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We introduce a simplified fabrication technique for Josephson junctions and demonstrate superconducting Xmon qubits with $T_1$ relaxation times averaging above 50$~��$s ($Q>$1.5$\times$ 10$^6$). Current shadow-evaporation techniques for aluminum-based Josephson junctions require a separate lithography step to deposit a patch that makes a galvanic, superconducting connection between the junction electrodes and the circuit wiring layer. The patch connection eliminates parasitic junctions, which otherwise contribute significantly to dielectric loss. In our patch-integrated cross-type (PICT) junction technique, we use one lithography step and one vacuum cycle to evaporate both the junction electrodes and the patch. In a study of more than 3600 junctions, we show an average resistance variation of 3.7$\%$ on a wafer that contains forty 0.5$\times$0.5-cm$^2$ chips, with junction areas ranging between 0.01 and 0.16 $��$m$^2$. The average on-chip spread in resistance is 2.7$\%$, with 20 chips varying between 1.4 and 2$\%$. For the junction sizes used for transmon qubits, we deduce a wafer-level transition-frequency variation of 1.7-2.5$\%$. We show that 60-70$\%$ of this variation is attributed to junction-area fluctuations, while the rest is caused by tunnel-junction inhomogeneity. Such high frequency predictability is a requirement for scaling-up the number of qubits in a quantum computer., 6 pages, 4 figures

We report the observation of Electromagnetically Induced Transparency (EIT) of a mechanical field, where a superconducting artificial atom is coupled to a 1D-transmission line for surface acoustic waves. An electromagnetic microwave drive is used as the control field, rendering the superconducting transmon qubit transparent to the acoustic probe beam. The strong frequency dependence of the acoustic coupling enables EIT in a ladder configuration due to the suppressed relaxation of the upper level. Our results show that superconducting circuits can be engineered to interact with acoustic fields in parameter regimes not readily accessible to purely electromagnetic systems.

The ability to control and measure the temperature of propagating microwave modes down to very low temperatures is indispensable for quantum information processing, and may open opportunities for studies of heat transport at the nanoscale, also in the quantum regime. Here we propose and experimentally demonstrate primary thermometry of propagating microwaves using a transmon-type superconducting circuit. Our device operates continuously, with a sensitivity down to $4\times 10^{-4}$ photons/$\sqrt{\mbox{Hz}}$ and a bandwidth of 40 MHz. We measure the thermal occupation of the modes of a highly attenuated coaxial cable in a range of 0.001 to 0.4 thermal photons, corresponding to a temperature range from 35 mK to 210 mK at a frequency around 5 GHz. To increase the radiation temperature in a controlled fashion, we either inject calibrated, wideband digital noise, or heat the device and its environment. This thermometry scheme can find applications in benchmarking and characterization of cryogenic microwave setups, temperature measurements in hybrid quantum systems, and quantum thermodynamics.

We numerically and experimentally investigate the phononic loss for superconducting resonators fabricated on a piezoelectric substrate. With the help of finite element method simulations, we calculate the energy loss due to electromechanical conversion into bulk and surface acoustic waves. This sets an upper limit for the resonator internal quality factor Q i. To validate the simulation, we fabricate quarter wavelength coplanar waveguide resonators on GaAs and measure Q i as function of frequency, power and temperature. We observe a linear increase of Q i with frequency, as predicted by the simulations for a constant electromechanical coupling. Additionally, Q i shows a weak power dependence and a negligible temperature dependence around 10 mK, excluding two level systems and non-equilibrium quasiparticles as the main source of losses at that temperature.

