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One-dimensional arrays of small-capacitance Josephson junctions exhibit a current-voltage curve that is characterized by a zero-current state for bias voltage below a threshold voltage {ital V}{sub {ital t}}. The threshold voltages can be modulated with an external magnetic field {ital B}, which tunes effective Josephson coupling between adjacent electrodes of the array. The dependance of {ital V}{sub {ital t}} on {ital B} is well explained by a model where {ital V}{sub {ital t}} is the injection voltage for a Cooper-pair charge soliton. {copyright} {ital 1996 The American Physical Society.}

We describe the fabrication and measurement of one‐dimensional series arrays of small capacitance Josephson junctions and superconducting quantum interference devices. These arrays exhibit a Coulomb blockade of Cooper pair tunneling when the Josephson coupling energy is the order of the charging energy. The effectiveness of a resistive coaxial cable (ThermocoaxTM made by Philips) for attenuation of high frequency electromagnetic noise is demonstrated. This noise influences the measured dc current–voltage characteristic of the array, having qualitatively the same effect as raising the temperature.

We report on a successful attempt to fabricate a Coulomb blockade electrometer consisting of an ultrasmall YBa2Cu3O7−δ (YBCO) island coupled to two gold electrodes through a high-resistance native surface tunnel barrier. A third electrode placed near the island was used as an electrostatic gate. Spectra typical for tunneling into the YBCO superconductor were reproducibly measured. At temperatures below 0.5 K the low-bias conductance was suppressed by the Coulomb blockade. The blockade could be periodically varied by the gate potential. An external magnetic field of up to 5 T strongly influenced the transport via the island but without any change in the period of the Coulomb oscillations.

We have experimentally investigated the higher-order correction terms for Coulomb blockade thermometry using two-dimensional arrays of small tunnel junctions and compared our results to the theory for one-dimensional arrays. We find that the correction term for the conductance dip is two times smaller, whereas the correction term for the half-width has a slightly higher value than the theoretical prediction.

We have studied the superconductor-insulator (SI) phase transition for two-dimensional (2D) arrays of small Josephson junctions in a weak magnetic field. The data were analyzed within the context of the theory of the magnetic-field-tuned SI transition in 2D superconductors. We show resistance scaling curves over several orders of magnitude for the 2D arrays. The critical exponent ${\mathit{z}}_{\mathit{B}}$ is determined to be 1.05, in good agreement with the theory. Moreover, the transverse (Hall) resistance at the critical field is found to be very small in comparison to the longitudinal resistance.

A parametric oscillator is an oscillating system in which one of the parameters, typically either the resonance frequency or damping, can be modulated by an external pump. Parametric oscillations can be found in a wide variety of systems including radiofrequency circuits, optical and mechanical systems, and even single electrons in a Penning trap. In recent years, interest in parametric oscillators has revived in many areas of physics, ranging from basic physics to applications. For instance, they are being used as quantum-limited amplifiers in an increasingly large number of experiments in quantum information and computing. At the same time, interest in their basic physics in the quantum regime, in which they are a model system for driven, nonlinear systems, has grown commensurately. This chapter gives a largely self-contained introduction to the theoretical description of the dynamics of parametric oscillators, both classical and quantum, and reviews some of the recent experimental work in superconducting circuits.

Measurements of the temperature and bias dependence of single-electron transistors (SETs) in a dilution refrigerator show that charge noise increases linearly with refrigerator temperature above a voltage-dependent threshold temperature, and that its low-temperature saturation is due to SET self-heating. We show further that the two-level fluctuators responsible for charge noise are in strong thermal contact with the electrons in the SET, which can be at a much higher temperature than the substrate. We suggest that the noise is caused by electrons tunneling between the SET metal and nearby potential wells.

