Session Q54: Superconductivity: Mesoscopic and Nanometer Scale

Designing high-impedance/low-noise superinductances for quantum electronics

Superinductances are essential circuit elements which enable the suppression of charge fluctuations in superconducting fluxonium qubits [1] and in other Josephson junction devices [2]. Commonly implemented as an array of Josephson junctions, superinductances have two main limitations. Firstly, the spurious capacitive coupling of the chain islands to ground lowers the plasma frequency of the chain, and consequently limits the operational bandwidth. Secondly, coherent quantum phase-slips (CQPS) [3] in the Josephson junction chain induce time dependent inductance fluctuations via the Aharonov-Casher effect [4]. We present the application of a novel lithographic technique [5] which enables the fabrication of arrays with optimal junction-capacitance to ground-capacitance ratio. We also present new superinductance designs which topologically suppress the CQPS, allowing the implementation of practically phase-slip free high inductance Josephson junction.\\[4pt] [1] Manucharyan et al., Science, 326 (2009)\\[0pt] [2] Guichard and Hekking, PRB, 81 (2010)\\[0pt] [3] Matveev et al. PRL, 89 (2002)\\[0pt] [4] Pop et al., arXiv:1105.6204 and Manucharyan et al., arXiv:1012.1928\\[0pt] [5] Lecocq et al., Nanotechnology, 22 (2011)   

Powerful coherent terahertz emission from $\rm{Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}}$ mesa array

Stacks of intrinsic Josephson junctions (IJJs) in high-temperature superconductors enable the fabrication of compact sources of coherent THz-radiation. Here we demonstrate 150 microwatts of radiation power at 0.51 THz, using three synchronized stacks patterned on a single $\rm{Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}}$ crystal. The emitted power scales roughly as the square of the number of energized stacks, while the total power spectrum is monochromatic to within observational limits. These results imply that the stacks radiate coherently.   

DC SQUID RF magnetometer with 200 MHz bandwidth

Because of periodic flux-to-voltage transfer function, Superconducting QUantum Interference Device (SQUID) magnetometers operate in a closed-loop regime [1], which linearizes the response, and increases the dynamic range and sensitivity. However, a transmission line delay between the SQUID and electronics fundamentally limits the closed-loop bandwidth at 20 MHz [1], although the intrinsic bandwidth of SQUIDs is in gigahertz range. We designed a DC SQUID based RF magnetometer capable of wideband sensing coherent magnetic fields up to 200 MHz. To overcome the closed-loop bandwidth limitation, we utilized a low-frequency flux-modulated closed-loop to simultaneously lock the quasi-static magnetic flux and provide AC bias for the RF flux. The SQUID RF voltage is processed by RF electronics based on a double lock-in technique. This yields a signal proportional to the amplitude and phase of the RF magnetic flux, with more than four decades of a linear response. For YBaCuO SQUID on bi-crystal SrTiO substrate at 77 K we achieved a flux noise density of 4 \textit{$\mu $}$\Phi _{0}$/$\surd $Hz at 190 MHz, which is similar to that measured at kHz frequencies with conventional flux-locked loop. [1] D. Drung, \textit{et al.}, Supercond. Sci. Technol. \textbf{19, }S235 (2006).   

Wideband S-parameter characterization of DC-SQUID amplifiers at GHz frequencies

Superconducting quantum interference devices (SQUIDs) are widely used as gain elements to achieve ultra-low noise amplification from DC to microwave frequencies. SQUID amplifiers typically have high input and output stray reactance and therefore proper impedance matching is needed to couple enough power into the device. Broadband impedance matching could be obtained by measuring the microwave S-parameters of a SQUID amplifier and then treating it as an equivalent ``black box'' to which standard microwave design techniques can be applied. In this talk measurement results of the full 2-port S-parameter matrix of DC-SQUID amplifiers operating at 20mK will be presented. Accurate microwave calibration was performed with an automated Through-Reflect-Line (TRL) calibration system, operating at 20mK. Input and output reflection coefficients as well as forward and backward transmission were characterized in the 1 to 8GHz range. Implications for the design of wideband SQUID amplifiers and device stability will be discussed.   

Characterization of Low Noise Superconducting Microwave Amplifiers

We have developed an experimental setup to characterize low noise amplifier chains with a six port low temperature microwave switch controlled via room temperature electronics. This switch allows us to do a traditional Y-factor measurement, a reflection measurement with an open input, or a measurement using a Shot Noise Tunnel Junction(SNTJ) device developed by J. Aumentado and L. Spietz at NIST. The SNTJ allows one to measure system gain, system noise, and junction temperature as fit parameters. The SNTJ is sensitive enough characterize amplifiers with noise due primarily to photon ground state fluctuations or quantum limited amplifiers. We first tested our setup using a HEMT and room temperature amplifier chain with calibrated noise and gain characteristics. We then characterized several iterations of the theoretically quantum limited SLUG amplifier developed by R. McDermott at U Wisconsin Madison.   

