Physical Review Applied

Online ISSN: 2331-7019
Publications
(Color online) (a) Atomic force microscope (AFM) image of a chemically and thermally prepared SrTiO 3 (111) surface. The surface is single terminated and unreconstructed, as shown in the RHEED image in (b). Longer annealing times result in a mixed terminated surface, as demonstrated in the AFM friction image (c) measured in contact mode. A 3 × 3 reconstruction of the surface can be deduced from the corresponding RHEED image in (d). 
Article
We report the existence of confined electronic states at the (110) and (111) surfaces of SrTiO3. Using angle-resolved photoemission spectroscopy, we find that the corresponding Fermi surfaces, subband masses, and orbital ordering are different from the ones at the (001) surface of SrTiO3. This occurs because the crystallographic symmetries of the surface and sub-surface planes, and the electron effective masses along the confinement direction, influence the symmetry of the electronic structure and the orbital ordering of the t2g manifold. Remarkably, our analysis of the data also reveals that the carrier concentration and thickness are similar for all three surface orientations, despite their different polarities. The orientational tuning of the microscopic properties of two-dimensional electron states at the surface of SrTiO3 echoes the tailoring of macroscopic (e.g. transport) properties reported recently in LaAlO3/SrTiO3 (110) and (111) interfaces, and is promising for searching new types of 2D electronic states in correlated-electron oxides.
 
(color online) Typical band structure and DOS of a c-Si/a-Si 3 N 3 . 5 :H interface sample in the presence of a single 
Article
Modern electronic devices are unthinkable without the well-controlled formation of interfaces at heterostructures. These often involve at least one amorphous material. Modeling such interfaces poses a significant challenge, since a meaningful result can only be expected by using huge models or by drawing from many statistically independent samples. Here we report on the results of high throughput calculations for interfaces between crystalline silicon (c-Si) and amorphous silicon nitride (a-Si$_3$N$_{3.5}$:H), which are omnipresent in commercially available solar cells. The findings reconcile only partly understood key features. At the interface, threefold coordinated Si atoms are present. These are caused by the structural mismatch between the amorphous and crystalline part. The local Fermi level of undoped c-Si lies well below that of a-SiN:H. To align the Fermi levels in the device, charge is transferred from the a-SiN:H part to the c-Si part resulting in an abundance of positively charged, threefold coordinated Si atoms at the interface. This explains the existence of a positive, fixed charge at the interface that repels holes.
 
(a) Trigonal prismatic unit cell of monolayer MoS 2 ; 
(a)–(c) Schematic of the phase change due to site permutation. Real-space configuration of the phase of the K- point Bloch state [exp( i K · r )] is represented, where circle, 
Article
We find that the motion of the valley electrons -- electronic states close to the ${\rm K}$ and ${\rm K'}$ points of the Brillouin zone -- is confined into two dimension when the layers of MoS$_{2}$ follow the 3R stacking, while in the 2H polytype the bands have dispersion in all the three dimensions. According to our first-principles band structure calculations, the valley states have no interlayer hopping in 3R-MoS$_{2}$, which is proved to be the consequence of the rotational symmetry of the Bloch functions. By measuring the reflectivity spectra and analyzing an anisotropic hydrogen atomic model, we confirm that the valley excitons in 3R-MoS$_{2}$ have two-dimensional hydrogen-like spectral series, and the spreads of the wave function are smaller than the interlayer distance. In contrast, the valley excitons in 2H-MoS$_{2}$ are well described by the three-dimensional model and thus not confined in a single layer. Our results indicate that the dimensionality of the valley degree of freedom can be controlled simply by the stacking geometry, which can be utilized in future valleytronics.
 
(Color online) SEM image of YBCO nanoSQUID-1. Vertical dashed line indicates position of the grain boundary intersecting the two SQUID arms. Horizontal arrows indicate paths for modulation current I mod across the constriction and bias current I across the grain boundary Josephson junctions. 
(Color online) SEM image of SQUID-3 with Fe-wire filled carbon nanotube positioned close to the SQUID loop. 
(Color online) Hysteresis loop Φ(H) of the Fenanowire detected with SQUID-3 (operated in FLL dc bias mode with cutoff frequency ∼ 190 kHz, at optimum working point with VΦ = 0.65 mV/Φ0). Switching of the magnetization occurs at ±µ0Hn = ±101 mT. The residual field µ0Hres = 4.0 mT was subtracted. Left axis indicates corresponding magnetization M = Φ/φM ; the dashed lines indicate the literature value of the saturation magnetization ±Ms. 
Article
We fabricated YBa$_2$Cu$_3$O$_7$ (YBCO) direct current (dc) nano superconducting quantum interference devices (nanoSQUIDs) based on grain boundary Josephson junctions by focused ion beam patterning. Characterization of electric transport and noise properties at 4.2$\,$K in magnetically shielded environment yields a very small inductance $L$ of a few pH for an optimized device geometry. This in turn results in very low values of flux noise $<50\,{\rm n}\Phi_0/{\rm Hz}^{1/2}$ in the thermal white noise limit, which yields spin sensitivities of a few $\mu_{\rm B}/{\rm Hz}^{1/2}$ ($\Phi_0$ is the magnetic flux quantum and $\mu_{\rm B}$ is the Bohr magneton). We observe frequency-dependent excess noise up to 7$\,$MHz, which can only partially be eliminated by bias reversal readout. This indicates the presence of fluctuators of unknown origin, possibly related to defect-induced spins in the SrTiO$_3$ substrate. We demonstrate the potential of using YBCO nanoSQUIDs for the investigation of small spin systems, by placing a 39$\,$nm diameter Fe nanowire, encapsulated in a carbon nanotube, on top of a non-optimized YBCO nanoSQUID and by measuring the magnetization reversal of the Fe nanowire via the change of magnetic flux coupled to the nanoSQUID. The measured flux signals upon magnetization reversal of the Fe nanowire are in very good agreement with estimated values, and the determined switching fields indicate magnetization reversal of the nanowire via curling mode.
 
Formation energy E f of NO as a function of the Fermi level under Zn-rich conditions, as obtained from different codes and supercell sizes.
Article
Despite the lack of reproducible experimental confirmation, group V elements have been considered as possible sources of \textit{p}-type doping in ZnO in the form of simple and complex defects. Using \textit{ab initio} calculations, based on state-of-the-art hybrid exchange-correlation functional, we studied a wide range of defects and defects complexes related with N, P, As and Sb impurities. We show that none of the candidates for \textit{p}-type doping can be considered a good source of holes in the valence band due to deep acceptor levels and low formation energies of compensating donor defects. In addition, we discuss the stability of complexes in different regimes.
 
Article
High fidelity coherent control of quantum systems is critical to building quantum devices and quantum computers. We provide a general optimal control framework for designing control sequences that account for hardware control distortions while maintaining robustness to environmental noise. We demonstrate the utility of our algorithm by presenting examples of robust quantum gates optimized in the presence of nonlinear distortions. We show that nonlinear classical controllers do not necessarily incur additional computational cost to pulse optimization, enabling more powerful quantum devices.
 
Article
We describe the coherent manipulation of harmonic oscillator and qubit modes using resonant trains of single flux quantum pulses in place of microwaves. We show that coherent rotations are obtained for pulse-to-pulse spacing equal to the period of the oscillator. We consider a protocol for preparing bright and dark harmonic oscillator pointer states. Next we analyze rotations of a two-state qubit system. We calculate gate errors due to timing jitter of the single flux quantum pulses and due to weak anharmonicity of the qubit. We show that gate fidelities in excess of 99.9% are achievable for sequence lengths of order 20 ns.
 
