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We demonstrate significant cooling of electrons in a nanostructure below 10mK by demagnetisation of thin-film copper on a silicon chip. Our approach overcomes the typical bottleneck of weak electron-phonon scattering by coupling the electrons directly to a bath of refrigerated nuclei, rather than cooling via phonons in the host lattice. Consequently, weak electron-phonon scattering becomes an advantage. It allows the electrons to be cooled for an experimentally useful period of time to temperatures colder than the dilution refrigerator platform, the incoming electrical connections, and the host lattice. There are efforts worldwide to reach sub-millikelvin electron temperatures in nanostructures to study coherent electronic phenomena and improve the operation of nanoelectronic devices. On-chip magnetic cooling is a promising approach to meet this challenge. The method can be used to reach low, local electron temperatures in other nanostructures, obviating the need to adapt traditional, large demagnetisation stages. We demonstrate the technique by applying it to a nanoelectronic primary thermometer that measures its internal electron temperature. Using an optimised demagnetisation process, we demonstrate cooling of the on-chip electrons from 9mK to below 5mK for over 1000 seconds.

In microwave engineering we are accustomed to thinking of the electromagnetic energy in our circuits as transmitted by waves. Now, new technologies are being developed that deal with signals at the level of single photons where this is no longer valid. Here we describe some of the challenges and opportunities in this rapidly developing field.

We present measurements of nanoelectronic Coulomb Blockade Thermometers that are optimised for operation below 10 mK. Their design incorporates on-chip electronic filters and cooling fins with a high electron-phonon coupling. By immersing the devices in the 3He/4He mixture of a dilution refrigerator, and by minimising electrical noise in the measurement circuit, the on-chip electron temperature reaches a value of 3.7 mK, the lowest yet measured in any nanoelectronic device. Above 7 mK the devices are in good thermal contact with their environment and are not susceptible to self-heating. Below 7 mK the device continues to provide accurate thermometry of the on-chip electron temperature, which is seen to diverge from the ambient temperature. In this regime the device provides valuable information about noise and heat-leaks from the environment, which points the way towards cooling nanoelectronic structures to lower temperatures.

Cooling nanoelectronic structures to millikelvin temperatures presents extreme challenges in maintaining thermal contact between the electrons in the device and an external cold bath. It is typically found that when nanoscale devices are cooled to ~10 mK the electrons are significantly overheated. Here we report the cooling of electrons in nanoelectronic Coulomb blockade thermometers below 4 mK. The low operating temperature is attributed to an optimized design that incorporates cooling fins with a high electron–phonon coupling and on-chip electronic filters, combined with low-noise electronic measurements. By immersing a Coulomb blockade thermometer in the 3He/4He refrigerant of a dilution refrigerator, we measure a lowest electron temperature of 3.7 mK and a trend to a saturated electron temperature approaching 3 mK. This work demonstrates how nanoelectronic samples can be cooled further into the low-millikelvin range.

As a step toward realizing the quantum current standard, single-electron pumping of parallel SINIS turnstiles was performed. Eleven SINIS turnstiles were fabricated on the same chip, and the number of operating SINIS turnstiles was increased one by one. As a result, 11 turnstiles were operated simultaneously up to 100 MHz, resulting in a pumped dc of 176.2 pA. We also did a linearity check toward parallelization. This paper for increasing the pumped current by parallelization will lead to practical use of stable and accurate dc sources with tiny values of the output current and the metrology triangle experiment for SI redefinition.

Real-time detection and generation of single microwave photons would be important in many quantum technology applications. For single-microwave-photon sources, much of the groundwork has been done already within the framework of circuit quantum electrodynamics (cQED) in the frequency range 4 GHz – 8 GHz. However, currently there are no detectors that can reliably resolve single microwave photon events, unlike at optical frequencies. In June 2013, we started a joint research project to develop both microwave detectors and sources working at the single-photon level as a final goal. All devices operate at cryogenic temperatures, most of them below 100 mK. We also aim at improving the performance of other cryoelectronic quantum devices by understanding and eliminating the detrimental effects caused by microwave radiation. The work is done in project MICROPHOTON of the European Metrology Research Programme (EMRP). The goals and status of the project will be described in the presentation.

Single electron pumping by a SINIS turnstile is investigated up to 1 GHz in various conditions to realize a quantum current standard. A standard deviation of the pumping current at 100 MHz is about 10-4 at base temperature of a dilution refrigerator. As expected, the plateau of the pumped current degrades as temperature increases because of the thermal smearing of Fermi energy of a normal metal island. On the other hand, an externally applied magnetic fields dramatically suppresses excess current on the single electron pumping plateau. This originates from the quasiparticle relaxation enhancement due to the penetration of the magnetic vortices into the superconducting leads.

