Few-electron Regime Research Articles

Charge state hysteresis in semiconductor quantum dots

Semiconductor quantum dots provide a two-dimensional analogy for real atoms and show promise for the implementation of scalable quantum computers. Here, we investigate the charge configurations in a silicon metal-oxide-semiconductor double quantum dot tunnel coupled to a single reservoir of electrons. By operating the system in the few-electron regime, the stability diagram shows hysteretic tunnelling events that depend on the history of the dots charge occupancy. We present a model which accounts for the observed hysteretic behaviour by extending the established description for transport in double dots coupled to two reservoirs. We demonstrate that this type of device operates like a single-electron memory latch.

Few-Electron Edge-State Quantum Dots in a Silicon Nanowire Field-Effect Transistor

We investigate the gate-induced onset of few-electron regime through the undoped channel of a silicon nanowire field-effect transistor. By combining low-temperature transport measurements and self-consistent calculations, we reveal the formation of one-dimensional conduction modes localized at the two upper edges of the channel. Charge traps in the gate dielectric cause electron localization along these edge modes, creating elongated quantum dots with characteristic lengths of ∼10 nm. We observe single-electron tunneling across two such dots in parallel, specifically one in each channel edge. We identify the filling of these quantum dots with the first few electrons, measuring addition energies of a few tens of millielectron volts and level spacings of the order of 1 meV, which we ascribe to the valley orbit splitting. The total removal of valley degeneracy leaves only a 2-fold spin degeneracy, making edge quantum dots potentially promising candidates for silicon spin qubits.

Frequency multiplexing for readout of spin qubits

We demonstrate a low loss, chip-level frequency multiplexing scheme for readout of scaled-up spin qubit devices. By integrating separate bias tees and resonator circuits on-chip for each readout channel, we realise dispersive gate-sensing in combination with charge detection based on two radio frequency quantum point contacts. We apply this approach to perform multiplexed readout of a double quantum dot in the few-electron regime and further demonstrate operation of a 10-channel multiplexing device. Limitations for scaling spin qubit readout to large numbers of multiplexed channels are discussed.

Charge States and Transition of Double Quantum Dot in the Few-Electron Regime

We present transport characterizations of a fully tunable double quantum dot over the few electron number down to zero, which is defined by means of surface gates on top of a GaAs/AlGaAs heterostructure. We also perform numerical simulations to map out the charge stability diagram, which is consistent with the measurements. The results demonstrate that the electric control in the double dot can be significantly enhanced by improving the design of the device structure, which provides potential advantages for quantum information processing.

Dispersive readout of a few-electron double quantum dot with fast RF gate sensors.

We report the dispersive charge-state readout of a double quantum dot in the few-electron regime using the in situ gate electrodes as sensitive detectors. We benchmark this gate sensing technique against the well established quantum point contact charge detector and find comparable performance with a bandwidth of ∼ 10 MHz and an equivalent charge sensitivity of ∼ 6.3 × 10(-3) e/sqrt[Hz]. Dispersive gate sensing alleviates the burden of separate charge detectors for quantum dot systems and promises to enable readout of qubits in scaled-up arrays.

Orbital and valley state spectra of a few-electron silicon quantum dot

Understanding interactions between orbital and valley quantum states in silicon nanodevices is crucial in assessing the prospects of spin-based qubits. We study the energy spectra of a few-electron silicon metal-oxide-semiconductor quantum dot using dynamic charge sensing and pulsed-voltage spectroscopy. The occupancy of the quantum dot is probed down to the single-electron level using a nearby single-electron transistor as a charge sensor. The energy of the first orbital excited state is found to decrease rapidly as the electron occupancy increases from N=1 to 4. By monitoring the sequential spin filling of the dot we extract a valley splitting of ~230 {\mu}eV, irrespective of electron number. This indicates that favorable conditions for qubit operation are in place in the few-electron regime.

A few-electron quadruple quantum dot in a closed loop

We report the realization of a quadruple quantum dot device in a square-like configuration where a single electron can be transferred on a closed path free of other electrons. By studying the stability diagrams of this system, we demonstrate that we are able to reach the few-electron regime and to control the electronic population of each quantum dot with gate voltages. This allows us to control the transfer of a single electron on a closed path inside the quadruple dot system. This work opens the route towards electron spin manipulation using spin-orbit interaction by moving an electron on complex paths free of electrons.

