Potential Qubits Research Articles

Potential molecular qubits with long coherence time constructed using bromo-substituted trityl radicals

Bromo-substituted trityl radicals with long room-temperature quantum coherence are expected to be the future generation of molecular-based quantum bits.

Calculation study of selective ion extraction from ion source with Paul-trap-type laser cooling device

A single-ion implantation (SII) system combining a laser-cooled linear-Paul-trap ion source (LPT-IS) and a two-stage acceleration lens is being developed to produce an array of nitrogen vacancy (NV) centers, which have recently attracted significant attention as potential qubits. The fabrication of the NV center array requires repeated implantations of a single nitrogen molecule ion (N2+) from SII into a diamond specimen with an energy on the order of a 10-keV and spatial precision of several tens of nanometers or less. To satisfy this requirement, N2+ ions with very low emittance must be selectively extracted from the LPT-IS. In this study, the selective extraction of N2+ ions is investigated using a three-dimensional multiparticle simulation by varying magnitudes and temporal profile of the voltage applied to the end plate electrodes of a conventional LPT. The emittance of the N2+ extracted from the LPT-IS is the lowest when N2+ ion is at the leading end of an array of N2+ and Ca+ ions in the string-like crystalline structure formed with sympathetic cooling. We show that a single N2+ ion with required energy can be extracted from the array of N2+ and Ca+ ions passing through the end plate electrode of the LPT-IS, without spoiling the emittance, by increasing the voltage applied to the end plate electrode at the precise required moment.

Persistent entanglement of valley exciton qubits in transition metal dichalcogenides integrated into a bimodal optical cavity

We report dissipative dynamics of two valley excitons residing in the $K$ and ${K}^{\ensuremath{'}}$ valleys of a bare ${\mathrm{WSe}}_{2}$ monolayer and the one being integrated into a bimodal optical cavity. In the former, only when the exciton-field detunings in the $K$ and ${K}^{\ensuremath{'}}$ valleys are rigorously equal (resonant detuning) can partially entangled stationary states be created. Otherwise, the concurrence of exciton qubits turns to zero. Remarkably, in the latter (the ${\mathrm{WSe}}_{2}$ monolayer in a bimodal optical cavity), the transfers of entanglement from one subsystem (exciton/light) to the other (light/exciton) take place. Hence a finite stationary concurrence of exciton qubits is always generated, independent of whether the exciton-field detuning in two valleys is resonant or nonresonant. In addition, it can even reach as high as 1 (maximally entangled state of two valley excitons). Since there no real system which has a strictly resonant detuning, an immersion of the ${\mathrm{WSe}}_{2}$ monolayer in a bimodal optical cavity provides an opportunity to overcome the challenge facing by the bare ${\mathrm{WSe}}_{2}$, opening a novel realm of potential qubits.

Dilute Gd hydroxycarbonate particles for localized spin qubit integration.

Molecular spins are considered as the quantum hardware to build hybrid quantum processors in which coupling to superconducting devices would provide the means to implement the necessary coherent manipulations. As an alternative to large magnetically-dilute crystals or concentrated nano-scale deposits of paramagnetic molecules that have been studied so far, the use of pre-formed sub-micronic spherical particles of a doped Gd@Y hydroxycarbonate is evaluated here. Particles with an adjustable number of spin carriers are prepared through the control of both particle size and doping. Bulk magnetic properties and continuous wave and time-domain-EPR spectroscopy show that the Gd spins in these particles are potential qubits with robust quantum coherence. Monolayers of densely-packed particles are then formed interfacially and transferred successfully to the surface of Nb superconducting resonators. Alternatively, these particles are disposed at controlled localizations as isolated groups of a few particles through Dip-Pen Nanolithography using colloidal organic dispersions as ink. Altogether, this study offers new material and methodologies relevant to the development of viable hybrid quantum processors.

Multireference Study of Optically Addressable Vanadium-Based Molecular Qubit Candidates.

