Kinetic inductance traveling wave amplifier designs for practical microwave readout applications

  1. A. Giachero,
  2. M. Visser,
  3. J. Wheeler,
  4. L. Howe,
  5. J. Gao,
  6. J. Austermann,
  7. J. Hubmayr,
  8. A. Nucciotti3,
  9. and J. Ullom
A Kinetic Inductance Traveling Wave amplifier (KIT) utilizes the nonlinear kinetic inductance of superconducting films, particularly Niobium Titanium Nitride (NbTiN), for parametric
amplification. These amplifiers achieve remarkable performance in terms of gain, bandwidth, compression power, and frequently approach the quantum limit for noise. However, most KIT demonstrations have been isolated from practical device readout systems. Using a KIT as the first amplifier in the readout chain of an unoptimized microwave SQUID multiplexer coupled to a transition-edge sensor microcalorimeter we see an initial improvement in the flux noise. One challenge in KIT integration is the considerable microwave pump power required to drive the non-linearity. To address this, we have initiated efforts to reduce the pump power by using thinner NbTiN films and an inverted microstrip transmission line design. In this article, we present the new transmission line design, fabrication procedure, and initial device characterization — including gain and added noise. These devices exhibit over 10 dB of gain with a 3 dB bandwidth of approximately 5.5-7.25 GHz, a maximum practical gain of 12 dB and typical gain ripple under 4 dB peak-to-peak. We observe an appreciable impedance mismatch in the NbTiN transmission line, which is likely the source of the majority of the gain ripple. Finally we perform an initial noise characterization and demonstrate system-added noise of three quanta or less over nearly the entire 3 dB bandwidth.

Performance of a Kinetic-Inductance Traveling-Wave Parametric Amplifier at 4 Kelvin: Toward an Alternative to Semiconductor Amplifiers

  1. M. Malnou,
  2. J. Aumentado,
  3. M. R. Vissers,
  4. J. D. Wheeler,
  5. J. Hubmayr,
  6. J. N. Ullom,
  7. and J. Gao
Most microwave readout architectures in quantum computing or sensing rely on a semiconductor amplifier at 4 K, typically a high-electron mobility transistor (HEMT). Despite its remarkable
noise performance, a conventional HEMT dissipates several milliwatts of power, posing a practical challenge to scale up the number of qubits or sensors addressed in these architectures. As an alternative, we present an amplification chain consisting of a kinetic-inductance traveling-wave parametric amplifier (KI-TWPA) placed at 4 K, followed by a HEMT placed at 70 K, and demonstrate a chain-added noise TΣ=6.3±0.5 K between 3.5 and 5.5 GHz. While, in principle, any parametric amplifier can be quantum limited even at 4 K, in practice we find the KI-TWPA’s performance limited by the temperature of its inputs, and by an excess of noise Tex=1.9 K. The dissipation of the KI-TWPA’s rf pump constitutes the main power load at 4 K and is about one percent that of a HEMT. These combined noise and power dissipation values pave the way for the KI-TWPA’s use as a replacement for semiconductor amplifiers.

Sub-Kelvin Thermometer for On-Chip Measurements of Microwave Devices Utilizing Two-Level Systems in Superconducting Microresonators

  1. J. Wheeler,
  2. M. R. Vissers,
  3. M. Malnou,
  4. J. Hubmayr,
  5. J. N. Ullom,
  6. and J. Gao
We present a superconducting microresonator thermometer based on two-level systems (TLS) that is drop-in compatible with cryogenic microwave systems. The operational temperature range
is 50-1000~mK (which may be extended to 5~mK), and the sensitivity (50-75~μK/Hz−−−√) is relatively uniform across this range. The miniature footprint that conveniently attaches to the feedline of a cryogenic microwave device facilitates the measurement of on-chip device temperature and requires no additional thermometry wiring or readout electronics. We demonstrate the practical use of these TLS thermometers to investigate static and transient chip heating in a kinetic inductance traveling-wave parametric amplifier operated with a strong pump tone. TLS thermometry may find broad application in cryogenic microwave devices such as superconducting qubits and detectors.

