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We introduce a solid-state qubit in which exchange interactions among confined electrons provide both the static longitudinal field and the oscillatory transverse field, allowing rapid and full qubit control via rf gate-voltage pulses. We demonstrate two-axis control at a detuning sweet spot, where leakage due to hyperfine coupling is suppressed by the large exchange gap. A π/2-gate time of 2.5 ns and a coherence time of 19 μs, using multipulse echo, are also demonstrated. Model calculations that include effects of hyperfine noise are in excellent quantitative agreement with experiment.

We present transport measurements in superconductor-nanowire devices with a gated constriction forming a quantum point contact. Zero-bias features in tunneling spectroscopy appear at finite magnetic fields, and oscillate in amplitude and split away from zero bias as a function of magnetic field and gate voltage. A crossover in magnetoconductance is observed: Magnetic fields above ~ 0.5 T enhance conductance in the low-conductance (tunneling) regime but suppress conductance in the high-conductance (multichannel) regime. We consider these results in the context of Majorana zero modes as well as alternatives, including Kondo effect and analogs of 0.7 structure in a disordered nanowire., Supplemental Material here: https://dl.dropbox.com/u/1742676/Churchill_Supplemental.pdf

We report initialization, complete electrical control, and single-shot readout of an exchange-only spin qubit. Full control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in under 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements, and non-orthogonal control axes., Comment: contains Supplementary Information

We describe the synthesis, materials characterization and dynamic nuclear polarization (DNP) of amorphous and crystalline silicon nanoparticles for use as hyperpolarized magnetic resonance imaging (MRI) agents. The particles were synthesized by means of a metathesis reaction between sodium silicide (Na4Si4) and silicon tetrachloride (SiCl4) and were surface functionalized with a variety of passivating ligands. The synthesis scheme results in particles of diameter ~10 nm with long size-adjusted 29Si spin lattice relaxation (T1) times (> 600 s), which are retained after hyperpolarization by low temperature DNP., Supporting material: https://dl.dropboxusercontent.com/u/1742676/Supporting_Atkins_v11.pdf

We investigated the low temperature transport properties of AlGaAs/GaAs quantum dot arrays. The coupling between dots and the electron density are controlled by a single gate covering the array. Below 1 K, the current - voltage (I - V) curves show multiple discontinuous jumps in the current, or `switching events', between different insulating and conducting states, which occur at gate-voltage and temperature dependent thresholds. Each single switching event is accompanied by hysteresis, and multiple switching events result in a hierarchy of hysteresis loops. A possible mechanism for this behavior, involving gate-to-dot tunneling, is discussed.

We report measurements of transport in a two-dimensional electron gas in a spatially random magnetic field in which the average magnetic field extends from the classical regime $〈{\ensuremath{\omega}}_{c}〉\ensuremath{\tau}l1$ into the quantum Hall regime. Experiments make use of a rough Nd-Fe-B permanent magnet on the surface of a GaAs heterostructure. Effective mass, transport and total scattering times, and $g$-factor-enhancement values (all measured from Shubnikov-de Haas oscillations) are comparable to those found for potential scattering in a uniform magnetic field.

The effective interaction between magnetic impurities in metals that can lead to various magnetic ground states often competes with a tendency for electrons near impurities to screen the local moment (Kondo effect). The simplest system exhibiting the richness of this competition, the two-impurity Kondo system, is here realized experimentally in the form of two quantum dots coupled through an open conducting region. We demonstrate non-local spin control by suppressing and splitting Kondo resonances in one quantum dot by changing electron number and coupling of the other dot. Results suggest an approach to non-local spin control relevant to quantum information processing., Comment: related articles available at http://marcuslab.harvard.edu

We investigate low temperature transport in $200\ifmmode\times\else\texttimes\fi{}200$ arrays of GaAs quantum dots in which coupling between dots and electron density is controlled by a single gate. Current-voltage curves obey a power law above a threshold voltage with exponent $\ensuremath{\sim}1.5$, and show discontinuous and hysteretic jumps in the current, or ``switching events.'' Multiple switching events result in a hierarchy of hysteresis loops. Switching and hysteresis decrease with increasing temperature and disappear above 1 K. A possible mechanism for the hysteresis involving gate-to-dot tunneling is discussed.

A magneto-optical sensor, the luminescent spin-valve transistor, is demonstrated, showing direct control of a light source using a magnetic field. By manipulating the relative magnetizations of thin-film ferromagnets in the transistor's base, the luminescence intensity is modulated by approximately 200%.

