We describe the fabrication and measurement of a micrometer-sized direct-current superconducting quantum interference device ͑dc-SQUID͒ in which the critical currents of each of the constriction-type Josephson junctions can be controlled independently and in situ via a process of nonequilibrium ͑hot-phonon͒ irradiation from a nanofabricated gated structure. The control mechanism is based on hot phonons which are injected into the superconducting microbridges from close proximity, but electrically isolated, normal-metal constrictions. We have also developed a one-dimensional computer model to analyze the behavior of micro-SQUID devices including situations in which we modify the asymmetry of the device. We show from the model that the experimental results are consistent with a change in effective length of the microbridge junctions with respect to the coherence length of the film. The experimental data, and its interpretation in relation to the micro-SQUID model, confirm that this technique, based on hot-phonon irradiation for controlling the critical current in Dayem bridge Josephson junctions, is compatible with the Josephson effect and a feasible method for post-fabrication parameter control in superconducting circuits using Dayem bridge Josephson junctions.

We electrically measure intrinsic silicon quantum dots with electrostatically defined tunnel barriers. The presence of both p-type and n-type ohmic contacts enables the accumulation of either electrons or holes. Thus we are able to study both transport regimes within the same device. We investigate the effect of the tunnel barriers and the electrostatically defined quantum dots. There is greater localisation of charge states under the tunnel barriers in the case of hole conduction leading to higher charge noise in the p-regime.Comment: in press at Appl. Phys. Let

As semiconductor device dimensions are reduced to the nanometer scale, effects of high defect density surfaces on the transport properties become important to the extent that the metallic character that prevails in large and highly doped structures is lost and the use of quantum dots for charge sensing becomes complex. Here we have investigated the mechanism behind the detection of electron motion inside an electrically isolated double quantum dot that is capacitively coupled to a single electron transistor, both fabricated from highly phosphorous doped silicon wafers. Despite, the absence of a direct charge transfer between the detector and the double dot structure, an efficient detection is obtained. In particular, unusually large Coulomb peak shifts in gate voltage are observed. Results are explained in terms of charge rearrangement and the presence of inelastic cotunneling via states at the periphery of the single electron transistor dot.

We report charge sensing measurements on a silicon quantum dot with a nearby silicon single electron transistor (SET) acting as an electrometer. The devices are electrostatically formed in bulk silicon using surface gates. We show that as an additional electron is added onto the quantum dot, a charge is induced on the SET of approximately 0.2e. These measurements are performed in the many electron regime, where we can count in excess of 20 charge additions onto the quantum dot.Silicon is a promising material in which to build a quantum computer 1 . This is primarily due to the long coherence time for electron spins, as demonstrated by electron spin resonance measurements on dopant ensembles 2,3 , and also expected for electrons in quantum dots (QDs) 4 . In recent experiments, the hyperfine interaction was demonstrated to limit spin coherence for electrons confined in gallium arsenide QDs 5 . With the motivation of going to nuclear spin free materials, much research effort is currently focussed on accessing the spin of electrons confined in silicon-based nanostructures 6,7 .Our approach to confining single electrons uses electrostatic gates to define QDs in intrinsic silicon 8 . Related techniques are also possible in silicon-on-insulator (SOI) 9 and silicon-germanium 10 . For both QDs and dopants a sensitive charge detector is an important experimental tool 11 . It allows confirmation that a single electron is confined in the potential. Such confirmation is also possible by electrical transport through a dot, but the geometry must ensure that there is still a measureable current at low electron numbers. An equally important advantage of the charge sensor is that it enables single shot measurement of electron spin states 12 -a useful property for quantum computation. In silicon-germanium a quantum point contact has been used for charge sensing on QDs in the few electron regime 13 . In SOI, electron occupancy on an isolated node has been measured with a single electron transistor (SET) at room temperature 14 , but not probed at cryogenic temperatures. In a narrow silicon metal-oxide-semiconductor field-effect transistor device, an aluminium SET has been used to detect charges on a self-aligned silicon SET 15 . In this Letter, we demonstrate charge sensing of a QD by a nearby SET co-fabricated in intrinsic silicon.Previously we have measured similar devices via electrical transport and observed both Coulomb blockade and orbital excited states in the island 8 . In addition we have embedded these structures in a radio-frequency circuit 16 and performed high-bandwidth charge measurement with the device configured as a radio-frequency SET 17 . In this work we operate the QD and SET at low frequencies using both dc and lock-in amplifiers. The measurements presented here were performed at T ≤ 100mK in a 0.2 T magnetic field, which is used to suppress superconductivity in the AlSi bondpads.Figure 1(a) shows a scanning electron microscope (SEM) image of a typical device. The fabrication process has been described previousl...

We experimentally study the transport properties of silicon quantum dots (QDs) fabricated from a highly doped n-type silicon-on-insulator wafer. Low noise electrical measurements using a low temperature complementary metal-oxide-semiconductor (LTCMOS) amplifier are performed at 4.2 K in liquid helium. Two series of Coulomb peaks are observed: long-period oscillations and fine structures, and both of them show clear source drain voltage dependence. We also observe two series of Coulomb diamonds having different periodicity. The obtained experimental results are well reproduced by a master equation analysis using a model of double QDs coupled in parallel.Single-electrons in quantum dots (QDs) have been proposed as one of the suitable candidates for realizing solidstate quantum bits (qubits). [1,2] Recently, the essential requirements of controlling and measuring single electron states have been achieved in GaAs-based semiconductor QDs. [3,4,5,6] On the other hand, Si-based QDs have attracted interest because a long electron-spin coherence time is expected owing to the small spin-orbit coupling in addition to the almost spin-zero nuclear background. [7] Despite the interest, the single-electron states have been less investigated in Si QDs [8,9,10,11] because the relatively heavy effective mass makes the manipulation of single electrons technically challenging. The larger effective mass of electron in Si than GaAs makes the confinement potential energy smaller. To obtain quantum confined electronic states, it is necessary to fabricate much smaller nanodevices. In addition, it is more difficult to make a clean two-dimensional electron gas in Si than GaAs and to avoid noise from the device itself because of the interface effect and impurities.In order to probe the single-electron states in such a Si system, it is important to measure nanodevices in a low-noise system. Measurement devices such as a transimpedance or charge integrator amplifier are more suitable for use in a low temperature system at 4.2 K owing to the variation of noise gain with temperature.In a nanoscale device, the confining potential structure might become complex. Therefore, it is important to perform a theoretical simulation in which a model that can reproduce the obtained experimental results is used. From the simulation we can extract the potential structure of the device and obtain data that can be fed back into the fabrication process.In this work, we experimentally study the transport properties of Si QDs fabricated from a highly doped ntype Si-on-insulator (SOI) wafer. Low-noise electrical measurements using a low-temperature complementary metal-oxide-semiconductor (LTCMOS) amplifier are performed at 4.2 K in liquid helium. [12] We successfully observed clear Coulomb oscillations and diamond-shaped Coulomb blockade regions. In particular, we observed a peculiar pattern of a large diamond containing several small diamonds in it. We calculate the transport properties by solving master equations for several QD systems to reproduce the obtained...