We have embedded an artificial atom, a superconducting transmon qubit, in a 1D open space and investigated the scattering properties of an incident microwave coherent state. By studying the statistics of the reflected and transmitted fields, we demonstrate that the scattered states can be nonclassical. In particular, by measuring the second-order correlation function, $g^{(2)}$, we show photon antibunching in the reflected field and superbunching in the transmitted field. We also compare the elastically and inelastically scattered fields using both phase-sensitive and phase-insensitive measurements., 5 pages, 3 figures

A quantum coherent interface between optical and microwave photons can be used as a basic building block within a future quantum information network. The interface is envisioned as an ensemble of rare-earth ions coupled to a superconducting resonator, allowing for coherent transfer between optical and microwave photons. Towards this end, we have realized a hybrid device coupling a Er$^{3+}$ doped Y$_2$SiO$_5$ crystal in a superconducting coplanar waveguide cavity. We observe a collective spin coupling of 4 MHz and a spin linewdith of down to 75 MHz., Comment: 5 pages, 4 figures

Fabrication of a thermal detector designed for high resolution spectroscopy of low-energy radiation is described. Details of the micromachining of pure silicon are given including the etching of the 0.1 mm 3 absorber and the doping process to obtain the temperature sensing thermistor which is an integrated part of the absorber. The cryogenic system, the signal processing at working temperatures around 50 mK and the data acquisition system are discussed. Theoretical aspects of the intrinsic resolving power are presented. The relation between the intrinsic detector resolution and particle energy at different working temperatures is derived. Measured resistance vs temperature is shown for thermistors made with different dopings on wafers with different initial wafer resistivity. The influence of the initial wafer purity on the R - T characteristic curves is discussed. The optimal implantation dose was determined for thermistors implanted in high purity wafers.

A crystalline, silicon, microelectronic, cryogenic, thermal detector for single quanta of low energy radiation and high resolution spectroscopy is being developed. Its volume is less than 0.1 mm 3 . The constituent parts of the detector are integrated in one unit by micromachining. The dependence of the resistance of the thermistor on temperature ( R - T characteristics) has been studied experimentally. The influence of changes in the implantation dose of the thermistor on the R - T characteristics has also been studied. To obtain the best energy resolution of the detector the optimal implantation dose of the thermistor has been determined. The amplified detector pulses were fed into a Macintosh computer via an especially developed computer interface. The operation of the detector was tested by sampling gamma, X-ray and electron spectra from a 109 Cd source.

We demonstrate a high-performance cryogenic amplifier based on a radio-frequency single-electron- transistor (rf-SET). The high charge sensitivity and large bandwidth of the rf-SET, along with low power dissipation, low capacitance and on-chip integrability, make it a good candidate for a general-purpose cryogenic amplifier for high impedance sources. We measure a large-gate rf-SET with an open-loop voltage noise of 30 nV/(Hz), among the lowest reported voltage noise figures for a SET. Using a closed-loop transimpedance configuration, the amplifier shows almost 2 orders of magnitude increase in dynamic range, a 3 dB bandwidth of 30 kHz, and a transimpedance gain of 50 V/μA for a cryogenic 1 MΩ load resistor. The performance of this amplifier is already sufficient for use as an integrated readout with some types of high-performance cryogenic detectors for astrophysics.

We have embedded an artificial atom, a superconducting transmon qubit, in an open transmission line and investigated the strong scattering of incident microwave photons (∼6GHz). When an input coherent state, with an average photon number N 1 is on resonance with the artificial atom, we observe extinction of up to 99.6% in the forward propagating field. We use two-tone spectroscopy to study scattering from excited states and we observe electromagnetically induced transparency (EIT). We then use EIT to make a single-photon router, where we can control to what output port an incoming signal is delivered. The maximum on-off ratio is around 99% with a rise and fall time on the order of nanoseconds, consistent with theoretical expectations. The router can easily be extended to have multiple output ports and it can be viewed as a rudimentary quantum node, an important step towards building quantum information networks. © 2011 American Physical Society., We acknowledge financial support from the Swedish Research Council, the Wallenberg foundation and from the EU through the European Research Council and the integrated project SOLID. B. P. acknowledges CSIC JAEPREDOC2009, FIS2009-10061, and QUITEMAD S2009- ESP-1594.

