Tunable Gate Research Articles

Dynamics of Majorana-based qubits operated with an array of tunable gates

We study the dynamics of Majorana zero modes that are shuttled via local tuning of the electrochemical potential in a superconducting wire. By performing time-dependent simulations of microscopic lattice models, we show that diabatic corrections associated with the moving Majorana modes are quantitatively captured by a simple Landau-Zener description. We further simulate a Rabi-oscillation protocol in a specific qubit design with four Majorana zero modes in a single wire and quantify constraints on the timescales for performing qubit operations in this setup. Our simulations utilize a Majorana representation of the system, which greatly simplifies simulations of superconductors at the mean-field level.

Room temperature continuous frequency tuning InGaAs/InP single-photon detector

The available high speed InGaAs/InP-based single-photon avalanche detectors (SPAD) are normally worked at fixed gate or narrow tunable gate frequency. However, a wide tunable gate frequency or even free running single photon detectors at high speed would be very useful in quantum key distribution, quantum entanglement distribution and so on. Here, we present a high speed InGaAs/InP-based single photon avalanche detector (SPAD) with tunable gate frequency from 900MHz to 1000MHz which also can work under the room temperature without any cooling setups. Instead of restricting the spike noise by self-differencing or filtering method, we use an Analog to Digital Converter(ADC) to sample the output voltage of the APD. Through the sampled voltage we can discriminate an avalanche signal from the noise. Based on the sampling method, a room temperature SPAD is implemented with a dark count rate of 2.5 × 10−5 per gate and afterpulse probability of 1.3%, given the condition that detection efficiency of 10.6%, dead time of 1ns, and clock frequency of 1GHz. The wide tunable gate frequency makes the SPAD very suitable for practical use and commercial producing.

Transient Evolutional Dynamics of Quantum-Dot Molecular Phase Coherence for Sensitive Optical Switching

Atomic phase coherence (quantum interference) in a multilevel atomic gas exhibits a number of interesting phenomena. Such an atomic quantum coherence effect can be generalized to a quantum-dot molecular dielectric. Two quantum dots form a quantum-dot molecule, which can be described by a three-level Λ-configuration model \(\{ |0\rangle ,|1\rangle ,|2\rangle \} \), i.e., the ground state of the molecule is the lower level |0〉 and the highly degenerate electronic states in the two quantum dots are the two upper levels \(|1\rangle ,|2\rangle \). The electromagnetic characteristics due to the |0〉–|1〉 transition can be controllably manipulated by a tunable gate voltage (control field) that drives the |2〉–|1〉 transition. When the gate voltage is switched on, the quantum-dot molecular state can evolve from one steady state (i.e., |0〉–|1〉 two-level dressed state) to another steady state (i.e., three-level coherent-population-trapping state). In this process, the electromagnetic characteristics of a quantum-dot mo...

Ag nanoprism metamaterial based tunable plasmonic logic gate

A metamaterial comprised of asymmetrically arranged Ag nanoprisms has been proposed and investigated in this paper. The metamaterial shows a polarization dependent PIT effect. The metamaterial with its periodic length along a y-axis less than 400 nm shows two tunable transmission dips. When the periodicity is increased above 400 nm, the metamaterial shows three adjustable transmission dips against the polarization state. By choosing different wavelengths of incident light, the metamaterial can be used as plasmonic Boolean logic gates. The working wavelengths of these plasmonic devices can then be adjusted by controlling the fillet radius of Ag nanoprism in metamaterial. These results offer great potential in developing polarization dependent tunable plasmonic information processing devices.

Imaging and tuning polarity at SrTiO3 domainwalls.

