Turnstile Device Research Articles

Research of the operation of the turnstile support-fastening device during the transportation of a long-dimensional heavy load

The article analyzes the motion of a wheelset along a path of unequal length with shaft wear of the rails. The authors have studied the influence of an external disturbing force on the parametric system “wheel-rail”. The considered problem is solved on the basis of a mathematical model of interaction of a pair of wheels and an unequal rail, taking into account the additive effect of unequal stiffness on a rail vehicle. It is shown that taking into account the unequal elasticity of the track in the longitudinal direction allows to determine more precisely the resonance regions and to evaluate the decrease or increase of the acceleration level and the amplitudes of the impact of the unsprung mass of the rail vehicle.

Anomalous single-electron transfer in common-gate quadruple-dot single-electron devices with asymmetric junction capacitances

In this paper, anomalous single-electron transfer in common-gate quadruple-dot turnstile devices with asymmetric junction capacitances is revealed. That is, the islands have the same total number of excess electrons at high and low gate voltages of the swing that transfers a single electron. In another situation, two electrons enter the islands from the source and two electrons leave the islands for the source and drain during a gate voltage swing cycle. First, stability diagrams of the turnstile devices are presented. Then, sequences of single-electron tunneling events by gate voltage swings are investigated, which demonstrate the above-mentioned anomalous single-electron transfer between the source and the drain. The anomalous single-electron transfer can be understood by regarding the four islands as “three virtual islands and a virtual source or drain electrode of a virtual triple-dot device”. The anomalous behaviors of the four islands are explained by the normal behavior of the virtual islands transferring a single electron and the behavior of the virtual electrode.

Single-common-gate triple-dot single-electron devices with side gate capacitances larger than the central one

Single-common-gate triple-dot single-electron devices that have homogeneous junction capacitances Cj and inhomogeneous but symmetrical gate capacitances have been studied. The turnstile operation is possible even if the side gate capacitance Cgs is larger than the central gate capacitance Cgc, although such structures are far from that of the conventional turnstile device with Cgs equal to zero. This is because an overlap between neighboring stability regions always exists for any Cgs/Cgc ratio, where a gate voltage (Vg) swing causes the turnstile operation. In particular, if Cgs/Cgc ratio is an integer, the stability diagrams have the periodic structure along the Vg axis and the overlap appears at the center of the periodic “unit cell”. The existence of the overlap is proved also for any noninteger Cgs/Cgc ratio. The case of Cgs less than Cgc is also investigated. If Cgs/Cgc decreases, the stability diagram approaches that for the conventional turnstile device.

Realization of Al tri-gate single electron turnstile co-integrated with a close proximity electrometer SET

This paper presents a novel fabrication process to realize high density silicon based quantum dot devices with close proximity Al and Si gates on ultrathin silicon-on-insulator for spin qubit applications. Al gates surrounding a Si nanowire channel can adjust tunnelling barrier height electrically, while Si plunger side gates enable precise control of the quantum dots potential. This device is fabricated using a multi-layer electron beam lithography process that is fully compatible with metal oxide semiconductor technology. Low temperature electrical measurements and Coulomb oscillation characteristics have demonstrated the capability of this structure to electrostatically define two coupled single electron transistors, one to be used as a turnstile device and the other as an electrometer.

Excitation of collective modes in a quantum flute

We use a generalized master equation (GME) formalism to describe the nonequilibrium time-dependent transport of Coulomb interacting electrons through a short quantum wire connected to semi-infinite biased leads. The contact strength between the leads and the wire is modulated by out-of-phase time-dependent potentials that simulate a turnstile device. We explore this setup by keeping the contact with one lead at a fixed location at one end of the wire, whereas the contact with the other lead is placed on various sites along the length of the wire. We study the propagation of sinusoidal and rectangular pulses. We find that the current profiles in both leads depend not only on the shape of the pulses, but also on the position of the second contact. The current reflects standing waves created by the contact potentials, like in a wind musical instrument (for example, a flute), but occurring on the background of the equilibrium charge distribution. The number of electrons in our quantum ``flute'' device varies between two and three. We find that for rectangular pulses the currents in the leads may flow against the bias for short time intervals, due to the higher harmonics of the charge response. The GME is solved numerically in small time steps without resorting to the traditional Markov and rotating wave approximations. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. The system (leads plus sample wire) is described by a lattice model.

Generalized Master equation approach to mesoscopic time-dependent transport

We use a generalized Master equation (GME) formalism to describe the non-equilibrium time-dependent transport through a short quantum wire connected to semi-infinite biased leads. The contact strength between the leads and the wire are modulated by out-of-phase time-dependent functions which simulate a turnstile device. One lead is fixed at one end of the sample whereas the other lead has a variable placement. The system is described by a lattice model. We find that the currents in both leads depend on the placement of the second lead. In the rather small bias regime we obtain transient currents flowing against the bias for short time intervals. The GME is solved numerically in small time steps without resorting to the traditional Markov and rotating wave approximations. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method.