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic "pinholes" in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlOx/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlOx/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current., 4 pages, 4 figures

We present measurements on hybrid single-electron turnstiles with superconducting leads contacting a normal island (SINIS). We observe Andreev tunneling of electrons influencing the current plateau characteristics of the turnstiles under radio-frequency pumping. The data is well accounted for by numerical simulations. We verify the dependence of the Andreev tunneling rate on the turnstile's charging energy. Increasing the charging energy effectively suppresses the Andreev current., 6 pages, 4 figures, 1 table Published version with various improvements to figures and text

In the same way that micro-mechanical resonators resemble guitar strings and drums, Surface Acoustic Waves (SAW) resemble the sound these instruments produce, but moving over a solid surface rather than through air. In contrast with oscillations in suspended resonators, such propagating mechanical waves have not before been studied near the quantum mechanical limits. Here, we demonstrate local probing of SAW with a displacement sensitivity of 30amRMS/sqrt(Hz) and detection sensitivity on the single-phonon level after averaging, at a frequency of 932MHz. Our probe is a piezoelectrically coupled Single Electron Transistor, which is sufficiently fast, non-destructive and localized to let us track pulses echoing back and forth in a long acoustic cavity, self-interfering and ringing the cavity up and down. We project that strong coupling to quantum circuits will allow new experiments, and hybrids utilizing the unique features of SAW. Prospects include quantum investigations of phonon-phonon interactions, and acoustic coupling to superconducting qubits, for which we present favourable estimates., Comment: 13 pages, 4 figures

We have fabricated an aluminum single-electron transistor and characterized it at frequencies up to 10 MHz by measuring the reflected signal from a resonant tank in which the transistor is embedded. We measured the charge sensitivity of this radio-frequency single-electron transistor to be 3.2×10−6 e/Hz, which corresponds to the uncoupled energy sensitivity of 4.8 ℏ. Our measurements indicate that with further improvements, the radio-frequency single-electron transistor could reach the shot-noise limit estimated to be about 1 ℏ.

We have measured the third derivative of the current-voltage characteristics, d^3I/dV^3, in a two-dimensional array of small tunnel junctions using a lock-in amplifier. We show that this derivative is zero at a voltage which scales linearly with the temperature and depends only on the temperature and natural constants, thus providing a primary thermometer. We demonstrate a measurement method which extracts the zero crossing voltage directly using a feedback circuit. This method requires only one voltage measurement, which makes it substantially faster than the original Coulomb blockade thermometry method., Comment: 3 pages, 4 figures. This article has been submitted to Applied Physics Letters (http://ojps.aip.org/aplo)

We report the observation of photon generation in a microwave cavity with a time-dependent boundary condition. Our system is a microfabricated quarter-wave coplanar waveguide cavity. The electrical length of the cavity is varied using the tunable inductance of a superconducting quantum interference device. It is measured in the quantum regime, where the temperature is significantly less than the resonance frequency (~ 5 GHz). When the length is modulated at approximately twice the static resonance frequency, spontaneous oscillations of the cavity field are observed. Time-resolved measurements of the dynamical state of the cavity show multiple stable states. The behavior is well described by theory. Connections to the dynamical Casimir effect are discussed., 5 pages, 3 Figures

We have fabricated a single Cooper-pair box (SCB) together with an on-chip lumped element resonator. By utilizing the quantum capacitance of the SCB, its state can be read out by detecting the phase of a radio-frequency (rf) signal reflected off the resonator. The resonator was optimized for fast readout. By studying quasiparticle tunneling events in the SCB, we have characterized the performance of the readout and found that we can perform a single shot parity measurement in approximately 50 ns. This is an order of magnitude faster than previously reported measurements., 7 pages, 5 figures

We have measured Current-Voltage (I-V) characteristics of a large number of two-dimensional arrays of small area Al-AlOx-Al junctions down to temperatures of 40 mK, recording the temperature dependence of the zero bias resistance R0(T). The normal state resistance of the junctions RN varies from 2 kΩ to 270 kΩ and the junction capacitance C from 0.2f F to 2.2f F. The arrays are classified into different types according to the EJ/Ec and the RQ/RN ratios, and mapped onto a parameter diagram. EJ is the Josephson coupling energy, Ec = e2/2C is the charging energy, and RQ = h/4e2 = 6.5 kΩ is the quantum resistance. For low resistance arrays, RQ/RN >> 1 with EJ/Ec > 1, the Josephson current prevails and the arrays are superconducting. When RQ/RN 0) depending on whether RN is greater than RQ or less than RQ respectively.