Article
Ultrasound-driven oscillating micro-bubbles have been used as active actuators in microfluidic devices to perform manifold tasks such as mixing, sorting and manipulation of microparticles. A common configuration consists on side-bubbles, created by trapping air pockets in blind channels perpendicular to the main channel direction. This configuration consists of acoustically excited bubbles with a semi-cylindrical shape that generate significant streaming flow. Due to the geometry of the channels, such flows have been generally considered as quasi two-dimensional. Similar assumptions are often made in many other microfluidic systems based on \emph{flat} micro-channels. However, in this paper we show that microparticle trajectories actually present a much richer behavior, with particularly strong out-of-plane dynamics in regions close to the microbubble interface. Using Astigmatism Particle Tracking Velocimetry, we reveal that the apparent planar streamlines are actually projections of a \emph{streamsurface} with a pseudo-toroidal shape. We therefore show that acoustic streaming cannot generally be assumed as a two-dimensional phenomenon in confined systems. The results have crucial consequences for most of the applications involving acoustic streaming as particle trapping, sorting and mixing.
 
Array of suspended (a) and deposited (b) parallel NWs in the FET configuration. The NWs are drawn in green, the two metallic electrodes in yellow, the substrate in gray, and the back gate in dark gray. The blue (red) spot in (b) indicates the substrate region that is cooled down (heated up) in the phonon-assisted activated regime, when a charge current flows from the left to the right electrode and the gate voltage is tuned so as to probe the lower edge of the NW impurity band.
Article
We study the thermoelectric effects in arrays of disordered nanowires in parallel, at temperatures where charge transport between localized states is thermally assisted by phonons. We obtain large power factors and electrical figures of merit, when the chemical potential probes the band edges of the nanowires, the large thermopowers self-averaging while the small electrical conductances add. The role of the parasitic phonon heat transport is estimated. We also show that phonon absorption and emission occur at opposite ends of the array in band-edge transport, a phenomenon which could be exploited for cooling hot spots in electronic circuits.
 
(Color online) (a) Out-of-plane ( 2 θ - ω scan ) XRD patterns for polycrystalline CFA full- Heusler alloy films on MgO-buffered Si/SiO 2 amorphous substrates with varied MgO buffer 
(Color online) TMR ratios as a function of MgO buffer thickness t MgO for polycrystalline CFA/MgO/CoFe MTJs with different thicknesses of the MgO barrier, t barr : 1.5 nm, 1.8 nm, and 2.0 nm. The 30-nm-thick CFA bottom electrodes were directly deposited on the MgO buffer layer and
(Color online) RA dependence of TMR ratios for polycrystalline CFA-MTJs with a 2-nm- thick CFA layer ( “ thin-CFA ” ) as a bottom electrode. The squared symbol indicates the TMR ratio for a polycrystalline CFA-MTJ with a 30-nm-thick CFA layer ( “ thick-CFA ” ). 
(a) Schematic illustration of the structure of polycrystalline CFA-MTJ nanopillar. (b) R-H loops for a polycrystalline CFA-MTJ nanopillar. Wide arrows show the magnetic configurations of bottom (free) and top (reference) electrodes of the MTJ, and narrow arrows indicate sweep direction of the applied magnetic field. (c) R-I loops for the MTJ at magnetic fields of 0,-11 and-20 Oe, respectively. Arrows indicate sweep direction of the applied current. (d) Representative R-I loops of the CFA-MTJ at applied magnetic field of-11 Oe. (e) and (f) Switching probabilities for I c,P->AP and
Figures and captions:
Article
We studied polycrystalline B2-type Co2FeAl (CFA) full-Heusler alloy based magnetic tunnel junctions (MTJs) fabricated on a Si/SiO2 amorphous substrate. Polycrystalline CFA films with a (001) orientation, a high B2 ordering, and a flat surface were achieved using a MgO buffer layer. A tunnel magnetoresistance (TMR) ratio up to 175% was obtained for an MTJ with a CFA/MgO/CoFe structure on a 7.5-nm-thick MgO buffer. Spin-transfer torque induced magnetization switching was achieved in the MTJs with a 2-nm-thick polycrystalline CFA film as a switching layer. Using a thermal activation model, the intrinsic critical current density (Jc0) was determined to be 8.2 x 10^6 A/cm^2, which is lower than 2.9 x 10^7 A/cm^2, the value for epitaxial CFA-MTJs [Appl. Phys. Lett. 100, 182403 (2012)]. We found that the Gilbert damping constant evaluated using ferromagnetic resonance measurements for the polycrystalline CFA film was ~0.015 and was almost independent of the CFA thickness (2~18 nm). The low Jc0 for the polycrystalline MTJ was mainly attributed to the low damping of the CFA layer compared with the value in the epitaxial one (~0.04).
 
Article
At strong pump powers, a semiconductor optical cavity passes through a Hopf bifurcation and undergoes self-oscillation. We simulate this device using semiclassical Langevin equations and assess the effect of quantum fluctuations on the dynamics. Below threshold, the cavity acts as a phase-insensitive linear amplifier, with noise $\sim 5\times$ larger than the Caves bound. Above threshold, the limit cycle acts as an analog memory, and the phase diffusion is $\sim 10\times$ larger than the bound set by the standard quantum limit. We also simulate entrainment of this oscillator and propose an optical Ising machine and classical CNOT gate based on the effect.
 
Article
We demonstrate fast readout of a double quantum dot (DQD) that is coupled to a superconducting resonator. Utilizing parametric amplification in a nonlinear operational mode, we improve the signal-to-noise ratio (SNR) by a factor of 2000 compared to the situation with the parametric amplifier turned off. With an integration time of 400 ns we achieve a SNR of 76. By studying SNR as a function of the integration time we extract an equivalent charge sensitivity of 8 x 10^{-5} e/root(Hz). The high SNR allows us to acquire a DQD charge stability diagram in just 20 ms. At such a high data rate, it is possible to acquire charge stability diagrams in a live "video-mode," enabling real time tuning of the DQD confinement potential.
 
Article
We study microfluidic self digitization in Hele-Shaw cells using pancake droplets anchored to surface tension traps. We show that above a critical flow rate, large anchored droplets break up to form two daughter droplets, one of which remains in the anchor. Below the critical flow velocity for breakup the shape of the anchored drop is given by an elastica equation that depends on the capillary number of the outer fluid. As the velocity crosses the critical value, the equation stops admitting a solution that satisfies the boundary conditions; the drop breaks up in spite of the neck still having finite width. A similar breaking event also takes place between the holes of an array of anchors, which we use to produce a 2D array of stationary drops in situ.
 
Article
We show that uniaxial color centers in silicon carbide with hexagonal lattice structure can be used to measure not only the strength but also the polar angle of the external magnetic field with respect to the defect axis with high precision. The method is based on the optical detection of multiple spin resonances in the silicon vacancy defect with quadruplet ground state. We achieve a perfect agreement between the experimental and calculated spin resonance spectra without any fitting parameters, providing angle resolution of a few degrees in the magnetic field range up to several millitesla. Our approach is suitable for ensembles as well as for single spin-3/2 color centers, allowing for vector magnetometry on the nanoscale at ambient conditions.
 
