Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Quantum fluctuations”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Spin Correlations in the Parent Phase of Li 1–x Fe x ⁢ ODFeSe

Elucidating spin correlations in the parent compounds of high-temperature superconductors is crucial for understanding superconductivity. We used neutron scattering to study spin correlations in Li 1–x Fe x ⁢ ODFeSe, an insulating material with reduced electron carriers compared to its superconducting counterpart (T c = 41 K), serving as the undoped parent compound. Our findings show a reduced total fluctuating moment in this insulator relative to FeSe and 122 iron pnictides, likely due to increased interlayer distances from intercalation, which enhance fluctuations and reduce the intensity of spin excitations. Moreover, we observed a V-shaped spin wavelike excitation dispersion, contrasting with the twisted hourglass pattern in the superconducting counterpart. Electron doping shifts spin excitation from (π,0) point to an incommensurate position towards (π,π) direction below 65 meV. In conclusion, this transition from V-shaped to hourglasslike dispersion, akin to behaviors in hole-doped cuprates, suggests a potential shared mechanism in magnetism and superconductivity across these diverse systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Universal coherence protection in a solid-state spin qubit

Decoherence limits the physical realization of qubits, and its mitigation is critical for the development of quantum science and technology. In this study, we construct a robust qubit embedded in a decoherence-protected subspace, obtained by applying microwave dressing to a clock transition of the ground-state electron spin of a silicon carbide divacancy defect. The qubit is universally protected from magnetic, electric, and temperature fluctuations, which account for nearly all relevant decoherence channels in the solid state. This culminates in an increase of the qubit’s inhomogeneous dephasing time by more than four orders of magnitude (to >22 milliseconds), while its Hahn-echo coherence time approaches 64 milliseconds. Requiring few key platform-independent components, this result suggests that substantial coherence improvements can be achieved in a wide selection of quantum architectures.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Rephasing spectral diffusion in time-bin spin-spin entanglement protocols

Generating high-fidelity spin-spin entanglement is an essential task of quantum repeater networks for the distribution of quantum information across long distances. Solid-state-based spin-photon interfaces are promising candidates to realize nodes of a quantum network, but are often limited by spectral diffusion of the optical transition, which results in phase errors on the entangled states. Here, we introduce a method to correct phase errors from quasi-static frequency fluctuations after the entangled state is generated, by shelving the emitters in the excited state to refocus the unknown phase. For quasi-static frequency fluctuations, the fidelity is determined only by the lifetime of the excited state used for shelving, making it particularly suitable for systems with a long-lived shelving state with correlated spectral diffusion. Such a shelving state may be found in Kramers doublet systems such as rare-earth emitters and color centers in Si or SiC interfaced with nanophotonic cavities with a strongly frequency-dependent Purcell enhancement. Furthermore, the protocol can be used to generate high-fidelity entangled spin pairs without reducing the rate of entanglement generation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2000

Statistical 3D 'Atomistic' Simulation of Decanano MOSFETs

A 3D statistical 'atomistic' simulation technique has been developed to study the effect of the random dopant induced parameter fluctuations in aggressively scaled MOSFETs. Efficient implementation of the 'atomistic' simulation approach has been used to investigate the threshold voltage standard deviation and lowering in the case of uniformly doped MOSFETs, and in fluctuation-resistant architectures utilising epitaxial-layers and delta-doping. The effect of the random doping in the polysilicon gate on the threshold voltage fluctuations has also been thoroughly investigated. The influence of a single-charge trapping on the channel conductivity in decanano MOSFETs is studied in the 'atomistic' framework as well. Quantum effects are taken into consideration in our 'atomistic' simulations using the density gradient formalism.