Electrostatic fields tune the ground state of interfaces between complex oxide materials. Electronic properties, such as conductivity and superconductivity, can be tuned and then used to create and control circuit elements and gate-defined devices. Here we show that naturally occurring twin boundaries, with properties that are different from their surrounding bulk, can tune the LaAlO3/SrTiO3 interface 2DEG at the nanoscale. In particular, SrTiO3 domain boundaries have the unusual distinction of remaining highly mobile down to low temperatures, and were recently suggested to be polar. Here we apply localized pressure to an individual SrTiO3 twin boundary and detect a change in LaAlO3/SrTiO3 interface current distribution. Our data directly confirm the existence of polarity at the twin boundaries, and demonstrate that they can serve as effective tunable gates. As the location of SrTiO3 domain walls can be controlled using external field stimuli, our findings suggest a novel approach to manipulate SrTiO3-based devices on the nanoscale.

Phase matching as a gate for photon entanglement

Phase matching is shown to provide a tunable gate that helps discriminate entangled states of light generated by four-wave mixing (FWM) in optical fibers against uncorrelated photons originating from Raman scattering. Two types of such gates are discussed. Phase-matching gates of the first type are possible in the normal dispersion regime, where FWM sidebands can be widely tuned by high-order dispersion management, enhancing the ratio of the entangled-photon output to the Raman noise. The photon-entanglement gates of the second type are created by dual-pump cross-phase-modulation-induced FWM sideband generation and can be tuned by group-velocity mismatch of the pump fields.

Simplified proposal for realizing a multiqubit tunable phase gate in circuit QED

We propose a scheme to realize a multiqubit tunable phase gate in a circuit QED setup where two resonators, each coupling with a qudit, are interconnected to a common qudit (d=4). In this proposal, only two levels of each qudit serve as the logical states and the other two levels are used for the gate realization. The proposal is efficient and simple because only a classical microwave pulse is needed, no matter how many qudits are involved, which significantly reduces experimental difficulty. In a nonresonant case, the tunable phase gate can be achieved readily, while under the resonant condition, a {\pi}-phase gate can be realized after a full cycle of Rabi oscillation where the gate speed is rather fast due to the resonant interaction. We have shown that the resulting effective dynamics allows for the creation of a high-fidelity phase gate. The influence of various decoherence processes such as the decay of the resonator mode and the relaxation of the qudits is investigated. Moreover, the proposed scheme can be easily generalized to realize N-qubit phase gate.

ZrO2 quantum dots/graphene phototransistors for deep UV detection

ZrO2 is a promising material for deep-UV detection application, yet the low carrier density and high crystal lattice defects hinder the device performance. Graphene is an attractive material for photoactive and charge transport layers in photodetectors, but the photoresponsivity have been limited by the weak light absorption, no wavelength selectivity. Here, we demonstrate heterostructure phototransistors consist of graphene as the conducting channel, which is covered by a film of ZrO2 quantum dots as the light absorption layer. Light absorption in the quantum dots layer creates electron-hole pairs. Under the built-in electric field, the electrons are trapped in ZrO2 acting as an additional light tunable gate, whereas the holes are transferred towards the graphene. The ZnO2/Graphene heterostructure, with a high photoresponsivity of 22AW−1 and wavelength selectivity to deep UV wavelength range (220–250nm) at low operating voltage, is promising to be integrated into solution processed and low-cost optoelectrical devices.

A Flexible Doubly Interpenetrated Metal-Organic Framework with Breathing Behavior and Tunable Gate Opening Effect by Introducing Co2+ into Zn4O Clusters.

A Zn4O clusters based flexible doubly interpenetrated metal-organic framework [(Zn4O)2(DCPB)6DMF]·2DMF·8H2O (JLU-Liu33, H2DCPB = 1,3-di(4-carboxyphenyl)benzene, DMF = N,N-dimethylformamide) with pcu topology has been solvothermally synthesized. Because of its flexible structure, JLU-Liu33 exhibits a breathing behavior upon N2 and CO2 adsorption at low temperature, and C2H6 and C3H8 adsorption at 273 and 298 K. Furthermore, by adopting the direct synthesis method, two isomorphic compounds-JLU-Liu33L and JLU-Liu33H-have been obtained by partial substituting Zn with different amounts of Co into the JLU-Liu33 framework. The gas adsorption study of Co-doped materials reveals that the gate opening effect of JLU-Liu33 can be modulated by introducing different contents of Co2+ into Zn4O clusters. Meanwhile, with the increasing amount of Co2+, the adsorption amount and isosteric enthalpy values for CO2 have been improved. It is worth mentioning that JLU-Liu33H exhibits commendable selectivity for CO2 over CH4 which may be a good candidate for industrial gas purification and air separation applications.