A three-wave mixing kinetic inductance traveling-wave amplifier with near-quantum-limited noise performance

  1. M. Malnou,
  2. M. R. Vissers,
  3. J. D. Wheeler,
  4. J. Aumentado,
  5. J. Hubmayr,
  6. J. N. Ullom,
  7. and J. Gao
We present a theoretical model and experimental characterization of a microwave kinetic inductance traveling-wave amplifier (KIT), whose noise performance, measured by a shot noise
thermometer, approaches the quantum limit. Biased with a dc current, the KIT operates in a three-wave mixing fashion, thereby reducing by several orders of magnitude the power of the microwave pump tone compared to conventional four-wave mixing KIT devices. It is built in an artificial transmission line intrinsically matched to 50 Ohms, whose dispersion allows for a controlled amplification bandwidth. We experimentally measure 17.6+1.1−1.4 dB of gain across a 2 GHz bandwidth, with an input 1 dB compression power of -63 dBm within that bandwidth, in qualitative agreement with theory. Using the KIT as the first amplifier in an amplification chain, we measure a system-added noise of 0.61±0.08 K between 3.5 and 5.5 GHz, about one eighth the noise obtained when using only a representative classical amplifier. The KIT contribution to this added noise is estimated to be 0.2±0.1 K, consistent with the quantum limit on amplifier added noise. This device is therefore suitable to read large arrays of microwave kinetic inductance detectors or thousands of superconducting qubits.

Traveling wave parametric amplifier with Josephson junctions using minimal resonator phase matching

  1. T. C. White,
  2. J. Y. Mutus,
  3. I.-C. Hoi,
  4. R. Barends,
  5. B. Campbell,
  6. Yu Chen,
  7. Z. Chen,
  8. B. Chiaro,
  9. A. Dunsworth,
  10. E. Jeffrey,
  11. J. Kelly,
  12. A. Megrant,
  13. C. Neill,
  14. P. J. J. O'Malley,
  15. P. Roushan,
  16. D. Sank,
  17. A. Vainsencher,
  18. J. Wenner,
  19. S. Chaudhuri,
  20. J. Gao,
  21. and John M. Martinis
Josephson parametric amplifiers have become a critical tool in superconducting device physics due to their high gain and quantum-limited noise. Traveling wave parametric amplifiers
(TWPAs) promise similar noise performance while allowing for significant increases in both bandwidth and dynamic range. We present a TWPA device based on an LC-ladder transmission line of Josephson junctions and parallel plate capacitors using low-loss amorphous silicon dielectric. Crucially, we have inserted λ/4 resonators at regular intervals along the transmission line in order to maintain the phase matching condition between pump, signal, and idler and increase gain. We achieve an average gain of 12\,dB across a 4\,GHz span, along with an average saturation power of -92\,dBm with noise approaching the quantum limit.

Improved superconducting qubit coherence using titanium nitride

  1. J. Chang,
  2. M. R. Vissers,
  3. A. D. Corcoles,
  4. M. Sandberg,
  5. J. Gao,
  6. David W. Abraham,
  7. Jerry M. Chow,
  8. Jay M. Gambetta,
  9. M. B. Rothwell,
  10. G. A. Keefe,
  11. Matthias Steffen,
  12. and D. P. Pappas
We demonstrate enhanced relaxation and dephasing times of transmon qubits, up to ~ 60 mu s by fabricating the interdigitated shunting capacitors using titanium nitride (TiN). Compared
to lift-off aluminum deposited simultaneously with the Josephson junction, this represents as much as a six-fold improvement and provides evidence that previous planar transmon coherence times are limited by surface losses from two-level system (TLS) defects residing at or near interfaces. Concurrently, we observe an anomalous temperature dependent frequency shift of TiN resonators which is inconsistent with the predicted TLS model.