Lafe Spietz

Dispersive photon blockade in a superconducting circuit.

Mediated photon-photon interactions are realized in a superconducting coplanar waveguide cavity coupled to a superconducting charge qubit. These nonresonant interactions blockade the transmission of photons through the cavity. This so-called dispersive photon blockade is characterized by measuring the total transmitted power while varying the energy spectrum of the photons incident on the cavity. A staircase with four distinct steps is observed and can be understood in an analogy with electron transport and the Coulomb blockade in quantum dots. This work differs from previous efforts in that the cavity-qubit excitations retain a photonic nature rather than a hybridization of qubit and photon and provides the needed tolerance to disorder for future condensed matter experiments.

Noise performance of the radio-frequency single-electron transistor

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.

Input impedance and gain of a gigahertz amplifier using a dc superconducting quantum interference device in a quarter wave resonator

Due to their superior noise performance, superconducting quantum interference devices (SQUIDs) are an attractive alternative to high electron mobility transistors for constructing ultra-low-noise microwave amplifiers for cryogenic use. We describe the use of a lumped element SQUID inductively coupled to a quarter wave resonator. The resonator acts as an impedance transformer and also makes it possible to accurately measure the input impedance and intrinsic microwave characteristics of the SQUID. We present a model for input impedance and gain, compare it to the measured scattering parameters, and describe how to use the model for the systematic design of low-noise microwave amplifiers with a wide range of performance characteristics.

Shot noise thermometry down to 10mK

The authors report measurements of the shot noise thermometer (SNT), a primary thermometer based on the electronic noise from a tunnel junction, in the range from 10to200mK. They demonstrate operation of the SNT down to 10mK with 10% accuracy at the lowest measured temperature. At 10mK, where for a measurement frequency of f=450MHz, hf=2.5kBT, the authors demonstrate that provided that quantum corrections are taken into account, the SNT continues to be a practical thermometer. They also show that self-heating is not a measurable problem and demonstrate a simplified readout of the SNT.

Superconducting quantum interference device amplifiers with over 27 GHz of gain-bandwidth product operated in the 4–8 GHz frequency range

We describe the performance of amplifiers in the 4–8 GHz range using direct current (dc) superconducting quantum interference devices (SQUIDs) in a lumped element configuration. We have used external impedance transformers to couple power into and out of the dc SQUIDs. By choosing appropriate values for coupling capacitors, resonator lengths and output component values, we have demonstrated useful gains in several frequency ranges with different bandwidths, showing over 27 GHz of power gain-bandwidth product. In this work, we describe our design for the 4–8 GHz range and present data demonstrating gain, bandwidth, dynamic range, and drift characteristics.

Noise performance of lumped element direct current superconducting quantum interference device amplifiers in the 4–8 GHz range

We report on the noise of a lumped element direct current superconducting quantum interference device amplifier. We show that the noise temperature in the 4–8 GHz range over ranges of tens of megahertz is below 1 K (three photons of added noise), characterize the overall behavior of the noise as a function of bias parameters, and discuss potential mechanisms which determine the noise performance in this amplifier. We show that this device can provide more than a factor of 10 improvement in practical system noise over existing phase-preserving microwave measurement systems in this frequency band.

A 4 : 1 Transmission-Line Impedance Transformer for Broadband Superconducting Circuits

We present a 4 : 1 superconducting transmission-line impedance transformer for cryogenic applications. The device transforms 25 Ω in the coplanar waveguide to 6.25 Ω in the microstrip and is designed to operate at 20 mK. Calibrated measurements in a dilution refrigerator demonstrate a -3 dB bandwidth from 1.6 to 13 GHz. In a modified Ruthroff design, a small capacitor is integrated at the input as a direct-current block, making it suitable for biasing and matching to low-impedance active circuits, such as superconducting quantum interference device (SQUID) amplifiers.

Two-port microwave calibration at millikelvin temperatures

In this work we introduce a system for 2-port microwave calibration at millikelvin temperatures operating at the coldest stage of a dilution refrigerator by use of an adapted thru-reflect-line algorithm. We show that this can be an effective tool for characterizing common 50 Ω microwave components with better than 0.1 dB accuracy at temperatures that are relevant to many current experiments in superconducting quantum information.

Observation of quantum oscillations in the photoassisted shot noise of a tunnel junction

We report measurements of the low frequency current fluctuations of a tunnel junction placed at very low temperature biased by a time-dependent voltage

Electron-photon correlations and the third moment of quantum noise

V(t)=V(1+\cos 2\pi\nu t)