Real-Time Controllable and Multifunctional Metasurfaces Utilizing Indium Tin Oxide Materials: A Phased Array Perspective

In this paper, two electrically tunable dual-band reflectarray antennas are designed by the integration of a thin layer of indium tin oxide (ITO) into plasmonic multi-sized and multilayer unit cells. The presented geometries include two gold nanoribbons located next to each other with different widths and backed by a stack of alumina-ITO-metallic ground plane and two pairs of vertically stacked gold nanoribbon-alumina-ITO on a metallic ground plane. The double-resonance nature of the double metal-insulator-metal unit-cells is exploited to achieve two distinct operating frequencies in which the control over phase of the reflected beam is obtained via the tunable gate biasing of ITO. An in-depth phased array analysis is developed with emphasis on the accomplishment of a reconfigurable antenna with robust beam scanning and focusing characteristics. The proposed structures can be considered as dual-band bifunctional reflectarray antenna that can operate at two distinct frequencies in the near-infrared regime with two different functionalities as bending and focusing.

Optical modulation characteristics of graphene supercapacitors at oblique incidence in visible-infrared region

Multi-angle modulation properties of electrolyte gated graphene visible-infrared modulators with supercapacitor structures have been examined in both experiment and theory. Both transmission and reflection geometries have been studied, and the results show that all the modulations of these devices are gate voltage and wavelength dependent. Maximum modulation depths for transmission device ∼3% and reflection modulator ∼5% are obtained, which can be attributed to the electrolyte assistant considerable change of the Fermi energy with tunable gate voltage. Moreover, little incident angle dependency can be observed with a certain gate voltage. Theoretical analysis based on the different tangential and normal optical responses of the quartz/graphene/eletrolyte interface has been used to explain the phenomena, which indicated a common effect of p-polarized and s-polarized light. This work offers useful insight into a multi-angle application of the graphene-based optical modulator in visible-infrared region.

Cation Dependent Surface Charge Regulation in Gated Nanofluidic Devices.

Surface charge governs nanoscale aqueous electrolyte transport, both in engineered analytical systems and in biological entities such as ion channels and ion pumps as a function of ion type and concentration. Embedded electrodes in a nanofluidic channel, isolated from the fluid in the channel by a dielectric layer, act as active, tunable gates to systematically modify local surface charge density at the interface between the nanochannel surface and the aqueous electrolyte solution, causing significant changes in measured nanochannel conductance. A systematic comparison of transport of monovalent electrolytes [potassium chloride (KCl), sodium chloride (NaCl)], 2:1 electrolytes [magnesium chloride (MgCl2), calcium chloride (CaCl2)], and electrolyte mixtures (KCl + CaCl2) through a gated nanofluidic device was performed. Ion-surface interactions between divalent Ca2+ and Mg2+ ions and the nanochannel walls reduced the native surface charge density by up to ∼4-5 times compared to the monovalent cations. In electrolyte mixtures, Ca2+ was the dominating cation with nanochannel conductance independent of KCl concentration. Systematic changes in local electrostatic surface state induced by the gate electrode are impacted by the divalent cation-surface interactions, limiting modulation of the local surface potential by the gate electrode and resulting in cation dependent nanoscale ion transport as seen through conductance measurements and numerical models.