We explore routing of propagating phonons in analogy with previous experiments on photons. Surface acoustic waves (SAWs) in the microwave regime are scattered by a superconducting transmon qubit. The transmon can be tuned on or off resonance with the incident SAW field using an external magnetic field or the Autler-Townes effect, and thus the reflection and transmission of the SAW field can be controlled in time. We observe 80% extinction in the transmission of the low power continuous signal and a 40 ns rise time of the router. The slow propagation speed of SAWs on solid surfaces allows for in-flight manipulations of the propagating phonons. The ability to route short, 100 ns, pulses enables new functionality, for instance to catch an acoustic phonon between two qubits and then release it in a controlled direction., 10 pages, 11 figures put into 4 figures, 1 table

Today, surface acoustic waves (SAWs) and bulk acoustic waves are already two of the very few phononic technologies of industrial relevance and can been found in a myriad of devices employing these nanoscale earthquakes on a chip. Acoustic radio frequency filters, for instance, are integral parts of wireless devices. SAWs in particular find applications in life sciences and microfluidics for sensing and mixing of tiny amounts of liquids. In addition to this continuously growing number of applications, SAWs are ideally suited to probe and control elementary excitations in condensed matter at the limit of single quantum excitations. Even collective excitations, classical or quantum are nowadays coherently interfaced by SAWs. This wide, highly diverse, interdisciplinary and continuously expanding spectrum literally unites advanced sensing and manipulation applications. Remarkably, SAW technology is inherently multiscale and spans from single atomic or nanoscopic units up even to the millimeter scale. The aim of this Roadmap is to present a snapshot of the present state of surface acoustic wave science and technology in 2019 and provide an opinion on the challenges and opportunities that the future holds from a group of renown experts, covering the interdisciplinary key areas, ranging from fundamental quantum effects to practical applications of acoustic devices in life science.

We present a comprehensive study of internal quality factors in superconducting stub-geometry three-dimensional cavities made of aluminum. We use wet etching, annealing, and electrochemical polishing to improve the as machined quality factor. We find that the dominant loss channel is split between the two-level system loss and an unknown source with a 40:60 proportion. A total of 17 cavities of different purity, resonance frequency, and size were studied. Our treatment results in reproducible cavities, with 10 of them showing internal quality factors above 80 x 10(6) at a power corresponding to an average of a single photon in the cavity. The best cavity has an internal quality factor of 115 x 10(6) at a single photon level. (C) 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:// creativecommons.org/licenses/by/4.0/).

We present an experimental demonstration as well as a theoretical model of an integrated circuit designed for the manipulation of a microwave field down to the single-photon level. The device is made of a superconducting resonator coupled to a transmission line via a second frequency-tunable resonator. The tunable resonator can be used as a tunable coupler between the fixed resonator and the transmission line. Moreover, the manipulation of the microwave field between the two resonators is possible. In particular, we demonstrate the swapping of the field from one resonator to the other by pulsing the frequency detuning between the two resonators. The behavior of the system, which determines how the device can be operated, is analyzed as a function of one key parameter of the system, the damping ratio of the coupled resonators. We show a good agreement between experiments and simulations, realized by solving a set of coupled differential equations., 10 pages, 6 figures

Entangled pairs of microwave photons are commonly produced in the narrow frequency band of a resonator, which represents a modified vacuum density of states. We generate and investigate the entanglement of a stream of photon pairs, generated in a semi-infinite broadband transmission line, terminated by a superconducting quantum interference device (SQUID). A weak pump signal modulates the SQUID inductance, resulting in a single time-varying boundary condition, and we detect all four quadratures of the microwave radiation emitted at two different frequencies separated by 0.7 GHz. Power calibration is done in situ, and we find positive logarithmic negativity and two-mode squeezing below the vacuum in the observed radiation, indicating entanglement.