Atom devices based on single dopants in silicon nanostructures

Silicon field-effect transistors have now reached gate lengths of only a few tens of nanometers, containing a countable number of dopants in the channel. Such technological trend brought us to a research stage on devices working with one or a few dopant atoms. In this work, we review our most recent studies on key atom devices with fundamental structures of silicon-on-insulator MOSFETs, such as single-dopant transistors, preliminary memory devices, single-electron turnstile devices and photonic devices, in which electron tunneling mediated by single dopant atoms is the essential transport mechanism. Furthermore, observation of individual dopant potential in the channel by Kelvin probe force microscopy is also presented. These results may pave the way for the development of a new device technology, i.e., single-dopant atom electronics.

Coulomb Blockade Conditions for Detailed Model of Single-Electron Turnstile Device Including Finite Self-Capacitances of Island Electrodes

We propose a detailed model of a single-electron turnstile device in which each island electrode has a finite self-capacitance. The Coulomb blockade (CB) regions are obtained by calculating the free energy of the system, and are graphically presented in the gate-charge versus bias-voltage plane. We confirm the CB regions obtained to be octagonal, instead of the quadrilateral CB regions obtained in the conventional model.

Quantized current in carbon nanotube quantum dots by surface acoustic waves

Single charge effects attract much attention with respect to possible applications as future transistor structures or for quantum cryptography. Here, surface acoustic wave mediated single charge transport through quantum dots (QDs) formed in a carbon nanotube (CNT) is presented. The CNT bridges metallic source and drain contacts on a piezoelectric host substrate, employing an acoustic alignment technique. The metal electrodes in contact with the nanotube provide tunnel barriers to the quasi one-dimensional electron system, hence forming a few QD in series due to the presence of defects within the CNT. The dynamic piezoelectric field associated with the surface wave leads to an acoustoelectric current through the system which turns out to be quantized like in a turnstile device.

Numerical study of turnstile operation in random-multidot-channel field-effect transistor

We have numerically studied the single-charge transfer operation in two-dimensional (2D) random-multidot-channel field-effect transistors (FETs) using orthodox theory of the Coulomb blockade phenomenon. The randomness of the multidot structure is reflected in the gate capacitance (Cg) in the equivalent circuit, embodying the dot-size disorder of the realistic devices developed in our laboratory. It was found that “turnstile operation” meaning that individual electron is transferred one by one from the source to the drain with a cycle of an alternating gate voltage can be performed in both random and homogeneous 2D multidot-channel FETs, although their equivalent circuits are significantly different from the ordinary four-junction turnstile device. By increasing the Cg randomness, some devices show that the average gate and drain bias condition (Vg0,Vd) which allows the turnstile operation is more relaxed. Consequently, the random-multidot-channel FET can work as a single-electron turnstile device.

Turnstile Behaviour of the Cooper Pair Pump

We have experimentally studied the behaviour of the so-called Cooper pair pump (CPP) with three Josephson junctions, in the limit of small Josephson coupling EJ < EC. These experiments show that the CPP can be operated as a traditional turnstile device yielding a gate-induced current 2ef in the direction of the bias voltage, by applying an RF-signal with frequency f to the two gates in phase, while residing at the degeneracy node of the gate plane. Accuracy of the CPP during this kind of operation was about 3% and the fundamental Landau-Zener limit was observed to lie above 20 MHz. We have also measured the current pumped through the array by rotating around the degeneracy node in the gate plane. We show that this reproduces the turnstile-kind of behavior. To overcome the contradiction between the obtained e-periodic DC-modulation and a pure 2e-behaviour in the RF-measurements, we base our observations on a general principle that the system always minimises its energy. It suggests that if the excess quasiparticles in the system have a freedom to tunnel, they will organize themselves to the configuration yielding the highest current.

Secure communication: quantum cryptography with a photon turnstile.

Quantum cryptography generates unbreakable cryptographic codes by encoding information using single photons, which until now have relied on highly attenuated lasers as sources1,2. But these sources can create pulses that contain more than one photon, making them vulnerable to eavesdropping by photon splitting3,4. Here we present an experimental demonstration of quantum cryptography that uses a photon turnstile device, which is more reliable for delivering photons one at a time. This device allows completely secure communication in circumstances under which this would be impossible with an attenuated laser.

A quantum dot single-photon turnstile device.

Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source.

Hybrid Circuit Simulator Including a Model for Single Electron Tunneling Devices

A hybrid circuit simulator has been developed that incorporates elements of single electron devices into the conventional circuit simulator SPICE (Simulation Program with Integrated Circuit Emphasis). The elements can consist of an arbitrary network of tunnel junctions and capacitors, whose characteristics are calculated using a master equation method. By employing the hybrid circuit simulator, we studied a turnstile device feeding the input of a complementary metal-oxide-semiconductor (CMOS) inverter, and were able to more successfully demonstrate the transfer of electrons through the turnstile one by one in SPICE.

