A design for a quantum gate performing transformations of a single electron spin is presented. The spin rotations are performed by the electron going around the closed loops in a gated semiconductor device. We demonstrate the operation of NOT, phase-flip and Hadamard quantum gates, i.e. the single-qubit gates which are most commonly used in the algorithms. The proposed devices employ the self-focusing effect for the electron wave packet interacting with the electron gas on the electrodes and the Rashba spin-orbit coupling. Due to the self-focusing effect the electron moves in a compact wave packet. The spin-orbit coupling translates the spatial motion of the electron into the rotations of the spin. The device does not require microwave radiation and operates using low constant voltages. It is therefore suitable for selective single-spin rotations in larger registers.
An idea for a nanodevice in which an arbitrary sequence of three basic quantum single qubit gates - negation, Hadamard and phase shift - can be performed on a single electron spin. The spin state is manipulated using the spin-orbit coupling and the electron trajectory is controlled by the electron wave function self-focusing mechanism due to the electron interaction with the charge induced on metal gates. We present results of simulations based on iterative solution of the time dependent Schrödinger equation in which the subsequent operations on the electron spin can be followed and controlled…
A novel spintronic nanodevice is proposed that is capable to manipulate the single heavy hole spin state in a coherent manner. It can act as a single quantum logic gate. The heavy hole spin transformations are realized by transporting the hole around closed loops defined by metal gates deposited on top of the nanodevice. The device exploits Dresselhaus spin orbit interaction which translates the spatial motion of the hole into a rotation of the spin. The proposed quantum gate operates on sub nanosecond time scales and requires only the application of a weak static voltage which allows for addr…
Using local scanning electrical techniques we study edge effects in side-gated Hall nanodevices made of epitaxial graphene. We demonstrate that lithographically defined edges of the graphene channel exhibit hole conduction within the narrow band of ~60-125 nm width, whereas the bulk of the material is electron doped. The effect is the most pronounced when the influence of atmospheric contamination is minimal. We also show that the electronic properties at the edges can be precisely tuned from hole to electron conduction by using moderate strength electrical fields created by side-gates. Howeve…
We study the electronic transport properties of a dual-gated bilayer graphene nanodevice via first principles calculations. We investigate the electric current as a function of gate length and temperature. Under the action of an external electrical field we show that even for gate lengths up 100 Ang., a non zero current is exhibited. The results can be explained by the presence of a tunneling regime due the remanescent states in the gap. We also discuss the conditions to reach the charge neutrality point in a system free of defects and extrinsic carrier doping.
The aim of presented research is to design a nanodevice, based on a MoS$_2$ monolayer, performing operations on a well-defined valley qubit. We show how to confine an electron in a gate induced quantum dot within the monolayer, and to perform the NOT operation on its valley degree of freedom. The operations are carried out all electrically via modulation of the confinement potential by oscillating voltages applied to the local gates. Such quantum dot structure is modeled realistically. Through these simulations we investigate the possibility of realization of a valley qubit in analogy with a r…
We propose a nanodevice for single-electron spin initialization. It is based on a gated planar semiconductor heterostructure with a quantum well and with potentials generated by voltages applied to local gates. Initially we insert an electron with arbitrary spin into the nanodevice. Next we perform a sequence of spin manipulations, after which the spin is set in a desired direction (e.g., the growth direction). The operations are done all-electrically, do not require any external fields and do not depend on the initial spin direction.
