Spin Quantum Computer Research Articles

High-quality Ge/SiGe heterostructure with atomically sharp interface grown by molecular beam epitaxy

Germanium is a versatile material for realization of spin and topological quantum computing. Here, we report on the epitaxial growth of an undoped Ge/SiGe heterostructure in which a hole quantum well is formed in the sandwiched Ge layer. The heterostructure is grown on Si (001) via molecular beam epitaxy (MBE). Atomic force microscopy characterizations display a flat surface with a root mean square roughness of 0.956 nm, and spherical aberration corrected transmission electron microscopy data show a sharp interface with a characteristic length of 0.49 nm. A mobility of up to 1.2 × 105 cm2 V−1 s−1 was achieved in the SiGe/Ge/SiGe two-dimensional hole gas (2DHG). The low percolation density of 3.70 × 1010 cm−2, light effective mass of 0.079 m0 (where m0 is the free electron mass), and large effective g-factor of 9.5 were obtained. These results show the potential of MBE-grown Ge 2DHG for semiconductor quantum computing.

(Invited) Reactivity on Endohedral Metallofullerenes

Endohedral metallofullerenes (EMFs) are fullerene molecules with unique host-guest structures that allow chemists to manipulate encapsulated metal ions with the arsenal of organic chemistry, for potential applications including spin-quantum computing, single-molecule magnets, dynamic nuclear polarization, artificial photosynthesis, magnetic resonance imaging contrast agents, and therapeutic agents. To unleash the power to do so, the development of new reactions for EMFs has been an ongoing task.A key step to improve the solubility of EMFs as well as to obtain new materials for multiple applications is via exohedral functionalization. In medicine, for instance, water-soluble gadofullerenols are used as powerful contrast agents.1 Besides, the external groups might impart complementary features to EMFs that tune their physical properties for use as acceptors in polymer solar cells or as donor-acceptor dyad conjugates.2 Among the most used methods to obtain EMF monoadducts are cycloaddition reactions and radical additions, including the Diels-Alder cycloaddition, Bingel-Hirsch cyclopropanation, Prato 1,3-dipolar addition, carbene addition, etc. In these reactions, fullerenes typically play a similar electron-accepting role, thus their reactivity and regioselectivity are highly consistent. However, the reactivity of EMFs is far more complicated than of empty fullerenes due to the relatively low molecular symmetry and the influence of the encapsulated species. To understand the role of metal-cage interactions on the reactivity of EMFs is desirable for tuning their chemical reactivity.Here, we report the functionalization of actinide endofullerenes, in particular, the mono-EMFs U@C s(4)-C82 and U@C 2v(9)-C82, which their chemical reactivities were investigated through the Bingel-Hirsch (nucleophilic addition) and carbene addition (electrophilic addition) reactions. Interestingly, only for U@C 2v(9)-C82, the Bingel-Hirsch reaction leads to mono-adduct and multi-adduct compounds. Another subgroup of EMF that we study its reactivity here is the nitride clusterfullerenes, especially the Lu3N@Ih-C80, with high stability and high production yields, that results its chemical modification to be challenging. In this case, the reaction studied is the inverse-electron demand Diels-Alder (IEDDA) reaction and it will be compared to the corresponding reactivity on C60 fullerene. We show that Lu3N@Ih-C80presents higher reactivity than for C60, as well as, different features. 1Bolskar, R. D.; Benedetto, A. F.; Wilson, L. J.; Alford, J. M. et al. J. Am. Chem. Soc. 2003, 125, 5471-5478. 2Ross, R. B.; Cardona, C. M.; Guldi, D. M. et al. Nature Mater. 2009, 8, 208-212. Figure 1

Intrinsic Ferromagnetic Semiconductors with High Saturation Magnetization from Hybrid Perovskites.

