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We report the fabrication and testing of arrays of nanoscale superconducting quantum interference devices (SQUIDs) directly written into a thin film of the high-transition temperature superconductor YBa2Cu-3O7−δ with a focused helium ion beam. We compare three array configurations with 400 nm by 400 nm nanoSQUIDs connected in series and parallel and a two-dimensional (2D) combination of both. Our electrical transport measurements show that series arrays of three nanoSQUIDs exhibit modulation voltages greater than 1 mV and that combining the devices in parallel greatly enhances the slope of the voltage–magnetic field characteristic. A 2D array with 3 SQUIDS in series and 7 in parallel exhibited a transfer function of 5.51 mV/mT. [ABSTRACT FROM AUTHOR]

We present two variants of a magnetic microcalorimeter with paramagnetic temperature sensors and integrated dc-superconducting quantum interference device readout for high-resolution x-ray emission spectroscopy. Each variant employs two overhanging gold absorbers with a sensitive area of 150 × 150 μm2 and a thickness of 3 μm, thus providing a thickness related quantum efficiency of >98% for photons up to 5 keV and >50% for photons up to 10 keV. The first variant operated nominally but suffered from Joule power dissipation of the Josephson junction shunt resistors, athermal phonon loss, and slew rate limitations of the overall setup. Overall, it only achieved an energy resolution of Δ E FWHM = 8.9 eV for 5.9 keV photons. In the second variant, we introduced an innovative tetrapod absorber geometry as well as a membrane technique for diverting dissipated heat away from the temperature sensors. When all mitigations are applied optimally, the second variant achieves an energy resolution of Δ E FWHM = 1.25 (18) eV for 5.9 keV photons and hence provides the present best energy resolving power E / Δ E FWHM among all existing energy-dispersive detectors for soft and tender x-rays. [ABSTRACT FROM AUTHOR]

Granular aluminum (grAl) is an applied quantum material. We present nano-superconducting quantum interference devices (nanoSQUIDs) based on grAl thin films. These devices exhibit non-hysteretic behavior, allowing conventional SQUID readout down to temperatures well below the critical temperature as well as detection properties comparable to those of Dayem bridge-based devices of greater complexity. Despite being much longer than the coherence length, the current–phase relation of these grAl constrictions appears to be single valued at least down to half their critical temperature. This suggests that grAl thin films should be described as a network of inter-grain Josephson junctions. [ABSTRACT FROM AUTHOR]

We demonstrate a 3-port Josephson parametric circulator matched to 50 Ω using second order Chebyshev networks. The device notably operates with two of its signal ports at the same frequency and uses only two out-of-phase pumps at a single frequency. As a consequence, when operated as an isolator, it does not require phase coherence between the pumps and the signal, thus simplifying the requirements for its integration into standard dispersive qubit readout setups. The device utilizes parametric couplers based on a balanced bridge of rf-superconducting quantum interference device arrays, which offer purely parametric coupling and high dynamic range. We characterize the device by measuring its full 3 × 3 S-matrix as a function of frequency and the relative phase between the two pumps. We find up to 15 dB nonreciprocity over a 200 MHz signal band, port match better than 10 dB, low insertion loss of 0.6 dB, and saturation power exceeding −80 dBm. [ABSTRACT FROM AUTHOR]

Topological materials are promising candidates in fault-tolerant quantum information processing architectures, making it essential to understand the dephasing mechanisms in these materials. Here, we investigate gated, nanoscale mesas fabricated on thin films of cadmium arsenide (Cd3As2), a three-dimensional Dirac semimetal that can be tuned into different topological phases. We observe two independent types of conductance oscillations, one as a function of the applied magnetic field and the other as a function of the gate voltage. Varying the dimensions of the nanostructures allows the discrimination of a variety of scenarios for similar oscillations previously reported in the literature. We conclude that the conductance oscillations are not a signature of topological boundary states per se, but rather are universal conductance fluctuations. These results broadly inform future interpretations of electronic quantum interference in mesoscopic devices made from topological materials. [ABSTRACT FROM AUTHOR]