We have investigated, both experimentally and numerically, single electron tunneling devices of interest for current standard applications. The devices consist of long arrays of small tunnel junctions with gate electrodes capacitively coupled to electrodes inside the arrays. Experiments on arrays of 25 small aluminum tunnel junctions with a single gate electrode capacitively coupled to the center of the array (an "array turnstile") have been performed at temperatures down to 40 mK. We observe sharp steps in the I-V characteristics at currents I = ±ef when an rf-signal of frequency f is applied to the gate electrode. The sharp steps can be obtained even without any compensation for background charge. Numerical simulations of a similar device (an "array pump") are also presented. This device consists of an array with two gate electrodes where two rf signals with a phase shift of 90° may be applied. The size of the voltage steps in the I-V characteristics are investigated as a function of several parameters. For instance, the dependence of electrode capacitance, number of junctions, and background charge is investigated.

We have systematically measured the shot noise in a single electron transistor (SET) as a function of bias and gate voltages. By embedding a SET in a resonance circuit we have been able to measure its shot noise at the resonance frequency 464 MHz, where the 1/f noise is negligible. We can extract the Fano factor which varies between 0.5 and 1 depending on the amount of Coulomb blockade in the SET, in very good agreement with the theory., Comment: 4 figures

We study relaxation and dephasing in a strongly driven two-level system interacting with its environment. We develop a theory which gives a straightforward physical picture of the complex dynamics of the system in terms of dressed states. In addition to the dressing of the energy diagram, we describe the dressing of relaxation and dephasing. We find a good quantitative agreement between the theoretical calculations and measurements of a superconducting qubit driven by an intense microwave field. The competition of various processes leads to a rich structure in the observed behavior, including signatures of population inversion., 5 pages, 2 figures

We present measurements of the ac response of a single-electron box (SEB). We apply an rf signal with a frequency larger than the tunneling rate and drive the system out of equilibrium. We observe much more dissipation in the SEB then one would expect from a simple circuit model. We can explain this in terms of a mechanism that we call the Sisyphus resistance. The Sisyphus resistance has a strong gate dependence which can be used for electrometery applications., 5 pages, 3 figures

We report on measurements of the current-voltage characteristics of ultrasmall Josephson junctions. The junctions were made of either Al or Pb alloy, and the leads connecting the junctions to the outside world were high resistance thin film microstrips fabricated on the chip very close to the junction. The high frequency impedance of these leads was sufficiently large to enable the observation in a single Josephson junction of the Coulomb blockade of Cooper pair tunneling, as well as evidence for the time correlation of Cooper pair tunneling (Bloch Oscillations).

Single charge tunnel effects have been seen in ultrasmall tunnel junctions. They include the Coulomb blockade, space and time correlation of tunnel events and the effect of the environment. This paper concentrates upon the newly found single electron (SET) and Cooper pair ('Bloch') oscillations, their implications and the criteria regarding the environment that have to be fulfilled for their observation.

We have fabricated and investigated submicron-size (0.01 and 0.04 μm 2 ) Pb-alloy single Josephson junctions attached to high-resistance thin-film NiCr or Cr leads which providethe necessary conditions for a strong charging erect. When the Josephson coupling energy is less than the charging energy, we find a novel shape of the current-voltage curves, with a Coulomb gap at small voltages, and a current peak at larger voltages. A theory based on the concept of incoherent Cooper-pair tunneling has explained the data well

The authors fabricated and tested the first version of the resistively coupled single-electron transistor (R-SET) based on the correlated transfer of single electrons in ultrasmall junctions. In this version, a one-dimensional array of 80*80 nm/sup 2/ Al/Al/sub x/O/sub y//Al junctions served as a resistor for transmission of the signal to the middle electrode of a pair of similar junctions. Of three tested R-SETs, all exhibited a real power gain, and one device showed a DC voltage gain of 0.85. The data are found to be in reasonable agreement with numerical simulations based on the orthodox theory of correlated tunneling taking into account the superconductivity of aluminum electrodes.

