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We have developed and tested a doubly tunable resonator, with the intention to simulate fast motion of the resonator boundaries in real space. Our device is a superconducting coplanar-waveguide half-wavelength microwave resonator, with fundamental resonant frequency ~5 GHz. Both of its ends are terminated by dc-SQUIDs, which serve as magnetic-flux-controlled inductances. Applying a flux to either SQUID allows tuning of the resonant frequency by approximately 700 MHz. By using two separate on-chip magnetic-flux lines, we modulate the SQUIDs with two tones of equal frequency, close to twice that of the resonator's fundamental mode. We observe photon generation, at the fundamental frequency, above a certain pump amplitude threshold. By varying the relative phase of the two pumps we are able to control the photon generation threshold, in good agreement with a theoretical model for the modulation of the boundary conditions. At the same time, some of our observations deviate from the theoretical predictions, which we attribute to parasitic couplings, resulting in current driving of the SQUIDs., Comment: 7 pages, 4 figures

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

The conversion efficiency of electric microwave signals into surface acoustic waves in different types of superconducting transducers is studied with the aim of quantum applications. We compare delay lines containing either conventional symmetric transducers (IDTs) or unidirectional transducers (UDTs) at 2.3 GHz and 10 mK. The UDT delay lines improve the insertion loss with 4.7 dB and a directivity of 22 dB is found for each UDT, indicating that 99.4 % of the acoustic power goes in the desired direction. The power lost in the undesired direction accounts for more than 90 % of the total loss in IDT delay lines, but only ~3 % percent of the total loss in the FEUDT delay lines., 4 pages (5 including references), 3 figures

In the dynamical Casimir effect (DCE), a boundary condition for the electromagnetic field is changed rapidly, resulting in generation of pairs of photons [1]. This was experimentally demonstrated in 2011 [2]. Both generation of photons and two-mode squeezing were observed. Quantum theory predicts that the photon pairs are entangled and that the resulting electromagnetic field is in a nonclassical state at zero temperature. Johansson et al. [3] recently suggested how to test entanglement of the DCE radiation by evaluating the log negativity. Here we present such an experimental test. We have measured a sample consisting of an open transmission line terminated by a superconducting quantum interference device (SQUID). The electrical length of the transmission line can be modulated, by changing the magnetic flux through the SQUID loop. Broadband radiation of photon pairs is observed along with strong two mode squeezing. The system is characterized at a temperature of < 20 mK, and the signal level is calibrated using the SQUID as a shot-noise source. We investigate the cross-correlations for the field at two different frequencies and we recover the covariance matrix. From this matrix we determine the log negativity, which shows weakly positive values indicating that the two modes are entangled. A source of entangled photons can be useful in quantum information applications, for instance to entangle qubits [4].

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

It has recently been demonstrated that surface acoustic waves (SAWs) can interact with superconducting qubits at the quantum level. SAW resonators in the GHz frequency range have also been found to have low loss at temperatures compatible with superconducting quantum circuits. These advances open up new possibilities to use the phonon degree of freedom to carry quantum information. In this chapter, we give a description of the basic SAW components needed to develop quantum circuits, where propagating or localized SAW-phonons are used both to study basic physics and to manipulate quantum information. Using phonons instead of photons offers new possibilities which make these quantum acoustic circuits very interesting. We discuss general considerations for SAW experiments at the quantum level and describe experiments both with SAW resonators and with interaction between SAWs and a qubit. We also discuss several potential future developments.

14 págs.; 4 figs.; Open Access funded by Creative Commons Atribution Licence 4.0, We show how to use relativistic motion to generate continuous variable Gaussian cluster states within cavity modes. Our results can be demonstrated experimentally using superconducting circuits where tuneable boundary conditions correspond to mirrors moving with velocities close to the speed of light. In particular, we propose the generation of a quadripartite square cluster state as a first example that can be readily implemented in the laboratory. Since cluster states are universal resources for universal one-way quantum computation, our results pave the way for relativistic quantum computation schemes., D.E.B. was in part supported by the UK Engineering and Physical Science Research Council grant number EP/J005762/1. I.F. and C.S acknowledge support from EPSRC (CAF Grant No. EP/G00496X/2 to I.F.). G.J. and P.D. would like to acknowledge funding from the Swedish Research Council and from the EU through the ERC and the projects PROMISCE and SQALEQIT. Financial support by Fundación General CSIC (Programa ComFuturo) is acknowledged by C.S.