Shot noise measurements in diffusive wire NSN structures

Subgap transport across a normal metal/superconductor interface requires a conversion from normal to supercurrent. We study the conversion process in superconducting wire samples by attaching two short diffusive normal metal wires at a distance comparable to the coherence length in the superconductor. In addition to ordinary Andreev reflection, nonlocal processes involving electronic states in both contacts (Crossed Andreev reflection, and Elastic Co-tunneling) are expected to contribute and to give rise to a cross-conductance signal in transport measurements. As one of the wire contacts is biased into the shot noise regime, a significant increase of current fluctuations is observed in the other (unbiased) contact. Only a small fraction of the noise measured in the two contacts is correlated. The magnitude of the correlated signal scales with the observed cross-conductance. We will compare our results with theoretical predictions for nonlocal transport and previous experimental work.   

Toward a Passive Lossless on-Chip Circulator in the 4-8 GHz Range

We present further progress toward constructing a passive microwave circulator from Josephson junction circuits. We have designed, built and tested a circuit which demonstrates the basic properties required to construct a gyrator in the 4-8 GHz range. We describe the theory and present the data on this circuit and point to the path from these results to practical passive lossless on-chip circulators.   

Calibration of a Microfabricated Phonon Spectrometer

Non-thermal distributions of phonons may be locally excited and detected in silicon micro- and nanostructures by decay of quasiparticles injected into an adjacent superconducting tunnel junction [1]. Using this technique, narrow frequency bands of phonons may be isolated and applied to investigate phonon transport through nanostructures at sub-kelvin temperatures [2]. In our prototype phonon spectrometer we have demonstrated spatial resolution below 1 micron and frequency resolution of ~10 GHz. We describe ways to control the spatial resolution, frequency resolution, frequency range, dynamic range and signal-to-noise ratio in this technique, by using different superconducting materials in the tunnel junctions and by inserting absorber materials into the phonon transmission path. This work is supported by DOE (DE-SC0001086). \\[4pt] [1] H. Kinder. Phys. Rev. Lett. 28, 1564 (1972)\\[0pt] [2] J. B. Hertzberg et al, Rev. Sci. Inst. 82, 104905 (2011).   

Microwave Spectroscopy of a Cooper-Pair Transistor Coupled to a Lumped-Element Resonator

We have studied the microwave response of a single Cooper-pair transistor (CPT) coupled to a lumped-element microwave resonator. The resonance frequency of this circuit, $f_{r}$ , was measured as a function of the charge $n_{g}$ induced on the CPT island by the gate electrode, and the phase difference across the CPT, $\phi_{B}$ , which was controlled by the magnetic flux in the superconducting loop containing the CPT. The observed $f_{r}(n_{g},\phi_{B})$ dependences reflect the variations of the CPT Josephson inductance with $n_{g}$ and $\phi_{B}$ as well as the CPT excitation when the microwaves induce transitions between different quantum states of the CPT. The results are in excellent agreement with our simulations based on the numerical diagonalization of the circuit Hamiltonian. This agreement over the whole range of $n_{g}$ and $\phi_{B}$ is unexpected, because the relevant energies vary widely, from 0.1K to 3K. The observed strong dependence $f_{r}(n_{g},\phi_{B})$ near the resonance excitation of the CPT provides a tool for sensitive charge measurements.   

Superconducting Low-Inductance Undulatory Galvanometer Microwave Amplifier: Theory

We present numerical studies of a phase-insensitive microwave linear amplifier based on the Superconducting Low-Inductance Undulatory Galvanometer (SLUG). Direct integration of the junction equations of motion provides access to the full scattering matrix of the SLUG element. We discuss the optimization of SLUG amplifiers and calculate amplifier gain and noise temperature in both the thermal and quantum regimes. The microwave SLUG amplifier is expected to achieve noise performance approaching the standard quantum limit in the frequency range from 5-10 GHz, with gain around 15 dB for a single-stage device and instantaneous bandwidth of order hundreds of MHz. We compare our numerical model with measured performance of state-of-the-art devices.   

