Antti Kemppinen

Environment-Assisted Tunneling as an Origin of the Dynes Density of States

We show that the effect of a high-temperature environment in current transport through a normal metal-insulator-superconductor tunnel junction can be described by an effective density of states in the superconductor. In the limit of a resistive low-Ohmic environment, this density of states reduces into the well-known Dynes form. Our theoretical result is supported by experiments in engineered environments. We apply our findings to improve the performance of a single-electron turnstile, a potential candidate for a metrological current source.

SINGLE-ELECTRON SHUTTLE BASED ON A SILICON QUANTUM DOT

We report on single-electron shuttling experiments with a silicon metal-oxide-semiconductor quantum dot at 300 mK. Our system consists of an accumulated electron layer at the Si/SiO2 interface below an aluminum top gate with two additional barrier gates used to deplete the electron gas locally and to define a quantum dot. Directional single-electron shuttling from the source to the drain lead is achieved by applying a dc source-drain bias while driving the barrier gates with an ac voltage of frequency fp. Current plateaus at integer levels of efp are observed up to fp=240 MHz operation frequencies. The observed results are explained by a sequential tunneling model, which suggests that the electron gas may be heated substantially by the ac driving voltage.

Experimental investigation of hybrid single-electron turnstiles with high charging energy

We present an experimental study of hybrid turnstiles with high charging energies in comparison to the superconducting gap. The device is modeled with the sequential tunneling approximation. The backtunneling effect is shown to limit the amplitude of the gate drive and thereby the maximum pumped current of the turnstile. We compare results obtained with sine and square wave drive and show how a fast rise time can suppress errors due to leakage current. Quantized current plateaus up to 160 pA are demonstrated.

Pumping properties of the hybrid single-electron transistor in dissipative environment

Pumping characteristics were studied of a hybrid normal-metal/ superconductor single-electron transistor embedded in high-Ohmic environment. Two 3 μm long microstrip resistors of CrOx with a sum resistance R≈80 k were placed adjacent to the transistor. Substantial improvement of pumping and a reduction of the subgap leakage were observed in the low-megahertz range. At higher frequencies (0.1-1 GHz), pumping performance deteriorated compared to reference devices without resistors by the slowdown of tunneling and by electronic heating.

Nanoampere pumping of Cooper pairs

The authors have employed a tunable Cooper-pair transistor, the sluice, with radio frequency control to pump current over a resistive circuit. They find that the charge transferred per pumping cycle can be controlled with the resolution of a single Cooper pair up to hundreds of pairs. The achieved nanoampere current features more than an order of magnitude improvement over the previously reported results and it is close to the theoretical maximum value for the measured sample.

Quantized current of a hybrid single-electron transistor with superconducting leads and a normal-metal island

We discuss the operation of the superconductor–insulator–normal-metal–insulator–superconductor (SINIS) turnstile. This voltage-biased hybrid single-electron transistor (SET) provides current quantization even with only one radio-frequency (rf) control parameter, namely the gate voltage of the single island. We give an overview of the main error mechanisms of the turnstile and consider its feasibility as a quantum current standard. We also present experimental results of pumping with the SINIS structure which show decreased leakage current compared to earlier measurements with the opposite NISIN structure.

Long hold times in a two-junction electron trap

The hold time τ of a single-electron trap is shown to increase significantly due to suppression of photon assisted tunneling events. Using two rf-tight radiation shields instead of a single one, we demonstrate increase of τ by a factor exceeding 103, up to about 10 h, for a trap with only two superconductor (S)—normal-metal (N) tunnel junctions and an on-chip resistor R ∼ 100 kΩ (R-SNS structure). In the normal state, the improved shielding made it possible to observe τ ∼ 100 s, which is in reasonable agreement with the quantum-leakage-limited level expected for the two-electron cotunneling process.

Single-Electronic Radio-Frequency Refrigerator

We demonstrate experimentally that a hybrid single-electron transistor with superconducting leads and a normal-metal island can be refrigerated by an alternating voltage applied to the gate electrode. The simultaneous measurement of the dc current induced by the rf gate through the device at a small bias voltage serves as an in situ thermometer.

Realization of a square-wave voltage with externally-shunted SIS Josephson junction arrays for a quantum AC voltage standard

A quantum-based ac voltage standard operating from about 1 Hz up to 10 kHz at 1 V is being developed. The standard is based on relating the output of a stable sine generator to a square wave obtained by biasing a nonhysteretic Josephson junction array alternately at steps n = -2 and n = +2. We have constructed a bias current source and a buffer amplifier connected to the array which enable fast switching between steps. In this paper, we describe the system used for generation of the square wave and present experimental results which show that the accuracy of the fundamental frequency component amplitude of the square wave is sufficient for realization of a standard with accuracy better than 1 /spl times/ 10/sup -6/.

Towards direct closure of the quantum metrological triangle

In this paper, plans and progress of a Finnish collaboration towards direct closure of the quantum metro- logical triangle are presented. The experimental concept and the present status of the main components, including two possibilities of the single charge transport device, Bloch oscillating transistor (BOT) based null detector, and a thin-film cryoresistor, are described.