One of the most surprising predictions of modern quantum theory is that the vacuum of space is not empty. In fact, quantum theory predicts that it teems with virtual particles flitting in and out of existence. While initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences, for instance producing the Lamb shift of atomic spectra and modifying the magnetic moment for the electron. This type of renormalization due to vacuum fluctuations is now central to our understanding of nature. However, these effects provide indirect evidence for the existence of vacuum fluctuations. From early on, it was discussed if it might instead be possible to more directly observe the virtual particles that compose the quantum vacuum. 40 years ago, Moore suggested that a mirror undergoing relativistic motion could convert virtual photons into directly observable real photons. This effect was later named the dynamical Casimir effect (DCE). Using a superconducting circuit, we have observed the DCE for the first time. The circuit consists of a coplanar transmission line with an electrical length that can be changed at a few percent of the speed of light. The length is changed by modulating the inductance of a superconducting quantum interference device (SQUID) at high frequencies (~11 GHz). In addition to observing the creation of real photons, we observe two-mode squeezing of the emitted radiation, which is a signature of the quantum character of the generation process., Comment: 12 pages, 3 figures

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.

We have observed energy level quantization in an all high critical temperature superconductor d-wave Josephson junction. From the measurements we have also extracted the quality factor Q of the junction which is of the order of 40. These results indicate that the role of dissipation mechanisms in high temperature superconductors has to be revised, and may also have consequences for the class of solid state “quiet” quantum bit with longer coherence time.

We demonstrate the operation of a differential single Cooper-pair box, a charge qubit consisting of two aluminum islands, isolated from ground, coupled by a pair of small-area Josephson junctions. We have tested four devices, all of which show evidence of quasiparticle poisoning. The devices are characterized with microwave spectroscopy and temperature dependence studies.

We propose and demonstrate a new read-out technique for a superconducting qubit by dispersively coupling it to a Josephson parametric oscillator. We employ a tunable quarter-wavelength superconducting resonator and modulate its resonant frequency at twice its value with an amplitude surpassing the threshold for parametric instability. We map the qubit states onto two distinct states of classical parametric oscillation: one oscillating state, with $185\pm15$ photons in the resonator, and one with zero oscillation amplitude. This high contrast obviates a following quantum-limited amplifier. We demonstrate proof-of-principle, single-shot readout performance, and present an error budget indicating that this method can surpass the fidelity threshold required for quantum computing., Comment: 11 pages, 5 figures

We investigate the backaction of superconducting single-electron transistor (SSET) continuously measuring a Cooper-pair box. Due to the minimized backaction of the SSET, we observe a 2e periodic Coulomb staircase according to the two-level system Hamiltonian of the Cooper-pair box. We demonstrate that we can control the quantum broadening of the ground state in-situ. We perform spectroscopy measurements and demonstrate that we have full control over the Cooper-pair box Hamiltonian. The ability to reduce the backaction is a necessary condition to use the SSET as a quantum state readout for the CPB as a qubit.

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 study decoherence of a Josephson charge qubit using fast pulses to perform qubit rotations. The gate charge dependence of the decoherence rate indicates that the low frequency fluctuations affecting the qubit are from charges distributed with a 1/f-type spectrum. Assuming the form Sq(ω) = α/|ω|, we find $$\sqrt \alpha$$ = 4 × 10−3e, which is slightly higher than the value found from very low frequency noise measurements. PACS numbers: 03.67.-a, 75.40.+r, 85.25.Cp.