Article
Spin-orbit coupling in ferromagnets gives rise to the anomalous Hall effect and the anisotropic magnetoresistance, both of which can be used to create spin-transfer torques in a similar manner as the spin Hall effect. In this paper we show how these effects can be used to reliably switch perpendicularly magnetized layers and to move domain walls. A drift-diffusion treatment of the anomalous Hall effect and the anisotropic magnetoresistance describes the spin currents that flow in directions perpendicular to the electric field. In systems with two ferromagnetic layers separated by a spacer layer, an in-plane electric field cause spin currents to be injected from one layer into the other, creating spin transfer torques. Unlike the related spin Hall effect in non-magnetic materials, the anomalous Hall effect and the anisotropic magnetoresistance allow control of the orientation of the injected spins, and hence torques, by changing the direction of the magnetization in the injecting layer. The torques on one layer show a rich angular dependence as a function of the orientation of the magnetization in the other layer. The control of the torques afforded by changing the orientation of the magnetization in a fixed layer makes it possible to reliably switch a perpendicularly magnetized free layer. Our calculated critical current densities for a representative CoFe/Cu/FePt structure show that the switching can be efficient for appropriate material choices. Similarly, control of the magnetization direction can drive domain wall motion, as shown for NiFe/Cu/NiFe structures.
 
Article
Using first-principles calculations, we propose a microscopic model to explain the reversible lithiation/delithiation of tin-oxide anodes in lithium-ion batteries. When the irreversible regime ends, the anode grains consist of layers of Li-oxide separated by Sn bilayers. During the following reversible lithiation, the Li-oxide undergoes two phase transformations that give rise to a Li-enrichment of the oxide and the formation of a SnLi composite. The anode grain structure stays layered and ordered with an effective theoretical reversible capacity of 4.5 Li per Sn atom. The predicted anode volume expansion and voltage profile agree well with experiments, contrary to existing models.
 
Inverse logarithmic slope of the measured transfer curves from Fig. 2. In the main panel, linear and saturation regimes manifest themselves in the known way: I d ∝ Vg and I d ∝ V 2 g , respectively. A magnification of the subthreshold region is shown in the inset. With increasing gate voltage, the inverse slope is approaching a subthreshold swing value of 65 mV/decade, very close to the theoretical limit of 58.5 mV/decade.
Article
Crystalline organic semiconductors, bonded by weak van der Waals forces, exhibit macroscopic properties that are very similar to those of inorganic semiconductors. While there are many open questions concerning the microscopic nature of charge transport, minimizing the density of trap states (trap DOS) is crucial to elucidate the intrinsic transport mechanism. We explore the limits of state-of-the-art organic crystals by measuring single crystalline rubrene field-effect transistors that show textbook like transfer characteristics, indicating a very low trap DOS. Particularly, the high purity of the crystals and the very clean interface to the gate dielectric are reflected in an unprecedentedly low subthreshold swing of $65$ ${\rm mV / decade}$, remarkably close to the fundamental limit of $58.5\,{\rm mV / decade}$. From the measured subthreshold behavior we have consistently quantified the trap DOS by two different methods, yielding an exceedingly low trap density of $D_{bulk} = 1 \times 10^{13}~{\rm cm^{-3}eV^{-1}}$ at an energy of $\sim0.62~{\rm eV}$. These numbers correspond to one trap per eV in $10^8$ rubrene molecules. The equivalent density of traps located at the interface is $D_{it} = 3 \times 10^{9}~{\rm cm^{-2}eV^{-1}}$ which puts them on par with the best crystalline ${\rm SiO_2/Si}$ field-effect transistors.
 
Article
Solid-state qubits have recently advanced to the level that enables them, in-principle, to be scaled-up into fault-tolerant quantum computers. As these physical qubits continue to advance, meeting the challenge of realising a quantum machine will also require the engineering of new classical hardware and control architectures with complexity far beyond the systems used in today's few-qubit experiments. Here, we report a micro-architecture for controlling and reading out qubits during the execution of a quantum algorithm such as an error correcting code. We demonstrate the basic principles of this architecture in a configuration that distributes components of the control system across different temperature stages of a dilution refrigerator, as determined by the available cooling power. The combined setup includes a cryogenic field-programmable gate array (FPGA) controlling a switching matrix at 20 millikelvin which, in turn, manipulates a semiconductor qubit.
 
Article
In this work, an effective quantum model based on the non-equilibrium Green's function formalism is used to investigate a selectively contacted high density quantum dot array in an wide band gap host matrix for operation as a quantum dot-enhanced single junction solar cell. By establishing a direct relation between nanostructure configuration and optoelectronic properties, the investigation reveals the influence of inter-dot and dot-contact coupling strength on the radiative rates and consequently on the ultimate performance of photovoltaic devices with finite quantum dot arrays as the active medium. The dominant effects originate in the dependence of the Joint Density of States on the inter-dot coupling in terms of band width and effective band gap.
 
Article
We report on experiments with a microfabricated surface trap designed for trapping a chain of ions in a ring. Uniform ion separation over most of the ring is achieved with a rotationally symmetric design and by measuring and suppressing undesired electric fields. After minimizing these fields the ions are confined primarily by an rf trapping pseudo-potential and their mutual Coulomb repulsion. The ring-shaped crystal consists of approximately 400 Ca$^+$ ions with an estimated average separation of 9 $\mu m$.
 
Article
Switching of the direction of the magnetic moment in a nanomagnet is studied within a modified Slonczewski's model that permits torsional oscillations of the magnet. We show that the latter may inhibit or assist the magnetization switching, depending on parameters. Three regimes have been studied: the switching by torsional oscillations alone, the switching by the spin-polarized current with torsional oscillations permitted, and the magnetization switching by the current combined with the mechanical twist. We show that switching of the magnetic moment is possible in all three cases and that allowing torsional oscillations of the magnet may have certain advantages for applications. Phase diagrams are computed that show the range of parameters required for the switching.
 
Article
Mass spectrometry is used in a wide range of scientific disciplines including proteomics, pharmaceutics, forensics, and fundamental physics and chemistry. Given this ubiquity, there is a worldwide effort to improve the efficiency and resolution of mass spectrometers. However, the performance of all techniques is ultimately limited by the initial phase-space distribution of the molecules being analyzed. Here, we dramatically reduce the width of this initial phase-space distribution by sympathetically cooling the input molecules with laser-cooled, co-trapped atomic ions, improving both the mass resolution and detection efficiency of a time-of-flight mass spectrometer by over an order of magnitude. Detailed molecular dynamics simulations verify the technique and aid with evaluating its effectiveness. Our technique appears to be applicable to other types of mass spectrometers.
 
Article
We demonstrate a dual-axis accelerometer and gyroscope atom interferometer, which forms the building blocks of a six-axis inertial measurement unit. By recapturing the atoms after the interferometer sequence, we maintain a large atom number at high data-rates of 50 to 100 measurements per second. Two cold ensembles are formed in trap zones located a few centimeters apart, and are launched toward one-another. During their ballistic trajectory, they are interrogated with a stimulated Raman sequence, detected, and recaptured in the opposing trap zone. We achieve sensitivities at $\mathrm{\mu \mathit{g} / \sqrt{Hz}}$ and $\mathrm{\mu rad / s / \sqrt{Hz}}$ levels, making this a compelling prospect for expanding the use of atom interferometer inertial sensors beyond benign laboratory environments.
 
Article
We report on transport in the 2$^{\text{nd}}$ Landau level in in-situ back-gated two-dimensional electron gases in GaAs/Al$_x$Ga$_{1-x}$As quantum wells. Minimization of gate leakage is the primary heterostructure design consideration. Leakage currents resulting in dissipation as small as a few pW can cause noticeable heating of the electrons at 10 mK, limiting the formation of novel correlated states. We show that when the heterostructure design is properly optimized, gate voltages as large as 4V can be applied with negligible gate leakage, allowing the density to be tuned over a large range from depletion to over 4 $\times$ 10$^{11}$ cm$^{-2}$. As a result, the strength of the $\nu = 5/2$ state can be continuously tuned from onset at n $\sim 1.2 \times 10^{11}$ cm$^{-2}$ to a maximum $\Delta_{5/2} = 625$ mK at n = $3.35 \times 10^{11}$ cm$^{-2}$. An unusual evolution of the reentrant integer quantum Hall states as a function of density is also reported. These devices can be expected to be useful in experiments aimed at proving the existence of non-Abelian phases useful for topological quantum computation.
 