An optical spin-valve effect is observed using sub-bandgap internal photoemission to generate and collect hot electrons in magnetic multilayers grown on n-Si. Approximately 1.5%-2.5% magnetoresistance is observed in this two-terminal device at low temperature, and this effect is reduced only to 1.1% at room temperature. A simple model is presented to explain the results.

We report a novel method for depositing patterned dielectric layers with sub-micron features using atomic layer deposition (ALD). The patterned films are superior to sputtered or evaporated films in continuity, smoothness, conformality, and minimum feature size. Films were deposited at 100-150C using several different precursors and patterned using either PMMA or photoresist. The low deposition temperature permits uniform film growth without significant outgassing or hardbaking of resist layers. A liftoff technique presented here gives sharp step edges with edge roughness as low as ~10 nm. We also measure dielectric constants (k) and breakdown fields for the high-k materials aluminum oxide (k ~ 8-9), hafnium oxide (k ~ 16-19) and zirconium oxide (k ~ 20-29), grown under similar low temperature conditions., related papers available at http://marcuslab.harvard.edu

We describe the design, fabrication, and operation of a GaAs-based heterostructure device which emits band gap luminescence from solid-state tunnel-junction ballistic injection of electrons with sub-bandgap energy. We find that, due to energy conservation requirements, a collector bias exceeding a threshold determined by the Schottky barrier height and sample band gap energy must be applied for luminescence emission. The consequences of these results for a hybrid scanning-probe microscopy and spectroscopy combining both ballistic electron emission microscopy and scanning tunneling luminescence are emphasized.

We investigate spin relaxation in a silicon double quantum dot via leakage current through Pauli blockade as a function of interdot detuning and magnetic field. A dip in leakage current as a function of magnetic field on a \sim 40 mT field scale is attributed to spin-orbit mediated spin relaxation. On a larger (\sim 400 mT) field scale, a peak in leakage current is seen in some, but not all, Pauli-blocked transitions, and is attributed to spin-flip cotunneling. Both dip and peak structure show good agreement between theory and experiment., supplementary material here: http://marcuslab.harvard.edu/SiDQDsupp.pdf

We report measurements of resistance oscillations in micron-scale antidots in both the integer and fractional quantum Hall regimes. In the integer regime, we conclude that oscillations are of the Coulomb type from the scaling of magnetic field period with the number of edges bound to the antidot. Based on both gate-voltage and field periods, we find at filling factor {\nu} = 2 a tunneling charge of e and two charged edges. Generalizing this picture to the fractional regime, we find (again, based on field and gate-voltage periods) at {\nu} = 2/3 a tunneling charge of (2/3)e and a single charged edge., Comment: related papers at http://marcuslab.harvard.edu

Direct dynamic nuclear polarization of 1H nuclei in frozen water and water-ethanol mixtures is demonstrated using silicon nanoparticles as the polarizing agent. Electron spins at dangling-bond sites near the silicon surface are identified as the source of the nuclear hyperpolarization. This novel polarization method open new avenues for the fabrication of surface engineered nanostructures to create high nuclear-spin polarized solutions without introducing contaminating radicals, and for the study of molecules adsorbed onto surfaces.

The spectrum of a segment of InAs nanowire, confined between two superconducting leads, was measured as function of gate voltage and superconducting phase difference using a third normal-metal tunnel probe. Sub-gap resonances for odd electron occupancy---interpreted as bound states involving a confined electron and a quasiparticle from the superconducting leads, reminiscent of Yu-Shiba-Rusinov states---evolve into Kondo-related resonances at higher magnetic fields. An additional zero bias peak of unknown origin is observed to coexist with the quasiparticle bound states., Comment: Supplementary information available here: https://dl.dropbox.com/u/1742676/Chang_Sup.pdf

We report measurements of universal-like conductance fluctuations in a quasiballistic approximately 2 mu m2 GaAs/AlGaAs quantum dot with adjustable point contact leads. Measurements cover a range of conductance G from nearly isolated, (G) approximately 0.1e2/h, to several modes in each lead, (G) approximately 5e2/h. The characteristic magnetic field scale of the fluctuations is found to increase with increasing mean conductance through the dot, consistent with a semiclassical picture of escape through the leads, and provides a means of estimating the phase breaking time tau phi for electrons inside the dot. The fluctuation amplitude is found to increase with increasing dot conductance, then begins to saturate once a few conducting channels are open.

