Dynamics of a dispersively coupled transmon qubit in the presence of a noise source embedded in the control line

  1. Antti Vaaranta,
  2. Marco Cattaneo,
  3. and Russell E. Lake
We describe transmon qubit dynamics in the presence of noise introduced by an impedance-matched resistor (50Ω) that is embedded in the qubit control line. To obtain the time evolution,
we rigorously derive the circuit Hamiltonian of the qubit, readout resonator and resistor by describing the latter as an infinite collection of bosonic modes through the Caldeira-Leggett model. Starting from this Jaynes-Cummings Hamiltonian with inductive coupling to the remote bath comprised of the resistor, we consistently obtain the Lindblad master equation for the qubit and resonator in the dispersive regime. We exploit the underlying symmetries of the master equation to transform the Liouvillian superoperator into a block diagonal matrix. The block diagonalization method reveals that the rate of exponential decoherence of the qubit is well-captured by the slowest decaying eigenmode of a single block of the Liouvillian superoperator, which can be easily computed. The model captures the often used dispersive strong limit approximation of the qubit decoherence rate being linearly proportional to the number of thermal photons in the readout resonator but predicts remarkably better decoherence rates when the dissipation rate of the resonator is increased beyond the dispersive strong regime. Our work provides a full quantitative description of the contribution to the qubit decoherence rate coming from the control line in chips that are currently employed in circuit QED laboratories, and suggests different possible ways to reduce this source of noise.

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.

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.

Overlap junctions for superconducting quantum electronics and amplifiers

  1. Mustafa Bal,
  2. Junling Long,
  3. Ruichen Zhao,
  4. Haozhi Wang,
  5. Sungoh Park,
  6. Corey Rae Harrington McRae,
  7. Tongyu Zhao,
  8. Russell E. Lake,
  9. Daniil Frolov,
  10. Roman Pilipenko,
  11. Silvia Zorzetti,
  12. Alexander Romanenko,
  13. and David P. Pappas
Due to their unique properties as lossless, nonlinear circuit elements, Josephson junctions lie at the heart of superconducting quantum information processing. Previously, we demonstrated
a two-layer, submicrometer-scale overlap junction fabrication process suitable for qubits with long coherence times. Here, we extend the overlap junction fabrication process to micrometer-scale junctions. This allows us to fabricate other superconducting quantum devices. For example, we demonstrate an overlap-junction-based Josephson parametric amplifier that uses only 2 layers. This efficient fabrication process yields frequency-tunable devices with negligible insertion loss and a gain of ~ 30 dB. Compared to other processes, the overlap junction allows for fabrication with minimal infrastructure, high yield, and state-of-the-art device performance.

Active protection of a superconducting qubit with an interferometric Josephson isolator

  1. Baleegh Abdo,
  2. Nicholas T. Bronn,
  3. Oblesh Jinka,
  4. Salvatore Olivadese,
  5. Antonio D. Corcoles,
  6. Vivekananda P. Adiga,
  7. Markus Brink,
  8. Russell E. Lake,
  9. Xian Wu,
  10. David P. Pappas,
  11. and Jerry M. Chow
Nonreciprocal microwave devices play several critical roles in high-fidelity, quantum-nondemolition (QND) measurement schemes. They separate input from output, impose unidirectional
routing of readout signals, and protect the quantum systems from unwanted noise originated by the output chain. However, state-of-the-art, cryogenic circulators and isolators are disadvantageous in scalable superconducting quantum processors because they use magnetic materials and strong magnetic fields. Here, we realize an active isolator formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. Nonreciprocity is generated by applying a phase gradient between the same-frequency pumps feeding the Josephson mixers, which play the role of the magnetic field in a Faraday medium. To demonstrate the applicability of this Josephson-based isolator for quantum measurements, we incorporate it into the output line of a superconducting qubit, coupled to a fast resonator and a Purcell filter. We also utilize a wideband, superconducting directional coupler for coupling the readout signals into and out of the qubit-resonator system and a quantum-limited Josephson amplifier for boosting the readout fidelity. By using this novel quantum setup, we demonstrate fast, high-fidelity, QND measurements of the qubit while providing more than 20 dB of protection against amplified noise reflected off the Josephson amplifier.

Quantum Gates for Propagating Microwave Photons

  1. Roope Kokkoniemi,
  2. Tuomas Ollikainen,
  3. Russell E. Lake,
  4. Sakari Saarenpää,
  5. Kuan Yen Tan,
  6. Janne I. Kokkala,
  7. Ceren B. Dağ,
  8. Joonas Govenius,
  9. and Mikko Möttönen
We report a generic scheme to implement transmission-type quantum gates for propagating microwave photons, based on a sequence of lumped-element components on transmission lines. By
choosing three equidistant superconducting quantum interference devices (SQUIDs) as the components on a single transmission line, we experimentally implement a magnetic-flux-tunable phase shifter and demonstrate that it produces a broad range of phase shifts and full transmission within the experimental uncertainty. Together with previously demonstrated beam splitters, these phase shifters can be utilized to implement arbitrary single-qubit gates. Furthermore, we theoretically show that replacing the SQUIDs by superconducting qubits, the phase shifter can be made strongly nonlinear, thus introducing deterministic photon–photon interactions. These results critically complement the previous demonstrations of on-demand single-photon sources and detectors, and hence pave the way for an all-microwave quantum computer based on propagating photons.