Quantum Nanodevices Research Articles

Real-time tracking of coherent oscillations of electrons in a nanodevice by photo-assisted tunnelling

Coherent collective oscillations of electrons excited in metallic nanostructures (localized surface plasmons) can confine incident light to atomic scales and enable strong light-matter interactions, which depend nonlinearly on the local field. Direct sampling of such collective electron oscillations in real-time is crucial to performing petahertz scale optical modulation, control, and readout in a quantum nanodevice. Here, we demonstrate real-time tracking of collective electron oscillations in an Au bowtie nanoantenna, by recording photo-assisted tunnelling currents generated by such oscillations in this quantum nanodevice. The collective electron oscillations show a noninstantaneous response to the driving laser fields with a T2 decay time of nearly 8 femtoseconds. The contributions of linear and nonlinear electron oscillations in the generated tunnelling currents were precisely determined. A phase control of electron oscillations in the nanodevice is illustrated. Functioning in ambient conditions, the excitation, phase control, and read-out of coherent electron oscillations pave the way toward on-chip light-wave electronics in quantum nanodevices.

Chiral, magnetic, molecule-based materials: A chemical path toward spintronics and quantum nanodevices

In this Perspective article, we briefly present the current state and discuss some future perspectives on hybrid molecular spintronics and smart molecular spintronics materials. We discuss the role of the chirality induced spin selectivity effect, where molecular chirality can be employed to manipulate spin states, potentially also including quantum coherent spin qubit states. We comment on some of the challenges and suggest future potential of magnetic biomolecules and chemical modification of 2D materials for these purposes.

Harnessing the Quantum Behavior of Spins on Surfaces.

The desire to control and measure individual quantum systems such as atoms and ions in a vacuum has led to significant scientific and engineering developments in the past decades that form the basis of today's quantum information science. Single atoms and molecules on surfaces, on the other hand, are heavily investigated by physicists, chemists, and material scientists in search of novel electronic and magnetic functionalities. These two paths crossed in 2015 when it wasfirst clearly demonstrated that individual spins on a surface can be coherently controlled and read out in an all-electrical fashion. The enabling technique is a combination of scanning tunneling microscopy (STM) and electron spin resonance, which offers unprecedented coherent controllability at the Angstrom length scale. This review aims to illustrate the essential ingredients that allow the quantum operations of single spins on surfaces. Three domains of applications of surface spins, namely quantum sensing, quantum control, and quantum simulation, are discussed with physical principles explained and examples presented. Enabled by the atomically-precise fabrication capability of STM, single spins on surfaces might one day lead to the realization of quantum nanodevices and artificial quantum materials at the atomicscale.

Anomalous low-energy properties in amorphous solids and the interplay of electric and elastic interactions of tunneling two-level systems

Tunneling two-level systems (TLSs), generic to amorphous solids, dictate the low-temperature properties of amorphous solids and dominate noise and decoherence in quantum nano-devices. The properties of the TLSs are generally described by the phenomenological standard tunneling model. Yet, significant deviations from the predictions of this model found experimentally suggest the need for a more precise model in describing TLSs. Here we show that the temperature dependence of the sound velocity, dielectric constant, specific heat, and thermal conductivity, can be explained using an energy-dependent TLS density of states reduced at low energies due to TLS-TLS interactions. This reduction is determined by the ratio between the strengths of the TLS-TLS interactions and the random potential, which is enhanced in systems with dominant electric dipolar interactions.

Random quantum batteries

Quantum nano-devices are fundamental systems in quantum thermodynamics that have been the subject of profound interest in recent years. Among these, quantum batteries play a very important role. In this paper we lay down a theory of random quantum batteries and provide a systematic way of computing the average work and work fluctuations in such devices by investigating their typical behavior. We show that the performance of random quantum batteries exhibits typicality and depends only on the spectral properties of the time evolving operator, the initial state and the measuring Hamiltonian. At given revival times a random quantum battery features a quantum advantage over classical random batteries. Our method is particularly apt to be used both for exactly solvable models like the Jaynes-Cummings model or in perturbation theory, e.g., systems subject to harmonic perturbations. We also study the setting of quantum adiabatic random batteries.