A quantum electric current standard is the last missing piece of the quantum metrology triangle. A suitable candidate for realizing this quantum current standard is a single-electron pumping device based on a superconductor/insulator/normal metal/insulator/superconductor (SINIS) turnstile. Here, we show the temperature dependence of the single-electron pumped current of a SINIS turnstile. As the operating temperatures are increased, the current plateaus of the single-electron pumping tilt because of increased back tunneling, which originates from the thermal broadening of energy levels in the normal metal island of the SINIS turnstile. Hence, we find that to use a SINIS turnstile for the current standard at more than 300 mK, it is necessary to increase the superconducting energy gaps of the lead material and the charging energy of the island.

In quantum information processing, knowledge of the noise in the system is crucial for high-precision manipulation and tomography of coherent quantum operations. Existing strategies for identifying this noise require the use of additional quantum devices or control pulses. We present a noise-identification method directly based on the system's non-Markovian response of an ensemble measurement to the noise. The noise spectrum is identified by reversing the response relationship in the frequency domain. For illustration, the method is applied to superconducting charge qubits, but it is equally applicable to any type of qubits. We find that the identification strategy recovers the well-known Fermi's golden rule under the lowest-order perturbation approximation, which corresponds to the Markovian limit when the measurement time is much longer than the noise correlation time. Beyond such approximation, it is possible to further improve the precision at the so-called optimal point by incorporating the transient response data in the non-Markovian regime. This method is verified with experimental data from coherent oscillations in a superconducting charge qubit.

The control electrons at the level of the elementary charge e was demonstrated experimentally already in the 1980s. Ever since, the production of an electrical current ef, or its integer multiple, at a drive frequency f has been in a focus of research for metrological purposes.This review discusses the generic physical phenomena and technical constraints that influence single-electron charge transport and presents a 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. The important issues of readout of singleelectron events and potential error correction schemes based on them are also discussed. Finally, an account is given 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 applications and uses of single-electron devices outside the metrological context are briefly discussed.

The control electrons at the level of the elementary charge e was demonstrated experimentally already in the 1980s. Ever since, the production of an electrical current ef, or its integer multiple, at a drive frequency f has been in a focus of research for metrological purposes.This review discusses the generic physical phenomena and technical constraints that influence single-electron charge transport and presents a 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. The important issues of readout of singleelectron events and potential error correction schemes based on them are also discussed. Finally, an account is given 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 applications and uses of single-electron devices outside the metrological context are briefly discussed.

We demonstrate nonadiabatic charge pumping utilizing a sequence of coherent oscillations between a superconducting island and two reservoirs. The pumping rate for each elementary cycle is limited by the coupling between the island and the reservoirs given by the Josephson energy. Our experimental and theoretical studies show that relaxation can be employed to reset the pump in order to avoid accumulation of errors due to nonideal control pulses. Thus our results demonstrate the effects of nonadiabatic quantum pumping and dissipation.

We review the current state of research into the development of nanomechanical resonators and their application as components of nanoelectromechanical and optomechanical systems for the highly sensitive detection and visualization of material structure. Methods for the manufacture and characterization of nanoscale mechanical resonators are described. We discuss quantum properties of nanomechanical resonators and dynamical cooling schemes for suppressing fluctuations in order to increase the threshold sensitivity of optomechanical and electromechanical systems.

A hundred years after the discovery of superconductivity, one fundamental prediction of the theory, coherent quantum phase slip (CQPS), has not been observed. CQPS is a phenomenon exactly dual(1) to the Josephson effect; whereas the latter is a coherent transfer of charges between superconducting leads(2,3), the former is a coherent transfer of vortices or fluxes across a superconducting wire. In contrast to previously reported observations(4-8) of incoherent phase slip, CQPS has been only a subject of theoretical study(9-12). Its experimental demonstration is made difficult by quasiparticle dissipation due to gapless excitations in nanowires or in vortex cores. This difficulty might be overcome by using certain strongly disordered superconductors near the superconductor-insulator transition. Here we report direct observation of CQPS in a narrow segment of a superconducting loop made of strongly disordered indium oxide; the effect is made manifest through the superposition of quantum states with different numbers of flux quanta(13). As with the Josephson effect, our observation should lead to new applications in superconducting electronics and quantum metrology(1,10,11).

