Individual Spins Research Articles

Modelling Conformational Flexibility in a Spectrally Addressable Molecular Multi-Qubit Model System.

Dipolar coupled multi-spin systems have the potential to be used as molecular qubits. Herein we report the synthesis of a molecular multi-qubit model system with three individually addressable, weakly interacting, spin centres of differing g-values. We use pulsed Electron Paramagnetic Resonance (EPR) techniques to characterise and separately address the individual electron spin qubits; CuII , Cr7 Ni ring and a nitroxide, to determine the strength of the inter-qubit dipolar interaction. Orientation selective Relaxation-Induced Dipolar Modulation Enhancement (os-RIDME) detecting across the CuII spectrum revealed a strongly correlated CuII -Cr7 Ni ring relationship; detecting on the nitroxide resonance measured both the nitroxide and CuII or nitroxide and Cr7 Ni ring correlations, with switchability of the interaction based on differing relaxation dynamics, indicating a handle for implementing EPR-based quantum information processing (QIP) algorithms.

Controllable spin-dynamic scenarios on zigzag carbon cross structure

We investigate the spin-dynamic properties of two carbon cross structures assembled from two carbon chains with two Ni atoms attached. Both the global spin transfer through the long-distance carbon cross and the local spin flip on the single Ni atom are achieved within the subpicosecond regime, demonstrating the feasibility of individual intrasite and intersite spin manipulation. We investigate the dependence of the two different types of spin-dynamics processes on an external magnetic field, and we compare them by analyzing the spin features of the populated intermediate states. The comparison explains well the addressability of the local spin flip and the conservation of the global spin transfer. In addition, by applying two identical laser pulses, we successfully enhance the sensitivity of the local spin flip with respect to the external magnetic field, which yields a higher spatial resolution of the individual spin addressing. The present investigation is a significant step towards integrated all-optical logic circuits based on nano-spintronics.

Interplay of magnetic states and hyperfine fields of iron dimers on MgO(001)

Individual nuclear spin states can have very long lifetimes and could be useful as qubits. Progress in this direction was achieved on MgO/Ag(001) via detection of the hyperfine interaction (HFI) of Fe, Ti and Cu adatoms using scanning tunneling microscopy. Previously, we systematically quantified from first-principles the HFI for the whole series of 3d transition adatoms (Sc-Cu) deposited on various ultra-thin insulators, establishing the trends of the computed HFI with respect to the filling of the magnetic s- and d-orbitals of the adatoms and on the bonding with the substrate. Here we explore the case of dimers by investigating the correlation between the HFI and the magnetic state of free standing Fe dimers, single Fe adatoms and dimers deposited on a bilayer of MgO(001). We find that the magnitude of the HFI can be controlled by switching the magnetic state of the dimers. For short Fe-Fe distances, the antiferromagnetic state enhances the HFI with respect to that of the ferromagnetic state. By increasing the distance between the magnetic atoms, a transition toward the opposite behavior is observed. Furthermore, we demonstrate the ability to substantially modify the HFI by atomic control of the location of the adatoms on the substrate. Our results establish the limits of applicability of the usual hyperfine hamiltonian and we propose an extension based on multiple scattering processes.

Local strain-dependent Zeeman splitting in GaN:Eu

The electronic spins of rare-earth materials are attractive candidates for spin qubits and quantum memories. To access individual spins, tuning of the g-factor is desirable. Here, we report on local strain-dependent g-factors of the 5D0–7F2 transitions of Eu3+ centers in GaN:Eu thin films. We have found a clear correlation between the effective g-factor and the emission energy shift induced by the local strain. The combination of micro-photoluminescence and scanning electron microscope/electron backscattering diffraction measurements has revealed that the compressive strain of 0.2%–0.4%, relative to a surrounding reference point, induces an energy shift of about 3 meV. The strain decreases the g-factor of the emission at 1.991 eV from 2.5 to 1.5, while the strain increases the g-factor of the emission at 1.994 eV from 1.1 to 1.7. The result suggests that the g-factor can be tuned by the local strain. On the basis of the strain-induced energy shift and the g-factor, we have identified the optical sites. The 5D0–7F2 transitions observed in this study consist of three optical sites with C3v symmetry and one site with C1h symmetry.