Ferromagnetic semiconductors (FMS) enable simultaneous control of both charge and spin transport of charge carriers, which have emerged as a class of highly desirable but rare material for applications in spin field-effect transistors and quantum computing. Organic-inorganic hybrid perovskite with high compositional adjustability and structural versatility can offer unique benefits in the design of FMS but has not been fully explored. Here we demonstrate a series of molecular FMSs based on two-dimensional organic-inorganic hybrid perovskite structure, namely (2ampy)CuCl4 , (3ampy)CuCl4 and (4ampy)CuCl4 , which exhibits high saturation magnetization, dramatic temperature-dependent conductivity change and tunable ferromagnetic resonance. Magnetic measurements revealed high saturation magnetization up to 18.56 emu/g for (4ampy)CuCl4 , which is one of the highest value among reported hybrid FMSs to date. Conductivity studies of the three FMSs demonstrate that the smaller adjacent octahedron distance in the two-dimensional layer results in higher conductivity. Systematic ferromagnetic resonance investigation shows that the gyromagnetic ratio and Landau factor values are strongly dependent on the types of organic cations used. This work demonstrates that two-dimensional hybrid perovskite materials can simultaneously possess both tunable long-range ferromagnetic ordering and semiconductivity, providing a straightforward strategy for designing and synthesizing high-performance intrinsic FMSs. This article is protected by copyright. All rights reserved.

Fidelity of a sequence of SWAP operations on a spin chain

We consider the ``transport'' of the state of a spin across a Heisenberg-coupled spin chain via the use of repeated SWAP gates, starting with one of two states---one in which the leftmost spin is down and the others up, and one in which the leftmost two spins are in a singlet state (i.e., they are entangled), and the others are again all up. More specifically, we ``transport'' the state of the leftmost spin in the first case and the next-to-leftmost spin in the second to the other end of the chain, and then back again. We accomplish our SWAP operations here by increasing the exchange coupling between the two spins that we operate on from a base value $J$ to a larger value $J_\text{SWAP}$ for a time $t=\pi\hbar/4J_\text{SWAP}$. We determine the fidelity of this sequence of operations in a number of situations---one in which only nearest-neighbor coupling exists between spins and there is no magnetic dipole-dipole coupling or noise (the most ideal case), one in which we introduce next-nearest-neighbor coupling, but none of the other effects, and one in which all of these effects are present. In the last case, the noise is assumed to be quasistatic, i.e., the exchange couplings are each drawn from a Gaussian distribution, truncated to only nonnegative values. We plot the fidelity as a function of $J_\text{SWAP}$ to illustrate various effects, namely crosstalk due to coupling to other spins, as well as noise, that are detrimental to our ability to perform a SWAP operation. Our theory should be useful to the ongoing experimental efforts in building semiconductor-based spin quantum computer architectures.

Trityl-Aryl-Nitroxide-Based Genuinely g-Engineered Biradicals, As Studied by Dynamic Nuclear Polarization, Multifrequency ESR/ENDOR, Arbitrary Wave Generator Pulse Microwave Waveform Spectroscopy, and Quantum Chemical Calculations.

Trityl and nitroxide radicals are connected by π-topologically controlled aryl linkers, generating genuinely g-engineered biradicals. They serve as a typical model for biradicals in which the exchange (J) and hyperfine interactions compete with the g-difference electronic Zeeman interactions. The magnetic properties underlying the biradical spin Hamiltonian for solution, including J's, have been determined by multifrequency CW-ESR and 1H ENDOR spectroscopy and compared with those obtained by quantum chemical calculations. The experimental J values were in good agreement with the quantum chemical calculations. The g-engineered biradicals have been tested as a prototype for AWG (Arbitrary Wave Generator)-based spin manipulation techniques, which enable GRAPE (GRAdient Pulse Engineering) microwave control of spins in molecular magnetic resonance spectroscopy for use in molecular spin quantum computers, demonstrating efficient signal enhancement of specific weakened hyperfine signals. Dynamic nuclear polarization (DNP) effects of the biradicals for 400 MHz nuclear magnetic resonance signal enhancement have been examined, giving efficiency factors of 30 for 1H and 27.8 for 13C nuclei. The marked DNP results show the feasibility of these biradicals for hyperpolarization.