Quantum plasmonic hot-electron injection in lateral WSe2/MoSe2 heterostructures

Lateral two-dimensional (2D) transitional metal dichalcogenide (TMD) heterostructures have recently attracted a wide attention as promising materials for optoelectronic nanodevices. Due to the nanoscale width of lateral heterojunctions, the study of their optical properties is challenging and requires using subwavelength optical characterization techniques. We investigated the photoresponse of a lateral 2D WSe2/MoSe2 heterostructure using tip-enhanced photoluminescence (TEPL) with nanoscale spatial resolution and with picoscale tip-sample distance dependence. We demonstrate the observation of quantum plasmonic effects in 2D heterostructures on a non-metallic substrate, and we report the nano-optical measurements of the lateral 2D TMD heterojunction width of ~ 150 nm and the charge tunneling distance of ~ 20 pm. Controlling the plasmonic tip location allows for both nano-optical imaging and plasmon-induced hot electron injection into the heterostructure. By adjusting the tip-sample distance, we demonstrated the controllability of the hot-electron injection via the competition of two quantum plasmonic photoluminescence (PL) enhancement and quenching mechanisms. The directional charge transport in the depletion region leads to the increased hot electron injection, enhancing the MoSe2 PL signal. The properties of the directional hot-electron injection in the quantum plasmonic regime make the lateral 2D MoSe2/WSe2 heterostructures promising for quantum nanodevices with tunable photoresponse.

Dynamical photo-induced electronic properties of molecular junctions.

Nanoscale molecular-electronic devices and machines are emerging as promising functional elements, naturally flexible and efficient, for next-generation technologies. A deeper understanding of carrier dynamics in molecular junctions is expected to benefit many fields of nanoelectronics and power devices. We determine time-resolved charge current flowing at the donor-acceptor interface in molecular junctions connected to metallic electrodes by means of quantum transport simulations. The current is induced by the interaction of the donor with a Gaussian-shape femtosecond laser pulse. Effects of the molecular internal coupling, metal-molecule tunneling, and light-donor coupling on photocurrent are discussed. We then define the time-resolved local density of states which is proposed as an efficient tool to describe the absorbing molecule in contact with metallic electrodes. Non-equilibrium reorganization of hybridized molecular orbitals through the light-donor interaction gives rise to two phenomena: the dynamical Rabi shift and the appearance of Floquet-like states. Such insights into the dynamical photoelectronic structure of molecules are of strong interest for ultrafast spectroscopy and open avenues toward the possibility of analyzing and controlling the internal properties of quantum nanodevices with pump-push photocurrent spectroscopy.

Phonon-induced dissipation and decoherence in solid-state quantum devices: Markovian versus non-Markovian treatments

Microscopic modeling of electronic phase coherence versus energy dissipation plays a crucial role in the design and optimization of new-generation electronic quantum nanodevices, like quantum-cascade light sources and quantum logic gates; in this context, non-Markovian density-matrix approaches are widely used simulation strategies. Here we show that such methods, along with valuable virtues, in some circumstances may exhibit potential limitations that need to be taken into account for a reliable description of quantum materials and related devices. More specifically, extending the analysis recently proposed in [EPL 112, 67005 (2015)] to high temperatures and degenerate conditions, we show that the usual mean-field treatment – employed to derive quantum-kinetic equations – in some cases may lead to anomalous results, characterized by decoherence suppression and positivity violations. By means of a simple two-level model, we show that such unexpected behaviors may affect zero-dimensional electronic systems coupled to dispersionless phonon modes, while such anomalies are expected to play a negligible role in nanosystems with higher dimensionality; these limitations are found to be significant in the low-density and low-temperature limit, while in the degenerate and/or finite-temperature regime – typical of many state-of-the-art quantum devices – their impact is strongly reduced.

Microscopic description of irreversibility in quantum Lorentz gas by complex spectral analysis of the Liouvillian outside the Hilbert space.