We study dynamics of a two-level superconducting quantum system, analogous to a natural atom in an open space, by measuring the evolution of its coherent and incoherent emission. The emitted waves containing full information about the states of the artificial atom are efficiently collected due to strong atom-transmission-line coupling. This allows us to do simultaneous measurements of all the quantum state projections and perform a full characterization of the system. We derive coherence times and extract the two-time correlation function from the dynamics of the coherent emission.

We have investigated charge transport in ultrasmall superconducting single and double Josephson junctions coupled to resonant modes of the electromagnetic environment. We observe pronounced current peaks in the transport characteristics of both types of devices and attribute them to the process involving simultaneous tunneling of Cooper pairs and photon emission into the resonant modes. The experimental data are well reproduced with the theoretical models.

We provide a direct proof of two-electron Andreev transitions in a superconductor-normal-metal tunnel junction by detecting them in a real-time electron counting experiment. Our results are consistent with ballistic Andreev transport with an order of magnitude higher rate than expected for a uniform barrier, suggesting that only part of the interface is effectively contributing to the transport. These findings are quantitatively supported by our direct current measurements in single-electron transistors with similar tunnel barriers.

An atom in open space can be detected by means of resonant absorption and reemission of electromagnetic waves, known as resonance fluorescence, which is a fundamental phenomenon of quantum optics. We report on the observation of scattering of propagating waves by a single artificial atom. The behavior of the artificial atom, a superconducting macroscopic two-level system, is in a quantitative agreement with the predictions of quantum optics for a pointlike scatterer interacting with the electromagnetic field in one-dimensional open space. The strong atom-field interaction as revealed in a high degree of extinction of propagating waves will allow applications of controllable artificial atoms in quantum optics and photonics.

We have suspended an Al based single-electron transistor (SET) whose island can resonate freely between the source and drain leads forming the clamps. In addition to the regular side gate, a bottom gate with a larger capacitance to the SET island is placed underneath to increase the SET coupling to mechanical motion. The device can be considered as a doubly clamped Al beam that can transduce mechanical vibrations into variations in the SET current. Our simulations based on the orthodox model, with the SET parameters estimated from the experiment, reproduce the observed transport characteristics in detail.

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.

We present experimental observation of electromagnetically induced transparency (EIT) on a single macroscopic artificial "atom'' (superconducting quantum system) coupled to open 1D space of a transmission line. Unlike in an optical media with many atoms, the single-atom EIT in 1D space is revealed in suppression of reflection of electromagnetic waves, rather than absorption. The observed almost 100% modulation of the reflection and transmission of propagating microwaves demonstrates full controllability of individual artificial atoms and a possibility to manipulate the atomic states. The system can be used as a switchable mirror of microwaves and opens a good perspective for its applications in photonic quantum information processing and other fields.

We report amplification of electromagnetic waves by a single artificial atom in open 1D space. Our three-level artificial atom-a superconducting quantum circuit-coupled to a transmission line presents an analog of a natural atom in open space. The system is the most fundamental quantum amplifier whose gain is limited by a spontaneous emission mechanism. The noise performance is determined by the quantum noise revealed in the spectrum of spontaneous emission, also characterized in our experiments.

We use tunneling spectroscopy in Al and Nb based superconductor-insulator-superconductor (SIS) junctions to demonstrate the existence of metallic quasiparticle states at the interface of a superconducting electrode and an oxide layer, introducing a new method to directly evaluate the interface quasiparticle areal density of states. Current-voltage (I-V) characteristics typically observed in metal-insulator-superconductor (MIS) junctions are observed in the subgap voltage region. The turn-on voltage of the MIS-type leakage is found to be determined by the superconducting electrode with a smaller gap energy (Delta), and its magnitude depends on the nature of the interface. Our experiment suggests that the interface plays a key role in contributing to the subgap leakage influencing the performance of superconducting circuits.

We have studied damping in polycrystalline Al nanomechanical resonators by measuring the temperature dependence of their resonance frequency and quality factor over a temperature range of 0.1-4 K. Two regimes are clearly distinguished with a crossover temperature of 1 K. Below 1 K we observe a logarithmic temperature dependence of the frequency and linear dependence of damping that cannot be explained by the existing standard models. We attribute these phenomena to the effect of the two-level systems characterized by the unexpectedly long (at least two orders of magnitude longer) relaxation times and discuss possible microscopic models for such systems. We conclude that the dynamics of the two-level systems is dominated by their interaction with one-dimensional phonon modes of the resonators.

