Presence Of Quantum Research Articles

Macroscopic quantum entanglement between an optomechanical cavity and a continuous field in presence of non-Markovian noise

Probing quantum entanglement with macroscopic objects allows us to test quantum mechanics in new regimes. One way to realize such behavior is to couple a macroscopic mechanical oscillator to a continuous light field via radiation pressure. In view of this, the system that is discussed comprises an optomechanical cavity driven by a coherent optical field in the unresolved sideband regime where we assume Gaussian states and dynamics. We develop a framework to quantify the amount of entanglement in the system numerically. Different from previous work, we treat non-Markovian noise and take into account both the continuous optical field and the cavity mode. We apply our framework to the case of the Advanced Laser Interferometer Gravitational-Wave Observatory and discuss the parameter regimes where entanglement exists, even in the presence of quantum and classical noises. Published by the American Physical Society 2024

On the Quantum Security of OCB

The OCB mode of operation for block ciphers has three variants, OCB1, OCB2 and OCB3. OCB1 and OCB3 can be used as secure authenticated encryption schemes whereas OCB2 has been shown to be classically insecure (Inoue et al., Crypto 2019). Even further, in the presence of quantum queries to the encryption functionality, a series of works by Kaplan et al. (Crypto 2016), Bhaumik et al. (Asiacrypt 2021) and Bonnetain et al. (Asiacrypt 2021) have shown how to break the unforgeability of the OCB modes. However, these works did not consider the confidentiality of OCB in the presence of quantum queries.We fill this gap by presenting the first formal analysis of the IND-qCPA security of OCB. In particular, we show the first attacks breaking the IND-qCPA security of the OCB modes. Surprisingly, we are able to prove that OCB2 is IND-qCPA secure when used without associated data, while relying on the assumption that the underlying block cipher is a quantum-secure pseudorandom permutation. Additionally, we present new quantum attacks breaking the universal unforgeability of OCB. Our analysis of OCB has implications for the post-quantum security of XTS, a well-known disk encryption standard, that was considered but mostly left open by Anand et al. (PQCrypto 2016).

Quantum analysis of second-order effects in superconducting travelling-wave parametric amplifiers

We have performed a quantum mechanical analysis of travelling-wave parametric amplifiers (TWPAs) in order to investigate five experimental phenomena related to their operations, namely the effect of impedance mismatch, the presence of upper idler modes, the presence of quantum and thermal noise, the generation of squeezed states, and the preservation of pre-squeezed states during amplification. Our analysis uses momentum operators to describe the spatial evolution of quantised modes along a TWPA. We calculate the restriction placed on pump amplitude as well as amplifier gain as a result of impedance mismatch between a TWPA and its external system. We apply our analysis to upper idler modes and demonstrate that they will result in suppressed gain. We show that an ideal TWPA is indeed quantum-limited—i.e. it introduces a half-quantum of zero-point fluctuation which is the minimum possible noise contribution for a phase-preserving linear amplifier. We analyse the thermal noise associated with a TWPA by considering the effect of distributed sources along an amplifier transmission line. Our analysis predicts a doubling of thermal noise in the high gain limit as a result of wave-mixing between signal and idler modes. We study the operation of a TWPA in the presence of a DC bias current, and have shown that highly squeezed states can in principle be generated. However, amplifying a pre-squeezed state using a non-degenerate TWPA generally reduces the squeezing advantage.

Terahertz surface modes and electron-phonon coupling on Bi2Se3 (111)

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator (TI) Bi$_2$Se$_3$ with particular emphasis on the low-energy region below 10 meV that has been difficult to resolve experimentally. By applying inelastic helium atom scattering (HAS), the entire phonon dispersion was determined and compared with density functional perturbation theory (DFPT) calculations. The intensity of the phonon modes is dominated by a strong Rayleigh mode, in contrast to previous experimental works. Moreover, also at variance with recent reports, no Kohn-anomaly is observed. These observations are in excellent agreement with DFPT calculations. Besides these results, the experimental data reveal$-$via bound-state resonance enhancement$-$two additional dispersion curves in the gap below the Rayleigh mode. They are possibly associated with an excitation of a surface electron density superstructure that we observe in HAS diffraction patterns. The electron-phonon coupling paramenter $\lambda$ = 0.23 derived from our temperature dependent Debye-Waller measurements compares well with values determined by angular resolved photoemission or Landau level spectroscopy. Our work opens up a new perspective for THz measurements on 2D materials as well as the investigation of subtle details (band bending, the presence of quantum well states) with respect to the electron-phonon coupling.