Superconductivity serves as a unique solid-state platform for electron interference at a device-relevant lengthscale, which is essential for quantum information and sensing technologies. As opposed to semiconducting transistors that are operated by voltage biasing at the nanometer scale, superconductive quantum devices cannot sustain voltage and are operated with magnetic fields, which impose a large device footprint, hindering miniaturization and scalability. Here, we introduce a system of superconducting materials and devices that have a common interface with a ferroelectric layer. An amorphous superconductor was chosen for reducing substrate-induced misfit strain and for allowing low-temperature growth. The common quantum pseudowavefunction of the superconducting electrons was controlled by the nonvolatile switchable polarization of the ferroelectric by means of voltage biasing. A controllable change of 21% in the critical temperature was demonstrated for a continuous film geometry. Moreover, a controllable change of 54% in the switching current of a superconducting quantum interference device was demonstrated. The ability to voltage bias superconducting devices together with the nonvolatile nature of this system paves the way to quantum-based memory devices. [ABSTRACT FROM AUTHOR]

We report on electron spin resonance spectroscopy measurements using a superconducting flux qubit with a sensing volume of 6 fl. The qubit is read out using a frequency-tunable Josephson bifurcation amplifier, which leads to an inferred measurement sensitivity of about 20 spins in a 1 s measurement. This sensitivity represents an order of magnitude improvement when compared to flux-qubit schemes using a direct current-superconducting quantum interference device switching readout. Furthermore, noise spectroscopy reveals that the sensitivity is limited by flicker (1 / f) flux noise. [ABSTRACT FROM AUTHOR]

We report on the experimental observation of period multiplication in parametrically driven tunable superconducting resonators. We modulate the magnetic flux through a superconducting quantum interference device, attached to a quarter-wavelength resonator, with frequencies nω close to multiples, n =2, 3, 4, and 5, of the resonator fundamental mode and observe intense output radiation at ω. The output field manifests n-fold degeneracy with respect to the phase, and the n states are phase shifted by 2π/n with respect to each other. Our demonstration verifies the theoretical prediction by Guo et al. [Phys. Rev. Lett. 111, 205303 (2013)] and paves the way for engineering complex macroscopic quantum cat states with microwave photons. [ABSTRACT FROM AUTHOR]

We report the performance of a reliable three-dimensional nanometer-sized Superconducting QUantum Interference Device (SQUID). The use of superconductor-isolator-superconductor Nb/ Al-AlOx/Nb Josephson tunnel junctions together with small SQUID loop dimensions permits a high modulation depth of the SQUID’s critical current and thus leads to very low intrinsic flux noise of the device. In particular, we present electrical characterization including detailed noise investigations. At 4.2 K, two-stage noise measurements with a SQUID as a low noise preamplifier result in a white flux noise of 51 nφ0/Hz1/2, which is equivalent to an energy resolution of 1.3 h, with h being Planck’s constant. Simulation of spin sensitivities results in about 1 μB/Hz1/2 for an electron spin positioned directly above the SQUID ring. [ABSTRACT FROM AUTHOR]

Integrated NbN direct current superconducting quantum interference device (DC-SQUID) magnetometers were developed based on high-quality epitaxial NbN/AlN/NbN Josephson junctions for SQUID applications operating at high temperatures. We report the current–voltage and voltage–flux characteristics and the noise performance of the NbN DC-SQUIDs for temperatures ranging from 4.2 to 9K. The critical current and voltage swing of the DC-SQUIDs decreased by 15% and 25%, respectively, as the temperature was increased from 4.2 to 9K. The white flux noise of the DC-SQUID magnetometer at 1 kHz increased from 3.9 μφ0/Hz1/2 at 4.2K to 4.8μφ0/Hz1/2 at 9K with 23% increase, corresponding to the magnetic field noise of 6.6 and 8.1 fT/Hz1/2, respectively. The results show that NbN DC-SQUIDs improve the tolerance of the operating temperatures and temperature fluctuations in SQUID applications. [ABSTRACT FROM AUTHOR]

A strong pre-polarization field, usually tenths of a milli-tesla in magnitude, is used to increase the signal-to-noise ratio in ordinary superconducting quantum interference device-based nuclear magnetic resonance/magnetic resonance imaging experiments. Here, we introduce an experimental approach using two techniques to remove the need for the pre-polarization field. A dynamic nuclear polarization (DNP) technique enables us to measure an enhanced resonance signal. In combination with a π/2 pulse to avoid the Bloch-Siegert effect in a micro-tesla field, we obtained an enhanced magnetic resonance image by using DNP technique with a 34.5 μT static external magnetic field without field cycling. In this approach, the problems of eddy current and flux trapping in the superconducting pickup coil, both due to the strong pre-polarization field, become negligible. [ABSTRACT FROM AUTHOR]

