arXiv subjects

A. R. Hamilton

Publications and source records attributed to A. R. Hamilton.

At least 91 records · Page 5Linked to original sources

arXiv2006

Superconductivity in Metal-mixed Ion-Implanted Polymer Films

Ion-implantation of normally insulating polymers offers an alternative to depositing conjugated organics onto plastic films to make electronic circuits. We used a 50 keV nitrogen ion beam to mix a thin 10 nm Sn/Sb alloy film into the sub-surface of polyetheretherketone (PEEK) and report the low temperature properties of this material. We observed metallic behavior, and the onset of superconductivity below 3 K. There are strong indications that the superconductivity does not result from a residual thin-film of alloy, but instead from a network of alloy grains coupled via a weakly conducting, ion-beam carbonized polymer matrix.

cond-mat.supr-con↗

arXiv2006

Controlled single electron transfer between Si:P dots

We demonstrate electrical control of Si:P double dots in which the potential is defined by nanoscale phosphorus doped regions. Each dot contains approximately 600 phosphorus atoms and has a diameter close to 30 nm. On application of a differential bias across the dots, electron transfer is observed, using single electron transistors in both dc- and rf-mode as charge detectors. With the possibility to scale the dots down to few and even single atoms these results open the way to a new class of precision-doped quantum dots in silicon.

cond-mat.mes-hall↗

arXiv2005

Electric field induced charge noise in doped silicon: ionization of phosphorus donors

We report low frequency charge noise measurement on silicon substrates with different phosphorus doping densities. The measurements are performed with aluminum single electron transistors (SETs) at millikelvin temperatures where the substrates are in the insulating regime. By measuring the SET Coulomb oscillations, we find a gate voltage dependent charge noise on the more heavily doped substrate. This charge noise, which is seen to have a 1/f spectrum, is attributed to the electric field induced tunneling of electrons from their phosphorus donor potentials.

cond-mat.mes-hall↗

arXiv2005

Coherent electronic transfer in quantum dot systems using adiabatic passage

We describe a scheme for using an all-electrical, rapid, adiabatic population transfer between two spatially separated dots in a triple-quantum dot system. The electron spends no time in the middle dot and does not change its energy during the transfer process. Although a coherent population transfer method, this scheme may well prove useful in incoherent electronic computation (for example quantum-dot cellular automata) where it may provide a coherent advantage to an otherwise incoherent device. It can also be thought of as a limiting case of type II quantum computing, where sufficient coherence exists for a single gate operation, but not for the preservation of superpositions after the operation. We extend our analysis to the case of many intervening dots and address the issue of transporting quantum information through a multi-dot system.

cond-mat.other↗

arXiv2005

Electrical readout of a spin qubit without double occupancy

We identify a novel mechanism to read out a single solid-state electron spin using an all-electrical spin-to-charge conversion in a closed system. Our scheme uses three donors and two electron spins, one spin is the qubit, the other is a reference. The population in the third, originally ionized, donor is monitored with an electrometer. Energy-dependent tunnelling of the reference spin to the ionized donor is used to determine the state of the qubit. In contrast to previous methods [e.g., B.E. Kane, Nature (London), \textbf{393}, 133 (1998)] we avoid double electron occupancy of any site within the system, thereby eliminating the possibility of unwanted electron loss from the system. The single spin readout scheme described here is applicable to both electron and nuclear spin based quantum computer architectures.

cond-mat.other↗

arXiv2005

Conductance quantization and the 0.7x2e2/h conductance anomaly in one-dimensional hole systems

We have studied ballistic transport in a 1D channel formed using surface gate techniques on a back-gated, high-mobility, bilayer 2D hole system. At millikelvin temperatures, robust conductance quantization is observed in the quantum wire formed in the top layer of the bilayer system, without the gate instabilities that have hampered previous studies of 1D hole systems. Using source drain bias spectroscopy, we have measured the 1D subband spacings, which are 5-10 times smaller than in comparable GaAs electron systems, but 2-3 times larger than in previous studies of 1D holes. We also report the first observation of the anomalous conductance plateau at G = 0.7 x 2e2/h in a 1D hole system.

cond-mat.mes-hall↗

arXiv2004

The effect of low-energy ion-implantation on the electrical transport properties of Si-SiO2 MOSFETs