Quantum information can be stored in micromechanical resonators, encoded as quanta of vibration known as phonons. The vibrational motion is then restricted to the stationary eigenmodes of the resonator, which thus serves as local storage for phonons. In contrast, we couple propagating phonons to an artificial atom in the quantum regime, and reproduce findings from quantum optics with sound taking over the role of light. Our results highlight the similarities between phonons and photons, but also point to new opportunities arising from the unique features of quantum mechanical sound. The low propagation speed of phonons should enable new dynamic schemes for processing quantum information, and the short wavelength allows regimes of atomic physics to be explored which cannot be reached in photonic systems., Comment: 30 pages, 6 figures, 1 table

We propose two different setups to generate single photons on demand using an atom in front of a mirror, along with either a beam-splitter or a tunable coupling. We show that photon generation efficiency ~99% is straightforward to achieve. The proposed schemes are simple and easily tunable in frequency. The operation is relatively insensitive to dephasing and can be easily extended to generate correlated pairs of photons. They can also in principle be used to generate any photonic qubit of the form $\mu |0 \rangle + \nu |1\rangle$ in arbitrary wave-packets, making them very attractive for quantum communication applications., Comment: 10 pages, Added appendix

The design, simulation and measurements of an on-chip Mach-Zehnder interferometer operating in the microwave regime are described. Using microwave signals in microfabricated superconducting Al waveguides, the concept of an interferometer is transferred from optics to on-chip. Tuning of the path length of one of the interferometer arms is executed through the tunable inductance of a SQUID. By placing one or more SQUIDs in the waveguide structure and by varying the magnetic flux through the SQUID loop, the total SQUID inductance can be tuned. In this way, a phase difference leading to destructive or constructive interference at the interferometer output can be achieved. Thorough software simulations were performed to determine the different design parameters, assign a desired working frequency and provide a reference for comparison with experimental results. Measurements at 300 mK show an effective working frequency close to the simulations, with a deviation smaller than 0.05 GHz. The behavior of the interferometer is very similar to the simulations as well. The on-off ratio exceeds 40 dB.

The optimum speed and sensitivity of a single-electron transistor (SET) depend crucially on the resistance-capacitance (RC) product of the tunnel junctions. We present a fabrication method of aluminum single-electron transistors with a high percentage of working devices (80%) and record low RC products: SETs with a typical charging energy of 15K and a resistance of 100kΩ. The oxygen pressure during junction oxidation was very low, 8∗10−4mBar, which resulted in devices with a high cut-off frequency (up to 40GHz). The devices were characterized at 4.2K and at 90mK. The noise was typical for an aluminum single-electron transistor (2.5∗10−4e∕Hz), and the gain (dID∕dQg) was high (54.5nA∕e).

We have analyzed six different cryogenic filters for applications in single electronics. Each filter is evaluated with respect to its construction, cryogenic compatibility, principle of operation, and performance. The filters were measured at 4.2 K in a 50 Ω environment between 20 kHz and 8 GHz. We have also combined some of the best properties of these filters into a single, highly effective filter that covers the entire frequency spectrum.

We present numerical results on the current mirror effect in a single-electron turnstile capacitively coupled with a one-dimensional (1D) array of small tunnel junctions. Contrary to conventional operation with a sinusoidal signal source, the single-electron turnstile is driven by charge solitons (and anti-solitons) transferred through the 1D array. Then, the current through the turnstile can be locked to the current through the 1D array; this is the current mirror effect. We numerically demonstrate the current mirror effect in our circuit, where a 4-junction turnstile and a 20-junction 1D array are coupled at their middle nodes via a coupling capacitor. We introduce a coupling parameter, Qc, to define the strength of coupling between the 1D array and the turnstile. We also introduce a current mirror index, CMI, to evaluate the quality of the current mirror effect. The dependence of CMI upon the soliton length, the current through the 1D array, and Qc is discussed.