We discuss magnetoconductance fluctuations in a ∼ 1 microm GaAS/AlxGa1−xAs quantum dot in the shape of a chaotic stadium billiard with point contact leads. Here we emphasize measurements carried out in the tunneling regime, where each lead carries less than one fully conducting channel. We discuss various crossover phenomena as the magnetoconductance fluctuations evolve from aperiodic structure at low fields (B ≲ 0.5 T) into periodic, Aharonov-Bohm-like oscillations at higher fields.

We report detailed measurements of reproducible, aperiodic fluctuations in the magnetoconductance of a submicron, stadium-shaped quantum dot with quantum point contacts. We study the prominent dip at B = 0 that is associated with a ballistic analog of the weak localization effect, and find its properties to be in agreement with semiclassical arguments.

Using single-shot charge detection in a GaAs double quantum dot, we investigate spin relaxation time (${T}_{1}$) and readout visibility of a two-electron singlet-triplet qubit following single-electron dynamic nuclear polarization (DNP). For magnetic fields up to 2 T, the DNP cycle is in all cases found to increase Overhauser field gradients, which in turn decrease ${T}_{1}$ and, consequently, reduce readout visibility. This effect was previously attributed to a suppression of singlet-triplet dephasing under a similar DNP cycle. A model describing relaxation after singlet-triplet mixing agrees well with experiment. Effects of pulse bandwidth on visibility are also investigated.

We investigate scaling of coherence time, T2, with the number of {\pi}-pulses, n_{\pi}, in a singlet- triplet spin qubit using Carr-Purcell-Meiboom-Gill (CPMG) and concatenated dynamical decoupling (CDD) pulse sequences. For an even numbers of CPMG pulses, we find a power law, T2 = (n_{\pi})^({\gamma}_e), with {\gamma}_e = 0.72\pm0.01, essentially independent of the envelope function used to extract T2. From this surprisingly robust value, a power-law model of the noise spectrum of the environment, S({\omega}) ~ {\omega}^(-{\beta}), yields {\beta} = {\gamma}_e/(1 - {\gamma}_e) = 2.6 \pm 0.1. Model values for T2(n_{\pi}) using {\beta} = 2.6 for CPMG with both even and odd n_{\pi} up to 32 and CDD orders 3 through 6 compare very well with experiment., Comment: related articles at http://marcuslab.harvard.edu

We explore a method for laser cooling and optical detection of excitations in a LC electrical circuit. Our approach uses a nanomechanical oscillator as a transducer between optical and electronic excitations. An experimentally feasible system with the oscillator capacitively coupled to the LC and at the same time interacting with light via an optomechanical force is shown to provide strong electro-mechanical coupling. Conditions for improved sensitivity and quantum limited readout of electrical signals with such an "optical loud speaker" are outlined., 5 pages, 2 figures

We investigate transport in a gate-defined graphene quantum point contact in the quantum Hall regime. Edge states confined to the interface of p and n regions in the graphene sheet are controllably brought together from opposite sides of the sample and allowed to mix in this split-gate geometry. Among the expected quantum Hall features, an unexpected additional plateau at 0.5 h/e^2 is observed. We propose that chaotic mixing of edge channels gives rise to the extra plateau., Comment: related papers at http://marcuslab.harvard.edu

We investigate the low-field relaxation of nuclear hyperpolarization in undoped and highly doped silicon microparticles at room temperature following removal from high field. For nominally undoped particles, two relaxation time scales are identified for ambient fields above 0.2 mT. The slower, T_1s, is roughly independent of ambient field; the faster, T_1f, decreases with increasing ambient field. A model in which nuclear spin relaxation occurs at the particle surface via a two-electron mechanism is shown to be in good agreement with the experimental data, particularly the field-independence of T_1s. For boron-doped particles, a single relaxation time scale is observed. This suggests that for doped particles, mobile carriers and bulk ionized acceptor sites, rather than paramagnetic surface states, are the dominant relaxation mechanisms. Relaxation times for the undoped particles are not affected by tumbling in a liquid solution., related papers at http://marcuslab.harvard.edu