Irreversible process of a weakly coupled one-dimensional quantum perfect Lorentz gas is studied on the basis of the fundamental laws of physics in terms of the complex spectral analysis associated with the resonance state of the Liouvillian. Without any phenomenological operations, such as a coarse-graining of space-time or a truncation of the higher order correlation, we obtained irreversible processes on a purely dynamical basis in all space and time scale including the microscopic atomic interaction range that is much smaller than the mean-free-length. The list of development of the complex spectral analysis of the Hamiltonian (instead of the Liouvillian) in quantum optical systems and in quantum nano-devices is also presented.

Electron–phonon dissipation in quantum nanodevices

Microscopic modelling of electronic-phase coherence versus energy dissipation plays a crucial role in the design and optimization of new-generation electronic quantum nanodevices, like quantum-cascade light sources and quantum logic gates; in this context, a variety of simulation strategies have been proposed and employed. The aim of this article is to discuss virtues versus intrinsic limitations of non-Markovian density-matrix approaches. More specifically, we shall show that the usual mean-field treatment employed to derive quantum-kinetic equations may lead to highly unphysical results, like negative distribution functions and non-dissipative carrier---optical phonon couplings. By means of a simple two-level model, we shall show that such limitations are expected to be particularly severe in zero-dimensional electronic systems--like quantum-dot nanostructures, potential constituents of quantum-computation devices--coupled to dispersionless phonon modes. Such a behaviour is in striking contrast with the case of Markovian treatments, where a proper combination of adiabatic limit and mean-field approximation guarantees a physically acceptable solution.

Thermal and quantum phase slips in niobium-nitride nanowires based on suspended carbon nanotubes

Superconducting nanowires have attracted considerable attention due to their unique quantum-mechanical properties, as well as their potential as next-generation quantum nanodevices, such as single-photon detectors, phase-slip (PS) qubits, and other hybrid structures. In this study, we present the results of one-dimensional (1D) superconductivity in nanowires fabricated by coating suspended carbon nanotubes with a superconducting thin niobium nitride (NbN) film. In the resistance-temperature characteristic curves, hallmarks of 1D superconductivity with PS events are observed with unconventional negative magnetoresistance. We also confirm that a crossover occurs between thermal and quantum PSs as the temperature is lowered.

Manifestation of percolation in high temperature superconductivity

Emergent advanced electronic and magnetic functionalities in novel materials appear in systems with a complex lattice structure. The key point is understanding the intrinsic effect of lattice fluctuations on the relevant electronic features in the range of 10–100meV near the Fermi level in new materials which is needed to develop advanced quantum nano-devices. This requires the control of structural inhomogeneity at multiple scales. Here we report some of the known advances in the field of percolative superconductivity. The necessity of the review is based on the growing consensus that the lack of an understanding of high temperature superconductivity is due to the few information on lattice fluctuations. In particular they could control the pseudo-gap phase, the electronic duality of holes in Fermi arcs and electrons in small Fermi pockets, multiple condensates in different points of the k-space. Moreover the emerging lattice granularity in cuprates shifts the search for the superconducting mechanism from a homogeneous superconductivity to a percolative superconductivity, therefore it is the scope of this review to provide further data to this kind of research.

Room temperature biological quantum random walk in phycocyanin nanowires

Quantum nano-structures are likely to become primary elements of future devices. However, there are a number of significant scientific challenges to real world applications of quantum devices. These include de-coherence that erodes operation of a quantum device and control issues. In nature, certain processes have been shown to use quantum mechanical processes for overcoming these barriers. One well-known example is the high energy transmission efficiency of photosynthetic light harvesting complexes. Utilizing such systems for fabricating nano-devices provides a new approach to creating self-assembled nano-energy guides. In this study, we use isolated phycocyanin (PC) proteins that can self-assemble into bundles of nanowires. We show two methods for controlling the organization of the bundles. These nanowires exhibit long range quantum energy transfer through hundreds of proteins. Such results provide new efficient building blocks for coupling to nano-devices, and shed light on distribution and the efficiency of energy transfer mechanisms in biological systems and its quantum nature.

