequality. Furthermore, we discuss how the unsuccessful measurement outcomes can be used in an experiment of delayed-choice entanglement swapping. An extension of testing the quantum steering inequality with the observers at three different times is also considered

quant-ph↗

arXiv2013

Radical-pair model of magnetoreception with spin-orbit coupling

The mechanism used by migratory birds to orientate themselves using the geomagnetic field is still a mystery in many species. The radical pair mechanism, in which very weak magnetic fields can influence certain types of spin-dependent chemical reactions, leading to biologically observable signals, has recently imposed itself as one of the most promising candidates for certain species. This is thanks both to its extreme sensitivity and its capacity to reproduce results from behavioral studies. Still, in order to gain a directional sensitivity, an anisotropic mechanism is needed. Recent proposals have explored the possibility that such an anisotropy is due to the electron-nucleus hyperfine interaction. In this work we explore a different possibility, in which the anisotropy is due to spin-orbit coupling between the electron spin and its angular momentum. We will show how a spin-orbit-coupling-based magnetic compass can have performances comparable with the usually-studied nuclear-hyperfine based mechanism. Our results could thus help researchers actively looking for candidate biological molecules which may host magnetoreceptive functions, both to describe magnetoreception in birds as well as to develop artificial chemical compass systems.

physics.bio-ph↗

arXiv2013

Feedback-induced nonlinearity and superconducting on-chip quantum optics

Quantum coherent feedback has been proven to be an efficient way to tune the dynamics of quantum optical systems and, recently, those of solid-state quantum circuits. Here, inspired by the recent progress of quantum feedback experiments, especially those in mesoscopic circuits, we prove that superconducting circuit QED systems, shunted with a coherent feedback loop, can change the dynamics of a superconducting transmission line resonator, i.e., a linear quantum cavity, and lead to strong on-chip nonlinear optical phenomena. We find that bistability can occur under the semiclassical approximation, and photon anti-bunching can be shown in the quantum regime. Our study presents new perspectives for engineering nonlinear quantum dynamics on a chip.

quant-ph↗

arXiv2013

Quantum internet using code division multiple access

A crucial open problem in large-scale quantum networks is how to efficiently transmit quantum data among many pairs of users via a common data-transmission medium. We propose a solution by developing a quantum code division multiple access (q-CDMA) approach in which quantum information is chaotically encoded to spread its spectral content, and then decoded via chaos synchronization to separate different sender-receiver pairs. In comparison to other existing approaches, such as frequency division multiple access (FDMA), the proposed q-CDMA can greatly increase the information rates per channel used, especially for very noisy quantum channels.

quant-ph↗

arXiv2013

Controlling a Nanowire Spin-Orbit Qubit via Electric-Dipole Spin Resonance

A semiconductor nanowire quantum dot with strong spin-orbit coupling (SOC) can be used to achieve a spin-orbit qubit. In contrast to a spin qubit, the spin-orbit qubit can respond to an external ac electric field, an effect called electric-dipole spin resonance. Here we develop a theory that can apply in the strong SOC regime. We find that there is an optimal SOC strength η_{opt}=\sqrt{2}/2, where the Rabi frequency induced by the ac electric field becomes maximal. Also, we show that both the level spacing and the Rabi frequency of the spin-orbit qubit have periodic responses to the direction of the external static magnetic field. These responses can be used to determine the SOC in the nanowire.

cond-mat.mes-hall↗

arXiv2013

Quantum memory using a hybrid circuit with flux qubits and NV centers

We propose how to realize high-fidelity quantum storage using a hybrid quantum architecture including two coupled flux qubits and a nitrogen-vacancy center ensemble (NVE). One of the flux qubits is considered as the quantum computing processor and the NVE serves as the quantum memory. By separating the computing and memory units, the influence of the quantum computing process on the quantum memory can be effectively eliminated, and hence the quantum storage of an arbitrary quantum state of the computing qubit could be achieved with high fidelity. Furthermore the present proposal is robust with respect to fluctuations of the system parameters, and it is experimentally feasibile with currently available technology.

quant-ph↗

arXiv2013

Photon blockade in quadratically coupled optomechanical systems

We study the steady-state photon statistics of a quadratically coupled optomechanical cavity, which is weakly driven by a monochromatic laser field. We examine the photon blockade by evaluating the second-order correlation function of the cavity photons. By restricting the system within the zero-, one-, and two-photon subspace, we obtain an approximate analytical expression for the correlation function. We also numerically investigate the correlation function by solving the quantum master equation including both optical and mechanical dissipations. The results show that, in the deep-resolved-sideband and single-photon strong-coupling regimes, the single-photon resonant driving will induce a photon blockade, which is limited by the thermal noise of the mechanical environment.