Strong electron-electron interactions in graphene are expected to result in multiple-excitation generation by the absorption of a single photon. We show that the impact of carrier multiplication on photocurrent response is enhanced by very inefficient electron cooling, resulting in an abundance of hot carriers. The hot-carrier-mediated energy transport dominates the photoresponse and manifests itself in quantum efficiencies that can exceed unity, as well as in a characteristic dependence of the photocurrent on gate voltages. The pattern of multiple photocurrent sign changes as a function of gate voltage provides a fingerprint of hot-carrier-dominated transport and carrier multiplication., Comment: 4 pgs, 2 fgs

We introduce two methods of extracting the rate of loss of electron phase coherence in ballistic quantum dot microstructures from statistical measures of magnetic-field-induced conductance fluctuation. The first method uses changes in the (magnetic-field) power spectrum of conductance fluctuations upon changing the average conductance through the dot. The second, based on a Hauser-Feshbach--like formula from nuclear physics, relates the phase-breaking rate to the amplitude of the conductance fluctuations. The two methods are then applied to experimental data from GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum dots in the shape of a stadium billiard. Preliminary results give phase-breaking rates that are consistent with those previously found in quasiballistic GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As microstructures.

We analyze the dynamics and statistical mechanics of attractor neural networks with ''distributed'' updating rules in which groups of one or more neurons are updated simultaneously. Such partially parallel updating schemes are a central feature of neural-network architectures that use many processors, implemented either on special multiprocessor hardware, or among many computers linked over a network. Several updating rules are classified and discussed;these rules generalize the parallel dynamics of the Little model and the one-at-a-time dynamics of the Hopfield model. Analytic results presented herein include a stability criterion that specifies sufficient conditions under which distributed dynamics lead to fixed-point attractors. For binary neurons with block-sequential updating and a Hebbian learning rule, the storage capacity is found as a function of the number of update groups. Several open problems are also discussed.

We investigate transport in locally gated graphene devices, where carriers are injected and collected along, rather than across, the gate edge. Tuning densities into the $p\mathrm{\text{\ensuremath{-}}}n$ regime significantly reduces resistance along the $p\mathrm{\text{\ensuremath{-}}}n$ interface, while resistance across the interface increases. This provides an experimental signature of snake states, which zigzag along the $p\mathrm{\text{\ensuremath{-}}}n$ interface and remain stable as applied perpendicular magnetic field approaches zero. Snake states appear as a peak in transverse resistance measured along the $p\mathrm{\text{\ensuremath{-}}}n$ interface. The generic role of snake states in disordered graphene is also discussed.

We experimentally demonstrate coherence recovery of singlet-triplet superpositions by interlacing qubit rotations between Carr-Purcell (CP) echo sequences. We then compare performance of Hahn, CP, concatenated dynamical decoupling (CDD) and Uhrig dynamical decoupling (UDD) for singlet recovery. In the present case, where gate noise and drift combined with spatially varying hyperfine coupling contribute significantly to dephasing, and pulses have limited bandwidth, CP and CDD yield comparable results, with T2 ~ 80 microseconds., Comment: related papers at http://marcuslab.harvard.edu

Coherent two-level systems, or qubits, based on electron spins in GaAs quantum dots are strongly coupled to the nuclear spins of the host lattice via the hyperfine interaction. Realizing nuclear spin control would likely improve electron spin coherence and potentially enable the nuclear environment to be harnessed for the long-term storage of quantum information. Toward this goal, we report experimental control of the relaxation of nuclear spin polarization in a gate-defined two-electron GaAs double quantum dot. A cyclic gate-pulse sequence transfers the spin of an electron pair to the host nuclear system, establishing a local nuclear polarization that relaxes on a time scale of seconds. We find nuclear relaxation depends on magnetic field and gate-controlled two-electron exchange, consistent with a model of electron mediated nuclear spin diffusion., Comment: related papers at http://marcuslab.harvard.edu

We theoretically investigate the controlled dynamic polarization of lattice nuclear spins in GaAs double quantum dots containing two electrons. Three regimes of long-term dynamics are identified, including the build up of a large difference in the Overhauser fields across the dots, the saturation of the nuclear polarization process associated with formation of so-called "dark states," and the elimination of the difference field. We show that in the case of unequal dots, build up of difference fields generally accompanies the nuclear polarization process, whereas for nearly identical dots, build up of difference fields competes with polarization saturation in dark states. The elimination of the difference field does not, in general, correspond to a stable steady state of the polarization process., Comment: 4 pages, 2 figures