We directly observe low-temperature non-equilibrium quasiparticle tunneling in a pair of charge qubits based on the single Cooper-pair box. We measure even- and odd-state dwell time distributions as a function of temperature, and interpret these results using a kinetic theory. While the even-state lifetime is exponentially distributed, the odd-state distribution is more heavily weighted to short times, implying that odd-to-even tunnel events are not described by a homogenous Poisson process. The mean odd-state dwell time increases sharply at low temperature, which is consistent with quasiparticles tunneling out of the island before reaching thermal equilibrium., Comment: Replaced Figure 1 with color version, corrected more typos. Version submitted to PRB

We demonstrate radio frequency single-electron transistors fabricated from epitaxially grown InAs/InP heterostructure nanowires. Two sets of double-barrier wires with different barrier thicknesses were grown. The wires were suspended 15 nm above a metal gate electrode. Electrical measurements on a high-resistance nanowire showed regularly spaced Coulomb oscillations at a gate voltage from -0.5 to at least 1.8 V. The charge sensitivity was measured to 32 microe rms Hz(-1/2) at 1.5 K. A low-resistance single-electron transistor showed regularly spaced oscillations only in a small gate-voltage region just before carrier depletion. This device had a charge sensitivity of 2.5 microe rms Hz(-1/2). At low frequencies this device showed a typical 1/f noise behavior, with a level extrapolated to 300 microe rms Hz(-1/2) at 10 Hz.

The time correlation of tunneling events has been predicted to occur in ultrasmall tunnel junctions: a periodic tunneling with a frequency determined by the current f=I/e. To test the predictions, we have fabricated one-dimensional arrays of Al/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{O}}_{\mathit{y}}$/Al tunnel junctions with areas down to 0.006 \ensuremath{\mu}${\mathrm{m}}^{2}$ corresponding to capacitances of \ensuremath{\sim}0.2 fF. Each array contained between 15 and 53 junctions. Current-voltage characteristics were measured at temperatures between 0.05 and 4.2 K. We observed phase locking of single-electron-tunneling oscillations to external microwaves in the frequency range 0.7\char21{}5 GHz. The phase locking manifested itself as peaks in the differential resistance at bias currents I=nef (n=\ifmmode\pm\else\textpm\fi{}1,\ifmmode\pm\else\textpm\fi{}2), corresponding to multiples of the microwave frequency. Numerical simulations show good agreement with our experimental results.

We study the influence of dissipation on the switching current statistics of moderately damped Josephson junctions. Different types of both low- and high-${T}_{c}$ junctions with controlled damping are studied. The damping parameter of the junctions is tuned in a wide range by changing temperature, magnetic field, and gate voltage and introducing a ferromagnetic layer or in situ capacitive shunting. A paradoxical collapse of switching current fluctuations with increasing $T$ occurs in all studied junctions. The phenomenon critically depends on dissipation in the junction and is explained by the interplay of two counteracting consequences of thermal fluctuations, which, on one hand, assist in premature switching into the resistive state and, on the other hand, help in retrapping back to the superconducting state. This is one of the rare examples of anticorrelation between temperature and the amplitude of fluctuations of a physically measurable quantity.

We have studied charge transport in a one-dimensional chain of small Josephson junctions using a single-electron transistor. We observe a crossover from time-correlated tunneling of single electrons to that of Cooper pairs as a function of both magnetic field and current. At relatively high magnetic field, single-electron transport dominates and the tunneling frequency is given by f=I/e, where I is the current through the chain and e is the electron's charge. As the magnetic field is lowered, the frequency gradually shifts to f=I/2e for I>200 fA, indicating Cooper-pair transport. For the parameters of the measured sample, we expect the Cooper-pair transport to be incoherent., 5 pages, 4 figures; v2: minor changes, clarifications, additions

In this work, we measure longitudinal dressed states of a superconducting qubit, the single Cooper-pair box, and an intense microwave field. The dressed states represent the hybridization of the qubit and photon degrees of freedom, and appear as avoided level crossings in the combined energy diagram. By embedding the circuit in an rf oscillator, we directly probe the dressed states. We measure their dressed gap as a function of photon number and microwave amplitude, finding good agreement with theory. In addition, we extract the relaxation and dephasing rates of these states., Comment: 5 pages, 3 figures