Molecular electron spin qubits with optical manipulation schemes are some of the most promising candidates for modern quantum technologies. Key values that determine a compound's viability for optical-spin initialization and readout include its singlet-triplet gap and zero-field splitting (ZFS) parameters. Generally, these values are very small in magnitude and are thus difficult to reproduce with theoretical methods. Here, we study a previously identified optically addressable molecular qubit, (C6F5)3trenVCNtBu (tren = tris(2-aminoethyl)amine), using the complete active space self-consistent field (CASSCF) and post-CASSCF methods: complete active space second-order perturbation theory (CASPT2), multiconfiguration pair-density functional theory (MC-PDFT), and hybrid MC-PDFT (HMC-PDFT). Of those methods, we successfully reproduce the singlet-triplet gap and ZFS parameters with reasonable accuracy using 0.5 HMC-PDFT and CASPT2. Four additional V3+ complexes with differing ligands were also investigated. We found that the ligands have minimal effect on the spin properties of the molecule and propose them to be optically addressable qubit candidates. These potential qubits are further analyzed in terms of ab initio ligand field theory (AILFT) to understand the influence of the ligands on the singlet-triplet gap and ZFS parameters.

First-principles study of transition metal dopants as spin qubits

Defect centers in solid-state materials have played an important role in the development of quantum information science as spin qubits. In recent years, many studies using optically detected magnetic resonance (ODMR) to manipulate the ground state spin of transition metal dopants have been reported. In this work, the tetrahedral ${d}^{2}$ and the octahedral ${d}^{3}$ transition metal ion systems are considered for the potential spin qubits, based on the ligand field theory and the basic requirements of general $\mathrm{\ensuremath{\Lambda}}$-type spin qubits. Then a computational scheme based on density functional theory is performed on the transition metal ligand structures $M{X}_{n}(M=\mathrm{V},\mathrm{Cr},\mathrm{Mn},\mathrm{Fe};X=\mathrm{N},\mathrm{O},\mathrm{C};\phantom{\rule{4pt}{0ex}}n=4, 6)$ in several typical hosts to analyze quantitatively their feasibility. The results on the charge transition levels, excitation, and excited-state structure relaxation energies of $M{X}_{n}$ show that some of them have the proper valence states to form the required ${d}^{2}$ or ${d}^{3}$ electronic systems and the appropriate excited-state level structure with the spin-selective transition. The results on the spontaneous emission lifetime show that the ${\mathrm{MnO}}_{4}$ structure is mostly suitable for spin polarization due to the fastest optical transition rates among these systems considered. Finally, some suggestions on determining potentially feasible systems activated by transition metal ions in solids are given.

Electronic structure study of divanadium complexes with rigid covalent coordination: potential molecular qubits with slow spin relaxation.

The electronic structures of homovalent [V2(μ-S2)2(R2dtc)4] (R = Et, iBu) and mixed-valent [V2(μ-S2)2(R2dtc)4]+ are reported here. The soft-donor, eight-coordinate ligand shell combined with the fully delocalised ground state provides a highly rigid and covalent environment that will nurture long spin relaxation times in vanadyl-based molecular qubits.

Coherent spin control of s-, p-, d- and f-electrons in a silicon quantum dot

Once the periodic properties of elements were unveiled, chemical behaviour could be understood in terms of the valence of atoms. Ideally, this rationale would extend to quantum dots, and quantum computation could be performed by merely controlling the outer-shell electrons of dot-based qubits. Imperfections in semiconductor materials disrupt this analogy, so real devices seldom display a systematic many-electron arrangement. We demonstrate here an electrostatically confined quantum dot that reveals a well defined shell structure. We observe four shells (31 electrons) with multiplicities given by spin and valley degrees of freedom. Various fillings containing a single valence electron—namely 1, 5, 13 and 25 electrons—are found to be potential qubits. An integrated micromagnet allows us to perform electrically-driven spin resonance (EDSR), leading to faster Rabi rotations and higher fidelity single qubit gates at higher shell states. We investigate the impact of orbital excitations on single qubits as a function of the dot deformation and exploit it for faster qubit control.

Hole spin echo envelope modulations

Hole spins in semiconductor quantum dots or bound to acceptor impurities show promise as potential qubits, partly because of their weak and anisotropic hyperfine couplings to proximal nuclear spins. Since the hyperfine coupling is weak, it can be difficult to measure. However, an anisotropic hyperfine coupling can give rise to a substantial spin-echo envelope modulation that can be Fourier-analyzed to accurately reveal the hyperfine tensor. Here, we give a general theoretical analysis for hole-spin-echo envelope modulation (HSEEM), and apply this analysis to the specific case of a boron-acceptor hole spin in silicon. For boron acceptor spins in unstrained silicon, both the hyperfine and Zeeman Hamiltonians are approximately isotropic leading to negligible envelope modulations. In contrast, in strained silicon, where light-hole spin qubits can be energetically isolated, we find the hyperfine Hamiltonian and $g$-tensor are sufficiently anisotropic to give spin-echo-envelope modulations. We show that there is an optimal magnetic-field orientation that maximizes the visibility of envelope modulations in this case. Based on microscopic estimates of the hyperfine coupling, we find that the maximum modulation depth can be substantial, reaching $\sim 10\%$, at a moderate laboratory magnetic field, $B\lesssim 200\,\mathrm{mT}$.