Controllable Tunneling of Light through a Quantum-Dot-Molecule Dielectric Film via Electromagnetically Induced Transparency

Since discrete multilevel transitions of quantum-dot molecules driven by external electromagnetic fields can exhibit quantum coherence effects, such an optical characteristic can be utilized to control propagation of electromagnetic wave through a quantum-dot molecule dielectric film. Since inner-dot tunneling in quantum-dot molecules can be controlled by a gate voltage, destructive quantum coherence among multilevel transitions in quantum-dot molecule would give rise to EIT (electromagnetically induced transparency). In this report, we shall investigate controllable on- and off-resonance tunneling effects of an incident electromagnetic wave through such a quan-tum-dot-molecule dielectric film, of which the optical response is tuned by the switchable gate voltage. We have found from the theoretical mechanism that a high gate voltage can cause the EIT phenomenon of quan-tum-dot-molecule systems, and under the condition of on-resonance light tunneling through the thin film, the probe field will propagation without loss if the probe frequency detuning is zero. By taking advantage of these effects sensitive to the tunable gate voltage, such quantum coherence would be inte-grated in certain photonic structures, and some devices such as photonic switching and transistors can be designed. Transient evolution of optical characteristics in the quantum-dot-molecule dielectric film (once the tunable gate voltage is turned on or off) is also considered in this report.

Direct Detection of Low Concentration DNA in High Ionic Strength Solution with AlGaN/GaN High Electron Mobility Transistors

In this research, the direct detection of DNA sequences in physiological salt concentration by FET based biosensor has been investigated. Traditionally, most of the DNA detection methods are optical measurements, such as fluorescence or Raman spectroscopy which are always expensive, bulky and hard to quantify. In contrast with the traditional method, we use AlGaN/GaN high electron mobility transistor (HEMT) to detect the target ssDNA. The III-IV compound semiconductor based transistor is chemically inert and possess really high resistivity to high salt environment and has low possibility of corrosion. The HEMT device is fabricated with four mask: MESA, SD metal, final metal and gate opening and presents a 3D structure which isolates the transistor channel and gold sensing region.(fig 1.) Moreover, with the isolated gold sensing area, the transistor can be extremely small as the gold sensing region stays in the large area. This kind of FET having high transconductance gain, can significantly improve the S/N ratio and sensitivity. Current measured in PBS buffer serves as the baseline of the biosensor. Probe ssDNA is immobilized on the gold gate electrode positioned at a fixed distance from the transistor channel. Device is operated with 2V drain voltage and a tunable gate pulse voltage. As the complementary DNA concentration, ranging from 1fM to 100nM, is introduced, the current gain of the device changes accordingly. The sensor can efficiently detect target DNA down to a low concentration of 100 fM. This ultrahigh sensitivity and low S/N ratio characteristics exhibited by the sensor makes it a competitive alternate choice compared to the traditional existing sensors. The miniaturized device design and ease of fabrication and testing indicate the potential of the sensor to be used as a commercial DNA biosensor. Figure 1

Cellular heterogeneity mediates inherent sensitivity–specificity tradeoff in cancer targeting by synthetic circuits

Synthetic gene circuits are emerging as a versatile means to target cancer with enhanced specificity by combinatorial integration of multiple expression markers. Such circuits must also be tuned to be highly sensitive because escape of even a few cells might be detrimental. However, the error rates of decision-making circuits in light of cellular variability in gene expression have so far remained unexplored. Here, we measure the single-cell response function of a tunable logic AND gate acting on two promoters in heterogeneous cell populations. Our analysis reveals an inherent tradeoff between specificity and sensitivity that is controlled by the AND gate amplification gain and activation threshold. We implement a tumor-mimicking cell-culture model of cancer cells emerging in a background of normal ones, and show that molecular parameters of the synthetic circuits control specificity and sensitivity in a killing assay. This suggests that, beyond the inherent tradeoff, synthetic circuits operating in a heterogeneous environment could be optimized to efficiently target malignant state with minimal loss of specificity.

Quantum Router for Single Photons Carrying Spin and Orbital Angular Momentum.