We propose a phase-controlled heat-flux quantum valve based on the proximity effect driven by a superconducting quantum interference proximity transistor (SQUIPT). Its operation relies on the phase-dependent quasiparticle density of states in the Josephson weak-link of the SQUIPT which controls thermal transport across the device. In a realistic Al/Cu-based setup the structure can provide efficient control of thermal current inducing temperature swings exceeding ~100 mK, and flux-to-temperature transfer coefficients up to ~500mK/Փ0 below 100 mK. The nanovalve performances improve by lowering the bath temperature, making the proposed structure a promising building-block for the implementation of coherent caloritronic devices operating below 1 K. [ABSTRACT FROM AUTHOR]

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. 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 µs 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. [ABSTRACT FROM AUTHOR]

We report experimental demonstration of tunable, monolithic, single-mode quantum cascade lasers (QCLs) at ~10 lm with a two-section etched slot structure. A single-mode tuning range of 77 cm-1 (785 nm), corresponding to ~7.8% of the relative tuning range, was realized with a ~20 dB side mode suppression ratio within the whole tuning range. Compared with integrated distributed feedback QCLs, our devices have the advantages of easy fabrication and a broader tuning range. Further theoretical analyses and numerical simulations show that it is possible to achieve a broad continuous tuning range by optimizing the slot structures. The proposed slot-waveguide design could provide an alternative but simple approach to the existing tuning schemes for realizing broadly continuous tunable single-mode QCLs. [ABSTRACT FROM AUTHOR]

The coercive field and angular dependence of the coercive field of single-grain Nd2Fe14B permanent magnets are computed using finite element micromagnetics. It is shown that the thickness of surface defects plays a critical role in determining the reversal process. For small defect thicknesses reversal is heavily driven by nucleation, whereas with increasing defect thickness domain wall de-pinning becomes more important. This change results in an observable shift between two well-known behavioral models. A similar trend is observed in experimental measurements of bulk samples, where an Nd-Cu infiltration process has been used to enhance coercivity by modifying the grain boundaries. When account is taken of the imperfect grain alignment of real magnets, the single-grain computed results appears to closely match experimental behaviour. [ABSTRACT FROM AUTHOR]

Magnetic field sensors based on two-dimensional arrays of superconducting quantum interference devices were constructed from magnesium diboride thin films. Each array contained over 30 000 Josephson junctions fabricated by ion damage of 30 nm weak links through an implant mask defined by nano-lithography. Current-biased devices exhibited very large voltage modulation as a function of magnetic field, with amplitudes as high as 8 mV. [ABSTRACT FROM AUTHOR]

Nano-Superconducting Quantum Interference Devices (nano-SQUIDs) are usually fabricated from a single layer of either Nb or Al. We describe here a simple method for fabricating suspended nano-bridges in Nb/Al thin-film bilayers. We use these suspended bridges, which act as Josephson weak links, to fabricate nano-SQUIDs which show critical current oscillations at temperatures up to 1.5K and magnetic flux densities up to over 20mT. These nano-SQUIDs exhibit flux modulation depths intermediate between all-Al and all-Nb devices, with some of the desirable characteristics of both. The suspended geometry is attractive for magnetic single nanoparticle measurements. [ABSTRACT FROM AUTHOR]

We present a contact free cross-correlation noise thermometer experimentally characterized for temperatures between 0.8 K and 45 μK. The noise source is a cold-worked copper cylinder. The fluctuations of magnetic fields due to the thermal motion of the electrons in the copper cylinder are simultaneously monitored by two superconducting quantum interference device magnetometers. A subsequent cross-correlation of both channels reduces the noise contribution of the amplifiers by more than one order of magnitude. This technique covers almost five orders of magnitude in temperature including ultra low temperatures, which were accessible only by platinum nuclear magnetic resonance thermometers so far. [ABSTRACT FROM AUTHOR]

We propose tunable superconducting split-ring resonators (SRRs) employing nonlinear Josephson inductance. A fraction of SRR is replaced by Nb-AlOx-Nb Josephson tunnel junctions connected in parallel and forming a superconducting quantum interference device, whose inductance is sensitive to the external dc magnetic field. Due to the lumped nature of the Josephson inductance, the SRR can be made very compact, and its resonance frequency can be tuned by applying magnetic field. We present the model, results of extensive electromagnetic-simulation and experimental data for the SRR weakly coupled to a transmission line within frequency range of 11-13 GHz. [ABSTRACT FROM AUTHOR]