We have analyzed a radio-frequency single-electron-transistor (RF-SET) circuit that includes a high-electron-mobility-transistor (HEMT)amplifier, coupled to the single-electron-transistor (SET) via an impedance transformer. We consider how power is transferred between different components of the circuit, model noise components, and analyze the operating conditions of practical importance. The results are compared with experimental data on SETs. Good agreement is obtained between our noise model and the experimental results. Our analysis shows, also, that the biggest improvement to the present RF-SETs will be achieved by increasing the charging energy and by lowering the HEMT amplifier noise contribution.

We present forward transmission measurements of spontaneous lipid bi-layer formation and the effect of enzymatic attack on such bi-layers at frequencies in the 20 kHz–1 GHz range. Measurement of the forward ac transmission parameter at a fixed frequency (1 MHz), on a suitably designed biochip, allows fast and simple real time monitoring of the kinetics of formation of lipid bi-layers on SiO 2 surfaces, through adsorption of vesicles from solution. The bi-layer formation modifies the electrical characteristics in the megahertz frequency regime, by adding an insulating layer on top of the electrode. The observed kinetics is consistent with bi-layer formation observed with, for example, the quartz crystal microbalance with dissipation (QCM-D) and the surface plasmon resonance (SPR) techniques. Measurements on the enzymatic action of phospholipase A 2 (PLA 2 ) on the breakdown of lipid bi-layers further demonstrates that the method can provide new information on bio-interfacial processes not obtainable by, for example, mass-sensing methods.

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.

There are several dualities between Josephson and single charge tunneling, being conjugate phenomena, where either the phase (flux) or the number (charge) states are well determined while the complementary entity (number, phase) is undetermined. A zero voltage, phase-dependent current flows in the Josephson junction up to a critical current value while no current flows in the Coulomb blockade region, up to a threshold voltage, in a single charge element having high resistance and small Josephson coupling. The dual of the extremely flux sensitive DC superconducting quantum interference device (SQUID) is the single charge transistor (SET). It has an outstanding charge sensitivity but its high impedance severely limits its bandwidth. A superconducting radio frequency version, RF-SET, is related to the common RF-SQUID and its bandwidth is improved orders of magnitude compared to the usual SET. A charge sensitivity of the order of 3×10 −6 e/√Hz has been demonstrated. It can be used, e.g., as a read out of the charge of a quantum “qu-bit”. The latter is, in this case, based upon the charge state of a small island, being a superposition of a charge or not a charge, which can be compared with a flux based qu-bit where currents go clock- or anti-clockwise in a SQUID loop.

We report a new method to study two level fluctuators (TLFs) by measuring the offset charge induced after applying a sudden step voltage to the gate electrode of a single electron transistor. The offset charge is measured for more than 20 hours for samples made on three different substrates. We find that the offset charge drift follows a logarithmic increase over four orders of magnitude in time and that the logarithmic slope increases linearly with the step voltage. The charge drift is independent of temperature, ruling out thermally activated TLFs and demonstrating that the charge fluctuations involve tunneling. These observations are in agreement with expectations for an ensemble of TLFs driven out of equilibrium. From our model, we extract the density of TLFs assuming either a volume density or a surface density., Comment: 5 pages, 5 figures

We show that the physics underlying the dynamical Casimir effect may generate multipartite quantum correlations. To achieve it, we propose a circuit quantum electrodynamics (cQED) scenario involving superconducting quantum interference devices (SQUIDs), cavities, and superconducting qubits, also called artificial atoms. Our results predict the generation of highly entangled states for two and three superconducting qubits in different geometric configurations with realistic parameters. This proposal paves the way for a scalable method of multipartite entanglement generation in cavity networks through dynamical Casimir physics., Improved version and references added. Accepted for publication in Physical Review Letters