Interfacing a Potential Purely Organic Molecular Quantum Bit with a Real-Life Surface.

By using a multidisciplinary and multitechnique approach, we have addressed the issue of attaching a molecular quantum bit to a real surface. First, we demonstrate that an organic derivative of the pyrene-Blatter radical is a potential molecular quantum bit. Our study of the interface of the pyrene-Blatter radical with a copper-based surface reveals that the spin of the interface layer is not canceled by the interaction with the surface and that the Blatter radical is resistant in presence of molecular water. Although the measured pyrene-Blatter derivative quantum coherence time is not the highest value known, this molecule is known as a "super stable" radical. Conversely, other potential qubits show poor thin film stability upon air exposure. Therefore, we discuss strategies to make molecular systems candidates as qubits competitive, bridging the gap between potential and real applications.

Structural Effects on the Spin Dynamics of Potential Molecular Qubits.

Control of spin-lattice magnetic relaxation is crucial to observe long quantum coherence in spin systems at reasonable temperatures. Such a control is most often extremely difficult to achieve, because of the coexistence of several relaxation mechanisms, that is direct, Raman, and Orbach. These are not always easy to relate to the energy states of the investigated system, because of the contribution to the relaxation of additional spin-phonon coupling phenomena mediated by intramolecular vibrations. In this work, we have investigated the effect of slight changes on the molecular structure of four vanadium(IV)-based potential spin qubits on their spin dynamics, studied by alternate current (AC) susceptometry. The analysis of the magnetic field dependence of the relaxation time correlates well with the low-energy vibrational modes experimentally detected by time-domain THz spectroscopy. This confirms and extends our preliminary observations on the role played by spin-vibration coupling in determining the fine structure of the spin-lattice relaxation time as a function of the magnetic field, for S = 1/2 potential spin qubits. This study represents a step forward in the use of low-energy vibrational spectroscopy as a prediction tool for the design of molecular spin qubits with long-lived quantum coherence. Indeed, quantum coherence times of ca. 4.0-6.0 μs in the 4-100 K range are observed for the best performing vanadyl derivatives identified through this multitechnique approach.