Enhanced Sheet Charge Density in DIBS Grown CdO Alloyed ZnO Buffer Based Heterostructure

In this letter, we report on achieving significantly high (~ $6{\times}$ ) sheet charge density (ns) in MgZnO/CdZnO heterostructure, as compared with that in MgZnO/ZnO, at lower Mg ( ${\le} 0.15$ ) compositions in barrier MgZnO layer, with both heterostructures grown by dual ion beam sputtering technique. Buffer CdZnO and barrier MgZnO layers are probed separately to investigate carrier density in defect prone sputtered layers. Capacitance-voltage measurement and temperature-dependent Hall measurement confirm the presence of quantum confined carrier density at barrier-buffer interface, suggesting that the enhancement in carrier densities in the heterostructure over individual layers is probably due to the formation of 2-D electron gas (2-DEG).

Improvement in Tracing Quantum Dot‐Conjugated Nanospheres for In Vivo Imaging by Eliminating Food Autofluorescence

Fluorescence imaging using fluorescent probes has demonstrated long‐term stability and brightness suitable for in vivo deep‐tissue imaging, but it also allows intense background fluorescence associated with food in the near‐infrared (IR) range. We investigated effects of changing rodent diet on food autofluorescence, in the presence of quantum dots‐conjugated magnetic nanospheres (QD‐MNSs). Replacement of a regular rodent diet with a purified diet has great improvement in removing autofluorescence in the near‐infrared range ideal for in vivo fluorescence imaging. By feeding a purified diet for eliminating ingredients impairing desirable fluorescence signals in the near‐IR range, food autofluorescence was clearly eliminated and fluorescence probes, QD‐MNSs, introduced by i.v. injection were effectively traced in a mouse by a distinctive signal‐to‐noise ratio.

Finite amplitude solitary structures of coupled kinetic Alfven-acoustic waves in dense plasmas

In this paper, we have investigated the nonlinear propagating coupled Kinetic Alfven-acoustic waves in a low beta degenerate quantum plasma in the presence of trapped Fermi electrons using the quantum hydrodynamic (QHD) model. By using the two potential theory and the Sagdeev potential approach, we have investigated the formation of solitary structures for coupled kinetic Alfven-acoustic waves in the presence of quantum mechanically trapped electrons. We have shown that there are regions of propagation and non-propagation for such solitary structures. We have also highlighted the differences between the classical and quantum mechanically trapped electrons. Interestingly, it has been found that the nature of the nonlinearity for the quantum mechanically trapped electrons is different from its classical counterpart. The results presented here may have applications in white dwarf asteroseismology as well as next generation laser-plasma experiments where low beta plasma condition is met.

Electrophoretic deposition of ZnO nanostructures: Au nanoclusters on Si substrates induce self-assembled nanowire growth

Fil: Sandoval, Claudia. Universidad Nacional de Tucuman. Facultad de Ciencias Exactas y Tecnologia. Departamento de Fisica. Departamento de Nanomateriales y Propiedades Dielectricas; Argentina

Contextual Risk and i ts Relevance in Economics

Uncertainty in economics still poses some fundamental problems illustrated, e.g., by the Allais and Ellsberg paradoxes. To overcome these difficulties, economists have introduced an interesting distinction between risk and ambiguity depending on the existence of a (classical Kolmogorovian) probabilistic structure modeling these uncertainty situations. On the other hand, evidence of everyday life suggests that context plays a fundamental role in human decisions under uncertainty. Moreover, it is well known from physics that any probabilistic structure modeling contextual interactions between entities structurally needs a non-Kolmogorovian quantum-like framework. In this paper we introduce the notion of contextual risk with the aim of modeling a substantial part of the situations in which usually only ambiguity is present. More precisely, we firstly introduce the essentials of an operational formalism called the hidden measurement approach in which probability is introduced as a consequence of fluctuations in the interaction between entities and contexts. Within the hidden measurement approach we propose a sphere model as a mathematical tool for situations in which contextual risk occurs. We show that a probabilistic model of this kind is necessarily non-Kolmogorovian, hence it requires either the formalism of quantum mechanics or a generalization of it. This insight is relevant, for it explains the presence of quantum or, better, quantum-like, structures in economics, as suggested by some authors, and can serve to solve the aforementioned paradoxes.

