T. M. Buehler

Density-Dependent Spin Polarization in Ultra-Low -Disorder Quantum Wires

There is controversy as to whether a one-dimensional (1D) electron gas can spin polarize in the absence of a magnetic field. Together with a simple model, we present conductance measurements on ultra-low-disorder quantum wires supportive of a spin polarization at B=0. A spin energy gap is indicated by the presence of a feature in the range (0.5-0.7)x2e(2)/h in conductance data. Importantly, it appears that the spin gap is not constant but a function of the electron density. Data obtained using a bias spectroscopy technique are consistent with the spin gap widening further as the Fermi level is increased.

Progress in silicon-based quantum computing

We review progress at the Australian Centre for Quantum Computer Technology towards the fabrication and demonstration of spin qubits and charge qubits based on phosphorus donor atoms embedded in intrinsic silicon. Fabrication is being pursued via two complementary pathways: a ‘top–down’ approach for near–term production of few–qubit demonstration devices and a ‘bottom–up’ approach for large–scale qubit arrays with sub–nanometre precision. The ‘top–down’ approach employs a low–energy (keV) ion beam to implant the phosphorus atoms. Single–atom control during implantation is achieved by monitoring on–chip detector electrodes, integrated within the device structure. In contrast, the ‘bottom–up’ approach uses scanning tunnelling microscope lithography and epitaxial silicon overgrowth to construct devices at an atomic scale. In both cases, surface electrodes control the qubit using voltage pulses, and dual single–electron transistors operating near the quantum limit provide fast read–out with spurious–signal rejection.

Correlated charge detection for readout of a solid-state quantum computer

The single-electron transistor (SET) is a prime candidate for reading out the final state of a qubit in a solid-state quantum computer. Such a measurement requires the detection of subelectron charge motion in the presence of random charging events. We present a detection scheme where the signals from two SETs are cross-correlated to suppress unwanted artifacts due to charge noise. This technique is demonstrated by using the two SETs to detect the charge state of two coupled metal dots, thereby simulating charge transfer and readout in a two-qubit system. These measurements indicate that for comparable buried dopant semiconductor architectures, the minimum measurement time required to distinguish between the two charge states is of the order of 10 ns.

Charge sensing in carbon-nanotube quantum dots on microsecond timescales

We report fast, simultaneous charge sensing and transport measurements of gate-defined carbon nanotube quantum dots. Aluminum radio-frequency (rf) single electron transistors capacitively coupled to the nanotube dot provide single-electron charge sensing on microsecond timescales. Simultaneously, rf reflectometry allows the fast measurement of transport through the nanotube dot. Charge stability diagrams for the nanotube dot in the Coulomb blockade regime show extended Coulomb diamonds into the high-bias regime, as well as even-odd filling effects, revealed in charge sensing data.

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 Δq>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.

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

Telegraph noise, which originates from the switching of charge between metastable trapping sites, becomes increasingly important as device sizes approach the nanoscale. For charge-based quantum computing, this noise may lead to decoherence and loss of readout 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 microsecond 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.

Development and operation of the twin radio frequency single electron transistor for cross-correlated charge detection

Ultrasensitive 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 subelectron 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 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 δqSET1=7.5μe∕Hz and δqSET2=4.4μe∕Hz.

Ion implanted Si:P double dot with gate tunable interdot coupling

We report on milli-Kelvin charge sensing measurements of a silicon double-dot system fabricated by phosphorus ion implantation. An aluminum single-electron transistor is capacitively coupled to each of the implanted dots enabling the charging behavior of the double-dot system to be studied independent of current transport. Using an electrostatic gate, the interdot coupling can be tuned from weak to strong coupling. In the weak interdot coupling regime, the system exhibits well-defined double-dot charging behavior. By contrast, in the strong interdot coupling regime, the system behaves as a single dot.

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 30nm. On application of a differential bias across the dots, electron transfer is observed, using single electron transistors in both dc and rf modes as charge detectors. With the possibility to scale the dots down to a few and even single atoms these results open the way to a new class of precision-doped quantum dots in silicon.

A 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 of using the nuclear spins of31P donors in28Si as qubits, controlled by metal surface gates and measured using single-electron transistors (SETs). The accurate registration of31P 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.