Microwave calibration of qubit drive line components at millikelvin temperatures

  1. Slawomir Simbierowicz,
  2. Volodymyr Y. Monarkha,
  3. Suren Singh,
  4. Nizar Messaoudi,
  5. Philip Krantz,
  6. and Russell E. Lake
Systematic errors in qubit state preparation arise due to non-idealities in the qubit control lines such as impedance mismatch. Using a data-based methodology of short-open-load calibration
at a temperature of 30 mK, we report calibrated 1-port scattering parameter data of individual qubit drive line components. At 5~GHz, cryogenic return losses of a 20-dB-attenuator, 10-dB-attenuator, a 230-mm-long 0.86-mm silver-plated cupronickel coaxial cable, and a 230-mm-long 0.86-mm NbTi coaxial cable were found to be 35+3−2 dB, 33+3−2 dB, 34+3−2 dB, and 29+2−1 dB respectively. For the same frequency, we also extract cryogenic insertion losses of 0.99+0.04−0.04 dB and 0.02+0.04−0.04 dB for the coaxial cables. We interpret the results using a master equation simulation of all XY gates performed on a single qubit. For example, we simulate a sequence of two 5 ns gate pulses (X & Y) through a 2-element Fabry-Pérot cavity with 400-mm path length directly preceding the qubit, and establish that the return loss of its reflective elements must be >9.42 dB (> 14.3 dB) to obtain 99.9 % (99.99 %) gate fidelity.

Broadband continuous variable entanglement generation using Kerr-free Josephson metamaterial

  1. Michael Perelshtein,
  2. Kirill Petrovnin,
  3. Visa Vesterinen,
  4. Sina Hamedani Raja,
  5. Ilari Lilja,
  6. Marco Will,
  7. Alexander Savin,
  8. Slawomir Simbierowicz,
  9. Robab Jabdaraghi,
  10. Janne Lehtinen,
  11. Leif Grönberg,
  12. Juha Hassel,
  13. Mika Prunnila,
  14. Joonas Govenius,
  15. Sorin Paraoanu,
  16. and Pertti Hakonen
Entangled microwave photons form a fundamental resource for quantum information processing and sensing with continuous variables. We use a low-loss Josephson metamaterial comprising
superconducting non-linear asymmetric inductive elements to generate frequency (colour) entangled photons from vacuum fluctuations at a rate of 11 mega entangled bits per second with a potential rate above gigabit per second. The device is operated as a traveling wave parametric amplifier under Kerr-relieving biasing conditions. Furthermore, we realize the first successfully demonstration of single-mode squeezing in such devices – 2.4±0.7 dB below the zero-point level at half of modulation frequency.

Characterizing cryogenic amplifiers with a matched temperature-variable noise source

  1. Slawomir Simbierowicz,
  2. Visa Vesterinen,
  3. Joshua Milem,
  4. Aleksi Lintunen,
  5. Mika Oksanen,
  6. Leif Roschier,
  7. Leif Grönberg,
  8. Juha Hassel,
  9. David Gunnarsson,
  10. and Russell E. Lake
We present a cryogenic microwave noise source with characteristic impedance of 50 Ω that can be installed in a coaxial line of a cryostat. The bath temperature of the noise source
is continuously variable between 0.1 K and 5 K without causing significant back-action heating on the sample space. As a proof-of-concept experiment, we perform Y-factor measurements of an amplifier cascade that includes a traveling wave parametric amplifier and a commercial high electron mobility transistor amplifier. We observe system noise temperatures as low as 680+20−200 mK at 5.7 GHz corresponding to 1.5+0.1−0.7 excess photons. The system we present has immediate applications in the validation of solid-state qubit readout lines.