We have fabricated an Al based single-electron transistor transistor (SET) whose island is suspended above the substrate. The structure can be considered as a two-in-one device containing a doubly clamped beam and a transducer that converts mechanical vibrations into variations in the SET current. In addition to the regular side gate, a bottom gate with a large capacitance, is placed beneath the SET island for increasing the SET coupling to the mechanical motion. The observed response is reproduced in our simulations, which are based on the orthodox model that considers the fundamental flexural mode of the resonating SET island.

We have performed transport measurements on Al and Nb based metal-oxide-metal tunnel junctions with an AlOx tunnel barrier and found a strong dependence of the effective potential barrier height on the oxide-metal interface properties. Our estimations of the barrier height based on a phenomenological Simmons' model are consistent with the values obtained from the first-principle calculations. The calculations clearly show that the barrier height is strongly affected by the formation of metal induced gap states originating from the hybridization between metallic bands and Al2O3 conduction band. These findings are important for nanoelectronic devices containing tunnel junctions with a thin insulating layer.

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.

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.

We present the simultaneous operation of ten single-electron turnstiles leading to one order of magnitude increase in current level up to 100 pA. Our analysis of device uniformity and background charge stability implies that the parallelization can be made without compromising the strict requirements of accuracy and current level set by quantum metrology. In addition, we discuss how offset charge instability limits the integration scale of single-electron turnstiles.

We show that the barrier profile of in situ grown AlOx tunnel barriers strongly depends on the material choices of the oxide-metal interface. By doing transport measurements on Al and Nb-based metal-oxide-metal tunnel junctions in a wide temperature range and using the phenomenological Simmons' model, we obtain barrier parameters that are qualitatively consistent with the values obtained from the first-principles calculations. The latter suggest that the formation of metal-induced gap states originating from the hybridization between the metallic bands and Al2O3 conduction band is responsible for the tunnel barrier modification. These findings are important for nanoelectronic devices containing tunnel junctions with a thin insulating layer.

We demonstrate electronic cooling of a suspended AuPd island using superconductor-insulator-normal metal tunnel junctions. This was achieved by developing a simple fabrication method for reliably releasing narrow submicron-sized metal beams. The process is based on reactive ion etching and uses a conducting substrate to avoid charge-up damage and is compatible with, e.g., conventional e-beam lithography, shadow-angle metal deposition, and oxide tunnel junctions. The devices function well and exhibit clear cooling, up to a factor of 2 at sub-Kelvin temperatures.

The field of solid-state quantum computation is expanding rapidly initiated by our original charge qubit demonstrations. Various types of solid-state qubits are being studied, and their coherent properties are improving. The goal of this review is to summarize achievements on Josephson charge qubits. We cover the results obtained in our joint group of NEC Nano Electronics Research Laboratories and RIKEN Advanced Science Institute, also referring to the works done by other groups. Starting from a short introduction, we describe the principle of the Josephson charge qubit, its manipulation and readout. We proceed with coupling of two charge qubits and implementation of a logic gate. We also discuss decoherence issues. Finally, we show how a charge qubit can be used as an artificial atom coupled to a resonator to demonstrate lasing action.

The field of solid-state quantum computation is expanding rapidly initiated by our original charge qubit demonstrations. Various types of solid-state qubits are being studied, and their coherent properties are improving. The goal of this review is to summarize achievements on Josephson charge qubits. We cover the results obtained in our joint group of NEC Nano Electronics Research Laboratories and RIKEN Advanced Science Institute, also referring to the works done by other groups. Starting from a short introduction, we describe the principle of the Josephson charge qubit, its manipulation and readout. We proceed with coupling of two charge qubits and implementation of a logic gate. We also discuss decoherence issues. Finally, we show how a charge qubit can be used as an artificial atom coupled to a resonator to demonstrate lasing action.

We have designed and experimentally implemented a circuit of inductively coupled superconducting charge qubits, where a Josephson junction is used as an inductance, and the coupling between the qubits is controlled by an applied magnetic flux. Spectroscopic measurements on the circuit are in good agreement with theoretical calculations. We observed anticrossings which originate from the coupling between the qubit and the plasma mode of the Josephson junction. Moreover, the size of the anticrossing depends on the external magnetic flux, which demonstrates the controllability of the coupling.

We developed a technology to fabricate fully metallic doubly clamped beams working as nanomechanical resonators. Measured with a magnetomotive detection scheme, the beams, made of polycrystalline metal films, show as good quality as previously reported ones made of single crystal materials, such as Si, GaAs, AlN, and SiC. Our method is compatible with the conventional fabrication process for nanoscale electronic circuits and thus offers a possibility of easily integrating the beams into superconducting charge and flux qubits and single-electron transistors as well as coupling them to coplanar waveguide resonators. (C) 2008 American Institute of Physics.