Article
The interaction of shear bands with crystalline nanoprecipitates in Cu-Zr-based metallic glasses is investigated by a combination of high-resolution TEM imaging and molecular dynamics computer simulations. Our results reveal different interaction mechanisms: Shear bands can dissolve precipitates, can wrap around crystalline obstacles or can be blocked depending on size and density of the precipitates. If the crystalline phase has a low yield strength, we also observe slip transfer through the precipitate. Based on the computational results and experimental findings a qualitative mechanism map is proposed that categorizes the various processes as a function of the critical stress for dislocation nucleation, precipitate size and distance.
 
Article
Employing first-principles calculations, we investigate efficiency of spin injection from a ferromagnetic (FM) electrode (Ni) into graphene and possible enhancement by using a barrier between the electrode and graphene. Three types of barriers, h-BN, Cu(111), and graphite, of various thickness (0-3 layers) are considered and the electrically biased conductance of the Ni/Barrier/Graphene junction are calculated. It is found that the minority spin transport channel of graphene can be strongly suppressed by the insulating h-BN barrier, resulting in a high spin injection efficiency. On the other hand, the calculated spin injection efficiencies of Ni/Cu/Graphene and Ni/Graphite/Graphene junctions are low, due to the spin conductance mismatch. Further examination on the electronic structure of the system reveals that the high spin injection efficiency in the presence of a tunnel barrier is due to its asymmetric effects on the two spin states of graphene.
 
Depiction of the effects of spin-orbit coupling in generating topologically insulating, potential high-performance thermoelectrics by means of opening of a gap in the electronic structure, with associated nonparabolicity. A material doped p type is depicted.
Calculated ranges of chemical potentials of the elements involved in Bi2Te2Se and related competing phases. The range of thermodynamical stability of Bi2Te2Se is defined by the trapezoid ABCD.
Article
Thermoelectric performance is of interest for numerous applications such as waste heat recovery and solid state energy conversion, and will be seen to be closely connected to topological insulator behavior. In this context we here report first principles transport and defect calculations for Bi$_{2}$Te$_{2}$Se in relation to Bi$_{2}$Te$_{3}$. The two compounds are found to contain remarkably different electronic structures in spite of being isostructural and isoelectronic. We discuss these results in terms of the topological insulator characteristics of these compounds.
 
Tight-binding model of a ferromagnet–insulator–topological-insulator (F-I-TI) junction with the ferromagnet on the [001] surface of the topological insulator, corresponding to setup A and B in Fig. 2.
Setups for TMR devices studied in this work. Different orientations of the crystal axes of the TI relative to the electrodes are given by the coordinate systems A–H, (a) for a three-dimensional TI and (b) for a two-dimensional TI. In (c) the crystallographic directions of the TI are shown.
Article
We theoretically investigate tunneling magnetoresistance (TMR) devices, which are probing the spin-momentum coupled nature of surface states of the three-dimensional topological insulator Bi2Se3. Theoretical calculations are performed based on a realistic tight-binding model for Bi2Se3. We study both three dimensional devices, which exploit the surface states of Bi2Se3, as well as two-dimensional devices, which exploit the edge states of thin Bi2Se3 strips. We demonstrate that the material properties of Bi2Se3 allow a TMR ratio at room temperature of the order of 1000%. Analytical formulas are derived that allow a quick estimate of the achievable TMR ratio in these devices. The devices can be used to measure the spin polarization of the topological surface states as an alternative to spin-ARPES. Unlike TMR devices based on magnetic tunnel junctions the present devices avoid the use of a second ferromagnetic electrode whose magnetization needs to be pinned.
 
Article
For two electrically small nonreciprocal scatterers an analytical electromagnetic model of polarizabilities is developed. Both particles are bianisotropic: the so-called Tellegen-omega particle and moving-chiral particle. Analytical results are compared to the full-wave numerical simulations. Both models satisfy to main physical restrictions and leave no doubts in the possibility to realize these particles experimentally. This paper is a necessary step towards applications of nonreciprocal bianisotropic particles such as perfect electromagnetic isolators, twist polarizers, thin-sheet phase shifters, and other devices.
 
Traditional thermocouple. A traditional thermocouple device consists of a n -type material (in green) and a p -type material (in red) connected electrically in series, but parallel with respect to the temperature gradient. The charge carriers move from the hot end to the cold end (black arrows), generating an electrical current (white dashed arrows). 
Bilayer exciton thermocouple. In a bilayer exciton thermocouple, as pre- sented in this paper, the p - and n -type materials are brought close together so that bilayer excitons can form. In the thermoelectric device, the excitons move from the hot side to the cold side (orange arrows) generating an electric counterflow (white dashed arrows). Due to the bosonic nature of the excitons, the exciton thermocouple can have a higher figure of merit than the traditional thermocouple of Fig. 1. 
Article
Currently, one of the major nanotechnological challenges is to design thermoelectric devices that have a high figure of merit. To that end, we propose to use bilayer excitons. Bilayer exciton systems are shown to have an improved thermopower and an enhanced electric counterflow and thermal conductivity, with respect to regular semiconductor-based thermoelectrics. Here we present a roadmap towards experimental realization of a bilayer exciton thermocouple. A bilayer exciton heterostructures of $p$- and $n$-doped Bi$_2$Te$_3$ can have a figure of merit $zT \sim 60$. Another material suggestion is to make a bilayer out of electron-doped SrTiO$_3$ and hole-doped Ca$_3$Co$_4$O$_9$.
 
nfluence of the pressure load on the Raman scattering intensity. Maps of (a) the integrated intensity of the G-mode feature IG and of (b) I2D/IG, the ratio of the integrated intensities of the 2D- and G-mode features recorded on the sample shown in Fig. 1, under a uniform pressure load of Δp=74  kPa. The step size is 250 nm. The border of the pit is represented by gray dashed circles. (c) High-resolution radial line scans of IG (black squares) and I2D (red circles), recorded across the pit at Δp=0  kPa (filled symbols) and Δp=74  kPa (open symbols), with a step size of 100 nm.
Article
We report a comprehensive micro-Raman study of a pressurized suspended graphene membrane that hermetically seals a circular pit, etched in a Si/SiO$_2$ substrate. Placing the sample under a uniform pressure load results in bulging of the graphene membrane and subsequent softening of the main Raman features, due to tensile strain. In such a microcavity, the intensity of the Raman features depends very sensitively on the distance between the graphene membrane and the Si substrate, which acts as the bottom mirror of the cavity. Thus, a spatially resolved analysis of the intensity of the G and 2D mode features as a function of the pressure load permits a direct reconstruction of the blister profile. An average strain is then deduced at each pressure. This allows a determination of the Gr\"{u}neisen parameters of $1.8\pm0.2$ and $2.4\pm0.2$ for the Raman G and 2D modes, respectively. The measured blister height is proportional to the cubic root of the pressure load, as predicted theoretically. The validation of this scaling provides a direct and accurate determination the Young's modulus of graphene with a purely optical, hence minimally invasive and contactless approach. We find a Young's modulus of $\left(1.05\pm 0.10\right) \rm TPa$ for monolayer graphene, in perfect match with previous nano-indentation measurements. This all optical approach opens avenues for pressure sensing using graphene and could readily be adapted to other emerging two dimensional membranes.
 
