1,359 research outputs found
Classical Optimizers for Noisy Intermediate-Scale Quantum Devices
We present a collection of optimizers tuned for usage on Noisy Intermediate-Scale Quantum (NISQ) devices. Optimizers have a range of applications in quantum computing, including the Variational Quantum Eigensolver (VQE) and Quantum Approximate Optimization (QAOA) algorithms. They are also used for calibration tasks, hyperparameter tuning, in machine learning, etc. We analyze the efficiency and effectiveness of different optimizers in a VQE case study. VQE is a hybrid algorithm, with a classical minimizer step driving the next evaluation on the quantum processor. While most results to date concentrated on tuning the quantum VQE circuit, we show that, in the presence of quantum noise, the classical minimizer step needs to be carefully chosen to obtain correct results. We explore state-of-the-art gradient-free optimizers capable of handling noisy, black-box, cost functions and stress-test them using a quantum circuit simulation environment with noise injection capabilities on individual gates. Our results indicate that specifically tuned optimizers are crucial to obtaining valid science results on NISQ hardware, and will likely remain necessary even for future fault tolerant circuits
All-optical switching of magnetic domains in Co/Gd heterostructures with Dzyaloshinskii-Moriya Interaction
Given the development of hybrid spintronic-photonic devices and chiral
magnetic structures, a combined interest in all-optical switching (AOS) of
magnetization and current-induced domain wall motion in synthetic ferrimagnetic
structures with strong Dzyaloshinskii-Moriya Interaction (DMI) is emerging. In
this study, we report a study on single-pulse all-optical toggle switching and
asymmetric bubble expansion in specially engineered Co/Gd-based multilayer
structures. In the absence of any external magnetic fields, we look into the
AOS properties and the potential role of the DMI on the AOS process as well as
the stability of optically written micro-magnetic domains. Particularly,
interesting dynamics are observed in moon-shaped structures written by two
successive laser pulses. The stability of domains resulting from an interplay
of the dipolar interaction and domain-wall energy are compared to simple
analytical models and micromagnetic simulations
Towards high all-optical data writing rates in synthetic ferrimagnets
Although all-optical magnetization switching with fs laser pulses has
garnered much technological interest, the ultimate data rates achievable have
scarcely been investigated. Recently it has been shown that after a switching
event in a GdCo alloy, a second laser pulse arriving 7 ps later can
consistently switch the magnetization. However, it is as of yet unknown whether
the same holds in layered ferrimagnetic systems, which hold much promise for
applications. In this work we investigate the minimum time delay required
between two subsequent switching events in synthetic ferrimagnetic Co/Gd
bilayers using two fs laser pulses. We experimentally demonstrate that the
minimum time delay needed for consistent switching can be as low as 10 ps.
Moreover, we demonstrate the importance of engineering heat diffusion away from
the magnetic material, as well as control over the laser pulse power. This
behavior is reproduced using modelling, where we find that the second switch
can occur even when the magnetization is not fully recovered. We further
confirm that heat diffusion is a critical factor in reducing the time delay for
the second switch, while also confirming a critical dependence on laser power
Boosting the Performance of WO3/nâSi Heterostructures for Photoelectrochemical Water Splitting: from the Role of Si to Interface Engineering
Metal oxide/Si heterostructures make up an exciting design route to highâperformance electrodes for photoelectrochemical (PEC) water splitting. By monochromatic light sources, contributions of the individual layers in WO3/nâSi heterostructures are untangled. It shows that band bending near the WO3/nâSi interface is instrumental in charge separation and transport, and in generating a photovoltage that drives the PEC process. A thin metal layer inserted at the WO3/nâSi interface helps in establishing the relation among the band bending depth, the photovoltage, and the PEC activity. This discovery breaks with the dominant Zâscheme design idea, which focuses on increasing the conductivity of an interface layer to facilitate charge transport, but ignores the potential profile around the interface. Based on the analysis, a highâworkâfunction metal is predicted to provide the best interface layer in WO3/nâSi heterojunctions. Indeed, the fabricated WO3/Pt/nâSi photoelectrodes exhibit a 2 times higher photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) and a 10 times enhancement at 1.6 V versus RHE compared to WO3/nâSi. Here, it is essential that the native SiO2 layer at the interface between Si and the metal is kept in order to prevent Fermi level pinning in the Schottky contact between the Si and the metal.</p
Deterministic all-optical magnetization writing facilitated by non-local transfer of spin angular momentum
Ever since the discovery of all-optical magnetization switching (AOS) around
a decade ago, this phenomenon of manipulating magnetization using only
femtosecond laser pulses has promised a large potential for future data storage
and logic devices. Two distinct mechanisms have been observed, where the final
magnetization state is either defined by the helicity of many incoming laser
pulses, or toggled by a single pulse. What has thus far been elusive, yet
essential for applications, is the deterministic writing of a specific
magnetization state with a single laser pulse. In this work we experimentally
demonstrate such a mechanism by making use of a spin polarized current which is
optically generated in a ferromagnetic reference layer, assisting or hindering
switching in an adjacent Co/Gd bilayer. We show deterministic writing of an
'up' and 'down' state using a sequence of 1 or 2 pulses, respectively.