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

A Superconducting QUantum Interference Device (SQUID) modulated by a fast oscillating magnetic flux can be used as a parametric amplifier, providing gain with very little added noise. Here, we develop linearized models to describe the parametrically flux-pumped SQUID in terms of an impedance. An unpumped SQUID acts as an inductance, the Josephson inductance, whereas a flux-pumped SQUID develops an additional, parallel element which we have coined the ``pumpistor.'' Parametric gain can be understood as a result of a negative resistance of the pumpistor. In the degenerate case, the gain is sensitive to the relative phase between the pump and signal. In the nondegenerate case, gain is independent of this phase. We develop our models first for degenerate parametric pumping in the three-wave and four-wave cases, where the pump frequency is either twice or equal to the signal frequency, respectively. We then derive expressions for the nondegenerate case where the pump frequency is not a multiple of the signal frequency, where it becomes necessary to consider idler tones which develop. For the nondegenerate three-wave case, we present an intuitive picture for a parametric amplifier containing a flux-pumped SQUID where current at the signal frequency depends upon the load impedance at an idler frequency. This understanding provides insight and readily testable predictions of circuits containing flux-pumped SQUIDs., Comment: 27 pages, 6 figures, 1 table

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 propose an implementation of a twin paradox scenario in superconducting circuits, with velocities as large as a few percent of the speed of light. Ultrafast modulation of the boundary conditions for the electromagnetic field in a microwave cavity simulates a clock moving at relativistic speeds. Since our cavity has a finite length, the setup allows us to investigate the role of clock size as well as interesting quantum effects on time dilation. In particular, our theoretical results show that the time dilation increases for larger cavity lengths and is shifted due to quantum particle creation., Comment: 6 pages, 3 figures. I. F. previously published as I. Fuentes-Guridi and I. Fuentes-Schuller

Quantum fluctuations of the vacuum are both a surprising and fundamental phenomenon of nature. Understood as virtual photons, they still have a very real impact, for instance, in the Casimir effects and the lifetimes of atoms. Engineering vacuum fluctuations is therefore becoming increasingly important to emerging technologies. Here, we shape vacuum fluctuations using a superconducting circuit analogue of a mirror, creating regions in space where they are suppressed. Moving an artificial atom through these regions and measuring the spontaneous emission lifetime of the atom provides us with the spectral density of the vacuum fluctuations. Using the paradigm of waveguide quantum electrodynamics, we significantly improve over previous studies of the interaction of an atom with its mirror image, observing a spectral density as low as 0.02 quanta, a factor of 50 below the mirrorless result. This demonstrates that we can hide the atom from the vacuum, even though it is exposed in free space.

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 study the radio-frequency single-electron transistor (rf-SET) as a readout device for charge qubits. We measure the charge sensitivity of an rf-SET to be $6.3\ensuremath{\mu}e/\sqrt{\mathrm{Hz}}$ and evaluate the backaction of the rf-SET on a single Cooper-pair box. This allows us to compare the needed measurement time with the mixing time of the qubit imposed by the measurement. We find that the mixing time can be substantially longer than the measurement time, which would allow readout of the state of the qubit in a single shot measurement.

Two separate research topics are discussed. We discuss noise measurements of single electron transistors and the gain dependence of the noise. We will also describe the fabrication and preliminary measurements on several different superconductor/two-dimensional electron gas/superconductor structures.

We report on the experimental study of the phase periodic resistance oscillations in an Andreev-type interferometer consisting of a mesoscopic normal metal ring (Ag) connected to a superconductor (Al). We observe a spectacular modulation of the oscillations due to an interplay of the phase transfer from the superconducting condensate to quasiparticles and interference phenomena in the normal ring. We discuss the results within the framework of recently developed theory of the long-range proximity effect.

From intrinsic tunneling spectroscopy, superconducting- and pseudo-gaps could be separated within a wide range of temperatures. We have found that the two gaps have different temperature ( T ) and magnetic field ( H ) dependencies. The superconducting gap closes both as T → T c and H → H c2 , while the pseudo-gap does not change neither with T , or H . This would speak in favor of two coexisting phenomena and against the precursor superconductivity scenario of the pseudo-gap.

