F. L. Lloyd

Quasiparticle heterodyne mixing in SIS tunnel junctions

The rapid onset of quasiparitcle tunneling current in superconductor‐insulator‐superconductor (Josephson) junctions at voltages above 2Δ/e is being used for millimeter‐wave heterodyne mixing. Junctions with a 2‐μm diameter and RN=50 Ω have little capacitive shunting at the signal frequency of 36 GHz. Because there is no series resistance, residual capacitance can be tuned out. Double sideband conversion efficiencies of 0.32 and mixer noise temperatures as low as TM⩽7 K=4hν/k have been observed. The results are compared with shot‐noise‐limited mixer theory. Photon‐assisted tunneling effects are seen which indicate the approach to photon‐noise‐limited operation.

Series-array Josephson voltage standards

Series arrays typically including 1500 Josephson junctions driven at 90 GHz have been used to generate quantized reference voltages in excess of 1 V. Such standards simplify the procedure and reduce the measurement uncertainities in the calibration of electrochemical cells.

Conversion gain in mm‐wave quasiparticle heterodyne mixers

We report the observation of heterodyne mixing in superconductor‐insulator‐superconductor tunnel junctions with significant conversion gain and with a noise temperature comparable to the photon noise limit. (Double sideband L−1=1.400.14, TM≲1.5 K at 36 GHz.) The mixing arises from the strong nonlinearity in the quasiparticle tunneling currents at voltages comparable to the full energy gap. Gain is observed when the onset of quasiparticle current is sufficiently sharp that quantum effects are important. The observed gain is in good quantitative agreement with calculations based on the work of Tucker. It should make possible the construction of photon‐noise‐limited microwave heterodyne receivers.

A practical Josephson voltage standard at 1 V

A series array of 1484 pairs of Josephson junctions, biased by microwaves at 72 GHz, is demonstrated to provide stable quantized voltages at the 1 V level. The niobium/lead-alloy junctions used in the array are not affected by thermal cycling.

Superconductor-insulator-superconductor quasiparticle junctions as microwave photon detectors

The strong nonlinearity of the quasiparticle tunneling current in superconductor‐insulator‐superconductor junctions near the full‐gap voltage 2Δ/e can be used for direct detection of microwave signals. Measurements at 36 GHz yielded a current responsivity of 3500 A/W, which is within a factor of 2 of the quantum‐limited value e/∇ω. The measured NEP of 2.6±0.8×10−16 W/,Hz1/2 is the lowest value reported to date and can probably be improved significantly. The experimental results are compared with both the standard classical analysis and photon‐assisted tunneling theory.

A Josephson array voltage standard at 10 V

The technology of Josephson voltage standards has been extended to an array of 14 184 junctions which is capable of generating over 150 000 quantized voltage levels spanning the range from -12 to +12 V. This makes possible the direct calibration of 10-V Zener reference standards without the use of a voltage divider.

Phase lock of a long Josephson junction to an external microwave source

A long Josephson junction dc biased on a zero‐field singularity and emitting radiation at microwave frequencies is irradiated with external microwave power. This power can be supplied either by a room‐temperature oscillator or by another long junction. We find that the oscillations of the junction can coherently lock to the external signal for frequency intervals ranging from 500 kHz to 50 MHz. The dependence of the width of these intervals of coherence on the external microwave power is measured for the case in which the power is generated by a room‐temperature oscillator.

Analog measurement applications for high speed Josephson switches

This paper reviews high speed analog applications of Josephson switching devices. The design and performance of two different analog sampling circuits is described. A method is proposed for delivering room temperature signals to these samplers with 30 GHz or more of bandwidth. An analog-to-digital converter based on quantum interference comparators is also described. This device has achieved conversion rates of 2 × 109samples per second.

A superconducting sampler for Josephson logic circuits

A method is described for automating a technique which is used to sample transition duration (rise time) in superconducting logic circuits. The method is based on measuring the time at which a biased Josephson junction switches under the influence of an applied signal. The system transition duration is limited primarily by time jitter which is estimated to be 7 ps. Transition durations of as little as 9 ps have been observed.

Precision of series‐array Josephson voltage standards

Comparison of two series‐array Josephson voltage standards operated at over 1 V shows that they differ in voltage by less than 2 parts in 1017.