Cavity Quantum Electrodynamics Effects with Nitrogen Vacancy Center Spins Coupled to Room Temperature Microwave Resonators

Cavity quantum electrodynamics (CQED) effects, such as Rabi splitting, Rabi oscillations, and superradiance, have been demonstrated with nitrogen vacancy (NV) center spins in diamond coupled to microwave resonators at cryogenic temperature. In this Letter, we explore the possibility to realize strong collective coupling and CQED effects with ensembles of NV spins at room temperature. Our calculations show that thermal excitation of the individual NV spins leads to population of collective Dicke states with low symmetry and a reduced collective coupling to the microwave resonators. Optical pumping can be applied to counteract the thermal excitation of the NV centers and to prepare the spin ensemble in Dicke states with high symmetry. The resulting strong coupling with high-quality resonators enables the study of intriguing CQED effects across the weak-to-strong coupling regime, and may have applications in quantum sensing and quantum information processing.

Site-specific electronic and magnetic excitations of the skyrmion material Cu2OSeO3

The manifestation of skyrmions in the Mott-insulator Cu2OSeO3 originates from a delicate balance between magnetic and electronic energy scales. As a result of these intertwined couplings, the two symmetry-inequivalent magnetic ions, Cu-I and Cu-II, bond into a spin S = 1 entangled tetrahedron. However, conceptualizing the unconventional properties of this material and the energy of the competing interactions is a challenging task due to the complexity of this system. Here we combine X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering to uncover the electronic and magnetic excitations of Cu2OSeO3 with site-specificity. We quantify the energies of the 3d crystal-field splitting for both Cu-I and Cu-II, fundamental for optimizing model Hamiltonians. Additionally, we unveil a site-specific magnetic mode, indicating that individual spin character is preserved within the entangled-tetrahedron picture. Our results thus provide experimental constraints for validating theories that describe the interactions of Cu2OSeO3, highlighting the site-selective capabilities of resonant spectroscopies.

Fourth RIT binary black hole simulations catalog: Extension to eccentric orbits

This fourth release of the RIT public catalog of numerical relativity black-hole-binary waveforms consists of 1881 accurate simulations that include 446 precessing and 611 nonprecessing quasicircular/inspiraling binary systems with mass ratios $q={m}_{1}/{m}_{2}$ in the range $1/128\ensuremath{\le}q\ensuremath{\le}1$ and individual spins up to $s/{m}^{2}=0.95$; and 824 in eccentric orbits in the range $0<e\ensuremath{\le}1$. The catalog also provides initial parameters of the binary, trajectory information, peak radiation, and final remnant black hole properties. The waveforms are corrected for the center of mass drifting and are extrapolated to future null infinity. As an application of this waveform catalog we reanalyze all of the peak radiation and remnant properties to find new, simple, correlations among them, valid in the presence of eccentricity, for practical astrophysical usage.

Long-lifetime spin excitations near domain walls in 1T-TaS2

SignificanceThere is an intense ongoing search for two-level quantum systems with long lifetimes for applications in quantum communication and computation. Much research has been focused on studying isolated spins in semiconductors or band insulators. Mott insulators provide an interesting alternative platform but have been far less explored. In this work we use a technique capable of resolving individual spins at atomic length scales, to measure the two-level switching of spin states in 1T-TaS2. We find quasi-1D chains of spin-1/2 electrons embedded in 1T-TaS2 which have exceptionally long lifetimes. The discovery of long-lived spin states in a tractable van der Waal material opens doors to using Mott systems in future quantum information applications.

A bright future for silicon in quantum technologies

Silicon is the most widely used material in microelectronic devices; integration of atomic impurities in silicon via doping during growth or ion implant is now widely used as it allows to form conventional transistors. Exploiting all the knowledge accumulated over the last 60 years in the context of the second quantum revolution that is now underway would help accelerate the commercialization of quantum technologies. Several works have already reported that silicon can be an optically active material with point-like defects emitting below the Si bandgap, both in ensemble emission and absorption in natural Si as well as in isotopically purified 28Si, even under electrical pumping. Very recently, the detection of individual impurities in silicon opened the door for further exploitation of this indirect bandgap material to applications in quantum technologies, including single photon emission at near-infrared frequency, matching the telecommunication band and optical detection of individual spins. Here, we describe the current state-of-the-art and discuss the forthcoming challenges and goals toward a reliable exploitation of these solid-state quantum-emitters in the context of quantum technologies. In particular, we examine opportunities, issues, and challenges in controlling defect formation and localization, extrinsic effects, and integration of optical devices.