Asenov, Asen↗
DOE OSTI2021

Proximate Quantum Spin Liquid on Designer Lattice

Complementary to bulk synthesis, here we propose a designer lattice with extremely high magnetic frustration and demonstrate the possible realization of a quantum spin liquid state from both experiments and theoretical calculations. In an ultrathin (111) CoCr 2 O 4 slice composed of three triangular and one kagome cation planes, the absence of a spin ordering or freezing transition is demonstrated down to 0.03 K, in the presence of strong antiferromagnetic correlations in the energy scale of 30 K between Co and Cr sublattices, leading to the frustration factor of similar to 1000. Persisting spin fluctuations are observed at low temperatures via low-energy muon spin relaxation. Our calculations further demonstrate the emergence of highly degenerate magnetic ground states at the 0 K limit, due to the competition among multiply altered exchange interactions. Furthermore, these results collectively indicate the realization of a proximate quantum spin liquid state on the synthetic lattice.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Local condensation of charge-4 ⁢e superconductivity at a nematic domain wall

In the fluctuation regime that precedes the onset of pairing in multi-component superconductors, such as nematic and chiral superconductors, the normal state is generally unstable towards the formation of charge-4e order – an exotic quantum state in which electrons form coherent quartets rather than Cooper pairs. However, charge-4e order is often suppressed by other competing composite orders, such as nematics. Importantly, the formation of nematic domains is unavoidable due to the long-range strains generated, leading to one-dimensional regions where the competing nematic order is suppressed. Here, we employ a real-space variational approach to demonstrate that, in such nematic domain walls, charge-4e order is locally condensed via a vestigial-order mechanism. Here, we explore the experimental manifestations of this effect and discuss materials in which it can be potentially observed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Extraordinary Magnetic Response of an Anisotropic 2D Antiferromagnet via Site Dilution

A prominent characteristic of 2D magnetic systems is the enhanced spin fluctuations, which reduce the ordering temperature. We report that a magnetic field of only 1000th of the Heisenberg superexchange interaction can induce a crossover, which for practical purposes is the effective ordering transition, at temperatures about 6 times the Néel transition in a site-diluted two-dimensional anisotropic quantum antiferromagnet. Such a strong magnetic response is enabled because the system directly enters the antiferromagnetically ordered state from the isotropic disordered state, skipping the intermediate anisotropic stage. The underlying mechanism is achieved on a pseudospin-half square lattice realized in the [(SrIrO3)1/(SrTiO3)2] superlattice thin film that is designed to linearly couple the staggered magnetization to external magnetic fields by virtue of the rotational symmetry-preserving Dzyaloshinskii–Moriya interaction. Furthermore, our model analysis shows that the skipping of the anisotropic regime despite finite anisotropy is due to the enhanced isotropic fluctuations under moderate dilution.

2D antiferromagnet↗
DOE OSTI2022

SiGe quantum wells with oscillating Ge concentrations for quantum dot qubits

Large-scale arrays of quantum-dot spin qubits in Si/SiGe quantum wells require large or tunable energy splittings of the valley states associated with degenerate conduction band minima. Existing proposals to deterministically enhance the valley splitting rely on sharp interfaces or modifications in the quantum well barriers that can be difficult to grow. Here, we propose and demonstrate a new heterostructure, the “Wiggle Well”, whose key feature is Ge concentration oscillations inside the quantum well. Experimentally, we show that placing Ge in the quantum well does not significantly impact our ability to form and manipulate single-electron quantum dots. We further observe large and widely tunable valley splittings, from 54 to 239 μ eV. Tight-binding calculations, and the tunability of the valley splitting, indicate that these results can mainly be attributed to random concentration fluctuations that are amplified by the presence of Ge alloy in the heterostructure, as opposed to a deterministic enhancement due to the concentration oscillations. Quantitative predictions for several other heterostructures point to the Wiggle Well as a robust method for reliably enhancing the valley splitting in future qubit devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Magic Gap Ratio for Optimally Robust Fermionic Condensation and Its Implications for High- $T_c$ Superconductivity