quant-ph↗

arXiv2013

Nonreciprocal light transmission in parity-time-symmetric whispering-gallery microcavities

Optical systems combining balanced loss and gain profiles provide a unique platform to implement classical analogues of quantum systems described by non-Hermitian parity-time- (PT-) symmetric Hamiltonians and to originate new synthetic materials with novel properties. To date, experimental works on PT-symmetric optical systems have been limited to waveguides in which resonances do not play a role. Here we report the first demonstration of PT-symmetry breaking in optical resonator systems by using two directly coupled on-chip optical whispering-gallery-mode (WGM) microtoroid silica resonators. Gain in one of the resonators is provided by optically pumping Erbium (Er3+) ions embedded in the silica matrix; the other resonator exhibits passive loss. The coupling strength between the resonators is adjusted by using nanopositioning stages to tune their distance. We have observed reciprocal behavior of the PT-symmetric system in the linear regime, as well as a transition to nonreciprocity in the PT symmetry-breaking phase transition due to the significant enhancement of nonlinearity in the broken-symmetry phase. Our results represent a significant advance towards a new generation of synthetic optical systems enabling on-chip manipulation and control of light propagation.

physics.optics↗

arXiv2013

Efficient tomography of quantum-optical Gaussian processes probed with a few coherent states

An arbitrary quantum-optical process (channel) can be completely characterized by probing it with coherent states using the recently developed coherent-state quantum process tomography (QPT) [Lobino et al., Science 322, 563 (2008)]. In general, precise QPT is possible if an infinite set of probes is available. Thus, realistic QPT of infinite-dimensional systems is approximate due to a finite experimentally-feasible set of coherent states and its related energy-cut-off approximation. We show with explicit formulas that one can completely identify a quantum-optical Gaussian process just with a few different coherent states without approximations like the energy cut-off. For tomography of multimode processes, our method exponentially reduces the number of different test states, compared with existing methods.

quant-ph↗

arXiv2013

Controlling single-photon transport in waveguides with finite cross-section

We study the transverse-size effect of a quasi-one-dimensional rectangular waveguide on the single-photon scattering on a two-level system. We calculate the transmission and reflection coefficients for single incident photons using the scattering formalism based on the Lippmann-Schwinger equation. When the transverse size of the waveguide is larger than a critical size, we find that the transverse mode will be involved in the single-photon scattering. Including the coupling to a higher traverse mode, we find that the photon in the lowest channel will be lost into the other channel, corresponding to the other transverse modes, when the input energy is larger than the maximum bound-state energy. Three kinds of resonance phenomena are predicted: single-photon resonance, photonic Feshbach resonance, and cutoff (minimum) frequency resonance. At these resonances, the input photon is completely reflected.

quant-ph↗

arXiv2013

Connection-state approach to pre- and post-selected quantum measurements

We discuss the concept of connection states (or connection matrices) that describe posterior ensembles, post-selected according to the outcomes of a quantum measurement. Connection matrices allow one to obtain results of any weak and some non-weak pre- and post-selected measurements, in the same manner as density matrices allow one to predict the results of conventional quantum measurements. Connection matrices are direct extensions of the density matrices and are generally non-Hermitian, which we show to be a direct consequence of quantum complementarity. This implies that the ultimate reason for unusual weak values is quantum complementarity. We show that connection matrices can be determined experimentally. We also show that retrodictive states are a special case of connection states. We propose a new method of tomography of quantum detectors.

quant-ph↗

arXiv2013

Electronic phase separation in iron pnictides

A mechanism for electronic phase separation in iron pnictides is proposed. It is based on the competition between commensurate and incommensurate spin-density-wave phases in a system with an imperfect doping-dependent nesting of a multi-sheeted Fermi surface. We model the Fermi surface by two elliptical electron pockets and three circular hole pockets. The interaction between a charge carrier in a hole band and a carrier in an electron band leads to the formation of spin-density-wave order. The commensurate spin density wave in the parent compound transforms to the incommensurate phase when doping is introduced. We show that, for certain parameter values, the uniform state is unstable with respect to phase separation. The resulting inhomogeneous state consists of regions of commensurate and incommensurate spin-density-wave phases. Our results are in qualitative agreement with recent observations of incommensurate spin density waves and electronic inhomogeneity in iron pnictides.