Synthesis of π-Extended Thiele’s and Chichibabin’s Hydrocarbons and Effect of the π-Congestion on Conformations and Electronic States

The biradicaloid of Chichibabin’s hydrocarbon exits in a unique thermal equilibrium between closed-shell singlet and open-shell triplet forms. Conceptually, the incorporation of nonplanar aromatic groups, such as anthraquinodimethane (AQD), in these species could bring about stabilization of the individual singlet and triplet spin biradicaloids by creating a high energy barrier for conformational interconversion between folded (singlet) and twisted (triplet) forms. Moreover, this alteration could introduce the possibility of controlling spin states through conformational changes induced by chemical and physical processes. Herein, we report the preparation of AQD-containing, π-extended Thiele’s (A-TH) and Chichibabin’s (A-CH) hydrocarbons, which have highly π-congested structures resulting from the presence of bulky 9-anthryl units. The π-congestion in these substances leads to steric frustration about carbon–carbon double bonds and creates flexible dynamic motion with a moderate activation barrier between folded singlet and twisted triplet states. These constraints make it possible to isolate the twisted triplet state of A-CH. In addition, simple mechanical grinding of the folded singlet of A-CH produces the twisted triplet.

Combining electron spin resonance spectroscopy with scanning tunneling microscopy at high magnetic fields.

The continuous increase in storage densities and the desire for quantum memories and computers push the limits of magnetic characterization techniques. Ultimately, a tool that is capable of coherently manipulating and detecting individual quantum spins is needed. Scanning tunneling microscopy (STM) is the only technique that unites the prerequisites of high spatial and energy resolution, low temperature, and high magnetic fields to achieve this goal. Limitations in the available frequency range for electron spin resonance STM (ESR-STM) mean that many instruments operate in the thermal noise regime. We resolve challenges in signal delivery to extend the operational frequency range of ESR-STM by more than a factor of two and up to 100 GHz, making the Zeeman energy the dominant energy scale at achievable cryogenic temperatures of a few hundred millikelvin. We present a general method for augmenting existing instruments into ESR-STM to investigate spin dynamics in the high-field limit. We demonstrate the performance of the instrument by analyzing inelastic tunneling in a junction driven by a microwave signal and provide proof of principle measurements for ESR-STM.

Clustering by quantum annealing on the three-level quantum elements qutrits

Clustering is grouping of data by the proximity of some properties. We report on the possibility of increasing the efficiency of clustering of points in a plane using artificial quantum neural networks after the replacement of the two-level neurons called qubits represented by the spins S = 1/2 by the three-level neurons called qutrits represented by the spins S = 1. The problem has been solved by the slow adiabatic change of the Hamiltonian in time. The methods for controlling a qutrit system using projection operators have been developed and the numerical simulation has been performed. The Hamiltonians for two well-known clustering methods, one-hot encoding and k-means, have been built. The first method has been used to partition a set of six points into three or two clusters and the second method, to partition a set of nine points into three clusters and seven points into four clusters. The simulation has shown that the clustering problem can be effectively solved on qutrits represented by the spins S = 1. The advantages of clustering on qutrits over that on qubits have been demonstrated. In particular, the number of qutrits required to represent $$N$$ data points is smaller than the number of qubits by a factor of $$\log_{2} N/\log_{3} N$$ . For qutrits, the simplest is to partition the data points into three clusters rather than two ones. At the data partition into more than three clusters, it has been proposed to number the clusters by the numbers of states of the corresponding multi-spin subsystems, instead of using the numbers of individual spins. This reduces even more the number of qutrits ( $$N\log_{3} K$$ instead of $$NK$$ ) required to implement the algorithm.