Superconducting Low-inductance Undulatory Galvanometer Microwave Amplifier

We describe a novel microwave linear amplifier based on the Superconducting Low-inductance Undulatory Galvanometer (SLUG). The compact SLUG element is straightforward to model at microwave frequencies, allowing separate optimization of the SLUG element and the resonant input matching network; we expect optimized devices based on high-Jc junctions to achieve gains around 15 dB in the range from 5-10 GHz, with instantaneous bandwidth of order hundreds of MHz and noise performance approaching the standard quantum limit. Using amplifiers based on low-Jc Al/AlO$_{x}$/Al junctions, we have achieved gain in excess of 20 dB at 3 GHz and greater than 15 dB at 9 GHz with bandwidth of several MHz. We discuss progress toward the incorporation of high-Jc Nb/AlOx/Nb junctions in the SLUG amplifier, and describe strategies to promote the thermalization of the SLUG shunt resistors at dilution refrigerator temperatures by integrating large-volume normal metal cooling fins with the shunts.   

Development and Implementation of a 1 GHz SQUID amplifier for the Axion Dark Matter Experiment

The Axion Dark Matter eXperiment (ADMX) was designed to detect ultra-weakly interacting relic axion particles by searching for their conversion to microwave photons in a resonant cavity positioned in a strong magnetic field. Given the extremely low expected axion-photon conversion power we have designed, built and operated a microwave receiver based on a Superconducting QUantum Interference Device (SQUID). We describe the implementation of a SQUID amplifier in the ADMX microwave receiver chain and discuss progress made at the Washington Micro-Fabrication Facility toward the production of SQUID amplifiers from a ${\rm Nb}-{\rm Al}_x{\rm O}_y-{\rm Nb}$ trilayer.   

Quantum pumping of electrons at the Josephson frequency

A macroscopic fluid pump works according to the law of Newtonian mechanics and transfers a large number of molecules per cycle. By contrast, a nano-scale charge pump can be thought as the ultimate miniaturization of a pump, with its operation being subject to quantum mechanics and with only few electrons or even fractions of electrons transfered per cycle. It generates a direct current in the absence of an applied voltage exploiting the time-dependence of some properties of a nano-scale conductor. So far, nano-scale pumps have been realised only in system exhibiting strong Coulombic effects, whereas evidence for pumping in the absence of Coulomb-blockade has been elusive. Here we report the experimental detection of charge flow in an unbiased InAs nanowire embedded in a superconducting quantum interference device (SQUID). In this system, pumping occurs via the cyclic modulation of the phase of the order parameter of different superconductors. The symmetry of the current with respect to the enclosed flux and SQUID current is a discriminating signature of pumping. Currents exceeding 20 pA are measured at 250 mK, and exhibit symmetries compatible with a pumping mechanism.   

NIS cooler platform: from 300 mK to 100 mK

Normal-insulator-superconducting (NIS) tunnel junctions allow cooling of the normal metal by electrons tunneling across the insulating barrier. With proper biasing, hot electrons leave the normal metal and cold electrons enter from the superconductor, lowering the electronic temperature. The heat dissipation is determined primarily by the quasiparticle relaxation in the superconducting leads. We explore the effects of magnetic field, electrode geometry, direct quasiparticle traps and other fabrication modifications on cooler effectiveness. The overall aim is to produce a general electronic cooler for efficient, solid state, cooling of small devices from 300 to 100 mK.   

Implementation and test of an Levitov's n-electron coherent source

Injecting a controlled number of electrons in a quantum ballistic conductor opens the way to new kind of quantum experiments, yet never done. It is well known that a voltage biased contact applied on a single mode quantum conductor, such as a Quantum Point Contact, inject continuously single electrons at a rate \textit{eV/h} , a remarkable property of the Fermi sea. Here we consider the injection of electrons during a very short time where is is expected that a voltage pulse with \textit{$\smallint $eV(t)dt = nh }injects exactly n-electrons, n being integer. If in addition, if the voltage pulse has the form of a Lorentzian shape in time, Levitov has shown that the n-electron are not accompanied by neutral spurious electron-hole pair excitations and thus form a minimal excitation n-electron source. The electron being indistinguishable if the time-scale is shorter than the coherence time, the Levitov's source is coherent and new quantum experiments involving interference with serveral electrons become at reach. We present here experimental realization of the n-electron source using short sub-nanosecond pulses and tests of the minimal excitation number using the shot noise created by repeatedly sending n-electrons toward a quantum point realized in clean ballistic 2D electrons. The square-wave, sine-wave and Lorentzian shape pulses are compared. This is also accompanied by photon-assisted current measurements. J. Dubois, T. Jullien, P. Roulleau, F. Portier, P. Roche, and D.C. Glattli, in preparation.