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 present an extensive experimental and theoretical study of the proximity effect in InAs nanowires connected to superconducting electrodes. We fabricate and investigate devices with suspended gate-controlled nanowires and nonsuspended nanowires, with a broad range of lengths and normal-state resistances. We analyze the main features of the current-voltage characteristics: the Josephson current, excess current, and subgap current as functions of length, temperature, magnetic field, and gate voltage, and compare them with theory. The Josephson critical current for a short-length device, L = 30 nm, exhibits a record high magnitude of 800 nA at low temperature that comes close to the theoretically expected value. The critical current in all other devices is typically reduced compared to the theoretical values. The excess current is consistent with the normal resistance data and agrees well with the theory. The subgap current shows a large number of structures; some of them are identified as subharmonic gap structures generated by multiple Andreev reflection. The other structures, detected in both suspended and nonsuspended devices, have the form of voltage steps at voltages that are independent of either the superconducting gap or length of the wire. By varying the gate voltage in suspended devices, we are able to observe a crossover from typical tunneling transport at large negative gate voltage, with suppressed subgap current and negative excess current, to pronounced proximity junction behavior at large positive gate voltage, with enhanced Josephson current and subgap conductance as well as a large positive excess current., 12 pages, 15 figures

We studied a mesoscopic cross-like normal metal structure connected to two superconducting (S) and two normal (N) reservoirs. We observed the Josephson effect under unusual conditions when there is no current through one of the two S/N interfaces. The potential difference between the S reservoirs was zero unless the voltage applied between S and N reservoirs exceeded a critical value although the electric potential in the N wire connecting the superconductors varied in a nonmonotonic way. The observed effects are discussed theoretically., Comment: 5 pages, 4 figures, to be published in PRB Rapid Communication

An experimental configuration is analyzed in order to studyplasma modes in a superconducting wire deposited ontoSrTiO3. A dispersion relation has been derived byevaluating the effect of the environment surrounding the wire.It corresponds to the one-dimensional dispersion law predictedfor an isolated superconducting wire. Preliminary measurementsare presented. They show 1D plasma modes in theoptimized experimental configuration here studied.

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.

One-dimensional (1D) arrays of small-capacitance Josephson junctions demonstrate a sharp transition, from Josephson-like behavior to the Coulomb blockade of Cooper-pair tunneling, as the effective Josephson coupling between nearest neighbors is tuned with an externally applied magnetic field. Comparing the zero-bias resistance of three arrays with 255, 127, and 63 junctions, we observe a critical behavior where the resistance, extrapolated to $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$, is independent of length at a critical magnetic field. Comparison is made with a theory of this $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ quantum phase transition, which maps to the 2D classical $\mathrm{XY}$ model.

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.

Normal (N) metallic (Ag) mesoscopic conductors with two superconducting (S) faces (Al), arranged mirror-symmetrically relative to the streamlines of the current, periodically switch into the normal state as the superconducting phase difference Δϕ between the NS boundaries approaches the values Δϕ =(2n+1)π, n=0,1,2,..., irrespective of temperature and applied voltage. For Δϕ =2nπ and low applied voltages the conductance passes through a maximum and approaches the normal value as temperature decreases (reentrance). As the voltage subsequently increases, the conductance increases and passes through a maximum. As the phase difference moves away from the values Δϕ=2nπ, the maxima shift in the direction of low temperatures and voltages. The latter result shows unequivocally that in our metal structures it is necessary to take into account the next-order corrections to the “weak” proximity effect approximation.

The effect of the electromagnetic environment on Coulomb blockade devices can be estimated through a simple horizon model where the interaction length is determined by the Heisenberg uncertainty principle. The advantage of this horizon model can be demonstrated in a device with two tunnel junctions in parallel where a cutoff voltage related to physical dimensions is observed. This model also explains the characteristics of single junctions and single-electron tunneling (SET) transistors. Several Al/AlO${}_{x}$/Al devices of various geometries were measured and the effective Coulomb barrier has been investigated through the offset voltage defined as ${V}_{\mathrm{off}}\ensuremath{\equiv}V\ensuremath{-}IdV/dI$. We have also used ${V}_{\mathrm{off}}$ to determine the capacitances of SET transistors, from which we determined microstrip capacitance as a function of island size.