Schematic representation of tunneling processes occurring in the G/h-BN/G heterostructure. Blue and green regions mark occupied and empty states, respectively. The arrows indicate a tunneling event from the T layer (initial point) to the B layer (end point). These points are shifted in energy by eV. Energy and momentum conservation laws define permitted initial curves in k space which are plotted in yellow. (a) At the minimum voltage for tunneling, only a point in k space can tunnel. (b) By increasing the voltage, the conservation curve is a hyperbola that resides in the conduction band. (c) At a critical voltage, the hyperbola collapses in a straight line, and a peak in the joint density of states occurs. (d) At larger voltages, the straight line becomes an ellipse, now residing in the valence band.
(Color online)Non-linear current for a graphene-hBNgraphene heterostructure. The density of electrons in both layers is n=5 × 10 12 cm −2. In panel a) we fix the top layer twist angle to θT =0.5 o and plot the current for different rotation angles of the bottom layer θB. Both angles are measured with respect the central hBN layer. In the inset we show the conduction to conduction (α=+ to β=+) contribution to the current. In panel b) both graphene layers are rotated the same angle θT =θB=3 o. The peak in the I(V ) indicates that coherent tunneling and negative differential conductivity can occur even when both graphene layers are fully aligned, provided there is a twisting with the hBN layer. In the calculation we use the band structure parameters[14] ∆1=3.33eV, ∆2=1.49eV, tCB=0.432eV and tCN =0.29eV and a value /τ =2.5meV.
(Color online)I(V ) curve or a G-BN-G structure with twisting angles θT =-0.5 o and θB=3 o , in presence of an inplane magnetic field corresponding to dd −2 a=5, being a the graphene lattice parameter. The density of carriers in both layers is n=5×10 1 2cm −2. The current for B =0 is shown as a black dashed line. Vertical lines indicate the critical voltages corresponding to the different magnetic field modified tunneling transfer wavevectors G i =j .
Article
We address the tunneling current in a graphene-hBN-graphene heterostructure as function of the twisting between the crystals. The twisting induces a modulation of the hopping amplitude between the graphene layers, that provides the extra momentum necessary to satisfy momentum and energy conservation and to activate coherent tunneling between the graphene electrodes. Conservation rules limit the tunneling to states with wavevectors lying at the conic curves defined by the intersection of two Dirac cones shifted in momentum and energy. There is a critical voltage where the intersection is a straight line, and the joint density of states presents a maximum. This reflects in a peak in the tunneling current and in a negative differential conductivity.
 
Article
We propose a class of linear elastic three-dimensional metamaterials for which the e?ective parameters bulk modulus and mass density can be adjusted independently over a large range|which is not possible for ordinary materials. First, we systematically evaluate the static mechanical properties and the phonon dispersion relations. We show that the two are quantitatively consistent in the long-wavelength limit. To demonstrate the feasibility, corresponding fabricated polymer microstructures are presented. Finally, we discuss calculations for laminates composed of alternating layers of two di?erent metamaterials with equal bulk modulus yet di?erent mass density. This leads to metamaterials with e?ectively anisotropic uniaxial dynamic mass density tensors.
 
Time-resolved thermometry. (a) Amplitudemodulated sinusoid used to drive the heating pulse (the frequency is not to scale) and (b) real-time response of the thermometer, obtained by recording the transmitted power P versus time for different values of the heating-pulse amplitude I pp H. The conversion from P into absolute electronic temperature Te is displayed on the right axis. Inset: Te at the end of the heating pulse (t = 520 µs) versus I pp H (triangles). The prediction of the thermal model [21] is shown for comparison (solid line). All the traces are taken at base temperature by averaging over 10 4 heating cycles and the voltage bias is V b = 0.17 mV. 
Thermal relaxation times. (a) Thermal relaxation time τ versus voltage bias V b for two different values of the bath temperature T bath. (b) Temperature dependence of τ , as estimated from relaxation after the falling edge (circles, the x axis is T bath ) as well as the rising edge of the pulse (triangles, the x axis is the temperature Te,0 at the end of the pulse). The error bars are obtained from the fits (see [21]). 
Thermal relaxation traces (circles, squares, triangles). The traces are shifted by their baseline after relaxation, scaled and plotted on a logarithmic scale. They are also horizontally offset by 150 µs for clarity. The full lines are exponential fits of the form A exp(t/τ ) + B to the data. The data in panels (a,b) correspond to the rising (a) and falling edges (b) of selected traces in Fig. 2 of the main text. Panels (c,d) present similar traces obtained at different bath temperatures T bath (c) and for different values of the voltage bias V b (d). All the traces are obtained by averaging over 2 · 10 5 heating cycles. 
Detail of a 10 ms long time trace taken under the same conditions as in Fig. 3 of the main text (dots). The full line is a fit of a double exponential A1 exp(−t/τ1) + A2 exp(−t/τ2) to the data. 
Measured (left) and simulated (right) transmittance of power |S21| 2 = P det /Pgen as a function of Pin and V b . 
Article
We demonstrate radiofrequency thermometry on a micrometer-sized metallic island below 100 mK. Our device is based on a normal metal-insulator-superconductor tunnel junction coupled to a resonator with transmission readout. In the first generation of the device, we achieve 100 {\mu}K/Hz^1/2 noise-equivalent temperature, limited by the first amplifier, with 10 MHz bandwidth. We measure the thermal relaxation time of the electron gas in the island, which we find to be of the order of 100 {\mu}s. Such a calorimetric detector, upon optimization, can be seamlessly integrated into superconducting circuits, with immediate applications in quantum-thermodynamics experiments down to single quanta of energy.
 
Article
Layered LiMnO2 and Li2MnO3 are of great interest for lithium-ion battery cathodes because of their high theoretical capacities. The practical application of these materials is, however, limited due to poor electrochemical performance. We herein report a comprehensive first-principles study of defect physics in LiMnO2 and Li2MnO3 using hybrid-density functional calculations. We find that manganese antisites have low formation energies in LiMnO2 and may act as nucleation sites for the formation of impurity phases. The antisites can also occur with high concentrations in Li2MnO3; however, unlike in LiMnO2, they can be eliminated by tuning the experimental conditions during preparation. Other intrinsic point defects may also occur and have an impact on the materials' properties and functioning. An analysis of the formation of lithium vacancies indicates that lithium extraction from LiMnO2 is associated with oxidation at the manganese site, resulting in the formation of manganese small hole polarons; whereas in Li2MnO3 the intrinsic delithiation mechanism involves oxidation at the oxygen site, leading to the formation of bound oxygen hole polarons η+O. The layered oxides are found to have no or negligible bandlike carriers and they cannot be doped n- or p-type. The electronic conduction proceeds through hopping of hole and/or electron polarons; the ionic conduction occurs through lithium monovacancy and/or divacancy migration mechanisms. Since η+O is not stable in the absence of negatively charged lithium vacancies in bulk Li2MnO3, the electronic conduction near the start of delithiation is likely to be poor. We suggest that the electronic conduction associated with η+O and, hence, the electrochemical performance of Li2MnO3 can be improved through nanostructuring and/or ion substitution.
 