SKIFFS: Superconducting Kinetic Inductance Field-Frequency Sensors for sensitive magnetometry in moderate background magnetic fields

We describe sensitive magnetometry using lumped-element resonators fabricated from a superconducting thin film of NbTiN. Taking advantage of the large kinetic inductance of the superconductor, we demonstrate a continuous resonance frequency shift of 27 MHz for a change in the magnetic field of 1.8 μT within a perpendicular background field of 60 mT. By using phase-sensitive readout of microwaves transmitted through the sensors, we measure phase shifts in real time with a sensitivity of 1°/nT. We present measurements of the noise spectral density of the sensors and find that their field sensitivity is at least within one to two orders of magnitude of superconducting quantum interference devices operating with zero background field. Our superconducting kinetic inductance field-frequency sensors enable real-time magnetometry in the presence of moderate perpendicular background fields up to at least 0.2 T. Applications for our sensors include the stabilization of magnetic fields in long coherence electron spin resonance measurements and quantum computation.

Bound states in the continuum and Fano antiresonance in electronic transport through a four-quantum-dot system

We study the electronic transport through a four-quantum-dot (FQD) structure with a diamond-like shape through nonequilibrium Green's function theory. It is observed that the bound state in the continuum (BIC) appears in this multiple QDs system, and the position of the BIC in the total density of states (TDOS) spectrum is tightly determined by the strength of the electronic hopping between the upper QD and the lower one. As the symmetry in the energy levels in these two QDs is broken, the BIC is suppressed to a general conductance peak with a finite width, and meanwhile a Fano-type antiresonance with a zero point appears in the conductance spectrum. These results will develop our understanding of the BICs and their spintronic device applications of spin filter and quantum computing.

Charge Sensing of Precisely Positioned P Donors in Si

Real-time sensing of (spin-dependent) single-electron tunneling is fundamental to electrical readout of qubit states in spin quantum computing. Here, we demonstrate the feasibility of detecting such single-electron tunneling events using an atomically planar charge sensing layout, which can be readily integrated in scalable quantum computing architectures with phosphorus-donor-based spin qubits in silicon (Si:P). Using scanning tunneling microscopy (STM) lithography on a Si(001) surface, we patterned a single-electron transistor (SET), both tunnel and electrostatically coupled to a coplanar ultrasmall quantum dot, the latter consisting of approximately four P donors. Charge transitions of the quantum dot could be detected both in time-averaged and single-shot current response of the SET. Single electron tunneling between the quantum dot and the SET island on a time-scale (τ ∼ ms) two-orders-of-magnitude faster than the spin-lattice relaxation time of a P donor in Si makes this device geometry suitable for projective readout of Si:P spin qubits. Crucial to scalability is the ability to reproducibly achieve sufficient electron tunnel rates and charge sensitivity of the SET. The inherent atomic-scale control of STM lithography bodes extremely well to precisely optimize both of these parameters.

An endohedral fullerene-based nuclear spin quantum computer

We propose a new scalable quantum computer architecture based on endohedral fullerene molecules. Qubits are encoded in the nuclear spins of the endohedral atoms, which posses even longer coherence times than the electron spins which are used as the qubits in previous proposals. To address the individual qubits, we use the hyperfine interaction, which distinguishes two modes (active and passive) of the nuclear spin. Two-qubit quantum gates are effectively implemented by employing the electronic dipolar interaction between adjacent molecules. The electron spins also assist in the qubit initialization and readout. Our architecture should be significantly easier to implement than earlier proposals for spin-based quantum computers, such as the concept of Kane [B.E. Kane, Nature 393 (1998) 133].

Triangular nanobeam photonic cavities in single-crystal diamond

Diamond photonics provides an attractive architecture to explore room temperature cavity quantum electrodynamics and to realize scalable multi-qubit computing. Here, we review the present state of diamond photonic technology. The design, fabrication and characterization of a novel nanobeam cavity produced in a single crystal diamond are demonstrated. The present cavity design, based on a triangular cross-section, allows vertical confinement and better signal collection efficiency than that of slab-based nanocavities and eliminates the need for a pre-existing membrane. The nanobeam is fabricated by focused-ion-beam (FIB) patterning. The cavity is characterized by confocal photoluminescence. The modes display quality factors of Q∼220 and deviate in wavelength by only ∼1.7 nm from the nitrogen-vacancy (NV−) color center zero phonon line (ZPL). The measured results are found to be in good agreement with three-dimensional finite-difference-time-domain (FDTD) calculations. A more advanced cavity design with Q=22 000 is modeled, showing the potential for high-Q implementations using the triangular geometry. The prospects of this concept and its application in spin non-demolition measurement and quantum computing are discussed.