Quantum router is an essential element in the quantum network. Here, we present a fully quantum router based on interaction free measurement and quantum dots. The signal photonic qubit can be routed to different output ports according to one control electronic qubit. Besides, our scheme is an interferometric method capable of routing single photons carrying either spin angular momentum (SAM) or orbital angular momentum (OAM), or simultaneously carrying SAM and OAM. Then we describe a cascaded multi-level quantum router to construct a one-to-many quantum router. Subsequently we analyze the success probability by using a tunable controlled phase gate. The implementation issues are also discussed to show that this scheme is feasible.

Ultracompact electro-optical logic gates based on graphene–silica metamaterial

We propose and simulate numerically a permittivity-tunable metamaterial channel, which is composed of alternating layers of graphene and silica. The real part of the permittivity of the proposed metamaterial can be tuned from a positive value to a negative one for a broadband width. Furthermore, optical waves can pass through the metamaterial channel only when its permittivity is tuned to zero. Inspired by this intriguing property of the graphene–silica metamaterial, three basic electro-optical logic gates, including NOT, NOR, and NAND gates, were proposed and numerically investigated by using the finite element method. Taking advantage of the permittivity-tunable property of graphene, the working wavelength of the proposed electro-optical logic gates can be actively controlled by tuning the external voltage applied on the graphene–silica metamaterial. These tunable and ultracompact electro-optical logic gates could benefit the development of nanoscale optical devices for highly integrated photonic circuits.

Excitonic and nematic instabilities on the surface of topological Kondo insulators

We study the effects of strong electron-electron interactions on the surface of cubic topological Kondo insulators (such as samarium hexaboride, SmB$_6$). Cubic topological Kondo insulators generally support three copies of massless Dirac nodes on the surface, but only two of them are energetically degenerate and exhibit an energy offset relative to the third one. With a tunable chemical potential, when the surface states host electron and hole pockets of comparable size, strong interactions may drive this system into rotational symmetry breaking nematic and translational symmetric breaking excitonic spin- or charge-density-wave phases, depending on the relative chirality of the Dirac cones. Taking a realistic surface band structure into account we analyze the associated Ginzburg-Landau theory and compute the mean field phase diagram for interacting surface states. Beyond mean field theory, this system can be described by a two-component isotropic Ashkin-Teller model at finite temperature, and we outline the phase diagram of this model. Our theory provides a possible explanation of recent measurements which detect a two-fold symmetric magnetoresistance and an upturn in surface resistivity with tunable gate voltage in SmB$_6$. Our discussion can also be germane to other cubic topological insulators, such as ytterbium hexaboride (YbB$_6$), plutonium hexaboride (PuB$_6$).

Quantum witness of high-speed low-noise single-photon detection.

We demonstrate high-speed and low-noise near-infrared single-photon detection by using a capacitance balancing circuit to achieve a high spike noise suppression for an InGaAs/InP avalanche photodiode. The single-photon detector could operate at a tunable gate repetition rate from 10 to 60 MHz. A peak detection efficiency of 34% has been achieved with a dark count rate of 9 × 10⁻³ per gate when the detection window was set to 1 ns. Additionally, quantum detector tomography has also been performed at 60 MHz of repetition rate and for the detection window of 1 ns, enabling to witness the quantum features of the detector with the help of a negative Wigner function. By varying the bias voltage of the detector, we further demonstrated a transition from the full-quantum to semi-classical regime.

Tunable all-optical gates in 2D discrete cavity solitons with local defect

We numerically studied the existence of discrete cavity solitons in a two-dimensional array of coupled optical cavities with Kerr nonlinearity influenced by a local Gaussian defect. Effect of the defect on soliton on/off switching by injecting a writing/erasing external beam is also discussed. The interaction of two adjacent solitons, mediated by a defect is considered and effect of the positive and negative defect on the independency interval is determined. Finally all-optical NOR and NAND logic gates are proposed which are tunable by well-adjusting the defect height.