We present a system which allows to tune the coupling between a superconducting resonator and a transmission line. This storage resonator is addressed through a second, coupling resonator, which is frequency-tunable and controlled by a magnetic flux applied to a superconducting quantum interference device (SQUID). We experimentally demonstrate that the lifetime of the storage resonator can be tuned by more than three orders of magnitude. A field can be stored for 18 {\mu}s when the coupling resonator is tuned off resonance and it can be released in 14 ns when the coupling resonator is tuned on resonance. The device allows capture, storage, and on-demand release of microwaves at a tunable rate., Comment: 5 pages, 3 figures

We characterize a novel Josephson parametric amplifier based on a flux-tunable quarter-wavelength resonator. The fundamental resonance frequency is ~1GHz, but we use higher modes of the resonator for our measurements. An on-chip tuning line allows for magnetic flux pumping of the amplifier. We investigate and compare degenerate parametric amplification, involving a single mode, and nondegenerate parametric amplification, using a pair of modes. We show that we reach quantum-limited noise performance in both cases, and we show that the added noise can be less than 0.5 added photons in the case of low gain.

We have fabricated an aluminium single-electron transistor?(SET) and characterized it at frequencies up to 8?MHz by measuring the reflected signal from a 331?MHz resonant tank in which the transistor is embedded. Within a bandwidth of 8?MHz, we measured the charge sensitivity of this radio-frequency SET, i.e.?rf-SET, to 6.3?10-6?e/(Hz)1/2, which corresponds to the uncoupled energy sensitivity of 13.

We have made Radio-Frequency Single-Electron Transistors (RF-SETs) with large input gates, and tested performance and modes of operation with the goal of using such devices as on-chip amplifiers for a variety of high impedance cryogenic photodetectors. We achieved /spl ap/100 kHz of closed-loop bandwidth for charge-locked-loop and transimpedance amplifier feedback configurations, and have combined amplifier outputs using a form of wavelength division multiplexing. With our choice of SET junction resistance, a 0.5 fF input gate capacitance gave a cotunneling-degraded charge noise of 1/spl times/10/sup -4/ e//spl radic/Hz, but a fairly low input voltage noise of 30 nV//spl radic/Hz.

We have fabricated Coulomb blockade devices which act as current mirrors. Each sample consists of two nominally identical one-dimensional arrays of small tunnel junctions. Each electrode of one array is capacitively coupled to two electrodes in the other array. By measuring the current voltage characteristics of the two arrays simultaneously, we observe a coupling between the currents in the two arrays. We have studied the properties both in the superconducting state and in the normal state, and we interpret the data in terms of correlated transport of charges in the two arrays. The locking between the currents in the two arrays is strongest for intermediate magnetic fields where the electrodes are still superconducting. At best we find current locking over a range of ±6.7 pA

We discuss the design for a submillimeter-wave photometer, using a combination of superconducting and single-electron devices, which would have high quantum efficiency, very low noise-equivalent powers, and eventually even submicrosecond timing resolution. The absorption of above-gap photons occurs in a small strip of superconducting Al, whose normal-state resistance can be matched efficiently to an antenna of a higher gap (Nb) superconductor. The quasiparticles produced by photon absorption are then confined via Andreev reflection, and forced to tunnel through a small SIS tunnel junction. The tunneling time is much shorter than the known (>10 /spl mu/s) quasiparticle recombination time, so collection efficiency will be high. The device sensitivity would be limited by the small subgap current in the high-quality Al/AlO/sub x//Al tunnel junction at temperatures (100 mK) well below T/sub c/. Scaling based on the larger junctions used in X-ray detector applications suggests that the total dark current can be

We describe a new mode of operation of the single electron transistor (SET). The so-called RF-SET (radio frequency-SET) is a dual of the RF-SQUID (radio frequency-superconducting quantum interference device). It has been operated at frequencies above 100 MHz with a very high charge sensitivity (1.2 × 10−5 e/√Hz. The large bandwidth, combined with a high sensitivity, will enable studies of the dynamics of mesoscopic systems on very short time scales.