Universal quantum logic gates in a scalable Ising spin quantum computer

We consider the model of quantum computer, which is represented as a Ising spin lattice, where qubits (spin-half systems) are separated by the isolators (two spin-half systems). In the idle mode or at the single bit operations the total spin of isolators is 0. There are no need of complicated protocols for correcting the phase and probability errors due to permanent interaction between the qubits. We present protocols for implementation of universal quantum gates with the rectangular radio-frequency pulses.

Implementation of molecular spin quantum computing by pulsed ENDOR technique: Direct observation of quantum entanglement and spinor

Pulsed electron nuclear double resonance (ENDOR) spin technology has been invoked for realization of a quantum computer (QC). Sample preparation of molecular spins, quantum operations, and measurements have been performed successfully, showing that ENDOR QC is feasible. We have employed a typical stable organic open-shell entity, malonyl radical, for pulsed ENDOR-based QC experiments. Time proportional phase incrementation (TPPI) technique was applied in order to characterize entangled states. The appearance of the entanglement between electron and nuclear spins was discussed based on the pulsed ENDOR-QC experiments with the TPPI detection. The generated entangled state was operated by both the microwave and RF pulses, demonstrating the appearance of a spinor property with four- π periodicity due to electron spin-1/2, in a straightforward manner for the first time. Interconversion from one entangled state to the other one via quantum operation was demonstrated.

Entangling spins by measuring charge: A parity-gate toolbox

The parity gate emerged recently as a promising resource for performing universal quantum computation with fermions using only linear interactions. Here we analyze the parity gate ($P$ gate) from a theoretical point of view in the context of quantum networks. We present several schemes for entanglement generation with $P$ gates and show that native networks simplify considerably the resources required for producing multiqubit entanglement, such as $n$--Greenberg-Horne-Zellinger (GHZ) states. Other applications include a Bell-state analyzer and teleportation. We also show that cluster state fusion can be performed deterministically with parity measurements. We then extend this analysis to hybrid quantum networks containing spin and mode qubits. Starting from an easy-to-prepare resource (spin-mode entanglement of single electrons) we show how to produce a spin $n$-GHZ state with linear elements (beam splitters and local spin flips) and charge-parity detectors; this state can be used as a resource in a spin quantum computer or as a precursor for constructing cluster states. Finally, we construct an alternative spin-controlled-$Z$ gate by using the mode degrees of freedom as ancill\ae{}.

Quantum theory for electron spin decoherence induced by nuclear spin dynamics in semiconductor quantum computer architectures: Spectral diffusion of localized electron spins in the nuclear solid-state environment

We consider the decoherence of a single localized electron spin due to its coupling to the lattice nuclear spin bath in a semiconductor quantum computer architecture. In the presence of an external magnetic field and at low temperatures, the dominant decoherence mechanism is the spectral diffusion of the electron spin resonance frequency due to the temporally fluctuating random magnetic field associated with the dipolar interaction induced flip-flops of nuclear spin pairs. The electron spin dephasing due to this random magnetic field depends intricately on the quantum dynamics of the nuclear spin bath, making the coupled decoherence problem difficult to solve. We provide a formally exact solution of this non-Markovian quantum decoherence problem which numerically calculates accurate spin decoherence at short times, which is of particular relevance in solid-state spin quantum computer architectures. A quantum cluster expansion method is developed, motivated, and tested for the problem of localized electron spin decoherence due to dipolar fluctuations of lattice nuclear spins. The method is presented with enough generality for possible application to other types of spin decoherence problems. We present numerical results which are in quantitative agreement with electron spin echo measurements in phosphorus doped silicon. We also present spin echo decay results for quantum dots in GaAs which differ qualitatively from that of the phosphorus doped silicon system. Our theoretical results provide the ultimate limit on the spin coherence (at least, as characterized by Hahn spin echo measurements) of localized electrons in semiconductors in the low temperature and the moderate to high magnetic field regime of interest in scalable semiconductor quantum computer architectures.