Bardeen-Schrieffer-Cooper (BCS) and Bose-Einstein condensation (BEC) occur at opposite limits of a continuum of pairing interaction strength between fermions. A crossover between these limits is readily observed in a cold atomic Fermi gas. Whether it occurs in other systems such as the high temperature superconducting cuprates has remained an open question. We uncover here unambiguous evidence for a BCS-BEC crossover in the cuprates by identifying a universal magic gap ratio 2Δ/k B T c ≈ 6.5 (where Δ is the pairing gap and T c is the transition temperature) at which paired fermion condensates become optimally robust. At this gap ratio, corresponding to the unitary point in a cold atomic Fermi gas, the measured condensate fraction N0 and the height of the jump δγ(T c ) in the coefficient γ of the fermionic specific heat at T c are strongly peaked. In the cuprates, δγ(T c ) is peaked at this gap ratio when Δ corresponds to the antinodal spectroscopic gap, thus reinforcing its interpretation as the pairing gap. We find the peak in δγ(T c ) also to coincide with a normal state maximum in γ, which is indicative of a pairing fluctuation pseudogap above T c .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1988

Fluctuation analysis of relativistic nucleus-nucleus collisions in emulsion chambers

An analytical technique was developed for identifying enhanced fluctuations in the angular distributions of secondary particles produced from relativistic nucleus-nucleus collisions. The method is applied under the assumption that the masses of the produced particles are small compared to their linear momenta. The importance of particles rests in the fact that enhanced fluctuations in the rapidity distributions is considered to be an experimental signal for the creation of the quark-gluon-plasma (QGP), a state of nuclear matter predicted from the quantum chromodynamics theory (QCD). In the approach, Monte Carlo simulations are employed that make use of a portable random member generator that allow the calculations to be performed on a desk-top computer. The method is illustrated with data taken from high altitude emulsion exposures and is immediately applicable to similar data from accelerator-based emulsion exposures.

Mcguire, Stephen C.↗
DOE OSTI2020

Detection of the clock transition in thulium atoms by using repump laser radiation

A population transfer from the upper level of the clock transition in thulium atoms to the ground level is demonstrated by using radiation with a wavelength of 402 nm. The method for detecting the clock transition excitation that is not affected by technical fluctuations of the number of atoms in an optical lattice is investigated. The hyperfine splitting of the |4f{sup 12}({sup 3}F{sub 2})5d{sub 5/2} 6s{sup 2}; J = 7/2〉 level in the thulium atom is measured to be 1480(9) MHz. The suggested method for population transfer can be applied to both the hyperfine components of the clock transition intended for compensating Zeeman effect in a thulium optical clock. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2024

Clock Synchronization Characterization of the Washington DC Metropolitan Area Quantum Network (DC-QNet)

Quantum networking protocols relying on interference and precise time-of-flight measurements require high-precision clock synchronization. This study describes the design, implementation, and characterization of two optical time transfer methods in a metropolitan-scale quantum networking research testbed. With active electronic stabilization, sub-picosecond time deviation (TDEV) was achieved at integration times between 1 s and 10 5 s over 53 km of deployed fiber. Over the same integration periods, 10-picosecond level TDEV was observed using the White Rabbit-Precision Time Protocol (WR-PTP) over 128 km. Measurement methods are described to understand the sources of environmental fluctuations on clock synchronization towards the development of in-situ compensation methods. Path delay gradients, chromatic dispersion, polarization drift, and optical power variations all contributed to clock synchronization errors. The results from this study will inform future work in the development of link noise management methods essential for enabling experimental research in developing practical quantum networking protocols.