We have directly measured the thickness distribution of the tunnel barriers in state-of-the-art Al/AlOx/Al tunnel junctions. From the distribution we can conclude that less than 10% of the junction area dominates the electron tunnelling. The barriers have been studied by transmission electron microscopy, specifically using atomic resolution annular dark field (ADF) scanning transmission electron microscopy (STEM) imaging. The direct observation of the local barrier thickness shows a Gaussian distribution of the barrier thickness variation along the junction, from ~1 to ~2nm. We have investigated how the thickness distribution varies with oxygen pressure (Po) and oxidation time (to) and we find, in agreement with resistance measurements, that an increased to has a larger impact on barrier thickness and its uniformity compared to an increased Po.

The interface between the Al bottom contact layer and Si substrates in Al based Josephson junctions is believed to have a significant effect on the noise observed in Al based superconducting devices. We have studied the atomic structure of it by transmission electron microscopy. An amorphous layer with a thickness of ∼5 nm was found between the bottom Al electrode and HF-treated Si substrate. It results from intermixing between Al, Si, and O. We also studied the chemical bonding states among the different species using energy loss near edge structure. The observations are of importance for the understanding of the origin of decoherence mechanisms in qubits based on these junctions.

We report fabrication as well as proof-of-concept experiments of a noninvasive sensor of weak nanoscale electric fields. The sensor is a single electron transistor (SET) placed at the tip of a noncontact atomic force microscope (AFM). This is a general technology to make any nanometer-sized lithography pattern at edges or tips of a cantilever. The height control of the AFM allows the SET to hover a few nanometers above the substrate, improving both the electric field sensitivity and lateral resolution of the electrometer. Our AFM-SET sensor is prepared by a scalable technology. It means that the probe can be routinely fabricated and replaced, if broken.

We present the fabrication and measurement of a radio frequency single electron transistor (rf-SET), that displays a very high charge sensitivity of 1.9 microlectrons/sqrt(Hz) at 4.2 K. At 40 mK, the charge sensitivity is 0.9 and 1.0 microlectrons/sqrt(Hz) in the superconducting and normal state respectively. The sensitivity was measured as a function of radio frequency amplitude at three different temperatures: 40 mK, 1.8 K and 4.2 K., 13 pages, 4 figures

Here we present the direct observation of macroscopic quantum properties in an all high-critical-temperature superconductor d-wave Josephson junction. Although dissipation caused by low-energy excitations is expected to strongly suppress macroscopic quantum effects, we demonstrate energy level quantization in our d-wave Josephson junction. The result indicates that the role of dissipation mechanisms in high-temperature superconductors has to be revised, and it may also have consequences for the class of solid-state “quiet” quantum bits with superior coherence time.

We have fabricated and characterized a new type of electrometer that couples two parallel single‐electron transistors (SETs) to a radio‐frequency tank circuit for use as a differential RF‐SET. We demonstrate operation of this device in summing, differential, and single‐SET operation modes. In differential mode, the device is sensitive to uncorrelated input signals while screening out correlated ones.

We investigate the pheonomenon of quasiparticle poisoning in a single Cooper‐pair box (SCB). We have designed, fabricated, and tested an SCB that demonstrates a transition between poisoned and unpoisoned Coulomb staircases, depending on the speed with which the gate charge is swept. Poisoning is shown to be suppressed at moderately high sweep rates. Coulomb staircases were measured for a variety of sweep rates, and quasiparticle tunneling rates were extracted from this data.

We study thermal fluctuation phenomena in small Bi-2212 intrinsic Josephson junctions. Being able to measure switching currents of a single intrinsic junction, we observe that its statistics can be very well described by thermal activation from a periodic Josephson potential with the sinusoidal current-phase relation. This is a direct evidence for the dc-intrinsic Josephson effect and confirmation of the tunneling nature of interlayer transport in strongly anisotropic high-temperature superconductors. Furthermore, the fluctuation-free critical current, extracted from the analysis of switching current statistics, exhibits a temperature dependence typical for superconductor-insulator-superconductor tunnel junctions.