Two-dimensional architectures for donor-based quantum computing

Through the introduction of a new electron spin transport mechanism, a 2D donor electron spin quantum computer architecture is proposed. This design addresses major technical issues in the original Kane design, including spatial oscillations in the exchange coupling strength and cross-talk in gate control. It is also expected that the introduction of a degree of non-locality in qubit gates will significantly improve the scaling fault-tolerant threshold over the nearest-neighbour linear array.

Creation of entanglement in a scalable spin quantum computer with long-range dipole-dipole interaction between qubits

Creation of entanglement is considered theoretically and numerically in an ensemble of spin chains with dipole-dipole interaction between the spins. The unwanted effect of the long-range dipole interaction is compensated by the optimal choice of the parameters of radio-frequency pulses implementing the protocol. The errors caused by (i) the influence of the environment, (ii) nonselective excitations, (iii) influence of different spin chains on each other, (iv) displacements of qubits from their perfect locations, and (v) fluctuations of the external magnetic field are estimated analytically and calculated numerically. For the perfectly entangled state the $z$ component $M$ of the magnetization of the whole system is equal to zero. The errors lead to a finite value of $M$. If the number of qubits in the system is large, $M$ can be detected experimentally. Using the fact that $M$ depends differently on the parameters of the system for each kind of error, varying these parameters would allow one to experimentally determine the most significant source of errors and to optimize correspondingly the quantum computer design in order to decrease the errors and $\ensuremath{\mid}M\ensuremath{\mid}$. Using our approach one can benchmark the quantum computer, decrease the errors, and prepare the quantum computer for implementation of more complex quantum algorithms.

Charge-Fluctuation-Induced Dephasing of Exchange-Coupled Spin Qubits

Exchange-coupled spin qubits in semiconductor nanostructures are shown to be vulnerable to dephasing caused by charge noise invariably present in the semiconductor environment. This decoherence of exchange gate by environmental charge fluctuations arises from the fundamental Coulombic nature of the Heisenberg coupling and presents a serious challenge to the scalability of the widely studied exchange gate solid state spin quantum computer architectures. We estimate dephasing times for coupled spin qubits in a wide range (from 1 ns up to >1 micros) depending on the exchange coupling strength and its sensitivity to charge fluctuations.

Spin diffusion and relaxation in solid state spin quantum computer

The processes of spin diffusion and relaxation are studied theoretically and numerically for quantum computation applications. Two possible realizations of a spin quantum computer (SQC) are analyzed: (i) a boundary spin chain in a 2D spin array and (ii) an isolated spin chain. In both cases, spin diffusion and relaxation are caused by a fast relaxing spin located outside the SQC. We have shown that in both cases the relaxation can be suppressed by an external non-uniform magnetic field. In the second case, our computer simulations have revealed various types of relaxation processes including the excitation of a random distribution of magnetic moments and the formation of stationary and moving domain walls. The region of optimal parameters for suppression of rapid spin relaxation is discussed.

Coherent control of cavity quantum electrodynamics for quantum nondemolition measurements and ultrafast cooling

Ultrafast initialization enables fault-tolerant processing of quantum information while QND readout enables scalable quantum computation. By spatially assembling photon resonators and wave-guides around an n-doped nanodot and by temporally designing optical pump pulses, an efficient quantum pathway can be established from an electron spin to a charged exciton to a cavity photon and finally to a flying photon in the waveguide. Such control of vacuum-nanodot coupling can be exploited for ultrafast initialization and QND readout of the spin, which are particularly compatible with the optically driven spin quantum computers.

Global control and fast solid-state donor electron spin quantum computing

We propose a scheme for quantum information processing based on donor electron spins in semiconductors, with an architecture complementary to the original Kane proposal. We show that a naive implementation of electron spin qubits provides only modest improvement over the Kane scheme, however through the introduction of global gate control we are able to take full advantage of the fast electron evolution timescales. We estimate that the latent clock speed is 100-1000 times that of the nuclear spin quantum computer with the ratio $T_{2}/T_{ops}$ approaching the $10^{6}$ level.