(Color online) Schematic of a hybrid device for coupling a doubly clamped diamond microbeam with a single, well-controlled embedded NV center spin to the CPW cavity field. (a) Top view of the configuration of the diamond beam positioned near z 0 ∼ L above the gap of the CPW cavity. 
a) Time evolution of the mean phonon number from numerically solving the master equation (6) under realistic parameters. (b) The final phonon number in a logarithmic scale as a function of g/κ. The parameters are chosen as those in (a).
Photon-motion coupling strength g and spin-motion coupling strength λ as a function of the length of the diamond beam. The relevant parameters are r ∼ 100 nm, E 0 ∼ 0.76 V=m, ½∂ x ~ e tr ðx;yފ ðx 0 ;y 0 Þ ∼ð2 μmÞ −1 , E p ∼10 V=μm, and ∂ x B∼10 7 T=m.
Article
We propose and analyze a hybrid device by integrating a microscale diamond beam with a single built-in nitrogen-vacancy (NV) center spin to a superconducting coplanar waveguide (CPW) cavity. We find that under an ac electric field the quantized motion of the diamond beam can strongly couple to the single cavity photons via polarization interaction. Together with the strong spin-motion interaction via a large magnetic field gradient, it provides a hybrid quantum device where the diamond resonator can strongly couple both to the single microwave cavity photons and to the single NV center spin. This enables coherent information transfer and effective coupling between the NV spin and the CPW cavity via mechanically dark polaritons. This hybrid spin-electromechanical device, with tunable couplings by external fields, offers a realistic platform for implementing quantum information with single NV spins, diamond mechanical resonators, and single microwave photons.
 
Article
Using a sub-millimetre sized YIG (Yttrium Iron Garnet) sphere mounted in a field-focusing cavity, we demonstrate ultra-strong coupling between magnon and photon modes at millikelvin temperatures with an ultra-high cooperativity of $10^5$ at microwave frequencies. The cavity is designed as a magnetic dipole using a novel patented multiple-post approach that effectively focuses the cavity magnetic field within the YIG crystal with very high filling factor. Coupling strength of 2~GHz is achieved for a bright cavity mode that constitutes about 10\% of the photon energy or 76 cavity linewidths and shows that ultra-strong coupling is possible in spin systems at microwave frequencies. With straight forward optimisations we show that this system has the potential to reach cooperativities of $10^7$, corresponding to a coupling strength of 5.2 GHz. Furthermore, a three-mode strong coupling regime is observed between a dark cavity mode and a magnon mode doublet pair, where the photon-magnon and magnon-magnon couplings are 143~MHz and 12.5~MHz respectively with bandwidths approaching 0.5~MHz.
 
Double resonance for (a) an L2 cavity with a ¼ 270 nm, (b) an L3 cavity with a ¼ 380 nm, (c) an L1 cavity with a ¼ 290 nm, and (d) an L5 cavity with a ¼ 300 nm. All data are taken at the center of the cavities using x-polarized excitation with P ¼ 1 mW. 
Article
We investigate the use of guided modes bound to defects in photonic crystals for achieving double resonances. Photoluminescence enhancement by more than three orders of magnitude has been observed when the excitation and emission wavelengths are simultaneously in resonance with the localized guided mode and cavity mode, respectively. We find that the localized guided modes are relatively insensitive to the size of the defect for one of the polarizations, allowing for flexible control over the wavelength combinations. This double resonance technique is expected to enable enhancement of photoluminescence and nonlinear wavelength conversion efficiencies in a wide variety of systems.
 
Article
Methods for the creation of thin amorphous silicon dioxide (aSiO2) layers on crystalline silicon substrates with very high densities of silicon dangling bonds (so called E' centers) have been explored and volume densities of [E']> 5x10^18 cm-3 throughout a 60nm thick film have been demonstrated by exposure of a thermal oxide layer to a low pressure Argon radio frequency plasma. While the generated high E' center densities can be annealed completely at 300C, they are comparatively stable at room temperature with a half life of about one month. Spin relaxation time measurements of these states between T = 5K and T = 70K show that the phase relaxation time T2 does not strongly depend on temperature and compared to SiO2 films of lower E' density, is significantly shortened. The longitudinal relaxation time T1 ~195(5)us at room temperature is in agreement with low-density SiO2. In contrast, T1 ~625(51)us at T = 5K is much shorter than in films of lower E' density. These results are discussed in the context of E' centers being used as probe spins for spin-selection rules based single spin-readout.
 
Schematic of the experimental setup for visualizing rain water channeling in model sandy soils. A sample cell partially filled with model sandy soils is suspended under a 2D rain source built by a linear array of glass capillaries inserted into the bottom of a plastic container with a separation of 1 cm. The sample cell can be shifted up or down to adjust the falling distance h of the rain droplets. Rain rate Q is determined by the water level in the plastic container. A gear pump is used for supplying water during the experiment.
mages (a)–(d) show deionized-water channeling in dry hydrophilic model sandy soils with bead diameters D varying from 0.18 to 1 mm, at a rain rate of Q=14.5  cm/h and a raindrop impinging speed of UT=1  m/s. The color images in the first row are the original images taken at steady states; the gray-scale images in the second row are obtained by subtracting the background images taken prior to the rain from the ones taken at steady states, converting to gray scale, and then enhancing the contrast. The sample packing in each image is 26 cm wide, 25 cm high, and 0.8 cm thick. As labeled in the images, zwet is the infiltration depth of rain water in soils, d is the water channel width, and d′ is the water channel separation. The saturated hydraulic conductivity κs [Eq. (2)] for these four samples are determined as 73  cm/h, 204  cm/h, 567  cm/h, and 2300  cm/h, respectively.
Gray-scale images (a)–(d) showing the influence of four different surface flatnesses, as labeled, on rain water channeling in a dry model sandy soil, 1-mm hydrophilic glass beads at a fixed rain condition. The rain rate is Q=14.5  cm/h, and the raindrop impinging speed is UT=1  m/s. The sample packing is 26 cm wide and 0.8 cm thick. The sample packing height varies due to the wavelike surface. More water channels may form when the wavelike surface creates an extra hydraulic pressure gradient on the wetting front.
Sequence of gray-scale images (a)–(e) at different times, as labeled, showing the influence of uniformly mixed hydrogel particle additives on the rain water channeling in a sandy soil composed of 1-mm hydrophilic glass beads; 0.1 wt% of dry hydrogel particles (0.3–0.5 mm in axis) are uniformly mixed into the top 10 cm of the dry model sandy soil before the rain. The rain rate is Q=14.5  cm/h, and the raindrop impinging speed is UT=1  m/s. The sample packing is 26 cm wide and 0.8 cm thick. The sample packing height is 25 cm (dry). The red line in each image indicates the boundary of the mixture and the pure model soil. The swelling of hydrogel particles extends the water channels.
Sequence of gray-scale images (a)–(e) at different times, as labeled, showing the influence of a layer of hydrogel particle additives on the rain water channeling in a sandy soil composed of 1-mm hydrophilic glass beads; 0.1 g of dry hydrogel particles (0.3–0.5 mm in axis) are placed in a layer under the top 10 cm of the dry model sandy soil before the rain. The rain rate is Q=14.5  cm/h, and the raindrop impinging speed is UT=1  m/s. The sample packing is 26 cm wide and 0.8 cm thick. The sample packing height is 25 cm (dry). The red line in each image indicates the location of the hydrogel particles. The swollen hydrogel particles partially clog the water channels, form a wet hydrogel layer across the sample packing, and create a fully saturated region in the soil above them.
Article
We visualize the formation of fingered flow in dry model sandy soils under different raining conditions using a quasi-2d experimental set-up, and systematically determine the impact of soil grain diameter and surface wetting property on water channelization phenomenon. The model sandy soils we use are random closely-packed glass beads with varied diameters and surface treatments. For hydrophilic sandy soils, our experiments show that rain water infiltrates into a shallow top layer of soil and creates a horizontal water wetting front that grows downward homogeneously until instabilities occur to form fingered flows. For hydrophobic sandy soils, in contrast, we observe that rain water ponds on the top of soil surface until the hydraulic pressure is strong enough to overcome the capillary repellency of soil and create narrow water channels that penetrate the soil packing. Varying the raindrop impinging speed has little influence on water channel formation. However, varying the rain rate causes significant changes in water infiltration depth, water channel width, and water channel separation. At a fixed raining condition, we combine the effects of grain diameter and surface hydrophobicity into a single parameter and determine its influence on water infiltration depth, water channel width, and water channel separation. We also demonstrate the efficiency of several soil water improvement methods that relate to rain water channelization phenomenon, including pre-wetting sandy soils at different level before rainfall, modifying soil surface flatness, and applying superabsorbent hydrogel particles as soil modifiers.
 