We investigate combined effects of spin-orbit coupling and magnetic field in carbon nanotubes containing one or more bends along their length. We show how bends can be used to provide electrical control of confined spins, while spins confined in straight segments remain insensitive to electric fields. Device geometries that allow general rotation of single spins are presented and analyzed. In addition, capacitive coupling along bends provides coherent spin-spin interaction, including between otherwise disconnected nanotubes, completing a universal set of one- and two-qubit gates., Comment: 6 pages, 5 figures

Single-shot measurement of the charge arrangement and spin state of a double quantum dot are reported with measurement times down to 100 ns. Sensing uses radio-frequency reflectometry of a proximal quantum dot in the Coulomb blockade regime. The sensor quantum dot is up to 30 times more sensitive than a comparable quantum point-contact sensor and yields three times greater signal to noise in rf single-shot measurements. Numerical modeling is qualitatively consistent with experiment and shows that the improved sensitivity of the sensor quantum dot results from reduced lifetime broadening and screening.

We investigate dynamic nuclear polarization in quantum point contacts (QPCs) in the integer and fractional quantum Hall regimes. Following the application of a dc bias, fractional plateaus in the QPC shift symmetrically about half filling of the lowest Landau level, \nu=1/2, suggesting an interpretation in terms of composite fermions. Polarizing and detecting at different filling factors indicates that Zeeman energy is reduced by the induced nuclear polarization. Mapping effects from integer to fractional regimes extends the composite fermion picture to include hyperfine coupling., Comment: related work at http://marcuslab.harvard.edu

We report detailed measurements of the low-temperature magnetoconductance in ballistic microstructures in the shape of a «chaotic» stadium and a circle with quantum-point-contact leads. Both structures show large, aperiodic, conductance fluctuations as a function of perpendicular magnetic field, and a zero-field resistance peak indicating geometry-dependent enhanced backscattering. Power spectra of fluctuations are consistent with recent semiclassical analyses based on quantum chaotic scattering, with the circle showing enhanced high-frequency spectral content

We analyze the collective behavior of feedback neural networks with discrete-time dynamics in which groups of one or more neurons may change their state simultaneously. Such partially parallel updating rules generalize beyond the one-at-a-time dynamics of the Hopfield model and the fully synchronous dynamics of the Little model. We derive sufficient conditions under which this type of distributed network dynamics leads to fixed point attractors only and report quantitative results on the emergent network properties.

We report nanoscale patterning of graphene using a helium ion microscope configured for lithography. Helium ion lithography is a direct-write lithography process, comparable to conventional focused ion beam patterning, with no resist or other material contacting the sample surface. In the present application, graphene samples on Si/SiO2 substrates are cut using helium ions, with computer controlled alignment, patterning, and exposure. Once suitable beam doses are determined, sharp edge profiles and clean etching are obtained, with little evident damage or doping to the sample. This technique provides fast lithography compatible with graphene, with approximately 15 nm feature sizes.

We report on the etching of graphene devices with a helium ion beam, including in situ electrical measurement during lithography. The etching process can be used to nanostructure and electrically isolate different regions in a graphene device, as demonstrated by etching a channel in a suspended graphene device with etched gaps down to about 10 nm. Graphene devices on silicon dioxide (SiO(2)) substrates etch with lower He ion doses and are found to have a residual conductivity after etching, which we attribute to contamination by hydrocarbons.

Two distinct types of magnetoresistance oscillations are observed in two electronic Fabry-Perot interferometers of different sizes in the integer quantum Hall regime. Measuring these oscillations as a function of magnetic field and gate voltages, we observe three signatures that distinguish the two types. The oscillations observed in a 2.0 square micron device are understood to arise from the Coulomb blockade mechanism, and those observed in an 18 square micron device from the Aharonov-Bohm mechanism. This work clarifies, provides ways to distinguish, and demonstrates control over, these distinct physical origins of resistance oscillations seen in electronic Fabry-Perot interferometers., related papers at http://marcuslab.harvard.edu

We use charge sensing of Pauli blockade (including spin and isospin) in a two-electron $^{13}\mathrm{C}$ nanotube double quantum dot to measure relaxation and dephasing times. The relaxation time ${T}_{1}$ first decreases with a parallel magnetic field and then goes through a minimum in a field of 1.4 T. We attribute both results to the spin-orbit-modified electronic spectrum of carbon nanotubes, which at high field enhances relaxation due to bending-mode phonons. The inhomogeneous dephasing time ${T}_{2}^{*}$ is consistent with previous data on hyperfine coupling strength in $^{13}\mathrm{C}$ nanotubes.