(Invited) Modelling and Simulation of Advanced Semiconductor Devices

I. Introduction Modeling and simulation of semiconductor devices have been a vital tool in driving the development and comprehensive understanding of the semiconductor physics and devices. In the last 4 decades’ analysis, using state-of-the-art device modeling tool has helped the process of miniaturization of MOSFET by characterizing their parameters, predicting their behavior on the system and circuit level. Additionally, modeling and simulation enhanced a deeper understanding of the critical issues surrounding the device scaling and its impact on overall system level functionality and reliability of its operation. To show their importance, in this paper, we present modeling and simulation of advanced semiconductor devices based on the realistic device structures. II. Model description The modeling of semiconductor devices is based on numerical solutions of a complex mathematical equation that describe their fundamental physical properties. In our TCAD tools, we use ranges of computational approaches. For example, we use the well-known and popular way of describing the carrier behavior, such as Drift Diffusion (DD) which can include Quantum Mechanical (QM) corrected models. Also in our work, we employ more computationally demanding methods such as Ensemble Monte Carlo (EMC) and Non-Equilibrium Green’s Function Formalism (NEGF) to simulate advanced atomistic devices where using the classical DD approach is not adequate to handle the quantum transport typical for the modern ultra-scaled transistors. Moreover, we perform a first principle (ab-initio) calculation from which one can extract physical parameters like band- gap and effective mass (m*) of electrons and holes to be used in the advanced modeling and simulation process. All the methods described above can evaluate the impact of various sources of statistical variability, random discrete dopants and their adverse impact on the device performance. III. Considered devices Advanced devices of current technology generations, such as FinFET and Silicon Nano Wire were born out of the necessity of continuing the scaling beyond the conventional MOSFET architecture. In this section, we present modeling and simulation of some of the existing important devices structures and new promising memory application devices. A. FinFET FinFET technology is massively adopted as the advanced logic node technology. Modeling and simulation of FinFETs and accurate predictions can significantly speed up the next technology node. 7nm technology node is a critical point due to its extremely complicated physics and technical difficulty in fabrication. Employing advanced TCAD tool is critically important in exploring the fundamental physical properties in the 7nm FinFET technology featured with Fin width of 5nm and height of 25nm. Accurate capturing of the charge distribution in the nanoscale channel is mandatory since the quantum effect plays a critical role regarding the performance. We achieved this by using solving 2D Poisson-Schrodinger equation in the cross-section of the device. The wave-functions in the corresponding sub-bands of each valley is calculated and the charge distribution is obtained for different gate biases. The profile of the charge distribution serves as the calibration target in the TCAD calibration tool for density-gradient coupled DD simulation. The calibration results provide accurate charge information for the further simulations. B. Z2FET Z2FET is one of the most complex structures, and yet very promising device to be used for application in a memory cell without connecting to extremal charge storage component (aka 1T-DRAM). Significant progress has been made in characterizing its memory behavior and analyzing its physical properties. These qualities can be exploited in the implementation of Z2FET in embedded 1T-DRAM application. To understand the complexity of this promising device, TCAD model is an invaluable asset. Although there are some experimental data that may help the characterization process, the use of numerical modeling is equally important in understanding the dynamics of carriers, how the DC operation of this device can be characterized in the different sections of the device. In this paper, we investigate the DC operation of Z2FET in conjunction with its memory behavior and how it can be used in 1-TDRAM memory cell. We present simulation based detail analysis of Z2FET in the main paper. C. Silicon Nano Wire Transistors Silicon nanowires have numerous potential applications, including transistors, memories, photovoltaics, biosensors and qubits. In this work, we demonstrate a solution by exploiting the quantum effects of a 1-Dimensional (1D) Si nanowire transistor and analyze 1D nanowires in a scalable, top-down Si technology.

Spin Dynamics and Low Energy Vibrations: Insights from Vanadyl-Based Potential Molecular Qubits.

Here we report the investigation of the magnetization dynamics of a vanadyl complex with diethyldithiocarbamate (Et2dtc-) ligands, namely [VO(Et2dtc)2] (1), in both solid-state and frozen solution. This showed an anomalous and unprecedentedly observed field dependence of the relaxation time, which was modeled with three contributions to the relaxation mechanism. The temperature dependence of the weight of the two processes dominating at low fields was found to well correlate with the low energy vibrations as determined by THz spectroscopy. This detailed experimental comparative study represents a fundamental step to understand the spin dynamics of potential molecular quantum bits, and enriches the guidelines to design molecule-based systems with enhanced quantum coherence.

Rational design of potential spin qubits manipulated by the valence tautomerism mechanism: quantum-chemical modeling of the trinuclear transition metal complexes with bischelate linkers

A new series of transition metal coordination compounds capable of manifesting the properties of logical elements of quantum computers has been computationally designed using the DFT UB3LYP*/6-311++G(d,p) calculations.

Tunneling Motion and Antiferroelectric Ordering of Lithium Cations Trapped inside Carbon Cages

Dielectric and X-ray diffraction measurements of [Li@C60](PF6) single crystals reveal the motion of the Li+ cations inside the C60 cages at low temperature. An increase in the dielectric permittivity below 100 K is consistent with a combined tunneling and hopping motion of the Li+ cation between two positions inside the C60 cage. A phase transition accompanied by a decrease in the dielectric permittivity at TC = 24 K is explained by an antiferroelectric ordering of the Li+ cations. The Li+ ordering is caused by interactions among electric dipole moments formed between the Li+ cations inside and the PF6− anions outside the C60 cages. The electric dipole moments that are switched by the Li+ tunneling and interact with each other are potential qubits in a quantum computer using electric dipole moments.

Design of defect spins in piezoelectric aluminum nitride for solid-state hybrid quantum technologies

Spin defects in wide-band gap semiconductors are promising systems for the realization of quantum bits, or qubits, in solid-state environments. To date, defect qubits have only been realized in materials with strong covalent bonds. Here, we introduce a strain-driven scheme to rationally design defect spins in functional ionic crystals, which may operate as potential qubits. In particular, using a combination of state-of-the-art ab-initio calculations based on hybrid density functional and many-body perturbation theory, we predicted that the negatively charged nitrogen vacancy center in piezoelectric aluminum nitride exhibits spin-triplet ground states under realistic uni- and bi-axial strain conditions; such states may be harnessed for the realization of qubits. The strain-driven strategy adopted here can be readily extended to a wide range of point defects in other wide-band gap semiconductors, paving the way to controlling the spin properties of defects in ionic systems for potential spintronic technologies.

