arXiv subjects

A. Kemppinen

Publications and source records attributed to A. Kemppinen.

At least 19 recordsLinked to original sources

arXiv2025

Fast optical data transfer into a Josephson junction array

We employ externally shunted Nb-AlO$_x$-Nb Josephson junctions for demonstrating a circuit that is suitable for an optically driven Josephson Arbitrary Waveform Synthesizer (JAWS). This technology enables overdamped junctions with characteristic frequencies above 100 GHz and critical currents of the order of 100 $\mu$A, which is promising, e.g., for low-dissipation optical control of quantum circuits such as superconducting quantum bits. Here we utilize a double-pulse technique to experimentally determine the maximum rate at which optical pulse data can be reliably delivered to the superconducting circuit. We demonstrate the feasibility of data transfer up to 60 Gbit/s, which is about factor 4 higher than for typical JAWS.

cond-mat.supr-con↗

arXiv2024

Efficient electronic cooling above 2 K by niobium-based superconducting tunnel junctions

Replacing the bulky cryoliquid-based cooling stages of cryoenabled instruments by chip-scale refrigeration is envisioned to disruptively reduce the system size similar to microprocessors did for computers. Electronic refrigerators based on superconducting tunnel junctions have been anticipated to provide a solution, but reaching the necessary above the 1-K operation temperature range has remained a goal out of reach for several decades. We show efficient electronic refrigeration by Al-AlO$_x$-Nb superconducting tunnel junctions starting from bath temperatures above 2 K. The junctions can deliver electronic cooling power up to approximately mW/mm$^2$, which enables us to demonstrate tunnel-current-driven electron temperature reduction from 2.4 K to below 1.6 K (34% relative cooling) against the phonon bath. Our work shows that the key material of integrated superconducting circuits - niobium - enables powerful cryogenic refrigerator technology. This result is a prerequisite for practical cryogenic chip-scale refrigerators and, at the same time, it introduces a new electrothermal tool for quantum heat-transport experiments.

cond-mat.supr-con↗

arXiv2022

Phonon-blocked junction refrigerators for cryogenic quantum devices

Refrigeration is an important enabler for quantum technology. The very low energy of the fundamental excitations typically utilized in quantum technology devices and systems requires temperature well below 1 K. Expensive cryostats are utilized in reaching sub-1 K regime and solid-state cooling solutions would revolutionize the field. New electronic micro-coolers based on phonon-blocked semiconductor-superconductor junctions could provide a viable route to such miniaturization. Here, we investigate the performance limits of these junction refrigerators.

quant-ph↗

arXiv2021

Cascaded superconducting junction refrigerators: optimization and performance limits

We demonstrate highly transparent silicon-vanadium and silicon-aluminum tunnel junctions with relatively low sub-gap leakage current and discuss how a trade-off typically encountered between transparency and leakage affects their refrigeration performance. We theoretically investigate cascaded superconducting tunnel junction refrigerators with two or more refrigeration stages. In particular, we develop an approximate method that takes into account self-heating effects but still allows us to optimize the cascade a single stage at a time. We design a cascade consisting of energy-efficient refrigeration stages, which makes cooling of, e.g., quantum devices from above 1 K to below 100 mK a realistic experimental target.

cond-mat.supr-con↗

arXiv2021

Coulomb Blockade Thermometry on a Wide Temperature Range

The Coulomb Blockade Thermometer (CBT) is a primary thermometer for cryogenic temperatures, with demonstrated operation from below 1 mK up to 60 K. Its performance as a primary thermometer has been verified at temperatures from 20 mK to 200 mK at uncertainty level below 1 % (k = 2). In a new project, our aim is to extend the metrologically verified temperature range of the primary CBT up to 25 K. We also demonstrate close-to-ideal operation of a CBT with only two tunnel junctions when the device is embedded in a low-impedance environment.

physics.app-ph↗

arXiv2020

Driving a low critical current Josephson junction array with a mode-locked laser

We demonstrate the operation of Josephson junction arrays (JJA) driven by optical pulses generated by a mode-locked laser and an optical time-division multiplexer. A commercial photodiode converts the optical pulses into electrical ones in liquid helium several cm from the JJA. The performance of our custom-made mode-locked laser is sufficient for driving a JJA with low critical current at multiple Shapiro steps. Our optical approach is a potential enabler for fast and energy-efficient pulse drive without expensive high-bandwidth electrical pulse pattern generator, and without high-bandwidth electrical cabling crossing temperature stages. Our measurements and simulations motivate an improved integration of photodiodes and JJAs using, e.g., flip-chip techniques, in order to improve both the understanding and fidelity of pulse-driven Josephson Arbitrary Waveform Synthesizers (JAWS).

physics.app-ph↗

arXiv2017

Superconducting MoSi nanowires

We have fabricated disordered superconducting nanowires of molybdenium silicide. A molybdenium nanowire is first deposited on top of silicon, and the alloy is formed by rapid thermal annealing. The method allows tuning of the crystal growth to optimise, e.g., the resistivity of the alloy for potential applications in quantum phase slip devices and superconducting nanowire single-photon detectors. The wires have effective diameters from 42 to 79 nm, enabling the observation of crossover from conventional superconductivity to regimes affected by thermal and quantum fluctuations. In the smallest diameter wire and at temperatures well below the superconducting critical temperature, we observe residual resistance and negative magnetoresistance, which can be considered as fingerprints of quantum phase slips.