Fabrication and characterization of a silicon metal-oxide-semiconductor based triple quantum dot

We fabricate electrostatically defined, few-electron triple quantum dot (TQD) devices in a silicon metal-oxide-semiconductor structure and obtain stability diagrams in the few-electron regime through charge detection by a nearby quantum point contact. We demonstrate the tunability of the TQD by achieving the quadruple points where all three dots are on resonance. The tuning evolutions are shown to be consistent with a constant interaction model. We identify quantum cellular automata phenomena near the quadruple point.

Radio frequency charge sensing in InAs nanowire double quantum dots

We demonstrate charge sensing of an InAs nanowire double quantum dot (DQD) coupled to a radio frequency (rf) circuit. We measure the rf signal reflected by the resonator using homodyne detection. Clear single dot and DQD behavior are observed in the resonator response. rf-reflectometry allows measurements of the DQD charge stability diagram in the few-electron regime even when the dc current through the device is too small to be measured. For a signal-to-noise ratio of one, we estimate a minimum charge detection time of 350 μs at interdot charge transitions and 9 μs for charge transitions with the leads.

Spectroscopy of Spin-Orbit Quantum Bits in Indium Antimonide Nanowires

A double quantum dot in the few-electron regime is achieved using local gating in an InSb nanowire. The spectrum of two-electron eigenstates is investigated using electric dipole spin resonance. Singlet-triplet level repulsion caused by spin-orbit interaction is observed. The size and the anisotropy of singlet-triplet repulsion are used to determine the magnitude and the orientation of the spin-orbit effective field in an InSb nanowire double dot. The obtained results are confirmed using spin blockade leakage current anisotropy and transport spectroscopy of individual quantum dots.

Top-gated germanium nanowire quantum dots in a few-electron regime

Top gated quantum dots (QDs) have been fabricated from n-type chemically synthesized germanium nanowires (GeNWs) by constricting its length with metal electrode contacts. With an intermediate HfO2 thin film, the constricted GeNW was fully covered by an Omega-shaped top-gate. The QD was probed and characterized by single-electron transport measurements at liquid helium temperature and has been found to reach a few-electron regime, in which the number of confined electrons was tunable from zero. The absolute zero-electron was confirmed with a charge stability diagram, and it was revealed that the extremely small QD arose from potential fluctuations due to phosphorus donors.

Single electron emission in two-phase xenon with application to the detection of coherent neutrino-nucleus scattering

We present an experimental study of single electron emission in ZEPLIN-III, a two-phase xenon experiment built to search for dark matter WIMPs, and discuss applications enabled by the excellent signal-to-noise ratio achieved in detecting this signature. Firstly, we demonstrate a practical method for precise measurement of the free electron lifetime in liquid xenon during normal operation of these detectors. Then, using a realistic detector response model and backgrounds, we assess the feasibility of deploying such an instrument for measuring coherent neutrino-nucleus elastic scattering using the ionisation channel in the few-electron regime. We conclude that it should be possible to measure this elusive neutrino signature above an ionisation threshold of $\sim$3 electrons both at a stopped pion source and at a nuclear reactor. Detectable signal rates are larger in the reactor case, but the triggered measurement and harder recoil energy spectrum afforded by the accelerator source enable lower overall background and fiducialisation of the active volume.

Numerical simulation study of electrostatically defined silicon double quantum dot device

Coupled quantum dots are of great interest for the application of quantum computing. The aspect needing attention is the preparation of well-defined quantum dots with small sizes and interdot distances. We propose a novel electrostatics method to form silicon double quantum dots. Three-dimensional numerical simulations were used to confirm the concept and study the mechanism controlling the tunnel barrier using the side gates. We estimate the electron number in each quantum dot to be less than five electrons. The prospect in creating ultrasmall quantum dots that operate at the few-electron regime, as well as exhibit processing simplicity is the great advantage of this method over those previously reported.

Charge state readout and hyperfine interaction in a few-electron InGaAs double quantum dot

A laterally defined InGaAs double quantum dot with an integrated charge readout sensor is realized in an InGaAs/InP heterostructure. The charge states of the double quantum dot are measured with the use of the charge readout sensor in the few-electron regime in which the current is too weak to be observable by direct measurements of electron transport through the double dot. We also measure the leakage current of the double quantum dot in the Pauli spin-blockade few-electron regime and study the singlet-triplet state mixing by the hyperfine coupling to the nuclear spins. The measurements of the leakage current in the weak external-magnetic-field region and for weak interdot couplings allow us to extract an effective nuclear magnetic field in the double-dot system. We also study spin relaxation and transport processes in the Pauli spin-blockade region at large external magnetic fields and observe transport through the excited triplet state in the few-electron double quantum dot.