We have fabricated and characterized a new type of electrometer that couples two parallel single-electron transistors (SETs) to a radio-frequency tank circuit for use as a differential RF-SET. We demonstrate operation of this device in summing, differential, and single-SET operation modes, and use it to measure a Coulomb staircase from a differential single Cooper-pair box. In differential mode, the device is sensitive to uncorrelated input signals while screening out correlated ones., 3 pages, 3 figures, submitted to Applied Physics Letters

We report an experimental observation of the backaction of a single-electron transistor (SET) measuring the Coulomb staircase of a single-electron box. As current flows through the SET, the charge state of the SET island fluctuates. These fluctuations capacitively couple to the box and cause changes in the position, width, and asymmetry of the Coulomb staircase. A sequential tunneling model accurately recreates these effects, confirming this mechanism of the backaction of a SET. This is a first step toward understanding the effects of quantum measurement on solid-state qubits.

We have fabricated a Cooper-pair transistor (CPT) with parameters such that for appropriate voltage biases, the sub-gap charge transport takes place via slow tunneling of quasiparticles that link two Josephson-coupled charge manifolds. In between the quasiparticle tunneling events, the CPT behaves essentially like a single Cooper-pair box (SCB). The effective capacitance of a SCB can be defined as the derivative of the induced charge with respect to gate voltage. This capacitance has two parts, the geometric capacitance, C_geom, and the quantum capacitance C_Q. The latter is due to the level anti-crossing caused by the Josephson coupling. It depends parametrically on the gate voltage and is dual to the Josephson inductance. Furthermore, it's magnitude may be substantially larger than C_geom. We have been able to detect C_Q in our CPT, by measuring the in-phase and quadrature rf-signal reflected from a resonant circuit in which the CPT is embedded. C_Q can be used as the basis of a charge qubit readout by placing a Cooper-pair box in such a resonant circuit., Comment: 3 figures

The escape rate from the zero voltage state in a superconducting Josephson junction (JJ) is determined by the temperature, but it saturates at low temperature due to macroscopic quantum tunneling (MQT). Complications due to $d$-wave symmetry in a high temperature superconductor, like low energy quasiparticles and an unconventional current-phase relation, may influence the escape rate. We report, for the first time to our knowledge, the observation of MQT in a ${\mathrm{Y}\mathrm{B}\mathrm{a}}_{2}{\mathrm{C}\mathrm{u}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ grain boundary biepitaxial JJ. This proves that dissipation can be significantly reduced by a proper junction configuration, which is of significance for quantum coherence.

We study switching current statistics in different moderately damped Josephson junctions: a paradoxical collapse of the thermal activation with increasing temperature is reported and explained by interplay of two conflicting consequences of thermal fluctuations, which can both assist in premature escape and help in retrapping back into the stationary state. We analyze the influence of dissipation on the thermal escape by tuning the damping parameter with a gate voltage, magnetic field, temperature and an in-situ capacitor., Comment: 4 pages, 4 figures

We have measured the fully 2e periodic Coulomb staircase of a single Cooper pair box (SCB) in superconducting quantum interference design geometry, using a radio-frequency single-electron transistor. We have determined the energies of the SCB with microwave spectroscopy and compared the calculated shape of the Coulomb staircases to the measured staircases. We find excellent agreement as the Josephson coupling energy is tuned by an external magnetic field.

Superconductor-Ferromagnet-Superconductor (S-F-S) Josephson junctions were fabricated by making a narrow cut through a S-F double layer using direct writing by Focused Ion Beam (FIB). Due to a high resolution (spot size smaller than 10 nm) of FIB, junctions with a small separation between superconducting electrodes ($\leq$ 30 nm) can be made. Such a short distance is sufficient for achieving a considerable proximity coupling through a diluted CuNi ferromagnet. We have successfully fabricated and studied S-F-S (Nb-CuNi-Nb) and S-S'-S (Nb-Nb/CuNi-Nb) junctions. Junctions exhibit clear Fraunhofer modulation of the critical current as a function of magnetic field, indicating good uniformity of the cut. By changing the depth of the cut, junctions with the $I_c R_n$ product ranging from 0.5 mV to $\sim 1\mu $V were fabricated., Comment: 5 pages, 5 figures, presentation at EUCAS-2003, to be published in Physica C