Moreover, we demonstrate the non-local origin of the effect by varying the
magnitude of the generated spin current. Our demonstration of deterministic
magnetization writing could provide an essential step towards the
implementation of future optically addressable spintronic memory devices
Exploiting variability for energy optimization of parallel programs
In this paper we present optimizations that use DVFS mechanisms to reduce the total energy usage in scientific applications. Our main insight is that noise is intrinsic to large scale parallel executions and it appears whenever shared resources are contended. The presence of noise allows us to identify and manipulate any program regions amenable to DVFS. When compared to previous energy optimizations that make per core decisions using predictions of the running time, our scheme uses a qualitative approach to recognize the signature of executions amenable to DVFS. By recognizing the "shape of variability" we can optimize codes with highly dynamic behavior, which pose challenges to all existing DVFS techniques. We validate our approach using offline and online analyses for one-sided and two-sided communication paradigms. We have applied our methods to NWChem, and we show best case improvements in energy use of 12% at no loss in performance when using online optimizations running on 720 Haswell cores with one-sided communication. With NWChem on MPI two-sided and offline analysis, capturing the initialization, we find energy savings of up to 20%, with less than 1% performance cost
Do tonic itch and pain stimuli draw attention towards their location?
Background. Although itch and pain are distinct experiences, both are unpleasant, may demand attention, and interfere with daily activities. Research investigating the role of attention in tonic itch and pain stimuli, particularly whether attention is drawn to the stimulus location, is scarce. Methods. In the somatosensory attention task, fifty-three healthy participants were exposed to 35-second electrical itch or pain stimuli on either the left or right wrist. Participants responded as quickly as possible to visual targets appearing at the stimulated location (ipsilateral trials) or the arm without stimulation (contralateral trials). During control blocks, participants performed the visual task without stimulation. Attention allocation at the itch and pain location is inferred when responses are faster ipsilaterally than contralaterally. Results. Results did not indicate that attention was directed towards or away from the itch and pain location. Notwithstanding, participants were slower during itch and pain than during control blocks. Conclusions. In contrast with our hypotheses, no indications were found for spatial attention allocation towards the somatosensory stimuli. This may relate to dynamic shifts in attention over the time course of the tonic sensations. Our secondary finding that itch and pain interfere with task performance is in-line with attention theories of bodily perception
Ultrafast single-pulse all-optical switching in synthetic ferrimagnetic Tb/Co/Gd multilayers
In this work, we investigate single-shot all-optical switching (AOS) in
Tb/Co/Gd/Co/Tb multilayers in an attempt to establish AOS in synthetic
ferrimagnets with high magnetic anisotropy. In particular, we study the effect
of varying Tb thicknesses to disentangle the role of the two rare earth
elements. Even though the role of magnetic compensation has been considered to
be crucial, we find that the threshold fluence for switching is largely
independent of the Tb content. Moreover, we identify the timescale for the
magnetization to cross zero to be within the first ps after laser excitation
using time-resolved MOKE. We conclude that the switching is governed mostly by
interactions between Co and Gd
Picosecond Switching of Optomagnetic Tunnel Junctions
Perpendicular magnetic tunnel junctions are one of the building blocks for
spintronic memories, which allow fast nonvolatile data access, offering
substantial potentials to revolutionize the mainstream computing architecture.
However, conventional switching mechanisms of such devices are fundamentally
hindered by spin polarized currents4, either spin transfer torque or spin orbit
torque with spin precession time limitation and excessive power dissipation.
These physical constraints significantly stimulate the advancement of modern
spintronics. Here, we report an optomagnetic tunnel junction using a
spintronic-photonic combination. This composite device incorporates an
all-optically switchable Co/Gd bilayer coupled to a CoFeB/MgO-based
perpendicular magnetic tunnel junction by the Ruderman-Kittel-Kasuya-Yosida
interaction. A picosecond all-optical operation of the optomagnetic tunnel
junction is explicitly confirmed by time-resolved measurements. Moreover, the
device shows a considerable tunnel magnetoresistance and thermal stability.
This proof-of-concept device represents an essential step towards ultrafast
spintronic memories with THz data access, as well as ultralow power
consumption.Comment: 18 pages, 3 figure
Aging and passivation of magnetic properties in Co/Gd bilayers
Synthetic ferrimagnets based on Co and Gd bear promise for directly bridging
the gap between volatile information in the photonic domain and non-volatile
information in the magnetic domain, without the need for any intermediary
electronic conversion. Specifically, these systems exhibit strong spin-orbit
torque effects, fast domain wall motion and single-pulse all-optical switching
of the magnetization. An important open challenge to bring these materials to
the brink of applications is to achieve long-term stability of their magnetic
properties. In this work, we address the time-evolution of the magnetic moment
and compensation temperature of magnetron sputter grown Pt/Co/Gd trilayers with
various capping layers. Over the course of three months, the net magnetic
moment and compensation temperature change significantly, which we attribute to
quenching of the Gd magnetization. We identify that intermixing of the capping
layer and Gd is primarily responsible for this effect, which can be alleviated
by choosing nitrides for capping as long as reduction of nitride to oxide is
properly addressed. In short, this work provides an overview of the relevant
aging effects that should be taken into account when designing synthetic
ferrimagnets based on Co and Gd for spintronic applications.Comment: 9 pages, 5 figure
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