Simulation Study of Charge Modulation in Coupled Quantum Dots in Silicon

We have calculated electron states in a lithographically defined Si double quantum dot (DQD) device. Electrons are confined near the upper interface in the silicon-on-insulator (SOI) layer by top gate and side gate voltages. Surface charge density, NS, at 4 K in DQDs is evaluated using experimental data of the gate voltage dependence of NS in a metal–oxide–semiconductor (MOS) transistor at 100 and 4 K. With optimum side gate biases, electrons are confined in QDs and coupling between QDs is controlled in a few-electron regime. We have also proposed that a charge sensor is required to read out the few-electron regime because no current flows in the DQD device.

Few-electron quantum dots in InGaAs quantum wells: Role of fluctuations

We study experimentally the electron transport properties of gated quantum dots formed in InGaAs/InP and InAsP/InP quantum well structures grown by chemical-beam epitaxy. For the case of the InGaAs quantum well, quantum dots form directly underneath narrow gate electrodes due to potential fluctuations. We measure the Coulomb-blockade diamonds in the few-electron regime of a single quantum dot and observe photon-assisted tunneling peaks under microwave irradiation. A singlet-triplet transition at high magnetic field and Coulomb-blockade effects in the quantum Hall regime are also observed. For the InAsP quantum well, an incidental triple quantum dot forms also due to potential fluctuations within a single dot layout. Tunable quadruple points are observed via transport measurements.

Disentangling the effects of spin-orbit and hyperfine interactions on spin blockade

We have achieved the few-electron regime in InAs nanowire double quantum dots. Spin blockade is observed for the first two half-filled orbitals, where the transport cycle is interrupted by forbidden transitions between triplet and singlet states. Partial lifting of spin blockade is explained by spin-orbit and hyperfine mechanisms that enable triplet to singlet transitions. The measurements over a wide range of interdot coupling and tunneling rates to the leads are well reproduced by a simple transport model. This allows us to separate and quantify the contributions of the spin-orbit and hyperfine interactions.

Singlet-triplet transition in a few-electron lateral In0.75Ga0.25As/In0.75Al0.25As quantum dot

The magnetic-field evolution of Coulomb blockade peaks in lateral In0.75Ga0.25As/In0.75Al0.25As quantum dots in the few-electron regime is reported. Quantum dots are defined by gates evaporated onto a 60-nm-thick hydrogen silsesquioxane insulating film. A gyromagnetic factor g∗≈4.4 is measured via zero-bias spin spectroscopy and a transition from singlet to triplet spin configuration is found at an in-plane magnetic field B=0.7 T. This observation opens the way to the manipulation of singlet and triplet states at moderate fields and its relevance for quantum information applications will be discussed.

Nature of electron states and symmetry breaking in quantum point contacts according tothe local spin density approximation

Electron states and local magnetization in quantum point contacts (QPCs) with differentgeometries and applied gate voltages are examined for a model GaAs/AlGaAs device.Using the local spin density approximation (LSDA) we recover ferromagnetic spatially splitsolutions in the pinch-off regime as well as antisymmetric solutions that occur withdecreasing gate voltage. These kinds of spin states, which may appear in a repeated fashionin the few-electron regime, are precursors to an extended ferromagnetic statethat may be associated with the 0.7 conductance anomaly. We briefly commenton some recent experiments indicating the presence of bound states (Yoon et al 2007 Phys. Rev. Lett. 99 136805). We have not found any indication of suchstates but suggest that the accumulations of spin and charge at the two endsof a QPC and associated singlet and triplet states are relevant in this context.

Top-gated few-electron double quantum dot in Si/SiGe

A few-electron quantum dot utilizing Schottky, metal top gates in a modulation doped Si/SiGe heterostructure was realized and non-linear transport through the dot was studied. By carefully tuning the capacitively coupled gates, the single quantum dot was transformed into two tunnel-coupled quantum dots in series. The resulting double quantum dot was tuned to the few-electron regime and the charge stability diagram was studied as a function of the gate voltages. Understanding of such a double dot system is essential for the practical implementation of exchange-mediated multi-qubit systems in silicon devices.