cond-mat.supr-con↗

arXiv2017

Characterising superconducting filters using residual microwave background

A normal metal - superconductor hybrid single-electron trap with tunable barrier is utilized as a tool for spectrum analysis at the extremely low signal levels, using only well filtered cryogenic microwave background as a photon source in the frequency range from about 50 to 210 GHz. We probe millimeter wave propagation in two superconducting systems: a Josephson junction array around its plasma frequency, and a superconducting titanium film in the limit when the photon energies are larger than the superconducting energy gap. This regime is relevant for improving the performance of cryogenic quantum devices but is hard to access with conventional techniques. We show that relatively simple models can be used to describe the essential properties of the studied components.

cond-mat.supr-con↗

arXiv2016

Reducing current noise in cryogenic experiments by vacuum-insulated cables

We measure the current noise of several cryogenic cables in a pulse tube based dilution refrigerator at frequencies between about 1~mHz and 50~kHz. We show that vibration-induced noise can be efficiently suppressed by using vacuum-insulated cables between room temperature and the 2nd pulse tube stage. A noise peak below 4 fA at the 1.4~Hz operation frequency of the pulse tube, and a white noise density of 0.44 fA/\sqrt{Hz} in the millihertz range are obtained.

physics.ins-det↗

arXiv2015

Accurate Coulomb Blockade Thermometry up to 60 Kelvin

We demonstrate experimentally a precise realization of Coulomb Blockade Thermometry (CBT) working at temperatures up to 60 K. Advances in nano fabrication methods using electron beam lithography allow us to fabricate a uniform arrays of sufficiently small tunnel junctions to guarantee an overall temperature reading uncertainty of about 1\%

cond-mat.mes-hall↗

arXiv2013

Single quasiparticle excitation dynamics on a superconducting island

We investigate single quasiparticle excitation dynamics on a small superconducting aluminum island connected to normal metallic leads by tunnel junctions. We find the island to be free of excitations within the measurement resolution allowing us to determine Cooper pair breaking rate to be less than 3 kHz. By tuning the Coulomb energy of the island to have an odd number of electrons, one of them remains unpaired. We detect it by measuring its relaxation rate via tunneling. By injecting electrons with a periodic gate voltage, we probe electron-phonon interaction and relaxation down to a single quasiparticle excitation pair, with a measured recombination rate of 8 kHz. Our experiment yields a strong test of BCS-theory in aluminum as the results are consistent with it without free parameters.

cond-mat.supr-con↗

arXiv2012

Single-electron current sources: towards a refined definition of ampere

Controlling electrons at the level of elementary charge $e$ has been demonstrated experimentally already in the 1980's. Ever since, producing an electrical current $ef$, or its integer multiple, at a drive frequency $f$ has been in a focus of research for metrological purposes. In this review we first discuss the generic physical phenomena and technical constraints that influence charge transport. We then present the broad variety of proposed realizations. Some of them have already proven experimentally to nearly fulfill the demanding needs, in terms of transfer errors and transfer rate, of quantum metrology of electrical quantities, whereas some others are currently "just" wild ideas, still often potentially competitive if technical constraints can be lifted. We also discuss the important issues of read-out of single-electron events and potential error correction schemes based on them. Finally, we give an account of the status of single-electron current sources in the bigger framework of electric quantum standards and of the future international SI system of units, and briefly discuss the applications and uses of single-electron devices outside the metrological context.

cond-mat.mes-hall↗

arXiv2012

Vanishing quasiparticle density in a hybrid Al/Cu/Al single-electron transistor

The achievable fidelity of many nanoelectronic devices based on superconducting aluminum is limited by either the density of residual nonequilibrium quasiparticles n_qp or the density of quasiparticle states in the gap, characterized by Dynes parameter γ. We infer upper bounds n_qp < 0.033 um^-3 and γ< 1.6*10^-7 from transport measurements performed on Al/AlOx/Cu single-electron transistors, improving previous results by an order of magnitude. Owing to efficient microwave shielding and quasiparticle relaxation, typical number of quasiparticles in the superconducting leads is zero.

cond-mat.supr-con↗

arXiv2011

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/SiO_2 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 and to the drain lead is achieved by applying a dc source-drain bias while driving the barrier gates with an ac voltage of frequency f_p. Current plateaus at integer levels of ef_p are observed up to f_p = 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.

quant-ph↗

arXiv2011

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 environmentally assisted tunneling events. Using two rf-tight radiation shields instead of a single one, we demonstrate increase of $τ$ by a factor exceeding $10^3$, up to about 10 hours, for a trap with only two superconductor (S) -- normal-metal (N) tunnel junctions and an on-chip resistor $R$ (R-SNS structure). In the normal state, the improved shielding made it possible to observe $τ\sim$ 100 s, which is in reasonable agreement with the quantum-leakage-limited level expected for the two-electron cotunneling process.

cond-mat.mes-hall↗

arXiv2010

Photon 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 (DOS) in the superconductor. In the limit of a resistive low-ohmic environment, this DOS 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.

cond-mat.supr-con↗

arXiv2009

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.

cond-mat.mes-hall↗

arXiv2009

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

Pumping characteristics were studied of the hybrid normal-metal/superconductor single-electron transistor embedded in a high-ohmic environment. Two 3 micrometer-long microstrip resistors of CrOx with a sum resistance R=80kOhm were placed adjacent to this hybrid device. Substantial improvement of pumping and reduction of the subgap leakage were observed in the low-MHz range. At higher frequencies 0.1-1GHz, a slowdown of tunneling due to the enhanced damping and electron heating negatively affected the pumping, as compared to the reference bare devices.

cond-mat.supr-con↗