cond-mat.str-el↗

arXiv2013

Photon-mediated electron transport in hybrid circuit-QED

We investigate photon-mediated transport processes in a hybrid circuit-QED structure consisting of two double quantum dots coupled to a common microwave cavity. Under suitable resonance conditions, electron transport in one double quantum dot is facilitated by the transport in the other dot via photon-mediated processes through the cavity. We calculate the average current in the quantum dots, the mean cavity photon occupation, and the current cross-correlations with both a full numerical simulation and a recursive perturbation scheme that allows us to include the influence of the cavity order-by-order in the couplings between the cavity and the quantum dot systems. We can then clearly identify the photon-mediated transport processes.

cond-mat.mes-hall↗

arXiv2013

Entanglement generation and quantum information transfer between spatially-separated qubits in different cavities

The generation and control of quantum states of spatially-separated qubits distributed in different cavities constitute fundamental tasks in cavity quantum electrodynamics. An interesting question in this context is how to prepare entanglement and realize quantum information transfer between qubits located at different cavities, which are important in large-scale quantum information processing. In this paper, we consider a physical system consisting of two cavities and three qubits. Two of the qubits are placed in two different cavities while the remaining one acts as a coupler, which is used to connect the two cavities. We propose an approach for generating quantum entanglement and implementing quantum information transfer between the two spatially-separated intercavity qubits. The quantum operations involved in this proposal are performed by a virtual photon process, and thus the cavity decay is greatly suppressed during the operations. In addition, to complete the present tasks, only one coupler qubit and one operation step are needed. Moreover, there is no need of applying classical pulses, so that the engineering complexity is much reduced and the operation procedure is greatly simplified. Finally, our numerical results illustrate that high-fidelity implementation of this proposal using superconducting phase qubits and one-dimenstion transmision line resonators is feasible for current circuit QED implementations. This proposal can also be applied to other types of superconducting qubits, including flux and charge qubits.

quant-ph↗

arXiv2013

Ballistic charge transport in graphene and light propagation in periodic dielectric structures with metamaterials: a comparative study

We explore the optical properties of periodic layered media containing left-handed metamaterials. This study is based on several analogies between the propagation of light in metamaterials and charge transport in graphene. We derive the conditions for these two problems become equivalent, i.e., the equations and the boundary conditions when the corresponding wave functions coincide. We show that the photonic band-gap structure of a periodic system built of alternating left- and right-handed dielectric slabs contains conical singularities similar to the Dirac points in the energy spectrum of charged quasiparticles in graphene. Such singularities in the zone structure of the infinite systems give rise to rather unusual properties of light transport in finite samples. In an insightful numerical experiment (the propagation of a Gaussian beam through a mixed stack of normal and meta-dielectrics) we simultaneously demonstrate four Dirac point-induced anomalies: (i) diffusion-like decay of the intensity at forbidden frequencies, (ii) focusing and defocussing of the beam, (iii) absence of the transverse shift of the beam, and (iv) a spatial analogue of the Zitterbewegung effect. All of these phenomena take place in media with non-zero average refractive index, and can be tuned by changing either the geometrical and electromagnetic parameters of the sample,or the frequency and the polarization of light.

physics.optics↗

arXiv2013

Self-propelled Janus particles in a ratchet: Numerical simulations

Brownian transport of self-propelled overdamped microswimmers (like Janus particles) in a two-dimensional periodically compartmentalized channel is numerically investigated for different compartment geometries, boundary collisional dynamics, and particle rotational diffusion. The resulting time-correlated active Brownian motion is subject to rectification in the presence of spatial asymmetry. We prove that ratcheting of Janus particles can be orders of magnitude stronger than for ordinary thermal potential ratchets and thus experimentally accessible. In particular, autonomous pumping of a large mixture of passive particles can be induced by just adding a small fraction of Janus particles.

physics.bio-ph↗

arXiv2013

Photon trajectories, anomalous velocities, and weak measurements: A classical interpretation

Recently, Kocsis et al. reported the observation of "average trajectories of single photons" in a two-slit interference experiment [Science 332, 1170 (2011)]. This was possible by using the quantum weak-measurements method, which implies averaging over many events, i.e., in fact, a multi-photon limit of classical linear optics. We give a classical-optics interpretation to this experiment and other related problems. It appears that weak measurements of the local momentum of photons made by Kocsis et al. represent measurements of the Poynting vector in an optical field. We consider both the real and imaginary parts of the local momentum, and show that their measurements have been realized in classical optics using small probe particles. We also examine the appearance of "anomalous" values of the local momentum: either negative (backflow) or exceeding the wavenumber (superluminal propagation). These features appear to be closely related to vortices and evanescent waves. Finally, we revisit a number of older works and find examples of photon-trajectories and anomalous-momentum measurements in various optical experiments.

quant-ph↗