We report repeated single-shot measurements of the two-electron spin state in a GaAs double quantum dot. The readout scheme allows measurement with fidelity above 90% with a 7 microsecond cycle time. Hyperfine-induced precession between singlet and triplet states of the two-electron system are directly observed, as nuclear Overhauser fields are quasi-static on the time scale of the measurement cycle. Repeated measurements on millisecond to second time scales reveal evolution of the nuclear environment., Comment: supplemental material at http://marcuslab.harvard.edu/papers/single_shot_sup.pdf

We investigate nonlinear transport in electronic Fabry-Perot interferometers in the integer quantum Hall regime. For interferometers sufficiently large that Coulomb blockade effects are absent, a checkerboard-like pattern of conductance oscillations as a function of dc bias and perpendicular magnetic field is observed. Edge-state velocities extracted from the checkerboard data are compared to model calculations and found to be consistent with a crossover from skipping orbits at low fields to E x B drift at high fields. Suppression of visibility as a function of bias and magnetic field is accounted for by including energy- and field-dependent dephasing of edge electrons., Comment: related papers at http://marcuslab.harvard.edu

We review a number of recent results on the dynamics and stability of networks of analog (graded-response) neurons. Topics are motivated by practical issues of implementation, especially concerning the use of electronic circuits and multiprocessor computers to build fast, stable neural networks. Stability criteria for symmetrically connected networks are presented which generalize the famous result “symmetric connections ⇒ fixed points only” to include neurons with time-delayed response as well as discrete-time dynamics with parallel updating. Example applications include long-range and nearest-neighbor 2D lattices with delayed lateral inhibition and analog associative memories with iterated-map dynamics. We also describe a small (3-neuron) electronic analog network that exhibits delay-induced chaos. Finally, analytical and numerical results are presented showing how lowering neuron gain can dramatically reduce the number of spurious attractors and thus improve the performance of an analog network.

Single-walled carbon nanotubes are synthesized by chemical vapor deposition of methane at controlled locations on a substrate using patterned catalytic islands. The combined synthesis and microfabrication technique presented here allows a large number of ohmically contacted nanotube devices with controllable length to be placed on a single substrate. Transport studies demonstrate ohmic contacting, giving two-terminal resistances as low as 20 kΩ at low temperatures.

A recently discovered mechanism of electric dipole spin resonance, mediated by the hyperfine interaction, is investigated experimentally and theoretically. The effect is studied using a spin-selective transition in a GaAs double quantum dot. The resonant frequency is sensitive to the instantaneous hyperfine effective field, revealing a nuclear polarization created by driving the resonance. A device incorporating a micromagnet exhibits a magnetic field difference between dots, allowing electrons in either dot to be addressed selectively. An unexplained additional signal at half the resonant frequency is presented., Submitted as part of a SST special issue on spin-orbit effects in spintronics. Related papers at http://marcuslab.harvard.edu

In quantum dots made from materials with nonzero nuclear spins, hyperfine coupling creates a fluctuating effective Zeeman field (Overhauser field) felt by electrons, which can be a dominant source of spin qubit decoherence. We characterize the spectral properties of the fluctuating Overhauser field in a GaAs double quantum dot by measuring correlation functions and power spectra of the rate of singlet-triplet mixing of two separated electrons. Away from zero field, spectral weight is concentrated below 10 Hz, with 1/f^2 dependence on frequency, f. This is consistent with a model of nuclear spin diffusion, and indicates that decoherence can be largely suppressed by echo techniques., related papers available at http://marcuslab.harvard.edu

Coherent spin states in semiconductor quantum dots offer promise as electrically controllable quantum bits (qubits) with scalable fabrication. For few-electron quantum dots made from gallium arsenide (GaAs), fluctuating nuclear spins in the host lattice are the dominant source of spin decoherence. We report a method of preparing the nuclear spin environment that suppresses the relevant component of nuclear spin fluctuations below its equilibrium value by a factor of ~ 70, extending the inhomogeneous dephasing time for the two-electron spin state beyond 1 microsecond. The nuclear state can be readily prepared by electrical gate manipulation and persists for > 10 seconds., Supporting material available at http://marcuslab.harvard.edu/papers.shtml