We report on electronic transport measurements of dual-gated nano-devices of the low-carrier density topological insulator Bi1.5Sb0.5Te1.7Se1.3. In all devices the upper and lower surface states are independently tunable to the Dirac point by the top and bottom gate electrodes. In thin devices, electric fields are found to penetrate through the bulk, indicating finite capacitive coupling between the surface states. A charging model allows us to use the penetrating electric field as a measurement of the inter-surface capacitance $C_{TI}$ and the surface state energy-density relationship $μ$(n),…
The effect of the up-conversion of the electronic energy level occupation was earlier interpreted as an implication of the multiple scattering of the charge carriers on the longitudinal optical phonons of the lattice vibrations in a small system like a quantum dot. In this work we study the influence of this effect on the electronic motion in a nanotransistor represented by a quantum dot connected to two electric wires and a gate electrode. We show that in an asymmetric nanotransistor the up-conversion effect gives rise to a spontaneous current between the source and the drain, or to an appear…
Inefficient screening of electric fields in nanoconductors makes electric manipulation of electronic transport in nanodevices possible. Accordingly, electrostatic (charge) gating is routinely used to affect and control the Coulomb electrostatics and quantum interference in modern nanodevices. Besides their charge, another (quantum mechanical) property of electrons - their spin - is at the heart of modern spintronics, a term implying that a number of magnetic and electrical properties of small systems are simultaneously harvested for device applications. In this review the possibility to achiev…
We propose a fast and accurate spin initialization method for a single electron trapped in an electrostatic quantum dot. The dot is created in a nanodevice composed of a catalytically grown indium antimonide (InSb) nanowire and nearby gates to which control voltages are applied. Initially we insert a single electron of arbitrary spin into the wire. Operations on spin are performed using the Rashba spin-orbit interaction induced by an electric field. First, a single pulse of voltages applied to lateral gates is used to split the electron wavepacket into two parts with opposite spin orientations…
The aim of presented research is to design a nanodevice based on a gate-defined quantum dot within a MoS$_2$ monolayer in which we confine a single electron. By applying control voltages to the device gates we modulate the confinement potential and force intervalley transitions. The present Rashba spin-orbit coupling additionally allows for spin operations. Moreover, both effects enable the spin-valley SWAP. The device structure is modeled realistically, taking into account feasible dot-forming potential and electric field that controls the Rasha coupling. Therefore, by performing reliable num…
In this paper, several nanodevices which realize basic single heavy hole qubit operations are proposed and supported by time dependent self consistent Poisson-Schrödinger calculations using a four band heavy hole-light hole model. In particular we propose a set of nanodevices which can act as Pauli X, Y, Z quantum gates and as a gate that acts similar as a Hadamard gate (i.e. it creates a balanced superposition of basis states but with an additional phase factor) on the heavy hole spin qubit. We also present the design and simulation of a gated semiconductor nanodevice which can realize an arb…
We examine spin-dependent displacement of a single electron, resulting in separation and relocation of the electron wavefunction components, and thus charge parts, corresponding to opposite spins. This separation is induced by a pulse of an electric field which generates varying Rashba type spin-orbit coupling. This mechanism is next implemented in a nanodevice based on a gated quantum dot defined within a quantum nanowire. The electric field pulse is generated by ultrafast changes of voltages, of the order of several hundred mV, applied to nearby gates. The device is modeled realistically wit…
A novel method and nanodevice are introduced that allows to rotate the single electron spin confined in a gated electrostatic InSb nanowire quantum dot. Proposed method does not require application of any (oscillating or static) external magnetic fields. Our proposal instead employs spatial and time modulation of confining potential induced by electric gates, which, in turn leads to oscillating Rashba type spin-orbit coupling. Moving electron back and forth in such a variable Rashba field allows for realization of spin rotations around two different axes separately without using an external ma…
We report low-temperature transport measurements performed on a planar Nb-InGaAs-Nb proximity Josephson junction hosting a gate-defined lateral quantum dot in the weak-link. We first study quasiparticle and Josephson transport through the open junction, when all gates are grounded. When the quantum dot is defined in the normal region by electrostatic depletion, cotunneling spectroscopy allows us to directly probe the energy gap induced in the two dimensional electron gas. Our data show good qualitative agreement with a model describing resonant tunneling through an Anderson impurity connected …
A nanodevice capable of separating spins of two electrons confined in a quantum dot formed in a gated semiconductor nanowire is proposed. Two electrons confined initially in a single quantum dot in the singlet state are transformed into the system of two electrons confined in two spatially separated quantum dots with opposite spins. In order to separate the electrons' spins we exploit transitions between the singlet and the triplet state which are induced by resonantly oscillating Rashba spin-obit coupling strength. The proposed device is all electrically controlled and the electron spin separ…
The atomically-precise controlled synthesis of graphene stripes embedded in hexagonal boron nitride opens up new possibilities for the construction of nanodevices with applications in sensing. Here, we explore properties related to electronic structure and quantum transport of a graphene nanoroad embedded in hexagonal boron nitride, using a combination of density functional theory and the non-equilibrium Green's functions method to calculate the electric conductance. We find that the graphene nanoribbon signature is preserved in the transmission spectra and that the local current is mainly con…
We propose an all-electrically controlled nanodevice - a gated semiconductor nanowire - capable of generating a coherent state of a single electron trapped in a harmonic oscillator or superposition of such coherent states - the Schrödinger cat state. In the proposed scheme, electron in the ground state of the harmonic potential is driven by resonantly oscillating Rashba spin-orbit coupling. This allows for the creation of the Schrödinger cat state with superposition amplitudes depending on the initial electron spin state. Such a method can be used for initialization of a single spin qubit defi…