Progressive quantum collapse

Instantaneous collapse of the wave function upon measurement of a single particle is one of the postulates of the Copenhagen interpretation of quantum mechanics. However, what happens when a many-body system in a macroscopic coherent state is measured one particle at a time? Here, we consider successive measurements of individual spins from a spin Bose condensate that starts in a Schrödinger cat state. When the spin measurements are done one particle at a time, the collapse of the spin condensate is not instantaneous but leads to probabilities for spin measurement that strongly depend on the previous measurements. What is surprising is that an almost complete collapse occurs in very few measurements. Even in a large system, a single cat component is emphasized quite quickly in the sequence of measurements. We examine the process by analysis of a simple two-Fock-state cat, as well as a cat state that has many components. Justification is given for our theoretical measurement process.

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.

Detection of individual spin species via frequency-modulated charge pumping.

We utilize a frequency-modulated charge pumping methodology to measure quickly and conveniently single "charge per cycle" in highly scaled Si/SiO2 metal-oxide-semiconductor field effect transistors. This is indicative of detection and manipulation of a single interface trap spin species located at the boundary between the SiO2 gate dielectric and Si substrate (almost certainly a Pb type center). This demonstration in sub-micrometer devices in which Dennard scaling of the gate oxide has yielded extremely large gate oxide leakage currents eliminates interference between the charge pumping current and the leakage phenomenon. The result is the ability to measure single trap charge pumping reliably and easily, which would otherwise be completely inaccessible due to oxide leakage. This work provides a unique and readily available avenue for single spin species detection and manipulation, which can be applied as a quantized standard of electrical current as well as to serve as a potentially useful platform for developing quantum engineering technologies. Finally, we discuss potential underlying physical mechanisms that are involved in producing a seemingly contradictory measure of both odd and even integer values for charge per cycle.

Estimating the final spin of binary black holes merger in STU supergravity

In this paper, we adopt the so-called Buonanno-Kidder-Lehner (BKL) recipe to estimate the final spin of a rotating binary black hole merger in STU supergravity. According to the BKL recipe, the final spin can be viewed as the sum of the individual spins plus the orbital angular momentum of the binary system which could be approximated as the angular momentum of a test particle orbiting at the innermost stable circular orbit around the final black hole. Unlike previous works, we consider the contribution of the orbital angular momentum of the binary system to the final spin by requiring the test particle to preserve the scaling symmetry in the Lagrangian of supergravity. We find some subtle differences between two cases corresponding to whether the symmetry is taken into account or not. In the equal initial spin configuration, when the initial black holes are non-spinning, the final spin of the merger is always larger than that in the case in which the symmetry is not imposed although the general behaviors are similar. The difference increases firstly and then decreases as the initial mass ratio approaches unity. Besides, as the initial spins exceed a threshold, the final spin is always smaller than that in the case where the scaling symmetry is not considered. The difference decreases constantly as the equal initial mass limit is approached. All these features exist in the merger of a binary STU black hole with different charge configurations. We also study the final spin's difference between different charge configurations and different initial spin configurations.

Interpretable AI forecasting for numerical relativity waveforms of quasicircular, spinning, nonprecessing binary black hole mergers

We present a deep-learning artificial intelligence model (AI) that is capable of learning and forecasting the late-inspiral, merger and ringdown of numerical relativity waveforms that describe quasicircular, spinning, nonprecessing binary black hole mergers. We used theNRHybSur3dq8 surrogate model to produce train, validation and test sets of $\ensuremath{\ell}=|m|=2$ waveforms that cover the parameter space of binary black hole mergers with mass ratios $q\ensuremath{\le}8$ and individual spins $|{s}_{{1,2}}^{z}|\ensuremath{\le}0.8$. These waveforms cover the time range $t\ensuremath{\in}[\ensuremath{-}5000\text{ }\text{ }\mathrm{M},130\text{ }\text{ }\mathrm{M}]$, where $t=0M$ marks the merger event, defined as the maximum value of the waveform amplitude. We harnessed the ThetaGPU supercomputer at the Argonne Leadership Computing Facility to train our AI model using a training set of 1.5 million waveforms. We used 16 NVIDIA DGX A100 nodes, each consisting of 8 NVIDIA A100 Tensor Core GPUs and 2 AMD Rome CPUs, to fully train our model within 3.5 h. Our findings show that artificial intelligence can accurately forecast the dynamical evolution of numerical relativity waveforms in the time range $t\ensuremath{\in}[\ensuremath{-}100\text{ }\text{ }\mathrm{M},130\text{ }\text{ }\mathrm{M}]$. Sampling a test set of 190,000 waveforms, we find that the average overlap between target and predicted waveforms is $\ensuremath{\gtrsim}99%$ over the entire parameter space under consideration. We also combined scientific visualization and accelerated computing to identify what components of our model take in knowledge from the early and late-time waveform evolution to accurately forecast the latter part of numerical relativity waveforms. This work aims to accelerate the creation of scalable, computationally efficient and interpretable artificial intelligence models for gravitational wave astrophysics.