All-optical control of a solid-state spin using coherent dark states

The study of individual quantum systems in solids, for use as quantum bits (qubits) and probes of decoherence, requires protocols for their initialization, unitary manipulation, and readout. In many solid-state quantum systems, these operations rely on disparate techniques that can vary widely depending on the particular qubit structure. One such qubit, the nitrogen-vacancy (NV) center spin in diamond, can be initialized and read out through its special spin-selective intersystem crossing, while microwave electron spin resonance techniques provide unitary spin rotations. Instead, we demonstrate an alternative, fully optical approach to these control protocols in an NV center that does not rely on its intersystem crossing. By tuning an NV center to an excited-state spin anticrossing at cryogenic temperatures, we use coherent population trapping and stimulated Raman techniques to realize initialization, readout, and unitary manipulation of a single spin. Each of these techniques can be performed directly along any arbitrarily chosen quantum basis, removing the need for extra control steps to map the spin to and from a preferred basis. Combining these protocols, we perform measurements of the NV center's spin coherence, a demonstration of this full optical control. Consisting solely of optical pulses, these techniques enable control within a smaller footprint and within photonic networks. Likewise, this unified approach obviates the need for both electron spin resonance manipulation and spin addressability through the intersystem crossing. This method could therefore be applied to a wide range of potential solid-state qubits, including those which currently lack a means to be addressed.

Quantum logic between remote quantum registers

We analyze two approaches to quantum state transfer in solid-state spin systems. First, we consider unpolarized spin-chains and extend previous analysis to various experimentally relevant imperfections, including quenched disorder, dynamical decoherence, and uncompensated long range coupling. In finite-length chains, the interplay between disorder-induced localization and decoherence yields a natural optimal channel fidelity, which we calculate. Long-range dipolar couplings induce a finite intrinsic lifetime for the mediating eigenmode; extensive numerical simulations of dipolar chains of lengths up to L=12 show remarkably high fidelity despite these decay processes. We further consider the extension of the protocol to bosonic systems of coupled oscillators. Second, we introduce a quantum mirror based architecture for universal quantum computing which exploits all of the spins in the system as potential qubits. While this dramatically increases the number of qubits available, the composite operations required to manipulate "dark" spin qubits significantly raise the error threshold for robust operation. Finally, as an example, we demonstrate that eigenmode-mediated state transfer can enable robust long-range logic between spatially separated Nitrogen-Vacancy registers in diamond; numerical simulations confirm that high fidelity gates are achievable even in the presence of moderate disorder.

Coherent control of indirect excitonic qubits in optically driven quantum dot molecules

We propose an optoelectronic scheme to define and manipulate an indirect neutral exciton qubit within a quantum dot molecule. We demonstrate coherent dynamics of indirect excitons resilient against decoherence effects, including direct exciton spontaneous recombination. For molecules with large interdot separation, the exciton dressed spectrum yields an often overlooked avoided crossing between spatially indirect exciton states. Effective two-level system Hamiltonians are extracted by Feshbach projection over the multilevel exciton configurations. An adiabatic manipulation of the qubit states is devised using time-dependent electric field sweeps. The exciton dynamics yields the necessary conditions for qubit initialization and near unitary rotations in the picosecond time scale, driven by the system internal dynamics. Despite the strong influence of laser excitation, charge tunneling, and interdot dipole-dipole interactions, the effective relaxation time of indirect excitons is much longer than the direct exciton spontaneous recombination time, rendering indirect excitons as potential elemental qubits in more complex schemes.

A Controllable Interaction Between Two-Level Systems Inside a Josephson Junction

Two-level system fluctuators (TLS's) in the tunnel barrier of a Josephson junction have recently been demonstrated to cause novel energy splittings in spectroscopic measurements of superconducting phase qubits. With their strong coupling to the Josephson junction and relatively long decoherence times, TLS's can be considered as potential qubits and demonstrate coherent quantum effects. Here, we study the effective interaction between the TLS qubits that is mediated by a Josephson junction resonator driven by an external microwave source. This effective interaction can enable controlled quantum logic gates between the TLS's. Our study can be extended to other superconducting resonators coupling with TLS's.