(Color online) Schematic of the standard imaging system and DOE collection system (not to scale). All percentages correspond to the transmittance or reflectivity at 397 nm. The product of each stage results in a total detection efficiency of a trapped fluorescing Ca + ion. 
(Color online) (a) Image of the compensated ion with corresponding DE SO and DE DOE (b) Image of an ion after ap- 
(Color online) (a) Image of a compensated ion (b) Image of a shifted ion due to applying 10 V RF to the center DC indicated by the star in Figure 1. (c) Ion after applying both an RF shift 
Article
One of the outstanding challenges for ion trap quantum information processing is to accurately detect the states of many ions in a scalable fashion. In the particular case of surface traps, geometric constraints make imaging perpendicular to the surface appealing for light collection at multiple locations with minimal cross-talk. In this report we describe an experiment integrating Diffractive Optic Elements (DOE's) with surface electrode traps, connected through in-vacuum multi-mode fibers. The square DOE's reported here were all designed with solid angle collection efficiencies of 3.58%; with all losses included a detection efficiency of 0.388% (1.02% excluding the PMT loss) was measured with a single Ca+ ion. The presence of the DOE had minimal effect on the stability of the ion, both in temporal variation of stray electric fields and in motional heating rates.
 
(a) Three-dimensional simulation of field distribution on our plasmonic DC structure. (b) Field distribution in single mode plasmonic waveguide with lateral size of 600nm × 600nm. (c) Field distribution of symmetric eigenmode in coupling section. (d) Field distribution of anti-symmetric eigenmode in coupling section. 
Article
Quantum photonic integrated circuits (QPICs) based on dielectric waveguides have been widely used in linear optical quantum computation. Recently, surface plasmons have been introduced to this application because they can confine and manipulate light beyond the diffraction limit. In this study, the on-chip quantum interference of two single surface plasmons was achieved using dielectric-loaded surface-plasmon-polariton waveguides. The high visibility (greater than 90%) proves the bosonic nature of single plasmons and emphasizes the feasibility of achieving basic quantum logic gates for linear optical quantum computation. The effect of intrinsic losses in plasmonic waveguides with regard to quantum information processing is also discussed. Although the influence of this effect was negligible in the current experiment, our studies reveal that such losses can dramatically reduce quantum interference visibility in certain cases; thus, quantum coherence must be carefully considered when designing QPIC devices.
 
(a) Dark-field microscope image of the quantum circuit on the Silicon-On-Insulator chip consisting of an integrated pump splitting circuit, ring resonator (Q~15k, FSR~5nm) entangled bi-photon source and Mach-Zehnder analysis circuit. (b) Schematic of the complete experimental setup. A pair of tunable lasers, Pump 1 (Blue Detuned-New Focus 6328) and Pump 2 (Red Detuned-Agilent 81642A) with 1mW of power, are polarized, combined and passed through pump clean-up filters (Clean-up Filter inset: ~128 dB rejection). A short section of High Index fiber (Nufern UHNA-7) was fusion spliced to optical fiber (SMF-28) to efficiently mode match to the Silicon waveguide achieving a total insertion loss of ~5dB (we have demonstrated <1.25dB fiber-chip coupling loss using this method before [10]). The relative phase () in the Mach-Zehnder was controlled by heating one of the spirals (by varying the current through shorted needle probes in contact with the spiral). Photons from the two outputs passed through bandpass pump rejection filters (Bandpass Filters inset: ~140 dB). The rejected pumps are sent to optical power meters to measure the classical signatures. The bi-photons are detected by free-running InGaAs Avalanche Photodiodes (idQuantique ID210-10% Efficiency, ID230-25% Efficiency) and correlated using a time-to-digital converter (Picoharp 300). (c) Transmission spectrum of the ring resonator (as measured from one of the outputs of the Mach-Zehnder interferometer when the relative phase is tuned for ~50/50 splitting) indicating the wavelengths of the two pump lasers [Pump 1 (blue) and Pump 2 (red)] as well as the resonant wavelength of the resulting bi-photons ("green").
a)–(f) Coincidences for different pump-laser wavelengths. (g),(h) Dependence of the coincidences on the wavelengths of the two pump lasers. The scales are different because the New Focus laser (blue scale with range −0.1 to 0.1 nm) has a very limited fine-tuning range.
Article
Here we demonstrate quantum interference of photons on a Silicon chip produced from a single ring resonator photon source. The source is seamlessly integrated with a Mach-Zehnder interferometer, which path entangles degenerate bi-photons produced via spontaneous four wave mixing in the Silicon ring resonator. The resulting bi-photon N00N state is controlled by varying the relative phase of the integrated Mach-Zehnder interferometer, resulting in high two-photon interference visibilities of V~96%. Furthermore, we show that the interference can be produced using pump wavelengths tuned to all of the ring resonances accessible with our tunable lasers (C+L band). This work is a key demonstration towards the simplified integration of multiple photon sources and quantum circuits together on a monolithic chip, in turn, enabling quantum information chips with much greater complexity and functionality.
 
Article
We analyze the design of a potential replacement technology for the commercial ferrite circulators that are ubiquitous in contemporary quantum superconducting microwave experiments. The lossless, lumped element design is capable of being integrated on chip with other superconducting microwave devices, thus circumventing the many performance-limiting aspects of ferrite circulators. The design is based on the dynamic modulation of DC superconducting microwave quantum interference devices (SQUIDs) that function as nearly linear, tunable inductors. The connection to familiar ferrite-based circulators is a simple frame boost in the internal dynamics' equation of motion. In addition to the general, schematic analysis, we also give an overview of many considerations necessary to achieve a practical design with a tunable center frequency in the 4-8 GHz frequency band, a bandwidth of 240 MHz, reflections at the -20 dB level, and a maximum signal power of approximately order 100 microwave photons per inverse bandwidth.
 
(a) Schematic of the generation of intensity correlated signal and idler beams via four wave mixing in the on-chip microring optical parametric oscillator (OPO). (b) Top: Optical spectrum analyzer scan of the light coupled out of the chip, at a pump power just above the parametric oscillation threshold. The Si3N4 ring is pumped at 1549.6 nm, generating signal and idler modes at 1540.2 nm and 1559.2 nm, respectively, which are 15 cavity modes away from the pump wavelength, as can be seen in the bottom figure. Bottom: Transmission spectrum of the ring for transverse electric (TE) polarization.
Experimental setup. EDFA: Erbium doped fibre amplifier. BPF: Bandpass filter. 
Article
We present the first demonstration of all-optical squeezing in an on-chip monolithically integrated CMOS-compatible platform. Our device consists of a low loss silicon nitride microring optical parametric oscillator (OPO) with a gigahertz cavity linewidth. We measure 1.7 dB (5 dB corrected for losses) of sub-shot noise quantum correlations between bright twin beams generated in the microring four-wave-mixing OPO pumped above threshold. This experiment demonstrates a compact, robust, and scalable platform for quantum optics and quantum information experiments on-chip.
 