Implementation of Quantum Level Addressability and Geometric Phase Manipulation in Aligned Endohedral Fullerene Qudits.

Endohedral nitrogen fullerenes have been proposed as building blocks for quantum information processing due to their long spin coherence time. However, addressability of the individual electron spin levels in such a multiplet system of 4 S3/2 has never been achieved because of the molecular isotropy and transition degeneracy among the Zeeman levels. Herein, by molecular engineering, we lifted the degeneracy by zero-field splitting effects and made the multiple transitions addressable by a liquid-crystal-assisted method. The endohedral nitrogen fullerene derivatives with rigid addends of spiro structure and large aspect ratios of regioselective bis-addition improve the ordering of the spin ensemble. These samples empower endohedral-fullerene-based qudits, in which the transitions between the 4 electron spin levels were respectively addressed and coherently manipulated. The quantum geometric phase manipulation, which has long been proposed for the advantages in error tolerance and gating speed, was implemented in a pure electron spin system using molecules for the first time.

Implementation of Quantum Level Addressability and Geometric Phase Manipulation in Aligned Endohedral Fullerene Qudits

AbstractEndohedral nitrogen fullerenes have been proposed as building blocks for quantum information processing due to their long spin coherence time. However, addressability of the individual electron spin levels in such a multiplet system of 4S3/2 has never been achieved because of the molecular isotropy and transition degeneracy among the Zeeman levels. Herein, by molecular engineering, we lifted the degeneracy by zero‐field splitting effects and made the multiple transitions addressable by a liquid‐crystal‐assisted method. The endohedral nitrogen fullerene derivatives with rigid addends of spiro structure and large aspect ratios of regioselective bis‐addition improve the ordering of the spin ensemble. These samples empower endohedral‐fullerene‐based qudits, in which the transitions between the 4 electron spin levels were respectively addressed and coherently manipulated. The quantum geometric phase manipulation, which has long been proposed for the advantages in error tolerance and gating speed, was implemented in a pure electron spin system using molecules for the first time.

Detecting spins by their fluorescence with a microwave photon counter.

Quantum emitters respond to resonant illumination by radiating part of the absorbed energy. A component of this radiation field is phase coherent with the driving tone, whereas another component is incoherent and consists of spontaneously emitted photons, forming the fluorescence signal1. Atoms, molecules and colour centres are routinely detected by their fluorescence at optical frequencies, with important applications in quantum technology2,3 and microscopy4-7. By contrast, electron spins are usually detected by the phase-coherent echoes that they emit in response to microwave driving pulses8. The incoherent part of their radiation-a stream of microwave photons spontaneously emitted upon individual spin relaxation events-has not been observed so far because of the low spin radiative decay rate and of the lack of single microwave photon detectors (SMPDs). Here using superconducting quantum devices, we demonstrate the detection of a small ensemble of donor spins in silicon by their fluorescence at microwave frequencies and millikelvin temperatures. We enhance their radiative decay rate by coupling them to a high-quality-factor and small-mode-volume superconducting resonator9, and we connect the device output to a newly developed SMPD10 based on a superconducting qubit. In addition, we show that the SMPD can be used to detect spin echoes and that standard spin characterization measurements (Rabi nutation and spectroscopy) can be achieved with both echo and fluorescence detection. We discuss the potential of SMPD detection as a method for magnetic resonance spectroscopy of small numbers of spins.