Using silicon MOSFETs with thin (5nm) thermally grown SiO2 gate dielectrics, we characterize the density of electrically active traps at low-temperature after 16keV phosphorus ion-implantation through the oxide. We find that, after rapid thermal annealing at 1000oC for 5 seconds, each implanted P ion contributes an additional 0.08 plus/minus 0.03 electrically active traps, whilst no increase in the number of traps is seen for comparable silicon implants. This result shows that the additional traps are ionized P donors, and not damage due to the implantation process. We also find, using the room temperature threshold voltage shift, that the electrical activation of donors at an implant density of 2x10^12 cm^-2 is ~100%.

cond-mat.mtrl-sci↗

arXiv2004

Single-Shot Readout with the Radio Frequency Single Electron Transistor in the Presence of Charge Noise

The radio frequency single electron transistor (rf-SET) possesses key requirements necessary for reading out a solid state quantum computer. This work explores the use of the rf-SET as a single-shot readout device in the presence of 1/f and telegraph charge noise. For a typical spectrum of 1/f noise we find that high fidelity, single-shot measurements are possible for signals q > 0.01e. For the case of telegraph noise, we present a cross-correlation measurement technique that uses two rf-SETs to suppress the effect of random switching events on readout. We demonstrate this technique by monitoring the charge state of a metal double dot system on microsecond time-scales. Such a scheme will be advantageous in achieving high readout fidelity in a solid state quantum computer.

cond-mat.mes-hall↗

arXiv2004

Observing sub-microsecond telegraph noise with the radio frequency single electron transistor

Telegraph noise, which originates from the switching of charge between meta-stable trapping sites, becomes increasingly important as device sizes approach the nano-scale. For charge-based quantum computing, this noise may lead to decoherence and loss of read out fidelity. Here we use a radio frequency single electron transistor (rf-SET) to probe the telegraph noise present in a typical semiconductor-based quantum computer architecture. We frequently observe micro-second telegraph noise, which is a strong function of the local electrostatic potential defined by surface gate biases. We present a method for studying telegraph noise using the rf-SET and show results for a charge trap in which the capture and emission of a single electron is controlled by the bias applied to a surface gate.

cond-mat.mes-hall↗

arXiv2004

Charge shelving and bias spectroscopy for the readout of a charge-qubit on the basis of superposition states

Charge-based qubits have been proposed as fundamental elements for quantum computers. One commonly proposed readout device is the single-electron transistor (SET). SETs can distinguish between localized charge states, but lack the sensitivity to directly distinguish superposition states, which have greatly enhanced coherence times compared with position states. We propose introducing a third dot, and exploiting energy dependent tunnelling from the qubit into this dot (bias spectroscopy) for pseudo-spin to charge conversion and superposition basis readout. We introduce an adiabatic fast passage-style charge pumping technique which enables efficient and robust readout via charge shelving, avoiding problems due to finite SET measurement time.

cond-mat.other↗

arXiv2004

Development and operation of the twin radio frequency single electron transistor for solid state qubit readout

Ultra-sensitive detectors and readout devices based on the radio frequency single electron transistor (rf-SET) combine near quantum-limited sensitivity with fast operation. Here we describe a twin rf-SET detector that uses two superconducting rf-SETs to perform fast, real-time cross-correlated measurements in order to distinguish sub-electron signals from charge noise on microsecond time-scales. The twin rf-SET makes use of two tuned resonance circuits to simultaneously and independently address both rf-SETs using wavelength division multiplexing (WDM) and a single cryogenic amplifier. We focus on the operation of the twin rf-SET as a charge detector and evaluate the cross-talk between the two resonance circuits. Real time suppression of charge noise is demonstrated by cross correlating the signals from the two rf-SETs. For the case of simultaneous operation, the rf-SETs had charge sensitivities of $δq_{SET1} = 7.5 μe/\sqrt{Hz}$ and $δq_{SET2} = 4.4 μe/\sqrt{Hz}$.

cond-mat.mes-hall↗

arXiv2004

Evolution of the bilayer nu = 1 quantum Hall state under charge imbalance

We use high-mobility bilayer hole systems with negligible tunneling to examine how the bilayer nu = 1 quantum Hall state evolves as charge is transferred from one layer to the other at constant total density. We map bilayer nu = 1 state stability versus imbalance for five total densities spanning the range from strongly interlayer coherent to incoherent. We observe competition between single-layer correlations and interlayer coherence. Most significantly, we find that bilayer systems that are incoherent at balance can develop spontaneous interlayer coherence with imbalance, in agreement with recent theoretical predictions.