PIKTime↗
DOE OSTI2021

Evolution of spin excitations from bulk to monolayer FeSe

Abstract In ultrathin films of FeSe grown on SrTiO 3 (FeSe/STO), the superconducting transition temperature T c is increased by almost an order of magnitude, raising questions on the pairing mechanism. As in other superconductors, antiferromagnetic spin fluctuations have been proposed to mediate SC making it essential to study the evolution of the spin dynamics of FeSe from the bulk to the ultrathin limit. Here, we investigate the spin excitations in bulk and monolayer FeSe/STO using resonant inelastic x-ray scattering (RIXS) and quantum Monte Carlo (QMC) calculations. Despite the absence of long-range magnetic order, bulk FeSe displays dispersive magnetic excitations reminiscent of other Fe-pnictides. Conversely, the spin excitations in FeSe/STO are gapped, dispersionless, and significantly hardened relative to its bulk counterpart. By comparing our RIXS results with simulations of a bilayer Hubbard model, we connect the evolution of the spin excitations to the Fermiology of the two systems revealing a remarkable reconfiguration of spin excitations in FeSe/STO, essential to understand the role of spin fluctuations in the pairing mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Subexponential Decay of Local Correlations from Diffusion-Limited Dephasing

Chaotic quantum systems at finite entropy density are expected to act as their own heat baths, rapidly dephasing local quantum superpositions. Here, we argue that in fact this dephasing is generically subexponential in one-dimensional systems with conservation laws: all local correlation functions decay as exp⁡[−𝒪⁡(𝑡 𝛼 )] with 0 ≤ 𝛼 ≤ 2/3, even when the operators are orthogonal to all hydrodynamic modes. The mechanism is diffusion-limited dephasing, in which rare low-entropy regions (“voids”) protect quantum coherences. This intrinsically quantum effect lies beyond standard hydrodynamics and disappears under extrinsic dephasing. In random charge-conserving circuits we find 𝛼 = 1/2, while in generic translation-invariant Floquet systems we bound 𝛼 ≤ 2/3. Our arguments are general, subject principally to the assumption that thermal fluctuations can create regions of zero entropy density. In systems with energy conservation, this assumption is automatically satisfied because of the third law of thermodynamics.

information scrambling↗
DOE OSTI2021

Localization crossover and subdiffusive transport in a classical facilitated network model of a disordered interacting quantum spin chain

Here, we consider the random-field Heisenberg model, a paradigmatic model for many-body localization (MBL) and add a Markovian dephasing bath coupled to the Anderson orbitals of the model's noninteracting limit. We map this system to a classical facilitated hopping model that is computationally tractable for large system sizes and investigate its dynamics. The classical model exhibits a robust crossover between an ergodic (thermal) phase and a frozen (localized) phase. The frozen phase is destabilized by thermal subregions (bubbles), which thermalize surrounding sites by providing a fluctuating interaction energy and so enable off-resonance particle transport. Investigating steady-state transport, we observe that the interplay between thermal and frozen bubbles leads to a clear transition between diffusive and subdiffusive regimes. This phenomenology both describes the MBL system coupled to a bath and provides a classical analog for the MBL transition in the corresponding quantum model, in that the classical model displays long local memory times. It also highlights the importance of the details of the bath coupling in studies of MBL systems coupled to thermal environments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Engineering high-coherence superconducting qubits

Advances in materials science and engineering have played a central role in the development of classical computers and will undoubtedly be critical in propelling the maturation of quantum information technologies. In approaches to quantum computation based on superconducting circuits, as one goes from bulk materials to functional devices, amorphous films and non-equilibrium excitations — electronic and phononic — are introduced, leading to dissipation and fluctuations that limit the computational power of state-of-the-art qubits and processors. In this Review, the major sources of decoherence in superconducting qubits are identified through an exploration of seminal qubit and resonator experiments. The proposed microscopic mechanisms associated with these imperfections are summarized, and directions for future research are discussed. The trade-offs between simple qubit primitives based on a single Josephson tunnel junction and more complex designs that use additional circuit elements, or new junction modalities, to reduce sensitivity to local noise sources are discussed, particularly in the context of materials optimization strategies for each architecture.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Non-equilibrium entropy production and information dissipation in a non-Markovian quantum dot

This study measures trajectory-level entropy production and information dissipation in a driven, non-Markovian quantum dot using time-resolved optical dynamics and machine-learning-based analysis. Although not a 2D-material system, it is relevant because it demonstrates quantitative extraction of nonequilibrium dynamics from nanoscale optical fluctuations, which is conceptually connected to the proposed studies of transient charge and spin dynamics at interfaces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