Interplay between energy dissipation and reservoir-induced thermalization in nonequilibrium quantum nanodevices

A solid state electronic nanodevice is an intrinsically open quantum system, exchanging both energy with the host material and carriers with connected reservoirs. Its out-of-equilibrium behavior is determined by a non-trivial interplay between electronic dissipation and decoherence induced by inelastic processes within the device, and the coupling of the latter to metallic electrodes. We propose a unified description, based on the density matrix formalism, that accounts for both these aspects, enabling to predict various steady-state as well as ultrafast nonequilibrium phenomena, nowadays experimentally accessible. More specifically, we derive a generalized density-matrix equation, particularly suitable for the design and optimization of a wide class of electronic and optoelectronic quantum devices. The power and flexibility of this approach is demonstrated with the application to a photoexcited triple-barrier nanodevice.

Beltrami fields at nanoscales in a quantum electron-hole plasma

The self-organization of electron-hole quantum plasmas is described. It is shown that steady state equilibrium is governed by a triple curl Beltrami field which is a superposition of three Beltrami fields. The results show the possibility of creating Beltrami fields of the order of nanoscale size. It is also shown that pressure gradients exist in the relaxed state and can be confined by a generalized Bernoulli mechanism. Our results are of strong relevance to quantum nano devices.

Virtual states and decoherence in single impurity atom solid state quantum device

When a single donor atom is embedded in solid state nanodevice, a rich variety of phenomena in the proximity of T = 0 emerges due to the long phase coherence times Such phenomena are probed by means of charge sensing via tunneling currents. Some aspects connected to the transition from quantum to classical in quantum nanodevices are overviewed: the realization of transport through virtual states, and the definition of the temperature T of a microscopic/mesoscopic system as a key parameter required in the master equation when decoherence is considered. I also discuss the opportunity to study the transition from quantum to classical offered by single atom impurity devices on a time scale below the decoherence time, with a particular emphasis on the experimental test of the Leggett-Garg inequalities.

Quantum nanodevices with carbon nanotubes

Quantum nanodevices with carbon nanotubes

Recent Progress and Surface-Related Key Issues in III-V Semiconductor Nanoelectronics

Recent trends toward a “ubiquitous network society” have opened up new horizons for III-V nanoelectronics based on nanostructures which will have much wider application areas than they do today. In this paper, recent progresses and issues of III-V nanoelectronics are reviewed, focusing on achievements by the author’s group. As for nanostructure fabrication, a combination of top-down and bottom-up approaches, instead of the traditional top-down approach applied to multi-layer epitaxial wafers is required to form high-density nanostructure networks. As such, a selective molecular beam epitaxy growth of nanowire networks is presented and discussed. Large-scale integration of quantum nanodevices requires a new combination of system architecture and hardware architecture, such as the hexagonal (BDD) quantum logic circuit approach. The remaining key issue is the removal of surface states and Fermi level pinning from surfaces of nanostructures. Use of a silicon interface control layer (Si ICL) is shown to be promising for this purpose.

Removal of Side-gating Effects in GaAs Quantum Nanodevices with Nano-Schottky Gates by Surface Passivation Using Si Interface Control Layer

For ultra-high-density integration of gate-controlled quantum devices, one possible phenomenon limiting the ultimate integration density is a side-gating effect which has not been properly addressed so far for nanodevices. The purpose of this paper is to attempt to remove the side-gating effect from the GaAs quantum wire transistors where we recently have found presence of anomalously large side-gating effect. We first present data on the side-gating and we explain its mechanism in terms of tunneling injection of electrons by field concentration at gate edges caused by Fermi level pinning. Then, we apply our Si interface control layer (Si ICL) passivation process to reduce the pinning. By applying the Si ICL technique via thin GaAs cap layer, the anomalously large side-gating was completely suppressed, and only electrostatic side-gating theoretically expected from the device structure remained. [DOI: 10.1380/ejssnt.2005.332]