cond-mat.mes-hall↗

arXiv2004

Interaction Correction to the Longitudinal Conductivity and Hall Resistivity in High Quality Two-Dimensional GaAs Electron and Hole Systems

We present a systematic study of the corrections to both the longitudinal conductivity and Hall resistivity due to electron-electron interactions in high quality GaAs systems using the recent theory of Zala et al. [Phys. Rev. B 64, 214204 (2001)]. We demonstrate that the interaction corrections to the longitudinal conductivity and Hall resistivity predicted by the theory are consistent with each other. This suggests that the anomalous metallic drop in resistivity at B=0 is due to interaction effects and supports the theory of Zala et al.

cond-mat.str-el↗

arXiv2003

Charge-based quantum computing using single donors in semiconductors

Solid-state quantum computer architectures with qubits encoded using single atoms are now feasible given recent advances in atomic doping of semiconductors. Here we present a charge qubit consisting of two dopant atoms in a semiconductor crystal, one of which is singly ionised. Surface electrodes control the qubit and a radio-frequency single electron transistor provides fast readout. The calculated single gate times, of order 50ps or less, are much shorter than the expected decoherence time. We propose universal one- and two-qubit gate operations for this system and discuss prospects for fabrication and scale up.

cond-mat↗

arXiv2003

Charge-based silicon quantum computer architectures using controlled single-ion implantation

We report a nanofabrication, control and measurement scheme for charge-based silicon quantum computing which utilises a new technique of controlled single ion implantation. Each qubit consists of two phosphorus dopant atoms ~50 nm apart, one of which is singly ionized. The lowest two energy states of the remaining electron form the logical states. Surface electrodes control the qubit using voltage pulses and dual single electron transistors operating near the quantum limit provide fast readout with spurious signal rejection. A low energy (keV) ion beam is used to implant the phosphorus atoms in high-purity Si. Single atom control during the implantation is achieved by monitoring on-chip detector electrodes, integrated within the device structure, while positional accuracy is provided by a nanomachined resist mask. We describe a construction process for implanted single atom and atom cluster devices with all components registered to better than 20 nm, together with electrical characterisation of the readout circuitry. We also discuss universal one- and two-qubit gate operations for this architecture, providing a possible path towards quantum computing in silicon.

cond-mat↗

arXiv2003

Current suppression in a double-island single-electron transistor for detection of degenerate charge configurations of a floating double-dot

We have investigated a double-island single-electron transistor (DISET) coupled to a floating metal double-dot (DD). Low-temperature transport measurements were used to map out the charge configurations of both the DISET and the DD. A suppression of the current through the DISET was observed whenever the charge configurations of the DISET and the DD were energetically co-degenerate. This effect was used to distinguish between degenerate and non-degenerate charge configurations of the double-dot. We also show that this detection scheme reduces the susceptibility of the DISET to interference from random charge noise.

cond-mat↗

arXiv2003

Maximizing the Hilbert space for a finite number of distinguishable quantum states

We consider a quantum system with a finite number of distinguishable quantum states, which may be partitioned freely by a number of quantum particles, assumed to be maximally entangled. We show that if we partition the system into a number of qudits, then the Hilbert space dimension is maximized when each quantum particle is allowed to represent a qudit of order $e$. We demonstrate that the dimensionality of an entangled system, constrained by the total number quantum states, partitioned so as to maximize the number of qutrits will always exceed the dimensionality of other qudit partitioning. We then show that if we relax the requirement of partitioning the system into qudits, but instead let the particles exist in any given state, that the Hilbert space dimension is greatly increased.

quant-ph↗

arXiv2002

Self-aligned fabrication process for silicon quantum computer devices

We describe a fabrication process for devices with few quantum bits (qubits), which are suitable for proof-of-principle demonstrations of silicon-based quantum computation. The devices follow the Kane proposal to use the nuclear spins of 31P donors in 28Si as qubits, controlled by metal surface gates and measured using single electron transistors (SETs). The accurate registration of 31P donors to control gates and read-out SETs is achieved through the use of a self-aligned process which incorporates electron beam patterning, ion implantation and triple-angle shadow-mask metal evaporation.

cond-mat↗