Zero-bias transfer of single electrons in a turnstile device designed with trapezoidal tunneling barriers

The transfer of single electrons has been theoretically studied in a turnstile device with four identical tunnel junctions of capacitance C and a gate capacitor of capacitance $C/2,$ designed with identical asymmetric tunneling barriers instead of symmetric tunneling barriers. The zero-current offset in the current-voltage characteristic of this turnstile is not at zero-bias voltage when the amplitude of an applied ac gate voltage is greater than $e/C;$ that is, electrons can be transferred through the device with an applied ac gate signal but without bias voltage. This unusual transfer characteristic is due to a rectification nature of electron tunneling in asymmetric tunneling barriers.

A single-photon turnstile device

Quantum-mechanical interference between indistinguishable quantum particles profoundly affects their arrival time and counting statistics. Photons from a thermal source tend to arrive together (bunching) and their counting distribution is broader than the classical Poisson limit1. Electrons from a thermal source, on the other hand, tend to arrive separately (anti-bunching) and their counting distribution is narrower than the classical Poisson limit2,3,4. Manipulation of quantum-statistical properties of photons with various non-classical sources is at the heart of quantum optics: features normally characteristic of fermions — such as anti-bunching, sub-poissonian and squeezing (sub-shot-noise) behaviours — have now been demonstrated5. A single-photon turnstile device was proposed6,7,8 to realize an effect similar to conductance quantization. Only one electron can occupy a single state owing to the Pauli exclusion principle and, for an electron waveguide that supports only one propagating transverse mode, this leads to the quantization of electrical conductance: the conductance of each propagating mode is then given by GQ = e2/h (where e is the charge of the electron and h is Planck's constant; ref. 9). Here we report experimental progress towards generation of a similar flow of single photons with a well regulated time interval.

New Structures of Quantized Current Plateaus in Semiconductor Turnstile Devices

Quantized current plateaus appear in the I - V characteristics of turnstile devices realized by modulating the tunnel barriers with a radio frequency (rf) signal of frequency f and phase difference π. We report two new results. (1) New fine structures resulting from the quantized level spacing Δ E appear. Then, the quantized current flows through the barriers via several excited states of the system with a fixed number of electrons, as the source drain voltage increases. These structures are smeared out when the temperature k B T becomes comparable to Δ E . However the quantized plateaus remain. The quantized plateaus are washed out when k B T becomes comparable to the charging energy e 2 / C , where C is the capacitance of the dot. (2) When the rf frequency f becomes larger than the leakage current J ( f =0)/ e , which is the time-averaged current at the frequency f =0, the time-averaged current J dc is quantized in the adiabatic regime. On the other hand, when f is smaller than J ( f =0)/ e , J dc is mu...

Single-photon turnstile device: simultaneous Coulomb blockade for electrons and holes

Utilizing simultaneous Coulomb blockade for electrons and holes in a p-n junction, we can realize a device where a single electron and a single hole are injected into the active region to produce a single photon with well-regulated time interval. The photons emitted from such a device can be studied with single photon counting detectors using a Si solid-state photomultiplier. We report locking of the photon emission with external driving pulse, in the regime where one electron and one hole are injected into the active region on the average.

Strongly Interacting Photons in a Nonlinear Cavity

We consider the dynamics of single photons in a nonlinear optical cavity. When the Kerr nonlinearities of atomic dark resonances are utilized, the cavity mode is well described by a spin- $1/2$ Hamiltonian. We show that it is possible to achieve coherent control of the cavity-mode wave function using $\ensuremath{\pi}$ pulses for single photons that switch the state of the cavity with very high accuracy. The underlying physics is best understood as the nonlinearity induced anticorrelation between single-photon injection/emission events, which we refer to as photon blockade. We also propose a method which uses these strong dispersive interactions to realize a single-photon turnstile device.

Role of Displacement Current in Quantum-Dot Turnstile Devices

The dynamics of the current and the electron number in quantum-dot turnstile devices, in which the heights of the tunneling barriers are modulated by external rf signals, is studied theoretically within the adiabatic approximation. The displacement current is more accurately quantized than the tunneling currents flowing through both the left and the right barriers, since the leakage current is compensated. We find numerically that an electron tunnels through the barrier before its height reaches the minimum value. The tunneling phases of rf signals, at which an electron can actually tunnel, change as the amplitude of rf signals and the barrier height and width are varied. We also discuss the conditions for the current quantization. The dc I-V curve which we obtain shows a plateau at each quantized current nef with width e2/C+ΔE and sharp steps between neighboring plateaus. Whenever the dc current is quantized, the change of the number of electrons in the quantum dot during one half-period of rf signal is an integer.