Absorption, photoluminescence, and resonant Rayleigh scattering probes of condensed microcavity polaritons

We investigate and compare different optical probes of a condensed state of microcavity polaritons in expected experimental conditions of nonresonant pumping. We show that the energy- and momentum-resolved resonant Rayleigh signal provides a distinctive probe of condensation as compared to, e.g., photoluminescence emission. In particular, the presence of a collective sound mode both above and below the chemical potential can be observed, as well as features directly related to the density of states of particle-hole-like excitations. Both resonant Rayleigh response and the absorption and photoluminescence are affected by the presence of quantum well disorder, which introduces a distribution of oscillator strengths between quantum well excitons at a given energy and cavity photons at a given momentum. As we show, this distribution makes it important that in the condensed regime, scattering by disorder is taken into account to all orders. We show that, in the low-density linear limit, this approach correctly describes inhomogeneous broadening of polaritons. In addition, in this limit, we extract a linear blueshift of the lower polariton versus density, with a coefficient determined by temperature and by a characteristic disorder length.

Short period magnetic coupling oscillations inCo∕Simultilayers: Role of crystallization and interface quality

In this work we show, in a transition-metal-semiconductor system, that both the interfacial quality and the crystallographic structure of the samples play an important role to allow the observation of the predicted properties of the system. To reduce the interfacial mixing, $\mathrm{Co}∕\mathrm{Si}$ multilayers were grown at $90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and different crystallization qualities have been obtained by depositing the samples on glass and silicon substrates. To be able to observe experimentally the short period magnetic coupling oscillations predicted by theoretical computations, the Si spacer layers must be in a crystalline form and the interfacial mixing to be reduced to 5 atomic planes. However, the role of the crystallographic structure or growth direction of the Si layer seems to be weak. This is in agreement with the theoretical computations suggesting that the magnetic properties of such a system are driven by the presence of quantum well states at the Fermi level of the semiconductor regardless of its crystallographic structure.

Thickness dependence of surface plasmon damping and dispersion in ultrathin Ag films

The thickness dependence of the surface plasmon damping and dispersion of atomically flat and ultrathin silver films deposited on the $\mathrm{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface was investigated by a combined high-resolution electron-energy-loss spectroscopy (HREELS) and scanning tunneling microscopy (STM) system. We found stronger plasmon energy dispersion with the momentum parallel to the surface $({q}_{\ensuremath{\Vert}})$ in thicker films, and a significant dependence of the damping edge on the film thickness. Both of them can be associated with the presence of quantum well states (QWS) in the direction perpendicular to the film surface, which influences the interband transitions between the lower and upper $5sp$ bands of silver.

Quantum manifestation of Lévy-type flights in a chaotic system

Abstract Semi-classical dynamics of quantum wave packets spreading is studied for a kicked rotor. Quantum flights are established for a specific, “magic” value of a chaos control parameter when the classical stickiness of trajectories is most effective. By studying of a survival probability and distribution of the accelerations we identify the presence of quantum Levy-type flights.

Theory of tunneling magnetoresistance in a junction with a nonmagnetic metallic interlayer

It is demonstrated by numerical evaluation of the real-space Kubo formula that the tunneling magnetoresistance (TMR) due to tunneling between two cobalt electrodes separated by a vacuum gap remains nonzero when one of the electrodes is covered with a copper layer. This contradicts the classical theory of tunneling that predicts zero TMR. It is shown that a nonzero TMR is due to quantum well states in the Cu layer that do not participate in transport. Since these only occur in the down-spin channel, their loss from transport creates a spin asymmetry of electrons tunneling from a Cu overlayer, i.e., nonzero TMR. This mechanism could provide an explanation of the observed nonzero TMR for junctions with Cu or Ag interlayers. A simple method for modifying the classical theory of tunneling so that it can describe correctly tunneling in the presence of quantum well states is proposed and implemented for the Co junction with a Cu interlayer.

Connection between Structure and Electronic Properties in Epitaxial Magnetic Layers

ABSTRACTThis study explores the consequences of structure on the electronic properties of magnetic multilayers. Epitaxial layers of Co and Cu are grown on Cu(100) in a new deposition system that couples sputter-deposition with MBE and contains a wide range of characterization tools, including RHEED, LEED, and Kerr effect. This system can be coupled in situ to spin-polarized, angle-resolved photoemission and to resonant, magnetic X-ray scattering, both employing synchrotron radiation. The interface structure turns out to be critical in determining the coercivity and the presence of quantum well states, which determine oscillatory magnetic coupling.