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

We show, in the context of single-photon detection, that an atomic three-level model for a transmon in a transmission line does not support the predictions of the nonlinear polarizability model known as the cross-Kerr effect. We show that the induced displacement of a probe in the presence or absence of a single photon in the signal field, cannot be resolved above the quantum noise in the probe. This strongly suggests that cross-Kerr media are not suitable for photon counting or related single-photon applications. Our results are presented in the context of a transmon in a one-dimensional microwave waveguide, but the conclusions also apply to optical systems.

We experimentally study the behavior of a parametrically pumped nonlinear oscillator, which is based on a superconducting quarter wavelength resonator, and is terminated by a flux-tunable superconducting quantum interference device. We extract parameters for two devices. In particular, we study the effect of the nonlinearities in the system and compare to theory. The Duffing nonlinearity is determined from the probe-power dependent frequency shift of the oscillator, and the nonlinearity related to the parametric flux pumping, is determined from the pump amplitude for the onset of parametric oscillations. Both nonlinearities depend on the parameters of the device and can be tuned in situ by the applied dc flux. We also suggest how to cancel the effect of by adding a small dc flux and a pump tone at twice the pump frequency.

We report conductance and supercurrent of InAs nanowires coupled to Al-superconducting electrodes with short channel lengths and good Ohmic contacts. The nanowires are suspended 15 nm above a local gate electrode. The charge density in the nanowires can be controlled by a small change in the gate voltage. For large negative gate voltages, the number of conducting channels is reduced gradually, and we observe a stepwise decrease of both conductance and critical current before the conductance vanishes completely.

In this work, we theoretically analyze a circuit quantum electrodynamics design where propagating quantum microwaves interact with a single artificial atom, a single Cooper-pair box. In particular, we derive a master equation in the so-called transmon regime, including coherent drives. Inspired by recent experiments, we then apply the master equation to describe the dynamics in both a two-level and a three-level approximation of the atom. In the two-level case, we also discuss how to measure photon antibunching in the reflected field and how it is affected by finite temperature and finite detection bandwidth. © IOP Publishing and Deutsche Physikalische Gesellschaft., This work was also supported by the Spanish MICINN project FIS2009-10061 and the CAM research consortium QUITEMAD S2009-ESP-1594. BP acknowledges support from the CSIC grant JAE-PREDOC2009.

We investigate quantum correlations in microwave radiation produced by the dynamical Casimir effect in a superconducting waveguide terminated and modulated by a superconducting quantum interference device. We apply nonclassicality tests and evaluate the entanglement for the predicted field states. For realistic circuit parameters, including thermal background noise, the results indicate that the produced radiation can be strictly nonclassical and can have a measurable amount of intermode entanglement. If measured experimentally, these nonclassicalilty indicators could give further evidence of the quantum nature of the dynamical Casimir radiation in these circuits., Comment: 5 pages, 3 figures

We address recent advances in microwave quantum optics with artificial atoms in one-dimensional (1D) open space. This field relies on the fact that the coupling between a superconducting artificial atom and propagating microwave photons in a 1D open transmission line can be made strong enough to observe quantum coherent effects, without using any cavity to confine the microwave photons. We investigate the scattering properties in such a system with resonant coherent microwaves. We observe the strong nonlinearity of the artificial atom and under strong driving we observe the Mollow triplet. By applying two resonant tones, we also observe the Autler-Townes splitting. Exploiting these effects, we demonstrate two quantum devices at the single-photon level in the microwave regime: the single-photon router and the photon-number filter. These devices provide important steps toward the realization of an on-chip quantum network. © IOP Publishing and Deutsche Physikalische Gesellschaft., We acknowledge financial support from the EU through the ERC and the project PROMISCE, from the Swedish Research Council and from the Wallenberg Foundation. B P acknowledges support from the CSIC grant JAE-PREDOC2009.