We have experimentally studied a superconducting flux qubit strongly coupled to a single mode of a high-quality transmission-type coplanar-waveguide resonator. The qubit-resonator coupling is revealed in the resonator transmission spectrum as a vacuum Rabi splitting. In the dispersive regime the qubit energy levels are spectroscopically probed. We observe a shift in the qubit transition frequency that linearly depends on the number of photons in the resonator, which is attributed to the ac-Zeeman shift. The observations are in agreement with the theoretical predictions.

The authors present a reliable process that allows them to fabricate all-niobium nanodevices in a reproducible and well controlled way. Deposition of niobium thin films is done through a suspended mask formed in a Ge layer supported by a thermally stable polymer called phenylene-ether-sulfone, while the tunnel barrier is formed by ion beam oxidation of Nb surface. They study the effect of the fabrication conditions on the quality of narrow Nb stripes by measuring their transition temperature. Using dc transport measurements, they characterize tunnel junctions as well as single-electron transistors (SETs) and extract basic junction parameters, such as the barrier height, width, and specific capacitance. Low-frequency charge noise of all-Nb SETs at 50mK50mK has a 1∕f1∕f power spectrum and the value of ∼4×10−3e/Hz1∕2∼4×10−3e∕Hz1∕2 at 1Hz1Hz.

Two different types of Cooper pair transistors, with Al and Nb islands, have been investigated in a tunable electromagnetic environment. The device with an Al island demonstrates gate charge modulation with 2e periodicity in a wide range of environmental impedances at bath temperatures below 340 mK. Contrary to the results of the Al sample, the authors were not able to detect 2e periodicity under any conditions on similar samples with Nb island. The authors attribute this to the material properties of Nb.

The authors have developed a fabrication method for suspended metallic single-electron transistors SETs utilizing a combination of conventional angle evaporation technique and ashing of the underlying organic polymer. The authors’ Al-based suspended devices exhibit clear Coulomb blockade effects typical for conventional SETs. The measured low-frequency charge noise is rather low but still within the range reported for conventional Al devices. We suggest that the noise level can be further reduced by decreasing the effective SET temperature.

We have fabricated ultrasmall all-Nb tunnel junctions and single electron transistors using shadow evaporation combined with in situ ion gun oxidation. Basic parameters of the Nb/Nb-oxide/Nb junctions, namely, the barrier height, width, and specific junction capacitance, are estimated from the transport characteristics.

We propose a novel method for suppression of quasiparticle poisoning in Al Coulomb blockade devices. The method is based on creation of a proper energy gap profile along the device. In contrast to the previously used techniques, the energy gap is controlled by the film thickness. Our transport measurements confirm that the quasiparticle poisoning is suppressed and clear 2e periodicity is observed only when the island is made much thinner than the leads. This result is consistent with the existing model and provides a simple method to suppress quasiparticle poisoning.

To verify the hypothesis about the common origin of the low-frequency 1 / f noise and the quantum f noise recently measured in the Josephson charge qubits, we study the temperature dependence of the 1 / f noise and decay of coherent oscillations. The T 2 dependence of the 1 / f noise is experimentally demonstrated, which supports the hypothesis. We also show that dephasing in the Josephson charge qubits off the electrostatic energy degeneracy point is consistently explained by the same low-frequency 1 / f noise that is observed in the transport measurements.

We demonstrate our recent experiments on Josephson charge qubits: Coherent manipulation of two electrostatically coupled qubits, controlled-NOT operation, single-shot readout, and study of decoherence mechanisms with help of the single-shot readout.

We have analyzed and measured the quantum coherent dynamics of a circuit containing two-coupled superconducting charge qubits. Each qubit is based on a Cooper pair box connected to a reservoir electrode through a Josephson junction. Two qubits are coupled electrostatically by a small island overlapping both Cooper pair boxes. Quantum state manipulation of the qubit circuit is done by applying non-adiabatic voltage pulses to the common gate. We read out each qubit by means of probe electrodes connected to Cooper pair boxes through high-Ohmic tunnel junctions. With such a setup, the measured pulse-induced probe currents are proportional to the probability for each qubit to have an extra Cooper pair after the manipulation. As expected from theory and observed experimentally, the measured pulse-induced current in each probe has two frequency components whose position on the frequency axis can be externally controlled. This is a result of the inter-qubit coupling which is also responsible for the avoided level crossing that we observed in the qubits’ spectra. Our simulations show that in the absence of decoherence and with a rectangular pulse shape, the system remains entangled most of the time reaching maximally entangled states at certain instances.