Schematics of the whole circuit designed and fabricated on 300 mm SOI wafers. The nanowire dominated by Coulomb blockade below 10 K is on the right. The CMOS circuit driving the gates is made of several sub-circuits. A voltage controlled oscillator (VCO), monitored through a frequency divider, feeds a clock generator also based on ring- oscillators. This generator delivers two delayed and phase- shifted RF signals, RF 1 and RF 2, which are further attenuated by capacitive dividers and added to external DC biases. The voltage supply V DD referenced to ground GN D is not 
a) View of the complete circuit layout with 12 contact pads, the bias resistors in red and capacitors as empty cyan squares between pads 2-3 and 4-5. b) Detailed view of the CMOS circuits located between pads 3 and 4. The top part is the VCO (≈ 130 FETs) and non-overlapping clock generator (≈ 100 FETs), while the bottom part is the frequency divider (≈ 400 FETs). Both are made with 1µm wide channels and 60 nm long gates as depicted in c). c) FET design with the gate level in red, the source and drain in purple and active area in green. d) Detailed view of the SET device (25 nm wide channel and 40 nm long gates).
Output frequency of the voltage-controlled oscillator measured at 300, 77 and 4.2 K after correction by the frequency divider (division by a factor of 65536). The maximum output frequency are 1.36 GHz at 300 K, 1.08 GHz at 77 K and 1.06 GHz at 4.2 K.
Article
Silicon-On-Insulator nanowire transistors of very small dimensions exhibit quantum effects like Coulomb blockade or single-dopant transport at low temperature. The same process also yields excellent field-effect transistors (FETs) for larger dimensions, allowing to design integrated circuits. Using the same process, we have co-integrated a FET-based ring oscillator circuit operating at cryogenic temperature which generates a radio-frequency (RF) signal on the gate of a nanoscale device showing Coulomb oscillations. We observe rectification of the RF signal, in good agreement with modeling.
 
Article
Strongly correlated electron systems such as the rare-earth nickelates (RNiO3, R = rare-earth element) can exhibit synapse-like continuous long term potentiation and depression when gated with ionic liquids; exploiting the extreme sensitivity of coupled charge, spin, orbital, and lattice degrees of freedom to stoichiometry. We present experimental real-time, device-level classical conditioning and unlearning using nickelate-based synaptic devices in an electronic circuit compatible with both excitatory and inhibitory neurons. We establish a physical model for the device behavior based on electric-field driven coupled ionic-electronic diffusion that can be utilized for design of more complex systems. We use the model to simulate a variety of associate and non-associative learning mechanisms, as well as a feedforward recurrent network for storing memory. Our circuit intuitively parallels biological neural architectures, and it can be readily generalized to other forms of cellular learning and extinction. The simulation of neural function with electronic device analogues may provide insight into biological processes such as decision making, learning and adaptation, while facilitating advanced parallel information processing in hardware.
 
(a) Inverting 2x fan-out circuit based on a nonlinear Kerr resonator. (b) Simulated output fields (blue, green) for a triangle-wave input (red), with E high = 50. The fields are averaged over a time inverval of 0.01. 
(a) Schematic for a D flip-flop, built from the primitive circuits described above. (b) Simulated "master" output (yellow) and final "slave" output (dark blue) in response to the main input (red) and clock (light blue) square-wave signals. The simulation used E high = 50, and the plotted fields are averaged over a time interval of 0.01. 
Circuit diagram for a 4-bit ripple counter. 
Article
A semiclassical simulation approach is presented for studying quantum noise in large-scale photonic circuits incorporating an ideal Kerr nonlinearity. A netlist-based circuit solver is used to generate matrices defining a set of stochastic differential equations, in which the resonator field variables represent random samplings of the Wigner quasi-probability distributions. Although the semiclassical approach involves making a large-photon-number approximation, tests on one- and two-resonator circuits indicate satisfactory agreement between the semiclassical and full-quantum simulation results in the parameter regime of interest. The semiclassical model is used to simulate random errors in a large-scale circuit that contains 88 resonators and hundreds of components in total, and functions as a 4-bit ripple counter. The error rate as a function of on-state photon number is examined, and it is observed that the quantum fluctuation amplitudes do not increase as signals propagate through the circuit, an important property for scalability.
 
Calculated effective potential between two colloids U(D) from Eq. (3) vs colloid surface separation D for varying temperatures T (a) and salt concentrations n0 (b). The location of the barrier peak at Dmax decreases with increasing distance from the coexistence line |T−Tt| or with increasing salt n0 (λD decreases). The mixture composition is such that |Tc−Tt|=10  K. In (a) n0=20  mM is constant, while in (b) |T−Tt|=6  K is constant. For the water–2,6-lutidine mixture containing the antagonistic salt NaBPh4, we used Tc=311  K, a=3.4  Å, C=χ/a, ϵ2,6−lutidine=6.9, ϵwater=79.5, Δγ=0.1kBTa−2, and Δu+=−Δu−=8. For the colloidal PS particles, we used R=1  μm and A=2×10−21  J.
Concentration of graphene dispersed in mixtures of water and acetonitrile containing 20  mM of NaBPh4 at t=0.5  h. UV-vis transmission measurements are performed at room temperature |T−Tt|>25  K. The dashed line is the critical water mole fraction. As clearly observed at concentrations above critical, 0.85≲ϕ≲0.95, exfoliated graphene sheets remain dispersed in the liquid mixture. Dispersed graphene sheets are found even after 3 months.
Article
We demonstrate a general non--Derjaguin-Landau-Verwey-Overbeek method to stabilize colloids in liquids. By this method, colloidal particles that initially form unstable suspension and sediment from the liquid are stabilized by the addition of salt to the suspending liquid. Yet, the salt is not expected to adsorb or directly interact with the surface of the colloids. For the method to work, the liquid should be a mixture, and the salt needs to be antagonistic such that each ion is preferentially solvated by a different component of the mixture. The stabilization may depend on the salt content, mixture composition, or distance from the mixture's coexistence line.
 
The second-order interference visibility V (2) as a func- 
b). We plot the interference visibility as a function of the filter bandwidth in Fig. 4. Three different regions can be distinguished in the data. In the first region with the filter bandwidth greater than 70 GHz, the visibility improves slowly when the filter narrows. In this region, the filter rejects only the spontaneous background and side-mode emissions, which are typically three orders of magnitude weaker than the lasing mode. The visibility improves from V (2) = 0 . 25 at 2 THz to 0.28 at 72.5 GHz. 
Article
Since the introduction of the decoy-state technique, phase-randomised weak coherent light pulses have been the key to increase the practicality of quantum-based communications. Their ultra-fast generation was accomplished via compact gain-switched (GS) lasers, leading to high key rates in quantum key distribution (QKD). Recently, the question arose of whether the same laser could be employed to achieve high-speed measurement-device-independent-QKD, a scheme that promises long-haul quantum communications immune to all detector attacks. For that, a challenging highvisibility interference between independent picosecond optical pulses is required. Here, we answer the above question in the affirmative by demonstrating high-visibility interference from two independent GS lasers triggered at 1GHz. The result is obtained through a careful characterization of the laser frequency chirp and time jitter. By relating these quantities to the interference visibility, we obtain a parameter-free verification of the experimental data and a numerical simulation of the achievable key rates. These findings are beneficial to other applications making use of GS lasers, including random number generation and standard QKD.
 
Top-cited authors
Thomas Kirchartz
  • Forschungszentrum Jülich
Yong Li
  • Tongji University
Alain Goriely
  • University of Oxford
Victor M Burlakov
  • University of Oxford
Tomas Leijtens
  • Stanford University