A new type of electrometer is described that uses a single-electron transistor (SET) and that allows large operating speeds and extremely high charge sensitivity. The SET readout was accomplished by measuring the damping of a 1.7-gigahertz resonant circuit in which the device is embedded, and in some ways is the electrostatic “dual” of the well-known radio-frequency superconducting quantum interference device. The device is more than two orders of magnitude faster than previous single-electron devices, with a constant gain from dc to greater than 100 megahertz. For a still-unoptimized device, a charge sensitivity of 1.2 × 10 −5 e / hertz was obtained at a frequency of 1.1 megahertz, which is about an order of magnitude better than a typical, 1/ f -noise-limited SET, and corresponds to an energy sensitivity (in joules per hertz) of about 41 ℏ︀.

Niobium thin film wires were fabricated using electron beam lithography with a four layer liftoff mask system, and subsequently thinned by anodisation. The resistance along the wire was monitored in situ and trimmed by controlling the anodisation voltage. Depending on the room temperature sheet resistance, samples showed either superconducting or insulating behaviour at low temperatures. A Coulomb blockade was observed for samples exceeding 6 kOhm per square. Samples were also made in a single electron transistor-like geometry with two weak links made by combined angular evaporation and anodisation. Their current-voltage characteristics could be modulated by a voltage applied to an overlapping gate., Comment: 15 pages, 10 EPS figures, LaTeX2e, to be published in Supercond. Sci. and Technol

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.

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 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 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 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.

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

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 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 describe fabrication and measurements of Single Electron Transistors operated in the Radio Frequency mode (RF-SET). We demonstrate very high sensitivity for RF-SETs and we evaluate the back-action from an RF-SET, when used as a read-out device for charge qubits. We conclude that single-shot read-out is possible with a signal to noise ratio greater than one, and that niobium qubits would substantially improve the signal to noise ratio. Furthermore we describe how the RF-SET could be used to read out a differential qubit in a gradiometer coupling.

We are developing superconducting direct detectors for submillimeter astronomy that can in principle detectindividual photons. These devices, Single Quasiparticle Photon Counter (SQPC), operate by measuring thequasiparticles generated when single Cooper-pairs are broken by absorption of a submillimeter photon. Thisphotoconductive type of device could yield high quantum efficiency, large responsivity, microsecond responsetimes, and sensitivities in the range of 10 -20 Watts per root Hertz. The use of antenna coupling to a smallabsorber also suggests the potential for novel instrument designs and scalability to imaging or spectroscopicarrays. We will describe the device concept, r ecent results on fabrication and electrical characterization of thesedetectors, issues related to saturation and optimization of the device parameters. Finally, we have developedpractical readout amplifiers for these high-impedance cryogenic detectors based on the Radio-FrequencySingle-Electron Transistor (RF-SET). We will describe results of a demonstration of a transimpedanceamplifier based on closed-loop operation of an RF-SET, and a demonstration of a wavelength-divisionmultiplexing scheme for the RF-SET. These developments will be a key ingredient in scaling to large arrays ofhigh-sensitivity detectors.Keywords: submillimeter, superconducting detector

We demonstrate that a continuously measured microelectronic circuit, the Cooper-pair box measured by a radio-frequency single-electron transistor, approximates a quantum two-level system. We extract the Hamiltonian of the circuit through resonant spectroscopy and measure the excited-state lifetime. The lifetime is more than 10(5) times longer than the inverse transition frequency of the two-level system, even though the measurement is active. This lifetime is also comparable to an estimate of the known upper limit, set by spontaneous emission, for this circuit.

Proper grounding between different ground planes in coplanar superconducting qubit circuits is important to avoid spurious resonances which increase decoherence. Here, the authors present a possible solution to suppress such undesired modes using superconducting aluminum air-bridges which have been fabricated on top of aluminum coplanar waveguide transmission lines. 3D electromagnetic simulations were done to guide the design of the air-bridges such that the input reflection (S11) of the bridges was kept at a minimum level. A fabrication method based on optical lithography techniques was developed and it resulted in air-bridges with a height of approximately 10 μm and lengths of up to 500 μm. The method can be generalized to arbitrary length air-bridge with heights even exceeding 15 μm.

We have operated a single-electron transistor (SET) in the radio-frequency mode. In this way both the bandwidth and the sensitivity of the traditional SET can be increased several orders of magnitude. By optimizing the system we reached a best sensitivity of 3.2×10 −6 e / Hz . We use this system to study charge noise up to 1 MHz and to demonstrate pulsed, short-time measurements of charge.

Several of the characteristic and interesting single charge tunneling effects have been observed in ID arrays of tunnel junctions, i. e., in series coupled arrays of many, closely spaced, ultrasmall junctions. For example, the time correlation of tunnel events was first observed in such an array. The interpretation of the results obtained with an array may be complicated as compared to a single junction, but the additional degrees of freedom for an array, like the formation of charge “solitons”, may be of advantage in developing the single charge effects and, in particular, for applications of these effects. One of the advantages of series coupled junctions is that it is easier to fabricate high resistance tunnel junctions closely connected to the junction under study than high resistance conventional resistors.

We have fabricated and characterized systems consisting of superconducting quantum bits (qubits) of the transmon type, coupled to a tunable transmission line resonator. The resonator consists of a superconducting quarter wavelength coplanar waveguide (CPW) resonator that is made tunable by terminating the transmission line to ground through a superconducting quantum interference device (SQUID). Here we present measurements on two devices where we observe Rabi oscillations as we drive the qubit, and vacuum Rabi splitting both as we tune the qubits into resonance with the resonator and as we tune the resonator into resonance with the qubits. We also observe coherent interaction between two resonant qubits. In addition, we demonstrate that we can calculate the response very accurately using a multilevel Hamiltonian.

We have fabricated and characterized tunable superconducting transmission line resonators. To change the resonance frequency, we modify the boundary condition at one end of the resonator through the tunable Josephson inductance of a superconducting quantum interference device. We demonstrate a large tuning range (several hundred megahertz), high quality factors (104), and that we can change the frequency of a few-photon field on a time scale orders of magnitude faster than the photon lifetime of the resonator. This demonstration has implications in a variety of applications.

We have fabricated two nominally identical one-dimensional arrays of small tunnel junctions, each with 10 junctions in series. Each electrode of one array is capacitively coupled to two electrodes in the other array. By measuring the current voltage characteristics of the two arrays simultaneously, we observe a coupling between the currents in the two arrays. In a narrow region of biasing conditions, we find that the currents are equally large but with opposite signs. We interprete this as correlated transport of electrons in one array, and holes in the other array. The experimental results show qualitative agreement with numerical simulations.

Integrated bolometers of a volume of about 0.05 mm 3 are fabricated by the technique of silicon micromachining. As thermistor serves a heavily phosphorus doped area implanted in the absorber surface. The optimum thermistor doping was determined with respect to a high resistance gradient, but at the same time with resistance values low enough for providing signals which are reasonably high. Measurements of low-energetic γ-rays and conversion electrons indicated the high sensitivity of our detectors, however unfortunately for picking up noise, too. The steps to improve the current energy resolution are reported.

We suggest a new type of temperature sensor in low temperature composite bolometers for detection of nuclear radiation. The new sensor is a two-dimensional (2D) array of ultrasmall (∼0.01μm 2 ) aluminum Josephson junctions, situated on a micro machined silicon absorber. The incident radiation creates phonons in the absorber, which in turn can excite either charge solitons or vortex solitons in the 2D array. The benefit of this sensor is the low operation temperature, which is determined by the activation energy for the solitons, and can easily be as low as 20 mK. A low operation temperature is essential since the specific heat of the absorber has a T 3 dependence. The activation energy can be set to the desired value by changing the individual junction sizes as well as the junction resistances. Furthermore, the optimal operation point may be trimmed by a moderate magnetic field. Since the atomic mass of aluminum is very close to that of silicon the Kapitza resistance between the absorber and the sensor is small. By keeping the aluminum electrodes in the superconducting state, the amount of additional electronic heat capacity from the sensor may also be kept low.